Differential Diagnosis for the U Wave on an EKG

The U wave is a small deflection in the EKG that appears after the T wave. Its presence and characteristics can be associated with various conditions. Here's a differential diagnosis categorized for clarity:

• Single Most Likely Diagnosis

  • Hypokalemia: This is often considered the most common cause of a prominent U wave. Hypokalemia affects the repolarization phase of the cardiac action potential, leading to the appearance or enhancement of the U wave on the EKG. The U wave is thought to represent late repolarization of the Purkinje fibers.

• Other Likely Diagnoses

  • Hypercalcemia: Elevated calcium levels can also affect the cardiac action potential, leading to changes in the EKG, including the appearance of U waves.
  • Bradycardia: Slow heart rates can make U waves more visible as the slower heart rate allows for a clearer distinction between the T wave and the U wave.
  • Anti-arrhythmic drugs (especially Class IA): Certain medications used to treat arrhythmias can affect the cardiac action potential and lead to the appearance of U waves.

• Do Not Miss Diagnoses

  • Toxicity from certain medications (e.g., quinidine, sotalol): While not the most common cause, missing toxicity from these medications can have serious consequences, including arrhythmias and cardiac arrest.
  • Cardiac glycoside toxicity: Digitalis toxicity can cause various EKG changes, including the presence of U waves, and is critical not to miss due to its potential for severe cardiac complications.

• Rare Diagnoses

  • Hypothyroidism: Although less common, hypothyroidism can lead to changes in the EKG, including U waves, due to its effects on the cardiovascular system and cardiac action potential.
  • Mitral valve prolapse: Some studies suggest that mitral valve prolapse may be associated with the presence of U waves, possibly due to the altered cardiac structure and function.
  • Cardiac amyloidosis: This condition, characterized by the deposition of amyloid proteins in the heart, can lead to various EKG abnormalities, including the presence of U waves, although it is a rare cause.