What are the ECG (electrocardiogram) changes associated with hypokalemia (low potassium levels)?

ECG Changes of Hypokalemia

Hypokalemia produces characteristic ECG changes including T-wave flattening, ST-segment depression, and prominent U waves, which can progress to life-threatening arrhythmias including ventricular tachycardia, torsades de pointes, ventricular fibrillation, and cardiac arrest. 1

Progressive ECG Changes Based on Severity

Hypokalemia produces a series of ECG changes that correlate with the severity of potassium depletion:

Mild Hypokalemia (3.0-3.5 mEq/L)

• T-wave flattening and decreased amplitude
• Early ST-segment depression
• Beginning of U-wave prominence

Moderate Hypokalemia (2.5-2.9 mEq/L)

• More pronounced T-wave flattening
• ST-segment depression (can mimic myocardial ischemia)
• Prominent U waves
• PR interval prolongation may begin

Severe Hypokalemia (<2.5 mEq/L)

• U-wave amplitude may exceed T-wave amplitude
• Significant ST-segment depression
• Prolonged PR interval
• Increased P wave amplitude
• Increased risk of significant arrhythmias 2, 1

Arrhythmias Associated with Hypokalemia

Hypokalemia creates an arrhythmogenic substrate through multiple electrophysiologic mechanisms:

• Supraventricular arrhythmias:

  • Atrial fibrillation
  • First or second-degree atrioventricular block

• Ventricular arrhythmias:

  • Premature ventricular contractions (PVCs)
  • Ventricular tachycardia
  • Torsades de pointes
  • Ventricular fibrillation
  • Cardiac arrest 2, 1

Clinical Implications

• ECG changes may be the first indicator of hypokalemia, even before symptoms appear
• Hypokalemia-induced ECG changes can mimic myocardial ischemia, creating "pseudoischemic" patterns 3
• The risk of arrhythmias increases substantially as potassium levels decrease
• Patients with heart failure should maintain potassium levels of at least 4 mEq/L 2

Monitoring Recommendations

Continuous cardiac monitoring is recommended for:

• Patients with moderate to severe hypokalemia (<3.0 mEq/L)
• Patients with cardiac disease
• Patients on digoxin (increased risk of digitalis toxicity)
• Patients with symptoms of hypokalemia 1

Important Considerations

• Check magnesium levels, as hypomagnesemia can exacerbate hypokalemia and its ECG manifestations 1
• Rapid correction of hypokalemia can cause cardiac arrhythmias
• Pseudonormalization of ECG changes during treatment can create a false sense of security 1
• ECG abnormalities are common in hypokalemic patients (present in approximately 40% of patients with K+ <3.5 mmol/L) 4

Pitfalls to Avoid

• Do not rely solely on ECG changes for diagnosis - confirm with serum potassium levels
• Do not overlook concurrent electrolyte abnormalities, especially hypomagnesemia
• Do not correct severe hypokalemia too rapidly - controlled infusion at 0.25 mmol/kg/hour is recommended for safety 5
• Do not assume that normalization of ECG changes means complete correction of potassium deficit - prolonged supplementation may be required 3

Recognizing these characteristic ECG patterns is crucial for early detection and treatment of hypokalemia, preventing potential morbidity and mortality from cardiac arrhythmias.