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

Variation in respiratory impedance may occur in mechanically ventilated patients. During pressure-targeted ventilatory support, this may lead to patient-ventilator asynchrony. We assessed the hypothesis that during pressure-support ventilation (PSV), preservation of minute ventilation ((V) over dot (E)) consequent to added mechanical loads would result in an increase in respiratory rate (RR) due to the large reduction in tidal volume (V-T). With proportional-assist ventilation (PAV), preservation of (V) over dot (E) would occur through the preservation of V-T, with a smaller effect on RR. We anticipated that this compensatory strategy would result in greater patient comfort and a reduce work of breathing. An increase in respiratory impedance was obtained by chest and abdominal binding in 10 patients during weaning from mechanical ventilation. (V) over dot (E) remained constant in both ventilatory modes after chest and abdominal compression. During PSV, this maintenance of V-E was obtained through a 58 +/- 3% increase in RR that compensated 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 < 0.001) during PAV. During both PSV and PAV, chest and abdominal compression caused 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 through increases in inspiratory effort after increases in mechanical loads is relatively preserved only during PAV. The ventilatory response to an added respiratory load during PSV required greater muscle effort than during PAV.