Follow these steps in making an ABG analysis:

1. Determine oxygenation status

Expected PaO2 for age
Supplemental oxygen--multiply percentage by 5 to get the expected PaO2
   (e.g. 0.40 face mask should yield a PaO2 of about 200)
Oxygen saturation

2. Determine acid-base balance

Normal pH
Acidotic (<7.35)
Alkalotic (>7.45)

3. Determine cause of acid-base imbalance

Respiratory
 Normally PCO2 is 35-45
 High PCO2 = respiratory acidosis (hypoventilation, COPD)
 Low PCO2 = respiratory alkalosis (hyperventilation)

 Metabolic
Usually bicarbonate (HCO3) is 24-28
High HCO3 = metabolic alkalosis (antacid overuse, NG suction, steroids,
   diuretics)
Low HCO3 = metabolic acidosis (DKA, lactic acidosis, renal failure,
   diarrhea)

4. Compensated or uncompensated

If the HCO3 or the CO2 are abnormal, look at the pH again to determine if acid-base imbalance is compensated or uncompensated. Normal pH means it's compensated, abnormal means it's uncompensated! Sometimes the body can keep the pH normal by compensating for respiratory problems with bicarbonate and metabolic problems by altering ventilation. This is why diabetics in DKA have Kussmaul's respirations, to offset the metabolic acidosis with a respiratory alkalosis. This is also why patients with COPD are often able to maintain a normal pH by increasing bicarbonate; it's a chronic problem, and as long as they are able to maintain a normal pH, we don't intervene with mechanical ventilation.

Sharon Kumm, University of Kansas School of Nursing, July 2004