Arterial Blood Gas (ABG)

indications, • Contraindications, Which Artery? Normal Values, Interpretation, ALLENS test, Treatment.

1.  Introduction:
 An arterial blood gas (ABG) is a test that measures the oxygen tension (PaO2), carbon dioxide tension
(PaCO2), acidity (pH), oxyhemoglobin saturation (SaO2), and bicarbonate (HCO3) concentration in arterial
blood.
 However, it is important to remember that results are not always definitive.
◦ A normal result does not exclude an acid-base disorder
◦ Additional information is often needed to make a conclusive diagnosis.
 The common indications for ABGs are:
1. Any respiratory distress/failure (acute or chronic)
2. Any severe illness which could lead to an acidotic state e.g.
- cardiac failure - liver failure - renal failure - hyperglycaemic state e.g. ketoacidosis
- multiorgan failure – sepsis – burns - poisons/toxins
3. Assessment of response to interventions such as ventilation
 Contraindications
1. Infection/lesion at the site
2. Poor collateral circulation (Negative allen test)
3. Anticoagulation
 Which Artery?
- The radial artery should be always the first choice.
- Other arteries (femoral, dorsalis pedis, brachial) can be used in emergencies.
Normal Values for ABGs:
 pH
- Measurement of acidity or alkalinity of the blood. based on the hydrogen (H+) ions present, -
The normal range is 7.35 to 7.45

 PaO2

- measure of the pressure of oxygen dissolved in the blood. It determines how well
oxygen is able to flow from the lungs into the blood.
- The normal range is (80 – 100 mmHg).
 SaO2
- measure of the amount of oxygen being carried by the hemoglobin in the red blood cells.
- The normal range is >94%
 PaCO2
- measure of the pressure of carbon dioxide dissolved in the blood. It determines how well
carbon dioxide is able to flow out of the body.
- The normal range is (35 – 45 mmHg)
 HCO3
- a chemical that helps prevent the pH of blood from becoming too acidic or too basic.
- The calculated value of the amount of bicarbonate in the bloodstream.
- The normal range is 22 – 26 mEq/L

2.  B.E.
- The base excess indicates the amount of excess or insufficient level of bicarbonate in the
system.
- The normal range is –2 to +2 ml litre
- (A negative base excess indicates a base deficit in the blood.)

 Steps to Arterial Blood Gas Interpretation
The arterial blood gas is used to evaluate both acid-base balance and oxygenation,
each representing separate conditions. Acidbase evaluation requires a focus on three of the
reported components: pH, PaCO2 and HCO3.
1. Step One
Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is
above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic
2. Step Two
If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a
respiratory or metabolic problem.

 To do this, assess the PaCO2 level.
- Remember that with a respiratory problem, as the pH decreases <7.35, the PaCO2
should rise. If the pH rises >7.45, the PaCO2 should fall

- Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite
directions, then the problem is primarily respiratory.

3. Step three
- Finally, assess the HCO3 value. Recall that with a metabolic problem,
- normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases,
HCO3 will decrease too.
- Compare the two values. If they are moving in the same direction, then the problem is
primarily metabolic



 Th e following chart summarizes the relationships between pH, PaCO2 and HCO3

3. Treatment:
1. Respiratory Acidosis:
- Bronchodilator drugs.
- Oxygen if the blood oxygen level is low.
- stop smoking.
2. Respiratory Alkalosis:
- Beta-adrenergic blockers may help control the manifestations of the h y peradrenergic
state that can lead to hyperventilation syndrome in some patients.

4. "Hyperadrenergic" is a state with abnormally elevated amount of excitation or
anxiety. A person would feel nervous, shaky, agitated, have low appetite, and
maybe feel a little nauseous.

3. Metabolic Acidosis:
- Fluid, insulin and electrolyte replacement is necessary for diabetic ketoacidosis
- Administration of bicarbonate and/or dialysis may be required for acidosis associated
with renal failure.
4. Metabolic Alkalosis:
- Supplement with Potassium to treat hypokalemia
- H+ blockers or PPIs if vomiting/NG suction to prevent further losses in H+ ions

ALLENS test performs to check that the patient can tolerate a temporary blockage of
the radial artery.
1. The hand is elevated and the patient/person is asked to make a fist for about 30 seconds.
2. Pressure is applied over the ulnar and the radial arteries so as to occlude both of them.
3. Still elevated, the hand is then opened. It should appear blanched (pallor can be observed at the
finger nails).
4. Ulnar pressure is released and the colour should return in 5 to 15 seconds.
The Allen's test is testing for abnormal circulation. If color returns as described above, the Allen's test is
considered to be "Positive". If color fails to return, the test is considered "Negative" and the ulnar
artery supply to the hand is not sufficient. The radial artery therefore cannot be safely cannulated.