Our preliminary modelling and experimental work reveals that where ventilatory inhomogeneity exists, the determined variables appear to be dependent on the period. The degree

of period dependency is likely to provide a robust index of ventilatory heterogeneity, and this will be developed in future work. Oxygen is used as an indicator gas in these studies. It is assumed that oxygen behaves much like an insoluble inert gas with respect to the diminution of the amplitude of its sinusoidal inspired concentration Selleckchem IPI 145 within the alveolar compartment. This is because in this analysis it is only the oscillatory components of the indicator concentration signal which is required for the analysis. The static or “DC” component of the signal can then be neglected. This was described in detail by Hahn (1996). The effect is independent of arterial oxyhaemoglobin saturation and concentration and there is no recirculation of the oscillatory signal in the venous blood. Fig. 3(a)–(c) shows the estimates for V  A, Q˙P, and V  D obtained using the continuous ventilation and the tidal ventilation

model at different forcing periods. learn more It can be seen that the estimates of Q˙P obtained using both the continuous ventilation model and the tidal ventilation model are similar for all forcing sinusoidal periods T = 2, 3, 4, 5 min. Similar behaviour can be observed in the estimates of VA at T = 2, 3, 4 min where the estimates of VA are close to the expected value, but VA estimates differ from expected values when T = 5 min. This may be due either to potential artifact from “venous recirculation”, or to the fact that the recovered values become frequency dependent if real data from inhomogeneously ventilated lungs are analysed in a single compartment model. The consistency of the results using both the continuous ventilation model and the tidal ventilation model for 2 ≤ T ≤ 4 suggests that this range is suitable for the forcing sinusoid. For both the continuous ventilation not model and the tidal ventilation model, VD is calculated by the proposed regression method using both CO2 and NO2 as described

in Section  4. The results of VD estimation are the same for both models, and are close to the expected value (0.25 L), indicating that the proposed improved Bohr equation method produces stable estimation of VD. However, we note that the estimated values of Q˙P appear smaller that the expected value of Q˙P of the volunteer (4.5 L/min). One possible reason is that the effect of “venous recirculation” of the N2O still exists to some degree, whereas both the continuous ventilation model and the tidal ventilation model assume that it is negligible. Another possible reason is that the equilibrium between the arterial and venous blood had not yet been established during the data collection, although nitrous oxide has low blood and tissue solubility.