Situation: A 57-year-old woman is on post-operative day 3 after surgery for a small bowel obstruction. She has a nasogastric (NG) tube on low intermittent suction that drained 1,800 mL in the past 24 hours, and she receives 5% dextrose in water at 80 mL/h. She is on subcutaneous enoxaparin to prevent venous thrombosis. This morning she reports numbness and tingling of her fingers. Her arterial blood gas (ABG) results on room air are (PaCO2 = partial pressure of carbon dioxide in arterial blood; HCO3− = bicarbonate): pH 7.49 PaCO2 47 mmHg HCO3− 35 mEq/L (Reference ranges: pH 7.35–7.45; PaCO2 35–45 mmHg; HCO3− 22–26 mEq/L) How should the nurse interpret these results?

The pH of 7.49 shows alkalosis. The HCO3− of 35 mEq/L is high and matches the alkaline pH, so the primary problem is metabolic alkalosis, here from the loss of hydrochloric acid through nasogastric suction. The PaCO2 of 47 mmHg is above normal because the lungs are retaining carbon dioxide to compensate, but the pH is still abnormal, so compensation is partial.

Step 1: Determine the primary disorder from the pH

The pH is 7.49, which is above the reference range of 7.35–7.45. This indicates alkalosis. The next question is whether the alkalosis is metabolic or respiratory in origin.

Step 2: Match the pH with the bicarbonate or the carbon dioxide

The HCO₃⁻ is 35 mEq/L, which is elevated above the normal range of 22–26 mEq/L. A high bicarbonate level drives the pH upward, so the primary problem is metabolic alkalosis. The PaCO₂ is also elevated at 47 mmHg, but carbon dioxide is an acid; retaining CO₂ would lower the pH, not raise it. Therefore, the elevated PaCO₂ is not the primary problem—it is the body’s response.

Step 3: Assess the degree of compensation

In metabolic alkalosis, the lungs compensate by hypoventilating to retain carbon dioxide and bring the pH back toward normal. The PaCO₂ of 47 mmHg shows this compensatory response is occurring. However, the pH remains 7.49, which is still outside the normal range. When the compensating system has responded but the pH has not returned to normal, the condition is classified as partially compensated. Full compensation would require the pH to fall back within 7.35–7.45.

Step 4: Link the ABG findings to the clinical scenario

The patient is on nasogastric suction, which has drained 1,800 mL in 24 hours. Gastric fluid is rich in hydrochloric acid (HCl). Loss of HCl through NG suction removes hydrogen ions and chloride from the body, directly generating a metabolic alkalosis. The numbness and tingling of the fingers is also consistent with alkalosis: as the blood becomes more alkaline, ionized calcium binds more readily to albumin, lowering the free (ionized) calcium level and producing symptoms of hypocalcemia such as paresthesias.

Step 5: Apply a systematic ABG interpretation approach

A stepwise method reduces errors when interpreting ABGs. The sequence is: evaluate the pH, identify the primary cause by matching the pH with either PaCO₂ or HCO₃⁻, then determine whether compensation is present and whether it is partial or full ^[1][2]. The table below summarizes the compensation patterns for metabolic alkalosis.

Compensation status	pH	PaCO₂	Interpretation
Uncompensated	Alkaline (above 7.45)	Normal (35–45 mmHg)	No respiratory response yet
Partially compensated	Alkaline (still above 7.45)	Elevated (above 45 mmHg)	Lungs are retaining CO₂ but pH not yet normal
Fully compensated	Normal (7.35–7.45)	Elevated (above 45 mmHg)	CO₂ retention has returned pH to normal

In this patient, the pH is still alkaline and the PaCO₂ is elevated, which matches the partially compensated metabolic alkalosis pattern ^[3]. The respiratory system is attempting to compensate, but the process is incomplete.

Step 6: Rule out the other options

Key point! Respiratory alkalosis would present with a low PaCO₂ and a high pH, not an elevated PaCO₂. Fully compensated metabolic alkalosis would require a normal pH, which is not the case here. Uncompensated metabolic alkalosis would show a normal PaCO₂, but this patient’s PaCO₂ is elevated, indicating that compensation has begun. The combination of high pH, high HCO₃⁻, and high PaCO₂ with an abnormal pH is the classic partially compensated metabolic alkalosis pattern.

Watch out! Do not interpret the elevated PaCO₂ as a primary respiratory acidosis. In a mixed picture, always match the pH direction with the parameter that could cause it. Since the pH is alkaline and HCO₃⁻ is high, the metabolic component is primary; the elevated PaCO₂ is the compensatory response to metabolic alkalosis ^[1][2].