Explain the rise in bicarbonate
Reading File
Finding Sources
Reading File
Reading File
Here is a full explanation of the rise in HCO3- seen in this case.
Why HCO3- Rises in Acute Respiratory Acidosis
The core problem
The patient's PCO2 is 72 mmHg (normal = 40). That excess CO2 drives the following reaction:
CO2 + H2O → H2CO3 → H⁺ + HCO3⁻
When CO2 accumulates (hypoventilation), this equilibrium shifts to the right, producing both H⁺ (causing the acidemia, pH 7.16) AND HCO3⁻. So even without any kidney involvement, the chemistry of CO2 dissolution itself generates a small rise in HCO3⁻.
Two phases of HCO3⁻ rise
1. Acute phase - chemical buffering (immediate)
The moment PCO2 rises, carbonic acid forms and is immediately buffered by hemoglobin and intracellular proteins:
- CO2 enters red blood cells, carbonic anhydrase rapidly converts it to H2CO3
- H2CO3 dissociates to H⁺ + HCO3⁻
- H⁺ is buffered by hemoglobin (the dominant non-bicarbonate buffer in blood)
- HCO3⁻ exits the RBC in exchange for Cl⁻ (the chloride shift)
- This raises plasma HCO3⁻ by ~1 mmol/L per 10 mmHg rise in PCO2
The slide uses the formula: Rise in HCO3 = ΔPaCO2 / 10 = 32 / 10 = 3.2 mEq/L expected
The observed rise is 3 mEq/L - which matches perfectly, confirming this is pure acute respiratory acidosis with no added metabolic disorder.
2. Chronic phase - renal compensation (takes 2-3 days to be complete)
If the respiratory acidosis persists beyond 6-12 hours, the kidneys kick in:
- Increased Na⁺/H⁺ exchange in the proximal tubule
- Increased ammonia (NH3) production and secretion (to buffer urinary H⁺)
- Increased HCO3⁻ reabsorption
- Net effect: HCO3⁻ rises by ~3.5 mmol/L per 10 mmHg rise in PCO2 (some sources say 4 mEq per 10 mmHg)
This renal compensation can bring pH partially back toward normal but does NOT fully normalize it.
Summary table
| Phase | Mechanism | Expected ΔHCO3⁻ per 10 mmHg ΔPaCO2 |
|---|---|---|
| Acute | Chemical buffering (Hb, proteins) + chloride shift | ~1 mEq/L |
| Chronic | Renal: ↑H⁺ excretion, ↑HCO3⁻ reabsorption | ~3.5 mEq/L |
What the slide is telling you
This case uses acute compensation formula (÷10) because the acidosis is presumably sudden. The expected HCO3⁻ rise = 3.2 mEq/L, and the observed rise = 3 mEq/L. Since observed ≈ expected, there is no additional metabolic alkalosis (which would push HCO3⁻ higher than expected) and no additional metabolic acidosis (which would keep HCO3⁻ lower than expected).
Bottom line: The HCO3⁻ rises because rising CO2 chemically shifts the bicarbonate buffer equation to the right, generating bicarbonate as a byproduct. In the acute setting, this is a physicochemical buffer response, not a kidney-driven one.
- Sabiston Textbook of Surgery, Respiratory Acidosis section: ΔHCO3⁻ = ΔPaCO2 × 0.1 (acute)
- Tietz Textbook of Laboratory Medicine, 7th Ed.: "In the acute phase, cHCO3 will increase by ~1 mmol/L for each 10 mm Hg rise in PCO2"
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