Proportional Assist Ventilation: Results of an Initial Clinical Trial

The response to proportional assist ventilation (PAV) was tested in four normal subjects during heavy exercise and in five ventilator-dependent patients recovering from assorted medical disorders. The apparatus consisted of a rolling-seal piston coupled to a motor that generated pressure in proportion to inspired flow and inspired volume, with the gains adjusted such that the proportionality between airway pressure (Paw) and instantaneous patient-generated pressure (Pmus) was approximately 1:1 (i.e., machine-amplified patient effort by a factor of 2). Normal subjects responded to PAV by decreasing their own effort, as judged from esophageal pressure, such that the changes in ventilation and breathing pattern were rather small ( e: 64.8 ± 3.6 during PAV versus 56.0 ± 4.3, p < 0.01; Vt: 2.39 ± 0.24 versus 2.02 ± 0.17, p < 0.05; f: 27.5 ± 1.9 versus 28.0 ± 2.2, NS). In patients, elastance ranged from 20 to 35 cm H2O cm/L, resistance ranged from 5 to 10 cm H2O/L/s, and maximal inspiratory pressure ranged from −16 to −65 cm H2O. After a period of observation during synchronized intermittent mechanical ventilation (SIMV) the patient was switched to PAV and maintained on it for 1 to 3 h. No patient had to be replaced on SIMV because of discomfort or deterioration in any of the monitored variables. During PAV peak airway pressure was less than half the value observed with the IMV breaths (16.6 ± 2.4 versus 35.4 ± 3.4 cm H2O, p < 0.001). Tidal volume during PAV was more than twice the patient's own volume during SIMV (0.63 ± 0.05 versus 0.23 ± 0.03 L, p < 0.001), and respiratory rate was less than the patient's own rate during SIMV (25.4 ± 3.2 versus 34.2 ± 3.5 breaths/min, p< 0.001). There were no differences in total minute ventilation (16.4 ± 1.2 versus 15.2 ± 8.0 L/min), PaCO2 (39.0 ± 2.9 versus 37.8 ± 4.0 mm Hg), or PaO2 (98 ± 5 versus 102 ± 3 mm Hg). There were also no significant changes in any of the monitored variables between early and late PAV. In summary, PAV is a feasible method of supporting ventilator-dependent patients that is well tolerated and can be implemented at much lower peak airway pressures. The spontaneously adopted breathing pattern during PAV does not appear to compromise gas exchange, at least over a few hours, in stable patients of the type studied here.