Compensation for increase in respiratory workload during mechanical ventilation - Pressure-support versus proportional-assist ventilation

Variation in respiratory impedance may occur in mechanically ventilated pat ients. During pressure-targeted ventilatory support, this may lead to patie nt-ventilator asynchrony. We assessed the hypothesis that during pressure-s upport ventilation (PSV), preservation of minute ventilation ((V) over dot (E)) consequent to added mechanical loads would result in an increase in re spiratory rate (RR) due to the large reduction in tidal volume (V-T). With proportional-assist ventilation (PAV), preservation of (V) over dot (E) wou ld occur through the preservation of V-T, with a smaller effect on RR. We a nticipated that this compensatory strategy would result in greater patient comfort and a reduce work of breathing. An increase in respiratory impedanc e was obtained by chest and abdominal binding in 10 patients during weaning from mechanical ventilation. (V) over dot (E) remained constant in both ve ntilatory modes after chest and abdominal compression. During PSV, this mai ntenance of V-E was obtained through a 58 +/- 3% increase in RR that compen sated for a 29 +/- 2% reduction in V-T. The magnitudes of the reduction in V-T (10 +/- 3%) and of the increase in RR (14 +/- 2%) were smaller (p < 0.0 01) during PAV. During both PSV and PAV, chest and abdominal compression ca used increases in both the pressure-time product (PTP) of the diaphragm per minute (142.9 +/- 26.9 cm H2O . s/min, PSV, and 117.6 +/- 16.4 cm H2O . s/ min, PAV) and per liter (13.4 +/- 2.5 cm H2O . s/L, PSV, and 9.6 +/- 0.7 cm H2O . s/L, PAV). These increments were greater (p < 0.001) during PSV than during PAV. The capability of keeping V-T and (V) over dot (E) constant th rough increases in inspiratory effort after increases in mechanical loads i s relatively preserved only during PAV. The ventilatory response to an adde d respiratory load during PSV required greater muscle effort than during PA V.