1 Chicago, USA 2 Düsseldorf, Germany
EditorI read with interest the article by Schindler and colleagues1 concerning the accuracy of feedback controlled oxygen delivery into a closed anaesthesia circuit for measurement of oxygen consumption (V·O2-PF). The accuracy of the V·O2-PF was compared with V·O2 measurement by the Fick method (V·O2-Fick) at different FIO2. They found that V·O2-PF significantly exceeded the Fick-derived V·O2 in patients by mean 52 (SD 40) ml min1. The lack of agreement between the two methods was attributed to two factors: the methodological imprecision of the Fick-derived V·O2, and an error caused by a small C(a-v)O2 and high Q·t. In fact, as I see it, the authors should recognize that V·O2-PF measurement other than at an FIO2 of 0.21 should not be considered as oxygen consumption: V·O2-PF measurement at an FIO2>0.21 should be considered as oxygen uptake. Higher FIO2 will result in higher oxygen uptake, because of the existence of the alveolar membrane.2 For the comparison of these two measurement methods, V·O2-PF and V·O2-Fick, in relation to oxygen consumption, the FIO2 should be limited to 0.21.
C.-Y. Lin
Chicago, USA
EditorWe are pleased that our article1 about measurement of oxygen consumption (V·O2) in an automated closed anaesthesia circuit created interest. In patients ventilated with inspired oxygen concentrations (FIO2) of 3050%, V·O2 measured with an automated closed anaesthesia circuit (V·O2-PF) significantly exceeded the values obtained simultaneously using the Fick principle (V·O2-FickPAT). Dr Lin is of the opinion that this bias results from the alveolar membrane causing an FIO2-dependent increase in oxygen uptake while oxygen consumption remains constant. Lins experiments with nitrous oxide and halothane2 showed that the inspiratory-to-expiratory concentration difference of both anaesthetics (which is a semi-quantitative measure of anaesthetic uptake) changed when the inspiratory concentration was altered in step-wise fashion. Each of several inspiratory concentrations of nitrous oxide and halothane was kept constant for 5 and 10 min, respectively. These results led to the opinion thatprovided both cardiac output and ventilation remained constantthe partial pressure gradient across the alveolar membrane is the sole factor controlling the rate of uptake. We agree with this interpretation of Lins experiments, which is one of the major points of Ficks law of diffusion. We also agree that V·O2 is likely to remain constant regardless of FIO2.
However, we disagree that FIO2 alone can explain the bias found in the patient part of our study. According to Ficks law of diffusion, the concentration gradient across a membrane determines the rate of uptake, but not the concentration on one side of it. If the FIO2 itself, rather than the gradient, determined oxygen uptake across the alveolar membrane, a bias should also have been found in the results we obtained in dogs ventilated with FIO2>0.21.1 In fact, we found agreement between the Fick-derived V·O2 and V·O2-PF in these animals unless the FIO2 exceeded 0.85.1
An increase in FIO2 temporarily increases the gradient across the alveolar membrane until a new steady state is reachedi.e. nitrogen is replaced by oxygen until blood and tissues are equilibrated with the new oxygen concentration. The amount of nitrogen to be replaced increases with the magnitude of the FIO2 step. When FIO2 is increased from 0.21 to 1.0, the exhaled nitrogen concentration is <1% after 26 min,3 indicating that blood and tissues rapidly equilibrate with the higher FIO2 levels. In our study on patients, FIO2 was allowed to equilibrate for at least 10 min before measurements started, so that any influence of nitrogen washout on the bias is unlikely. We did comment on this point1 when discussing the FIO2-dependent bias in the animal part of the study. Furthermore, although not explicitly stated, all patients were ventilated with oxygen 100% before tracheal intubation as part of our clinical routine. In the presence of disequilibrium between alveolar and bloodtissue oxygen pressure, V·O2-FickPAT would have exceeded V·O2-PF during anaesthesia alone, and there should have been no bias during the anaesthesia plus major surgery, which was measured after >1 h of equilibrium. We did not, however, find any systematic difference between these two conditions.
Lins critique is based on pharmacokinetic data referring to nitrous oxide and halothane, while we studied oxygen. Data referring to one inhalational agent cannot be used to predict the uptake of another, because each agent has its unique pharmacokinetic profile affecting the rate of diffusion across the alveolar membrane, the rate of equilibration between inspiratory and alveolar concentration, the uptake into the blood, and the distribution to the tissues. This is especially true for oxygen, which is chemically bound to haemoglobin. Its transport capacity exceeds that of all inhalational anaesthetics by several magnitudes, as these others are only bound physically to plasma.
In summary, we agree with Dr Lin (and Ficks law of diffusion) that the partial pressure gradient across the alveolar membrane mainly influences the diffusion rate, and that increasing the inspired concentration of a gas to be taken up by the body temporarily increases this gradientuntil a new steady state is reached. We disagree that this effect explains the bias observed in the patient part of our study, because V·O2-FickPAT and V·O2-PF were measured simultaneously when steady state conditions could be reasonably assumed.
A. W. Schindler
T. W. L. Scheeren
O. Picker
M. Doehn
J. Tarnow
Düsseldorf, Germany
References
1 Schindler AW, Scheeren TWL, Picker O, Doehn M, Tarnow J. Accuracy of feedback-controlled oxygen delivery into a closed anaesthesia circuit for measurement of oxygen consumption. Br J Anaesth 2003; 90: 28190
2 Lin CY. Can we practice safe, simple closed-circuit anaesthesia without extensive monitoring or calculus? In: Droh R, Spintge R, eds. Closed-Circuit System and Other Innovations in Anaesthesia. Berlin, New York, London, Paris, Tokyo: Springer-Verlag, 1986; 7084
3 Carmichael FJ, Cruise CJ, Crago RR, Paluck S. Preoxygenation: a study of denitrogenation. Anesth Analg 1989; 68: 4069[ISI][Medline]