Multiple Occlusion Technique

The multiple occlusion technique [22, 23] uses a sequence of different-sized volume-controlled inflations with an end-inspiratory pause to allow semi-static pressure measurements. Intrinsic PEEP is determined before each inflation to ensure that the lung volume and the end-expiratory pressure are stable. Pressure and volume are plotted for each end-inspiratory pause to form a static P-V curve. If expiratory interruptions are also done, a static expiratory P-V curve is obtained. Because the pressure and volume values are obtained during standard, but varying breaths, the artifacts caused by gas exchange do not occur, and no hysteresis is found even in ARDS patients. Although the largest tidal volumes used with this technique may cause recruitment, this is limited because the time at high pressure is very short. The entire set of measurements may take 5^10 min.

The inspiratory occlusion technique offers the advantage of avoiding disconnection of the patient from the ventilator, and it allows measurements from any level of PEEP. Since the beginning of the 1990s, this technique has been extensively used to determine the lower and upper inflection points on the P-V curve [24, 25] and to quantify the effect of PEEP on alveolar recruitment in patients with ARDS [26]. The sudden change in pressure occurring with interruption of flow is accompanied by rapid damped oscillations and a subsequent further transient change in pressure to a stable plateau. These two phases have been interpreted on the basis of two- compartment models of respiratory mechanics [27, 28].