Analysis of dynamic intratidal compliance in a lung collapse model
Background: For mechanical ventilation to be lung-protective, an accepted suggestion is to place the tidal volume (VT) between the lower and upper inflection point of the airway pressure-volume relation. The drawback of this approach is, however, that the pressure-volume relation is assessed under quasistatic, no-flow conditions, which the lungs never experience during ventilation. Intratidal nonlinearity must be assessed under real (i.e., dynamic) conditions. With the dynamic gliding-SLICE technique that generates a high-resolution description of intratidal mechanics, the current study analyzed the profile of the compliance of the respiratory system (CRS). Methods: In 12 anesthetized piglets with lung collapse, the pressure-volume relation was acquired at different levels of positive end-expiratory pressure (PEEP: 0, 5, 10, and 15 cm H2O). Lung collapse was assessed by computed tomography and the intratidal course of CRS using the gliding-SLICE method. Results: Depending on PEEP, CRS showed characteristic profiles. With low PEEP, CRS increased up to 20% above the compliance at early inspiration, suggesting intratidal recruitment; whereas a profile of decreasing CRS, signaling overdistension, occurred with VT > 5 ml/kg and high PEEP levels. At the highest volume range, CRS was up to 60% less than the maximum. With PEEP 10 cm H2O, CRS was high and did not decrease before 5 ml/kg VT was delivered. Conclusions: The profile of dynamic CRS reflects nonlinear intratidal mechanics of the respiratory system. The SLICE analysis has the potential to detect intratidal recruitment and overdistension. This might help in finding a combination of PEEP and VT level that is protective from a lung-mechanics perspective. Copyright © 2011, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins. Anesthesiology.