What determines coronary artery perfusion?

Determinants of Coronary Artery Perfusion

Coronary artery perfusion is primarily determined by the difference between aortic diastolic pressure and left ventricular diastolic pressure (or central venous pressure), which creates the coronary perfusion pressure gradient. 1

Key Determinants of Coronary Perfusion

1. Coronary Perfusion Pressure

• Coronary perfusion occurs almost exclusively during diastole, making diastolic blood pressure the primary determinant of coronary perfusion pressure 1
• The perfusion pressure gradient is calculated as:
  • Aortic diastolic pressure minus left ventricular end-diastolic pressure (LVEDP) 1, 2
  • This gradient drives blood flow through the coronary arteries

2. Coronary Vascular Resistance

Resistance is determined by several factors:

• Blood viscosity 2
• Anatomy and geometry of the coronary vascular bed 2
• Epicardial vessel stenosis (R1 resistance) 3
• Small arteries and arterioles (R2 resistance) 3
• Intramyocardial capillary system (R3 resistance) 3

3. Coronary Autoregulation

• Coronary circulation demonstrates autoregulation, where a fall in perfusion pressure triggers coronary vasodilation to maintain constant blood flow 1
• The autoregulatory capacity has limits - when maximal vasodilation occurs, further decreases in perfusion pressure will reduce coronary flow 1
• In patients with coronary artery disease, the lower autoregulatory limit is shifted upward, requiring higher perfusion pressures to maintain adequate flow 1

4. Myocardial Oxygen Demand

• Increased myocardial oxygen demand leads to coronary vasodilation and increased blood flow 3, 4
• Factors affecting myocardial oxygen demand:
  • Heart rate (tachycardia increases basal flow and decreases hyperemic flow) 1
  • Contractility
  • Wall tension (affected by preload and afterload)
  • Systolic blood pressure (increases myocardial oxygen demand) 1, 4

5. Coronary Flow Reserve (CFR)

• CFR represents the capacity to increase coronary blood flow above baseline levels during maximal vasodilation 3
• Normal CFR is approximately 3-5 fold above baseline in healthy adults 3, 5
• CFR is reduced by:
  • Coronary stenosis 3, 5
  • Left ventricular hypertrophy 5
  • Microvascular disease 3
  • Tachycardia (reduces CFR by 10% for each 15-beat increase in heart rate) 3, 1

Clinical Implications

Subendocardial Vulnerability

• The subendocardial layers of the left ventricle are more vulnerable to ischemia 2
• These layers depend entirely on diastolic coronary flow 2
• In conditions with elevated LVEDP or reduced diastolic pressure, subendocardial perfusion is compromised first

Impact of Arterial Stiffness

• Wide pulse pressure reflects significant arterial stiffness, which increases cardiac workload 1
• High systolic pressure increases myocardial oxygen demand while low diastolic pressure potentially compromises coronary perfusion 1

Pharmacological Considerations

• Nitroglycerin dilates both arterial and venous beds, with venous effects predominating 4
• While nitroglycerin reduces blood pressure, effective coronary perfusion pressure is usually maintained 4
• However, excessive blood pressure reduction or increased heart rate that decreases diastolic filling time can compromise coronary perfusion 4

Pitfalls and Caveats

1. Relying solely on anatomical assessment of coronary stenoses can be misleading, as functional significance varies widely for stenoses between 50-90% 1

2. In patients with coronary artery disease and left ventricular hypertrophy, excessive lowering of diastolic blood pressure (below mid-80s mmHg) may increase risk of myocardial infarction due to reduced coronary perfusion pressure 5

3. Rapid reduction in diastolic BP may be more hazardous than gradual reduction in patients with combined hypertension and coronary artery disease 1

4. During cardiopulmonary resuscitation, coronary perfusion depends on maintaining an adequate aortic-to-right atrial pressure gradient during "diastole" 6