Arterial Blood Gases (ABG)

Arterial Blood Gases is a blood investigation which majorly measures the acid-base balance and oxygenation of an arterial blood sample. ABG may not be a priority investigation in congenital heart disease when compared to an ECG (electrocardiography) and ECHO. However, when done, it might help with appreciating reduced oxygen saturation and changes in acid-base balance.

Blood gas can be used to assess:

i. Respiratory compromise

ii. A Patient’s status peri- and post- cardiopulmonary arrest

iii. Patient’s with medical conditions that cause metabolic derangements (Diabetic Ketoacidosis, Sepsis, Renal failure, drug overdose etc.)

iv. How effective Oxygen therapy, Fluid and electrolyte replacement and ventilatory support have been.

v. Levels of dyshemogblobins e.g. carboxyhemoglobin and methemoglobin

Components of the test

The major components of the test include pH, Partial pressure of arterial oxygen (PaO₂ ), Partial pressure of arterial Carbondioxide (PaCO₂), Bicarbonate and Arterial oxygen saturation. Arterial blood electrolytes, glucose and lactate may also be included.

pH: Measures the hydrogen ion concentration in arterial blood. The normal range is cited between 7.35-7.45. Values below 7.35 signify acidosis and values above 7.45 signify alkalosis.

PaCO₂: This forms the respiratory component of the test and shows how well the lungs are handling Carbondioxide in the body or otherwise. Normal range is 35-45mmHg A high value (>45mmHg) signifies respiratory acidosis meaning the lungs are not excreting CO₂ as required or it is being produced in excess of the rate at which the lungs can clear it and vice versa for alkalosis.

Bicarbonate: This forms the metabolic/renal component of the test. Bicarbonate is produced in the kidneys and therefore plays a major role in acid-base balance. Normal range is 22-26 mEq/L.

A reduction in bicarbonate concentration signifies metabolic acidosis and an increase in bicarbonate concentration signifies metabolic alkalosis.

PaO₂: A measure of Oxygen dissolved in arterial blood. Values range between 80-100mmhg. <60% signifies hypoxia

Oxygen Saturation (SaO₂): Oxygen saturation represents the oxygen binding capacity of hemoglobin. Normal oxygen saturation should range between 95-100% however in practice >92% might be allowed as adequate. Factors that affect oxygen saturation are; Temperature, pH, Carbondioxide levels and 2,3 diphosphoglycerate levels.

Causes of derangements:

Respiratory acidosis: Hypoventilation, Respiratory infection, Severe outflow obstruction (Asthma or COPD), Pulmonary edema, Pneumothorax, Neuromuscular disorders, CNS depression, spinal cord injury, chest wall injury.

Respiratory alkalosis: Hyperventilation, Pain, anxiety, hypoxia, pulmonary embolism.

Metabolic acidosis: Renal failure, diabetic ketoacidosis, Lactic acidosis, sepsis, shock, diarrhea, drugs, toxins.

Metabolic alkalosis: Diuretics, corticosteroids, excess vomiting, dehydration, liver failure, hypokalemia.

Contraindications

• Presence of severe peripheral vascular disease.
• Severe Coagulopathy (problems with blood clotting to stop bleeding) or patients on anticoagulation therapy e.g. warfarin
• Patient on thrombolytic agents.
• Local infection or distorted anatomy at possible puncture site.