Laboratory validation of the M-COVX metabolic module in measurement of oxygen uptake.

A practical method of breath-by-breath monitoring of metabolic gas exchange has previously been developed by GE Healthcare and can now be easily incorporated into existing anaesthetic and critical care monitoring (M-COVX). Previous research using this device has shown good accuracy and precision between the M-COVX measurements and a traditional measurement of gas uptake at the mouth and also against the reverse Fick method during cardiac surgery and critical care, but its accuracy in the paediatric situation and across a range of ventilatory settings awaits validation. We tested the M-COVX metabolic monitor in the laboratory comparing its measurement to a traditional Haldane transformation across a wide range of oxygen consumption values, from 50 ml/minute to just under 300 ml/minute, typical of those expected in anaesthetised adults and children. The M-COVX device showed acceptable accuracy with an overall mean bias of -3.3% (range -15.1 to +4.2%, P = 0.21). Excellent linearity was found, by y = 0.96x + 0.5 ml/minute, r = 0.99. The device showed acceptable robustness to ventilatory changes examined, including changes in respiratory rate, I:E ratio, FiO2 up to 75% and simulated spontaneous breathing. However any induced leak from around the simulated endotracheal tube caused a significant error in paediatric scenarios.

The rate of alveolar-capillary uptake of sevoflurane and nitrous oxide following anaesthetic induction.

The rate of anaesthetic gas uptake from the breathing system has been extensively measured, but this does not reflect the true rate of early gas uptake by pulmonary blood, which drives inhalational induction of anaesthesia. In eight patients undergoing coronary bypass surgery, we measured the rate of alveolar-capillary uptake of anaesthetic gases up to 30 min following introduction of 0.5% sevoflurane and 33% nitrous oxide using the reverse Fick method, in which blood partial pressures were measured using a headspace equilibration technique. Simultaneous measurements of gas uptake from the breathing system were made by indirect calorimetry. Measured rates of sevoflurane and nitrous oxide uptake from the breathing system agreed well with previously described formulae when adjusted for inspired concentration. The time course of alveolar-capillary gas uptake followed a characteristic rising curve peaking at 3-4 min and then exponentially declining, and for nitrous oxide was significantly higher than previously estimated.

In vivo validation of the M-COVX metabolic monitor in patients under anaesthesia.

A practical method of breath-by-breath monitoring of metabolic gas exchange has been developed by GE Healthcare/Datex Ohmeda and incorporated into existing anaesthetic and critical care monitoring systems (M-COVXO). This device relates flow measurements made at the mouth by pneumotachograph to measurements of inspired and expired gas composition by matching the two waveforms thereby allowing continuous, breath-by-breath monitoring of an intubated patient's oxygen uptake and carbon dioxide production. Given that there is a paucity of data comparing this new device against methods more widely used clinically, we tested the device on 11 patients undergoing cardiopulmonary bypass surgery. Using a standard anaesthetic machine (Datex Ohmeda Excel 210 SE) with a semi-closed circle absorber system, oxygen uptake was measured at the mouth continuously throughout the operation at approximately six-second intervals. The data were compared against the reverse Fick method and against standard indirect calorimetry using the Haldane transformation. When compared to the calculated reverse Fick oxygen uptake, a mean difference of +16.5% was found pre-bypass and +9.9% post-bypass, consistent with uptake of oxygen by lung tissue, which is not taken into account by the reverse Fick method. Measurements made comparing the M-COVX metabolic monitor against standard Haldane showed a mean difference of +5.1% pre-bypass and -2.1% post-bypass. Given the ease with which this device can be incorporated into existing anaesthetic monitoring systems and its accuracy in measuring oxygen uptake, the M-COTVX module is an attractive addition to existing perioperative monitoring.

Non-invasive metabolic monitoring of patients under anaesthesia by continuous indirect calorimetry--an in vivo trial of a new method.

BACKGROUND: Oxygen uptake is an important form of metabolic monitoring for patients under anaesthesia. In critically ill patients oxygen uptake has been shown to provide valuable clinical information in directed therapy and acts as a useful monitor of cardiovascular dysfunction. A new method of continuous real time monitoring of metabolic gas exchange was tested in patients during anaesthesia. METHODS: Using a standard anaesthetic machine with attached semi-closed circle absorber system, oxygen uptake was measured continuously throughout surgery in 30 patients undergoing cardiopulmonary bypass surgery and compared with paired measurements made with the reverse Fick method. The method is an indirect calorimetry technique which uses fresh gas rotameters for control, regulation and measurement of the gas flows into the system, with continuous sampling of mixed exhaust gas. RESULTS: When compared with the reverse Fick method the oxygen uptake showed a mean difference (and sd) of 20.7 ml min(-1) or 12.1% (25.3 ml min(-1)) pre-bypass and 13.9 ml min(-1) or 8.1% (27.0 ml min(-1)) post-bypass. This bias is consistent with previous studies comparing oxygen uptake measured at the mouth against oxygen uptake by reverse Fick, which have shown a difference of approximately 10-15% accounted for by the consumption of oxygen by lung tissue. CONCLUSIONS: As the method allows continuous measurement of gas exchange and can be adapted to a modern anaesthetic workstation it is an attractive method for use in clinical setting.

Persisting concentrating and second gas effects on oxygenation during N2O anaesthesia.

Theoretical modelling predicts that the concentrating effect of nitrous oxide (N2O) uptake on alveolar oxygenation is a persisting phenomenon at typical levels of ventilation - perfusion (V/Q) inhomogeneity under anaesthesia. We sought clinical confirmation of this in 20 anaesthetised patients. Arterial oxygen pressure (P(aO2)) was measured after a minimum of 30 min of relaxant general anaesthesia with an inspired oxygen (F(I O2)) of 30%. Patients were randomly allocated to two groups. The intervention group had N2O introduced following baseline blood gas measurements, and the control group continued breathing an identical F(I O2) in nitrogen (N2). The primary outcome variable was change in P(aO2). Mean (SD) in P(aO2) was increased by 1.80 (1.80) kPa after receiving a mean of 47.5 min of N2O compared with baseline conditions breathing O2/N2 (p = 0.01). This change was significantly greater (p = 0.03) than that in the control group: + 0.09 (1.37) kPa, p = 0.83 and confirms the presence of significant persisting concentrating and second gas effects.

Accuracy of the Foldes-Biro equation for measurement of oxygen uptake during anaesthesia: a laboratory simulation.

Laboratory benchtop testing was conducted of the Biro method for measurement of oxygen uptake from an anaesthetic breathing system by patients during low flow anaesthesia. A high precision flow and gas concentration measurement system was employed in conjunction with a physical gas exchange simulation model to test the theoretical basis of the method under controlled conditions. Simulated oxygen uptake values between 200 and 350 ml x min(-1) were modelled. The Biro-derived measurement of simulated O(2) uptake significantly underestimated the target value (mean difference -88.5 ml x min(-1), or -31.7%). The bias was directly proportional to the simulated O(2) uptake. The Biro method has a systematic bias that cannot be explained by random measurement imprecision.