Effect of Hemorrhage on Blood Flow to Marrow and Osseous Tissue in Conscious Rabbits

The purpose of this experiment was to characterize cardiovascular responses in skeletal tissue 15 minutes and 16 hours following reversible hemorrhagic hypotension and to validate a quantitative method for measuring blood flow rates in marrow and bone. Cardiac output and regional blood flow determinations were made using the radioactive microsphere technique. Microspheres (15 u dia) were injected into unanesthetized rabbits via a chronically implanted left atrial catheter. Blood flow and cardiac output measurements were made by injecting individual microsphere isotopes, each with a different label (®^Sr, 5 1 c r Gr l ^ C e ) , at three specified time intervals: first, as a pretreatment control value; second, 15 minutes following a standardized non-fatal hemorrhage (20 ml blood/kg body wt); and third, 16 hours post-hemorrhage. Results of four standard validations used to test reliability of the microsphere method as applied in this study showed close correlation between blood flow and microsphere distribution. On the day of the experiment both central ear arteries were cannulated. One catheter was used for blood pressure and heart rate recordings. The other v e s se l provided arterial blood for PCO2 , PO2 , pH and hematocrit measurements and also served as the site for collection of free-flowing reference samples used to calculate cardiac output. Each animal was sacrificed immediately following the third isotope injection and the heart, both kidneys, spleen and both femurs (marrow and bone separated) were removed, weighed fresh and ashed overnight at 550° C. Every tissue contained three isotopes, each representing a flow rate measurement during one of the time periods. Dissolved soft tissue samples and bone ash were counted and appropriate equations used to calculate percentage distribution of cardiac output, blood flow and tissue res istance for each time interval. Fifteen minutes after hemorrhage there was a significant decrease in cardiac output, blood pressure, arterial pCC\u3e2 ; while hematocrit and heart rate were significantly increased. These changes were accompanied by significantly reduced blood flows to the heart, kidney, spleen, whole bone, marrow and osseous tissue with corresponding resistance increases in all tissues except the heart. Spleen, whole bone, marrow and osseous tissue received a decreased percentage distribution of cardiac output while there was an increased percentage going to the heart. Renal flow fractions remained unchanged. Sixteen hours following hemorrhage cardiac output, heart rate and arterial pCC\u3e2 returned to normal while pC\u3e2 increased and hematocrit decreased. Arterial pH was unchanged at both post-hemorrhage measurements. Heart and marrow blood flows were significantly increased and kidney flow rates continued to be lower than control values. Perfusion of whole bone, spleen and osseous tissue returned to pretreatment levels. Tissue res istance decreased in the heart, spleen, whole bone and marrow but remained elevated in the renal vascular bed. Osseous tissue resistance decreased but not significantly. Percentage distribution of cardiac output increased in the heart, whole bone and marrow and decreased in the kidney while the spleen and osseous tissue values returned to normal. Normal hemodynamic properties of bone showed parallel blood supplies to marrow and osseous tissue with higher vascular resistance in the latter. It was estimated that total skeletal tissue plus marrow received 16% of resting cardiac output. The response 15 minutes post-hemorrhage demonstrated the characteristic decrease in regional blood perfusion with a relative preservation of flow to the heart. There was a uniform reduction of blood flow within the femur giving no evidence of preferential shunting away from osseous tissue in favor of marrow. Sixteen hours after hemorrhage blood flow to the various soft t is sues reflected continuation of a stress state where both vasodilatory metabolic and vasoconstrictive neurohumoral factors were still operative. A selective blood flow increase not seen in surrounding osseous tissue was observed in marrow. The response seen in marrow may be a preparatory mechanism for the increased metabolism associated with erythropoietic and reticuloendothelial activation and may represent a direct vasoproliferative effec t of erythropoietin on marrow vasculature