Type 1 vs Type 2 Respiratory Failure

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Respiratory failure occurs when there is a failureΒ of gas exchange and/or ventilation, leading to abnormalities in arterial oxygen partial pressure (PaO2) and arterial carbon dioxide partial pressure (PaCO2) on arterial blood gas. A variety of conditions can affect the respiratory system andΒ cause respiratory failure.Β 

There are twoΒ broad categories of respiratory failure, which reflect different underlying pathophysiological processes:

  • Type 1 respiratory failure involves hypoxaemia (PaO2 <8 kPa / 60mmHg) with normocapnia (PaCO2 <6.0 kPa / 45mmHg)
  • Type 2 respiratory failure involves hypoxaemia (PaO2 <8 kPa / 60mmHg) with hypercapnia (PaCO2 >6.0 kPa / 45mmHg)
Mnemonic – type 1 and type 2 respiratory failure

An easy way to remember type 1 vs. type 2 respiratory failure when looking at an arterial blood gas is:

  • TypeΒ 1Β respiratory failure affectsΒ 1 valueΒ (PaO2 ↓)
  • Type 2Β respiratory failure affectsΒ 2 values (PaO2 ↓ and PaCO2 ↑)
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Type 1 respiratory failure

Type 1 respiratory failure involvesΒ hypoxaemia (PaO2 <8 kPa / 60mmHg) with normocapnia (PaCO2 <6.0 kPa / 45mmHg).

It usually occurs due to ventilation/perfusion (V/Q) mismatch –Β the volume of air flowing in and out of the lungs is not matched with the flow of blood to the lung tissue.

As a result of the ventilation/perfusion mismatch, PaO2 falls, and PaCO2 rises. The rise in PaCO2 rapidly triggers an increase in a patient’s overall alveolar ventilation, which corrects the PaCO2 but not the PaO2 due to the different shapes of the CO2 and O2 dissociation curves.

The final result is hypoxaemia (PaO2 < 8 kPa / 60mmHg) with normocapnia (PaCO2 < 6.0 kPa / 45mmHg).

Causes of type 1 respiratory failure

Examples of VQ mismatch include:

  • Reduced ventilation and normal perfusion (e.g. pneumonia, pulmonary oedema, bronchoconstriction)
  • Reduced perfusion with normal ventilation (e.g. pulmonary embolism)

Type 2 respiratory failureΒ 

Type 2 respiratory failure involvesΒ hypoxaemia (PaO2 is <8 kPa / 60mmHg) with hypercapnia (PaCO2 >6.0 kPa / 45mmHg).

It occurs as a result of alveolar hypoventilation, which prevents patients from being able to adequately oxygenate and eliminate CO2 from their blood.

This leads to PaO2 fallingΒ (due to lack of oxygenation) and PaCO2 risingΒ (due to lack of ventilation and elimination of CO2).

Causes of type 2 respiratory failure

Hypoventilation can occur for several reasons, including:

  • Increased resistance as a result of airway obstruction (e.g. COPD)
  • Reduced compliance of the lung tissue/chest wall (e.g. pneumonia, rib fractures, obesity)
  • Reduced strength of the respiratory muscles (e.g. Guillain-BarrΓ©, motor neurone disease)
  • Reduced respiratory drive (e.g. opioids and other sedatives)


Table 1. An overview of type 1 vs. type 2 respiratory failure.

  Type 1 Type 2
PaO2 Low Low
PaCO2 Normal or decreased High
Pathophysiology Ventilation/perfusion (V/Q) mismatch Alveolar hypoventilation


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