Respiratory effects of a calibrated volume-limited pressure-variable ventilator during surgery

¹ Department of Anesthesiology, New England Center Hospital, and Department of Surgery (Anesthesiology), Tufts University School of Medicine, Boston, Massachusetts

The application of a calibrated volume-limited, pressure-variable ventilator was studied in 34 patients during surgery. Tidal volume and respiratory frequency were determined by means of the Radford nomogram and ventilation was controlled throughout the intra-abdominal and intrathoracic procedures. There was no resistance to the expiratory flow of gases and the recoil of the lung was not impeded during the expiratory phase. The tidal volume was unchanged while the airway pressure varied in relationship to the change of the nonelastic and elastic resistance of the lung-thorax system. The ratio of the duration of the expiratory flow of gas to the duration of the expiratory pause varied in accordance with the changes of the airway resistance and was not altered by the ventilator. This allowed a sufficient duration of the pause period for compensatory circulatory adjustment. Adequate alveolar ventilation was maintained during surgery from 1–6 hours as indicated by the level of the arterial pCO₂ (range: 30–48 mm Hg).