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

Peri-operative oxygen consumption revisited: An observational study in elderly patients undergoing major abdominal surgery

BACKGROUND

Monitoring oxygen consumption (VO2) is neither recommended nor included in peri-operative haemodynamic algorithms aiming at optimising oxygen delivery (DO2) in major abdominal surgery. Estimates of peri-operative VO2 changes are uncertain in earlier publications and have limited generalisability in the current high-risk surgical population. In a prospective non-interventional observational study in elderly patients undergoing major abdominal procedures, we investigated the change of VO2 after induction of anaesthesia and secondarily, the further changes during and after surgery in relation to DO2 and estimated oxygen extraction ratio (O2ER) by routine monitoring.

METHODS

VO2 was determined by indirect calorimetry (QuarkRMR) in 20 patients more than 65 years (ASA II to IV), scheduled for elective open upper abdominal surgery with combined epidural and general anaesthesia. Data were collected during 20-minute periods pre-operatively and after anaesthesia induction, with subsequent measurements during surgery and postoperatively. Simultaneously, DO2 was monitored using LiDCOplus. The O2ER was estimated from arterial-central venous oxygen content calculation. Mixed models were used to analyse the peri-operative changes.

RESULTS

VO2 decreased after induction of anaesthesia by a mean of 34% (95% CI, 28 to 39). After 2 h of surgery, VO2 was reduced by 24% (95% CI, 20 to 27) compared with the awake baseline measurements. Pre-operative mean DO2 was 440 (95% CI, 396 to 483) ml min m and decreased by a mean of 37% (95% CI, 30 to 43) during anaesthesia. The estimated O2ER did not change intra-operatively 0.24 (95% CI, 0.21 to 0.26) but increased postoperatively to 0.31 (95% CI, 0.27 to 0.36). The changes of VO2 were parallel with changes of DO2 and O2ER in the intra-operative period.

CONCLUSION

General anaesthesia reduced VO2 by approximately a third in elderly patients undergoing major abdominal surgery. Parallel changes of intra-operative VO2 and delivery were demonstrated while oxygen extraction was low. The relevance of these changes needs further assessment in relation to outcomes and haemodynamic interventions.

TRIAL REGISTRATION

Clinicaltrials.gov NCT03355118.

The relation between oxygen consumption and the equilibrated inspired oxygen fraction in an anesthetic circle breathing system: a mathematic formulation & laboratory simulations

Measuring a patients' oxygen consumption (VO2) is valuable in critical care and during anesthesia. Up to now, there has been no satisfactory equation describing the relation between the VO2, the fresh gas, and F(I)O(2) in a semi-closed circle breathing system. By adopting a "volume-weighted average concentration" approach and stepwise calculations, we have proposed an equation. We constructed a model with known simulated O(2) consumption ((SIM)VO2) to test our equation and two other previous methods (Biro's and Azami's). After 32 different laboratory scenarios, the %-error of the calculated VO2 ((CAL)VO2) from our method is -4.0 +/- 2.9%, which is significantly better than those from Azami's method (-8.8 +/- 6.2%, p < 0.01) and from Biro's method (-27.4 +/- 5.1%, p < 0.01). We also produce a Bland-Altman analysis of our (CAL)VO2 and (SIM)VO2. The 95% limits of agreement are -18.6-3.3 mL/min with a mean bias of -7.7 mL/min, which shows a good agreement. Our equation also explains the difference between F(I)O(2) and the oxygen concentration of the fresh gas in a semi-closed circle breathing system.

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.

Continuous indirect calorimetry--a laboratory simulation of a new method for non-invasive metabolic monitoring of patients under anaesthesia or in critical care

A method was tested which permits continuous real time monitoring of O(2) uptake in patients attached to a breathing system. 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. Testing of this approach was conducted using a lung gas exchange simulator, in order to determine its accuracy and precision under controlled conditions, when compared to a range of simulated O(2) uptake values. The overall mean bias (standard error) was -1.3 mL min(-1) (0.3) and the standard deviation was 6.5. The performance of the method was found to be consistent across a wide range of fresh gas flow rates and O(2) concentrations from 30 to 80%. The method warrants in vivo testing under clinical conditions.