The Diaphragm Acts as a Brake during Expiration to Prevent Lung Collapse

Rationale: The diaphragm is the major inspiratory muscle and is assumed to relax during expiration. However, electrical postinspiratory activity has been observed. Whether there is an expiratory diaphragmatic contraction that preserves lung patency has yet to be explored. Objectives: We hypothesized the occurrence of an expiratory diaphragmatic contraction directed at stabilizing peripheral airways and preventing or reducing cyclic expiratory lung collapse. Methods: Mild acute respiratory distress syndrome was induced in 10 anesthetized, spontaneously breathing pigs. Lung volume was decreased by lowering end‐expiratory airway pressure in a stepwise manner. We recorded the diaphragmatic electric activity during expiration, dynamic computed tomographic scans, and respiratory mechanics. In five pigs, the same protocol was repeated during mechanical ventilation after muscle paralysis. Measurements and Main Results: Diaphragmatic electric activity during expiration increased by decreasing end‐expiratory lung volume during spontaneous breathing. This enhanced the diaphragm muscle force, to a greater extent with lower lung volume, indicating a diaphragmatic electromechanical coupling during spontaneous expiration. In turn, the resulting diaphragmatic contraction delayed and reduced the expiratory collapse and increased lung aeration compared with mechanical ventilation with muscle paralysis and absence of diaphragmatic activity. Conclusions: The diaphragm is an important regulator of expiration. Its expiratory activity seems to preserve lung volume and to protect against lung collapse. The loss of diaphragmatic expiratory contraction during mechanical ventilation and muscle paralysis may be a contributing factor to unsuccessful respiratory support.