Yes, serum bicarbonate will be low in respiratory alkalosis.
In respiratory alkalosis, serum bicarbonate decreases as a compensatory response to the primary reduction in PaCO2, with the magnitude of decrease depending on whether the condition is acute or chronic.
Understanding the Mechanism
Respiratory alkalosis occurs when hyperventilation reduces arterial CO2 tension (PaCO2), causing the blood pH to rise above 7.45 1. The body responds through two phases:
Acute Phase (Minutes to Hours)
- Immediate chemical buffering occurs as the CO2 equation shifts: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3⁻
- When CO2 drops, this equilibrium shifts leftward, consuming bicarbonate
- For every 1 mm Hg decrease in PaCO2, bicarbonate drops by approximately 0.2 mEq/L in acute respiratory alkalosis 2
Chronic Phase (Days)
- Renal compensation develops over 2-3 days as the kidneys reduce bicarbonate reabsorption and increase bicarbonate excretion 3, 4
- For every 1 mm Hg decrease in PaCO2, bicarbonate decreases by approximately 0.4-0.5 mEq/L in chronic respiratory alkalosis 3
- This represents a more substantial drop than in the acute phase
Clinical Quantification
A landmark study in normal subjects demonstrated that chronic hyperventilation at altitude produced a bicarbonate decrease of 0.41 mmol/L for each 1 mm Hg reduction in PaCO2 3. Importantly, this relationship held true even in subjects with pre-existing metabolic acidosis, though the percent reduction in PaCO2 always exceeded the percent reduction in bicarbonate, meaning pH still increased 3.
Critical Diagnostic Pitfall
Low bicarbonate does NOT automatically mean metabolic acidosis. Chronic respiratory alkalosis is frequently misdiagnosed as metabolic acidosis and inappropriately treated with alkali therapy 4. The key distinguishing features are:
- Respiratory alkalosis: Low HCO3⁻ + Low PaCO2 + High pH (>7.45)
- Metabolic acidosis: Low HCO3⁻ + Low PaCO2 (compensatory) + Low pH (<7.35)
The urine anion gap can help differentiate: it will be negative in metabolic acidosis with appropriate renal acid excretion, but this tool has limitations in chronic respiratory alkalosis 4.
Associated Metabolic Effects
Respiratory alkalosis produces multiple secondary metabolic changes beyond bicarbonate reduction 2:
- Mild lactic acidosis may develop
- Potassium, phosphate, and calcium handling is altered
- These changes can further complicate the acid-base picture
Bottom Line for Clinical Practice
When you see a low bicarbonate level, always check the arterial blood gas before assuming metabolic acidosis. If pH is elevated (>7.45) and PaCO2 is reduced (<35 mm Hg), you're dealing with respiratory alkalosis—not metabolic acidosis—and the low bicarbonate is an appropriate compensatory response that should not be treated with bicarbonate administration 4, 2.