Volume-dependent compliance and ventilation-perfusion mismatch in surfactant-depleted isolated rabbit lungs

Objective: Volume-dependent alterations of lung compliance are usually stud ied over a very large volume range. However, the course of compliance withi n the comparably small tidal volume (intratidal compliance-volume curve) ma y also provide relevant information about the impact of mechanical ventilat ion on pulmonary gas exchange. Consequently, we determined the association of the distribution of ventilation and perfusion with the intratidal compli ance-volume curve after modification of positive end-expiratory pressure (P EEP). Design: Repeated measurements in randomized order. Setting: An animal laboratory. Subjects: Isolated perfused rabbit lungs (n = 14). Interventions: Surfactant was removed by bronchoalveolar lavage. The lungs were ventilated thereafter with a constant tidal volume (10 mL/kg body weig ht). Five levels of PEEP (0-4 cm H2O) were applied in random order for 20 m ins each. Measurements and Main Results: The intratidal compliance-volume curve was d etermined with the slice method for each PEEP level. Concurrently, pulmonar y gas exchange was assessed by the multiple inert gas elimination technique . At a PEEP of 0-1 cm H2O, the intratidal compliance-volume curve was forme d a how with downward concavity. At a PEEP of 2 cm H2O, concavity was minim al or compliance was almost constant, whereas higher PEEP levels (3-4 cm H2 O) resulted in a decrease of compliance within tidal inflation, Pulmonary g as exchange did not differ between PEEP levels of of 0, 1, and 2 cm H2O. Pu lmonary shunt was lowest and perfusion of alveoli with a normal ventilation -perfusion was highest at a PEEP of 3-4 cm H2O. Deadspace ventilation did n ot change significantly but tended to increase with PEEP. Conclusions: An increase of compliance at the very beginning of tidal infla tion was associated with impaired pulmonary gas exchange, indicating insuff icient alveolar recruitment by the PEEP level. Consequently, the lowest PEE P level preventing alveolar atelectasis could be detected by analyzing the course of compliance within tidal volume without the need for total lung in flation.