Although distribution of ventilation may be assessed by both N2 washout (NW) and frequency dependence of lung compliance [C(L)] the mathematical link between these tests has not been established. In a mechanical lung model consisting of two compartments of equal volume but variable time constants, alveolar dilution factors for N2 (w) were measured at different cycling frequencies. Using equations relating time constants of the model to expected compartmental NW, a good experimental fit was obtained. As two compartments of NW developed, C(L) decreased. At fast frequencies, a single compartment was found as the tidal volume of the slow compartment became smaller than its dead space; C(L) fell to 50% of its static value. Pendelluft and common dead space had only minimal effects on N2 clearance delay in the model. In 5 dogs with graded obstruction in a main bronchus and in 5 with large beads in lobar bronchi, increasing respiratory rate by transvenous phrenic nerve stimulation produced both fall in C(L) and increased N2 clearance delay. In 9 of 10 asymptomatic young smokers, NW at 6 frequencies from 10 to 110 breaths per minute revealed two compartments at one or more frequencies whereas in 10 non smokers the NW was a single exponential at all frequencies. Although NW in smokers did not completely follow the prediction from the model, frequency dependence of NW still appears to be a good non invasive method to detect small differences in time constants within the lung.
|State||Published - Dec 1 1973|
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