Mechanical Factors in Distribution of Pulmonary Ventilation

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Abstract
The assumption is made that the lungs can be represented mechanically by a population of pathways each consisting of a compliance, C, and a resistance, R, connected in series. A theoretical analysis of the behavior of such a system is developed using methods which are available for analagous electrical circuits. This analysis indicates that unless the separate pathways have similar time constants (RC product) they will, when exposed to the same driving pressure, be unevenly ventilated both temporally and spatially. In a system composed of pathways having unequal time constants, the distribution of ventilation and the effective compliance and resistance will vary with frequency of breathing. This behavior was demonstrated by a mechanical model consisting of two parallel pathways. In measurements on normal human subjects the effective compliance of the lungs was shown to be invariant with frequency of breathing. In individuals with induced broncho-spasm and in patients with emphysema or asthma effective compliance decreased with increasing frequency. In the light of the theory these results suggest that in normal lungs the time constants of the separate pathways are similar and that ventilation is evenly distributed at all frequencies, but that in the abnormal cases studied inequalities of time constants exist and that distribution of ventilation alters with changes in breathing frequency.