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# Respiratory acidosis

The lungs do not remove enough CO₂ and the blood turns acidic; it happens in COPD, sedative overdose and muscle weakness.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Diaphragm](https://durgical.com/en/anatomy/diaphragm/)
- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Right upper lobe](https://durgical.com/en/anatomy/superior-lobe-of-right-lung/)
- [Right middle lobe](https://durgical.com/en/anatomy/middle-lobe-of-right-lung/)
- [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/)
- [Left upper lobe](https://durgical.com/en/anatomy/superior-lobe-of-left-lung/)
- [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/)
- [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/)
- [Phrenic nerves](https://durgical.com/en/anatomy/phrenic-nerve/)

## In biochemistry

- [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/)
- [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/)
- [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/)

## In the clinical exam

- [Respiratory distress and respiratory failure: the signs](https://durgical.com/en/clinical-exam/respiratory-distress-and-respiratory-failure-the-signs/)

## In physiology

- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)
- [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/)
- [Mixed acid–base disorders](https://durgical.com/en/physiology/mixed-acid-base-disorders/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [Control of breathing, chemoreceptors and lung reflexes](https://durgical.com/en/physiology/control-of-breathing-chemoreceptors-and-lung-reflexes/)
- [Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy](https://durgical.com/en/physiology/oxygenation-spo-pao-cyanosis-and-oxygen-therapy/)
- [Hypoxemic and hypercapnic respiratory failure](https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/)

## What it is

A primary rise in pCO₂ (hypercapnia) from alveolar hypoventilation, with a fall in pH; it may be acute or chronic and [kidney](https://durgical.com/en/anatomy/kidney/) -compensated.

## Who gets it

Common in COPD exacerbations, a highly prevalent disease in Colombia from smoking and wood-smoke exposure, and in opioid or sedative poisoning. Severe obesity adds cases of chronic hypoventilation.

## Causes

- Depressed respiratory centre: opioids, benzodiazepines, alcohol, anaesthesia, brainstem injury
- Neuromuscular disease: Guillain-Barré, myasthenia, high [spinal cord](https://durgical.com/en/anatomy/spinal-cord/) injury, amyotrophic lateral sclerosis
- Lung or airway disease: COPD, severe asthma, extensive pneumonia, advanced pulmonary oedema
- Chest wall: obesity (obesity hypoventilation syndrome), kyphoscoliosis, flail chest
- Inadequate mechanical ventilation

## Risk factors

- COPD from tobacco or wood smoke
- Opioid or sedative use
- Severe obesity and sleep apnoea
- Neuromuscular diseases

## Pathophysiology

CO₂ production outpaces its removal and pCO₂ rises; through the carbonic anhydrase reaction H⁺ and HCO₃⁻ increase. In the acute form only cell buffers act and HCO₃⁻ rises just 1 mmol/L per 10 mmHg; in the chronic form the kidney increases H⁺ and ammonium excretion and reabsorbs more HCO₃⁻ (about 3.5–4 per 10 mmHg) over 3–5 days. CO₂ crosses the blood–brain barrier, acidifies cerebrospinal fluid and dilates cerebral vessels: headache, drowsiness and, in severe cases, coma from CO₂ narcosis. On room air it is nearly always accompanied by hypoxaemia.

## Classification

- Acute: low pH with near-normal HCO₃⁻
- Chronic: near-normal pH with high HCO₃⁻
- Acute-on-chronic: COPD exacerbation

## Symptoms and signs

- Breathlessness or, conversely, slow shallow breathing (sedatives)
- Morning headache (chronic hypoventilation)
- Drowsiness, confusion
- Flapping tremor (asterixis)
- Warm, flushed [skin](https://durgical.com/en/anatomy/skin/) from vasodilatation

## Red flags

- Progressive drowsiness or coma
- Slow breathing after opioids
- Very low pH with rising pCO₂
- Respiratory muscle fatigue (paradoxical breathing)

## Diagnosis

**Clinical**
History of COPD, sedatives or muscle weakness; respiratory rate, accessory muscle use and level of consciousness.

**Tests**
Arterial gases (high pCO₂, low pH, HCO₃⁻ by chronicity), oximetry, electrolytes, toxicology, lung or neuromuscular function tests once stable.

**Imaging**
[Chest](https://durgical.com/en/anatomy/region/chest/) X-ray for pneumonia, oedema or pneumothorax; CT according to the cause.

## Treatment

**Acute**
Secure the airway and ventilation; opioid antagonist (naloxone) for overdose

**Procedure**
Non-invasive ventilation in COPD exacerbations or obesity hypoventilation; intubation and mechanical ventilation if it fails

**Medical**
Treat the cause: bronchodilators and corticosteroids in COPD or asthma, antibiotics in pneumonia, immunotherapy in Guillain-Barré

**Follow-up**
Controlled oxygen with a moderate saturation target in COPD to avoid worsening hypercapnia; slow correction of chronic pCO₂

## Complications

- Coma from CO₂ narcosis
- Arrhythmias
- Respiratory arrest
- Post-hypercapnic alkalosis if over-ventilated

## Prognosis

Depends on the cause; acute hypercapnia needing ventilation in COPD marks advanced disease.

## Prevention

- Stopping smoking and reducing wood-smoke exposure
- Careful use of opioids and sedatives
- Treating sleep apnoea

## Test yourself

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

## What separates acute from chronic respiratory acidosis?

HCO₃⁻: it rises 1 mmol/L per 10 mmHg of pCO₂ in the acute form and 3.5–4 in the chronic form, thanks to renal compensation.

## Why can uncontrolled oxygen worsen hypercapnia in COPD?

It worsens ventilation–perfusion mismatch and the Haldane effect, and in some patients it reduces respiratory drive.

## What causes headache and drowsiness in hypercapnia?

CO₂ crosses the blood–brain barrier, acidifies cerebrospinal fluid and dilates cerebral vessels.

## Which drugs cause acute respiratory acidosis by central depression?

Opioids, benzodiazepines and other sedatives, as well as alcohol.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/conditions/respiratory-acidosis/)

## Related conditions

- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Obstructive sleep apnoea](https://durgical.com/en/conditions/obstructive-sleep-apnoea/)
- [Guillain-Barré syndrome](https://durgical.com/en/conditions/guillain-barre-syndrome/)
- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Obesity](https://durgical.com/en/conditions/obesity/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Respiratory acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000092.htm) (MedlinePlus, US National Library of Medicine, 2026); [Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Source: https://durgical.com/en/conditions/respiratory-acidosis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/respiratory-acidosis/