Measurement of pressure-volume curves in patients on mechanical ventilation: methods and significance

Abstract

Physiological background concerning mechanics of the respiratory system, techniques of measurement and clinical implications of pressure-volume curve measurement in mechanically ventilated patients are discussed in the present review. The significance of lower and upper inflection points, the assessment of positive end-expiratory pressure (PEEP)-induced alveolar recruitment and overdistension and rationale for optimizing ventilatory settings in patients with acute lung injury are presented. Evidence suggests that the continuous flow method is a simple and reliable technique for measuring pressure-volume curves at the bedside. In patients with acute respiratory failure, determination of lower and upper inflection points and measurement of respiratory compliance should become a part of the routine assessment of lung injury severity, allowing a bedside monitoring of the evolution of the lung disease and an optimization of mechanical ventilation.

Figure 1

Inspiratory occlusion technique as described by Levy et al [8]. The patient is on controlled ventilation with a constant flow. Between two measurements, the lung volume is standardized by maintaining the ventilatory parameters constant. The intrinsic PEEP (PEEPi) is determined before each inflation followed by an end-inspiratory occlusion. The plateau pressure (Pst) is obtained a few seconds after the occlusion. From Levy et al [8].

Figure 2

Recording obtained from the ventilator display screen (César ventilator, Taema, Antony, France) during measurement of the pressure-volume curve with a constant flow technique (9 l/min). The diagram on the right shows that the patient is ventilated in a volume controlled (VAC) mode at a constant inspiratory flow with a tidal volume of 1480 ml, a respiratory frequency of 4.9 breaths/min and a fractional inspired oxygen of 97%. These settings deliver a constant flow of 9 l/min (diagram in the middle) over a period of 9.6 s. The pressure-volume curve is seen on the left side of the screen. The lower inflection point and the slope are determined with the help of two cursors. From Lu et al [10].

Figure 3

Respiratory (left panel) and pulmonary (right panel) pressure-volume curves obtained by the supersyringe technique (clear circle), the inspiratory occlusion technique (clear square) and the constant flow technique at 3 (filled circle) and 9 (clear triangle) l/min flow. The curves obtained using the constant flow technique at 9 l/min are slightly shifted to the right due to the generation of a resistive pressure. The curves obtained by the other methods are not significantly different. From Lu et al [10].

Figure 4

Respiratory (left panel) and pulmonary (right panel) pressure-volume curves at ZEEP in patients with acute respiratory failure or ARDS presenting with (n =8, clear square) or without (n =6, filled square) a lower inflection point. Both the compliances were significantly lower in patients with a lower inflection point. Paw, airway pressure measured the trachea; Pes, oesophageal pressure measured in the lower third of the oesophagus. From Vieira et al [31].

Figure 5

Pressure-volume curves obtained in ZEEP conditions and at PEEP values of 5, 10 and 15 cmH₂O in two patients with ARDS. In the patientwho had a convex curve in ZEEP conditions (top left panel), PEEP did not induce any alveolar recruitment. When the different PEEP levels were applied the pressure-volume curves appeared superimposed on the curve obtained in ZEEP conditions, indicating an over-distension of the lung (bottom left panel). In the patient who had a concave curve in ZEEP conditions (top right panel) PEEP displaced the curve upwards, indicating alveolar recruitment (bottom right). The increase in lung volume between ZEEP and PEEP for a given alveolar pressure (20 cmH₂O) is the recruited lung volume. From Ranieri et al [17].

Figure 6

Changes in the volume of overdistended areas (upper panel) and in the volume of nonaerated areas (lower panel) induced by two PEEP levels in patients with (n =8, clear circle) and without (n =6, filled circle) a lowerinflection point. PEEP₁, lower inflection point +2 cmH₂O or +10 cmH₂O in the absence of lower inflection point; PEEP₂, lowerinflection point +7 cmH₂O or +15 cmH₂O in the absence of a lowerinflection point. The lung volumes were measured by an analysis of the volume distribution of CT attenuations with a spiral thoracic CT scan. Alveolar recruitment is defined as the reduction of the nonaerated lung zones with a CT attenuation ranging between -100 and +100 HU. Pulmonary over-distension is defined as the appearance of lung regions with CT attenuation less than -900 HU. In patients without a lower inflection point, increasing levels of PEEP are associated with PEEP-induced lung over-distension. From Vieira et al [31].

Figure 7

Percentage of patients with ARDS and ventilated with PEEP in whom the plateau pressure exceeds the upper inflection point. In 50% of the patients, the upper inflection point is attained or exceeded for aplateau pressure of 25 cmH₂O. From Roupie et al [4].