Continuous measurement of pulmonary gas exchange during general anaesthesia in man

Abstract

We report a system for the continuous measurement of oxygen uptake (VO2) and carbon dioxide output (VCO2) during open-circuit anaesthesia. Gas concentrations were measured by a mass-spectrometer, and expired flow by a pneumotachograph. The values measured by the system were compared in vitro to values produced by a nitrogen-dilution technique. Excellent correlations were found. Continuous measurements were performed in 21 patients anaesthetized for abdominal surgery. Compared to pure intravenous anaesthesia (flunitrazepam-fentanyl), anaesthesia including the administration of nitrous oxide or nitrous oxide and halothane led to more pronounced and sustained decreases in VO2 and core temperature, with a better cardiovascular stability. Two hours postoperatively, VO2 was not different from preoperative values. After a transient increase at the onset of anaesthesia, the respiratory exchange ratio (VCO2/VO2) returned to preoperative values, and then remained subsequently unchanged. Based on these observations, the system described provides an accurate approach to noninvasively monitoring the pulmonary gas exchange in the operating room.

Metabolic gas exchange during different surgical procedures

“…In the emergency laparotomy and in the arthroscopy groups a decline in VCO, was seen despite of a rise in Vo,. These findings are in accordance with the studies of others [6,9], in which Vco2 started at a relatively high level at the beginning of surgery but declined thereafter. These authors suggest that relative hyperventilation was responsible for this but the phenomenon was not observed in the elective laparotomy patients despite the use of a similar anaesthetic technique.…”

“…In another study involving patients undergoing gastrointestinal surgery, an increase in 30, similar to that found in the present study was seen in patients during anaesthesia with a benzodiazepine/fentanyl combination. This surgically induced change in oxygen consumption was blunted in patients in whom N 2 0 or halothane was added to the intravenous anaesthetic agents [6]. Thus, it is evident that both the degree of surgical stress and the anaesthetic technique may alter oxygen consumption during surgery.…”

“…The continuous gas exchange measurements were delayed until at least 15 min after tracheal intubation in order to avoid the effects of intubation on metabolic gas exchange [5]. This delay has been reported to eliminate the difficulties of measuring gas exchange during changes in FEN, [6]. Baseline data on metabolic gas exchange were measured during the 2 min before skin incision during general anaesthesia.…”

Emergence from isoflurane/N2O or isoflurane anaesthesia

“…Volatile agents such as halothane (23), enflurane (24, 25), and isoflurane (25, 26) generally reduce pre-anaesthetic oxygen uptake by 20-35%. Postoperatively, the stress associated with return to conciousness and recovery of pain sensation, and the thermogenesis needed to correct a temperature fall occurring during surgery are main factors known to increase metabolic rate (26,27). In our study, the carbon dioxide elimination rate was 185 and 145 ml 70 kg-' min-* at the end of a 1.3 MAC isoflurane / N20 and isoflurane anaesthesia, respectively.…”

Metabolic gas exchange during gynaecological laparotomy and laparoscopy

“…7 This time lag has also been reported to eliminate the difficulties regarding measurements of gas exchange during changes in FEN2.2 After 15 min of anaesthesia the nitrogen body stores are in steady state so that the relative error between computed and measured VO2 is <3%. For details see discussion by Viale et al 2 Baseline data on metabolic gas exchange were measured during the two minutes prior to skin incision during general anaesthesia. Metabolic gas exchange was thereafter measured continuously each minute as long as surgery was performed.…”