Pressure Support Ventilation Decreases Inspiratory Work of Breathing During General Anesthesia and Spontaneous Ventilation

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Abstract
Eive CMV. We compared the inspiratory work of breathing of anesthetized patients who breathed with pressure support ventilation (PSV) with that associated with a demand gas flow and a standard anesthesia circle system. We studied nine consenting patients who underwent general inhaled anesthesia with or without regional supplementation. An anesthesia/ ventilator system (Siemens 900D, Solna, Sweden) provided PSV (5 cm H2O) or demand gas flow during spontaneous inspiration. Gas flow during demand breathing and PSV was initiated when inspiration produced a 2-cm H2O reduction in airway pressure. An anesthesia machine (Dräger Narkomed 3, Telford, Pa.) provided a gas flow rate of 6 L/min through a standard semiclosed circle system. Airway pressure, airway gas flow rate, and esophageal pressure were continuously transduced, and data or signals were conveyed to a computer. Tidal volume and respiratory rate were computed from the flow curve. The inspiratory work of breathing was calculated as the integral of the area subserved by a plot of esophageal pressure and tidal volume during inspiration. Heart rate and mean arterial blood pressure were recorded, and arterial blood was sampled for gas tension and pH analysis. No differences were found in pHa, Paco2, Pao2, tidal volume, respiratory rate, heart rate, or mean arterial blood pressure among the three modes of ventilation. Inspiratory work of breathing was less when patients received PSV (171 ± 65 mJ/L) (mean ± SD) compared with breathing with a demand valve (690 ± 231 mJ/L) or circle system (532 ± 160 mJ/L) (P < 0.05). Mean airway pressure was more with PSV (5.4 ± 1.0 cm H2O) than with demand flow (0.1 ± 1.4 cm H2O) or with the circle system (0.4 ± 1.0 cm H2O) (P < 0.05); Fio2 remained constant at 0.4 ± 0.05. We conclude that PSV with 5 cm H2O can be used to decrease aspiratory work in spontaneously breathing patients during general anesthesia compared with spontaneous breathing with a standard circle system or demand gas flow and that the reduction is not associated with deleterious homodynamic effects. Address correspondence to Dr. Christie, Department of Anesthesiology, University of South Florida College of Medicine, MDC Box 59, 12901 Bruce B. Downs Boulevard, Tampa, FL 33612. Presented in part at the Annual Meeting of the International Anesthesia Research Society, March 8--12, 1991, San Antonio, Texas. Accepted for publication March 4, 1992. © 1992 International Anesthesia Research Society...

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