Pressure-support ventilation and diaphragm shortening in the rat model.

Little is known about how pressure-support ventilation affects diaphragm performance because there is no direct measurement of diaphragm function in the clinical setting. An indicator of diaphragm performance or work is the product of diaphragm muscle shortening and intrathoracic pressure during inspiration. We studied the effect of pressure-support ventilation on diaphragm shortening, diaphragm work, and other cardiopulmonary parameters. In 15 anesthetized Sprague-Dawley male rats, pressure support was increased from 0 to 10 cm H2O in 2 cm increments. Increasing pressure support from 0 to 10 cm H2O resulted in respiratory rate decreasing 25%, tidal volume increasing 148%, minute ventilation increasing 91%, end-tidal carbon dioxide decreasing 5%, and cardiac output decreasing 30%. Progressively increasing pressure support to 10 cm H2O was accompanied by decreases in the end-inspiratory pressure without significant increases in diaphragm shortening. Therefore, diaphragm work was decreased. The lack of an increase in diaphragm shortening in the presence of an increase in tidal volume indicated that there was an augmentation of thoracic volume in the coronal and/or horizontal axes instead of the cephalocaudal axes throughout inspiration. These findings may be useful to nurse anesthetists in the understanding of diaphragm work when patients are being ventilated with pressure-support ventilation.