In an elderly postoperative colectomy patient with dehydration, increased skin turgor, postural hypotension and an arterial blood gas showing pH 7.8, bicarbonate 48 mmol/L, PCO₂ 44 mm Hg and low PO₂, what is the acid‑base interpretation?

Metabolic Alkalosis with Respiratory Compensation

This arterial blood gas demonstrates metabolic alkalosis with respiratory compensation (Answer D). The markedly elevated pH (7.8) and bicarbonate (48 mmol/L) confirm a primary metabolic alkalosis, while the elevated PCO₂ (44 mmHg) represents the body's compensatory hypoventilation attempt—though this compensation is only partial. 1

Systematic ABG Interpretation

Step 1: Identify the primary acid-base disturbance

• A pH of 7.8 is significantly above the normal range (7.35–7.45), confirming alkalemia and ruling out any primary acidotic process. 2
• The bicarbonate of 48 mmol/L is markedly elevated above the normal range of 22–26 mEq/L, identifying this as a primary metabolic disorder. 2, 3
• Serum bicarbonate values >26 mEq/L with pH >7.45 definitively indicate metabolic alkalosis as the primary disturbance. 2

Step 2: Assess the degree of compensation

• In isolated metabolic alkalosis, the expected compensatory rise in PCO₂ is approximately 0.7 mmHg for each 1 mmol/L increase in bicarbonate above 24 mmol/L. 1
• With a bicarbonate of 48 mmol/L (24 mmol/L above normal), the predicted PCO₂ should be approximately 57 mmHg (40 + [24 × 0.7]). 1
• The observed PCO₂ of 44 mmHg is elevated above normal (35–45 mmHg) but falls short of the expected 57 mmHg, indicating partial respiratory compensation. 1, 2
• The elevated PCO₂ (rather than a reduced value) distinguishes this from primary respiratory alkalosis and confirms it is a compensatory response. 1

Step 3: Rule out alternative interpretations

• Not uncompensated metabolic alkalosis (Answer A): The PCO₂ of 44 mmHg is elevated above the normal range, demonstrating that respiratory compensation has begun. 2
• Not compensated metabolic & respiratory alkalosis (Answer B): A PCO₂ of 44 mmHg represents hypoventilation (compensation), not a concurrent respiratory alkalosis which would require a low PCO₂. 2, 4
• Not respiratory alkalosis with metabolic compensation (Answer C): The pH of 7.8 with markedly elevated bicarbonate identifies the metabolic component as primary; respiratory alkalosis would show a low PCO₂ as the primary abnormality. 2, 4

Clinical Context: Contraction Alkalosis

This postoperative colectomy patient's presentation is classic for contraction alkalosis:

• Volume depletion from postoperative fluid losses leads to "contraction alkalosis," where the kidneys avidly reabsorb sodium together with bicarbonate to preserve intravascular volume and electroneutrality. 1
• The clinical signs—dehydration, increased skin turgor (likely meant to indicate decreased turgor), and postural hypotension—all signal marked volume contraction that sustains renal bicarbonate retention. 1, 2
• Concurrent chloride loss (from gastrointestinal drainage or nasogastric suction) impairs the kidney's ability to excrete bicarbonate, further perpetuating the alkalosis. 1, 2

Clinical Significance and Urgency

A pH of 7.8 places this patient in a high-risk category:

• Severe alkalemia (pH >7.60) is associated with approximately 48% mortality in reported cohorts, making this a medical emergency requiring prompt intervention. 1
• Elevated pH increases the risk of cardiac arrhythmias and causes a leftward shift of the oxyhemoglobin dissociation curve, reducing tissue oxygen delivery despite adequate arterial oxygen content. 2
• The low PO₂ in this context likely results from hypoventilation (the compensatory response) or postoperative atelectasis, and supplemental oxygen is indicated. 1, 2

Management Priorities

Volume resuscitation with isotonic saline is the cornerstone of treatment:

• Administer 0.9% NaCl to restore intravascular volume and supply chloride, which enables the kidneys to excrete excess bicarbonate. 1
• Chloride repletion is essential because adequate chloride availability is required for the kidneys to eliminate bicarbonate and resolve the alkalosis. 1
• Direct administration of acidifying agents to lower bicarbonate is discouraged; correcting the underlying volume and chloride deficits allows physiologic mechanisms to resolve the alkalosis. 1

Electrolyte monitoring is critical:

• Check serum potassium urgently, as hypokalemia commonly coexists with metabolic alkalosis and worsens the alkalemic state. 1
• Monitor for surgical complications (anastomotic leak, intra-abdominal infection) that may be contributing to ongoing fluid losses. 2

Common Pitfalls to Avoid

• Do not withhold oxygen in an attempt to correct the alkalosis by allowing PCO₂ to rise further; hypoxemia always takes precedence over acid-base correction. 2
• Do not misinterpret the elevated PCO₂ as indicating a primary respiratory acidosis; in the context of severe alkalemia and markedly elevated bicarbonate, the elevated PCO₂ is clearly compensatory. 1, 2
• Do not administer bicarbonate or other alkalinizing agents, which would worsen the already severe alkalemia. 1