Constancy of physiological dead space during high-frequency ventilation.

A ventilator, specially designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, New Zealand White rabbits at ventilatory frequencies up to 22 Hz. A total of eleven studies were carried out on seven animals. Determinations of the minute ventilation required to maintain a normal steady-state PCO2, together with the FECO2 and arterial PO2, PCO2, and pH, were made at a number of frequencies. The results so obtained were used to calculate the physiological dead space. It was found that this value remained remarkably constant for each individual rabbit over a range of ventilatory frequencies from 1 to 22 Hz. Our findings in rabbits are in contrast to those of other workers (in dogs) that adequate gas exchange can be maintained at high ventilatory frequencies with the use of tidal volumes much smaller than the volume of anatomical dead space.