Angina Pectoris: Why Pain Radiates to the Left Arm + Full Pathophysiology

1. What is Angina Pectoris?

Robbins Pathologic Basis of Disease, p. 1152-1154

2. Why Does Pain Radiate to the Left Arm? (Referred Pain Mechanism)

A. Embryological / Dermatomal Convergence Theory (Guyton & Hall)

"The reason for this distribution of pain is that during embryonic life, the heart originates in the neck, as do the arms. Therefore, both the heart and these surface areas of the body receive pain nerve fibers from the same spinal cord segments (C8-T5)."

• Guyton and Hall Textbook of Medical Physiology, p. 274

B. Convergence-Projection Theory (Ganong / Harrison's)

"The basis for referred pain may be convergence of somatic and visceral pain fibers on the same second-order neurons in the dorsal horn that project to the thalamus and then to the somatosensory cortex. When the visceral stimulus is prolonged, facilitation of activity from the somatic fiber endings occurs... the brain cannot determine whether the stimulus came from the viscera or from the area of referral."

• Ganong's Review of Medical Physiology, p. 1199

Step-by-Step Pathway of Referred Pain in Angina:

3. Full Pathophysiology of Angina Pectoris

Underlying Cause: Supply-Demand Mismatch

• Atherosclerotic coronary artery stenosis (most common) - fixed narrowing reduces baseline perfusion reserve
• Coronary vasospasm (Prinzmetal/variant angina)
• Thrombosis (unstable angina)
• Anaemia or hypoxia reducing oxygen-carrying capacity

• Physical exertion (increases heart rate, stroke volume, contractility)
• Emotional stress - sympathetic activation
• Cold exposure - peripheral vasoconstriction raises afterload
• Full stomach - vagal stimulation + increased cardiac work
• Tachycardia of any cause - reduces diastolic perfusion time

Sequence of Events (Ischemic Cascade):

Coronary stenosis / spasm
        ↓
Reduced subendocardial blood flow (subendocardium most vulnerable - highest wall tension)
        ↓
O2 supply < O2 demand
        ↓
Anaerobic glycolysis → lactic acid accumulation → intracellular acidosis
        ↓
ATP depletion → failure of Na+/K+ ATPase → cellular K+ loss
        ↓
Diastolic dysfunction (impaired relaxation) FIRST, then systolic dysfunction
        ↓
ECG changes: ST depression (subendocardial ischemia) or ST elevation (transmural)
        ↓
Release of adenosine, bradykinin, histamine, kinins → stimulate cardiac nociceptors
        ↓
Pain: substernal + referred to left arm/jaw/neck
        ↓
If ischemia resolves (rest / nitroglycerin) → NO necrosis (angina)
If ischemia persists (>20-40 min) → myocyte death → MI

Chemical Mediators of Anginal Pain:

Robbins Pathologic Basis of Disease: "Anginal pain is a consequence of ischemia-induced release of adenosine, bradykinin, and other molecules that stimulate sympathetic and vagal afferent nerves."

4. Three Patterns of Angina Pectoris

Robbins Pathologic Basis of Disease, p. 1155-1160

5. Clinical Features Summary

• Location: Substernal / retrosternal, described as pressure, heaviness, or squeezing
• Radiation: Left arm (especially ulnar aspect of forearm and hand), left jaw, neck, back, interscapular region - NOT below umbilicus or above mandible
• Duration: Typically 2-5 minutes (stable); up to 15-20 minutes
• Levine's sign: Patient places clenched fist over sternum when describing pain
• Precipitants: Exertion, emotion, cold, heavy meal
• Relief: Rest within 1-5 min; sublingual nitroglycerin within minutes

Harrison's Principles of Internal Medicine, 22nd Ed., p. 2136

Summary - Why Left Arm?

1. Embryological origin - the heart and arm develop from the same cervical segments (C8-T5), so they share the same spinal nerve supply
2. Convergence-projection - cardiac visceral pain fibers and arm somatic pain fibers converge onto the same dorsal horn neurons (T1-T4); the brain, having much more experience with arm pain than heart pain, mislocates the source to the arm