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Background. Intellivent-adaptive support ventilation (ASV) is a closed-loop, fully automatic method of mechanical ventilation. This advanced mode of ventilation adjusts ventilation and oxygenation parameters according to patient weight, lung function (as assessed by the ventilator) and continuous input of end-tidal carbon dioxide and oxygen saturation. Our study compares the efficacy of this new mode with ASV and synchronised intermittent mandatory ventilation (SIMV) modes.Methods. We conducted a within-group comparison of three modes of ventilation, ASV, Intellivent-ASV and SIMV, using a Hamilton S1 ventilator (Hamilton Medical, Switzerland). Subjects were ventilated for 2 hours on each mode, and at the end of each 2-hour period, parameters of ventilation and haemodynamics were measured.Results. Twenty subjects participated in this study. Their mean age was 67.3 years (range 22 - 82 years). The most common diagnosis at presentation was pneumonia (55%), followed by chronic obstructive pulmonary disease (16%) and acute respiratory distress syndrome (11%). Mean (standard deviation) levels of positive end-expiratory pressure (PEEP) were significantly higher in the Intellivent-ASV group (7.6 (5) v. 5.1 (2) and 5.2 (2) cm H2O in the ASV and SIMV groups, respectively (p