Arterial blood gases (ABGs) are used to monitor patients on ventilators, monitor critically ill non-ventilator patients, establish preoperative baseline parameters and regulate electrolyte therapy. They provide valuable information for assessing and managing a patient’s respiratory and metabolic acid-base balance.
Before going into interpreting ABG results, let’s first take a look at the components of an ABG:
- pH is inversely proportional to the actual hydrogen ion concentration of blood. So, as the hydrogen ion concentration decreases, the pH goes up. A pH of 7.35-7.45 is a normal value; greater than 7.45 is considered alkalosis and less than 7.35 is acidosis.
- PCO₂ is a measure of the partial pressure of CO₂ in the blood. This is the respiratory component in acid-base determination because it is primarily controlled by the lungs. As the CO₂ level increases, pH decreases and vice versa. The faster and more deeply the patient breathes, the more CO₂ is blown off causing PCO₂ levels to drop. This can be used as a compensatory mechanism for respiratory acidosis. Alternatively, in metabolic acidosis, the respiratory rate may decrease to retain CO2 and decrease pH.
- HCO₃- (bicarbonate ion) is regulated by the kidneys and used as a measure of the metabolic component of acid-base equilibrium. As HCO₃- increases, the pH also increases- HCO₃- is elevated in metabolic alkalosis and decreased in metabolic acidosis. The kidneys are also used to compensate for respiratory acid-base imbalances. In respiratory acidosis, the kidneys attempt to compensate by retaining excess amounts of HCO₃- and in respiratory alkalosis, the kidneys excrete more HCO₃- out of the body in an attempt to lower the pH.
- PO₂ is an indirect measure of the O₂ content of the arterial blood. The normal value for PO₂ is 80-100 mmHg. This measure is useful in determining the effectiveness of O₂ therapy.
Now that we covered the basics, here are three easy steps you can use for interpreting the majority of ABGs:
Step 1: Evaluate the pH to determine Acidosis or Alkalosis.
Step 2: Look at the PCO₂ to determine respiratory effect
If the PCO₂ is high in a patient who has acidosis, the patient has respiratory acidosis and if their PCO₂ is low with a high pH, the patient has respiratory alkalosis. If the PCO₂ is low in a patient who has been said to have acidosis, the patient has metabolic acidosis and is compensating by blowing off CO₂. If their PCO₂ is high with a high pH, the patient has metabolic alkalosis and is compensating by retaining CO₂. If that lost you (I got lost a bit just writing it), the chart below simplifies the thought process and is right most of the time.
Step 3: Assume metabolic cause when respiratory is ruled out
Use the HCO₃- to verify metabolic effect (normal is 21-28 mmHg).
PATIENT SAFETY NOTE: After an ABG draw, pressure should be held or applied to the site for 3-5 minutes. The draw puts a patient at risk for excessive bleeding and hematoma formation.
Reference: Pagana, K. D. & Pagana J. P. (2014). Mosby’s Manual of Diagnostic and Laboratory Tests, 5th Edition. [Pageburstl]. Retrieved from https://pageburstls.elsevier.com/#/books/978-0-323-08949-4/