Linear approximation of Brody's equation to predict oxygen consumption.

Abstract

We recently described simple rules for syringe delivery of volatile anesthetics into a closed drcuit [2], based on the square-root-of-time model described by Lowe and Ernst [3]. Our technique uses a series of constant-rate infusions. During each constant-rate stage, the same total amount of volatile anesthetic is delivered to the circuit as that predicted by Lowe's equations. We noticed empirically that there was a near-linear relationship between the rate of delivery required in the first 5 min of such an infusion and body weight. Assuming a 6-L circuit, the rate of delivery of liquid isoflurane for the first 5 min can be approximated by: Rate = 14.38 + 0.43w ml/hr ~ where w is the weight in kilograms. The table shows the delivery rates predicted from Low& equations and from our linear approximation. Also included in the table are the relative error of this approximation and the relative error of the linear approximation of Arndt and colleagues to the Brody equation. Both sets of errors are well within acceptable limits for clinical practice and suggest that such linear approximations may make useful alternatives to more complex equations, tables [4], or nomograms [5]. As outlined by Arndt and colleagues, Lowe's method for closed-circuit anesthesia includes a term for cardiac output using the Brody equation; therefore, the corre