Inspiratory Work Of Breathing In Ventilated Preterm Infants

In ventilated newborns, part of the inspiratory work of breathing (WOB) may be due to the inspiratory efforts preceding inspiratory ventilator flow. This study was designed to quantify the contribution of these efforts to WOB. WOB was evaluated in six intubated preterm infants ventilated by the Drager Babylog 8000. The ventilatory modes studied were intermittent manda tory ventilation (IMV), continuous positive airway pressure (CPAP), and assist-control ventilation at 10 (ACV(10)) and 15 (ACV(15)) cm H2O peak pressure. Mouth flow (V) and esophageal pressure (Pe) were recorded, and WOB was estimated from the area delineated by the esophageal pressure-volume curve, where volume is the time integral of V. Calculation of WOB started either at the onset of the infant's inspiratory flow (WOBf), or at the beginning of the infant's inspiratory muscle efforts, detected on Pe and confirmed on the V tracing (WOBm). WOBm was found to be significantly higher than WOBf under all ventilatory conditions studied. The difference in work of breathing (Delta W) between WOBm and WOBf did not depend on the type of ventilatory mode. When Delta W was related to WOBm, it amounted to about 30% of WOBf in IMV and CPAP, and 60% in ACV (P < 0.05, ACV(15) vs. IMV). These results suggest that, in preterm infants connected to a ventilator, inspiratory efforts preceding flow inspiration might account for a large fraction of the inspiratory work of breathing. (C) 1996 Wiley-Liss. Inc.