Electrolytes essential for myocardial rhythm and contractility: 1 Calcium; 2 Potassium; 3 Magnesium; 4 Sodium. Which combination is correct?

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Electrical stability and the heart’s ability to contract depend mainly on calcium, potassium, and magnesium. Calcium entry into cardiac muscle triggers the contraction process: it initiates calcium-induced calcium release from the sarcoplasmic reticulum, increases intracellular calcium, and allows cross-bridge cycling for strong contraction. Potassium sets the resting membrane potential and drives repolarization; disturbances in potassium balance disrupt the timing of electrical impulses and can cause arrhythmias. Magnesium acts as a stabilizer, influencing ion channel function and serving as a cofactor for enzymes like Na+/K+-ATPase, helping to prevent excessive excitability and torsades de pointes; it also modulates calcium handling and helps maintain regular rhythm. Sodium, while essential for the initiation and propagation of the action potential, is not the primary regulator of contractile force in the same direct way as the others. Together, calcium, potassium, and magnesium most directly govern myocardial rhythm and contractility.