Comparative studies of IPPV and HFPPV with PEEP in critical care patients. II: Studies on intrapulmonary gas distribution.

The ventilatory patterns of a conventional ventilator for volume-controlled ventilation (SV-900) and a low-compression ventilator utilizing the pneumatic valve principle for pressure/flow-generated, volume-controlled ventilation (system H) were studied in a lung model and in 10 patients with respiratory failure. System H was used at frequencies of 20 (H-20) and 60 (H-60 = high-frequency positive-pressure ventilation [HFPPV]) breath/min, and SV-900 at a frequency of 20 (SV-20) breath/min. With system H, inspiration constituted 22% (no inspiratory pause) and with SV-900, 25% (with 10% inspiratory pause). System H delivers an instantaneous accelerating flow which rapidly decelerates during the second part of the inspiratory phase. Maximal flow rates studied were 1.3 (H-60), 0.9 (H-20), and 0.7 (SV-20) L/sec. Thus, HFPPV delivers an effective tidal volume with highest linear velocity. This increased velocity increases gas mixing by increasing turbulence in conducting airways. In the 10 patients with respiratory failure, intrapulmonary gas distribution (measured as the nitrogen washout delay) was improved from 106% during SV-20 to 74% with H-60 (p less than 0.05). H-60 also increased carbon dioxide elimination in the 2 patients with the most severe pulmonary dysfunction.