[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Respiratory](https://durgical.com/en/physiology/#s-resp) › Hypoxemic and hypercapnic respiratory failure

[Physiology](https://durgical.com/en/physiology/) · [Respiratory](https://durgical.com/en/physiology/#s-resp)

# Hypoxemic and hypercapnic respiratory failure

- Mechanism
- Exam priority: medium

The lungs can fail in two ways: they can't get oxygen into the blood (type 1), or they can't clear CO₂ because breathing is too weak (type 2).

Updated 2026-11-01

Also called: Type 1 and type 2 respiratory failure

**See it in 3D:** [Diaphragm](https://durgical.com/en/anatomy/diaphragm/) · [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)

## Key points

- Type 1: PaO₂ 45–50 mmHg with pH
- Type 1: areas with perfusion but no ventilation (filled or collapsed alveoli) add unoxygenated blood.
- Hypoxemia drives ventilation; CO₂, more diffusible, leaves easily and PaCO₂ stays normal or low.
- Type 2: alveolar ventilation falls from lack of drive (opioids, brainstem injury), weakness (Guillain-Barré) or fatigue against a high load (COPD).
- Because PaCO₂ is inversely proportional to alveolar ventilation, CO₂ accumulates as ventilation falls.
- Retained CO₂ displaces O₂ in the alveolus (alveolar gas equation), so PaO₂ falls too.
- Acute respiratory acidosis depresses consciousness and contractility; if it becomes chronic, the kidney compensates with bicarbonate.

### Oxygen–hemoglobin dissociation curve

**P50** 26.8 mmHg **Arterial blood** 97 % **Mixed venous blood** 75 % **O₂ given up to the tissues** 23 %

Normal curve. Tap a point (arterial, venous or P50) or move a control.

- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues

A PaO₂ of 60 mmHg sits at the edge of the steep part of the curve (≈90 % saturation): below it, every mmHg counts.

## The two types compared

| Feature | Type 1 (hypoxemic) | Type 2 (hypercapnic) |
| --- | --- | --- |
| PaO₂ | Low | Low |
| PaCO₂ | Normal or low | High |
| A–a gradient | Raised | Normal if pure hypoventilation |
| Mechanism | V/Q, shunt, diffusion | Alveolar hypoventilation |
| [Examples](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/) | ARDS, pneumonia, embolism, pulmonary edema | COPD exacerbation, opioids, Guillain-Barré |

## Clinical notes

- COPD patient growing drowsy: suspect acute-on-chronic hypercapnia; the blood gas shows low pH with high bicarbonate.
- ARDS: refractory hypoxemia with bilateral infiltrates; the PaO₂/FiO₂ ratio grades severity.
- Opioid overdose: slow breathing, pinpoint pupils, hypercapnia; naloxone reverses the effect at the receptor.
- Nursing and NCLEX: drowsiness and morning headache can be the first signs of CO₂ retention.

## Memory aid

Type 1 = the exchanger fails (1 gas off); type 2 = the pump fails (2 gases off).

## Test yourself

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

## Why does opioid hypoventilation have a normal A–a gradient?

The lung is healthy: PaO₂ falls only because accumulated CO₂ takes up space in the alveolus, and blood equilibrates with that alveolar gas.

## Why does hypoxemic failure usually come with a low PaCO₂?

Hypoxemia stimulates chemoreceptors and raises ventilation; CO₂ is cleared easily even from areas with V/Q mismatch.

## What separates acute from chronic hypercapnia on a blood gas?

In acute hypercapnia pH clearly falls; in chronic hypercapnia the kidney has retained bicarbonate for days and pH is near normal.

## Why does shunt hypoxemia improve little with oxygen while COPD hypoxemia does improve?

In shunt, blood passes unventilated alveoli that never see the O₂; in V/Q mismatch poorly ventilated alveoli still get some, and extra O₂ raises their PO₂.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Diaphragm](https://durgical.com/en/anatomy/diaphragm/)
- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)

## Related topics

- [The causes of hypoxemia](https://durgical.com/en/physiology/the-causes-of-hypoxemia/)
- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [Ventilation–perfusion matching and West zones](https://durgical.com/en/physiology/ventilation-perfusion-matching-and-west-zones/)
- [Respiratory acidosis and alkalosis](https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/)
- [The work of breathing](https://durgical.com/en/physiology/the-work-of-breathing/)
- [Control of breathing, chemoreceptors and lung reflexes](https://durgical.com/en/physiology/control-of-breathing-chemoreceptors-and-lung-reflexes/)
- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)

## Related conditions

- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)
- [Guillain-Barré syndrome](https://durgical.com/en/conditions/guillain-barre-syndrome/)
- [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/)

## In Pharmacology

- [Opioids and antagonists](https://durgical.com/en/pharmacology/classes/opioids-and-antagonists/)
- [Morphine](https://durgical.com/en/pharmacology/morphine/)
- [Fentanyl](https://durgical.com/en/pharmacology/fentanyl/)
- [Naloxone](https://durgical.com/en/pharmacology/naloxone/)

[← 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: [22.4 Gas Exchange](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-4-gas-exchange) (OpenStax (Rice University), 2022); [Acute Respiratory Distress Syndrome](https://www.ncbi.nlm.nih.gov/books/NBK436002/) (StatPearls (NCBI Bookshelf), 2026); [Chronic Obstructive Pulmonary Disease (COPD)](https://www.ncbi.nlm.nih.gov/books/NBK559281/) (StatPearls (NCBI Bookshelf), 2026); [Respiratory acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000092.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 43). 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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Source: https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/