Original communication| Volume 75, ISSUE 4, P508-516, April 1974

The adequacy of myocardial oxygen delivery in acute normovolemic anemia

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      Subendocardial ischemia with patent coronary arteries is recognized increasingly as a cause of previously unexplained cardiac failure. Subendocardial muscle flow is diastolic, and subendocardial oxygen delivery is determined by oxygen content as well as coronary blood flow. Hemodilution (open-heart surgery, refusal of transfusion) reduces oxygen content by acutely lowering the hemoglobin levels; this study assesses its effects on the adequacy of myocardial oxygen delivery. In ten anesthetized dogs, we produced anemia by exchanging blood with a dextran solution while measuring aortic and left atrial blood pressure, cardiac output, coronary blood flow, and endocardial and epicardial EKG's. Myocardial blood flow distribution, aortic blood pressure, and cardiac output remained normal with hemoglobins ranging from 14 to 5 Gm. percent. Myocardial oxygen delivery was maintained by increasing coronary blood flow from 75 to 127 c.c. per 100 Gm. per minute. Coronary flow became redistributed away from subendocardial muscle (P < 0.025) and myocardial failure occurred with hemoglobins below 5 Gm. percent. Failure was associated with ischemic endocardial EKG's despite total left ventricular coronary blood flow increasing to 310 percent above control levels. We conclude that (1) oxygen delivery to the entire left ventricle remains adequate over a wide range of hemoglobin levels in normal hearts, and (2) subendocardial ischemia and cardiac failure occur when coronary blood flow becomes maldistributed and cannot compensate for decreased oxygen-carrying capacity of the blood. We would expect these adverse changes to occur at higher hemoglobin levels with valvular or ischemic heart disease, abnormal circulatory dynamics, or increased myocardial oxygen requirement.
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