In an electrocardiogram, the deflection that represents atrial depolarization is the:

• A. QRS complex
• B. T wave
• C. P wave
• D. U wave

Answer: (C)

Atrial depolarization is represented by the P wave on the electrocardiogram. It’s the small, rounded deflection that appears before the QRS complex and reflects the electrical activation of the atria as the impulse from the sinoatrial node spreads through the atrial myocardium, triggering atrial contraction. The QRS complex that follows represents ventricular depolarization, which is larger and sharper due to the greater muscle mass of the ventricles, while the T wave reflects ventricular repolarization. The U wave, when present, is a more variable, less consistent deflection.