In all patients a flow-directed pulmonary artery catheter (Edwards Swan-Ganz 7F) and a radial artery catheter had been placed previously for hemodynamic monitoring. The CO was calculated with a computer (Edwards 9250 A), as the mean of 5 measurements obtained by injecting 10 ml of dextrose solution. All vascular pressures were measured via quartz transducer (Hewlett-Packard 1290). Arterial blood and mixed-venous blood gas levels were measured (Corning 178 and CO-Oximeter 2500) immediately after CO measurements. The Do and oxygen consumption (Vo) were calculated using standard formulas. Blood lactate concentration was determined using an enzymatic method. Airway pressures were measured with a differential pressure transducer (Validyne DP 45, ±56 cm HaO) connected close to the proximal end of the endotracheal tube. Plateau pressure (Pplat) and PEEPi were measured 2 s after end-inspiratory and end-expiratory occlusions, respectively. The mean airway pressure (mPaw) was measured by electronic filtering of the airway pressure signal. Airway pressure tracing was recorded on a multichannel polygraph (Could ТА 550). Tidal volume (Vt) was measured by means of the inspiratory flow transducer of the ventilator.
Protocol of the Study
Three ventilatory modes were tested in a randomized order: (1) VCV with inspiratory time of 33 percent, inspiratory pause of 0, and expiratory time of 67 percent; (2) PCV with inspiratory time of 33 percent, inspiratory pause of 0, and expiratory time of 67 percent; and (3) PC-IRV with inspiratory time of 67 percent, inspiratory pause of 0, and expiratory time of 33 percent. In each patient, the following parameters were kept constant in the three modes: inspired oxygen fraction (Flo); respiratory rate (RR); Vt; and PEEPt. The level of PEEPt was adjusted by manipulating the level of PEEP. In PCV and PC-IRV, Vt was adjusted by manipulating the level of inspiratory pressure. The Vt, RR, and PEEPt were maintained at the level they were before entry in the study (ie, as requested by the attending physician) while patients were on VCV Intrapulmonary Thyroid
In each ventilatory mode, measurements were performed after a 1-h period of stabilization. The blood lactate level was measured at the beginning of the study.
Informed consent was obtained from the patients relative. The study was approved by the ethics committee of our institution.
Results were compared by one-way variance analysis with repeated measures, complemented, if significant, by an intergroup comparison using the Scheffe test. The a risk chosen for analysis was 0.05. Results are expressed as the mean ± SEM.