What are the early ECG (electrocardiogram) changes in a case of non-bilious vomiting due to gastric outlet obstruction (GOO) with hypokalemia?

Early ECG Changes in Hypokalemia Due to Gastric Outlet Obstruction

The most common early ECG change in a case of non-bilious vomiting due to gastric outlet obstruction (GOO) with hypokalemia is flattening or depression of the T wave (option D: T wave sagging).

Pathophysiology of Hypokalemia in GOO

Gastric outlet obstruction causes persistent non-bilious vomiting which leads to significant electrolyte abnormalities, particularly hypokalemia. This occurs through several mechanisms:

• Loss of potassium-rich gastric contents
• Activation of the renin-angiotensin-aldosterone system due to volume depletion
• Metabolic alkalosis which shifts potassium intracellularly

ECG Changes in Hypokalemia

The electrocardiographic manifestations of hypokalemia follow a progressive pattern based on severity:

1. Early changes (mild to moderate hypokalemia, 3.0-3.5 mEq/L):

   • T wave flattening/sagging (earliest change)
   • ST-segment depression
   • Increased amplitude of P waves

2. Later changes (more severe hypokalemia, <3.0 mEq/L):

   • Prominent U waves
   • Prolonged PR interval
   • QT interval prolongation
   • Fusion of T and U waves

According to the American Heart Association guidelines, hypokalemia produces ECG changes including "broadening of the T waves, ST-segment depression, and prominent U waves" 1. The earliest of these changes is T wave flattening or sagging, which occurs before the development of prominent U waves or other more advanced changes.

Clinical Significance

Recognizing early ECG changes of hypokalemia in GOO is critical because:

• Hypokalemia increases the risk of cardiac arrhythmias, including:

  • First or second-degree atrioventricular block
  • Atrial fibrillation
  • Ventricular arrhythmias (PVCs, VT, torsades de pointes)
  • Ventricular fibrillation and cardiac arrest 1

• Patients with GOO often develop severe metabolic alkalosis alongside hypokalemia, which can worsen cardiac electrical instability

Common Pitfalls to Avoid

1. Confusing U waves with elevated T waves: U waves are separate deflections after the T wave, while elevated/peaked T waves are typically seen in hyperkalemia, not hypokalemia.

2. Missing hypokalemia in the setting of digoxin therapy: Patients on digoxin are particularly susceptible to arrhythmias when hypokalemic.

3. Focusing only on potassium: GOO patients often have multiple electrolyte abnormalities (hypokalemia, hypochloremia, metabolic alkalosis, and sometimes hypocalcemia) that can all affect the ECG.

4. Delayed treatment: Severe hypokalemia in GOO can rapidly progress to life-threatening arrhythmias if not promptly corrected.

The correct answer is D: T wave sagging, as this represents the earliest ECG manifestation of hypokalemia that would be seen in a patient with non-bilious vomiting due to gastric outlet obstruction.