The theoretical ideal fresh‐gas flow sequence at the start of low‐flow anaesthesia

Abstract

SummaryA spreadsheet model of a circle breathing system and a 70-kg anaesthetised 'standard man' has been used to simulate the first 20 min of low-flow anaesthesia with halothane, enflurane, isoflurane, sevoflurane and desflurane in oxygen. It is shown that, with the fresh-gas flow set initially equal to the total ventilation and the fresh-gas partial pressure to 3 MAC, the end-expired partial pressure can be raised to 1 MAC in 1 min with desflurane and sevoflurane, 1.5 min with isoflurane, 2.5 min with enflurane and 4 min with halothane. Sequences of lower fresh-gas flow and partial pressure settings are given for then maintaining 1 MAC end-expired partial pressure, with a minimum usage of anaesthetic, e.g. 13 ml of liquid desflurane in 20 min (of which only 33% is taken up by the patient) if the minimum acceptable flow is 1 l.min ¹1 , or 8 ml (with 57% in the patient) if the minimum is 250 ml.min ¹1 .

Clinical evaluation of the Mapleson theoretical ideal fresh gas flow sequence at the start of low‐flow anaesthesia with isoflurane, sevoflurane and desflurane*

“…The computer simulation indicated that if the initial vaporiser setting is high (3 MAC) and if the target is an end-expired partial pressure of 1 MAC, the fresh gas flow can be reduced to low levels within 1±2 min for the three least soluble anaesthetic agents, isoflurane, sevoflurane and desflurane [1]. In general, the results of our clinical study support this (Fig.…”

“…2). It would be interesting to explore the reasons for these two observations, perhaps with the help of the Mapleson spreadsheet program [1].…”

“…The Mapleson concept of an initial high fresh gas flow and high vaporiser settings, followed first by reduced high fresh gas flow, as followed in this clinical study, results in P E H an values close to or slightly higher than predicted in the spreadsheet model. Mapleson used a computer spreadsheet model to predict the theoretical ideal fresh gas flow sequence at the start of low-flow anaesthesia [1]. Physiological and pharmacokinetic parameters for an anaesthetised 70 kg`standard man' were used [2±5].…”

“…Mapleson used a computer spreadsheet model to predict the theoretical ideal fresh gas flow sequence at the start of low-flow anaesthesia [1]. Physiological and pharmacokinetic parameters for an anaesthetised 70 kg`standard man' were used [2±5].…”

The volatile consumption: uptake ratio as a measure of the efficiency of semi‐closed circle breathing system usage

“…However, it is commonplace to observe the employment of higher than necessary fresh gas flows via semi-closed circle breathing systems despite the availability of validated, pragmatic, optimal initial flow and vaporiser setting regimens [1,2]. This results in widespread cost inefficiencies, excessive environmental pollution and poor breathing circuit conditioning [3].…”