The hazardous effects of alveolar hypocapnia on lung mechanics during weaning from cardiopulmonary bypass.

The bronchoconstrictive effects of alveolar hypocapnia during weaning from cardiopulmonary bypass (CPB) were investigated in patients undergoing elective coronary artery revascularization. Thirty patients were randomly assigned into two equal groups. In both groups, mechanical ventilation was initiated for 3 min prior to weaning from CPB with the venous pressure low. This kept the pulmonary vascular bed empty, resulting in alveolar hypocapnia (ETCO2 < 2 kPa). Peak airway pressure (P(peak)) and plateau pressures (P(plateau)) were recorded. In group 1, 5% CO2 was added to the inspiratory gas mixture and the ETCO2 allowed to rise (ETCO2 > 3.3 kPa). The ventilation pressure measurements were recorded again after 3 min stabilization. In group 2, the venous pressure was increased to allow the pulmonary venous bed to fill and the ventilation pressures recorded after a 3 min period of stabilization. In group 1, the ventilatory pressures dropped significantly (p < 0.001) when the alveolar hypocapnia was reversed with added CO2 (P(peak) 19.71 +/- 5.7 to 12.31 +/- 2.8 cmH2O and P(plateau) 13.15 +/- 3.28 to 9.15 +/- 2.23 cmH2O). In group 2, a similar effect was achieved by allowing filling of the pulmonary vascular bed (P(peak) 17.46 +/- 4.72 to 11.92 +/- 3.03 cmH2O and P(plateau) 13.93 +/- 4.10 to 9.37 +/- 3.00 cmH2O). These results suggest that filling the pulmonary vascular bed prior to initiating ventilation, when weaning from CPB, prevents the otherwise deleterious effects of alveolar hypocapnia, resulting in raised bronchomotor tonus and raised airway pressures.