Abstract
Brody’s formula for estimating oxygen consumption is rarely used in low flow anaesthesia and most anaesthetists assume that 250 ml/min is sufficient for every adult. However there is a simple method of monitoring fairly accurately the oxygen consumption in real time, using the current standard equipment and monitoring in anaesthesia. The FiO2 and End Tidal oxygen concentrations are measured almost routinely with the modern anaesthetic machines. At sea level, these figures are almost exactly the same as percentages. Therefore, if the difference between FiO2 and ETO2 is calculated as a proportion of the minute ventilation, this gives the oxygen consumption ((FiO2 ETO2) 9 MV). I have found that the actual amount of oxygen taken up varies considerably between patients of similar weights and at different times during anaesthesia. Sometimes up to 450 ml/min. This simple calculation adds to the safety of low flow anaesthesia.
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