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 Physiological Determinants

Pressure Gradient Components

• Perfusion pressure: Normally equals diastolic coronary pressure, which is calculated as:
  • Aortic pressure minus left ventricular diastolic pressure or central venous pressure 1
  • In normal coronary arteries, aortic pressure is transmitted completely to distal regions without appreciable pressure loss 1

Flow Dynamics in Coronary Stenosis

When coronary stenosis is present, several factors affect perfusion:

1. Pressure drop across stenosis: Determined by:

   • Viscous friction along the entrance of a lesion (increases linearly with flow - Poiseuille's Law)
   • Convective acceleration along the lesion (increases with the square of flow - Bernoulli's Law) 1
   • Flow separation and eddy formation at the stenosis exit 1

2. Stenosis characteristics:

   • Severity of stenosis affects the pressure gradient
   • Visual assessment of stenosis severity correlates poorly with physiological significance 1
   • Quantitative coronary analysis (QCA) measurements are often discordant with the functional significance of lesions 1

Autoregulation and Coronary Flow Reserve

Coronary flow is normally autoregulated to maintain constant blood flow despite changes in perfusion pressure:

• Within normal physiological blood pressure range, there is an approximately linear relationship between perfusion pressure and blood flow when coronary resistance is minimized 1
• As perfusion pressure falls, coronary arterioles dilate to maintain flow 2
• Under basal conditions, a five-fold increase in coronary flow can occur (flow reserve of five) 2
• In the presence of an obstructive epicardial coronary stenosis, perfusion pressure is reduced, leading to compensatory vasodilation 1

Clinical Implications

Impact of Pathological Conditions

• Coronary flow reserve is markedly impaired in the presence of:

  • Severe coronary artery stenosis
  • Left ventricular hypertrophy (LVH) 2

• When both severe stenosis and LVH are present, a fall in perfusion pressure (diastolic blood pressure) can result in:

  • Decreased coronary flow
  • ECG changes
  • Ventricular dysfunction 2

Measurement of Functional Significance

• Fractional Flow Reserve (FFR) is used to assess the physiological significance of coronary stenosis:
  • FFR = distal coronary pressure/aortic pressure (measured during maximal hyperemia) 1
  • Normal FFR value is 1.0 for every patient and coronary artery 1
  • FFR ≤ 0.75 is associated with inducible ischemia (specificity 100%)
  • FFR ≥ 0.80 indicates absence of inducible ischemia in most patients (sensitivity 90%) 1

Common Pitfalls and Caveats

1. Assessing stenosis severity:

   • Visual assessment of stenosis severity is subjective and correlates poorly with physiological significance 1
   • Even quantitative coronary analysis may not accurately reflect the functional impact of a lesion 1

2. Coronary Flow Reserve limitations:

   • CFR is affected by both epicardial and microvascular components
   • CFR can be altered by changes in basal or hyperemic flow due to hemodynamics, loading conditions, and contractility 1
   • Tachycardia increases basal flow and decreases hyperemic flow, reducing CFR by 10% for each 15-beat increase in heart rate 1

3. Clinical considerations:

   • In patients with coronary artery disease and LVH, lowering diastolic blood pressure below the low-mid 80s may increase risk of myocardial infarction (J-curve relationship) 2
   • In advanced heart failure, low coronary perfusion pressure (<40 mmHg) is associated with adverse outcomes 3

Understanding these determinants of coronary perfusion is essential for proper assessment and management of patients with suspected or known coronary artery disease, particularly when making decisions about revascularization based on the functional significance of coronary lesions.