Measurement of cardio-respiratory function using single frequency inspiratory gas concentration forcing signals

The possibility of using inspiratory, oscillating inert gas concentration forcing signals to measure the average lung ventilation-perfusion ratio was proposed by Zwart and colleagues (1976, 1978). These authors used halothane as the forcing gas, at sub-anaesthetic concentra­tions around 0.02% v/v, but this introduced severe measurement problems at these very low concentrations. Recently Hahn et al (1993) have described a similar but improved respiratory model which uses nitrous oxide (N2O) as the forcing gas. The advantages of nitrous oxide over halothane are that it is free from biological toxicity; it is easy to measure N2O with conventional gas analysis apparatus (especially at the concentrations used by Hahn et al (1993), around 2–10% v/v); and because nitrous oxide can be measured continuously in blood, the mathematical model can be extended to include shunt blood flow. In this technique, a three-compartment continuous ventilation mathematical model describes the imposition of a forced oscillating inspired inert gas concentration, Pi(t), which oscillates sinusoidally over a range of frequencies (0.6 to 6 min-1), and the subsequent measurement of the attenuation of the end-expired inert gas concentration, Pé(t), with changing forcing frequency (Figure 1). The plot of Pé/Piagainst forcing frequency (the Bode plot) is then used to derive alveolar volume, Va, and pulmonary blood flow, Qp. Dead space, Vds, is derived from the sinusoidal mixed expired indicator gas signal. We have further modified this technique by using a single forcing monosinusoid (forcing frequency 0.5 min-1) of two inert gases to measure cardio-respiratory lung function. This binary inert gas oscillation technique uses Argon, which is relatively insoluble in blood, to derive alveolar volume and dead space; and nitrous oxide, which is soluble in blood, to measure pulmonary blood flow.