Respiratory alkalosis compensation: acute vs chronic
Two patients can breathe the same way and still have different bicarbonate. Respiratory alkalosis compensation comes in two waves, and the kidneys need days to deliver the second.
Chapters (9)
Predict the expected HCO₃⁻ fall in acute versus chronic respiratory alkalosis and identify common causes of hyperventilation.
The short version
- Respiratory alkalosis is a primary fall in PaCO₂ from breathing more than the body needs; the pH may be high or near normal.
- Acute compensation lowers HCO₃⁻ by about 2 mEq/L for every 10 mm Hg fall in PaCO₂; chronic compensation lowers it by about 4.
- Expected HCO₃⁻ = 24 − slope × (40 − PaCO₂) / 10, with slope 2 if acute and 4 if chronic.
- Acute compensation rarely takes HCO₃⁻ below about 18, and chronic compensation rarely below about 12–15; a lower value points to a second problem, a metabolic acidosis.
- In very recent hyperventilation, bicarbonate can still be close to 24, so a near-normal bicarbonate does not rule out respiratory alkalosis.
Same PaCO₂, different bicarbonate
Two patients are breathing fast. Both have a PaCO₂ of 28 mm Hg. One has a bicarbonate of 22; the other has a bicarbonate of 19. Why the difference?
This lesson is about respiratory alkalosis compensation: how far bicarbonate falls in acute versus chronic respiratory alkalosis, and which causes of fast breathing to know.
The mental model: the HCO₃⁻ / CO₂ ratio and the lungs
Respiratory alkalosis is a primary fall in PaCO₂, caused by breathing more than the body needs, with or without a compensatory fall in HCO₃⁻. The pH may be high, or near normal.[1]
Think of the ratio: bicarbonate on top, carbon dioxide on the bottom. Carbon dioxide drops, so the ratio climbs and the pH goes up. The body fights back by lowering bicarbonate, which pulls the ratio down again.[1][3]
The baseline for the series is a bicarbonate of 24 and a PaCO₂ of 40.[3][4]
Respiratory alkalosis compensation: acute vs chronic
The response comes in two waves.
Wave one is acute. Within minutes, buffers release hydrogen ions, and those use up some of the bicarbonate.[1] Bicarbonate falls about 2 for every 10 mm Hg that PaCO₂ falls. Sources quote 1 to 2; this lesson uses 2.[2][3]
Wave two is chronic. Over 2 to 3 days, the kidneys excrete less acid, and bicarbonate falls further.[1] Bicarbonate falls about 4 for every 10 mm Hg. Sources say 4 to 5; this lesson uses 4.[2][3]
| Acute | Chronic | |
|---|---|---|
| Timing | Within minutes | Over 2–3 days |
| Mechanism | Buffers release H⁺, which uses up bicarbonate | Kidneys excrete less acid |
| HCO₃⁻ fall per 10 mm Hg fall in PaCO₂ | About 2 (sources 1–2) | About 4 (sources 4–5) |
| Rough floor for HCO₃⁻ | About 18 | About 12–15 |
So chronic compensation is roughly twice as strong as acute, because the kidneys need time.
The three-step recipe for expected HCO₃⁻
slope = 2 if acute, 4 if chronic; 24 and 40 are the baseline HCO₃⁻ (mEq/L) and PaCO₂ (mm Hg)
- Find the fall in PaCO₂Forty minus the measured value.
- Divide by tenThis counts the tens of mm Hg that PaCO₂ has fallen.
- Multiply and subtractMultiply by 2 if acute, by 4 if chronic, and subtract that from 24.
Worked pair: PaCO₂ 28 gives HCO₃⁻ of about 22 or about 19
- Fall in PaCO₂: 40 − 28 = 12, which is 1.2 tens
- Acute: 1.2 × 2 = 2.4, so HCO₃⁻ ≈ 24 − 2.4 ≈ 22
- Chronic: 1.2 × 4 = 4.8, so HCO₃⁻ ≈ 24 − 4.8 ≈ 19
Limits of compensation: a lower bicarbonate means a second problem
There are limits. Acute compensation rarely takes bicarbonate below about 18. Chronic compensation rarely goes below about 12 to 15.[3]
A bicarbonate lower than expected points to a second problem, a metabolic acidosis.[3]
Causes of hyperventilation
- Anxiety and pain.[1][3]
- Hypoxaemia, such as at high altitude, and pulmonary embolism.[1][3]
- Sepsis, pregnancy and liver disease, driven by cytokines, progesterone and circulating toxins.[3]
- Salicylate poisoning.[3]
- Central nervous system disease, like stroke.[1][3]
- Mechanical ventilation set too fast.[1]
Practice question
Original · USMLE styleA 31-year-old man is evaluated at a high-altitude clinic because of a mild headache and fatigue. He arrived at an altitude of 4200 m (13,780 ft) 5 days ago. He has no vomiting, diarrhea, or medication use. Temperature is 36.9 C (98.4 F), pulse is 92/min, respirations are 20/min, and blood pressure is 118/74 mm Hg. Oxygen saturation is 88% on room air. Laboratory studies show:
| Serum Na⁺ | 138 mEq/L |
|---|---|
| Serum Cl⁻ | 108 mEq/L |
| Serum HCO₃⁻ | 18 mEq/L |
| Arterial pH (room air) | 7.46 |
| Arterial PCO₂ (room air) | 26 mm Hg |
Common questions
How much does bicarbonate fall in acute respiratory alkalosis?
About 2 mEq/L for every 10 mm Hg fall in PaCO₂. Sources quote 1 to 2; this lesson uses 2.[2][3]
How much does bicarbonate fall in chronic respiratory alkalosis?
About 4 mEq/L for every 10 mm Hg fall in PaCO₂. Sources say 4 to 5; this lesson uses 4.[2][3]
How long does chronic respiratory alkalosis compensation take?
Over 2 to 3 days, as the kidneys excrete less acid and bicarbonate falls further. Acute buffering happens within minutes.[1]
What is the lowest bicarbonate that compensation alone should produce?
Acute compensation rarely takes bicarbonate below about 18, and chronic compensation rarely below about 12 to 15. A lower value points to a second problem, a metabolic acidosis.[3]
Transcript
Read the full narration, by chapter653 words
0:00Two patients, same PaCO2 of 28, different bicarbonate
Two patients are breathing fast. Both have a PaCO2 of 28 millimetres of mercury. One has a bicarbonate of 22. The other has a bicarbonate of 19. Why the difference? Today: how far bicarbonate falls in acute versus chronic respiratory alkalosis, and which causes of fast breathing to know.
0:24The mental model: the HCO3⁻ / CO2 ratio and the lungs
First, the mental model. Respiratory alkalosis is a primary fall in PaCO2, caused by breathing more than the body needs. The pH may be high, or near normal. Think of the ratio: bicarbonate on top, carbon dioxide on the bottom. Carbon dioxide drops, so the ratio climbs, and the pH goes up. The body fights back by lowering bicarbonate, which pulls the ratio down again. Our baseline is a bicarbonate of 24 and a PaCO2 of 40. Now the key idea.
0:58Two waves: acute (−2 per 10 mm Hg) and chronic (−4 per 10 mm Hg)
The response comes in two waves. Wave one is acute. Within minutes, buffers release hydrogen ions, and those use up some of the bicarbonate. The rule: bicarbonate falls about two for every 10 mm Hg that the PaCO2 falls. Sources quote one to two. We'll use two. Wave two is chronic. Over 2 to 3 days, the kidneys excrete less acid, and bicarbonate falls further. The rule: bicarbonate falls about four for every 10. Sources say four to five. We'll use four. So chronic compensation is roughly twice as strong as acute, because the kidneys need time.
1:43The three-step recipe
Here's the recipe. Step one: find the fall in PaCO2. Forty minus the measured value. Step two: divide by 10. Step three: multiply by two if acute, by four if chronic, and subtract that from 24. Back to our two patients.
2:01Worked pair: PaCO2 28 gives HCO3⁻ about 22 (acute) and about 19 (chronic)
Forty minus 28 is a fall of 12. That is 1.2 tens. Acute: 1.2 times two is 2.4, so bicarbonate is about 22. Chronic: 1.2 times four is 4.8, so bicarbonate is about 19. Same breathing, different clock. There are limits.
2:25Limits of compensation: a lower bicarbonate means a second problem
Acute compensation rarely takes bicarbonate below about 18. Chronic compensation rarely goes below about 12 to 15. A bicarbonate lower than expected points to a second problem, a metabolic acidosis.
2:41Causes of hyperventilation
So what makes people hyperventilate? Anxiety and pain. Hypoxaemia, such as at high altitude, and pulmonary embolism. Sepsis, pregnancy, and liver disease, driven by cytokines, progesterone and circulating toxins. Salicylate poisoning. Central nervous system disease, like stroke. And mechanical ventilation set too fast.
3:06Practice question: five days at 4200 m
Your turn. A 31-year-old man has been at an altitude of 4200 metres for five days. He has a mild headache and fatigue, and takes no medications. Respiratory rate is 20, and oxygen saturation is 88%. Sodium 138, chloride 108, bicarbonate 18. Arterial blood gas: pH 7.46, PaCO2 26. Which best describes his acid-base status? A, acute respiratory alkalosis. B, chronic respiratory alkalosis with appropriate renal compensation. C, acute respiratory alkalosis with a metabolic acidosis. D, metabolic acidosis with respiratory compensation. E, chronic respiratory alkalosis with a metabolic alkalosis. Pause here and decide. His PaCO2 fell by 14, so the chronic rule predicts a bicarbonate near 18. A is out: acute compensation predicts about 21, and 5 days is long enough for the kidneys to respond. C is out for the same reason: with 5 days at altitude, a bicarbonate of 18 is the chronic expectation, not an extra acidosis. D is out: his pH is alkalaemic, and for a bicarbonate of 18 Winters' formula would predict a PaCO2 near 35, but his is 26. E is out: an added metabolic alkalosis would leave bicarbonate above the predicted 18. So the answer is B: a chronic respiratory alkalosis, appropriately compensated, driven by hypoxaemia at altitude. Three things to keep.
4:59High-yield recap and next lesson
One: in respiratory alkalosis, bicarbonate falls about two per 10 acutely, and about four per 10 chronically. Two: the kidneys take days, so a bicarbonate near 24 can still occur in very recent hyperventilation. Three: know the causes, from anxiety and pain to hypoxaemia, pulmonary embolism, sepsis, pregnancy, liver disease, salicylates and the brain. Next up: salicylate poisoning, a classic mixed disorder.
Sources
Numbers in the text, e.g. [1], point here- Respiratory Alkalosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/respiratory-alkalosis
- Acid-Base Disorders (compensation table). MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders
- Nickson C. Respiratory Alkalosis. Life in the Fast Lane (LITFL). litfl.com/respiratory-alkalosis/
- Reference baseline (pH 7.40, PaCO2 40, HCO3⁻ 24) and Winters' formula carried from the series: Medservatory EP 01 and EP 03.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): lungs. Kidney icon by Servier (via Bioicons, bioicons.com), licensed under CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/); adapted (recoloured, outlined, resized). All other diagrams are original; example lab values are illustrative.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-014.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01ABG interpretation, made simple5:35
- EP 02Anion gap explained: high vs normal gap metabolic acidosis5:25
- EP 03Winters' formula: the expected PaCO₂ in metabolic acidosis5:10
- EP 04Delta ratio: find the hidden second acid–base disorder5:09
- EP 05New bicarbonate: how the kidney excretes acid with ammonium5:23
- EP 06High anion gap metabolic acidosis causes, sorted by the anion5:54
- EP 07Osmolar gap: how toxic alcohols hide from the anion gap5:31
- EP 08Urine anion gap: telling diarrhea from RTA5:33
- EP 09Renal tubular acidosis: tell RTA types 1, 2 and 4 apart5:24
- EP 10Metabolic alkalosis: generation, maintenance and urine chloride5:30
- EP 11Vomiting metabolic alkalosis: why the urine turns acid5:36
- EP 12Bartter vs Gitelman: NKCC2, NCC and urine calcium5:21
- EP 13Respiratory acidosis compensation: acute vs chronic5:22
- EP 14Respiratory alkalosis compensation: acute vs chronic5:33
- EP 15Metabolic alkalosis compensation: the expected PaCO₂5:12
- EP 16Salicylate poisoning acid-base: a classic mixed disorder5:31
- EP 17Type A vs type B lactic acidosis: split it by mechanism5:33
- EP 18DKA anion gap explained: from high gap to hyperchloraemic acidosis5:42
- EP 19Acidosis and hyperkalemia: how pH shifts potassium5:43
- EP 20ABG interpretation in 6 steps: the acid-base algorithm5:32
- EP 21Acid-base practice cases: three ABG vignettes, one algorithm5:44