This study showed that both medical and surgical patients assigned randomly to either an IMV or T-piece mode could be weaned from mechanical ventilation over the same period. Two-thirds of the patients who were weaned in 2 h were postsurgical patients who were ventilated for <72 h. The majority of these patients had undergone uncomplicated coronary artery bypass grafting (CABG) and usually were weaned as soon as they awoke from anesthesia. This represents a group of patients that usually present minimal problems in weaning from mechanical ventilation rapidly and may represent a bias to the hypothesis. Ideally, the use of a similar protocol with a large group of long-term ventilator patients might uncover differences in success between the two modes of weaning.
Careful attention was addressed to study design to minimize variables that could affect weaning. Past criticism of weaning studies has centered on the work of breathing when different ventilators, circuitry, and endotracheal tubes were employed. All patients had at least a 7.0-mm internal diameter endotracheal tube in place (usually 7.5 mm), and all were ventilated and weaned with a Bennett MA-1 volume ventilator, with a low resistance H valve replacing the demand valve for the IMV circuit. A potential criticism is a bias designed into the study relating to the arbitrary designation of time zero to begin each weaning attempt.
This was necessary in order to compare a set of identifiable weaning times. IMV proponents may argue that an IMV wean begins immediately and needs no weaning parameter measurements, that the patient is continually challenged to wean and, therefore, may have been weaned successfully prior to being identified by specific criteria (although possibly requiring more blood gas determinations). Most of the patients in group A could not have initiated weaning sooner, because they were still under the influence of anesthesia.
Bedside spontaneous ventilation parameters are used to assist the clinician in identifying patients who should be able to be weaned from mechanical ventilation. In the present study, the criteria chosen were those commonly used by clinicians caring for patients receiving mechanical ventilation and were set conservatively to maintain a high sensitivity and avoid the false positive wean/extubation. Only three patients were weaned successfully despite inadequate weaning parameters (false negatives).