Blood gases,

Blood gases are commonly measured, and with a logical, systematic approach they may be easily interpreted.

Blood gases provide information about:

• ventilation
• oxygenation and alveolar-arterial O2 gradient
• acid-base status

Note that to convert from kiloPascals to millimetres of mercury, multiply by 7.5.

The normal range for PaO2 is 11.3-13.3 kPa (82,5 to 97mmHg). A partial pressure less than 8 kPa (60mmHg) defines respiratory failure.

Arterial hypoxaemia is commonly due to pulmonary disease:

• poor gas transfer:
• type I respiratory failure
• PaCO2 is typically low
• hypoventilation:
• type II respiratory failure
• PaCO2 is typically raised

Arterial hypoxaemia may occur when the lungs are normal:

• low inspired partial pressure of oxygen e.g. altitude
• ventilation-perfusion mismatch
• right to left shunts:
• congenital heart disease
• arteriovenous malformation of the pulmonary vessels

Measurement of the alveolar-arterial (A-a) gradient is useful in hypoxic patients and may help to distinguish hypoxaemia due to pulmonary disease from other causes of hypoxaemia.

The Alveolar : arterial oxygen gradient is an index of the efficiency of gas transfer across the alveolar membrane. It is the difference in oxygen tension across the alveolar membrane.

• A-a (gradient) = PAO2 (partial pressure alveolar O2) - PaO2 (partial pressure arterial O2)

The alveolar partial pressure of oxygen, PAO2 is given as:

• PAO2 = (FiO2 - PCO2)/R where R is approximately 0.8 and FiO2 is 20 kPa on room air.

Otherwise the A-a gradient may be estimated by the formula:

• PiO2 (inspired PO2) - ( PaO2 (arterial PO2) + PaCO2 (arterial PCO2) )

The partial pressure of inspired O2 is approximately its percentage in kPa.

The upper limit for this gradient varies with age between 1.5 kPa and 3.0 kPa. A gradient in excess of 3.0 kPa suggests impaired gas exchange.