A reconciliation of continuous and tidal ventilation gas exchange models

Continuous-ventilation mathematical gas exchange models are widely used since their analytical equations are amenable to physiological interpretation. They describe qualitatively the respiratory system's response to changing physiological conditions, but do not calculate accurate values for respiratory parameters when experimental tidal ventilation expired gas data are inserted into their analytical expressions. A simple mathematical expression is presented to reconcile continuous and tidal ventilation gas exchange models. Tidal ventilation experimental data can then be inserted into conventional continuous ventilation equations to produce more accurate measures of lung volume. This hypothesis is tested with controlled experimental tidal ventilation tracer gas data obtained from both wash-out and forced inspired sinusoid experiments, using a mechanical lung model with known volume; tidal volume, VT; and series 'airway' dead space VD. We show that the subtraction of 1/2 (VT + VD) from the lung volume calculated from the continuous ventilation theory can produce lung volume measurements which agree with the true lung volume to within +/-5%, for physiological lung volume values, for both wash-out and forced sinusoid techniques.