Metabolic and Respiratory Variables during Pressure Support versus Synchronized Intermittent Mandatory Ventilation

Abstract

Background: Mechanically ventilated patients interact and respond differently to different modes of ventilatory support. Objectives: To assess changes in metabolic and respiratory variables during equivalent changes with either pressure support ventilation (PSV) or volume-cycled synchronized intermittent mandatory ventilation (SIMV) in non-tracheostomized patients without known obstructive pulmonary disease receiving short-term mechanical ventilation in the intensive care unit. Methods: Fourteen patients receiving volume-cycled SIMV at 12 breaths/min (SIMV_100%) were included in the study. The PSV level (PSV_100%) resulting in a minute volume and respiratory rate equivalent to that during SIMV_100% was determined for each patient. Then each patient underwent trials at 66% and 33% of initial ventilator support with volume-cycled SIMV (SIMV_66% and SIMV_33%) and PSV (PSV_66% and PSV_33%) in random order. At the end of each trial, oxygen consumption (v̇O₂), carbon dioxide production (v̇CO₂), measured energy expenditures (MEE), peak inspiratory flow, total respiratory frequency, tidal volume, minute ventilation, occlusion pressure (P_0.1) and inspiratory duty cycle (T_i/T_tot) were measured. Results: There were smaller changes in v̇O₂, v̇CO₂ and MEE when equivalent changes were applied with PSV (15.7 ± 4.4; 12.5 ± 2.2 and 15 ± 3.5%) compared with volume-cycled SIMV (32.7 ± 7.7; 23 ± 5.2 and 30.7 ± 6.8%; p < 0.05). P_0.1 and T_i/T_tot were significantly smaller during PSV (2.64 ± 0.28 and 0.38 ± 0.03 cm H₂O) than volume-cycled SIMV (4.01 ± 0.21 and 0.43 ± 0.02 cm H₂O; p < 0.05). Conclusions: Changes in the level of PSV resulted in smaller changes in metabolic and respiratory variables compared with equivalent changes in the level of volume-cycled SIMV support. PSV may be more suitable for progressive respiratory muscle reloading.