When interpreting the ABG results, one must first know the five major components of the ABG to be addressed: oxygen saturation (SaO2), partial pressure of oxygen (PaO2), acidity or alkalinity (pH), partial pressure of carbon dioxide (PaCO2), and bicarbonate ions concentration (HCO3).
Table 2: Definitions important to know when Interpreting ABG
- Acidemia – arterial pH less than 7.35
- Acidosis – lowering of the extracellular fluid pH caused by an elevated PCO2 or a reduced HCO3
- Metabolic acidosis – reduction in pH and serum HCO3
- Respiratory acidosis – reduction in pH with an elevation of the arterial PCO2
- Alkalemia – arterial pH above 7.45
- Alkalosis – elevation of the extracellular fluid pH caused by a fall in PCO2 or a rise in HCO3
- Metabolic alkalosis - elevation in pH and serum HCO3
- Respiratory alkalosis – elevation of the pH with a reduction in the arterial PCO2
- Mixed acid-base disorder – more than one acid-base disorder at the same time
- Anion gap = (Na) - (Cl + HCO3)
- Normal range is 8-16 mEq/L
The four main acid-base disorders are respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.
Respiratory alkalosis and acidosis may be classified as acute or chronic. It takes up to five days for the renal system to compensate for respiratory disorders.
The acid-base balance of the blood is maintained by two areas of the body, which are the lungs and the kidneys. The lower pH represents acidosis, and the higher pH represents alkalosis, with the normal pH range from 7.35-7.45.
The PaO2 evaluates the oxygen in plasma and has an 80-95 mm Hg normal range. The PaO2 does not measure the amount of oxygen attached to the hemoglobin. SaO2 measures the amount of oxygen attached to the hemoglobin. The normal range is 95-99% and generally should be above 90%.
PaCO2 evaluates the ventilation component. The normal range is 35-45 mm Hg. However, the value is inversely related to ventilation. For example, decreased ventilation has a higher value, and increased ventilation has a lower value. Therefore, hyperventilation causes alkalosis because the patient is blowing off carbon dioxide, and hypoventilation causes acidosis because the patient is retaining carbon dioxide. The body adjusts for these conditions by changing the respiratory rate (Romeu-Bordas et al., 2017).
HCO3 (bicarbonate) is regulated by the kidneys and evaluates the metabolic component. The normal range is 22-26 mEq/L. Below 22 mEq/L is considered acidosis, and above 26 mEq/L is alkalosis. The body can adjust to the abnormalities in the HCO3 levels but not as quickly as it can to the abnormal PaCO2 levels. Several days could be required to make the necessary adjustments to bring the HCO3 levels to a normal range (Byrd, 2018).
Table 3: Normal ABG Values
|PaCO2||35-45 mm Hg|
|PaO2||80-95 mm Hg|
Four conditions are evaluated based on the ABG: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. As we explore these conditions, the potential causes, the ABG values, and the compensatory mechanisms, we will better understand what is happening within the body.