UNIVERSITÀ DI PISA
Dipartimento di Scienze Veterinarie
Corso di Laurea Magistrale in Medicina Veterinaria
“Analisi dell’emogas arterioso e venoso e degli elettroliti nel Gabbiano Reale
Mediterraneo (Larus michahellis) durante la chirurgia ortopedica”
“Analysis of arterial and venous blood gases and electrolytes in the Yellow-legged
gull (Larus michahellis) during orthopedic surgery"
Candidato: Silvia Galli
Relatori:
Prof. Grazia Guidi
Dott. Renato Ceccherelli
Ai miei amati nonni
To my beloved grandparents
Index
Riassunto
p.1
Abstract
p.2
Introduction
p.3
CRUMA
p.3
Species object of the study: The Yellow-legged gull
p.3
Anatomy mentions of the yellow-legged gull
p.5
Cardiovascular System
p.5
Respiratory System
p.6
Exchange of gases
p.6
Notes on Physiology
p.8
Blood Gases and electrolytes
p.9
Anaesthesia Mentions
p.26
Premedication drugs
p.26
Opioids
p.27
Benzodiazepines
p.27
Inhalational Anaesthetics
p.28
Anesthesiological Circuits
p.29
Monitoring of the patient
p.30
Materials and Methods
p.33
Other analyses
p.40
Statistical analyses
p.41
Comparison between arterial and venous samples
p.52
Discussion and Conclusion
p.58
Bibliography
p.70
Riassunto
Lo scopo della tesi è di valutare le variazioni dell’emogasanalisi nel Gabbiano Reale Mediterraneo (Larus michahellis) durante la chirurgia ortopedica e di valutare le differenze tra l’emogas arterioso e quello venoso per stabilire se possono essere usati indifferentemente in corso di chirurgia.
Lo studio, condotto presso il CRUMA LIPU (Centro recupero uccelli marini ed acquatici – Lega Italiana Protezione Uccelli) di Livorno, ha coinvolto 10 individui di gabbiano reale mediterraneo sottoposti a chirurgia ortopedica a seguito di traumi di varia origine e successivamente liberati dopo opportuna riabilitazione. I prelievi sono stato effettuati subito dopo l’induzione anestetica e dopo 60 minuti dall’induzione a livello della vena metatarsale e tramite un catetere arterioso a livello dell’arteria ulnare profonda. L’anestesia prevedeva premedicazione con Sufentanyl (20mcg/kg) e Midazolam (2mg/kg) ed induzione con isoflurano (3,5%) in flusso di ossigeno di 1.5 L/min. Il paziente è stato sottoposto a ventilazione controllata a pressione positiva (allo scopo di mantenere la concentrazione di anidride carbonica di fine espirazione (ETCO2) tra 30 e 45 mmHg) e fluidoterapia di mantenimento con Ringer
Lattato per tutta la durata della chirurgia. I campioni di sangue sono stati analizzati a 30 minuti dal prelievo e conservati anaerobicamente in un refrigeratore portatile con panetti di ghiaccio fino al momento dell’analisi.
I campioni sono stati analizzati per i valori di pH, pressione parziale di anidride carbonica (pCO2), pressione
parziale di ossigeno (pO2), ematocrito (Hct), concentrazione totale di emoglobina (ctHb), saturazione di
ossigeno (sO2), concentrazione plasmatica di bicarbonato (HCO3-), eccesso di basi (BE), anion gap (AG),
concentrazione plasmatica di glucosio (cGlu), concentrazione plasmatica di lattati (cLat), osmolalità del plasma (mOsm), sodio, potassio, cloro e calcio ionizzato.
L’analisi del sangue arterioso ha rilevato un aumento significativo per i valori di sodio ed una diminuzione significativa per i valori di ctHb ed Hct. Nei campioni di sangue venoso è stata trovata una diminuzione significativa per i valori di pCO2, HCO3-, LAC, mOsm e un aumento significativo per il valore di sO2.
È stata osservata una correlazione positiva tra i valori di ETCO2 e pCO2, quest’ultimo sottostimando il valore
di ETCO2 in media di 1.5mmHg.
Lo studio ha rivelato importanti differenze significative, per quasi tutti i valori, tra il sangue arterioso e quello venoso, ad eccezione di sodio, mOsm, Hct e ctHb.
Durante la chirurgia tutti gli animali sono rimasti stabili e non hanno mostrato complicazioni, indicando che il protocollo anestetico utilizzato, associato alla ventilazione controllata e ai fluidi di mantenimento, risulta ottimale per questa specie e che non comporta importanti variazioni dell’equilibrio acido-base.
In questo studio non esiste un accordo clinicamente valido tra i valori dell’emogas arterioso e venoso, indicando che anche i singoli valori ottenuti non possono essere utilizzati intercambiabilmente.
I risultati specie-specifici riportati in questo studio possono servire da riferimento per la fisiologia e l’anestesia del Gabbiano Reale Mediterraneo e fornire un contributo al crescente database di informazioni relative a molte specie di uccelli.
Abstract
The aim of this study was to evaluate the eventual differences in blood gas analysis for the Yellow-legged gull (Larus michahellis) during orthopedic surgery and to evaluate the differences between arterial and venous blood gas to determine whether they can be used interchangeably during surgery.
The study, conducted at CRUMA LIPU (Wildlife Rescue Centre for sea and water birds – Italian League for the protection of birds), involved 10 individuals of yellow-legged gulls undergoing orthopedic surgery due to different injuries and subsequently released after appropriate rehabilitation.
The blood samples were collected immediately and after 60 minutes from anesthetic induction, from the metatarsal vein and from an arterial catheter placed in the deep ulnar artery. Birds were premedicated with Sufentanyl (20mcg/kg) and Midazolam (2mg/kg) and inducted with isoflurane (3.5%) at an oxygen flow of 1.5L/min. A mechanical ventilation with intermittent positive pressure was provided to maintain end tidal CO2 (ETCO2) between 30 and 45 mmHg and maintenance fluid therapy was provided throughout the
duration of the surgery. Blood samples were analysed after 30 minutes from collection and anaerobically stored in a portable ice cooler until analysis. Samples were analysed for pH, partial pressure of carbon dioxide (pCO2), partial pressure of oxygen (pO2), total haemoglobin concentration (ctHb), blood oxygen
saturation (sO2), bicarbonate plasma concentration (HCO3-), base excess (BE), anion gap (AG), plasma
glucose concentration (cGlu), plasma lactate concentration (cLat), plasma osmolality (mOsm) and electrolytes values of sodium, potassium, chlorine and ionized calcium.
Arterial blood analysis revealed a significant increase in sodium values and a significant decrease in ctHb and Hct values. In venous blood samples, a significant decrease was found for the values of pCO2, HCO3-,
LAC, mOsm and a significant increase for the value of sO2.
A positive correlation was observed between the values of ETCO2 and pCO2, the latter underestimating the
ETCO2 value of 1.5mmHg on average.
The study revealed clinically significant differences, for almost all values, between arterial and venous blood, except for sodium, mOsm, Hct and ctHb values.
During the surgery, all the animals remained stable and showed no complications, indicating that the anesthetic protocol used, associated with mechanical ventilation and maintenance fluids, is optimal for this species and is not associated with important variations in the acid-base equilibrium. In this study, the agreement between arterial and venous blood gas analysis was too poor and unpredictable to be clinically useful and single values cannot be used interchangeably.
The analysis of blood gas analytes documented in this thesis and the specie-specific values reported will help to provide the baseline data of the physiology and anaesthetic management of yellow legged gulls and can be used as a reference to the growing pool of vital information in bird’s species as well as provide a basis for further research.
Introduction
CRUMA (Centro Recupero Uccelli Marini ed Acquatici – Wildlife Rescue Centre for sea and water birds) – LIPU
CRUMA is a Rescue and Recovery Centre located in Livorno (Tuscany, Italy) and it is part of the Italian League for the Protection of Birds (LIPU).
It is a major reference point for wildlife animals rescue in Tuscany and for veterinary practice all over Italy, as it is provided with a large clinic where injured or critically ill animals can undergo first aid, medical examination and surgeries. The centre offers recovery mainly to all the local avifauna (from pigeons to raptors) but also to local amphibians, reptiles and wild mammals such as bats, hedgehogs, squirrels, badgers, foxes and larger herbivores, recovering more than 4.000 specimen per years (CRUMA database).
The aim of CRUMA is to protect the Italian wildlife, taking care of the animals recovered and then, after a period of rehabilitation and adaptation, reintroducing them safely into nature.
Species object of the study: The Yellow-legged gull
The Yellow-legged gull (Larus michahellis, J.F. Naumann, 1840) (Fig.1.1) is a large gull distributed in Europe, the Middle East and North Africa and has recently achieved the recognition as a distinct species, with the distinction in two subspecies: Larus michahellis michahellis and Larus michahellis
atlantis. The first one is diffused within the Mediterranean region (Gill and Donsker, 2016)
This seagull displays large migratory patterns in summer and autumn between inshore sites and coasts of central and western Europe (where it spends winter time), but a large number of individuals get through the winter within the Mediterranean region, being mainly sedentary in Italy, with larger movements between June-October and December-April in relation to reproductive sites. It is common the presence of individuals from northern Europe during winter (Bird Life International, 2014)
The actual population in Italy it is estimated to be around 150.000-350.000 individuals, widely distributed in coastal areas, urban sites, port towns, beaches, rivers, lakes, dumps sites, cultivated areas and wetlands (Brichetti and Fracasso, 2006).
In Livorno, where CRUMA is located, the yellow-legged seagull lives and colonizes all places within the city, easily found on roofs and attics and feeding on the streets (Arcamone and Franceschi, 2006). Due to these habit, it is easy for this species to share its territory with humans and consequently to be subjected to injuries by accident or on purpose. The main injuries these animals are subjected to are fractures, caused mainly by collisions with cars, electric cables or illegal gunshots (CRUMA Database). It is also very common to find intoxicated or poisoned animals, as this species is vulnerable to hydrocarbons (Bird Life International, 2014).
Figure 1.1 A Yellow-legged seagull (Larus michahellis michahellis)
Anatomy mentions of the yellow-legged gull
Cardiovascular System
Birds have very efficient cardiovascular systems that permit them to meet the metabolic demands of flight (and running, swimming, or diving). The cardiovascular system not only delivers oxygen to
body cells (and removes metabolic wastes) but also plays an important role in maintaining a bird's body temperature.
Birds, like mammals, have a 4-chambered heart, with complete separation of oxygenated and de-oxygenated blood. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. Because the left ventricle must generate greater pressure to pump blood throughout the body (in contrast to the right ventricle that pumps blood to the lungs), the walls of the left ventricle are much thicker and muscular.
Birds tend to have larger hearts than mammals (relative to body size and mass), with a heart rate at rest from 150 to 350 beats per minute and low peripheral resistances. The relatively large heart is necessary to meet the high metabolic demands of flight. Among birds, smaller birds have relatively larger hearts than larger birds. Hummingbirds have the largest hearts of all birds, with a percentage of heart weight versus body mass of 20%, and a heart rate up to 1000 bpm (Aspinall and Cappello, 2009)
In order to maintain the high arterial pressures the body needs (108-250 mmHg), arterial vessels are less flexible than those of mammals, causing them to easily break (especially when stressed), and therefore conducing to aortic breakdown, heart failure, and bleeding (Welty, 1982). Hypoxia, hypercapnia, and anaesthetics (depending on the type and dose) may depress the cardiovascular function causing critical situations for the life of the bird (Bufalari et al. 2012).
The total blood volume of birds may vary according to the species but it is generally between 5% and 13% of the body weight: for that it is recommended to avoid taking more than 8% of total blood volume in case of blood sampling (O’Malley, 2005). Blood collection is not a simple task in birds, as they are easily stressed and feature fragile and hardly accessible blood vessels: thus, it is recommended to use sedation while carrying out such manipulations (Ludders and Matthews, 2007; Cunningham, 2006).
Differences between the red blood cells of birds and mammals, which had developed an aerobic metabolism, emerged in the Triassic, when the oxygen content in the atmosphere was approximately 50% lower than current levels and even lower than in the Jurassic period (when birds evolved). Under these conditions, natural selection favoured the loss of nuclei in the red blood cells of mammals (making the cells smaller and allowing capillaries to become even smaller in diameter) and change to a biconcave shape (increasing the amount of surface area and enhancing diffusion into and out of the red blood cells). Birds, with their efficient respiratory
system, evolved during the Jurassic when the oxygen content in the Earth atmosphere approached the present level, so there was no selective pressure to eliminate nuclei from their red blood cells or change in shape (Gavrilov, 2013).
The avian cardiovascular system can quickly respond to changes in levels of activity (e.g., resting vs. flying) via changes in heart rate, cardiac output and blood flow (by vasoconstriction and vasodilatation of vessels) (Machida and Aohagi, 2001).
Respiratory System
The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration but not directly involved in the exchange of gases. The air sacs permit a unidirectional flow of air through the lungs, meaning that the air that moves through them is largely 'fresh' air and has a higher oxygen content. In contrast, mammals have a bidirectional air flow, with air moving back and forth in and out the lungs. As a result, air coming into a mammal’s lungs is mixed with old air, that has been in the lungs for a while and this mixed air has less oxygen. Instead, in bird lungs, more oxygen is available to diffuse into the blood. The avian pulmonary system uses "flow-through ventilation," relying on the flexible air sacs that act like bellows to move air through the almost completely rigid lungs. As said, air sacs do not take part in the actual oxygen exchange, but do greatly enhance its efficiency and allow for the high metabolic rates found in birds. This system also keeps the volume of air in the lungs nearly constant (Aspinall and Cappello, 2009; Ludders and Matthews, 2007; Cunningham, 2006).
Exchange of gases
In the avian lungs, oxygen diffuses from the air capillaries into the blood and carbon dioxide from the blood into the air capillaries. This exchange is very efficient in birds: the complex arrangement of blood and air capillaries in the lungs creates a substantial surface area through which gases can diffuse. The surface area available for exchange (SAE) varies with bird size: smaller birds have a greater SAE per unit mass than do larger birds (Maina, 2008). Among mammals, there is also a negative relationship between SAE and body size, with smaller mammals having a greater SAE per
unit mass than larger mammals. However, for birds and mammals of similar size, the SAE of birds is generally about 15% greater (Maina et al. 1989).
A second reason why gas exchange in avian lungs is so efficient is that the blood-gas barrier through which gases diffuse is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness. Among terrestrial vertebrates, the blood-gas barrier is the thinnest in birds and, among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds. For example, the blood-gas barrier is 0.09 μm thick in Violet-eared Hummingbirds (Colibri coruscans) and 0.56 μm thick in Ostriches (Struthio camelus) (West 2009).
The efficiency of gas exchange in avian lungs is also guaranteed by a process called cross-current exchange (Piper & Scheid, 1975). Air passing through air capillaries and blood moving through blood capillaries generally travel at right angles to each other in what is called cross-current flow (Makanya and Djonov 2009). As a result, oxygen diffuses from the air capillaries into the blood at many points along the length of the parabronchia, resulting in a greater concentration of oxygen (i.e., higher partial pressures) in the blood leaving the lungs than is possible in the alveolar lungs of mammals.
Breathing is done by a 4-stroke pump mechanism, with a predominantly unidirectional flow. During the inhalation process the air is transported directly to the caudal air sacs without crossing the exchange tissue, while the air that was already subjected to hematosis passes into the cranial sacs.
During exhalation the air passes from the caudal air sacs to the exchange tissue, while the one present in the cranial sacs passes outward.
Thanks to these functional-anatomic characteristics, the oxygen partial pressure of blood leaving the lungs (PaO2) is higher than that of the exhaled (PeO2), while in a system such as the one in
mammals it reaches an equilibrium (Powell and Whittow, 2000).
This mechanism is optimal for gases exchanges and for this reason birds can breathe with no problems at higher altitudes (up to 8000 meters high, where oxygen partial pressure is reduced by one-third with respect to the value at sea level) (Aspinall and Cappello, 2009; Ludders and Matthews, 2007; Cunningham, 2006).
Notes on Physiology
In birds, as in mammals, mainly two organs are responsible for maintaining the normal acid-base equilibrium and for compensating in case of acid-base disturbance: the lungs and the kidneys. Lungs regulate the CO2 concentration in blood: higher CO2 is correlated with more acidic pH
values. Respiratory acid-base alterations that increase or decrease CO2 concentration are generally
correlated with dysfunction of neurologic, respiratory o musculoskeletal systems. Kidneys possess a lot of regulatory mechanisms that control the amount of acid or base excreted in urine. The gastrointestinal tract, the cellular metabolism and liver dysfunctions can contribute to alterations of acid and bases within the body (Montesinos and Ardiaca 2013).
Thanks to the volatile nature of CO2 that can be exhaled from the body very quickly, the
respiratory system is able to compensate rapidly for non-respiratory disturbances. Renal function, on the other hands, is responsible for regulating acid or base excretion from the body but takes longer to compensate for respiratory disturbance although it can be very efficient if given enough time. In severely ill patients, alterations may be present in multiple body systems and compensation may results inadequate or absent leading to mixed acid-base disturbances (Montesinos and Ardiaca 2013).
Most birds maintain an alkaline arterial pH of approximately 7.5, despite a constant metabolic production of acid. Most of this acid is excreted as respiratory CO2. However, non-volatile acids
(i.e., H2SO4 and H3PO4) are also a threat to acid-base homeostasis and the kidneys must excrete
the protons equivalent to these metabolic end products. Thus, avian urine is typically acidic, with a pH in the range of 5.5 to 7.5. The renal defence of arterial alkaline pH is presumed to consist of two components: conservation of the base (bicarbonate) and excretion of acid (H+, largely
buffered). Three compounds (ammonia, phosphate, and urate) serve as the primary urinary buffers in birds (Montesinos and Ardiaca 2013).
Blood Gases and electrolytes
Physiologic parameters such as blood gas analytes and electrolytes are important factors to be used in veterinary clinical care of both mammals and birds, as they allow the assessment of breathing efficiency, tissue oxygenation, acid-base balance, cardiovascular system status and the overall health of the animal and can be an important tool in the diagnostic evaluation and therapeutic intervention in critically ill birds (Day 2002, Bateman 2008).
In emergency situation, blood gas analysis facilitates a rapid diagnosis of metabolic and respiratory disorders from a small volume of blood, allowing vital therapeutic intervention and diagnostic decision making (Ardiaca et al. 2013). The monitoring of this parameters in anaesthetised birds is essential for the recognition and the rapid correction of any complication that may affect the normal body functions (Chemonges and Filippich 1999, Jaensch et al. 2001).
Venous blood gas analysis reflects the acid-base balance at a cellular level, as it contains cellular waste products, whereas arterial blood gas analysis is the gold standard for assessing an animal’s ventilation status, tissue perfusion and efficiency of the respiratory gas exchange in the lungs (Malatesha et al. 2007, Montesinos et al. 2013, Schnellbacher et al. 2014, Bateman 2008).
However, there are limited amount of data available for avian blood gas values: few arterial blood gas studies have been performed on different species of birds (determining only values of pH, pCO2 and pO2) (Montesinos et al. 2013) (Table 1.1) while more data analysis on arterial blood were
Bird Species pH pCO2 PO2
Female Black Bantam chicken 7.48 29.90 -
Female White Leghorn chicken 7.52 33.00 82.00
Male White Rock chicken 7.53 29.20 -
Mallard duck 7.56 30.80 81.00 Muscovy duck 7.49 38.00 82.00 Muscovy duck 7.46 36.90 96.10 Pekin duck 7.46 28.00 93.50 Pekin duck 7.48 33.80 100.00 Emu 7.45 33.80 99.70 Bar-headed goose 7.47 31.60 92.50 Domestic goose 7.52 32.00 97.00 Herring gull 7.56 27.20 - Red-tailed hawk 7.49 27.00 108.00 Burrowing owl 7.46 32.60 97.60 White pelican 7.50 28.50 - Adelie penguin 7.51 36.90 83.80 Chinstrap penguin 7.52 37.10 89.10 Gentoo penguin 7.49 40.90 77.10 Pigeon 7.50 40.90 77.10 Roadrunner 7.58 24.50 - Abdim’s stork 7.56 27.00 - Mute swan 7.50 27.10 91.30 Turkey vulture 7.10 27.50 - Amazon Parrots 7.45 22.10 98.10
Table 1.1 Arterial Blood gases and pH in non-anesthetized birds breathing air. Data from Powerll FL. Respiration. In: Caussey Whittow G, editor. Sturkie’s avian physiology. 5th edition. San Diego (CA): Academic
pH 7.5 pCO2 (mmHg) 22.1 pO2 (mmHg) 98.1 HCO3 (mmol/L) 14.8 sO2 (%) 96.2 Hb (g/dL) 13.2 Hct (%) 38.7 BE (mmol/Lt) -7.9 Na (mmol/Lt) 147.4 K (mmol/Lt) 3.5 iCa (mmol/Lt) 0.8
Respiration Rate (Breaths/min) 82
Temperature (°C) 41.8
Table 1.2 Values of arterial blood acid-base status and electrolytes in non-anesthetized Amazon Parrots (Amazona aestiva). Data from Paula VV, Fantoni DT, Otsuki DA, et al. Blood and Electrolyte values for Amazon parrots. Pesq. Vet Bras 28 (2): 108-112 (2008)
Only one study investigated arterial and venous blood gases and electrolytes of Gyr Falcons under isoflurane anaesthesia (Raghav et al. 2015) (Table 1.3).
Arterial Venous pH 7.42 7.42 pCO2 (mmHg) 36.76 35.45 pO2 (mmHg) 203.70 140.30 HCO3 (mmol/L) 23.60 22.40 sO2 (%) 99.86 98.56 Glu (mg/dl) 325.10 317.00 Hb (g/dL) 13.90 14.31 Hct (%) 41.03 42.10 BE (mmol/Lt) - -0.93 AG 0.36 - Na (mmol/Lt) 146.50 148.00 K (mmol/Lt) 3.35 3.27 iCa (mmol/Lt) 1.07 1.05
Table 1.3 Arterial and Venous blood gases in Gyr Falcons under anaesthesia. Data from Raghav R., Middleton R., Ahamed R., Arjunan R., Caliendo V., Analysis of arterial and venous blood gases in healthy Gyr falcons (Falco rusticolus) under anaesthesia, Journal of Avian Medicine and Surgery, 29 (4): 290-297 (2015)
A higher number of studies investigated venous blood gases in different species of birds, like chickens, ducks, pigeons, psittacine birds, passerine birds, falcons and vultures, by using different protocols and methodologies (Kenny et al. 2015, Raghav et al. 2015, Steinmetz et al. 2007,
Schnellbacher et al. 2014, Heatley et al. 2005, Olanrewaju et al. 2006, Arca-Ruibal et al. 2007, Schoemaker et al. 2007, Paula et al. 2008, Harms et al. 2012, Heatley et al. 2013).
Some studies were conducted under anaesthesia: one on ducks (Ludders et al. in 1990) reporting only pH, pCO2 and pO2 values and two on Sulphur-Crested Cockatoos (Chemonges et al. 2012) and
African Grey Parrots (Edling et al. 2001) reporting more data. McKinney (2003) sampled 70 healthy falcons to establish normal parameters on venous blood and the falcons were anesthetized using isoflurane. Arca-Ruibal and colleagues (2007) described blood gases and electrolytes in 59 healthy falcons under ketamine-medetomidine anaesthesia. Most of these studies compared the effects of anaesthetic conditions, exercise or changes in altitude in the bird subjects compared with their baseline values (Table 1.4, 1.5, 1.6, 1.7).
To our knowledge, there are no arterial and venous blood gas and electrolytes data available in literature for the yellow-legged gull.
Table 1.5 Venous blood gases data from different species of birds. See tables for references.
Table 1.7 Venous blood gases data from different species of birds. See tables for references.
Even if the analysis of arterial samples is the gold standard, this procedure is not done routinely due to the fact that this is not practical in most bird species, as it requires an arterial catheter placement under anaesthesia and there are more potential negative health complications and side effects from arterial venipuncture (Malatesha et al. 2007, Desmarchelier et al. 2007, Schnellbacher et al. 2014, Kenny et al. 2015).
A high degree of agreement for pH, pCO2, HCO3-, base excess, and lactate is reported when
comparing arterial and central venous values and these findings might allow venous sampling to be used for measuring these variables in certain setting. However, there is poor correlation comparing arterial pO2 with venous pO2 (Silverstein and Hopper 2009, Cho 2014, Yildizas et al.
2004, Malatesha et al 2007, Middleton et al. 2006, Kelly et al. 2010), suggesting that this parameter should not be considered valid when evaluated from venous blood. In human patients, there are conflicting studies and there is no data confirming that this level of agreement is maintained in shock states or in patients with severe hemodynamic compromise (Kelly 2010, Adrogue et al. 1989). There is only one study reported in birds that compared venous and arterial pCO2, pO2 and pH on healthy Gyr Falcons (Raghav et al. 2015), and even if a good degree of
agreement was found for pH and pCO2 values, this should be considered when evaluating venous
blood in an avian patient in shock state.
Blood gas analysers have become a useful tool in the diagnosis and treatment of avian acid-base conditions: the most used instruments to do so are point-of-care blood gas analyser that can measure directly the pH, partial pressure of oxygen (pO2), partial pressure of carbon dioxide
(pCO2) and can calculate haemoglobin saturation with oxygen (sO2), bicarbonate concentration
(HCO3-), total CO2 concentration and base excess (cBase or BE) of the extracellular fluid
(Montesinos et al. 2013, Chemonges 2012, Gardhouse and Eshar 2016, Apula et al. 2008, Kenny et al. 2015, Mckinney 2003).
Oxygen and carbon dioxide are usually reported as partial pressure, since according to Henry’s law the partial pressure of a gas is proportional to its concentration at a given temperature and pressure. However, as temperature decrease, the solubility of oxygen and carbon dioxide in blood increases, meaning that the relationship of partial pressure to the total content of O2 and CO2
within the fluid changes. The opposite is true if the temperature increases (Bacher 2005). Thus, results from blood gas analysis can be altered by several factors, including the higher core body temperature found in birds. Blood gas analysers have temperature conversion formulas within their software to correct for core body temperature differences, however there is only one study (Schoemaker et al. 2007) determining the significantly high correlation and accuracy of this formula in racing pigeons with a mean cloacal temperature of 42.1°C. That finding is assumed to be applicable in most species of birds but, because it is not yet validated, it can influence the results of most studies (Raghav et al. 2015).
Blood pH represent the concentration of H+ ions and the overall balance of all the acids and bases
within the body. It is the first parameter to consider in the evaluation of the acid-base equilibrium of a patient. It ranges between 0 and 14: many physiologic processes are optimized at or near a pH of 7.4. It is influenced by respiratory (pCO2) and metabolic (HCO3-) components and its value
express the balance between respiratory, renal and blood buffer systems (D’ Orazio et al. 2009, Silverstain et al. 2009, Burkitt et al. 2012).
The alteration of pH must be correlated to pCO2 and HCO3- values. The pCO2 reflects the
respiratory component of a certain pH alteration, while HCO3- reflects the metabolic component
(Burkitt et al 2012, Silverstain et al. 2009).
Reduced pH value (acidemia) may result from a metabolic disorder and the condition is called metabolic acidosis. If the patient breathes spontaneously, the acidosis is generally partially compensated by hyperventilation, which results in a reduction in pCO2 values.
Reduced pH value can also be related to a respiratory disorder (respiratory acidosis). If this condition persists, the excretion of bicarbonate by the kidneys is reduced in order to counteract partially or totally the acidosis by increasing the concentration of bicarbonate in the blood. A compensated respiratory acidosis is characterized by a slightly lower pH, with a high pCO2 and a
high concentration of bicarbonate (Silverstain et al. 2009).
High pH value (alkaline) may be due to a metabolic disorder (metabolic alkalosis). In patients breathing spontaneously, alkalosis can be compensated by a slight decrease in ventilation, which results in a slight increase in pCO2.
High pH value may be also related to a respiratory disorder (respiratory alkalosis) The cause can be hyperventilation with excessive lung elimination of CO2 (Burkitt et al 2012, Silverstain et al. 2009).
A good rule is that pH generally changes in the same direction as the primary disorder (Montesinos and Ardiaca 2013).
Thanks to compensatory mechanisms, a pH value near the standard does not exclude the presence of an acid-base imbalance. Therefore, even when the pH is normal, the assessment of the acid-base equilibrium must always include pCO2 and HCO3- values and then the anion gap and
base excess (Silverstain et al. 2009).
Partial pressure of carbon dioxide (pCO2) provides essential information regarding ventilation. It
represents the proportion of carbon dioxide in gas phase that is in equilibrium with the blood. It reflects the adequacy of ventilation in relation to metabolism rate and it is used to assess the respiratory component of an acid-base imbalance as well to evaluate the efficacy of pulmonary gases exchange. It is an excellent marker to evaluate the level of CO2 in the tissues (balance
between arterial pCO2, metabolic production of CO2 and tissue perfusion) (Burkitt et al 2012,
A partial pressure of pCO2 > 45 mm Hg (hypercapnia) in most domestic species is associated with
hypoventilation and indicate respiratory acidosis, associated with anaesthesia, sedation, altered
breathing mechanics (e.g. insufficient artificial ventilation), improper air flow through the airways for upper or lower airway obstruction (e.g. a mass in the coelomic cavity compressing the air sacs), acid-base balance alterations, trauma, edema (Burkitt et al. 2012).
Hyperventilation, which can lead to respiratory alkalosis, is characterized by decreases in pCO2 <
35 mm Hg in most domestic species (hypocapnia). Common causes of hyperventilation include hypoxemia, pulmonary disease, pain, anxiety, excessive manual or mechanical ventilation, or compensatory mechanisms for metabolic acidosis (Montesinos et al. 2013). Avian venous pCO2
levels are typically lower than in mammals (Calder and Schmidt 1968) and in many studies lower values were found, suggesting that this could be normal for avian species (Paula et al. 2008, Montesinos et al. 2013, Raghav et al. 2015, Kenny et al. 2015). One explanation can be hyperventilation following capture or a more complete ventilation for avian species compared to mammals (Calder and Schmidt 1968).
Partial pressure of oxygen (pO2) gives a good indication of tissue perfusion. It represents the
oxygen in the gaseous phase that is in equilibrium with the blood and is used to assess the extent to which the body is able to absorb oxygen in the lungs. pO2 should be evaluated together with
oxygen saturation (sO2) and total haemoglobin content (ctHb) in order to have a complete
assessment of oxygen availability (Burkitt et al. 2012). It should be carefully evaluated if the patient is under oxygen therapy (D’Orazio et al. 2009).
Low values (arterial hypoxia) indicate an inadequate intake of oxygen within the lungs due to pulmonary, circulatory or respiratory anomalies. Tissue hypoxemia occurs when pO2 values are <
80 mm Hg. Presence of persistently hypoxemic conditions can be life threatening, and a pO2 value
< 60 mm Hg warrants immediate therapeutic intervention.
High values may occur during oxygen therapy (modification of FiO2). This condition involves the
risk of oxygen toxicity (free radical formation) if maintained at high levels. An elevated pO2 can
also derive from an artefact when there are one or more air bubbles in the sample (atmospheric pO2 can increase the value of blood pO2 up to 150 mmHg). pO2 measured on arterial blood
samples allows to assess the adequacy of respiratory exchanges and oxygen transport in the patient (Di Bartola et al. 2012, Silverstain et al.2009, D’Orazio et al. 2009, Burkitt et al. 2012).
Total haemoglobin concentration (ctHb) is the amount of haemoglobin (Hb) actually present in
the blood. Haemoglobin is a globular protein containing an “eme” ring with an iron atom that allows it to carry the gases and it represents the measurement of the potential capacity of the blood to charge oxygen (Burkitt et al. 2012). Blood gas analysers calculate the ctHb and determine the amount of oxygen actually delivered by the arterial blood (Blood gases analysis Manual 2005). Decreased ctHb indicates a reduced blood capability to transport oxygen, hence a reduction in oxygen content within arterial blood with a risk of tissue hypoxia. Frequent causes of low ctHb values are those that cause anaemia: primitive forms (impaired production of red blood cells) or secondary forms (haemolysis, haemorrhage, dilution or hyper-hydration). The physiological mechanisms for the compensation of a low ctHb concentration is represented by an increase in cardiac output and an increase in red blood cell production (Blood gases analysis Manual, 2005). Increased ctHb indicates high blood viscosity. Frequent cause of high ctHb values is polycythaemia (true polycythaemia, dehydration, chronic pneumopathy, chronic heart disease, high altitude residence, athletic fitness condition). An increased blood viscosity can increase the cardiac load, causing circulatory failure and in extreme cases microcirculation failure (Blood gases analysis Manual, 2005).
However, normal ctHb does not necessarily guarantee a normal oxygen transport capacity. In fact, if high concentrations of abnormal haemoglobin are present, the actual transport capacity will be significantly reduced. ctHb should be evaluated together with haematocrit values (Hct) (better if obtained with a microhematocrit analyser) and total proteins in order to properly evaluate the mentioned blood disorders (polycythaemia, anaemia) (Blood gases analysis Manual, 2005)
Blood Oxygen saturation (sO2) is the percentage of oxygenated haemoglobin in relation to the
amount of haemoglobin actually capable of transporting oxygen.
Low sO2 indicates a decrease in lung oxygen uptake, sepsis, cytotoxic hypoxia (Silverstain et al.
2009).
High or normal sO2 indicates an adequate O2 transport capacity and a potential risk of hyperoxia.
Normal values of sO2 does not guarantee proper blood oxygenation as blood oxygen content can
be reduced due to low haemoglobin concentrations. Therefore, to better evaluate the respiratory function, ctHb should be considered. This parameter assumes clinical significance when measured
on arterial blood samples (in association with pO2 and ctHb) or mixed arterial and venous blood
(Blood gases analysis Manual 2005, Silverstain et al. 2009).
Bicarbonate plasma concentration (HCO3-): the metabolic contribution to the acid-base balance
can be assessed with HCO3- and the base excess of the extracellular fluid. HCO3- is a basic
component of the blood buffer system and its alteration allows to identify acid-base equilibrium, metabolic disorders and, together with chlorine and proteins, it plays an important role in maintaining the electrical neutrality of extracellular and intracellular fluids. HCO3- level are
regulated by the kidneys: it increases with the presence of alkalosis and decreases in the presence of acidosis. HCO3- is contained within the Anion Gap formula (Blood gases analysis Manual, 2005).
Raghav et al. 2015 reported in anesthetized Gyr Falcons a baseline mean HCO3- reference range of
23.6 ± 4.22 mmol/L in arterial blood.
As a reference, values that are below this range may indicate metabolic acidosis or compensation of a respiratory alkalosis, whereas values greater than this range may indicate metabolic alkalosis or compensation of a respiratory acidosis. Metabolic acidosis can be caused by increased generation of acidic metabolic products such as lactate or ketones or the inability of the kidneys to eliminate those products. As the acid increases, it becomes buffered, resulting in a subsequent decrease in the HCO3- levels. Metabolic acidosis can also occur through a direct loss of bicarbonate
from the gastrointestinal tract or kidneys (Di Bartola et al. 2006).
Base excess (BE or cBase) is defined as the amount of strong acid or alkali required to titrate 1 L of
blood to pH 7.40 at 37°C while the partial pressure of carbon dioxide is held constant at 40 mmHg. It is valued in mmol/Lt (Blood gases analysis Manual 2005).
It is the most accurate index to evaluate the metabolic contribution to an acid-base imbalance because it allows to estimate the quantitative impact of buffer bases in blood, isolating the metabolic component from respiratory influences. Buffer bases represent the total blood buffer capacity (Burkitt et al. 2012).
Its value is independent from respiratory activity and therefore help to differentiate between respiratory and metabolic components and is useful to calculate the amount of buffer required to treat patients with acidosis (Montesinos et al. 2013). A negative BE (BE deficit) is consistent with metabolic acidosis and indicates an excess of non-carbonic acids, while a positive value indicates a
deficit in non-carbonic acids (DiBartola 2012, Fischbach and Dunning 2009, Russel et al. 1996). An increase in BE above normal ranges (BE excess) implies metabolic alkalosis and respiratory acidosis (Bailey and Pablo 1998).
It is reported that metabolic acidosis occurs in spontaneously ventilating bird during anaesthesia and an increase in BE values is recorded in cockatoos (Phalen et al. 1997, Chemonges 2012). During intermittent positive pressure ventilation (IPPV), BE values are negative just after induction and then shift toward positive values during anaesthesia, suggesting a metabolic acidosis soon after induction. Therefore, IPPV can reverse the further development of metabolic acidosis, accordingly to the positive shift in BE values that indicate respiratory acidosis (Chemonges, 2012).
Anion Gap (AG) is an adjunct to blood gas evaluation that helps differentiate causes of metabolic
acidosis. It is calculated as the difference between the measured plasma concentration of the major positively charged ions (cations) and the major negatively charged ions (anions):
AG = (Na+ + K+) – (Cl- + HCO 3-)
The body always attempts to maintain the electroneutrality, so the concentration of serum cations equals that of anions. When there is an increase in unmeasured anions or in the Cl- concentration,
the HCO3- decreases to maintain electrochemical balance. Thus, the AG can be used to categorize
metabolic acidosis as increased (elevated AG acidosis) or hyperchloremic (normal AG acidosis). In humans, the reference range is 8-16 mmol/L, in dogs is 12-24 mmol/L while in cats is 13-27 mmol/L (Silverstein and Hopper 2009, DiBartola 2012, Fischbach and Dunning 2009, Madias 1986). Increases in the production of organic acids containing unmeasured anions in blood can cause an increase in the AG and is associated with metabolic acidosis and mixed disorders (DiBartola 2012, Madias 1986). Examples include lactic acidosis, ethylene glycol (antifreeze) poisoning, and salicylic poisoning (Silverstein and Hopper 2009, DiBartola 2012, Sherwin 2002, Carlson and Bruss 2008, Montesinos and Ardiaca 2013).
A decrease in the AG can be caused by hypoproteinemia, hyponatremia and an increase in uncounted cations. In mixed disorders, the various components can give rise to a compensation that keep the AG to normal values even in the presence of high free acid concentration (Silverstain et al. 2009, Burkitt et al. 2009, Willard and Tvedten 2005).
Metabolic acidosis characterized by a normal AG arises when chloride, which is routinely measured, is added to the blood (e.g. dilutional acidosis with aggressive sodium chloride fluid administration) or when HCO3- loss from the body (e.g. diarrhea) is replaced with chloride to
maintain electrochemical balance (Montesinos and Ardiaca 2013).
Plasma Glucose Concentration (cGlu): glucose is the primary source of energy for the body.
Consequently, the concentration of glucose in the blood plays a central role in metabolism and its maintenance is essential for survival. Its blood concentration depends on the balance between consumption, diet intake and intestinal absorption, as well as it synthesis within the body. Reduced glucose levels (hypoglycemia) induce an acute medical condition characterized by several specific signs and symptoms including neurological signs (Blood gases analysis Manual, 2005). The normal glucose range in avian patients is 180–350 mg/dL, much higher than in mammals (Hochleithner, 1994).
Plasma Lactate Concentration (cLat): lactate is mainly formed as a final product of cellular
anaerobic glycolysis, although small amounts are produced during aerobic metabolism (Sharkey and Wellman 2013). The orientation of cellular metabolism toward anaerobic glycolysis and lactate production is related to inadequate oxygen supply, so lactate can be considered as a marker of the imbalance between oxygen demand in tissues and the blood supply for this gas. However, it is not a specific indicator of the availability of arterial oxygen but is important to monitor the adequacy of tissue oxygenation (Blood gases analysis Manual, 2005). In normal conditions of liver and kidney clearance, lactate half-life is less than one hour and the presence of high levels occur only in case of long-term tissue hypoxia. However, it should be considered that its half-life may be doubled during hepatic/renal alterations and sepsis (Bertoli, 2012).
Low levels of lactate (hyperlactatemia) have no clinical significance since the minimum value considered normal is 0 mmol/L (DiBartola, 2012).
High levels of lactate (hyperlactatemia) can be caused by local or systemic hypoperfusion and severe arterial blood supply deficiency (hypoxemia). High levels may also occur in the event of alkalosis, hypoglycaemia, systemic diseases (hepatic failure and sepsis), seizures, administration of drugs and stress myopathy (Silvertstain et al. 2009, Sharkey and Wellman 2013, Hill and Miller 2013). Severe hyperlactatemia (>8 mmol/L in domestic animals) is defined as lactic acidosis and
results in metabolic acidosis with increased anion gap and can be lethal if protracted in time (Silvertstain et al. 2009).
In emergency situations, lactate monitoring is a way to verify the adequacy of a patient’s treatment in critical conditions. Decreasing or persistently low levels of lactatemia indicate the efficacy of the treatment, but if the availability of arterial O2 is compromised, measures need to be
taken to improve it. Increasing or persistently high levels point to the ineffectiveness of the treatment. It has been shown, both in human and veterinary medicine, that the level of lactatemia represents a good prognostic index of intra-hospital mortality (Silvertstain et al. 2009, Sharkey and Wellman 2013).
Plasma osmolality (mOsm): Osmolarity represents the total concentration of all dissolved solutes
in water and is described as the number of osmoles per litre of solution, expressed in milliosmol/L. Osmolality, on the other hand, represents the total concentration of all solutes dissolved in any solution and is described as the number of solutes per kilo of solvents, expressed in milliosmol/kg (mOsm/Kg). In the blood, these two measures can be considered equivalent, as the main solvent is water and 1 Kg of water = 1 Lt (Stockham and Scott 2002, Wellman et al. 2006). Plasma osmolality is the quantification of the number of osmotically active particles present in the plasma. The main solutes responsible for osmotic pressure (and therefore for the osmolality) in the extracellular compartment (intravascular and interstitial) are sodium, chlorine, glucose, urea and uric acid, while for the intracellular compartment are potassium and magnesium (Silverstain et al. 2009, Burkitt et al. 2012, Willard and Tvedten 2005, Stockham and Scott 2002). Plasma or serum osmolality can be used to infer osmotic and electrolyte disorders that an animal experiences in response to a disease process and a loss of water and electrolytes (Guyton and Hall 2006).
The normal range of serum osmolality in humans is 285-295 mOsm/kg. The measured osmolality should not exceed the predicted by more than 10 mOsm/kg. A difference of more than 10 mOsm/kg is considered an osmolal gap (Seifarth 2004).
A reduction in mOsm (hypo-osmolality) may occur in all cases that bring to hyponatremia such as hyperhydration (fluid retention, excess fluid therapy, psychogenic polydipsia), dehydration, ascites from severe hepatic disease, heart failure, renal disease, hypoadrenocorticism, diuretic administration (Silverstain et al. 2009, Willard and Tvedten 2005). An increase in mOsm (hyper-osmolality) may occur in case of hypernatremia, hyperglycaemia, diabetic keto-acidosis, severe
hyperazotemia, lacto-acidosis, hyperphosphatemia (related to kidney failure), administration of mannitol, injection of drugs contrast, intoxications. It can have serious neurological consequences (Silverstain et al. 2009, Burkitt et al. 2012, Willard and Tvedten 2005).
A study on African grey parrots, Hispaniolan Amazon parrots, and Red-fronted Macaws (Beaufrère et al. 2011) suggests that plasma osmolality is slightly higher in parrots than in mammals, with the presence of species-specific differences. In table 1.8 are reported the values of osmolality and reference ranges obtained in the cited study.
Species Mean (mOsm/Kg) Reference Range (mOSM/Kg)
African grey parrot 306 288-324
Hispaniolan Amazon parrots 327 308-345
Red-fronted Macaws 304 223-369
Table 1.8 Plasma osmolality values in 3 different species of parrots (Beaufrère et al. 2011)
Plasma protein and various electrolytes including phosphate serve as physiological buffers in the blood (Bailey and Pablo 1998, Steinmetz et al. 2007). Values of Na+ and K+ differ significantly in
relation to the assay method (Heatley et al.2005).
Sodium (Na+) is the main extracellular cation. Its primary functions in the body are the
transmission of nerve impulses, chemical maintenance of osmotic pressure and acid-base balance. At the cellular membrane level, Na+ acts by creating the electric potential, thus maintaining the
transmission of nerve impulses and neuromuscular excitability. The body tends to maintain the total base content unchanged and, even in case of a disease, only slight variations are found (Burkitt et al. 2012). Changes in the concentration of Na+ can have a significant impact on plasma
osmolality (Marks and Taboada, 1998). Kidneys regulate Na+ and water balance in the body (Di
Bartola, 1998).
Low Na+ values (hyponatraemia) indicate water excess within the body and a massive loss of
High Na+ values (hypernatremia) are associated with loss of water usually with the maintenance of
electrolytes.
Sudden changes in sodium values are life threatening, as they can cause damage to the central nervous system that cannot activate its self defence mechanisms (Burkitt et al. 2012).
Potassium (K+) is the main intracellular cation and plays a critical role in intracellular buffering and
in the neuromuscular transmission. Imbalances in K+ manifest as skeletal muscle dysfunction
(Phillips and Polzin, 1998). Ninety percent of potassium is found within the cell so its blood values directly indicates the real concentration in the body. Damage to the cells can lead to an increase of potassium in blood. Low K+ levels indicate a massive loss of the cation (Burkitt et al. 2012).
Chlorine (Cl-) is an anion found mainly in the extracellular fluids and helps to regulate the osmotic
pressure and water equilibrium (together with sodium) and the acid-base balance (Burkitt et al. 2012). Chlorine value doesn’t change in case of metabolic acidosis when other anions increase (Gough 2007). The relative concentration of chloride in extracellular fluid affects the acid-base status, and changes in bicarbonate can occur if chloride is altered (Russel et al. 1996).
Calcium (Ca2+) is a regulatory ion and plays a role in neuronal excitability, muscle contraction and
blood coagulation (Dhupa and Proulx, 1998). Within the blood, calcium is distributed in the form of free calcium ions (50%), protein-bound calcium (mainly albumin 40%) and calcium linked to anions (bicarbonate, citrate, phosphate and lactate 10%). However, the body only uses free ionized calcium to carry out vital processes such as muscle contraction, cardiac function, nerve impulse transmission, and blood coagulation. In clinical practice, it is more important to detect a calcium deficiency than an excess, as it may endanger the life of the patient, causing severe hypotension and cardiopulmonary arrest (Burkitt et al. 2012). The concentration of ionized Ca2+ is
closely influenced by the pH of the sample because calcium directly competes with H+ for the
binding to active protein sites. As the pH drops, the protein-bound calcium decreases, resulting in increased ionic calcium concentration in the sample. This occurs both in vivo (in subjects with acidosis) and in vitro in case of prolonged storage of the sample (anaerobic glycolysis of the erythrocytes). Likewise, an increase in pH results in a reduction of ionized calcium in both systemic
alkalosis and when the sample remains in contact with air for a long time, causing the loss of CO2
(0.036 mmol/Lt for each pH increase of 0.1 unit)
Phosphorous is the major intracellular anion existing as organic or inorganic phosphate and plays
an integral role in many metabolic processes (Rosol and Capen, 1996).
Typically, pH changes arising from one component (e.g. metabolic) are opposed by changes in another component (e.g. respiratory) to maintain the proper ratio of metabolic to respiratory contribution to overall pH. For example, with metabolic acidosis, the HCO3- concentration
decreases, thereby lowering the HCO3- to pCO2 ratio and resulting in alkalemia. Therefore, the
body compensates by decreasing the pCO2 or by hyperventilating to maintain the ratio. The
respiratory component thus compensates for the metabolic acidosis in an attempt to raise the pH to neutral (Montesinos and Ardiaca 2013).
Physiologic compensation rarely completely resolves the primary acid-base problem and never leads to overcompensation. Therefore, the pH typically deviates from neutral, even after adequate compensation, although it can be within the reference ranges in patients with mild acid-base disorders. Metabolic acidosis is the most common acid-base disturbance in canine and feline medicine. If metabolic acidosis is the primary disturbance, it will be represented by a lower pH, a negative BE or lower HCO3 - concentration, and a compensatory decrease in the pCO2 in an
attempt to blow off the excess acid load (Montesinos and Ardiaca 2013).
Table 1.9 summarizes the four primary acid-base disorders and their compensatory changes.
Conditions Primary disorder Compensation
Low pH and low HCO3- (negative BE) Metabolic acidosis Decreased PCO2
High pH and High HCO3- (positive BE) Metabolic alkalosis Increased PCO2
Low pH and High PCO2 Respiratory acidosis Increased HCO3 - (BE)
High pH and low PCO2 Respiratory alkalosis Decreased HCO3 - (BE)
Anaesthesia Mentions
Recently, the increased interest and efforts in the conservation of wild animals led to major efforts in understanding how safely perform anaesthetics procedures for surgeries in wild and exotic birds. It is therefore essential for the veterinary practitioner to know the optimal combination of injectable and inhalable anaesthetics, having also frequently to manage patients easily subjected to stress that may need little sedation for manipulation (Bufalari et al. 2012)
Avian patients are particularly fascinating from an anesthesiological point of view, as they are characterized by peculiarities not detectable in any other animals. Knowledge of the anatomy and physiological aspects typical of these animals is therefore necessary to better manage the different phases of the anesthesiological procedure while maintaining an adequate level of safety (Bufalari et al. 2012). As already described, the cardiovascular and respiratory systems are considerably different from those of mammals.
Before any type of sedation or complete anaesthesia, each volatile should be subjected to visual and physical examination and the state of nutrition should be evaluated by palpation of the keel (keel sharpness is an excellent indicator of muscle mass and body fat), although other signs of illness may appear after an adaptation phase. While awakening and recovery after the anaesthesia, it can occur that the patient vigorously flutter the wings and consequently break them down, so it is essential to balance the inhalational and the injectable anaesthesia appropriately. After any type of anaesthesia, avian patients should be kept warm in an environment not too bright, preferably in a padded box, and should be placed in sternal recumbency (Bufalari et al. 2012, Hall et al. 2001).
Premedication drugs
In avian medicine, the use of drugs such as sedatives, tranquilizer and analgesics has several benefits, like:
- anxiolytic;
- sparing-effects on drugs doses used for induction procedures; - analgesia;
-sparing-effects on inhalational anaesthetics (as the reduction of the amount of inhaled anaesthetics required), with a decrease in adverse effects on the cardiovascular system (arrhythmogenicity and hypotension) (Gunkel & Lafortune, 2005);
- gentle and gradual awakening;
The most common class of drugs used for premedication are benzodiazepines and opioids (Longley, 2008)
Opioids
The use of analgesics, such as opioids, reduces the concentration of the inhaled anaesthetic necessary to induce and maintain the anaesthesia of the patient. Opioids are currently the most effective class of analgesic drugs and are most commonly used in the management of the pain during surgeries (Dyson, 2008).
Among these, sufentanil is used in veterinary medicine as an analgesic component of anaesthesia, as a supplement to inhalational anaesthetics or for total intravenous anaesthesia as a primary agent. This drug is to be considered as an elective medication when used to obtain an anaesthetic protocol with optimal cardiovascular stability or when intra-surgical pain is intense and not easily manageable with the most common opioids (Bufalari and Cerasoli, 2012). It has high analgesic power: studies in the dog have shown that its analgesic power is 625 higher than morphine and 5-10 times higher than fentanyl (Hall et al., 2001). Unlike other opioids, sufentanil does not appear to cause severe alterations in the cardiovascular function: cardiac output, peripheral resistances and blood pressure remain in the reference values. However, it determines dose-dependent respiratory depression characterized by reduced respiratory rate which may evolve in apnea, decrease in tidal volume and increase in ETCO2. As with other opioids, it can easily be antagonized
by naloxone (Hall et al., 2001; West et al.,2014)
Benzodiazepines
Benzodiazepines such as diazepam and midazolam possess sedative, anti-anxiety, and muscle relaxant properties and have sparing effect on the MAC of halogens but do not cause analgesia (Whittow, 2000). The few side effects on the cardiovascular system and the possibility of being antagonized (flumazenil) make these tranquilizers ideal for the administration in conjunction with
anaesthetic agents both during induction and during maintenance of the anaesthetic plan (Gunkel & Lafortune, 2005).
Midazolam is absorbed quickly and completely by the muscles (Hawkins & Pascoe, 2007). In Canadian geese (Branta canadensis) a dose of 2 mg/kg induces a degree of sedation sufficient to allow radiographic examination as an alternative to general anaesthesia with isoflurane. At this dosage, midazolam did not cause significant changes in heart rate, pressure values, pH, arterial blood gas tension, and temperature (Valverde and Honeyman, 1990). In the quail (Coturnix
coturnix) doses from 2 to 6 mg/kg IM produce immobility and sedation for a duration of 40-60
minutes, without significant cardio-respiratory effects (Day & Roge, 1996). In pigeons, midazolam (14-16mg/kg IM) produces deep sedation and reduces the isoflurane MAC by about one third (Smith et al., 1993). In some cases, the behaviour of the animal does not reflect an important degree of pre-anaesthetic sedation, but during manipulations the birds appear much less likely to react (Paul-Murphy and Fialkowski, 2001). The anxiolytic properties of midazolam have proved to be useful in reducing mortality and morbidity following the capture of wild species subjected to stress induced rhabdomyolysis (Ward et al. 2011).
Inhalational Anaesthetics
Inhaled anaesthesia represents the elective technique in birds. The most commonly used drugs in avian species possess many benefits, the main ones being rapid induction times, easy and rapid change in the depth of the anesthesiological plane, and rapid waking up times with only few side effects.
The ease of modulating the anaesthetic plan is more evident in birds compared to other species, due to the great efficiency of the mechanisms regulating the pulmonary gas exchange (Gunkel and Lafortune, 2005). The main side effects of these anaesthetics are dose dependent depression of the central nervous system, cardiovascular and respiratory system (Longley, 2008). Birds do not possess an alveolar type lung, so the term "minimal alveolar concentration" (MAC) is inappropriate. Therefore, MAC in avian patients is defined as the "minimal anaesthetic concentration" at the end of the expiration, necessary to prevent any movements in the 50% of the subjects following a painful supramaximal stimulus (Phair et al. 2012). The MAC of halogenated anaesthetics does not differ much among the avian species (Ludders and Mathews,
1996). For isoflurane, MAC is 1.32% in the mallard (Anas platyrhynchos), 1.34% in the Sandhill crane (Grus canadensis), 1.25% in the domestic chicken (Gallus gallus domesticus), 1.44% in Cacatua (Cacatua spp.), 2.05% in the red-tailed hawk (Buteo jamaicensis). In some species, it is particularly low as in the thick-billed parrot (Rhynchopsitta pachyrhyncha) with a value of 1.07% and in the cinereous vulture (Aegypius monachus) with a value of 1.06% (Hawkins & Pascoe, 2007; Pavez et al. 2011, Mercado et al. 2008, Kim et al. 2011).
When a bird is hypoventilated, it is difficult to keep the anaesthetic plane stable (Ludders, 1998). Intermittent Positive Pressure Ventilation (IPPV) in mammals adversely affects cardiac function by creating a positive intrathoracic pressure that reduces the venous blood return to the heart by lowering the mean arterial pressure. However, the effects of IPPV in birds are not yet fully clear (Hawking & Pascoe, 2007). Studies on African grey parrots indicated that IPPV and continuous capnographic monitoring can be used to prevent respiratory acidosis during isoflurane anaesthesia (Edling et al. 2001). During general anaesthesia with inhaled agents, ventilation should always be assisted or controlled, even to ensure stability (Ludders, 1998). Administration of anaesthetic mixtures during maintenance can be through masking or intubation. Intubation offers countless advantages, keeps full airways control, allows IPPV and greatly reduces environmental pollution.
Isoflurane is the most widely used inhaled anaesthetic in the avian practice (Hawkins & Pascoe, 2007). The induction with the mask occurs in 1-2 minutes when administered at concentrations of 3-5%. Awakening is similarly rapid, although it is related to the duration of anaesthesia (Edling 2005). Isoflurane is safe and has minor side effects, but has a negative impact on the cardiovascular system and has been observed a dose dependent hypotension (Longley, 2008, Hawkins & Pascoe, 2007).
Anesthesiological Circuits
The anesthesiological circuits vary depending on the species. The circuit should have a reduced dead space, little resistance and possess a non-rebreathing system. The best circuits used in avian practice are Bain, Modified Rees and Ayre’s T-piece (Longley, 2008). Because the tidal volume in birds is not sufficient to move gases in a closed system, opened or semi-opened circuits are
necessary. To remove the CO2 from the system and avoid rebreathing, it is preferable to use
relatively high gas flows (Coles, 1997). The oxygen flow in a non-rebreathing system should be around 150-200 ml/kg/min (Edling, 2005).
Monitoring of the patient
The monitoring of the patient is a fundamental aspect of anaesthesia (Hawkings & Pascoe, 2007). Like in other species, an uneven trend or a decline in the monitored parameters indicates an aggravation of the physiological conditions of the patient. In birds, such condition can cause death and therefore the role of the anaesthesiologist is fundamental. Electrocardiography, monitoring of arterial blood pressure and capnography are fundamental for a safe anaesthesia (Longley, 2008).
The heart frequency is highly variable among species and individuals. It can be monitored with a stethoscope or with an esophageal probe. The peripheral pulse can be detected from the brachial artery, tibial cranial artery, carotid and sometimes in the palatine one (Hawkins & Pascoe, 2007).
Blood pressure (BP) can be measured either by a direct technique or invasive blood pressure (IBP) or with an indirect one, using a doppler probe. The probe should be located on the superficial ulnar artery, deep radial artery or cranial tibial artery; a small sleeve is then attached to a sphygmomanometer placed next to the probe (Fig. 1.2). With this technique, the systolic pressure measured in 8 species of non-anaesthetized psittacides was 113-157mmHg, and was 96-140mmHg when anesthetized (Lichtenberger, 2005).
Figure 1.2 Measurement of systolic blood pressure with a doppler probe on the deep radial artery in a yellow-legged seagull (CRUMA).
The invasive technique is more complex: in fact, arterial catheterization has several difficulties in many species of birds, due to the small size and the relative inaccessibility of the arteries (Degernes, 2008). This practice allows to evaluate the BP continuously, in addition to allow practitioner to perform blood gas analysis to obtain pH data and blood gases values. The catheter can be introduced through a percutaneous way, although it is easier to visualize and confirm the correct positioning by using a cut-down procedure. In many species during arterial catheterization, significant arteriospasm may occur. To counteract this adverse effect, it is possible to apply an anaesthetic cream 30 minutes before catheterization. Otherwise, with the cut-down technique, a 2% lidocaine solution can be used directly on the artery (Hawkins & Pascoe, 2007).
Even if it is the preferred anaesthetic, isoflurane can cause, in birds, a respiratory depression at concentration required for surgery (Ludders et al. 1990). Because of the potential problems associated with consequent hypoventilation, it is important to monitor closely the ventilatory status of birds during anaesthesia. Experimental studies indicated that the solely monitoring of the respiratory rate does not provide an adequate assessment of ventilation in birds, because isoflurane and high inspired oxygen concentration have the effect of reducing the tidal volume, despite the respiratory rate increases or remains unchanged, thus resulting in hypoventilation (Seaman et al. 1994, Ludders et al. 1995). The respiratory pattern is usually fast and irregular when anaesthesia is superficial and becomes slow and regular when is reached a steady state of
