Normal P Wave Height in ECG Leads
The normal P wave amplitude should be ≤2.5 mm in the limb leads (particularly leads II, III, and aVF) and ≤1.5 mm in the precordial leads (particularly V1 and V2), with P waves exceeding these thresholds indicating atrial abnormality. 1
Standard Normal Values by Lead
Limb Leads
- Lead II, III, and aVF: P wave amplitude should be ≤2.5 mm 1
- A P wave >2.5 mm in lead II is characteristic of right atrial abnormality and represents a specific threshold for pathology 1, 2
- The normal P wave axis ranges from 0° to 90° in the frontal plane 3
Precordial Leads
- Lead V1: P wave amplitude should be ≤1.5 mm for the initial positive component 1, 2
- Prominent initial positivity of the P wave in V1 or V2 (≥1.5 mm or 0.15 mV) indicates right atrial abnormality 1, 2
- Lead V1 typically shows a biphasic morphology in normal individuals 3
P Wave Duration
- Normal P wave duration: <120 ms (less than 3 small boxes on standard ECG paper) 1, 4
- P wave duration ≥120 ms indicates left atrial abnormality or intraatrial conduction delay 1, 4
Clinical Context for Abnormal P Waves
Right Atrial Abnormality
When P waves exceed normal height thresholds, this suggests right atrial abnormality, characterized by:
- Tall, peaked P waves in lead II (>2.5 mm) with pointed appearance 1, 2
- Prominent initial positivity in V1 or V2 (≥1.5 mm) 1, 2
- Rightward shift of P-wave vector 1, 2
- Normal P wave duration (distinguishing it from left atrial abnormality) 1, 2
Left Atrial Abnormality
In contrast, left atrial abnormality presents with:
- P wave duration ≥120 ms 1, 4
- Double-peaked or notched P wave (≥40 ms between peaks) 1, 4
- Negative terminal component in V1 (P terminal force) 1, 4
- Amplitude may be normal, but duration is prolonged 1
Important Clinical Pitfalls
Common mistake: Confusing increased P wave amplitude with increased duration. Right atrial abnormality increases amplitude with normal duration, while left atrial abnormality increases duration with variable amplitude 1, 2.
Terminology: Use "atrial abnormality" rather than "enlargement," "overload," "strain," or "hypertrophy" when describing ECG findings, as recommended by the American College of Cardiology 1, 4, 2.
Prognostic significance: In patients with pulmonary arterial hypertension, a P-wave amplitude in lead II of ≥0.25 mV (2.5 mm) is associated with a 2.8-fold greater risk of death over 6 years 1.
Measurement Technique
- Measure P wave amplitude from the isoelectric baseline to the peak of the P wave 1
- Use standard calibration (10 mm = 1 mV) 1
- Assess P wave morphology in multiple leads for accurate diagnosis 1, 4
- The vertical sternal lead system may provide larger P wave area for better detection in ambulatory monitoring 5