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CDHB

Respiratory Failure

Definition

Respiratory failure is defined as occurring with PaO₂ <60 mm Hg, or PaCO₂ >50 mm Hg in a patient at rest breathing air. Respiratory failure is not a disease but reflects the inability of the lungs to maintain normal gas exchange.

Note: A kilopascal (kPa) is the SI unit of pressure. 1 mm Hg equals 0.13 kPa and 1 kPa equals 7.5 mm Hg.

Classification

• Type I Respiratory Failure (gas exchange/hypoxaemic) - causes include pulmonary oedema, infections, inflammatory lung disease and pulmonary embolism.
• Type II Respiratory Failure (ventilatory/hypercapnic) - causes include COPD, asthma, massive obesity, kyphoscoliosis, CNS depression due to drugs, neuromuscular disease and pneumothorax.

  Both types of respiratory failure may be acute or chronic.

Patient assessment: The underlying cause for the respiratory failure must be determined to enable appropriate treatment in each case.

Blood gas interpretation may be assisted by the following diagram (which is also discussed under Acidosis / Alkalosis):

A-a gradient in the assessment of hypoxaemia

Calculation of the A-a gradient assists in differentiating between hypoventilation and V/Q mismatching as the source of hypoxaemia. It predicts the degree of shunt by comparing the partial pressure of O₂ in the (A) alveoli to that in the (a) artery. The difference between them gives us an idea how well the oxygen is moving from the alveoli to the arterial blood.

Normal A-a gradient for a young adult is <20. The A-a gradient increases with age. For elderly patients (>75 yrs) a normal A-a gradient is <25.

Calculation of the A-a Gradient

Note: A-a gradient is best calculated from a blood gas taken on room air (FiO₂ = 0.21). Whilst on supplemental oxygen it is usually difficult to obtain an accurate assessment of FiO₂ due to variability in actual % of oxygen delivery.

Topic Code: 1624