2.1 Cardiac Ischaemia

Definition and Description of Stable coronary artery disease (SCAD)

• SCAD is a form of chronic coronary artery disease, characterised by fixed atherosclerotic plaques in the epicardial coronary arteries

  These plaques cause progressive narrowing of the coronary vessels, restricting blood flow during periods of increased demand.

• Also referred to in recent guidelines as part of the Chronic Coronary Syndromes (CCS)

• SCAD presents with predictable angina during exertion and is relieved by rest or nitrates.

Symptoms of Angina: Pathological and pathophysiological basis

• Angina results from myocardial ischemia, where oxygen demand exceeds oxygen supply to the myocardium

• Pathology involves:

  • Fixed narrowing of coronary arteries from atherosclerotic plaques

  • Endothelial dysfunction, reducing vasodilator availability and promoting vasoconstriction.

• Pathophysiological features:

  • Angina typically manifests as constrictive chest pain, radiating to jaw, neck, or arm.

  • Triggered by exertion or stress (increased demand)

  • Relieved by rest (reduced demand) or nitrates (vasodilation)

Oxygen supply-demand mismatch in cardiac myocytes

• O2 supply depends on coronary blood flow, which may be compromised by:

  • Stenosis severity (length, diameter reduction).

  • Impaired vasodilation (blood vessels, particularly arteries, cannot dilate properly in response to signals that would normally cause them to) due to endothelial dysfunction

• O2 demand increases with heart rate, wall stress, or contractility

• In SCAD, the narrowed vessels can’t increase flow adequately during exertion, leading to relative ischemia (oxygen supply to a tissue is insufficient to meet its increased demand, even though baseline blood flow might still be present).

Anaerobic metabolism in ischemia and pain mechanism

• In ischemia, oxidative phosphrylation is impaired due to restricted O2.

• The myocardium shifts to anaerobic glycolysis (glucose is broken down into pyruvate to produce ATP without O2)

  • Increases glucose use, lactate production

  • Decreases ATP production efficiency

• Byproducts of anaerobic metabolism:

  • Lactic acid, H+ ions, adenosine accumulate

  • These metabolites stimulate nociceptive nerve endings, triggering anginal pain (chest pain or discomfort that occurs when the heart muscle

Dermatomal distribution (C3-T5) and referred cardiac pain

• Visceral afferents from the heart enter the spinal cord at C3-T5 levels

• These segments also receive somatic input from:

  • Neck, jaw (C3-C5)

  • Chest and inner arms (T1-T5)

• The CNS may misinterpret cardiac pain as originating from these areas, leading to referred pain:

  • Common sites: left arm, jaw, neck, shoulder, epigastrium.