Frequency dependence of dead space during high-frequency ventilation in rhesus monkeys

A rotary valve ventilator, designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, rhesus monkey at ventilatory frequencies up to 20 Hz. A total of five studies were carried out on three 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 Bohr (physiological) dead space. Dead space remained approximately constant at close to the value determined during spontaneous ventilation for each individual animal in the range of ventilatory frequencies from below 1 to around 5 Hz, but decreased somewhat with increasing frequency above 5 Hz. The calculated physiological dead space at 15 Hz was about 70% of the value at normal respiratory frequencies. These findings in primates, obtained using a ventilator system which allows very accurate determinations of expired gas volume and content, when compared with those from our previous studies in rabbits and dogs, provide further evidence that the relationship between efficiency of gas exchange and ventilatory frequency during HFV is highly species-specific.