Static measurements

The term 'static' refers to the conditions of no flow where the respiratory system is not moving. Increments of lung volume are delivered from a large (1.5 liters) syringe which inflates the lungs at the end of expiration. During deflation, the lung is emptied in similar steps and the pressure is recorded.

The pressure-volume curve (Fig 3) obtained using the supersyringe technique displays two general characteristics which are amplified when the lungs are stiff.

Firstly, pressure must be applied to the lungs to open them and this opening pressure can be clearly seen as the first inflexion point of the graph. Inflation from this point is generally linearly related to the pressure difference. During inflation there is a point where lung volume reaches its maximum. When this happens, high pressures are needed to change volume further. This is the second (upper) inflexion point. Patterns of ventilation which allow the patient to fall below the lower inflexion point would entail opening and closing of alveoli. If ventilation is set such that the upper inflexion point is exceeded, high-pressure damage would be expected. Ideal ventilation would entail the application of external positive end-expiratory pressure (PEEP) to prevent alveolar collapse together with ventilation over the linear pressure-volume range, avoiding high inflation pressures and alveolar distension.

Fig. 3 A pressure-volume tracing obtained from a patient with stiff lungs by means of the supersyringe technique. The inflation curve has two clearly separate components and the inflexion point is indicated. The pressure at the inflexion point is used to assess the external PEEP required to prevent airway closure. The inspiratory and expiratory curves are separated due to hysteresis. This difference in volume is related to gas trapping.

Pressure-volume curves of the lung are very sensitive to changes in lung stiffness. This is seen frequently when the lung tissue is damaged, particularly when the response is fibrosis. The changes seen in the acute respiratory distress syndrome provide a good example of the use of pressure-volume curves. In health, the lungs are held open at the relaxed end-expiratory point (functional residual capacity) and inflation occurs at transpleural pressure changes of 2 to 5 cmH 2O. The first change related to increasing lung stiffness in the acute respiratory distress syndrome is the appearance of a lower inflexion point, suggesting that alveolar collapse has occurred at the relaxed end-expiratory point. As the syndrome progresses the slope of the pressure-volume curve changes, suggesting that compliance has decreased. Furthermore, the gap between the inflation and deflation curves (hysteresis) increases (§La/, 1984; §§y.d.on §La/ 19.9.1).

Several therapeutic strategies are available, and the conceptual type of ventilation has been described as ventilation for a 'baby lung'. As with neonates and infants, alveolar collapse occurs readily. Recruitment of partially ventilated or collapsed lungs is difficult, but this problem can be minimized by titrating external PEEP appropriately. Overexpansion of the shrinking 'normal lung' in adults can be avoided if the compliance is measured frequently. During recovery, pressure and volume limitations imposed by lung mechanics can be normalized in a timely manner.

A quasistatic pressure-volume curve can be obtained without ventilator disconnection. If the ventilator can be adjusted so that ventilation pauses and the pressure in the airway can be measured, the compliance of the lung and chest wall can be plotted. The volume of successive breaths can be adjusted so that higher volumes are achieved. The agreement between this approach and the supersyringe technique is good, although care must be taken with all ventilator measurements that the calibration of pressure and volume sensors is accurate ( Sy.d..0W §ta/, 1991).

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