DETERMINANTS OF MYOCARDIAL OXYGEN DELIVERY IN RESPONSE TO MODERATE AND SEVERE REDUCTIONS IN HEMATOCRIT
Authors: Kiel Alexander M , Goodwill Adam G , Noblet Jillian N , Sassoon Daniel J , Tune Johnathan D 
Published: November 2023

The objective of this investigation was to examine coronary responses to moderate and severe reductions in hematocrit and arterial oxygen content. Experiments were performed in open‐chest anesthetized swine in which coronary blood flow, arterial pressure, and coronary arterial and venous blood gases were assessed in response to progressive hemodilution through serial 500 ml exchanges of arterial blood with simultaneous venous infusion of the synthetic plasma volume expander Hespan (6% hetastarch in 0.9% sodium chloride at 37°C). Values were recorded following stabilization after each 500 ml exchange. Hemodilution resulted in progressive reductions in arterial hematocrit from 31 ± 1% to 6 ± 1% (P < 0.001) with accompanying reductions in arterial oxygen content from 14.1 ± 0.5 ml/dl to 4.8 ± 0.1 ml/dl (P < 0.001). Arterial PO2 was unaffected by hemodilution (average 170 ± 20 mmHg; P = 0.31). Over this range of reductions in arterial oxygen content there was a continual increase in coronary flow (0.41 ± 0.07 ml/min/g to 1.69 ± 0.15 ml/min/g; P < 0.001) without changes in either mean arterial pressure (93 ± 5 mmHg to 83 ± 5 mmHg; P = 0.628) or myocardial oxygen consumption (47 ± 9 ml O2/min/g to 61 ± 5 ml O2/min/g; P = 0.329). Heart rate increased from 66 ± 6 beats/min at baseline to 94 ± 8 beats/min (P = 0.016). Despite the changes in coronary blood flow, myocardial oxygen delivery was not significantly increased by euvolemic anemia (56.9 ± 9.43 μl O2/min/g vs. 80.2 ± 6.38 μl O2/min/g; P = 0.14). Moreover, coronary venous PO2 remained constant across this range of arterial oxygen contents (20.3 mmHg ± 2 vs. 19.4 ± 1 mmHg; P = 0.24). In a separate set of studies, identical hemodilution experiments were performed with a crystalloid solution (sodium chloride 0.9%) in place of the colloid solution (Hespan). In these swine, mean arterial pressure decreased from 74 ± 6 mmHg to 26 ± 2 mmHg (P < 0.001) as arterial oxygen content was reduced from 15.8 ± 0.4 to 8.8 ± 0.3 ml/dl (P < 0.001). Despite these changes, coronary blood flow remained stable (0.49 ± 0.02 to 0.42 ml/min/g; P = 0.40) while myocardial oxygen delivery was significantly decreased from 77 ± 4 to 36 ± 6 ml O2/min/g (P < 0.001). Taken together, these findings demonstrate that coronary blood flow progressively increases as arterial oxygen content falls below 10 ml/dl, as long as arterial pressure remains > 40 mmHg. This increase in coronary blood flow is able to sustain myocardial oxygen delivery and consumption over a wide range of arterial oxygen contents (~4–17 ml/dl). The mechanisms responsible for the preserved balance between myocardial oxygen delivery and metabolism remain to be determined.Support or Funding InformationThis study was supported by U01HL118738 and the Purdue University Ross Fellowship. Dr. Goodwill was supported by American Heart Association 13POST1681001813 (A. Goodwill, PI). Ms. Noblet was supported by TL1 TR000162 (A. Shekhar, PI) from the National Institutes of Health, National Center for Advancing Translational Sciences, Clinical and Translational Sciences Award.