Understanding the P Wave: The First Electrical Impulse of the Heart

The P wave on an electrocardiogram (ECG) represents atrial depolarization – the electrical activation of the atria.

Origin & Pathway

• Sinoatrial (SA) node: Primary pacemaker (right atrial wall near SVC); spontaneous phase 4 depolarization → action potential.
• Atrial conduction: Spreads via Bachmann's bundle (interatrial) and internodal tracts to AV node.
• Timing: Atrial depolarization lasts ~80–110 ms; PR interval (SA node to ventricular depolarization) = 120–200 ms.

ECG Characteristics

• Amplitude: <0.25 mV (2.5 mm) in limb leads.
• Duration: <120 ms (3 small squares).
• Morphology: Upright in I, II, aVF; biphasic in V1 (initial +, terminal –); inverted in aVR.
• Axis: Normal 0° to +75°.

Clinical Significance

• Absent P waves: Atrial fibrillation, junctional rhythm, hyperkalemia.
• Abnormal morphology: Left atrial enlargement (broad, notched "P mitrale"), right atrial enlargement (tall, peaked "P pulmonale"), ectopic atrial rhythm.
• PR interval: Short (WPW), long (AV block), variable (2nd degree AV block).

The P wave is the ECG signature of atrial electrical activation, initiating each cardiac cycle.