What is the most likely causative electrolyte disturbance in a patient presenting with persistent vomiting and abdominal distension?

Potassium is the Most Likely Causative Electrolyte Disturbance

Hypokalemia (potassium <3.5 mEq/L) is the most likely causative electrolyte disturbance in a patient presenting with persistent vomiting and abdominal distension, as potassium loss from vomiting is a well-established mechanism that leads to significant depletion. 1, 2

Mechanism of Potassium Loss in Vomiting

• Persistent vomiting causes direct potassium loss from gastrointestinal secretions, which normally contain significant amounts of potassium 1, 2
• Potassium depletion develops rapidly when the rate of loss through gastrointestinal tract exceeds intake, particularly with severe vomiting 2
• Vomiting-induced hypokalemia is typically accompanied by concomitant chloride loss and manifests as metabolic alkalosis 2
• Secondary hyperaldosteronism resulting from sodium depletion (also caused by vomiting) further increases urinary potassium losses, compounding the deficit 1

Clinical Significance and Cardiac Risk

• Hypokalemia is the most common electrolyte abnormality encountered clinically, present in up to 20% of hospitalized patients 3
• Even moderate hypokalemia can cause cardiac arrhythmias, prominent U-waves on ECG, and disturbances of cardiac rhythm (primarily ectopic beats) 1, 4, 2
• The American Heart Association recommends continuous ECG monitoring for patients with moderate to severe electrolyte imbalances 4
• Hypokalemia causes broadening of T waves, ST-segment depression, prominent U waves, and QT interval prolongation, increasing the risk of ventricular arrhythmias 1, 4

Why Not Sodium or Magnesium as Primary Cause

• While vomiting does cause sodium loss, hyponatremia typically presents with different symptoms (nausea, dizziness, altered mental status, seizures in severe cases) rather than the combination of persistent vomiting and abdominal distension 5, 6
• Magnesium depletion can occur with vomiting but is less commonly the primary causative disturbance 1
• Hypomagnesemia often coexists with hypokalemia and should be corrected concurrently, but potassium is the primary electrolyte lost through gastrointestinal losses 1, 4

Diagnostic Approach

• Check serum potassium levels immediately, along with other electrolytes (sodium, chloride, magnesium, calcium) 1
• Obtain an ECG to assess for hypokalemic changes (U waves, ST depression, T wave flattening, QT prolongation) 1, 4
• Assess volume status clinically to determine if hypovolemia is present 1
• Measure urine potassium if the diagnosis is unclear; urine potassium <20 mmol/L suggests extrarenal losses (gastrointestinal) 1

Treatment Priorities

• For symptomatic patients or those with ECG abnormalities, intravenous potassium replacement with cardiac monitoring is indicated 3, 7
• Oral potassium replacement is appropriate for asymptomatic patients with mild hypokalemia (3.0-3.5 mEq/L) 3
• Correct hypovolemia with isotonic fluids (balanced crystalloids preferred over 0.9% saline) 1
• Check and correct concurrent magnesium deficiency, as hypomagnesemia impairs potassium repletion 1, 4
• Address the underlying cause of vomiting to prevent ongoing losses 7

Common Pitfall to Avoid

• Do not overlook concurrent hypomagnesemia, which is frequently present with vomiting-induced hypokalemia and must be corrected for successful potassium repletion 1, 4