What are the EKG findings in hypokalemia?

EKG Findings of Hypokalemia

The classic EKG findings of hypokalemia include prominent U waves, T-wave flattening or broadening, ST-segment depression, and QT interval prolongation, with these changes becoming progressively more pronounced as potassium levels decline below 3.5 mEq/L. 1, 2

Primary EKG Manifestations

The characteristic electrocardiographic changes in hypokalemia appear in a concentration-dependent manner:

Classic Findings (Mild to Moderate Hypokalemia: 2.5-3.5 mEq/L)

• T-wave flattening or broadening is one of the earliest manifestations 1, 2
• ST-segment depression with a characteristic "sagging" appearance 1, 3
• Prominent U waves (>1 mm), particularly visible in leads V2 and V3 1, 2, 4
• A U wave >0.5 mm in lead II or >1.0 mm in lead V3 is considered abnormal 1
• U waves larger than the T wave in the same lead indicate significant hypokalemia 5

Severe Hypokalemia Findings (<2.5 mEq/L)

• Giant U waves that can merge with T waves, creating a notched appearance on the T-wave upstroke 3
• This T-U wave fusion can be mistaken for QT prolongation or ischemic changes 6, 3
• PR interval prolongation 4
• Increased P-wave amplitude 4
• The T wave may become merely a notch on the upstroke of a giant U wave at potassium levels below 2.5 mEq/L 3

Associated Arrhythmias

Hypokalemia predisposes to multiple arrhythmias through increased automaticity and reentrant mechanisms:

• Premature ventricular contractions (PVCs) 2, 5
• Atrial fibrillation 7, 2
• First or second-degree atrioventricular block 7, 2
• Ventricular tachycardia 2, 5
• Torsades de pointes 2
• Ventricular fibrillation and cardiac arrest in severe cases 2, 5
• Atrial premature complexes are common 3

Clinical Correlation and Severity

The severity of EKG changes correlates with potassium levels:

• 3.0-3.5 mEq/L (mild): One or more characteristic findings may be present 3
• 2.5-2.9 mEq/L (moderate): Multiple findings typically present 1
• <2.5 mEq/L (severe): All three classic findings (ST depression, T-wave flattening, prominent U waves) are common 1, 3

Important Clinical Pitfalls

Pseudoischemic Changes

A critical pitfall is mistaking hypokalemia-induced EKG changes for myocardial ischemia. The ST-segment depression and apparent QT prolongation (actually T-U wave fusion) can mimic ischemic patterns 6, 3. The key distinguishing feature is the presence of prominent U waves, which are not typical of ischemia.

Coexisting Hypomagnesemia

Hypomagnesemia frequently coexists with hypokalemia and can exacerbate cardiac effects, including QT prolongation and increased risk of torsades de pointes 1, 2. Failure to check and correct magnesium levels may lead to treatment resistance 2.

Increased Digitalis Toxicity Risk

Patients taking digoxin have significantly increased risk of digitalis toxicity even with mild hypokalemia, making EKG monitoring particularly critical in this population 7, 2.

Monitoring Recommendations

Continuous ECG monitoring is recommended for patients with moderate to severe hypokalemia (<3.0 mEq/L) and for any patient with hypokalemia who demonstrates EKG abnormalities, regardless of potassium level. 1, 2

Additional high-risk scenarios requiring continuous monitoring include:

• Patients with cardiac comorbidities 1
• Patients on digoxin 7, 2
• Heart failure patients (who should maintain potassium ≥4.0 mEq/L) 1, 2

Electrophysiologic Mechanisms

Hypokalemia increases resting membrane potential, prolongs action potential duration, increases the refractory period disproportionately to action potential duration, increases threshold potential, increases automaticity, and decreases conductivity—all of which create conditions favorable for both reentrant and automatic arrhythmias 5.