What are the characteristic ECG (electrocardiogram) findings in a patient with hypokalemia (low potassium levels)?

ECG Findings in Hypokalemia

The characteristic ECG findings of hypokalemia include prominent U waves (especially in leads V2-V4), ST-segment depression, T-wave flattening or inversion, and QT interval prolongation, with these changes becoming progressively more pronounced as potassium levels decline below 3.5 mEq/L. 1

Primary ECG Manifestations

U Waves

• Prominent U waves (>1 mm) are the hallmark finding of hypokalemia, particularly visible in the mid-precordial leads V2-V4 1, 2
• A U wave >0.5 mm in lead II or >1.0 mm in lead V3 is considered abnormal and suggestive of hypokalemia 1
• U waves larger than the T wave in the same lead, with associated ST-segment depression, are diagnostic criteria for hypokalemia 3
• At severe hypokalemia levels (<2.5 mEq/L), the T-wave can become a notch on the upstroke of a giant U-wave, sometimes mistaken for ischemic changes 4

ST-Segment Changes

• ST-segment depression occurs as a primary repolarization abnormality due to altered plateau phase of the ventricular action potential 1
• "Sagging" ST-segments are characteristic, distinct from ischemia-related ST changes 4
• These changes are most prominent in the mid-precordial leads 2

T-Wave Abnormalities

• T-wave flattening is common, occurring in approximately 27% of hypokalemic patients 5
• T-wave broadening and inversion can occur 1, 5
• Dynamic changes in T-wave morphology are characteristic 2

QT Interval Prolongation

• QT interval prolongation occurs in approximately 14% of hypokalemic patients and increases the risk of ventricular arrhythmias 1, 5

Additional ECG Findings

Conduction Changes

• PR interval prolongation can occur 2
• Increased amplitude of the P wave may be present 2
• QRS widening is not typical of hypokalemia (this is more characteristic of hyperkalemia) 1

Arrhythmias

• Atrial premature complexes are common 4
• First or second-degree atrioventricular block can develop 6
• Atrial fibrillation may occur 6, 4
• Ventricular arrhythmias including PVCs, ventricular tachycardia, torsades de pointes, and ventricular fibrillation can develop, particularly in severe cases 6

Severity-Based ECG Patterns

Mild Hypokalemia (3.0-3.5 mEq/L)

• One or more of the characteristic findings (U waves, ST depression, T-wave changes) may be present 4
• T-wave flattening and ST-segment depression are early findings 6

Moderate Hypokalemia (2.5-2.9 mEq/L)

• Multiple ECG abnormalities typically present 1
• Increased risk of arrhythmias 1

Severe Hypokalemia (<2.5 mEq/L)

• All three classic findings (prominent U waves, ST depression, T-wave flattening) are commonly present 4
• Giant U-waves with T-wave notching may occur 4
• High risk of life-threatening ventricular arrhythmias 2

Clinical Significance and Monitoring

Risk Stratification

• ECG abnormalities are present in approximately 40% of hypokalemic patients 5
• Patients with ECG abnormalities, cardiac disease, heart failure, or those on digoxin require emergency evaluation even with mild hypokalemia 6
• Continuous cardiac monitoring is recommended for patients with moderate to severe hypokalemia, abnormal 12-lead ECG findings, or cardiac comorbidities 1

Important Caveats

• Individual variability exists in ECG manifestations, requiring careful clinical correlation 1
• ECG changes can mimic myocardial ischemia, particularly the combination of ST-segment depression and apparent QT prolongation (which may actually be a prominent U-wave) 4, 7
• ECG abnormalities completely resolve with potassium replacement 4, 7
• The presence of hypomagnesemia (which frequently coexists) can contribute to QT prolongation and increase the risk of torsades de pointes 1, 6

Target Potassium Levels

• Heart failure patients should maintain potassium ≥4.0 mEq/L to prevent cardiac arrhythmias 1, 6
• Post-cardiac arrest patients should maintain potassium between 4.0-4.5 mmol/L to prevent ventricular arrhythmias 6