Respiratory failure (types I and II)

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What is Respiratory failure?

Respiratory failure is a disease of the lungs.

The respiratory system basically consists of a gas exchanging organ (the lungs) and a ventilatory pump (respiratory muscles and the thorax). Either or both of these can fail and cause respiratory failure.

Respiratory failure occurs when gas echange at the lungs is sufficiently impaired to cause a drop in blood levels of oxgyen (hypoxaemia); this may occur with or without an increase in carbon dioxide levels. The definition of respiratory failure is PaO2 <8kPa (60mmHg), or PaCO2 >7kPa (55mmHg).

Respiratory failure is divided into type I and type II.

Type I respiratory failure involves low oxygen, and normal or low carbon dioxide levels.
Type II respiratory failure involves low oxygen, with high carbon dioxide.

Who gets Respiratory failure?

Respiratory failure is common, as it occurs in any severe lung disease - it can also occur as a part of multi-organ failure.

Predisposing Factors

Causes of Type I respiratory failure: disease that damage lung tissue, including pulmonary oedema, pneumonia, acute respiratory distress syndrome, and chronic pulmonary fibrosing alveoloitis.

Causes of Type II respiratory failure: the most common cause is chronic obstructive pulmonary disease (COPD). Others include chest-wall deformities, respiratory muscle weakness (e.g. Guillain-Barre syndrome) and central depression of the respiratory centre (e.g. heroin overdose).

Progression

Type I respiratory failure occurs because of damage to lung tissue. This lung damage prevents adequate oxygenation of the blood (hypoxaemia); however, the remaining normal lung is still sufficient to excrete the carbon dioxide being produced by tissue metabolism. This is possible because less functioning lung tissue is required for carbon dioxide excretion than is needed for oxygenation of the blood.

Type II respiratory failure is also known as 'ventilatory failure'. It occurs when alveolar ventilation is insufficient to excrete the carbon dioxide being produced. Inadequate ventilation is due to reduced ventilatory effort, or inability to overcome increased resistance to ventilation - it affects the lung as a whole, and thus carbon dioxide accumulates.

Complications include: damage to vital organs due to hypoxaemia, CNS depression due to increased carbon dioxide levels, respiratory acidosis (carbon dioxide retention). This is ultimately fatal unless treated. Complications due to treatment may also occur.

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