Assessment of cardiorespiratory function using oscillating inert gas forcing signals

Abstract

A theoretical model (Hahn et al. J. Appl. Physiol. 75: 1863-1876, 1993) predicts that the amplitudes of the argon and nitrous oxide inspired, end-expired, and mixed expired sinusoids at forcing periods in the range of 2-3 min (frequency 0.3-0.5 min-1) can be used directly to measure airway dead space, lung alveolar volume, and pulmonary blood flow. We tested the ability of this procedure to measure these parameters continuously by feeding monosinusoidal argon and nitrous oxide forcing signals (6 +/- 4% vol/vol) into the inspired airstream of nine anesthetized ventilated dogs. Close agreement was found between single-breath and sinusoid airway dead space measurements (mean difference 15 +/- 6%, 95% confidence limit), N2 washout and sinusoid alveolar volume (mean difference 4 +/- 6%, 95% confidence limit), and thermal dilution and sinusoid pulmonary blood flow (mean difference 12 +/- 11%, 95% confidence limit). The application of 1 kPa positive end-expiratory pressure increased airway dead space by 12% and alveolar volume from 0.8 to 1.1 liters but did not alter pulmonary blood flow, as measured by both the sinusoid and comparator techniques. Our findings show that the noninvasive sinusoid technique can be used to measure cardiorespiratory lung function and allows changes in function to be resolved in 2 min.

Modelling Inert Gas Exchange in Tissue and Mixed–Venous Blood Return to the Lungs

“…The mixed-venous blood partial pressure of an inert gas is calculated using the partial pressure of the gas leaving each body compartment, and so the use of the ODE mathematical model in the past could have led to errors in the cardiorespiratory variables estimated using experimental techniques that use this ODE (Barton et al, 1998;Gavaghan & Hahn, 1995Hahn et al, 1993;Williams et al, 1994Williams et al, , 1997Williams et al, , 1998Zwart et al, 1975). The shape of the body compartment also a!ects the solution of the PDEs.…”

“…as used with the sine-wave technique for inert gases (Barton et al, 1988;Gavaghan & Hahn, 1995Hahn et al, 1993;Williams et al, 1994Williams et al, , 1998Zwart et al, 1975). In this case, we are interested in the venous blood partial pressure, P T (t), that #ows back to the lungs.…”

“…A further need for an accurate description is to calculate the content of a given gas in the mixedvenous blood #owing from the body compartments back to the lungs. This is used in the sine-wave technique, an experimental technique for investigating various cardio-respiratory parameters (Barton et al, 1988;Gavaghan & Hahn, 1995Hahn et al, 1993;Williams et al, 1994Williams et al, , 1997Williams et al, , 1998Zwart et al, 1975).…”

“…The sine-wave technique (Barton et al, 1988;Gavaghan & Hahn, 1995Hahn et al, 1993;Williams et al, 1994Williams et al, , 1997Williams et al, , 1998Zwart et al, 1975) is an experimental technique designed to investigate various cardiorespiratory parameters. When being investigated with the sine-wave technique, a subject is given an inspired gas mixture containing one or more marker gases whose partial pressure varies with a sinusoidal perturbation with time about a "xed mean concentration, with a period typically between 1 and 5 min.…”

“…All of this work then uses the ODE describing gas partial pressure in the body compartment to calculate the amplitude of the resulting perturbation to the mixed-venous blood signal. Most authors, (Barton et al, 1988;Gavaghan & Hahn, 1995Hahn et al, 1993;Williams et al, 1994Williams et al, , 1997Zwart et al, 1975) on the basis of the attenuation predicted by the ODE, assume that the perturbation in the concentration of the marker gases in mixed-venous blood is negligible, and so assume that the partial pressure of the marker gas in mixed-venous blood is constant. Another work (Williams et al, 1998) has used the ODE to predict the amplitude of the perturbation to the partial pressure of mixed-venous blood under conditions where it is not negligible.…”