Calcium Ions and Cardiac Electrophysiology: Role in Heart Rate Stability

Calcium (Ca²⁺) is the pivotal intracellular messenger linking electrical excitation to mechanical contraction in the heart (excitation-contraction coupling). Its precise regulation maintains rhythmic, stable heartbeats.

Cellular Mechanism

1. Action potential (AP) plateau (Phase 2): L-type Ca²⁺ channels open → Ca²⁺ influx (I_Ca,L).
2. Calcium-induced calcium release (CICR): Trigger Ca²⁺ binds ryanodine receptors (RyR2) on sarcoplasmic reticulum (SR) → massive Ca²⁺ release → contraction.
3. Relaxation: SERCA pumps Ca²⁺ back into SR; NCX extrudes Ca²⁺ from cell; PMCA minor role.

Effect of Ca²⁺ Concentration on Heart Rate & Rhythm

• Normal [Ca²⁺]ₑ (2.2–2.6 mM ionized): Stable AP duration, regular rhythm, normal contractility.
• Hypocalcemia (↓ ionized Ca²⁺): Prolongs AP plateau (Phase 2) → QT prolongation → risk of torsades de pointes. May cause bradycardia, reduced contractility.
• Hypercalcemia (↑ ionized Ca²⁺): Shortens AP plateau → QT shortening. Can cause bradycardia, AV block, arrhythmias. Severe: cardiac arrest in systole.

Clinical Context

• Corrected calcium = measured Ca + 0.8 × (4.0 – albumin). Ionized Ca²⁺ is physiologically active.
• Conditions: hypoparathyroidism, vitamin D deficiency, renal failure, massive transfusion (citrate), pancreatitis → hypocalcemia. Hyperparathyroidism, malignancy, thiazides, vitamin D excess → hypercalcemia.
• Digoxin toxicity risk ↑ with hypercalcemia; hypocalcemia protects against digoxin arrhythmias.

Key Takeaway

Calcium homeostasis is essential for electrical stability. Both extremes are pro-arrhythmic. Monitoring ionized calcium is critical in critical illness, renal failure, and massive transfusion.