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

ECG Changes in Hypokalemia

Hypokalemia produces characteristic ECG changes including T-wave flattening, ST-segment depression, and prominent U waves, which can progress to dangerous ventricular arrhythmias if left untreated. 1

Definition and Classification

• Hypokalemia is defined as serum potassium level <3.5 mEq/L 1
• Severity classification:
  • Mild: 3.0-3.5 mEq/L
  • Moderate: 2.5-2.9 mEq/L
  • Severe: <2.5 mEq/L 1

Characteristic ECG Changes

• Primary ECG manifestations:

  • Flattening of T waves 1
  • ST-segment depression 1
  • Prominent U waves (most characteristic finding) 1
  • Broadening/widening of T waves 1
  • QTc interval prolongation 2

• Progressive ECG changes with worsening hypokalemia:

  • T-wave flattening occurs in approximately 27% of hypokalemic patients 2
  • ST-segment depression occurs in approximately 16% of hypokalemic patients 2
  • QTc prolongation occurs in approximately 14% of hypokalemic patients 2
  • These changes can create a "pseudoischemic" pattern that resolves with potassium correction 3

Associated Arrhythmias

• First or second-degree atrioventricular block 1
• Atrial fibrillation 1
• Ventricular arrhythmias:
  • Premature ventricular contractions (PVCs) 1
  • Ventricular tachycardia (VT) 1
  • Torsades de pointes (TdP) 1
  • Ventricular fibrillation (VF) 1
  • Cardiac arrest 1

Clinical Significance and Monitoring

• ECG abnormalities are present in approximately 40% of patients with hypokalemia 2
• Patients with heart failure should maintain potassium levels of at least 4 mEq/L to reduce arrhythmia risk 1
• Risk of arrhythmias is significantly increased in patients taking digoxin 1
• Continuous ECG monitoring is recommended for:
  • Patients with moderate to severe hypokalemia (<3.0 mEq/L) 1
  • Patients with any degree of hypokalemia who show ECG abnormalities 1
  • Patients with rapid potassium losses who may be symptomatic even with mild hypokalemia 1

Special Considerations

• Hypokalemia is often associated with hypomagnesemia, which can exacerbate cardiac effects 1
• Rapid correction of severe hypokalemia with IV potassium is not recommended during cardiac arrest as its effects are unknown and potentially harmful 1
• ECG changes may persist until potassium levels are fully normalized, requiring prolonged supplementation in some cases 3, 4
• Tachycardia (heart rate >100 bpm) in hypokalemic patients is associated with increased risk of mortality and ICU admission 2

Common Pitfalls

• ECG changes may be subtle in mild hypokalemia and easily overlooked 5
• Not all patients with hypokalemia will exhibit ECG changes, especially if the decrease in potassium is gradual 2
• Hypokalemia-induced ECG changes can mimic myocardial ischemia (pseudoischemic pattern) leading to diagnostic confusion 3
• Failure to check magnesium levels when hypokalemia is present may lead to treatment resistance, as hypomagnesemia often coexists and needs correction 1
• Potassium depletion develops slowly in patients on diuretic therapy but can develop rapidly with severe diarrhea and vomiting 6