How would you describe the pH compensation in a patient with respiratory alkalosis and lab results showing alkalosis, hypocapnia, and low bicarbonate level?

Respiratory Alkalosis with Partial Compensation

The patient's blood gas values indicate respiratory alkalosis with partial compensation (option C).

Analysis of Blood Gas Values

The patient's arterial blood gas results show:

• pH = 7.55 (elevated, indicating alkalosis)
• pCO2 = 20 mmHg (decreased, indicating respiratory alkalosis)
• HCO3- = 17 mmol/L (decreased, indicating metabolic compensation)

Interpretation Framework:

1. Determine primary disorder: The markedly low pCO2 (20 mmHg) with elevated pH (7.55) confirms primary respiratory alkalosis
2. Assess compensation: The decreased bicarbonate (17 mmol/L) indicates renal compensation is occurring

Physiological Compensation Mechanism

In respiratory alkalosis, the kidneys respond by decreasing bicarbonate reabsorption to normalize pH. This compensation follows predictable patterns:

• For acute respiratory alkalosis: HCO3- decreases by 0.2 mEq/L for each 1 mmHg decrease in pCO2 1
• For chronic respiratory alkalosis: HCO3- decreases by approximately 0.4-0.5 mEq/L for each 1 mmHg decrease in pCO2 2

Expected Compensation Calculation:

• Normal pCO2 is approximately 40 mmHg
• The decrease in pCO2 is 20 mmHg (40 - 20 = 20)
• Expected HCO3- decrease:
  • Acute: 20 × 0.2 = 4 mEq/L (expected HCO3- ≈ 20-21 mEq/L)
  • Chronic: 20 × 0.4 = 8 mEq/L (expected HCO3- ≈ 16-17 mEq/L)

The patient's actual HCO3- of 17 mmol/L suggests partial to near-complete compensation consistent with a chronic process (approximately 3 weeks duration, matching the clinical history).

Clinical Correlation

The patient's presentation strongly suggests pulmonary embolism:

• 22-year-old female on oral contraceptives (risk factor)
• Chest pain and shortness of breath for 3 weeks
• Tachypnea (23 breaths/min)
• Positive D-dimer
• Respiratory alkalosis with partial compensation

Respiratory alkalosis in this case is driven by hyperventilation (tachypnea) due to hypoxic stimulation and anxiety from the pulmonary embolism 3. The 3-week duration has allowed time for renal compensation to begin but not fully normalize pH.

Why Not Other Options?

• Not fully compensated (option A): The pH remains significantly elevated at 7.55 (normal 7.35-7.45), indicating compensation is incomplete
• Not uncompensated (option B): The decreased bicarbonate (17 mmol/L) clearly shows renal compensation is occurring

Common Pitfalls in Blood Gas Interpretation

1. Failing to identify the primary disorder before assessing compensation
2. Not considering the time course (acute vs. chronic) when evaluating expected compensation
3. Confusing partial compensation with mixed acid-base disorders
4. Overlooking clinical context when interpreting laboratory values

Clinical Implications

The respiratory alkalosis with partial compensation in this patient requires urgent evaluation for pulmonary embolism. The positive D-dimer, risk factors, and clinical presentation strongly support this diagnosis. Management should focus on:

• Anticoagulation therapy
• Oxygen supplementation targeting SpO2 94-98% 4
• Consideration of thrombolysis if hemodynamically unstable
• Addressing the underlying cause rather than the acid-base disturbance itself

The partial compensation indicates this is not an acute event, consistent with the 3-week history, and suggests the body's compensatory mechanisms are appropriately responding to the primary respiratory alkalosis.