Negative P Wave in Lead I

A negative P wave in lead I should always prompt a question about proper lead placement. About 99% of the time, a negative P wave in lead I is due to false poling of the precordial leads. A negative P wave in lead 1 may represent a left atrial rhythm anomaly 1% of the time.

Right Atrial Enlargement Secondary to Chronic Pulmonary Hypertension

Pulmonary hypertension can be caused by a chronic pulmonary embolism where the embolus has restricted the blood flow in the pulmonary arteries, causing an increase in pulmonary artery blood pressure. COPD can also cause chronic pulmonary hypertension. 

P waves with abnormally increased amplitude are the associated ECG findings with chronic pulmonary hypertension. The P wave amplitude is commonly increased the most in lead II (especially when compared to leads I and III). Some of the precordial leads may also exhibit an increase in P wave amplitude.

Atrial Infarction

Atrial infarction is rare. Its presence induces a deformed P wave with a positive or negative segment in between the end of the P wave and the Q wave.

Rare Atrial Enlargement

Right atrial overload is seen the following: 

• Tricuspid atresia – negative P waves in V₁. 
• Ebstein anomaly – giant P waves in the inferior leads II, III, and aVF; abnormalities in the QRS complex and repolarization

A left atrial overload can mimic P pulmonale if the action potential from the left atrium projects into the inferior leads. The second hump of the biphasic P wave is higher than the first part. In this case, a slightly accentuated terminal P-negativity in V₁ can help distinguish left atrial overload from P pulmonale.