Assessment of cardiorespiratory function using oscillating inert gas forcing signals

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.