Does Respiratory Alkalosis Cause Metabolic Acidosis?
No, respiratory alkalosis does not cause metabolic acidosis—it triggers a compensatory metabolic response that decreases bicarbonate levels, but this is not a true metabolic acidosis. 1
Understanding the Physiological Response
When respiratory alkalosis occurs (from hyperventilation), the body responds through two distinct mechanisms:
Acute Compensation (Minutes to Hours)
- The immediate response involves a decrease in plasma bicarbonate of approximately 0.2 mEq/L for each 1 mmHg decrease in PaCO2 2
- This bicarbonate reduction is a compensatory mechanism to normalize pH, not a pathological metabolic acidosis 1
- The compensation occurs rapidly through cellular buffering systems and begins within minutes 1
The Lactic Acid Exception
There is one important clinical scenario where respiratory alkalosis can be associated with metabolic acidosis:
- Respiratory alkalosis produces a mild lactic acidosis as one of its metabolic consequences 3
- In panic disorder patients with severe hyperventilation, lactic acid production increases significantly, with plasma bicarbonate decreasing by 0.41 mEq/L for each 1 mmHg drop in PaCO2 (double the normal compensatory response) 4
- Mean lactate levels in these patients reach 2.59 ± 1.50 mmol/L, with some patients reaching as high as 7.78 mmol/L 4
- This lactate accumulation correlates directly with the degree of hypocapnia 4
Clinical Distinction: Compensation vs. Causation
What Actually Happens
- Respiratory alkalosis causes compensatory bicarbonate reduction, not metabolic acidosis 1, 2
- The decreased bicarbonate is an appropriate physiological response to maintain pH homeostasis 2
- In healthy subjects, metabolic alkalosis (elevated bicarbonate) actually decreases respiratory drive and minute ventilation through reduced neural respiratory drive 5
The Opposite Direction
- Metabolic alkalosis causes compensatory hypoventilation (not hyperventilation), with PaCO2 rising by 0.7 mmHg for each 1 mEq/L increase in plasma bicarbonate 2
- This demonstrates the body compensates in the opposite direction from what the question suggests 2
Critical Clinical Context
In Septic Shock
- Early sepsis typically presents with respiratory alkalosis from centrally mediated hyperventilation 6
- As sepsis progresses, patients develop metabolic acidosis (from tissue hypoperfusion and lactate production) AND may develop respiratory acidosis (from respiratory muscle fatigue or lung disease) 6
- These are concurrent processes, not causally related—the metabolic acidosis comes from shock, not from the preceding respiratory alkalosis 6
Common Pitfall
Do not confuse the compensatory decrease in bicarbonate during respiratory alkalosis with a primary metabolic acidosis. The key distinguishing feature is that in true metabolic acidosis, the bicarbonate drop is the primary problem with respiratory compensation following, whereas in respiratory alkalosis, the bicarbonate drop is secondary to the primary respiratory problem 1, 7.