LITHUANIAN UNIVERSITY OF HEALTH SCIENCES VETERINARY ACADEMY Faculty of Veterinary Medicine

LITHUANIAN UNIVERSITY OF HEALTH SCIENCES VETERINARY

ACADEMY

Faculty of Veterinary Medicine

Andrea-Giulia Piercecchi

THE INFLUENCE OF PREMEDICATION ON PHYSIOLOGICAL PARAMETERS DURING GENERAL ANAESTHESIA IN HORSE.

PREMEDIKCIJOS ĮTAKA ŽIRGŲ FIZIOLOGINIAMS PARAMETRAMS BENDROSIOS ANESTESIJOS METU

MASTER THESES

of Integrated Studies of Veterinary Medicine

Supervisor

2 THE WORK WAS DONE IN THE DEPARTMENT OF LARGE ANIMAL CLINIC

CONFIRMATION OF THE INDEPENDENCE OF DONE WORK

I confirm that the presented Master Thesis “THE INFLUENCE OF PREMEDICATION ON

PHYSIOLOGICAL PARAMETERS DURING GENERAL ANAESTHESIA IN HORSE”.

1. has been done by me;

2. has not been used in any other Lithuanian or foreign university;

3. I have not used any other sources not indicated in the work and I present the complete list of the used literature.

2019-01-04 Andrea-Giulia Piercecchi

(date) (author’s name, surname) (signature)

CONFIRMATION ABOUT RESPONSIBILITY FOR CORRECTNESS OF THE ENGLISH LANGUAGE IN THE DONE WORK

I confirm the correctness of the English language in the done work. 2019-01-04 Andrrea-Giulia Piercecchi

(date) (author’s name, surname) (signature)

CONCLUSION OF THE SUPERVISOR REGARDING DEFENCE OF THE MASTER THESIS

2019-01-04 Zoja Miknienė

(date) (supervisor’s name, surname) (signature)

THE MASTER THESIS HAVE BEEN APPROVED IN THE DEPARTMENT/CLINIC/INSTITUTE

Large Animal Clinic

(date of approval) (name, surname of the head of department/clinic/institute)

(signature)

Reviewer of the Master Thesis

(name, surname) (signatures)

Evaluation of defence commission of the Master Thesis:

(date) (name, surname of the secretary of the defence commission)

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TABLE OF CONTENT

1.1 Preoperative evaluation-history, physical examination, blood work ... 9

1.2 Preparation ... 10

1.3 Selection of anaesthetic agents ... 10

1.4 Premedication and induction ... 11

1.5 Intubation ... 12

1.6 Maintenance ... 13

1.6.1 Inhalation anaesthesia ... 13

1.6.2 Total intravenous anaesthesia ... 13

1.7 Monitoring ... 14

1.8 Recovery ... 15

2 METHODOLOGY ... 17

2.1 Research workload, location and methods ... 17

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SUMMARY

THE INFLUENCE OF PREMEDICATION ON PHYSIOLOGICAL PARAMETERS DURING GENERAL ANAESTHESIA IN HORSE

Andrea-Giulia Piercecchi Master’s Thesis

There are no doubts that general anaesthesia in horse involves a major risk. However, with an appropriately deliberate anaesthesia plan these risks and complications can be prevented and managed.

The aim of this work is to determine the influence of premedication on physiological parameters during general anaesthesia in 16 horses undergoing closed recumbent castration. This is a retrospective cohort study where I, with the help of my supervisor, gathered 16 anaesthesia protocols of horses undergoing castration at the large animal clinic of LUHS in Kaunas, Lithuania, 7 of which were premedicated with xylazine and 9 were premedicated with romifidine. The physiological parameters assessed were heart rate, respiratory rate, capillary refill time, SpO2, blood lactate concentration and temperature. These parameters were monitored at 5 minutes interval throughout the surgical procedure. Post anaesthetic recovery was also observed and discussed.

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SANTRAUKA

PREMEDIKCIJOS ĮTAKA ŽIRGŲ FIZIOLOGINIAMS PARAMETRAMS BENDROSIOS ANESTESIJOS METU

Andrea-Giulia Piercecchi Magistro baigiamasis darbas

Nėra jokių abejonių, kad arkliams taikoma bendra anestezijayra susijusi su nemaža rizika.Tačiauvadovaujantis gerai apgalvotu anestezijos planu galima užkirsti kelią šiai rizikai ir komplikacijoms bei jas valdyti.

Šio darbo tikslas yra nustatytipremedikacijos įtaką 16 arklių fiziologiniams parametrams taikant bendrą anesteziją gyvūnams atliekantuždarą kastraciją gulimoje padėtyje.Tairetrospektyvus kohortinis tyrimas, kai, padedant mano vadovui, mesišanalizavome 16 arkliųanestezijos protokolų, kuriemsLietuvos sveikatos mokslų universitetoStambiųjų gyvūnų klinikoje, esančioje Kaune, Lietuvoje,buvo atlikta uždara kastracija. Septyniems iš šešiolikos arklių buvo taikoma premedikacija naudojantksilaziną, o devyniems buvo taikoma premedikacija naudojant romifidiną.Visos operacijos metu5 minučių intervalubuvo stebimi tokie fiziologiniai parametrai, kaipširdies susitraukimų dažnis, kvėpavimo dažnis, kapiliarų prisipildymo laikas,kraujo SpO2,laktato koncentracija kraujyje ir kūno temperatūra.Taip pat buvo stebimas ir aptartasatsigavimas po anestezijos.

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ABBREVIATIONS

PAM – Post-anaesthetic myopathy

NSAID – Non-steroidal anti-inflammatory drugs CRT – Capillary refill time

RR – Respiratory rate i.v. – Intravenously i.m. – Intramuscularly

TIVA –Total intravenous anaesthesia PH – Potential hydrogen

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INTRODUCTION

It is well-known that general anaesthesia entails a significant risk regarding both morbidity and mortality in all animal species. However, this risk predominates distinctly in horses [1].

Morbidity suggests an anaesthetic-related consequential outcome such as an illness or disease not present before anaesthesia. These complications may differ in severity, yet they are always considered to have a harmful or negative impact on the post-anaesthetic status of the horse. However, mortality implicates death as a result of general anaesthesia [2].

The mortality rate from peri-anaesthetic complications was particularly high in horses in comparison to humans, dogs, cats and even rabbits. The anaesthesia-related mortality rate of healthy horses undergoing elective surgery has been reported to be 1 percent on average (ranging from 0.8 – 1.8 percent depending on study design). Clearly, these numbers are much higher in emergency cases especially those requiring abdominal intervention ranging from 7.8 – 19.2 percent. Compared to other animal species, the incidence of death among horses is 100 – 1000 times greater than in humans (0.01 – 0.001 percent), 20 times greater than in dogs (0.05 percent), 10 times greater than in cats (0.11 percent) with only rabbit having a slightly similar mortality risk (0.73 percent) [3].

There are as many as 22 different causes of death reported in non-emergency cases, of which the three most common causes were cardiac arrest (32.8 percent), fracture during recovery (23.3 percent) and PAM (7.1 percent) [4].

Complications can arise at any point during anaesthesia despite anything. Therefore, it is important to have a good understanding of the anaesthetic equipment and the physiological and pharmacological impacts of anaesthesia on the horse’s organism. A ground rule for prevention and management of complications includes a thoroughly planned anaesthetic strategy, careful monitoring, and early recognition of abnormal signs as well as having knowledge on how to proceed in any situation [5].

The aim of the work: To analyse physiological changes in horses during general

anaesthesia using different agents as premedication.

The tasks of the work:

1. To collect samples and data from horses undergoing castration to use for analysis of

physiological changes during general anaesthesia induced with different premedication.

2. To identify changes in physiological parameters in horses during general anaesthesia. 3. To study and compare samples from the distinct categories.

8 Objectives:

1. To establish if the choice of premedication prior to general anaesthesia has an impact on the

physiological state of the horse based on HR, RR, CRT, SpO2, blood lactate concentration and temperature.

2. To determine how these parameters are affected and if this has a correlation with the choice

of pre-anaesthetic agent.

3. To determine the impact of pre-anaesthesia on recovery based on attempt to stand up

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1 LITTERATURE REVIEW

1.1 Preoperative evaluation-history, physical examination, blood work

Before considering general anaesthesia, a total preoperative health assessment is fundamental. A full history must be taken where the veterinarian will obtain information about the patients’ previous experiences to anaesthesia or sedation and the horses’ response to it, if such an experience exists, and if there is any noteworthy illness or injury that might interfere with the anaesthetic procedure. Any medication that the horse is currently on must be mentioned as well, in order to anticipate undesired interactions between different medications and the anaesthetic agents and to avoid complications that might arise if this information is being withheld by the owner. An example of this is the administration of aminoglycoside antibiotics before or during anaesthesia. This antibiotic is a neuromuscular blocking agent and can possibly lead to depressed pulmonary ventilation. Another example is NSAIDs when administered over a prolonged period which can cause unwanted effects. Finally, the owner must explain any current issues that the patient might experience if so exist [6].

Once a proper history is collected, a physical examination is carried out. Here mucous membranes are checked for colour, moisture and CRT. The colour should be pink and the gums moist. CRT should not be longer than 2 seconds in a healthy horse. Moving on, an assessment of the organ systems is performed concentrating on the respiratory as well as the cardiovascular systems to check for presence or absence of cardiovascular and respiratory abnormalities. The respiratory rate as well as heart rate and sounds are evaluated with the help of a stethoscope. A healthy horse at rest has a normal respiratory rate of 8 – 12 breaths per minute and the heart rate should be at 32 – 36 beats per minute in an adult horse (slightly higher in foals and yearlings). It is important to listen carefully and exclude any deviant respiratory or cardiac sounds. The temperature is measured rectally with a digital thermometer; normal body temperature is around 37.5 – 38.6 degrees celsius [7].

The gut sounds are also assessed with the aid of a stethoscope. Gut sounds must always be present in healthy horses; the absence of gut sounds is indicative for poor wellbeing, usually infection, colic or other gastrointestinal disturbances. Make sure to auscultate on both sides and at all four sites [8].

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before putting the patient through general anaesthesia and help control risk factors and anticipate possible complications that the patient might experience [9].

1.2 Preparation

Preparation for surgery involves fasting up to 12 hours before surgery and withholding of water up to 2 hours before surgery. Another important preparatory aspect is the mouth rinsing before intubation to avoid solid material in the oral cavity to be pushed down and reach the trachea and lungs once the horse is intubated with an endotracheal tube. Hooves should be cleaned or covered to avoid dirt droppings from hooves during surgery and shoes are preferably removed to avoid traumatic damage to the horse itself and staff when falling asleep and trying to stand up after anaesthesia [10]. The last stage in this step is placing the jugular catheter, through where the drugs will be administered. Size of the catheter depends on the size of the horse; however, a 12 – 14-gauge size is generally used in majority of adult horses [11].

Fig. 1 Visual illustration of jugular vein catherization in horse [1].

1.3 Selection of anaesthetic agents

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Moving on, the safety of the environment should be considered; where the anaesthetic procedure will take place, on field, in the operation room, in stable and so on, and if the animal will be standing or lying down during the procedure. Finally, a couple of factors influencing the choice of anaesthetics is the availability of anaesthetic agents in said country and the number of workers and their competence concerning anaesthesia [12].

1.4 Premedication and induction

The main mechanism of action of alpha 2 adrenoreceptor agonists is to inhibit the release of neurotransmitters from the neurons; this is done by binding to presynaptic alpha 2 adrenoreceptor agonists, resulting in reduced sympathetic outflow, analgesia and sedation [13].

Romifidine is a sedative substance belonging to the alpha 2 adrenoreceptor agonist group. It is primarily used for premedication, sedation and analgesia of horses and it has the longest duration of sedative effect compared to detomidine and xylazine of about 60 – 120 minutes [14, 15]. Its action differs a bit from other alpha 2 adrenoreceptor agonists such as xylazine and detomidine. It appears to be less hypnotic and seems to produce less ataxia compared to other sedatives within the same group. Due to the reduced presence and sometimes even complete lack of ataxia, it is highly recommended in standing procedures in horses [16].

The degree of loss of sensation to stimuli during sedation seems to be equal among romifidine, detomidine and xylazine. However, romifidine requires lower dosage to obtain the equivalent effect acquired by detomidine and xylazine. The duration of analgesic action is not enough for more painful procedures, and in these cases romifidine should be combined with stronger analgesics such as butorphanol or ketamine. It has a few side effects including bradycardia, hypotension and respiratory depression [17].

Xylazine is the least potent sedative agent among alpha 2 adrenoreceptor agonists [18]. The duration of sedation is dose-dependent and usually lasts about 30 minutes [15]. Xylazine produces a great degree of ataxia especially right after administration but is the least cardiovascular depressive agent among alpha 2 adrenoreceptor agonists [19]. It also has the least specific alpha 2 agonist receptor affinity in comparison to romifidine (highest affinity) and detomidine (second highest affinity). Similar to the other agents, it has some cardiopulmonary side effects [20]

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Detomidine is mainly used in standing surgical and non-surgical procedures in horses. It has light dose-dependent analgesic properties in respect to both depth and duration; however, it is preferred to be combined with an opioid such as butorphanol for more painful procedures. It causes both bradycardia and respiratory depression right after administration, but usually goes back to normal within a few minutes [21].

Besides providing sedative, analgesic and muscle relaxing properties, alpha 2 adrenoreceptor agonists works synergistically with injectable and inhalant anaesthetic agents and allow the reduction of the induction dose significantly [22].

The most common administration route of premedicative drugs in horses is i.m. or i.v. However, i.m. is preferred since it is taken up much slower by the body than i.v. administration. This will give a slow, much deeper onset of relaxation with fewer side effects compared to i.v. injection [23].

Induction is the next step; this step provides unconsciousness in the horse [24]. The most preferred anaesthetic agent for induction is ketamine [25]. It is important to make sure that the horse is adequately premedicated with an alpha 2 adrenoreceptor agonist, since the administration of ketamine alone can lead to excitement and convulsions in horses [26]. Due to this significant disadvantage, ketamine is often combined with a muscle relaxant such as benzodiazepines or guaifenesin to achieve a more controlled and safe anaesthesia. Benzodiazepines are usually preferred since large volumes of guaifenesin is required to achieve an effective dosage in order to provide an adequate state of muscle relaxation in horses [25]. This combination of anaesthetic agents can be mixed into a single syringe and administered i.v. as a bolus about 5 minutes after premedication. Be certain that the horse is in a safe and quiet environment during induction of anaesthesia to avoid injuries; a specially padded box is favoured [24], [26].

1.5 Intubation

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of foreign materials into the lungs since it allows the airways to be inflated under positive pressure [28].

Fig. 2 Illustration of a size chart for endotracheal tube in horse [2].

1.6 Maintenance

1.6.1 Inhalation anaesthesia

Inhalational anaesthetics such as sevoflurane, isoflurane and halothane are often administered in the form of gas through the endotracheal tube at a continuous rate for maintenance of general anaesthesia of horses [29]. All these anaesthetic agents are known to cause a dose-dependent cardiovascular and respiratory depression and should therefore be used cautiously [30]. These agents produce unconsciousness by affecting the central nervous system, but unfortunately, they have no analgesic effects. The gas is inhaled and rapidly distributed into the bloodstream and other tissues, including the brain. The absorption, distribution and the elimination of the unmetabolized agents occurs rapidly through the lungs which allows fast induction, awakening and recovery from anaesthesia [23].

Halothane has shown to have the least respiratory depressive properties compared to isoflurane which is a slightly stronger respiratory depressive agent among halothane, sevoflurane and isoflurane. Additionally, halothane is the most hemodynamic depressive and arrhythmic causing agent, while isoflurane and sevoflurane have a milder effect on the cardiovascular system [23]. The recovery time and quality are also the slowest and the best respectively when using halothane. However, when using isoflurane and sevoflurane, the recovery time is faster [31], the quality of recovery is poorer in isoflurane in comparison to sevoflurane [32].

The only practical way to deliver inhalational anaesthesia to a horse is through a closed or semi closed system [33].

1.6.2 Total intravenous anaesthesia

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TIVA is an alternative technique of achieving general anaesthesia causing unconsciousness, analgesia and muscle relaxation through i.v. administration of a specific combination of drugs, without the necessity of the inhalation machine [35]. This may be achieved in two different ways: repeated bolus injection or constant rate infusion [36].

This involves the combination of three drugs providing the perfect balance between anaesthesia, analgesia and muscle relaxation. Usually the optimal combination consists of ketamine, an alpha 2adrenoreceptor agonist (such as xylazine, detomidine or romifidine) and guaifenesin [35].

Guaifenesin is diluted in isotonic solution or 5 percent glucose solution, and thereafter ketamine and xylazine are added. This combination is administered as a constant rate infusion calculated depending on the horse’s weight and the volume of infusion [34]. However, the availability of guaifenesin is limited and can therefore be substituted with similar anticonvulsants such as diazepam or midazolam. Midazolam should be used at lower concentrations since higher ataxia after standing up is a common finding among horses medicated with higher concentrations [37].

It is important not to use an excessively high concentration of guaifenesin since this will cause severe ataxia at recovery. Oxygen supplementation is recommended since TIVA does not deliver oxygen. It is recommended to limit the duration and usage of the “triple drip” to not more than 60 minutes [34].

Propofol can be used for TIVA as a replacement for ketamine; however, it is not recommended in horses due to possible excitement, involuntary muscle contractions, hypoventilation and poor reliability [37].

1.7 Monitoring

Various physiological changes can be noticed in horses when put under general anaesthesia. Various alterations of different parameters of the body systems can be observed. General anaesthetic agents have been proven to have a reversible effect on the central nervous system; accordingly, this affects the central homeostatic mechanism leading to a cardiovascular and respiratory depression [38]. Other noticeable changes that might be seen is decreased in body temperature, changes in mucous membrane colour and CRT, as well as changes in levels of oxygen saturation [36.]. Monitoring of arterial blood gases, blood lactate levels and PH is an important indication of what is happening at the tissue level [25].

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palpation of the peripheral and superficial arteries’ strength and rhythm, auscultation of heart, arterial blood pressure and mucous membrane colour, moisture and CRT [40].

An increase in arterial blood pressure can indicate a decrease in depth of anaesthesia. Mucous membranes’ colour should be pink, and CRT should be less than 2 seconds; in cases when CRT is more than 2 seconds it is an indication of insufficient or poor oxygenation, perfusion or dehydration. The position of the eye during general anaesthesia is a very helpful indication of the depth of anaesthesia. Depth of inhalational anaesthesia is relied on the position of the eye [23]. At desired optimal surgical depth, one eye or both should be rotated forward, and eyelid should be closed and moist. If nystagmus is present, anaesthesia is too light. If eye is open and central, it is too deep [10].

Visual monitoring of the RR, depth and rhythm is a fundamental estimation of proper breathing during anaesthesia [41]. Respiration should be regular in spontaneous breathing and not induced by surgical stimuli [25.]. Hypothermia is one of the most common perioperative physiological changes since the thermoregulatory defence mechanisms such as vasoconstriction and shivering are suppressed during general anaesthesia. Therefore, the occasional monitoring of the body temperature is essential to keep it under control. Lastly, oxygen saturation must be continuously monitored with the help of pulse oximetry SpO2 [38]; a level of 95 percent or higher is favourable. In anaesthetized horses, the respiratory system is usually compromised affecting the SpO2 level. A value below 90 percent indicates poor oxygen saturation and thereby hypoxemia [42.].

A significant metabolic stress response is commonly induced by anaesthesia in horses. Therefore, monitoring of blood glucose and lactate levels before, during and after surgery by taking blood samples or express tests can be a useful and necessary indication for the horse’s wellbeing when undergoing surgery [43]. Increased values of lactate can be seen in cases of tissue hypoxia, underlying diseases, drugs or toxins and hereditary metabolic diseases [44]

1.8 Recovery

The recovery phase starts once the patient is disconnected from the anaesthesia machine and placed in a special recovery box with properly padded walls and dry, non-slippery floor. The horse must be gently positioned in lateral recumbency in the recovery stall and supervised until it is ready to stand on its own. When the patient shows signs of spontaneous deglutition, extubate and close stall to let the horse move onto sternal recumbency and attempt to stand up. Recovery time and quality depends on age, temperament, breed and duration of anaesthesia and procedure as well as type of anaesthetics used [45].

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is making a big effort such as standing up, the staff must be ready to support the effort accordingly. Studies have been made on importance of assistance of horses during recovery to prevent injuries commonly occurring when the animal wakes up from anaesthesia. The outcome showed that the importance of assistance is very variable among different techniques largely related to the experience of the personnel assisting [46].

Studies have been made on the lighting condition in horses’ surrounding during recovery, and the results showed no significant difference between light and dark environments during the recovery phase [47].

The duration of recovery is affected by other factors such as horse´s physical status, inhalational anaesthetic used, duration of anaesthesia, surgery type, the depth of maintained anaesthesia and pre-anaesthetic used. In maintenance with inhalational pre-anaesthetics such as sevoflurane and isoflurane, the recovery time between detachment from inhalation machine and standing up is shorter than with halothane. Additionally, a few more factors to keep in mind that might affect the time and quality of recovery are the choice of pre-anaesthetic and anaesthetic drugs and their routes of administration. Horses premedicated with i.m. or i.v. detomidine and i.m. xylazine prior to induction remains recumbent for longer than horses premedicated with i.v. xylazine [48].

It has been shown that the postanaesthetic administration of alpha 2 adrenoceptor agonists result in a greater quality of recovery from inhalation anaesthesia (isoflurane) compared to horses that were only given saline solution. Among the alpha 2 adrenoceptor agonists tested (romifidine, xylazine and detomidine), the group postmedicated with romifidine gave the greatest degree of sedation and the best recovery, but also an increase of heart rate shortly after standing up. The group postmedicated with saline solution resulted in the quickest recovery but also the most ataxic and uncoordinated recovery compared to the groups medicated with an alpha 2 adrenoceptor agonists in which the degree of incoordination and ataxia was equivalent [49].

Additionally, horses premedicated with romifidine before inhalational anaesthesia (isoflurane) with or without postmedication with romifidine leads to the best recovery phase in comparison to horses premedicated with xylazine with or without postanaesthetic sedation with xylazine, giving the worst recovery especially without postanaesthetic sedation. This is based on the number of attempts to rise and the degree of coordination during the recovery phase after inhalational anaesthesia [50].

The recovery is therefore improved by the administration of postanaesthetic sedation with romifidine, particularly at a higher dose in contrast to xylazine [51].

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2 METHODOLOGY

2.1 Research workload, location and methods

This research work was been carried out in the large animal clinic of LUHS in Kaunas, Lithuania. The test objects were 16 selected horse brought to the clinic for castration, nine of which were premedicated with romifidine and the other seven with xylazine. The horses were monitored for changes in physiological parameters throughout the entire duration of the surgical procedure and during recovery. The cases were collected at the large animal clinic of LUHS over the period of 7 years.

The collection of data stared in October 2012 and continued randomly throughout the years depending on the availability of the drugs until April 2018. The data was collected to analyse the changes in certain physiological parameters during general anaesthesia and to determine if the choice of premedication has an influence on these parameters.

2.2 Research objectives

16 horses were examined and selected for investigation: nine of which were premedicated with romifidine and the other seven were premedicated with xylazine. All horses were clinically healthy and of the same gender, male. The age ranged between 6 months to 6 years. The breed, size and colour varied. They all underwent the same surgical procedure; closed recumbent castration, (Table 1).

Table 1. Horses investigated in this research

Horse no. Premedication Age Physiological parameters examined

1 Xylazine 6 years HR, RR, CRT, Temperature, Lactate, SpO2

2 Xylazine 5 years HR, RR, CRT, Temperature, Lactate, SpO2

3 Xylazine 2 years HR, RR, CRT, Temperature, Lactate, SpO2

4 Xylazine 4.5 years HR, RR, CRT, Temperature, Lactate, SpO2

5 Xylazine 4 years HR, RR, CRT, Temperature, Lactate, SpO2

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7 Xylazine 4 years HR, RR, CRT, Temperature, Lactate, SpO2

8 Romifidine 6 months HR, RR, CRT, Temperature, Lactate, SpO2

9 Romifidine 3 years HR, RR, CRT, Temperature, Lactate, SpO2

10 Romifidine 4 years HR, RR, CRT, Temperature, Lactate, SpO2

11 Romifidine 3.5 years HR, RR, CRT, Temperature, Lactate, SpO2

12 Romifidine 3 years HR, RR, CRT, Temperature, Lactate, SpO2

13 Romifidine 2.5 years HR, RR, CRT, Temperature, Lactate, SpO2

14 Romifidine 3 years HR, RR, CRT, Temperature, Lactate, SpO2

15 Romifidine 4 years HR, RR, CRT, Temperature, Lactate, SpO2

16 Romifidine 3 years HR, RR, CRT, Temperature, Lactate, SpO2

2.3 Collection of data

Horses presented for closed recumbent castration were enrolled in the study. Horses were included in the study if their owners agreed to participate.

Food, but not water, was withheld for 12 h prior to anaesthesia. Once they arrived to the clinic the veterinarian performed a thorough clinical examination where heart rate, breathing rate, CRT, colour of mucosa, rectal temperature and peristaltic movements were carefully examined to get an overall health status of the horse. A blood sample and a medical history was taken.

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box. Following 3 – 5 minutes after premedication, anaesthesia induction was performed with diazepam (0.05 mg/kg iv Apaurin) combined with ketamine (2.2 mg/kg iv Bioketan).

Following induction, the airway was intubated with an appropriately sized and lubricated cuffed endotracheal tube. Immediately after intubation, the horse was fixed and moved onto the surgery table and the endotracheal tube was connected to closed system large animal anaesthesia machine of which incorporated a 30 L re-breathing bag filled with oxygen. The oxygen flow throughout anaesthesia was 8 – 10 ml/kg/min. Anaesthesia was maintained with sevoflurane (Sevurane). Approximately 5 minutes elapsed between induction of anaesthesia and the beginning of sevoflurane administration. During anaesthesia the following variables were monitored: HR, RR, CRT, Temperature, Lactase, SpO2. Values were recorded at 5 minute intervals.

All horses could breathe Spontaneously. The latter was based on responses to surgical stimulation, the ‘depth’ of anaesthesia as judged by ocular signs, e.g. nystagmus, rotation of the eye, palpebral and corneal reflexes.

Sevoflurane was discontinued at the end of surgery. Horses were moved and hoisted in lateral recumbency into a darkened and padded recovery stall. Intravenous xylazine with ketamine was administered as needed to facilitate moving and rising. The endotracheal tube was not removed until animals showed deglutition reflex.

The time, and the number of attempts to achieve sternal recumbency and standing were recorded. The time when satisfactory coordination was present, e.g. when the horse could walk and maintain balance, was recorded as well.

2.4 Statistical analysis

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3 RESULTS

3.1 Physiological parameters

The study analysed the data of 16 horses undergoing two different anaesthesia protocols. All horses were of various breeds and age, and these aspects were not taken into consideration in this study.

The first group consisted of seven stallions undergoing closed recumbent castration, premedicated with xylazine. Their age ranged between 2 and 6 years. The second group consisted of nine stallions undergoing the same procedure, premedicated with romifidine. Their age ranged from 6 months to 4 years of age. The average duration of the procedure was 58 minutes, varying between 47 – 75 minutes.

Below are multiple linear comparative graphs showing the influence of premedication on the physiological parameters of horse over time.

Heart rate

Fig.3 Changes in mean HR (unit: bpm) over time (unit: minutes) between different groups. Group 1 Xylazine mean HR: X0 (34.0), X15 (32.7), X30 (33.4), X45 (35.1).

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This linear diagram represents how the HR changes over a period of 45 minutes (which is the approximate duration of the surgical procedure) between groups 1 and 2. We can clearly see that in both groups the HR is decreasing after 15 minutes and then increasing again until it normalises. However, a more drastic decrease in mean HR is seen between 0 minutes and 15 minutes when using romifidine compared to xylazine. Note that the y-axis does not start from 0.

The statistical significance P > 0.05 (0.456) was determined, meaning that there is no significant difference of the mean HR value and the premedication of choice.

Respiratory rate

Fig. 4 Changes in mean RR (unit: bpm) over time (unit: minutes) between different groups. Group 1 Xylazine mean RR: X0 (10.7), X15 (9.3), X30 (8.0), X45 (7.5)

Group 2 Romifidine mean RR: R0 (16.5), R15 (13.0), R30 (9.9), R45 (9.8) P < 0.05

In this diagram we can see the changes in RR over time in both group 1 and 2. The graph shows a steady decrease of number of breaths between 0 minutes and 45 minutes. Group 1 has a steadier decline while group 2 has a sharper decline between 0 minute and 30 minutes. The mean respiratory rate in group 1 is always within the normal range while we can see hyperpnea at 0 minutes (16.5) and 15 minutes (13) in group 2. Note that the y-axis does not start from 0.

22 CRT

Fig. 5 Changes in mean CRT (unit: seconds) over time (unit: minutes) between different groups. Group 1 Xylazine mean CRT: X0 (1.8), X15 (1.5), X30 (1.4), X45 (1.1)

Group 2 Romifidine mean CRT: R0 (2.2), R15 (2.0), R30 (1.9), R45 (1.5) P<0.05

This linear graph expresses the variation in CRT value over time in group 1 and 2. We can see similar patterns of decreased levels of CRT in both groups. Only in group 2 did the CRT exceed 2 seconds at the beginning of general anaesthesia. Otherwise the CRT value remained within the normal range throughout the procedure.

23 SpO2

Fig. 6 Changes in mean SpO2(unit: percent) over time (minutes) between different groups. Group 1 Xylazine mean SpO2: X0 (97.6), X15 (98.1), X30 (97.7), X45 (98.3)

Group 2 Romifidine mean SpO2: R0 (99.8), R15 (91.2), R30 (86.0), R45 (89.0) P < 0.05

In this graph we compare the variation of SpO2 over time between group 1 and 2. Here we can see a relatively constant variation of values ranging between 97.6 – 98.3 percent in group 1. The values expressed in group 2 ranges between 99.8 – 89.0 percent during the entire procedure with a dip reaching as low as 86 percent after 30 minutes. Romifidine had an obvious impact on the SpO2 value meaning poor oxygen haemoglobin saturation resulting in hypoxia. Pay attention to the fact that the y-axis starts at 85 percent since this gives a more magnified view of the differences between groups.

24 Lactate

Fig. 7 Changes in mean Lactate (unit: mmol/l) over time (unit: minutes) between different groups.

Group 1 Xylazine mean Lactate: X0 (1.1), X15 (1.12), X30 (1.3), X45 (1.4) Group 2 Romifidine mean Lactate: R0 (0.7), R15 (0.9), R30 (1.1), R45 (1.3) P < 0.05

This graph represents the variation of the blood lactate concentration value over time in group 1 and 2. Blood lactate concentration increased during anaesthesia for both groups, reaching the highest level at 45 minutes of general anaesthesia, all values are within the normal range, none of the mean values are exceeding 2 mmol/l. However, the mean values in group 1 are markedly higher compared group 2 throughout the entire duration of the anaesthetic process. Moderate hyperlactatemia was noted in one horse from group 1 (horse no. 5) at 0 minutes measuring 2.3 mmol/l.

25 Temperature

Fig. 8 Changes in mean temperature (unit: degrees Celsius) over time (minutes)between groups.

Group 1 Xylazine mean Temperature: X0 (37.8), X15 (37.54), X30 (37.35), X45 (37.2) Group 2 Romifidine mean Temperature: R0 (37.9), R15 (37.7), R30 (37.4), R45 (37.1) P > 0.05

This graph represents the variation of temperature with time in group 1 and 2. The mean temperature declined linearly over time. We can see an overall faster decrease of temperature in group 2 especially between 30 minutes and 45 minutes, starting at a higher initial temperature and finishing at a lower temperature in comparison to group 1. Pay attention to the fact that the y-axis starts at 37 degrees and displays only a range of 1 degree to show a clearer view of the differences between groups. Also, it is important to keep in mind that the season and environmental temperature impact on the core body temperature of the horses investigated was not taken into consideration.

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3.2 Recovery

Time from disconnection of inhalation anaesthesia to deglutition reflex

Fig. 9 Mean time from disconnection of sevoflurane to deglutition reflex appears (unit: minutes) between groups.

Group 1 Xylazine mean time from disconnection of sevoflurane to deglutition reflex, min X (13.1) Group 2 Romifidine mean time from disconnection of sevoflurane to deglutition reflex, min R (20.3)

P < 0.05

This graph represents the time that it took for the horse to show signs of swallowing after being disconnected from the inhalational anaesthesia machine. Signs of swallowing indicates that the patient is waking up and it’s time to extubate. We can see that the mean time is clearly longer in group 1 compared to group 2. Horses from group 2 woke up on average 7.2 minutes faster than horses in group 1.

27 Time from deglutition reflex to stand up

Fig. 10 Mean time from appearance of deglutition reflex to stand up (unit: minutes) between groups.

Group 1 Xylazine mean time from deglutition reflex to stand up, min X (12.7) Group 2 Romifidine mean time from deglutition reflex to stand up, min R (5.25) P< 0.05

This graph illustrates the difference in mean time from deglutition reflex appeared until the horse was able to stand up on its own. Even for this aspect the duration of time was considerably longer in group 1 compared to group 2. On average the horses from group 2 stood up 7.45 minutes faster than horse from group 1.

28 Tries to stand up

Fig. 11 Mean tries to stand up between groups. Group 1 Xylazine mean tries to stand up X (2.1) Group 2 Romifidine mean tries to stand up R (1.0) P < 0.05

This graph shows the mean tries for the horse to stand during recovery. As clearly illustrated in the bar graph horses from group 1 required on average 2.1 tries while horses from group 2 required only 1 try. This is more than double the amount of tries in group 1 compared to group 2.

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4 DISSCUSSION

Unquestionably, the administration of an alpha 2 adrenoreceptor agonist such as xylazine and romifidine causes profound bradycardia and respiratory depression in horses. In most cases, it was noted that a fast decline of the heart rate had already occurred 1-minute post-administration. Even though theoretically this physiological impact seems to be alike among all agents belonging to this group, it appears that xylazine has the least effect on the HR compared to an equal dose of romifidine. The persistence of bradycardia is stated to be longer following the administration of romifidine in comparison to xylazine [52]. The cardiovascular impact is explained to occur in 2 phases. Characteristic events for the first phase include hypertension due to increased resistance of peripheral vascularisation and reflex bradycardia. The second phase is mediated by central receptors causing reduction of sympathetic muscle tone, HR, cardiac output and blood pressure. Heart blocks occur frequently, and the effect is more distinct in high dose i.v. administration compared to low dose i.m. administration of these agents. [53].

A decrease in cardiac output has been reported in horses after i.v. administration of xylazine, usually within 5 minutes, followed by a slow return to normal. This decrease in cardiac output possibly occurs as a result of bradycardia [52]

A study comparing romifidine and xylazine as premedication for short term anaesthesia induced with ketamine in combination with diazepam reported that the HR was decreased 5,10,15 minutes after induction in both groups. However, the HR was significantly lower in the group premedicated with romifidine. In this study, the HR fell noticeably following sedation and induction. In both groups, bradycardia occurred; however, it was more obvious in the group premedicated with romifidine [54]. The results in this study are compatible to results of previous studies explaining the reason behind a rapid decrease in HR after 15 minutes. It also clarifies that despite the similarities between the agents belonging to the alpha 2 adrenoreceptor agonists, the physiological effects differ. Romifidine has a more expressed bradycardic impact compared to xylazine. In this statistical analysis, the p-value was more than 0.05. This can be explained by the variation of samples evaluated in each group. Also, a bigger number of samples in general would provide a safer statistical statement.

An additional parameter indicating adequate cardiovascular function is the CRT. The CRT helps attain information about peripheral tissue perfusion [55]. CRT should be less than 2 seconds. A prolonged CRT is characteristic for low blood pressure and cardiac output as well as vasoconstriction [5].

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that the CRT was higher in group 2 compared to group 1 during the entire procedure, especially at minute 0, meaning that the tissue perfusion capacity using romifidine is poorer compared to xylazine; this can be explained by the potent bradycardic effect of romifidine.

The alpha 2 adrenoreceptor agonists also interfere with the function of the respiratory system during sedation and general anaesthesia [57]. The impact of alpha 2 adrenoreceptor agonists on the pulmonary function is however less pronounced compared to the cardiovascular impact, mainly affecting the RR. Though, the respiratory depressive mechanism of other agents is usually enhanced by alpha 2 adrenoreceptor agonists [53]. A study was carried out comparing detomidine, romifidine, xylazine and their impact on physiological parameters at a minimum dosage demonstrating that the RR value for all three groups decreased. The major alterations were observed in detomidine followed by romifidine and xylazine [58]. Another additional study comparing the cardiopulmonary effects using romifidine and xylazine as premedication each induced with ketamine for short term anaesthesia in horse showed that the RR was decreasing; however, the changes between the groups were similar. [59]

In this study a significant respiratory depression was observed throughout the anaesthetic procedure for both groups. However, the RR was decreasing at a faster rate between minute 0 and minute 30 in group 2. We can also see a slight to moderate state of elevated RR between minute 0 and minute 15 in group 2.

SpO2, or arterial oxygen saturation is determined by the degree of oxygen being carried by haemoglobin measured in percentage by a pulse oximetry [55]. Recumbency in combination with anaesthesia leads to impaired oxygen supply and thereby hypoxia [60]. The vasoconstrictive properties arising as a result of alpha 2 adrenoreceptor agonist administration can impair the readings of the pulse oximetry giving false low saturation readings [53]. SpO2 alone is not a very informative parameter and should be assessed in combination with the state of the entire animal, taking in consideration factors that might interfere with the measured values; hypothermia, hypovolemia and vasoconstrictions to mention a few [61].

As clearly seen in the graphs, even though the CRT indicates adequate peripheral perfusion in both groups, it is not informative about oxygen saturation of the arterial blood; meaning that despite enough blood being carried to peripheral tissues, the oxygen level in the blood might not be enough to sustain a normal cellular function. This is called hypoxemia and can lead to irreversible cell damage.

31

value might not be accurate, and the actual arterial oxygen saturation might be lower than the value given by the pulse oximetry [61].

A likely cause of increased lactate concentration in blood is anaerobic gluconeogenesis as a result of poor muscle perfusion in association with poor oxygen delivery to the tissues during general anaesthesia [62]. The causes of hyperlactatemia are divided into two types; Type A occurring as a result of tissue hypoxia and Type B refers to a state in which hypoxia is not the reason of hyperlactatemia. During monitoring of general anaesthesia of elective surgery, Type A hyperlactatemia is solely observed. Oxygen delivery to the surrounding tissues depends on the cardiac output and oxygen content in the arterial circulation [63]

The results presented by this study indicates a strong correlation (-0.505) between blood lactate concentration and SpO2 for group 2 and a weak correlation (-0.262) in group 1. This is understandable since the concentration of lactate in the blood is dependent on the oxygen saturation SpO2, or rather the amount of oxygen in the blood during general anaesthesia. In the graph presented in the results (Fig. 4), it is noted that there is a higher increase of blood lactate concentration in group 2 compared to group 1 throughout the entire duration of anaesthesia, with a more acute inclination between 0 minutes and 30 minutes when the SpO2 value steeped. A rather light increase of the lactate level between 30 minutes and 45 minutes was seen when the SpO2 value was normalising again. Likewise, this is highly correlated to the RR (-0.645) and how well the patient is breathing throughout the procedure. It is also indicative that the SpO2 levels showed in the graph (Fig. 6) are accurate and reliable values.

It is common for body temperature to decrease following the administration of alpha 2 adrenoreceptor agonists [52]. This response is assumed to be due to the alpha 2 adrenoreceptor agonist causing CNS depression, reduction in muscle activity and loss of vasomotor function. All these impaired functions lead to heat loss and is an inevitable consequence of general anaesthesia [61, 64]. Hypothermia is the term describing decreased body temperature and is an indicator of poor tissue perfusion and reduced cardiac output [56].

A study on hypothermia induced by general anaesthesia when premedicated with xylazine was carried out reporting that hypothermia occurred in all anaesthetized horses except for those undergoing procedures of less than 45 minutes, regardless of inhalational anaesthetic used and surgical procedure performed, however at different degrees [65].

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and HR in group 2 at minute 0 (-0.735) and minute 15 (-0.587); explaining that the temperature was mostly affected by the HR only during the first 15 minutes. Group 2 was also strongly correlated to the CTR (0.603), RR (0.603), SpO2 (0.478) and lactate (0.832). This is comprehensible since hypothermia is affected by cardiac output and tissue perfusion, hence CRT. In addition, hypothermia affects the arterial oxygen saturation which in turn has a major impact on blood lactate concentration. The reason why the temperature was more affected in group 2 might be due to romifidine’s stronger influence on the cardiovascular system.

A study comparing pre-anaesthetic influence on recovery following general anaesthesia using romifidine and xylazine as premedicants points out that horses were presented with a more stable stance and shorter duration of recovery period following premedication with romifidine compared to xylazine. Furthermore, the group premedicated with romifidine displayed a tendency for fewer attempts to stand up at recovery. Overall premedication using romifidine showed to have a better impact on the recovery phase [66].

The results in this study are comparable to earlier studies presenting the same results. Horses belonging to group 2 expressed an overall faster and better-quality recovery compared to horses in group 1.

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CONCLUSIONS

1. By the results provided in this study, it can be concluded that the premedication of choice prior to general anaesthesia does affect the physiology of the horse in respect to HR, RR, CRT, SpO2, lactate and temperature.

2. The choice of premedication has a significant impact on the physiological parameters; RR, CRT, SpO2 and lactate. These values are statistically significant, meaning that there is a relationship between physiological parameters and choice of premedication. The effects are deduced to be decreased respiratory rate, CRT and SpO2 and increased lactate value.

However, it can be established that despite not being statistically significant, it does affect the HR and temperature likewise.

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ACKNOWLEGEMENT

Firstly, I would like to express my outmost appreciation to the Lithuanian University of Health Sciences, Veterinary Academy and the Department of Large Animal Clinic for welcoming me, giving me the opportunity and providing me the tools necessary to carry out my master’s thesis.

Most importantly I would like to express my sincerest gratitude to my supervisor Dr. Zoja Mikniene for her guidance, help and support throughout the duration of my study.

Additionally, I would like to give out a big thank you to Dr. Evaldas Šlyžius and Dr. Sigita Keržiene who patiently helped me with my statistical analysis.

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