Tidal Volumes Required to Maintain Isocapnia at Frequencies from 3 to 30 Hz in the Dog

We ventilated seven mongrel dogs with high frequency oscillatory ventilation (HFO) at frequencies from 3 to 30 Hz. At each frequency, the tidal volume required to achieve isocapnia (VTiso) was measured by plethysmography. In an individual dog, VTiso could be related to frequency by an equation of the form VTiso = KfA. A was similar for the seven dogs, A = -0.54 +/- 0.03. K varied from 179 to 325 cc- Hz-A, reflecting differences between the dogs in dead space, CO2 production and gradient for gas transport. This relation is consistent with that predicted from the data of previous investigators who used different ventilators, circuits and methodology. From 3 to 30 Hz, VTiso decreased from 1.2 to 0.4 times anatomic plus equipment dead space, but minute volumes required to maintain isocapnia increased from 6 to 18 times those required during conventional ventilation. We conclude that low tidal volume ventilation is also high minute volume ventilation.