Blood gas arterial

Keywords: Arterial blood gas, ABG, pC02, Blood Gas pCO2, pO2, Blood Gas pO2, Blood Gas pH, pH blood gas, pH - arterial blood

Description:

Tests include pO2, pCO2, pH and various derived parameters eg, base excess.

Most benchtop analysers also measure haemoglobin, oxygen saturation, and met- and carboxy-haemoglobin. 

See also Haemoximetry.

Specimen:

1 mL arterial blood collected anaerobically in a heparinised syringe and transported rapidly to the laboratory in a capped syringe with the needle removed.

An appropriately collected 'arterialised' capillary blood specimen may have values close to those of arterial blood.

Fractional concentration of inspired O[FiO2] should be specified on request form.

Method:

Selective electrodes, chemiluminescence.

Reference Interval:

pO2: 11.0-13.5 kPa (80-100 mmHg) (varies with age)

pCO2: 4.6-6.0 kPa (35-45 mmHg)

pH: 7.36-7.44 (36-44 nmol/L)

Base excess: (-3) to (+3) mmol/L

Alveolar-arterial pO2 difference: <3.3 kPa (<25 mmHg) if FiO2  = 0.21 (ie, room air).

Application:

Assessment of cardiopulmonary function, acid-base balance.

Interpretation:

FiO2 and patient’s temperature must be known.

Decreased pO2 is seen with hypo­ventilation, ventilation/perfusion mismatch, alveolar-capillary block and right to left shunts.

Increased pO2 may be seen with hyperventilation or oxygen therapy.

Decreased pCO2 (respiratory alkalosis) is usually a compensatory phenomenon in metabolic acidosis, but may be a primary abnormality; in both situations it is due to hyperventilation.

Increased pCO2 (respiratory acidosis) occurs in respiratory failure, but is also seen as a compensatory phenomenon, caused by hypoventilation, in metabolic alkalosis.

Decreased pH indicates a net acidaemia and increased pH indicates a net alkalaemia. The acid-base balance component (viz metabolic or respiratory) that is in the same direction as the pH change is the primary abnormality in acid-base imbalance.

Base excess is decreased in metabolic acidosis and compensated respiratory alkalosis.

It is increased in metabolic alkalosis or compensated respiratory acidosis.

Alveolar-arterial pO2 difference is elevated in all causes of hypoxia except hypoventilation. The reference interval is defined only for room air.

Reference:

Walmsley RN et al. Cases in Chemical Pathology. 2nd ed. Singapore: PG Publishing, 1988.

Breen PH. Anesthesiol Clin North Am 2001; 19: 885-906.