Alterations in ventilatory pattern and ratio of dead-space to tidal volume

The effects of alterations in ventilatory pattern on the simultaneously measured physiologic and anatomic dead-spaces (VDphys and VDan, respectively) and the dead-space to tidal volume ratio (VD-VT) were studied in 17 healthy normal subjects (13 men, four women, ages 21 to 36 years). There were no significant changes in VDan with increases in respiratory frequency (f) or tidal volume (VT). The VDphys increased (mean change +0.153 L, p less than 0.05) with increase in VT (mean increase +0.84 L, p less than 0.01), but did not alter significantly with a twofold increase in f, at control VT. Increase in VT significantly reduced VD/VT (mean change -10.4 percent, p less than 0.05), but increase in f, at control VT, did not significantly alter VD/VT. These results indicate that in normal subjects, increase in VT alters ventilation/perfusion matching in the lungs, whereas an increase in f, at constant VT, has no effect on ventilation/perfusion matching. Increases in VD/VT cannot, therefore, be ascribed to alterations in ventilatory pattern where either VT, or f, or both are increased.