The right ventricular responses to mild hypocarbia and hypercarbia were studied in 18 anesthetized and paralyzed patients following coronary artery bypass surgery. Maintaining constant tidal volume (8 ml.kg-1), FIO2 (0.5), and PEEP (5 cm H2O), the ventilator rate was varied to sequentially produce: 1) normocarbia (PaCO2, 38.3 +/- 2.5 mmHg; mean +/- SD), 2) hypocarbia (PaCO2, 33.2 +/- 2.8 mmHg), 3) hypercarbia (PaCO2, 49.8 +/- 2.9 mmHg) and 4) normocarbia (PaCO2, 38.8 +/- 3.6 mmHg). Pulmonary and right ventricular hemodynamics were assessed using a rapid-response pulmonary artery catheter after 10 min of stabilization at each PaCO2. Pulmonary and right ventricular hemodynamics remained unaffected by slight hypocarbia. In contrast, hypercarbia increased pulmonary vascular resistance by 54% (P less than 0.001) and mean pulmonary artery pressure by 34% (P less than 0.001). This was accompanied by a 24% (P less than 0.001) increase in right ventricular end-diastolic volume, a 38% (P less than 0.001) increase in right ventricular end-systolic volume, and a 20% decrease (P less than 0.001) in right ventricular ejection fraction. Despite an increase in right ventricular afterload, stroke volume was maintained unchanged because of a 45% (P less than 0.001) increase in right ventricular stroke work index. Although the patients maintained pulmonary blood flow during hypercarbia using preload augmentation, compensatory reserve might be exceeded in patients with more compromised right ventricular function.