British Journal of Anaesthesia, Vol 76, Issue 2 284-291, Copyright © 1996 by The Board of Management and Trustees of the British Journal of Anaesthesia
A. D. Farmery and P. G. Roe
We have developed a model to describe the rate of oxyhaemoglobin
desaturation during apnoea. This model takes into account the non-
steady-state kinetics which pertain to this situation. We first derived a
mathematical expression for instantaneous oxygen flux rate from the
alveolar compartment. We then derived an expression to describe the effect
of shunt on this flux. The effect of circulation time on real- time
arterial mixed venous oxygen content difference and oxygen flux in the lung
was determined graphically. We finally described a manoeuvre to accommodate
the effect of the Bohr shift which is related to the increase in FACO2
during apnoea. We present plots of arterial oxyhaemoglobin saturation
(SaO2) vs duration of apnoea to illustrate the individual effects of the
initial fractional concentration of oxygen in the alveolus (FAO2initial),
alveolar volume (VA), shunt fraction (QS/QT), oxygen consumption rate
(VO2), total blood volume (QT) and haemoglobin concentration (Hb). The
model is illustrated by examples of paediatric, morbidly obese and
post-operative scenarios. The postoperative scenario is particularly
notable for the effect of a combination of small changes in individual
variables leading to a large overall effect on the rate of oxyhaemoglobin
desaturation.
LABORATORY INVESTIGATIONS
A model to describe the rate of oxyhaemoglobin desaturation during apnoea
Department of Anaesthesia, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ
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