ABG Introduction

ABG Course Overview

Arterial blood gases are critical in assessing ventilation, acid-base balance, and oxygenation. The most important part of analyzing lab values is remembering to treat the patient, not the numbers. Lab results should always be correlated with good clinical data. Accurate history and physical examinations are a clinician's best resources.

**Normal ABG Values (at sea level)**
Variables	Ranges	Quick Reference Page
CaO₂	16 to 22 ml O₂/dL	16 to 22 ml of O₂ in 100 ml of blood
pH	7.35 and 7.45	pH reflects the concentration of hydrogen ions (H+) in solution
PaO₂	80 - 100 mmHg	Partial pressure of O₂ dissolved in arterial plasma
PaCO₂	35-45 mmHg	Partial pressure of CO₂ dissolved in arterial plasma
HCO₃	22-26 mEq/L	arterial Bicarbonate
SaO₂	95-100%	Arterial oxygen saturation
Cleveland Clinic (2022) Arterial Blood Gas (ABG) retrieved from https://my.clevelandclinic.org/health/diagnostics/22409-arterial-blood-gas-abg

What is an arterial blood gas (ABG) test?

The ABG blood test requires a small sample of arterial blood to measure the levels of oxygen, carbon dioxide and acid-base balance of your blood. An ABG usually includes the following measurements:

Arterial oxygen content (CaO2): This measures the amount of oxygen in arterial blood.
pH: This measures the balance of acids and bases in your blood, known as your blood pH level. The pH of blood is usually between 7.35 and 7.45. If it’s lower than that, your blood is considered too acidic. Your blood is considered too basic (alkaline) if it's higher than that range.
Partial pressure of oxygen (PaO2): measures the pressure of oxygen dissolved in your blood plasma. It helps show how well oxygen moves from your lungs to your bloodstream.
Partial pressure of carbon dioxide (PaCO2): This measures the amount of carbon dioxide in your blood and how well carbon dioxide can move out of your body.
Oxygen saturation (O2Sat): This measures how much hemoglobin in your blood is carrying oxygen. Hemoglobin is a protein in your red blood cells that carries oxygen from your lungs to the rest of your body.
Bicarbonate (HCO3): This is calculated using the measured values of pH and PaCO2 to determine the amount of the basic compound made from carbon dioxide (CO2.)
Hemoglobin: This measures the amount of hemoglobin, the protein responsible for carrying oxygen to your cells and your blood.

A modified Allen test is necessary before an ABG is drawn from either of the upper extremities to verify adequate collateral flow. Alternatively, the use of pulse oximetry and duplex ultrasound can verify adequate collateral circulation.

The most common arterial puncture site is the radial artery, which is superficial and easily palpable over the radial styloid process. The test is performed on the unilateral upper extremity chosen for the procedure (Figure:Modified Allen Test). The second most common site after the radial artery is the brachial artery. The third potential site for drawing an ABG is the femoral artery, but it is not preferred because it is a deeper puncture that is in close proximity to major veins and nerves.

It is best to approach arterial blood gas interpretation systematically. Interpretation leads to understanding the degree or severity of abnormalities, whether the abnormalities are acute or chronic, and whether the primary disorder is metabolic or respiratory in origin (castro et al, 2024).

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The normal ABG value for pH is about 7.4

References

Castro D, Patil SM, Zubair M, et al. Arterial Blood Gas. [Updated 2024 Jan 8]. In: StatPearls [Internet]. Treasure sland (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK536919/

Cleveland Clinic (2022) Atrerial Blood Gas ABG accessed retrieved from https://my.clevelandclinic.org/health/diagnostics/22409-arterial-blood-gas-abg)