Lung compliance changes on high-frequency ventilation in normal dogs

To test the hypothesis that high-frequency ventilation (HFV) promotes lung stability we compared the temporal course of dynamic lung compliance changes after two inflations on HFV with those occurring on conventional mechanical ventilation (CMV) at two different lung volumes, specifically with and without 5 cmH₂O positive end-expiratory pressure (PEEP). In our first set of experiments we ventilated six anesthetized paralyzed dogs first with CMV, then with HFV, then again with CMV using tidal volumes of 15 ml/kg at rates of 16-18 times/min for CMV and less than 90 ml and a rate of 15 Hz for HFV. In our second set of experiments we ventilated six dogs for 4 h, the 1st h with CMV at 0 cmH₂O end-expiratory pressure, the 2nd h with CMV with 5 cmH₂O PEEP, the 3rd h with HFV at the same mean pleural pressure, and the 4th h again with CMV with 5 cmH₂O PEEP. We found the decreases in dynamic compliance with time following hyperinflations were similar on HFV and CMV (P > 0.5) at both lung volumes. With the lower lung volume the initial dynamic compliance following hyperinflation also tended to fall progressively from one hour to the next despite the inflations. However, with PEEP the initial dynamic compliance over successive hours tended to rise from one hour to the next. We found that changes in dynamic compliance were not necessarily reflected in the venous admixture or alveolar to arterial O₂ partial pressure gradients. We thus conclude that lung stability in normal dogs is not improved during HFV, and blood gases cannot be used to predict compliance changes.