Patient-ventilator interactions during volume-support ventilation: asynchrony and tidal volume instability--a report of three cases.

During pressure-support ventilation, tidal volume (V(T)) can vary according to the level of the patient's respiratory effort and modifications of the thoraco-pulmonary mechanics. To keep V(T) as constant as possible, the Siemens Servo 300 ventilator proposes an original modification of pressure-support ventilation, called volume-support ventilation (VSV). VSV is a pressure-limited mode of ventilation that uses V(T) as a feedback control: the pressure support level is continuously adjusted to deliver a preset V(T). Thus, the ventilator adapts the inspiratory pressure level, breath by breath, to changes in the patient's inspiratory effort and the mechanical thoraco-pulmonary properties. The clinician sets V(T) and respiratory frequency, and the ventilator calculates a preset minute volume. It has been shown that ineffective respiratory efforts can occur during pressure-support ventilation.

Effect of bi-level positive airway pressure (BiPAP) nasal ventilation on the postoperative pulmonary restrictive syndrome in obese patients undergoing gastroplasty.

STUDY OBJECTIVE: Upper abdominal surgery results in a postoperative restrictive pulmonary syndrome. Bi-level positive airway pressure (BiPAP System; Respironics Inc; Murrysville, Pa), which combines pressure support ventilation and positive end-expiratory pressure via a nasal mask, could allow alveolar recruitment during inspiration and prevent expiratory alveolar collapse, and therefore limit the postoperative pulmonary restrictive syndrome. This study investigated the effect of BiPAP on postoperative pulmonary function in obese patients after gastroplasty. DESIGN: Prospective controlled randomized study. SETTING: GI surgical ward in a university hospital. PATIENTS: Thirty-three morbidly obese patients scheduled for gastroplasty were studied. INTERVENTION: The patients were assigned to one of three techniques of ventilatory support during the first 24 h postoperatively: O2 via a face mask, BiPAP System 8/4, with inspiratory and expiratory positive airway pressure set at 8 and 4 cm H2O, respectively, or BiPAP System 12/4 set at 12 and 4 cm H2O. Pulmonary function (FVC, FEV1, and peak expiratory flow rate [PEFR]) were measured the day before surgery, 24 h after surgery, and on days 2 and 3. Oxygen saturation by pulse oximeter (SpO2) was also recorded during room air breathing. RESULTS: Three patients were excluded. After surgery, FVC, FEV1, PEFR, and SpO2 significantly decreased in the three groups. On day 1, FVC and FEV1 were significantly improved in the group BiPAP System 12/4, as compared with no BiPAP; SpO2 was also significantly improved. After removal of BiPAP System 12/4, these benefits were maintained, allowing faster recovery of pulmonary function. No significant effects were observed on PEFR. BiPAP System 8/4 had no significant effect on the postoperative pulmonary restrictive syndrome. CONCLUSION: Prophylactic use of BiPAP System 12/4 during the first 24 h postoperatively significantly reduces pulmonary dysfunction after gastroplasty in obese patients and accelerates reestablishment of preoperative pulmonary function.