COMPARATIVE PHYSIOLOGICAL-EFFECTS OF NONINVASIVE ASSIST-CONTROL AND PRESSURE SUPPORT VENTILATION IN ACUTE HYPERCAPNIC RESPIRATORY-FAILURE

Study objective: To compare the effects of noninvasive assist-control ventilation (ACV) and pressure support ventilation (PSV) by nasal mask on respiratory physiologic parameters and comfort in acute hypercapni c respiratory failure (AHRF). Design: A prospective randomized study. Setting: A medical ICU. Patients and interventions: Fifteen patients w ith COPD and AHRF were consecutively and randomly assigned to two noni nvasive ventilation (NIV) sequences with ACV and PSV mode, spontaneous breathing (SE) via nasal mask being used as control, ACV and PSV sett ings were always subsequently adjusted according to patient's toleranc e and air leaks, Fraction of inspired oxygen did not change between th e sequences. Measurements and results: ACV and PSV mode strongly decre ased the inspiratory effort in comparison with SB. The total inspirato ry work of breathing (WOBinsp) expressed as WOBinsp/tidal volume (VT) and WOBinsp/respiratory rate (RR), the pressure time product (PTP), an d esophageal pressure variations (Delta Pes) were the most discriminan t parameters (p<0.001), ACV most reduced WOBinsp/VT (p<0.05), Delta Pe s (p<0.05), and PTP (0.01) compared with PSV mode. The surface diaphra gmatic electromyogram activity was also decreased >32% as compared wit h control values (p<0.01), with no difference between the two modes, S imultaneously, NIV significantly improved breathing pattern (p<0.01) w ith no difference between ACV and PSV for VT, RR, minute ventilation, and total cycle duration. As compared to SE, respiratory acidosis was similarly improved by both modes. The respiratory comfort assessed by visual analog scale was less with ACV (57.23 +/- 30.12 mm) than with S B (75.15 +/- 18.25 mm) (p<0.05) and PSV mode (81.62 +/- 25.2 mm) (p<0. 01) in our patients. Conclusions: During NIV for AHRF using settings a dapted to patient's clinical tolerance and mask air leaks, both ACV an d PSV mode pro tide respiratory muscle rest and similarly improve brea thing pattern and gas exchange, However, these physiologic effects are achieved with a lower inspiratory workload but at the expense of a hi gher respirator) discomfort with ACV than with PSV mode.