Electrolytes Lost with Vomiting
Chloride is the primary electrolyte lost with vomiting, along with significant losses of potassium and hydrogen ions, leading to the characteristic metabolic alkalosis seen in patients with persistent vomiting. 1, 2
Pathophysiology of Electrolyte Loss
When gastric contents are lost through vomiting, the following occurs:
- Chloride depletion is the hallmark finding, as gastric secretions contain high concentrations of hydrochloric acid 1, 2
- Potassium is lost concomitantly with chloride, and this depletion develops rapidly with severe vomiting 1
- Hydrogen ion loss leads to metabolic alkalosis, creating the classic hypokalemic metabolic alkalosis pattern 2
- The characteristic urine electrolyte pattern shows high sodium and potassium with chloride concentration less than 5 mmol/L 2
Clinical Significance
Potassium depletion due to vomiting is usually accompanied by concomitant loss of chloride and manifested by hypokalemia and metabolic alkalosis 1. This differs fundamentally from diarrheal losses, where bicarbonate loss predominates and results in metabolic acidosis 3.
Key Distinguishing Features:
- Vomiting causes excessive chloride loss as the primary electrolyte disturbance 3
- Sodium and potassium losses occur but are secondary to the chloride depletion 1, 2
- The acid-base disturbance is metabolic alkalosis (not acidosis), which helps differentiate vomiting from diarrhea 2
Treatment Implications
Volume depletion from vomiting requires replacement with isotonic fluids containing sodium, potassium, and chloride 4. The potassium deficit must be corrected alongside chloride replacement, as potassium depletion associated with metabolic alkalosis requires supplemental potassium chloride specifically 1.
Common Pitfall:
Do not overlook that bicarbonate is actually retained (not lost) in vomiting, causing alkalosis rather than being depleted 2. This is the opposite of what occurs with diarrhea, where bicarbonate loss causes acidosis 3.
Answer: Chloride is the correct choice, as it represents the primary and most characteristic electrolyte loss with vomiting.