What is the estimated compensatory pCO2 (partial pressure of carbon dioxide) for a patient with metabolic alkalosis?

Calculating Compensatory pCO2 in Metabolic Alkalosis

For metabolic alkalosis, the expected compensatory pCO2 can be calculated using the formula: Expected pCO2 (mm Hg) = 0.7 × [HCO3-] + 20 (± 2 mm Hg), which predicts the degree of respiratory compensation through hypoventilation. 1

Understanding the Compensatory Mechanism

• Metabolic alkalosis triggers compensatory hypoventilation, which increases arterial pCO2 to help normalize pH by retaining CO2 2, 3
• The body responds to elevated bicarbonate by reducing alveolar ventilation entirely through decreased tidal volume, while respiratory frequency remains unchanged 1
• This compensatory response is consistent and predictable in metabolic alkalosis, unlike earlier controversies suggested 1

The Calculation Formula

Expected pCO2 = 0.7 × [HCO3-] + 20 (± 2 mm Hg) 1

• This linear relationship between pCO2 and plasma bicarbonate was established in controlled studies with a correlation coefficient of 0.95 1
• The formula applies across the spectrum of metabolic alkalosis severity 1
• A measured pCO2 within ±2 mm Hg of the calculated value indicates appropriate respiratory compensation 1

Clinical Application

To use this formula:

• Obtain the patient's serum bicarbonate level from arterial blood gas or basic metabolic panel 2
• Multiply the bicarbonate value by 0.7 and add 20 1
• Compare the calculated expected pCO2 to the measured arterial pCO2 1

If the measured pCO2 differs significantly from expected:

• Higher than expected pCO2: Suggests a concurrent respiratory acidosis (mixed disorder) 2, 3
• Lower than expected pCO2: Suggests inadequate compensation or concurrent respiratory alkalosis (mixed disorder) 2, 3

Important Caveats

• Metabolic alkalosis is characterized by pH >7.45 and elevated HCO3-, with compensatory elevation in pCO2 4, 2
• The compensatory increase in pCO2 never fully normalizes the pH in pure metabolic alkalosis 2, 3
• Severe metabolic alkalosis (pH ≥7.55) is associated with significantly increased mortality in critically ill patients 2
• The ventilatory response to CO2 is diminished during metabolic alkalosis, which facilitates the compensatory hypoventilation 1
• Hypoxemia may develop as a consequence of hypoventilation, though arterial PO2 changes do not alter the ventilatory compensation 1

Maintenance Factors

For metabolic alkalosis to persist, both generation and maintenance mechanisms must be present 2, 5:

• Volume contraction 2
• Hypochloremia 2, 5
• Hypokalemia 2, 5
• Low glomerular filtration rate 2
• Aldosterone excess 2

These factors impair renal bicarbonate excretion and must be addressed in treatment 2, 5