Study objective: To determine if reducing respiratory muscle load or increasing tidal volume (VT) with pressure support (PS) would improve ventilatory efficiency by reducing respiratory rate (RR) and resting PaCO2.
Design: Prospective, and patient-controlled, with randomized treatment crossover.
Setting: University cancer center and research institute.
Patients: ASA status I and II patients scheduled to undergo inhalation anesthesia for operations in which neuromuscular relaxation is undesirable.
Interventions: Propofol and succinylcholine were used to induce anesthesia and facilitate tracheal intubation. General anesthesia was maintained with desflurane, nitrous oxide, and oxygen. Patients breathed spontaneously at atmospheric pressure for 15 minutes and then randomly received alternate 15-minute trials of PS adjusted to provide either 5 cmH2O or a level titrated to produce VT of 8 ml/kg body weight. Then patients breathed spontaneously at atmospheric pressure for 15 minutes.
Measurements and main results: In 20 patients aged 52 +/- 11 years and weighing 72 +/- 18 kg, airway gas flow and pressure, esophageal pressure, arterial blood gas tensions and pH, and mean arterial pressure (MAP) were measured. Heart rate (HR), VT, RR, minute ventilation (VE), carbon dioxide production (VCO2), ratio of dead space to VT (VD/VT), and inspiratory work of breathing (WI) were calculated. End-tidal desflurane (6.7 +/- 0.4%), VCO2 (142 +/- 39 ml/min), HR, and MAP were unchanged throughout the study. WI, RR, VE, VD/VT, and PaCO2 were reduced significantly when PS was titrated to produce a VT of 8 ml/kg and spontaneous breathing before and after PS trials (p < 0.05).
Conclusions: PS titrated to produce a near normal VT improves the efficiency of spontaneous breathing by lowering RR and PaCO2 while preserving hemodynamic homeostasis in patients undergoing inhalation anesthesia. Lesser PS levels will unload the respiratory muscles, but have no effect on RR or PaCO2.