Myocardial and systemic responses to arterial hypoxemia during cardiac tamponade

Experiments were performed on 14 anesthetized, open-chest dogs to assess myocardial and systemic responses to cardiac tamponade alone (TAMP) and combined with arterial hypoxemia (HYP). Regional blood flow (RBF) was measured with radioactive microspheres and used to compute regional O2 supply. Myocardial oxygen and lactate extraction were determined. Myocardial oxygen consumption (MVO2) was calculated with Fick equation. An increase in pericardial pressure, sufficient to reduce mean aortic pressure (MAP) by 20%, caused proportional decreases in myocardial RBF and MVO2 but had no effect on endo-to-epi flow ratio or on myocardial lactate extraction. TAMP alone decreased RBF and O2 supply in kidney, splanchnic organs, skeletal muscle, and skin, but it had no effect in brain. HYP (arterial PO2, 35 +/- 2 mmHg) during TAMP restored MAP and caused transmurally uniform increases in myocardial RBF that were adequate to maintain MVO2 and lactate extraction. RBF increased sufficiently in brain to maintain regional O2 supply, whereas unchanged or inadequate increases in RBF in other tissues accentuated reductions in O2 supply. During combined TAMP and HYP, local vasodilator mechanisms were capable of maintaining adequate oxygen supply in myocardium and brain but not apparently in the nonvital tissues where these mechanisms were antagonized by reflex vasoconstriction.