Response of ventilator-dependent patients to different levels of proportional assist

Proportional-assist ventilation (PAV) is a form of ventilatory support in which airway pressure increases in proportion to patient effort. Because it effectively reduces the mechanical load to an adjustable extent, PAV permits the study of the pattern of breathing in patients with respiratory disease when unconstrained by abnormal respiratory mechanics. We studied 11 patients with assorted medical problems requiring ventilatory support. The patients were switched to PAV, and the level of support was varied from near-maximal levels to the lowest tolerable level. Each level was maintained for several minutes while ventilation (VE), tidal volume (VT), and respiratory rate (f) were monitored. The breathing pattern observed with the highest assist varied substantially among patients. The ranges (and means) of VE, VT, and f were 5.6-18.7 (12.8) l/min, 203-844 (517) ml, and 18-33 (25) breaths/min, respectively. The correlation between VT and VE at the highest assist was very high (r = 0.91), suggesting that ventilatory demand is the most important determinant of VT variability. There were no systematic changes in breathing pattern as the level of assist was altered; at the highest and lowest levels of support, VE, VT, and f were, respectively, 12.8 +/- 5.4 (SD) vs. 11.6 +/- 4.3 l/min, 517 +/- 217 vs. 459 +/- 175 ml, and 25.0 +/- 4.2 vs. 25.7 +/- 3.9 breaths/min. These results indicate that within each patient, in a given state, there exist unique values for a desired VE, VT, and f that are largely independent of the mechanical load; if assist is increased, patient effort is decreased to maintain the desired ventilatory targets.