Frequency dependence of dead space during high-frequency ventilation in dogs

A rotary value ventilator, designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, mongrel dogs at ventilatory frequencies up to 20 Hz. A total of eleven studies were carried out on four animals. Determinations of the minute ventilation required to maintain a normal steady-state P CO 2 , together with the Fe CO 2 and arterial P CO 2 , P CO 2 and pH, were made at a number of frequencies. The results so obtained were used to calculate the Bohr (physiological) dead space. Dead space remained approximately constant at close to the value measured during spontaneous ventilation for each individual dog in the range of ventilatory frequencies from below 1 to around 5 Hz, but decreased with increasing frequency above 5 Hz. The calculated physiological dead space at 15Hz was about half of the value at normal respiratory frequencies. These findings in dogs, obtained using a ventilator system which allows very accurate determinations of expired gas volume and content, are consistent with published results for this species from other laboratories. They contrast with our earlier findings in rabbits that dead space remains constant with increasing frequency, suggesting that the effects of high-frequency ventilation on CO 2 transport are species-dependent.