Since the advent of positive pressure ventilation, various methods for elective withdrawal of ventilatory support (weaning) have been proposed. The introduction of intermittent mandatory ventilation (IMV) as a method of weaning has initiated a controversy concerning the optimum weaning mode. Several studies have examined the attributes of T-piece and IMV weaning with inconclusive results. Criticism of these studies has included machine variability in relation to the work of breathing, improper application of the two weaning methods, and the lack of a heterogeneous study population. We report the results of a prospective, randomized trial of medical and surgical patients who were weaned from mechanical ventilation by either T-piece or IMV. To test the hypothesis that patients satisfying specific ventilatory and gas transfer criteria can wean rapidly by either IMV or T-piece, a protocol was developed to ensure equal weaning time for all successful weaning attempts.
Over a 16-month period all adult patients requiring mechanical ventilation at the three hospitals in the Medical University of South Carolina system (Medical University Hospital, Charleston Veterans Administration Medical Center, and Charleston Memorial Hospital) were randomized into two groups following informed consent. All patients were ventilated with the same ventilator, a Bennett MA-1 (Puritan-Bennett Corp). IMV was administered with an H valve replacing the 9200 demand valve for the IMV circuit (Air Life-American Pharmaseal Co). read
To eliminate bias during the stabilization period of mechanical ventilation due to possible differences in work of breathing using the two modes of ventilation, the two groups were randomized according to months. During odd months (January, March, May, etc), patients were assigned to assisted mandatory ventilation (AMV) and during even months (February, April, June, etc) to the IMV mode. The stabilization period was defined as the time required for mechanical ventilation before the primary physician decided that the patient was “clinically stable.” During the stabilization period, the primary physicians specified the rate, Vt, FIo2, and level of PEEP. Only the ventilatory mode was predetermined by protocol; the patient remained on the initial mode of ventilation until weaning was initiated. All patients had a ^7.0 mm endotracheal tube in place before weaning was initiated. The weaning phase of the protocol was started when the patient was stable clinically and specific gas transfer and acid-base parameters were satisfied: (1) a PaO, of >55 mm Hg on a Flo* of <0.50; (2) a pH 2>7.30 and S7.50; (3) PEEP ^5 cm HaO; and (4) respiratory rate ^36/min.