Early ECG Changes in Hypokalemia
The correct answer is A: Depressed ST segment + prolonged PR, prominent U waves, and flat T waves represent the early ECG changes in hypokalemia. 1
Classic ECG Manifestations
The early electrocardiographic changes in hypokalemia follow a characteristic pattern that includes:
- ST-segment depression is one of the hallmark early findings 1, 2
- T-wave flattening (flat T waves) occurs as potassium levels decline 1, 2
- Prominent U waves (>1 mm), particularly visible in leads V2 and V3, are highly characteristic of hypokalemia 1
- PR interval prolongation can occur along with increased P wave amplitude 3
These changes are often best visualized in the mid-precordial leads (V2-V4) 3.
Why Other Options Are Incorrect
Option B (Elevated T wave) is incorrect because hypokalemia causes T-wave flattening, not elevation. Elevated or peaked T waves are characteristic of hyperkalemia, not hypokalemia 1
Option C (P wave prominent) alone is incomplete. While P wave amplitude can increase in hypokalemia, this is not the primary or most characteristic early finding 3
Option D (T wave sagging) is imprecise terminology. The correct description is T-wave flattening or broadening, not "sagging" 1
Clinical Significance of U Waves
The American College of Cardiology defines abnormal U waves as:
U waves appearing larger than the T wave in the same lead (with associated ST-segment depression) are particularly suggestive of hypokalemia 4.
Progression of ECG Changes
As hypokalemia worsens, additional changes develop:
- QT interval prolongation becomes more apparent 1
- Increased risk of ventricular arrhythmias, including PVCs, ventricular tachycardia, and potentially ventricular fibrillation 2
- First or second-degree AV block may occur in more severe cases 2
Common Pitfall
A critical error is confusing hypokalemia with hyperkalemia ECG changes. Remember that peaked T waves indicate hyperkalemia, while flat T waves with prominent U waves indicate hypokalemia 1. This distinction is essential for appropriate emergency management.