Distribution of ventilation-perfusion ratios in dogs with normal and abnormal lungs

Abstract

We have recently described a new method for measuring distributions of ventilation-perfusion ratios (VA/Q) based on inert gas elimination. Here we report the initial application of the method in normal dogs and in dogs with pulmonary embolism, pulmonary edema, and pneumonia. Characteristic distributions appropriate to the known effects of each lesion were observed. Comparison with traditional indices of gas exchange revealed that the arterial PO2 calculated from the distributions agreed well with measured values, as did the shunts indicated by the method and by the arterial PO2 while breathing 100 per cent 02. Also the Bohr dead space closely matched the dispersion of ventilation in realtion to VA/Q. Assumptions made in the method were critically evaluated and appear justified. These include the existence of a steady state of gas exchange, an alveolar-end-capillary diffusion equilibration, and the fact that all of the observered VA/Q inequality occurs between gas exchange units in parallel. However, theoretical analysis suggests that the method can detect failure of diffusion equilbration across the blood-gas barrier should it exist. These results suggest that the method is well-suited to clinical investigation of patients with pulmonary disease.

Gas exchange after pulmonary thromboemoblization in dogs.

“…This is consistent with the observation that the highest mean pulmonary artery pressure was 25.8 mm Hg (mean for all dogs). Other studies 12 ' 15 have shown that, at considerably higher pulmonary artery pressures associated with more severe embolism, pulmonary edema may develop. Under these conditions, additional factors may contribute to hypoxemia, such as perfusion of poorly ventilated or unventilated alveoli and failure of diffusion equilibration.…”

“…The preembolization distributions for both dogs in Figures 1 and 2 are unimodal and similar to those already described for supine, anesthetized, and mechanically ventilated dogs. 15 There is no ventilation or blood flow to very high or very low VA/Q ratios. The shunts measured before embolization, 2.6% and 0.0% for the dogs of Figures 1 and 2, respectively, remained unchanged after embolization.…”

“…This was done by computer as previously described. 15 ' 20 For the 17 dogs of groups A and B, 180 pairs of measured blood gases with the corresponding retentions and excretions were available, and Figure 4 compares the measured arterial Po 2 and the predicted arterial Po 2 . Note that the line of best fit and the line of identity lie close to each other.…”

“…It is possible, in the few cases in Figure 4 where the arterial Po 2 was less than 50 mm Hg and the expected values greater than measured, that a small component of the hypoxemia was not due to ventilation-perfusion inequality but to diffusion impairment. 15 …”

Simulating hypoxia and modelling the airway

“…A well-known example of this kind of inverse modelling in our discipline is the Multiple Inert Gas Elimination Technique (MIGET) to estimate ventilation ⁄ perfusion (V ⁄ Q) distributions in the lung. Here, measurements of arterial and venous partial pressures of various infused dissolved inert gases are made, and a computer determines which possible distribution or permutation of V ⁄ Q ratios (from many thousands of possibilities) is most consistent with the actual measurements [6]. Another example is pulse contour analysis for the determination of cardiac output [7].…”