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

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 ((.)VO2), carbon dioxide production ((.)VCO2), 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 (.)VO2, (.)VCO2 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 H(2)O) 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.