Oxygen respiratory gas analysis by sinewave measurement: a theoretical model. Hahn, Clive E W. Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Woodstock Road, Oxford, OX2 6HE, United Kingdom
APStracts 3:0141A, 1996.
A sinusoidal forcing function inert gas-exchange model (Hahn et al. J. Appl. Physiol. 75: 1863-1876, 1993) is modified by replacing the inspired inert gas with oxygen, which then behaves mathematically in the gas phase as if it were an inert gas. A simple perturbation theory is developed which relates the ratios of the amplitudes of the inspired, end-expired and mixed-expired oxygen sinewave oscillations to the airways dead space volume and lung alveolar volume. These relationships are independent of oxygen consumption, the gas-exchange ratio, and the mean inspired (FIO2) and expired oxygen partial pressures. The model also predicts that blood flow shunt fraction (S/T) is directly related to the oxygen sinewave amplitude perturbations transmitted to end-expired air, arterial and mixed -venous blood through two simple equations. When the mean FIO2 is sufficiently high for arterial haemoglobin to be fully saturated, oxygen behaves mathematically in the blood like a low solubility inert gas, and the amplitudes of the arterial and end-expired sinewave perturbations are directly related to S/T. This relationship is independent of the mean arterial and mixed-venous oxygen partial pressures, and is also free from mixed-venous perturbation effects at high forcing frequencies. When arterial blood is not fully saturated, the theory predicts that S/T is directly related to the ratio of the amplitudes of the induced saturation sinusoids in arterial and mixed venous blood. The model therefore predicts that (a) on-line calculation of airways dead space and end-expired lung volume can be made by the addition of an oxygen sinewave perturbation component to the mean FIO2; (b) S/T can be measured from the resultant oxygen perturbation sinewave amplitudes in the expired gas and in arterial and mixed-venous blood, and is independent of the mean blood oxygen partial pressure and oxyhaemoglobin saturation values. These calculations can be updated at the sinewave forcing period, typically 2-4 min. 

Received 6 January 1995; accepted in final form 21 February 1996.
APS Manuscript Number A25-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96