What is the Utility of Monitoring Pulmonary Mechanics in the Treatment of Patients with Acute Respiratory Failure?

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of Monitoring Pulmonary Mechanics in the Treatment of Patients with Acute Respiratory Failure?

S. Benito, M. Subirana, J. M. García

Introduction

Monitoring is to use an instrument with the purpose of making a measurement, rather than of providing treatment. The variable measured can be of interest from a physiopathologic point of view, for later adjustment, or it may be associated with patient outcome. In either case, the adjustment of the variables should be to the patient’s benefit.

Pulmonary mechanics is the method of physiological measures used for diagnostic purposes and which allows exploration of the mechanical properties of the total respiratory system. It can be adapted to patients under mechanical ventilation, as they can be easily sedated in order to obtain the measurement. Reduced to the simplicity of a loop, it provides information about the complexity of the thoracic system: the lung, airways, ribcage, respiratory muscles, and abdomen. Pulmonary mechanics analyses the respiratory movement; the components involved are described in the equation of the movement, these being the elastic properties, the resistance to flow, and inertia. For clinical study, inertia can be ruled out. Pulmo- nary parenchyma is responsible for the elastic behaviour and the airway is responsible for airflow resistance. In order to eliminate the non-elastic forces, the exploration must be performed under static conditions.

Evidence-based medicine (EBM) methodology has been extensively developed in recent years, and applied to the evaluation of diagnostic tests. In this chapter on the utility of pulmonary mechanics monitoring, the EBM strategy has been used and results are commented on.

Study Identification

The databases MEDLINE, CINAHL, and the Cochrane Library were consulted by means of a structured electronic search. A total of 255 article summaries and four systematic reviews were obtained. Once the relevant abstracts were read, the originals were recovered and, from their bibliography, additional articles were obtained. For this chapter, 64 original articles were analyzed, covering a period of thirty years.

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Evaluation of Diagnostic Examination Studies

According to the EBM criteria (Table 1) for critical appraisal, diagnostic studies must be compared with a standard reference and in the population of patients that will be later used in clinical practice. The studies must provide the rates of prob- ability of the examination results or the necessary data for their calculation. They should be reproducible and useful for patient management and contribute to modification of treatment. The degree of recommendation and the levels of evidence for the diagnostic examinations differ depending on the topic in question.

Systematic reviews prioritize cohort studies that validate the quality of the test and are of high specificity.

Critical appraisal of the 64 retrieved articles shows they do not meet the EBM criteria and should be rejected. To analyze diagnostic tests based on clinical studies and final outcomes, EBM is possibly a very well consolidated methodology but it is less useful in evaluating results from basic research and physiopathology which includes pulmonary mechanics studies.

Table 1. Oxford Centre for Evidence-based Medicine Levels of Evidence for Diagnosis (May 2001), adapted from http://www.cebm.net/levels_of_evidence.asp#levels

Level

1a SR (with homogeneity) of Level 1 diagnostic studies; CDR*

with 1b studies from different clinical centres

1b Validating cohort study with good reference standards;

or CRD* tested within one clinical centre 1c Absolute SpPins and SnNouts

2a SR (with homogeneity) of Level >2 diagnostic studies 2b Exploratory cohort study with good reference standards;

CDR* after derivation, or validated only on split-sample** or databases 3a SR (with homogeneity) of 3b and better studies

3b Non-consecutive study; or without consistently applied reference standards 4 Case-control study, poor or non-independent reference standard

5 Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”

SR: systematic review; *: Clinical Decision Rule -These are algorithms or scoring systems which lead to a prognostic estimation or a diagnostic category. $ An “Absolute SpPin” is a diagnostic finding whose Specificity is so high that a Positive result rules-in the diagnosis. An “Absolute SnNout” is a diagnostic finding whose Sensitivity is so high that a Negative result rules-out the diagnosis. ** Split-sample validation is achieved by collecting all the information in a single tranche, then artificially dividing this into “derivation” and “validation” samples.

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Analysis of Intervention: Compliance How is it Measured?

1. Total compliance of the respiratory system: We obtain the value of the compli- ance by dividing the tidal volume by the total positive end-expiratory pressure (PEEP, which includes intrinsic PEEP) subtracted from the pressure measurement at zero flow with a pause of 1.2 sec at the end of inspiration (Fig 1). The compliance value depends on the value of the increase in induced volume and the pulmonary volume, specifically of the functional residual capacity (FRC) and the increase produced by the PEEP applied [1].

2. Pressure-Volume (P/V) loop: The patient should be supine, sedated, and para- lyzed. After suctioning the secretions and producing a deep insufflation the patient is connected to a syringe of 2 l and to a pressure transducer, while the volume is set at FRC. 100 millilitres are insufflated; after a pause of 1 second, another 100 millilitres are insufflated, until reaching 25 ml/kg or 40 cmH2O of airway pressure (Paw). Deflection follows this point and is performed with the same pauses and decrements. The pause points are joined [2].

What Information does it Give?

The P/V loop yields the lower inflection point (LIP) and upper inflection point (UIP), the compliance in various portions of the loop, and hysteresis. The pulmonary distensibility status from FRC to total lung volume (TLV), can be explored by means of the P/V loop in its static condition, by tidal volume loops at different PEEP increases, and by an airway pressure curve at the same tidal volume value and increses of PEEP values (Fig. 2).

Fig. 1. Total compliance of the respiratory system during mechanical ventilation

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Is it the Only Way to obtain these Data?

In addition to the supersyringe system [1], systems of continuous flow and occlusion have been described [3, 4].

What are the risks of the measurement?

Performing a P/V loop technique is well tolerated, although it can cause changes in oxygenation and hemodynamics. For this reason, patients must be strictly monitored during the maneuver [5].

Utility of Measurement of Pulmonary Mechanics

Is the P/V Loop Measurement Effective in Patients with Acute Respiratory Failure?

Models of the P/V loop have been described to correspond with the thorax X-ray in patients with different stages of acute respiratory distress syndrome (ARDS) [2]. The behaviour of the P/V loop has been related to the amount of healthy lung and the recruitment of pulmonary zones when using varying values of PEEP [6, 7], as well as to the pulmonary or extrapulmonary origin of the ARDS [8].

Is the Compliance Measurement in Patients

with Acute Respiratory Failure Effective in Deciding the Value of PEEP to set on the Ventilator?

Table 2 shows articles that used the inflection pressure as a value to decide the level of PEEP in mechanically ventilated patients. Note that for some authors, LIP is the value of PEEP to improve oxygenation [2] and ventilation [9]. Others Fig 2. P/V loop and, at differ- ent PEEP increases, tidal volume loops and airway pressure curves.

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recommend setting the level of PEEP to improve arterial blood gases and only in some cases with the loop [10]; still other authors recommend the use of discretely superior values [6, 14, 15]. For others recruitment that is tried with PEEP cannot be assured with PEEP levels equal to the LIP [11–13]. The work of Amato and colleagues [15] deserves a separate analysis. It was a randomized controlled trial, with a ventilatory technique of lung protection in which it used a value of PEEP calculated from the P/V loop. With this modality a mortality reduction in relation to the control group was observed.

Table 2. PEEP versus LIP

Study Design Pathology Nº Technique Result

Matamis [2] Cross-sectional ARDS 19 Syringe LIP = PEEP

Gattinoni [6] Cross-sectional ARF 20 Syringe Best PEEP> LIP most of the recruitment Blanch [9] Cross-sectional ARF 13 Syringe LIP= PEEP =↓ (PaCo2-

PetCO2)

Brunet [10] Cross-sectional ARDS 8 Syringe PEEP = blood gases;

optimized P-Vc Jonson [11] Cross-sectional ALI 11 Computer- Recruitment occurs far

controlled above LIP ventilator

Richard [12] Cross-sectional ALI 15 Computer- PEEP = or above controlled LIP→ alveolar ventilator instability

Maggiore [13] Cross-sectional ALI 16 Computer- LIP is a poor predictor controlled of alveolar Closure ventilator

Mancini [14] Cross-sectional ARDS 8 Syringe P flex + 2↑ oxygenation (PVS)

Amato [15] Randomized ARDS 53 Insp. Static PEEP= LIP+2; protective P/V curve ventilation↓ mortality ARDS: acute respiratory dictress syndrome; ARF: acute respiratory failure ; ALI: acute lung injury;

LIP: lower inflection point; PEEP: positive end-expiratory pressure; P/V: pressure/volume

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Is the Measurement of Compliance Effective in Diminishing the Pulmonary Injury induced

by the Ventilator in Patients with Acute Respiratory Failure?

Richard et al. [12] analyzed the risk of alveolar derecruitment with low tidal volumes when the pulmonary protection modality is used. They concluded that to avoid derecruitment, a PEEP level equal to LIP +4 can be used, or periodic recruitment maneuvers can be made. Takeuchi et al. [16] analyzed the most appropriate method to determine PEEP during the ventilatory strategy of lung protection. Although an experimental model is not applicable for an EBM review, the conclusions may be worthy of consideration for this question. PEEP calculated on the basis of the P/V loop diminished pulmonary injury as compared with the PEEP calculated by the suitable oxygenation method, and the 2 cmH2O below the LIP was more effective.

Conclusion

• The P/V loop provides information about the state of the lung in the ARDS, and has helped to improve our understanding of the lung in mechanical ventilation.

• The most adequate area to ventilate the patient is possibly between the LIP and the UIP. This would suggest using the PEEP and the tidal volume placed outside the zones of derecruitment and overdistension.

• In its present state, the technique cannot be recommended for general use. In addition, due to a lack of suitable studies, its clinical implications are not well known.

Future investigation

• Systems to perform a P/V loop with continuous flow by means of the ventilator.

• Standardization of the maneuver. Previous history of volume (insufflation, opening maneuver) in zero end-expiratory pressure (ZEEP), insufflation to 40 cmH2O, deflation.

• Help with ventilation decision-making (tidal volume, PEEP). Comparative studies in homogenous patients (LIP, +2, +4, hysteresis, UIP) and with final clinical results.

References

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2. Matamis D, Lemaire F, Harf A, et al (1984) Total respiratory pressure-volume curves in the adult respiratory distress syndrome. Chest 86:58–66

3. Mankikian B, Lemaire F, Benito S, et al (1983) A new device for measurement of pulmonary pressure-volume curves in patients on mechanical ventilation. Crit Care Med 11:897–901

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10. Brunet F, Jeanbourquin D, Monchi M, et al (1995) Should mechanical ventilation be optimized to blood gases, lung mechanics, or thoracic CT scan? Am J Respir Crit Care Med 152:524–530 11. Jonson B, Richard JC, Straus C, Mancebo J, Lemaire F, Brochard L (1999) Pressure-volume curves and compliance in acute lung injury. Evidence of recruitment above the lower inflection point. Am J Respir Crit Care Med 159:1172–1178

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