Arterial Blood Gas (ABG): Internal Medicine for the Ignorant

Arterial Blood Gas (ABG) Interpretation

An arterial blood gas (ABG) is the measurement of pH and partial pressure of both oxygen and carbon dioxide from a sample of arterial blood. The blood is aspirated into a blood gas analyzer which passes electrical current across three different electrodes. Each electrode measures conductivity / resistance within the blood and the software compares the results with a reference range for corresponding blood gas values. So, blood is collected and fed into the machine. The machine electrifies the blood (essentially) and measures how much current passes through at each electrode. The values from each electrode are then taken by the analyzer’s software and compared and matched on a data table. The end result is whatever number matched the measured electrical current passed through the blood. Pretty cool. 

You may or may not have heard of Dalton’s law. It says that when you add up all of the partial pressures (represented as “Pa” in front of the gas, i.e. PaCO2 or PaO2) of gasses within a mixture, you get the total gas pressure within the liquid. It’s not the most elegant but makes complete sense. If you have two gasses occupying space within a mixture, the pressure of one gas plus the pressure of the other gas equals the total gas pressure within the mixture. Simple addition. So, don’t let partial pressure confuse you. When we say partial pressure, just remind yourself that we’re measuring the partial pressure of the mixture and the whole pressure of the gas being measured within the mixture.

Now, on to ABG interpretation. 

First, we’re going to assume that the ABG result is valid. If you’re interested in evaluating the validity of the results, the Henderson-Hasselbach formula can be used and there are multiple online calculators to do so. 

Second, remind yourself that alkalosis and acidosis can exist even when the pH is normal. Just remember that. Then, look at the pH.

The pH is usually consistent with the primary problem (acidosis or alkalosis).

Third, evaluate the source (respiratory or metabolic) of the pH disturbance.

This gives you the general idea of whether the acid/base disturbance is primarily respiratory or metabolic. However, the human body is an amazing biological machine that has many, many, many mechanisms in place to achieve homeostasis (required for continued healthy living) and that attempt to compensate for and balance the acid/base mixture. So, be aware that metabolic or respiratory compensation can and does occur and ABG results can sometimes look confusing. There are many online calculators that can help in determining compensation. 

From this point, you could calculate an anion gap, AG, (the difference between positively and negatively charged ions in the blood) using the formula AG=(Na-Cl+HCO)-12. However, most modern metabolic panels generate this for us anyway.

Now, a word about arterial blood gas versus venous blood gas. In most circumstances (i.e. outside of hypercapnia), a venous blood gas will correlate well with the same pH, PaCO2 (PCO2), bicarbonate (HCO3), and base excess as an arterial blood gas. There will be a poor correlation with PaO2 and you can rely on the SpO2 reading from pulse oximetry if an ABG is challenging to obtain. In the case of severe respiratory distress, the gold standard for evaluating blood gasses is the ABG.

FUN FACT: In an effort to increase the number of physicians in the military during the Korean war, President Truman signed into law the Doctor Draft which required two years of military service for medical students after completing their internship. The draft was kept in place for a while after the Korean war ended in 1953. In that same year, Dr. John Severinghaus was drafted into the National Institutes of Health (NIH) as the director of anesthesia research. While there, he began working on the first automated laboratory devices for measuring gas pressures and pH within blood.

Internal Medicine for the Ignorant

This article is an excerpt from our upcoming course, Internal Medicine for the Ignorant, a course designed for everyone involved in clinical practice that covers 11 body systems, 65 internal medicine diagnoses and their management, as well as over 50 of the most common lab tests and their interpretation and clinical application. You’ll learn about the history of modern-day labs too!

Scheduled Release: November 2024

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