ECG Findings in Hypokalemia
In a patient with hypokalemia, the ECG will most likely show ST-segment depression, along with T-wave flattening, prominent U waves, and QT interval prolongation. 1
Characteristic ECG Changes in Hypokalemia
Hypokalemia produces a predictable sequence of electrocardiographic abnormalities that directly result from altered ventricular repolarization. ST-segment depression is a primary repolarization abnormality caused by hypokalemia's effect on the plateau phase of the ventricular action potential. 2 This occurs because low potassium levels fundamentally change how ventricular myocytes repolarize, creating the characteristic depression pattern seen on ECG. 2
The complete spectrum of ECG findings in hypokalemia includes:
- ST-segment depression (horizontal or downsloping) 2, 1
- T-wave flattening or broadening 1, 3
- Prominent U waves (>1 mm, particularly in leads V2-V3) 1, 3
- QT interval prolongation 1
- PR interval prolongation 3
- Increased P-wave amplitude 3
Why ST Depression Occurs (Not the Other Options)
ST-segment elevation is NOT a feature of hypokalemia—it occurs with acute myocardial infarction, pericarditis, and other conditions involving injury currents between ischemic and non-ischemic zones. 2 Hypokalemia causes ST depression, not elevation. 2, 1
Peaked T waves are characteristic of hyperkalemia, not hypokalemia. 1 In hypokalemia, T waves become flattened or inverted, which is the opposite morphology. 1, 3 This is a critical distinction that prevents dangerous treatment errors.
While QT prolongation does occur in hypokalemia 1, it is not the most prominent or earliest finding. ST-segment depression and U-wave prominence typically appear first and are more diagnostically specific. 1, 3
Severity Correlation
The severity of ECG abnormalities correlates with the degree of hypokalemia:
- Mild hypokalemia (3.0-3.5 mEq/L): Minimal changes, possibly subtle T-wave flattening 1
- Moderate hypokalemia (2.5-2.9 mEq/L): ST-segment depression, T-wave flattening, prominent U waves 1, 4
- Severe hypokalemia (<2.5 mEq/L): Marked ST depression, pronounced U waves, QT prolongation, risk of life-threatening arrhythmias 1, 3
In a multicenter cohort study of 79,599 patients, ECG abnormalities were present in 40% of hypokalemic patients, with T-wave flattening occurring in 27%, ST-segment depression in 16%, and QTc prolongation in 14%. 4 These findings demonstrate that ST-segment depression is one of the most common and clinically significant ECG manifestations of hypokalemia. 4
Clinical Significance and Arrhythmia Risk
Hypokalemia-induced ECG changes indicate increased risk of ventricular arrhythmias, including ventricular tachycardia, torsades de pointes, and ventricular fibrillation. 1, 5, 6 The combination of ST-segment depression with prominent U waves is particularly concerning because it reflects significant potassium depletion affecting cardiac repolarization. 1, 3
The arrhythmogenic mechanism involves:
- Increased resting membrane potential 6
- Prolonged action potential duration 6
- Increased automaticity 6
- Decreased conductivity 6
- Enhanced refractory period creating substrate for reentrant arrhythmias 6
Important Clinical Caveat
Severe hypokalemia with marked ST-segment depression can mimic myocardial ischemia, particularly when accompanied by chest discomfort. 7 A detailed history (looking for diuretic use, vomiting, diarrhea, or other causes of potassium loss) and serum potassium measurement are essential to avoid misdiagnosis. 7 The ST-segment abnormalities will resolve with potassium correction, unlike true ischemic changes. 7
Any patient with potassium <2.5 mEq/L AND ECG changes requires immediate aggressive treatment with continuous cardiac monitoring due to extreme risk of life-threatening arrhythmias. 8