Mastering Your ABGs

Analyzing arterial blood gases (ABGs) is something you’ll have to learn if you’re working to become a nurse or a physician. It is an important test that will come up on many exams throughout school, but it does not end once your exam on it is over or once you’ve graduated from nursing or medical school. You’ll deal with them again in your nursing/medical practice.

Explaining ABGs

ABGs is a blood test that measures the pH, the levels of oxygen and carbon dioxide from an artery. This test is often done to check the function of a patient’s lungs and how well oxygen can move into their blood. It is a common test done in the ICU and ER.  

The test can indicate respiratory issues but is also a great indicator of metabolic trouble.  The body needs to keep pH in a very narrow range for our systems to function. It does this by compensating using whichever of the two systems is working correctly, so if there is trouble in the respiratory system, the body can only compensate with the metabolic process, or vice-versa. Compensation from the respiratory system happens quickly, but if we need to compensate metabolically things progress far slower.

According to Nurse.org, ABGs are drawn for a variety of reasons including:

• Lung Failure
• Kidney Failure
• Uncontrolled Diabetes
• Asthma 
• Chronic Obstructive Pulmonary Disease (COPD)
• Hemorrhage 

If you are struggling with analyzing ABGs, here are some ways to make learning them easier. 

Memorize Values

The first step is to learn and memorize what normal ABG values look like. According to the National Institute of Health, standard ABG values are: 

• pH: 7.35 - 7.45
• Bicarbonate (HCO3): 22-26 mEq/L
• Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
• Partial pressure of oxygen (PaO2): 80 - 100 mmHg
• Oxygen saturation (O2 Sat): 95% to 100%
• Base excess (BE) -2 to +2 mEq/L. 

After you’ve learned these values, you can begin to properly determine what is normal or abnormal in an acid-base balance. 

Determining pH

Acids are continually produced during normal metabolism while bases neutralize and promote the excretion of the acids. The regulation of acids and bases is necessary to maintain balance, also known as homeostasis. Anything lower than a pH of 7.35 is considered acidic or “acidotic” and anything higher than a pH of 7.45 is considered basic or “alkaline.”

How and why is this significant to the studies of a nursing student, medical student or to the work of a nurse or a doctor? Patients with diabetes mellitus, COPD and kidney disease frequently develop acid-base imbalances. COPD patients normally retain more C02, and our view of what is normal for them has to be adjusted accordingly.

An acid-base imbalance is not a disease per se but a symptom of an underlying health issue. The acidity or alkalinity of a solution depends on its hydrogen ion (H+) concentration. The higher H+ leads to acidity. The lower H+ leads go alkalinity. The pH of 7 is neutral. Acid is less than pH of 7 and alkaline is pH greater than 7. Our blood is slightly alkaline because again, the normal arterial pH is 7.35 to 7.45.

Our acid-base regulation has 3 mechanisms: (1) the buffer system, (2) the respiratory system, and (3) the renal system. It’s all connected to the human body and therefore it is so important to learn. 

Analyze the AGBs

You can analyze the ABGs using the ROME method. ROME stands for Respiratory Opposite Metabolic Equal. How does it work? You think of respiratory when one is high and the other one is low. You think of metabolic when ABGs are on equal values.

Are you confused yet? How does it work? Remember ROME, meaning when pH is low and PaCO2 is high and the HCO3 is normal, the patient’s ABG is in Respiratory Acidosis.  The HC03 may or may not be in the normal range, depending on the body’s compensation.

Next is to analyze the compensation of the ABGs. The PaO2 will represent the type of compensation the patient is experiencing on their ABGs. The compensation can be thought of as the body’s way or an attempt at correcting an imbalance!

Now let’s practice interpreting ABGs first!

Practice, Practice, Practice!

• pH 7.25, PaCO2 40, and HCO3 -18

If you interpret the above you will notice that a pH of 7.25 indicates acidosis. PaCO2 is within normal limits (WNL) and HCO3 is lower than the normal range. If the cause is respiratory, the pH and the PaCO2 levels will be in opposite directions but in this case, they’re not. Therefore, it is a case of metabolic acidosis.

• pH 7.34, HCO3- 9, and PaCO2 35

A pH of 7.34 indicates acidosis. An HCO3- level of 9 is lower than the normal range. The PaCO2 is in the normal range. Therefore, it is a case of metabolic acidosis.

Now let’s practice interpreting the compensation!

If the pH level is not within or close to the normal range, then it is a “partial” compensation. If the pH is within the normal range, then is “full” compensation.

• pH 7.43, PaCO2 28 mmHg, and HCO3 18 mEq/L. 

Fully Compensated Respiratory Alkalosis

It is being compensated because the HC03 has moved to the acidotic side to compensate for the acid problem in the respiratory system and the PH has returned to normal.