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[Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal)

# Mixed acid–base disorders

- Measurement
- Exam priority: medium

Sometimes two or three acid–base problems happen at once; they show up because the compensation doesn’t fit what is expected.

Updated 2026-11-01

Also called: Double and triple disorders, Inappropriate compensation

## Key points

- Compensation does not overcorrect: a normal pH with abnormal PaCO₂ and HCO₃⁻ suggests a mixed disorder.
- PaCO₂ and HCO₃⁻ moving in opposite directions = two primary disorders adding up (pH very abnormal).
- Compensation outside the expected range = a second respiratory or metabolic disorder.
- Respiratory acidosis and respiratory alkalosis cannot coexist; metabolic acidosis and metabolic alkalosis can.
- Rise in anion gap larger than the fall in HCO₃⁻: on top of the high anion gap acidosis there is a metabolic alkalosis.
- Classic examples: salicylates and sepsis (respiratory alkalosis + high anion gap acidosis); cardiac arrest (respiratory + lactic acidosis).

## What it is

A mixed disorder is two or more primary disorders at the same time. Physiology provides the clues: compensation takes time and has limits, never overshoots normal pH and almost never fully normalises it. If PaCO₂ or HCO₃⁻ falls outside the expected compensation, another process is present. This page covers why mixed disorders happen and how to spot them; the steps and formulas are in Biochemistry.

## Key facts

- Respiratory compensation starts in minutes and is complete in 12–24 hours; renal compensation takes 3–5 days.
- So the same high PaCO₂ comes with a different HCO₃⁻ in the first hours than after several days: duration changes what is expected.
- Chronic respiratory alkalosis comes closest to a normal pH (for example in pregnancy or at altitude).
- In Bogotá and other high cities baseline PaCO₂ is lower: comparing with local values avoids diagnosing a mixed disorder that isn’t there.

## Combinations that get asked

| Situation | Disorders | Why |
| --- | --- | --- |
| [Salicylate poisoning (adult)](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/) | Respiratory alkalosis + high anion gap acidosis | Direct respiratory-centre stimulation and metabolic uncoupling with organic acids |
| [Sepsis](https://durgical.com/en/conditions/sepsis-and-septic-shock/) | Respiratory alkalosis + lactic acidosis | Mediators and fever drive ventilation; hypoperfusion makes lactate |
| [Cardiorespiratory arrest](https://durgical.com/en/conditions/lactic-acidosis/) | Respiratory acidosis + lactic acidosis | No ventilation and no perfusion |
| [Ketoacidosis with vomiting](https://durgical.com/en/conditions/diabetic-ketoacidosis/) | High anion gap acidosis + metabolic alkalosis | Keto-acids on one side, HCl loss on the other |
| [COPD on a diuretic](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/) | Respiratory acidosis + metabolic alkalosis | CO₂ retention plus volume contraction and hypokalaemia |
| [Advanced cirrhosis with vomiting or diuretics](https://durgical.com/en/conditions/cirrhosis/) | Respiratory alkalosis + metabolic alkalosis | Hyperventilation from toxins and progesterone, plus acid loss |

## Clinical notes

- A patient with ketoacidosis and a normal PaCO₂ is not “well compensated”: respiratory compensation is missing (fatigue, sedation, pneumonia) and the patient is at risk.
- In COPD with chronic hypercapnia, a raised HCO₃⁻ is expected; if it climbs much higher, a metabolic alkalosis has been added (diuretics, vomiting).

## Memory aid

“The body draws, it doesn’t win”: compensation brings pH toward 7.40 but never past it; if pH crosses or sits exactly on 7.40, look for a second disorder.

## Test yourself

Think of the answer, then tap the question to see it.

## A patient has pH 7.40, PaCO₂ 25 mmHg and HCO₃⁻ 15 mmol/L. Why is this not a simple compensated disorder?

Compensation almost never fully normalises pH. Both values badly deranged with an exactly normal pH point to two primary disorders (metabolic acidosis plus respiratory alkalosis), for example from salicylates or sepsis.

## Why can respiratory acidosis and respiratory alkalosis not coexist?

PaCO₂ has a single value at any moment: high, low or normal. HCO₃⁻, by contrast, can receive an acid load and an H⁺ loss at the same time.

## In ketoacidosis the anion gap rises by 18 mmol/L but HCO₃⁻ falls by only 6. What does that indicate?

There was more HCO₃⁻ than expected: alongside the high anion gap acidosis there is a metabolic alkalosis (for example from vomiting).

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## Related topics

- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)
- [Metabolic acidosis, the anion gap and compensation](https://durgical.com/en/physiology/metabolic-acidosis-the-anion-gap-and-compensation/)
- [Metabolic alkalosis and its compensation](https://durgical.com/en/physiology/metabolic-alkalosis-and-its-compensation/)
- [Respiratory acidosis and alkalosis](https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/)
- [Buffers and the Henderson–Hasselbalch equation](https://durgical.com/en/physiology/buffers-and-the-henderson-hasselbalch-equation/)

## Related conditions

- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [High anion gap metabolic acidosis](https://durgical.com/en/conditions/high-anion-gap-metabolic-acidosis/)
- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)
- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)
- [Metabolic alkalosis](https://durgical.com/en/conditions/metabolic-alkalosis/)
- [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/)
- [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)

## In Biochemistry

- [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/)
- [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/)
- [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [26.5 Disorders of Acid-Base Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-5-disorders-of-acid-base-balance) (OpenStax (Rice University), 2022); [Bacterial Sepsis](https://www.ncbi.nlm.nih.gov/books/NBK537054/) (StatPearls (NCBI Bookshelf), 2024); [Respiratory alkalosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000111.htm) (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 31). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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