Changes in ventilation, oxygen uptake, and carbon dioxide output during recovery from isoflurane anesthesia

Recovery from inhalation anesthesia is often marked by the occurrence of postoperative tremor that resembles shivering, which is known to be associated with an increase in oxygen uptake (VO2), CO2 output (VCO2), and minute ventilation (VE). This study determined the time course of the ventilatory changes observed during the first hour of recovery from isoflurane anesthesia. Ten patients (ASA PS 1) scheduled for minor orthopedic surgery (knee arthroscopy) were included in this study. Anesthesia was induced with thiopental (5 mg/kg) and maintained with 70% N2O and isoflurane (1-2%) in oxygen, allowing spontaneous ventilation. In the recovery room, after N2O had been discontinued, patients were connected to a Beckman Metabolic measurement cart, which allowed a continuous monitoring of VE, VO2, VCO2, and PETCO2. Postoperative tremor was observed in all patients within 7.1 +/- 1.2 min (mean +/- SEM) after isoflurane discontinuation and was associated with a marked increase in the following: VO2, from 173 +/- 26 ml/min at the end of anesthesia to 457 +/- 88 ml/min; VCO2, from 149 +/- 18 ml/min at the end of anesthesia to 573 +/- 98 ml/min; and VE, from 6.8 +/- 0.7 l/min at the end of anesthesia to 16.6 +/- 2.8 l/min (values obtained 20 min after isoflurane discontinuation). In three patients during intense shivering, VO2, VCO2, and VE reached peak values higher than 800 ml/min, 1,300 ml/min and 30 l/min, respectively. This study shows that postoperative tremor following isoflurane anesthesia may be associated with prolonged and large increases in oxygen uptake, CO2 output, and minute ventilation.