Physiological dead space during high-frequency ventilation in dogs

Tidal volumes used in high-frequency ventilation (HFV) may be smaller than anatomic dead space, but since gas exchange does take place, physiological dead space [V(D)] must be smaller than tidal volume [V(T)]. We quantified changes in V(D) in three dogs at constant alveolar ventilation using the Bohr equation as V(T) was varied from 3 to 15 ml/kg and frequency (f) from 0.2 to 8 Hz, ranges that include normal as well as HFV. We found that V(D) was relatively constant at tidal volumes associated with normal ventilation (7-15 ml/kg) but fell sharply as V(T) was reduced further to tidal volumes associated with HFV (< 7 ml/kg). The frequency required to maintain constant alveolar ventilation increased slowly as tidal volume was decreased from 15 to 7 ml/kg but rose sharply with attendant rapid increases in minute ventilation as tidal volumes were decreased to < 7 ml/kg. At tidal volumes < 7 ml/kg, the data deviated substantially from the conventional alveolar ventilation equation [f(V(T)-V(D)) = constant] but fit well a model derived previously for HFV. This model predicts that gas exchange with volumes smaller than dead space should vary approximately as the product of f and V(T²).