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Coronary hemodynamic responses during local hemodilution in canine hearts
1Department of Physiology and Biophysics, University of Illinois College of Medicine, Chicago.
Insights
Hemodilution increases coronary blood flow (CBF) by reducing blood viscosity and causing vasodilation. This effect persists even during simulated coronary insufficiency, though subendocardial perfusion may remain impaired.
Area of Science:
- Cardiovascular Physiology
- Hemodynamics
- Coronary Circulation
Background:
- Hemodilution's impact on coronary hemodynamics is complex, often confounded by systemic responses in previous studies.
- Understanding the direct effects of hemodilution on coronary blood flow (CBF) and its distribution is crucial for clinical applications.
Purpose of the Study:
- To isolate and evaluate the direct effects of hemodilution on coronary hemodynamics.
- To assess the influence of hemodilution on coronary blood flow, transmural distribution, and dilator reserve capacity.
Main Methods:
- Experiments were conducted in anesthetized, open-chest dogs with selective perfusion of the left anterior descending coronary artery (LAD).
- Coronary blood flow (CBF) was measured using an electromagnetic flowmeter, and transmural distribution was assessed with radioactive microspheres.
- Perfusion pressure was manipulated to simulate normal and insufficient coronary conditions, with hemodilution achieved using lactated Ringer solution.
Main Results:
- Under normal perfusion pressure, hemodilution progressively increased CBF uniformly across the myocardium, nonlinearly related to hematocrit.
- Increased reactive hyperemic flow and reduced dilator reserve indicated contributions from decreased viscosity and vasodilation.
- Hypotension reduced CBF, particularly in the subendocardium; hemodilution normalized overall CBF but relative subendocardial hypoperfusion persisted.
Conclusions:
- Hemodilution directly enhances coronary blood flow through reduced viscosity and vasodilation, independent of systemic changes.
- While hemodilution improves overall coronary flow, it may not fully resolve regional hypoperfusion, especially in the subendocardium during hypotension.
- This study provides fundamental insights into hemodilution's effects on coronary hemodynamics in a controlled experimental setting.
Abstract:
To evaluate the effect of hemodilution per se on coronary hemodynamics, experiments were performed in 36 anesthetized, open-chest dogs whose left anterior descending coronary artery (LAD) was perfused selectively with either normal arterial blood or arterial blood diluted with lactated Ringer solution. LAD blood flow (CBF) was measured with an electromagnetic flowmeter and its transmural distribution assessed with 15-microns radioactive microspheres. LAD perfusion pressure was set at the normal level (approximately 100 mmHg) or at 50% of that level to simulate coronary insufficiency. Dilator reserve capacity was calculated from ratio of reactive hyperemic peak flow following release of 90-s occlusion to control (preocclusion) flow. Systemic hemodynamic parameters were maintained near control values during coronary hemodilution. With perfusion pressure normal, graded hemodilution caused progressive, transmurally uniform increases in CBF that showed a nonlinear relationship to inflow hematocrit. Increased peak reactive hyperemic flow and decreased dilator reserve ratio indicated that both reduced viscosity and vasodilation contributed to increased CBF during hemodilution. Hypotension alone reduced CBF, with greater effect in the subendocardium. Additional hemodilution returned CBF to normotensive value, but relative subendocardial hypoperfusion persisted. The present study provides fundamental information on effects of hemodilution on coronary hemodynamics without the systemic responses that complicated previous studies utilizing whole body exchange transfusions.