PATIENT-VENTILATOR INTERACTION DURING ACUTE HYPERCAPNIA - PRESSURE-SUPPORT VS PROPORTIONAL-ASSIST VENTILATION

The objective of this study was to compare patient-ventilator interact ion during pressure-support ventilation (PSV) and proportional-assist ventilation (PAV) in tile course of increased ventilatory requirement obtained by adding a dead space in 12 patients on weaning from mechani cal ventilation. With PSV. the level of unloading was provided by sett ing the inspiratory pressure at 20 and 10 cmH(2)O, whereas with PAV th e level of unloading was at 80 and 40% of the elastic and resistive lo ad. Hypercapnia increased (P < 0.001) tidal swing of esophageal pressu re and pressure-time product per breath at both levels of PSV and PAV. During PSV, application of dead space increased ventilation (V-E) dur ing PSV (67 +/- 4 and 145 +/- 5% during 20 and 10 cmH(2)O PSV, respect ively, P < 0.001). This was due to a relevant increase in respiratory rate (48 +/- 4 and 103 +/- 5%C during 20 and 10 cmH(2)O PSV, respectiv ely, P < 0.001), whereas the increase in tidal volume (VT) played a sm all role (13 +/- 1 and 21 +/- 2% during 20 and 10 cmH(2)O PSV, respect ively P < 0.001). With PAV, the increase in V-E consequent to hypercap nia (27 +/- 3 and 64 +/- 4% during 80 and 40% PAV, respectively, P < 0 .001) was related to the increase in VT (32 +/- 1 and 66 +/- 2% during 80 and 40% PAV, respectively, P < 0.001), respiratory rate remaining unchanged. The increase in pressure-time product per minute and per li ter consequent to acute hypercapnia and the sense of breathlessness we re significantly (P < 0.001) higher during PSV than during PAV Our dat a show that, after hypercapnic stimulation of tile respiratory drive, tile capability to increase VE through changes in VT modulated by vari ations in inspiratory muscle effort is preserved only during PAV; the compensatory strategy used to increase VE during PSV requires greater muscle effort and causes more pronounced patient discomfort than durin g PAV.