Anesth Analg 2004;99:306
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000124863.77921.1B
LETTERS TO THE EDITOR
Oxygen Consumption: Validation of the Closed-Circuit PhysioFlex® Anaesthesia Machine
Jan Hofland, MD PhD, and
Robert Tenbrinck, MD PhD
Department of Anesthesiology, Academic Medical Hospital, University of Amsterdam, Amsterdam, the Netherlands
To the Editor:
With interest we read the article by González-Arévalo et al. (1), about oxygen consumption ( O2) validation of the PhysioFlex® closed-circuit anesthesia machine.
Surprisingly, the authors selected ICU patients for their study although the PhysioFlex® anesthesia machine is designed to measure O2 during anesthesia. Thus validation of the apparatus needs to be done in patients during anesthesia for surgery and not in ICU patients. Furthermore, the choice made by the authors to use volume-controlled mechanical ventilation (MV) without giving neuromuscular blockade could have induced patient-ventilator mismatches and seems an uncommon clinical practice. The PhysioFlex® is only equipped with controlled MV modes (IPPV and PCV); it has no support MV modes available that are commonly used today in ICU practice.
To avoid these problems we selected patients undergoing general anesthesia for two different surgical interventions and compared the O2 measurements with simultaneously measured Fick-derived data (2,3). Both investigated surgical procedures induced a large variety in hemodynamics, consequently varying the range of our measurements [O2 PhysioFlex®: 68–359 (2), and 22–400 mL/min (3)]. This variation was greater than the range studied by González-Arévalo et al., which may illustrate our above-mentioned concern.
References
- González-Arévalo A, Gómez-Arnau JI, dela Cruz J, et al. Oxygen consumption measurement: agreement between the closed-circuit PhysioFlex anesthesia machine and the Deltatrac II indirect calorimeter. Anesth Analg 2003; 97: 1680–5.[Abstract/Free Full Text]
- Hofland J, Tenbrinck R, van Eijck CHJ, et al. Comparison of closed circuit and Fick-derived oxygen consumption in patients undergoing simultaneous aortocaval occlusion. Anaesthesia 2003; 58: 377–84.[Medline]
- Hofland J, Tenbrinck R, Erdmann W. Comparison of closed-circuit and Fick-derived oxygen consumption during anaesthesia for liver transplantation in patients. Adv Exp Med Biol 2003; 540: 305–11.[Medline]
Response
Antonio González-Arévalo, MD PhD,
Juan I. Gómez-Arnau, MD PhD, and
Santiago García-del-Valle, MD
Department of Anesthesia and Critical Care, Fundación Hospital Alcorcón, Madrid, Spain
In Response:
We appreciate the comments on our study (1) offered by Hofland and Tenbrinck. The purpose of our work was to compare the PhysioFlex anesthesia machine O2 measurement with the "gold standard" O2 measurement in intubated and mechanically ventilated patients (2). For this reason we chose the Deltatrac II indirect calorimeter, and we searched for the conditions in which this instrument have demonstrated to measure the O2 accurately (3). We decided to make the comparison in critical care patients, rather than in anesthetized patients, because of the difficulty in achieving periods of stable O2 during surgery, and because the Deltatrac measures O2 with nonrebreathing respirators, so it does not work with the circle breathing systems used in anesthesia. We rejected the comparison with O2 estimation using the Fick equation due to its drawbacks: 1) the accuracy of the estimation depends on many measurements (O2 content in arterial and venous blood) and on the technique used to determine cardiac output, so the possibilities of error are not negligible; 2) it does not take into account the O2 consumed by the lungs.
It is not a usual practice in our hospital to administer neuromuscular blocking agents to mechanically ventilated patients, so we decided, in order to maintain the treatment routine, not to give such drugs to the patients in our study. Moreover, patients in our study received sedative and analgesic drugs (1) (Table 1 in our article) at clinically suitable doses to tolerate mechanical ventilation. The 10-minute periods of stable recording, required to make the measurements, were achieved in all patients. Thus, we think that performing the study without the use of neuromuscular blockade does not preclude the accuracy of our measurements.
Our opinion is that O2 measurement has increasing significance during anesthesia. The future role of closed circuit anesthesia machines within this scope of monitoring, which will surely be important, should be established by further studies.
References
- Gonzalez-Arevalo A, Gomez-Arnau JI, delaCruz J, et al. Oxygen consumption measurement: agreement between the closed-circuit PhysioFlex anesthesia machine and the Deltatrac II indirect calorimeter. Anesth Analg 2003; 97: 1680–5.
- Walsh TS. Recent advances in gas exchange measurement in intensive care patients. Br J Anaesth 2003; 91: 120–31.[Free Full Text]
- Takala J, Keinänen O, Väisänen P, Kari A. Measurement of gas exchange in intensive care: laboratory and clinical validation of a new device. Crit Care Med 1989; 17: 1041–7.[Web of Science][Medline]
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