Introduction to Arterial Blood Gas (ABG) NCLEX^® Review

At first, it may often be difficult to read ABGs as the subject depends on an understanding of complex physiological processes. The complexity of this topic may often steer nurses and other practitioners into believing that blood gas interpretation may be the job of respiratory therapists or reserved for physicians to understand.

Instead, it’s important for all healthcare workers to have a strong understanding of ABGs as this can dramatically impact the outcomes of clients hospitalized with respiratory or metabolic acidosis. This ABG NCLEX^® Review aims to provide readers with a thorough understanding of arterial blood gases in order to prepare for both daily practice as a nurse and for the NCLEX^® exam.

Key Concepts for ABG

When interpreting ABGs the first step will always be to determine whether the client has normal, acidic, or basic (alkalotic) measurements. The first step will be to observe the pH of the blood via laboratory tests.

The next step will be to evaluate the partial pressure of carbon dioxide (PaCO2) and the concentration of bicarbonate (HCO3) in the serum. Refer below for the normal values of these laboratory parameters to aid in your evaluation of blood gases.

• Normal pH: 7.35 – 7.45
• PaCO2: 35-45 mmHg
• Bicarbonate (HCO3): 21-27 mEq/L

When a client presents with a pH that is out of the normal range, you can interpret the value as either acidic or basic. Acidosis is observed when the pH is below the normal range < 7.35 whereas alkalosis or (basic blood gas) is measured as > 7.45.

PaCO2 in general is referring to the concentration of carbon dioxide in the blood. Abnormalities in these readings will indicate that the client has a respiratory issue instead of a metabolic one. In short, it may suffice to know that higher concentrations of carbon dioxide will increase the acidity of the blood (therefore lowering the pH).

Thus, values > 45 mmHg can be interpreted as respiratory acidosis whereas values < 35 mmHg are considered respiratory alkalosis. Notice that high values, in this case, suggest acidosis, in contrast to pH where high values suggest alkalosis.

You may recall from basic chemistry that bicarbonate (HCO3) is a basic chemical compound that raises pH (decreasing acidity). Therefore, elevated bicarbonate readings (> 27 mEq/L) will increase pH, causing alkalosis. In this case, you can interpret these values as metabolic alkalosis as it has nothing to do with the expiration or inhalation of CO2. Clients who have values < 22 mmHg may have metabolic acidosis as the concentration of bicarbonate in the blood is low, therefore lowering the blood pH and causing acidosis.

Unfortunately, when reading blood gases, it may be necessary for practitioners to take additional considerations with respect to the bodies tendency to compensate for pH abnormalities. This may complicate matters as it may appear that the pH of the blood is within normal limits (e.g. between 7.35-7.45) but still indicative of potential underlying issues.

Compensation mechanisms often make interpretation of ABGs very difficult, so it is important to keep some basic concepts in mind. First, clients that present with primary metabolic acidosis may have a disorder causing decreased bicarbonate. The body will compensate this by increasing the rate of expiration of carbon dioxide via the lungs to drive the pH of the blood up.

Clients that present with primary metabolic alkalosis may have increased levels of bicarbonate in the body. The body will compensate for this by reducing the ventilation of carbon dioxide, thus retaining more pCO2 in the blood causing compensation of the pH abnormality.

Compensation of primary respiratory alkalosis and acidosis generally involves the kidney’s ability to either retain bicarbonate or increase the excretion of it. Retention of bicarbonate of course will increase the pH of the blood where excretion of it will decrease it.

For example: A client may have respiratory acidosis as they were initially failing to expire the appropriate amount of carbon dioxide from the body. As a result, the kidneys will opt to retain more bicarbonate in order to increase the pH of the body and temporarily resolve the blood pH.

Upon arriving to the hospital, it may appear that the client has a normal pH despite having high readings of both carbon dioxide and bicarbonate. This would be an example of fully compensated respiratory acidosis.

Causes of ABG Abnormalities

When evaluating clients, there are many underlying causes of blood gas abnormalities. It’s imperative to know the various causes of these abnormalities as ultimately the goal will be to treat the cause rather than the symptoms.

Metabolic acidosis can be caused by several underlying conditions that can contribute to the increased acidity observed in the blood. Below are several examples of causes to keep in mind when evaluating clients with this condition.

• Salicylate poisoning (e.g. aspirin overdose)
• Lactic acidosis (caused by poor oxygen perfusion of tissue increase anaerobic respiration)
• Uremia (caused by renal failure)
• Medications
  • Antidiabetic medications – metformin
  • Antibiotics – linezolid
  • Anti-seizure – topiramate
• Diabetic or alcoholic acidosis
• Gas or heavy metal poisoning

Respiratory acidosis involves the failure of the lungs to expire carbon dioxide from the body – therefore increasing the acidity of the blood (reducing the pH). This can be caused by varying types of respiratory failure.

Respiratory failure occurs when the lungs fail to fully oxygenate the blood (leading to low pO2) causing hypoxia or failure of the lungs to properly ventilate causing hypoventilation. Below are several examples of causes of respiratory acidosis.

Poor oxygen diffusion (hypoxemic respiratory failure) 

• Alveolar hypoventilation (e.g. opiate overdose, benzodiazepine overdose)
• Ventilation-perfusion mismatch (e.g. COPD, cystic fibrosis)
• Impaired diffusion (e.g. diffuse parenchymal lung disease)
• Low partial pressure of inspired oxygen (e.g. high altitude)

Hypoventilation 

• Musculoskeletal problems (e.g. muscular dystrophy)
• Airway obstruction (e.g. epiglottitis)
• Neuropathy (e.g. motor neuron disease, Guillain-Barre syndrome)
• Neuromuscular junction problem (e.g. myasthenia gravis)
• Extrapulmonary issues (e.g. severe ascites)
• Respiratory center suppression (e.g. brainstem stroke, propofol overdose)
• Spinal cord tension

ABG Conclusion

Blood gases often prove to be a difficult yet very important subject to understand when treating clients with abnormal blood gases. Blood gas abnormalities can often be caused by a variety of conditions that can often be life-threatening for the client.

For this reason, it is imperative to not only know how to recognize clients with ABG values that are out of the ordinary – but to also screen for the cause in order to resolve the issue and improve your client’s outcomes. Hopefully, this ABG NCLEX^® Review will serve as a good overview of the subject matter to aid in your understanding of the topic and serve you in your future practice.

References:

1. Arterial Blood Gases. In: Post T, ed. UpToDate. Waltham, MA.: UpToDate; 2020. www.uptodate.com. Accessed May 5th, 2020.
2. Yap CY, Aw TC. Arterial Blood Gases. Proceedings of Singapore Healthcare. 2011;20(3):227-235. doi:10.1177/201010581102000313.
3. ABA Keywords. OpenAnesthesia.https://www.openanesthesia.org/aspirin_toxicity_treatment/. Accessed May 10, 2020.