Mild Hypercapnia After Uncomplicated Heart Surgery Is Not Associated With Hemodynamic Compromise

Objective: The aim of this study was to examine the effects of small changes in PaCO₂ on hemodynamic parameters after uncomplicated heart surgery with cardiopulmonary bypass. Design: This was a prospective, randomized crossover study. Setting: A large academic medical center. Participants: Twenty-four subjects who were scheduled for elective cardiac surgery were enrolled in this study. Interventions: Each subject underwent the normal procedures that are associated with cardiac surgery. General anesthesia, including muscle relaxation, were continued in the immediate postoperative period. Measured tidal volumes and minute ventilation were kept constant for the duration of the study. Target PaCO₂ concentrations of 30, 40, and 50 mmHg were achieved by adding varying amounts of exogenous CO₂ gas to the inhaled oxygen. Various measurements were made at each target PaCO₂, including cardiac index, mixed venous oxygen saturation, blood pressure, heart rate, and pulmonary artery pressure. Measurements and Main Results: Twenty-four patients were enrolled. Seven were withdrawn before commencement of the study. The cardiac index increased when the PaCO₂ was increased from 30 to 40 mmHg (p < 0.001) and remained unchanged between 40 and 50 mmHg. Mixed venous oxygen saturation increased (p < 0.001) with elevations in PaCO₂ up to 50 mmHg and decreased again when the PaCO₂ was returned to 30 mmHg. The blood pressure decreased (p < 0.001) with increasing PaCO₂. The pulmonary pressure increased (p < 0.001) with elevations in PaCO₂. No patient became hemodynamically unstable or had any arrhythmias. Conclusion: The findings of this study suggest that unless there is a specific contraindication to mild hypercapnia, such as pulmonary hypertension or hemodynamic instability, concerns about mild respiratory acidosis should not prevent weaning of sedation and mechanical ventilation after uncomplicated heart surgery.