Interpreting the blood gas
The arterial pH is 7.51 (high), the bicarbonate is 34 mEq/L (high), and the PaCO2 is 47 mmHg (slightly high). A high pH with a high HCO3− is metabolic alkalosis. The raised PaCO2 moves in the same direction as the HCO3− and represents respiratory compensation: the client hypoventilates slightly to retain carbon dioxide and pull the pH back toward normal. With low chloride and low potassium, this pattern fits loss of gastric acid and chloride through repeated vomiting in a high small-bowel obstruction.
Why vomiting causes metabolic alkalosis
Gastric juice is rich in hydrochloric acid. When it is lost through repeated vomiting or nasogastric suction, the body loses hydrogen ions and chloride, and bicarbonate rises. Volume depletion then drives the kidneys to retain sodium, which they do partly by excreting potassium and hydrogen ions, worsening both hypokalemia and alkalosis. Low chloride also limits the kidneys' ability to excrete excess bicarbonate. The result is the classic triad of metabolic alkalosis, hypochloremia, and hypokalemia seen here: chloride 89 mEq/L and potassium 3.0 mEq/L.
| Process | Expected pH | Expected HCO3− | Expected PaCO2 |
|---|
| Vomiting of gastric contents (high obstruction) | High | High | Slightly high (compensation) |
| Loss of bicarbonate-rich lower intestinal fluid | Low | Low | Low (compensation) |
| Lactic acidosis from bowel ischemia | Low | Low | Low (compensation) |
| Hyperventilation from pain | High | Normal or slightly low | Low |
Why the other processes do not fit
Loss of bicarbonate-rich fluid from the lower intestine tends toward metabolic acidosis, not a high HCO3−. Bowel ischemia produces lactic acidosis with a low pH and low HCO3−; it is a serious complication of obstruction, but these values do not show it. Watch out! Rapid breathing from pain would cause respiratory alkalosis, which would lower the PaCO2; here the PaCO2 is raised, so the primary problem is metabolic, not respiratory.
Nursing implications
Management corrects the cause and the deficits: isotonic saline restores volume and chloride, and potassium is replaced as ordered, after adequate urine output is confirmed. The nurse measures NG output accurately, because continued suction adds to acid and chloride losses, monitors serial electrolytes and cardiac rhythm for hypokalemia, and watches for signs that suggest strangulation, such as fever, rising pain, or a developing metabolic acidosis.
Exam takeaway
Key point! Vomiting or NG suction: metabolic alkalosis with hypochloremia and hypokalemia. Check the direction of pH and HCO3− first; a PaCO2 that moves the same way is compensation.