Basal oxygen flow

Abstract

To minimise the risk of delivering a hypoxic mixture from the modern anaesthesia machine, two safeguards are perceived to be in operation: first, the ‘hypoxic guard’ and, secondly, a basal oxygen flow. The former is achieved by an interconnection between the oxygen and nitrous oxide delivery system to provide a fixed minimum ratio, the mechanism varying according to the manufacturer. The latter as ‘… an oxygen flow of 200–300 ml. min−1 maintained as an attempt to meet the requirements for minimal oxygen consumption of the patient’ with ‘minimum oxygen flow incorporated into safe anaesthesia machines’[1]. This setting was promoted as the minimum for adults first by Ohmeda then Dräger and became, effectively, a standard though none was formally created [2], and we incorporated it into our departmental comprehensive machine check [3]. The latest edition of Ward's Anaesthetic Equipment [4] considering Anti-hypoxia Devices includes details of minimum standing flow of 175–250 ml.min−1 for Ohmeda and basal flow of 200–300 ml.min−1 for M & IE. We have recently acquired new Penlon anaesthesia machines with ‘basal flow’ set at 100–150 ml.min−1, and I have discussed this issue with them who advised upon the significant engineering limitations which I understand have also led Ohmeda to re-examine the concept of providing only 60 ml.min−1 for the Aestiva 5. The purpose of ‘basal flow’ never having been agreed or standardised, does it have a value any more? The Association of Anaesthetists' latest monitoring guidelines mandate the use of an oxygen analyser to continuously monitor the composition of the gas mixture delivered to the patient [5] and the Royal College of Anaesthetists has issued a Safety Notice on Prevention of Hypoxic gas mixtures [6] which also mandates the same. Perhaps now is the time to formally abandon the concept of ‘basal flow’ and avoid thereby the confusion created by manufacturers' varied interpretations?