What are the EKG (electrocardiogram) findings in hypokalemia?

EKG Findings in Hypokalemia

Hypokalemia causes characteristic EKG 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

Primary EKG Manifestations

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

• T-wave changes:

  • Flattening of T waves 1, 2
  • Decreased T-wave amplitude 1

• ST-segment changes:

  • ST-segment depression 1, 2
  • Can mimic myocardial ischemia (pseudoischemic changes) 3

• U-wave abnormalities:

  • Prominent U waves (most characteristic finding) 1, 2
  • Best visualized in mid-precordial leads (V2-V4) 2
  • In severe hypokalemia (K+ <2.7 mmol/L), U-wave amplitude may exceed T-wave amplitude 1
  • U waves may merge with T waves creating a fusion pattern 1

• Other interval changes:

  • Prolonged PR interval 2
  • Increased P-wave amplitude 2

Severity-Based EKG Changes

The EKG manifestations typically correlate with the degree of hypokalemia:

• Mild hypokalemia (3.0-3.5 mEq/L):

  • Subtle T-wave flattening
  • Early ST-segment depression
  • Appearance of U waves

• Moderate hypokalemia (2.5-2.9 mEq/L):

  • More pronounced T-wave flattening
  • ST-segment depression
  • Prominent U waves

• Severe hypokalemia (<2.5 mEq/L):

  • U-wave amplitude may exceed T-wave amplitude
  • T-U wave fusion
  • Risk of significant arrhythmias increases substantially 1

Arrhythmias Associated with Hypokalemia

Hypokalemia creates an arrhythmogenic substrate through multiple electrophysiologic mechanisms:

• Supraventricular arrhythmias:

  • First or second-degree atrioventricular block
  • Atrial fibrillation 1

• Ventricular arrhythmias:

  • Premature ventricular contractions (PVCs)
  • Ventricular tachycardia (VT)
  • Torsades de pointes (TdP)
  • Ventricular fibrillation (VF)
  • Cardiac arrest 1, 4

Electrophysiologic Mechanisms

Hypokalemia promotes arrhythmias through several mechanisms:

• Increases resting membrane potential
• Increases action potential duration
• Increases refractory period (to a greater degree than action potential duration)
• Increases threshold potential and automaticity
• Decreases conductivity 4

This combination of effects creates conditions favorable for both reentrant and automatic arrhythmias 4.

Clinical Significance and Monitoring

• EKG changes may be present in up to 40% of patients with hypokalemia 5

• The most common EKG abnormalities in hypokalemic patients are:

  • T-wave flattening (27%)
  • ST-segment depression (16%)
  • QTc prolongation (14%) 5

• Continuous cardiac monitoring is recommended for:

  • Moderate to severe hypokalemia (<3.0 mEq/L)
  • Patients with cardiac disease
  • Patients on digoxin
  • Patients with symptoms of hypokalemia 1, 6

Pitfalls and Caveats

1. Variable presentation: EKG manifestations of hyperkalemia vary between individuals and may not be predictable 1

2. Confounding factors: Concurrent conditions like ischemia, medication effects, or other electrolyte abnormalities may mask or mimic hypokalemic EKG changes

3. Poor prognostic value: Recent research suggests that while EKG abnormalities are common in hypokalemia, they may be poor prognostic markers for short-term adverse events under current standard of care 5

4. Magnesium levels: Always check magnesium levels, as hypomagnesemia can exacerbate hypokalemia and its EKG manifestations 6

5. Rapid correction risks: Overly rapid correction of hypokalemia can cause cardiac arrhythmias 6

6. Pseudonormalization: Some EKG changes may normalize despite persistent hypokalemia, creating a false sense of security

By recognizing these characteristic EKG patterns, clinicians can promptly identify and address hypokalemia before it progresses to potentially life-threatening arrhythmias.