Effects of acute hypercapnia on the central and peripheral circulation of conscious sheep

We studied the cardiorespiratory effects of acute hypercapnia in 10 unanesthetized sheep. After a 15-min exposure to either 7.3 or 10% CO₂ in air, we measured arterial blood gases, minute ventilation (V̇E), O₂ consumption (V̇O₂), cardiac output (Q̇), heart rate (HR), an index of left ventricular contractility [(dP/dt)/P], and vascular pressures. In addition, regional flows to all major organs were determined by injecting 15-μm radiolabeled microspheres into the left heart. Exposure to 7.3% CO₂ (arterial CO₂ partial pressure, PaCO₂ 58 Torr) resulted in increased V̇E, (dP/dt)/P, and higher blood flows to the brain and respiratory muscles. All other variables remained unchanged. Exposure to 10% CO₂ (PaCO₂ 75 Torr) resulted in a further augmentation of V̇E and a 48% increase in Q̇, which was associated with a tachycardia, a decrease in systemic vascular resistance, and an increase in V̇O₂. Coronary and respiratory muscle flows increased, but all other variables remained unchanged. Thus the hemodynamic effects of hypercapnia are not related linearly to the level of PaCO₂.