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To compare the effects of adaptive support ventilation (ASV) and synchronized intermittent mandatory ventilation plus pressure support (SIMV-PS) on patient-ventilator interactions in patients undergoing partial ventilatory support.Prospective, crossover interventional study.Medical intensive care unit, university tertiary care center.Ten patients, intubated and mechanically ventilated for acute respiratory failure of diverse causes, in the early weaning period, ventilated with SIMV-PS and clinically detectable sternocleidomastoid activity suggesting increased inspiratory load and patient-ventilator dyssynchrony.Measurement of respiratory mechanics, P0.1, sternocleidomastoid electromyographic activity, arterial blood gases, and systemic hemodynamics in three conditions: 1) after 45 mins with SIMV-PS (SIMV-PS 1); 2) after 45 mins with ASV, set to deliver the same minute-ventilation as during SIMV-PS; 3) 45 mins after return to SIMV-PS (SIMV-PS 2), with settings identical to those of the first SIMV-PS period.The same minute ventilation was observed during ASV (11.4 ± 3.1 l/min [mean ± sd]) as during SIMV-PS 1 (11.6 ± 3.5 L/min) and SIMV-PS 2 (10.8 ± 3.4 L/min). No parameter was significantly different between SIMV-PS 1 and 2, hence subsequent results refer to ASV vs. SIMV-PS 1. During ASV, tidal volume increased (538 ± 91 vs. 671 ± 100 mL, p < .05) and total respiratory rate decreased (22 ± 7 vs. 17 ± 3 breaths/min, p < .05) vs. SIMV-PS. However, spontaneous respiratory rate increased in six patients, decreased in four, and remained unchanged in one. P0.1 decreased during ASV in all patients except three in whom no change was noted (1.8 ± 0.9 vs. 1.1 ± 1 cm H2O, p < .05). During ASV, sternocleidomastoid electromyogram activity was markedly reduced (electromyogram index, where SIMV-PS 1 = 100, ASV 34 ± 41, SIMV-PS 2 89 ± 36, p < .02) as was palpable muscle activity. No changes were noted in arterial blood gases, pH, or mean systemic pressure during the trial.In patients undergoing partial ventilatory support, with clinical and electromyographic signs of increased respiratory muscle loading, ASV provided levels of minute ventilation comparable to those of SIMV-PS. However, with ASV, central respiratory drive and sternocleidomastoid activity were markedly reduced, suggesting decreased inspiratory load and improved patient-ventilator interactions. These preliminary results warrant further testing of ASV for partial ventilatory support.