Metabolic alkalosis compensation: the expected PaCO₂
In metabolic alkalosis the lungs compensate by breathing less, but only weakly. Use the expected PaCO₂ to tell appropriate compensation from a second respiratory disorder.
Chapters (8)
Apply the expected PaCO₂ rise for metabolic alkalosis (about 0.7 mm Hg per 1 mEq/L rise in HCO₃⁻) and recognise when hypoventilation is excessive or inadequate.
The short version
- In metabolic alkalosis, PaCO₂ rises about 0.7 mm Hg for every 1 mEq/L that bicarbonate rises.
- Expected PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24), and it should not climb much above about 55 mm Hg.
- A PaCO₂ higher than expected means a concomitant respiratory acidosis: too little ventilation.
- A PaCO₂ lower than expected means a concomitant respiratory alkalosis: too much ventilation.
- A PaCO₂ inside the window means appropriate compensation and one disorder only.
Is it metabolic alkalosis compensation, or something else?
A man has been vomiting for four days. His bicarbonate is 36 and his PaCO₂ is 62 mm Hg. Is that his lungs compensating, as they should, or is something else going on?
This lesson is about metabolic alkalosis compensation: the expected PaCO₂ is about 0.7 mm Hg higher for every 1 mEq/L that bicarbonate rises. Once you can predict it, you can spot too much, or too little, ventilation.[2][3]
The mental model: the HCO₃⁻ / CO₂ ratio and the lungs
In metabolic alkalosis, the primary event is a rise in bicarbonate, with or without a compensatory rise in PaCO₂. The pH may be high, or nearly normal.[1][2]
Think of the ratio: bicarbonate on top, carbon dioxide on the bottom. Bicarbonate climbs, so the ratio climbs, and the pH goes up.
The lungs respond the opposite way from acidosis. They breathe less, so carbon dioxide builds up and pulls the ratio back down.[2][3] The baseline for the series is a bicarbonate of 24 and a PaCO₂ of 40.[4]
The rule: about +0.7 mm Hg PaCO₂ per +1 mEq/L HCO₃⁻
PaCO₂ rises about 0.7 mm Hg for every 1 mEq/L that bicarbonate rises. Sources quote a range of 0.6 to 0.75; this lesson uses 0.7.[2][3]
Compare acidosis: in Winters' formula the slope is 1.5. So compensation in metabolic alkalosis is weaker, and more variable between patients.[2][3]
The three-step recipe for expected PaCO₂, and the ceiling
40 = baseline PaCO₂ (mm Hg); 24 = baseline HCO₃⁻ (mEq/L); 0.7 = mm Hg per 1 mEq/L
- Find the riseBicarbonate minus 24.
- Multiply by 0.7This is the expected rise in PaCO₂.
- Add it to 40The result is the expected PaCO₂.
There is also a ceiling. PaCO₂ should not climb much above about 55 mm Hg. Some sources say 55 to 60, and higher values have been reported.[2][3]
Some resources use 0.7 × HCO₃⁻ + 20, which runs about 3 mm Hg lower (45 at HCO₃⁻ 36). This lesson uses the delta method; either way the patient's 62 is far too high.[3]
Worked example: bicarbonate 36, expected PaCO₂ about 48
- Bicarbonate is 36
- 36 − 24 = a rise of 12
- 12 × 0.7 = 8.4
- 40 + 8.4 = 48.4, about 48
So the expected PaCO₂ is near 48. His is 62: far above the prediction, and above the ceiling.
Reading a PaCO₂ above, below or inside the expected window
| Measured PaCO₂ | Meaning |
|---|---|
| Higher than expected | Concomitant respiratory acidosis: too little ventilation, too much carbon dioxide held back |
| Lower than expected | Concomitant respiratory alkalosis: too much ventilation, too much carbon dioxide blown off |
| Inside the window | Appropriate compensation, and one disorder only |
The expected window is the PaCO₂ predicted by the rule, about 47 to 49 mm Hg when HCO₃⁻ is 36 (using slopes of 0.6 to 0.75). A PaCO₂ outside the window is the sign of a second, respiratory disorder.[2][3]
Practice question
Original · USMLE styleA 58-year-old man is evaluated in the emergency department because of repeated vomiting for 4 days. Examination shows a distended upper abdomen with a succussion splash, and gastric outlet obstruction is diagnosed. He is given intravenous morphine for pain, and 30 minutes later he is drowsy but rousable. Temperature is 36.8 C (98.2 F), pulse is 104/min, respirations are 8/min, and blood pressure is 102/64 mm Hg. Laboratory studies show:
| Serum Na⁺ | 138 mEq/L |
|---|---|
| Serum K⁺ | 3.0 mEq/L |
| Serum Cl⁻ | 90 mEq/L |
| Serum HCO₃⁻ | 36 mEq/L |
| Arterial pH (room air) | 7.39 |
| Arterial PCO₂ (room air) | 62 mm Hg |
Common questions
What is the expected PaCO₂ in metabolic alkalosis?
Expected PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) mm Hg. PaCO₂ rises about 0.7 mm Hg for every 1 mEq/L that bicarbonate rises.
How high can PaCO₂ rise in metabolic alkalosis compensation?
It should not climb much above about 55 mm Hg. Some sources say 55 to 60, and higher values have been reported.
Why is compensation weaker than in metabolic acidosis?
In Winters' formula the slope is 1.5, while in metabolic alkalosis it is about 0.7. The response is also more variable between patients.
What does a PaCO₂ higher than expected mean in metabolic alkalosis?
It means a concomitant respiratory acidosis: too little ventilation, so too much carbon dioxide is held back. A PaCO₂ lower than expected means a concomitant respiratory alkalosis.
Transcript
Read the full narration, by chapter590 words
0:00Vomiting for four days: is that his lungs compensating?
A man has been vomiting for 4 days. His bicarbonate is 36, and his PaCO2 is 62 millimetres of mercury. Is that his lungs compensating, as they should? Or is something else going on? Today: the expected PaCO2 in metabolic alkalosis, about 0.7 for every one that bicarbonate rises, and how to spot too much, or too little, ventilation.
0:27The mental model: the HCO3⁻ / CO2 ratio and the lungs
First, the mental model. In metabolic alkalosis, the primary event is a rise in bicarbonate, with or without a compensatory rise in PaCO2. The pH may be high, or nearly normal. Think of the ratio: bicarbonate on top, carbon dioxide on the bottom. Bicarbonate climbs, so the ratio climbs, and the pH goes up. The lungs respond the opposite way from acidosis: they breathe less, so carbon dioxide builds up and pulls the ratio back down. Our baseline is a bicarbonate of 24 and a PaCO2 of 40. Now, how far should the PaCO2 rise?
1:09The rule: about +0.7 mm Hg PaCO2 per +1 mEq/L HCO3⁻
The rule: about 0.7 millimetres of mercury, for every one milliequivalent per litre that bicarbonate rises. Sources quote a range, 0.6 to 0.75. We'll use 0.7. Compare acidosis: in Winters' formula the slope is 1.5. So metabolic alkalosis compensation is weaker, and more variable between patients.
1:36The three-step recipe and the ceiling at about 55
Here's the recipe. Step one: find the rise. Bicarbonate minus 24. Step two: multiply by 0.7. Step three: add that to 40. There's also a ceiling. PaCO2 should not climb much above about 55. Some sources say 55 to 60, and higher values have been reported.
1:59Worked example: bicarbonate 36, expected PaCO2 about 48
Let's run our patient. Bicarbonate is 36. Thirty-six minus 24 is a rise of 12. Twelve times 0.7 is 8.4. Forty plus 8.4 is about 48. So we expect a PaCO2 near 48. His is 62. That is far above the prediction, and above the ceiling.
2:24Reading a PaCO2 above, below or inside the window
Now the interpretation. A PaCO2 higher than expected means a concomitant respiratory acidosis: too little ventilation, too much carbon dioxide held back. A PaCO2 lower than expected means a concomitant respiratory alkalosis: too much ventilation, too much carbon dioxide blown off. A PaCO2 inside the window means appropriate compensation, and one disorder only.
2:54Practice question
Your turn. A 58-year-old man has had 4 days of repeated vomiting from a gastric outlet obstruction. After morphine for pain, he becomes drowsy, with a respiratory rate of eight. Serum sodium 138, potassium 3.0, chloride 90, bicarbonate 36. Arterial blood gas on room air: pH 7.39, PaCO2 62. Which best describes his acid-base status? A, metabolic alkalosis with appropriate respiratory compensation. B, metabolic alkalosis with a concomitant respiratory alkalosis. C, metabolic alkalosis with a concomitant respiratory acidosis. D, chronic respiratory acidosis alone. E, metabolic acidosis with respiratory compensation. Pause here and decide. The expected PaCO2 is 40 plus 12 times 0.7: about 48. E is out: his bicarbonate is high, not low, and vomiting loses acid. A is out: appropriate compensation would give about 48, and his is 62. B is out: respiratory alkalosis would push the PaCO2 below the window, not above it. D is out: vomiting with low chloride makes the bicarbonate rise the primary event, not a response to carbon dioxide. So the answer is C: a metabolic alkalosis with too little ventilation, a respiratory acidosis on top, probably from the morphine.
4:38High-yield recap and next lesson
Three things to keep. One: in metabolic alkalosis, PaCO2 rises about 0.7 per one of bicarbonate. Two: expected PaCO2 is 40 plus 0.7 times the rise in bicarbonate, and it should not exceed about 55. Three: higher than expected means a second, respiratory acidosis; lower means respiratory alkalosis. Next up: stepwise blood gas interpretation, one algorithm for any gas.
Sources
Numbers in the text, e.g. [1], point here- Lewis JL III. Metabolic Alkalosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-alkalosis
- Lewis JL III. 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. Metabolic Alkalosis. Life in the Fast Lane (LITFL), 11 Oct 2025. litfl.com/metabolic-alkalosis/
- Reference baseline (pH 7.40, PaCO2 40, HCO3⁻ 24) and the Winters' formula contrast 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. 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-015.
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