Type 1 vs Type 2 Respiratory Failure

Introduction

Respiratory failure occurs when there is a failure 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 and cause respiratory failure. 

There are two 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)

Type 1 respiratory failure

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

It usually occurs due to ventilation/perfusion (V/Q) mismatch – 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 failure 

Type 2 respiratory failure involves 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 falling (due to lack of oxygenation) and PaCO₂ rising (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-Barré, motor neurone disease)
• Reduced respiratory drive (e.g. opioids and other sedatives)

Summary

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