Arterial Blood Gas - An Overview

What Is Arterial Blood Gas?

Arterial blood gas (ABG) is a diagnostic test for assessing the blood's oxygen and carbon dioxide levels and determining the body's acid-base balance (pH). ABG is considered the standard test for analyzing oxygenation and ventilation status. The patient’s partial pressure of oxygen (PaO2) is used for assessing oxygenation status, and the partial pressure of carbon dioxide (PaCO2) gives the ventilation status.

For ABG, blood is taken only from an artery, unlike other blood gas analysis tests where blood may be taken from an artery, vein, or capillary. Other names for ABG are

• Blood gas test.

• Oxygen saturation test.

ABG is used for assessing respiratory, metabolic, and circulatory disorders. The lungs and kidneys play an essential role in maintaining the acid-base balance of the body. ABG helps in evaluating the acid-base balance of blood and thus helps in the diagnosis of kidney disorders. Lungs are responsible for gaseous exchange and removing carbon dioxide from the blood (carbon dioxide is an acidic waste product of metabolism). This carbon dioxide then gets eliminated from the body through exhaled air. Furthermore, oxygen from the inhaled air is taken into the blood through the gaseous exchange in the lungs. So, assessing the level of oxygen and carbon dioxide in the blood helps in determining the status of the lungs. ABG thus has various applications in pulmonary medicine, endocrinology, emergency medicine, cardiology, and more.

What Are the Indications for ABG?

ABG is used as a diagnostic tool in the following conditions:

• Acid-base imbalances resulting in:

  • Confusion.

  • Muscle cramps.

  • Nausea.

  • Vomiting.

  • Rapid breathing.

  • Abnormal heart rhythm.

  • Fatigue.

• Lung disorders include the following:

  • COPD (Chronic Obstructive Pulmonary Disease).

  • Asthma.

  • Myasthenia gravis.

  • Cystic fibrosis.

• In patients who have undergone oxygen therapy.

• In patients having burns in the airway.

• Carbon monoxide poisoning cases.

• In case of head and neck injuries presenting with breathing difficulties.

• Inhalation injury.

• For patients in shock (hypovolemic shock).

• In cases of a drug overdose.

• In newborns with breathing difficulty.

• In smokers, it is used to know the risk of congestive heart failure.

• Congenital metabolic disorders.

• In patients who have undergone a near-drowning accident.

How Is the Test Carried Out?

The test is carried out as follows:

• ABG is carried out by a healthcare provider, mostly a respiratory therapist. Blood is collected from the artery in the wrist (radial artery), arm (brachial artery), or groin (femoral artery).

• If ABG is to be done in a newborn, the sample is collected from the umbilical cord or baby’s heel.

• Ultrasound imaging may be used for detecting the artery.

• A modified Allen test is performed before ABG, in which the patient is asked to hold the hand high with a clenched fist. The therapist then applies pressure to the ulnar and radial arteries for 30 seconds to occlude the blood flow. After 5 seconds, the patient is asked to unclench the fist. On unclenching, the palm appears pale. Then the pressure on the ulnar artery is released by the therapist and after 10 to 15 seconds palm returns to its original color. This indicates good blood flow in the hand. If the palm does not return to its original color, then ulnar blood flow is not sufficient; in that case, the radial artery should not be punctured.

• After locating the artery, the area is cleaned and disinfected.

• The required blood sample is collected using a needle, and a bandage is applied at the injection site.

Laboratory Procedures for ABG

The laboratory procedures after sample collection (blood collection) are

• The blood specimen collected through an arterial puncture or from an indwelling arterial catheter is sent for analysis.

• The blood sample is placed on ice and analyzed immediately to avoid errors.

• The analysis is done using an automated blood gas analyzer.

• The results are obtained in about 10 to 15 minutes.

Which Are the Parameters Measured Using an ABG?

ABG help in determining the following parameters, which are called the ABG components, includes the following:

• The partial pressure of oxygen in arterial blood (PaO2).

• The partial pressure of carbon dioxide in arterial blood (PaCO2).

• The acid-base balance of the blood (pH).

• Bicarbonate concentration in arterial blood (HCO3).

• Relative excess or deficit of base in arterial blood (base excess/deficit).

• Arterial oxygen saturation (SaO2).

• Haemoglobin in blood.

What Are the Normal Values?

The normal value range varies according to age (neonates to old age) and also among laboratories. The normal value range is as follows:

• PaO2 - 75 to 100 mm Hg.

• PaCO2 - 354 to 5 mm Hg

• HCO3 - 22 to 26 meq/L.

• pH - 7.35 to 7.45.

• Base excess or deficit - -4 to +2

• SaO2 - 95 to 100 %.

Interpretation of Result

For values of ABG parameters which does not fall within the normal range, the result interpretation is as follows:

• If pH is increased, it indicates alkalosis; if pH is decreased, it indicates acidosis.

• If pCO2 is increased, it indicates respiratory acidosis; if pCO2 is decreased, it indicates respiratory alkalosis.

• If HCO3 is increased, it indicates metabolic alkalosis; if decreased, it indicates metabolic acidosis.

• If taken, the ABG results are compared with previous ABG results to assess the success of treatment.

What Are the Associated Risks?

The risks associated with ABG are fewer and include bleeding and soreness at the injection site. Damage to a nerve or an artery can occur in some rare cases. Patients are advised to avoid lifting heavy objects for about 24 hours after the procedure.

What Are the Steps to Be Taken to Avoid Incorrect Results?

ABG can result in incorrect results due to improper sampling techniques or processing errors. However, the possibility of errors can be reduced by taking the following steps:

• Proper quality control of the automated blood gas analyzer.

• ABG is a procedure that involves multiple health professionals like nurses, respiratory therapists, etc. So proper coordination and communication between the team are important.

• Proper sample transportation (otherwise, sample contamination can occur).

• Avoid air bubble trapping during blood sample collection.

Conclusion

ABG is the standard diagnostic test for evaluating the patient's ventilation, oxygenation, and acid-base status. Though noninvasive tests have replaced it, it is still used as a confirmatory test. ABG is also useful in evaluating the success of treatment intervention and the progression of cardiopulmonary disorders. Though of great clinical significance, there are chances for incorrect results due to improper sampling techniques or processing errors. This is one of the major disadvantages of ABG. However, proper care in the procedure and attention to detail in processing can easily overcome this disadvantage.