Cardiorespiratory effects of high frequency intermittent positive pressure ventilation in the dog

The cardiorespiratory effects of increasing respiratory frequency were investigated in two groups of dogs ventilated mechanically with a system which minimized apparatus and anatomical deadspace. In one group minute volume was constant whilst frequency was increased; in the second group minute volume was increased to maintain normal PaCO2. The reduction in tidal volume with increasing frequency was accompanied by a reduction in physiological deadspace. However, the reduction in deadspace was proportionally less than the decrease in tidal volume, so that the deadspace/tidal volume ratio increased. The peak airway pressures were minimal at a frequency of 45 b.p.m. At frequencies of 60 b.p.m. and more, the lung failed to empty so that peak airway pressures were increased and cardiac output decreased.