The Systematic ABG Interpretation Method

Normal values: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L. First, determine if pH is acidotic (under 7.35) or alkalotic (over 7.45). Second, check PaCO2 — if it moves opposite to the expected direction for the pH, respiratory is the primary problem; if HCO3 moves in the same direction as the pH abnormality would predict, metabolic is primary. Third, check the other value for compensation — if it shifts to normalize pH, compensation is occurring; full compensation returns pH to normal range while partial compensation improves it without fully normalizing.

The Four Primary Disorders

Respiratory acidosis (low pH, high CO2) results from hypoventilation — COPD, opioid overdose, respiratory depression — treated by improving ventilation. Respiratory alkalosis (high pH, low CO2) results from hyperventilation — anxiety, pain, high altitude, early sepsis — treated by addressing the underlying cause and possibly rebreathing techniques. Metabolic acidosis (low pH, low HCO3) results from DKA, renal failure, diarrhea, or lactic acidosis — treated by correcting the underlying cause, sometimes bicarbonate in severe cases. Metabolic alkalosis (high pH, high HCO3) results from vomiting, NG suction, or excessive antacid/diuretic use — treated with fluid and electrolyte replacement.

Clinical Application and Common Traps

Kussmaul respirations are the body's respiratory compensation for metabolic acidosis (blowing off CO2 to raise pH), so seeing rapid deep breathing with low HCO3 confirms compensated metabolic acidosis, not a primary respiratory problem — a frequent NCLEX trap. A patient who is vomiting and hypokalemic points to metabolic alkalosis from acid and potassium loss. Anxious hyperventilating patients develop respiratory alkalosis with tingling and lightheadedness from cerebral vasoconstriction. Always correlate the ABG with the clinical picture rather than interpreting numbers in isolation, since the history usually confirms which value is the primary disturbance.