If volume is delivered during inspiration by a constant (square-wave) flow, a tracing of pressure against time can be used to measure compliance. As no account is taken of the effect of airways resistance on peak airway pressure, or of the effect of intrinsic positive end-expiratory pressure (PEEP), this measurement can be thought of as 'effective' compliance. As flow is constant, the same volume is added for all time periods and the time access can be regarded as a linear analog of volume. The tracing can be regarded as a pressure-volume tracing (Fig 3).
Fig. 3 Airway pressure of a mechanical breath under conditions of constant flow. With an inspiratory pause and occlusion, airway pressure declines to a plateau. If no respiratory effort occurs, this is the static recoil pressure of the lung and chest wall. The gradient of the airway pressure-time curve is termed the effective dynamic compliance. It is offset from alveolar pressure by the effect of airflow resistance. Ideally, the gradients of airway and alveolar pressure are parallel. This can be checked when the peak plateau pressure equals the initial upstroke of the curve.
At the onset of inspiration no flow occurs, and this is the starting point for pressure measurement. The pressure is measured again at the end of the square-wave flow. The slope of the line is the 'effective dynamic compliance' (as the system is in motion). Care should be taken to ensure that the upstroke is linear when resistance or intrinsic PEEP is high.
The 'effective dynamic compliance' can be verified from the airway pressure-time tracings. If a pause is inserted at the end of inspiration, airway pressure declines and, if sufficient time is allowed, the pressure in the airway will reflect the compliance of the lung. As the system is now stationary, this value is related to the total static compliance of the respiratory system. The slopes of alveolar pressure and 'effective dynamic compliance' often match ( Fig.3). Errors can occur if the inspiratory pause is brief relative to the time constants of the lung. When expiratory time is delayed, airway pressure will slowly equilibrate with alveolar pressure and care must be taken to establish a plateau.
Many ventilators compute compliance automatically. Although there is a danger that such systems can be misinterpreted, good agreement has been found when compliance is measured automatically (S.y.d.ow..e.t..a.l 1991).
The pressure-volume curve of the lung and chest wall can be constructed formally by measuring the inflation pressure when the lung is inflated with small aliquots
(volume 200 ml) from a large calibrated syringe (1-2000 ml) (MMam.ls.e.t..,a.l 1984). Pressure is measured during inflation and deflation and several slopes can be determined. This technique has been used to demonstrate the changes in compliance in acute respiratory distress syndrome, and has been applied to determine the inspiratory inflection point indicative of the point where the airways start to open.
Although dynamic compliance is best measured as the transpulmonary pressure ( Paw - Pes), the technique requires an esophageal balloon catheter. When the patient is relaxed and supine, the weight of the mediastinal contents creates an offset increase in pressure. The absolute measurement of Cdyn would also be affected by the presence of esophageal contractions which can be detected as an intermittent positive-pressure deflection.
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