From Respiratory Wiki
The interpretation of ABG’s is an important skill that Physicians, Nurses, Respiratory Therapists and other healthcare providers use to determine the optimal course of treatment for critically ill patients. In order to help healthcare providers, I have developed an easy 4 step process to interpreting abnormal ABG’s.
Before interpreting an ABG it is important to know the acceptable range of normal values.
|Arterial||7.35 to 7.45||80 to 100mmHg||35 to 45mmHg||22 to 26mEq/L||-2 to +2mEq/L|
|Venous||7.32 to 7.42||28 to 48mmHg||38 to 52mmHg||19 to 25mEq/L||-2 to +2mEq/L|
Step 1 - Is the pH acidic or alkalotic?
pH is a measure of the amount of hydrogen ions in a liquid solution on a scale of 0-14. The closer a pH is to 0 the more acidic it is considered, while a pH closer to 14 is considered alkalotic. Based on the normal pH of a healthy adult we can assume that:
pH less than 7.35 indicates acidosis
pH greater than 7.45 indicates alkalosis
If the pH is within normal limits but the other ABG values are abnormal you would continue with your analysis.
Step 2 - Is the the cause of the abnormality respiratory or metabolic?
The two primary compounds that affect the acid-base balance in our body are carbon dioxide \(CO_2\) and bicarbonate \(HCO_3\).
An abnormal \(CO_2\) indicates a respiratory cause and an abnormal \(HCO_3\) indicates a metabolic cause.
An ↑\(CO_2\) in the blood will cause an acidosis and an ↑\(HCO_3\) in the blood will cause an alkalosis.
Conversely, a ↓\(CO_2\) will cause alkalosis and a ↓\(HCO_3\) will cause acidosis.
It is possible that you have a combined acidosis that would manifest as an ↑\(CO_2\) and ↓\(HCO_3\) and a combined alkalosis ↓\(CO_2\) and ↑\(HCO_3\).
Heres a handy dandy chart to make it easier:
Step 3 - is the pH compensated, partially compensated, or uncompensated?
Since \(CO_2\) and \(HCO_3\) are opposing forces in the acid-base balance, their respective systems will often attempt to correct a prolonged abnormality. Combined acidosis or alkalosis can not be compensated.
A compensated pH will be within normal limits with both an abnormal \(CO_2\) and/or \(HCO_3\)
A partially compensated pH will be abnormal but shows an attempt to correct the imbalance. For example an acidic pH of 7.30 with a \(CO_2\) of 65 (acidic) and a \(HCO_3\) of 30 (alkalotic).
An uncompensated pH will be abnormal with no attempt at compensation. Either \(CO_2\) or \(HCO_3\) will be normal while the other value is abnormal.
Step 4 - Is the patient hypoxic?
Examine the \(PaO_2\)
\(PaO_2\) > 100 is High
\(PaO_2\) 80-100 Normal
\(PaO_2\) 60-80 Mild
\(PaO_2\) 40-60 Moderate
\(PaO_2\) <40 Severe
Though this 4 step process makes it easy for healthcare providers to interpret ABG's, the acid-base balance is complex in that many different factors contribute to its homeostasis. This simplified process is not intended to replace a comprehensive education in ABG interpretation, but should be used as a reference to reinforce existing knowledge.