ABG Interpretation Made Simple: A Step-by-Step Guide to Arterial Blood Gas Analysis

Before you can interpret an abnormal ABG, you must know normal. There are three values you need to commit to memory: pH normal range is 7.35-7.45 (below 7.35 is acidosis, above 7.45 is alkalosis). PaCO2 (partial pressure of carbon dioxide) normal range is 35-45 mmHg — this is the respiratory component. CO2 is an acid (it combines with water to form carbonic acid), so high CO2 means more acid (respiratory acidosis) and low CO2 means less acid (respiratory alkalosis). HCO3 (bicarbonate) normal range is 22-26 mEq/L — this is the metabolic component. Bicarbonate is a base, so high HCO3 means more base (metabolic alkalosis) and low HCO3 means less base (metabolic acidosis). There is also PaO2 (partial pressure of oxygen), with a normal range of 80-100 mmHg, which tells you about oxygenation. PaO2 does not directly affect acid-base status but is critical for the complete clinical picture. A PaO2 below 60 mmHg indicates hypoxemia and is a clinical emergency regardless of the acid-base findings.

This method works for every ABG you will ever see — on the NCLEX, in clinical rotations, and in practice. Step 1: Look at the pH. Is it acidotic (below 7.35), alkalotic (above 7.45), or normal (7.35-7.45)? If the pH is normal but the PaCO2 and HCO3 are both abnormal, there is a compensated disorder — look at which side of 7.40 the pH falls. Below 7.40 suggests a primary acidosis with compensation; above 7.40 suggests a primary alkalosis with compensation. Step 2: Match the pH to the cause. Look at PaCO2 and HCO3. Which one matches the direction of the pH abnormality? If the pH is acidotic and the PaCO2 is high (above 45), the primary problem is respiratory acidosis — the lungs are retaining CO2. If the pH is acidotic and the HCO3 is low (below 22), the primary problem is metabolic acidosis — the body is losing bicarbonate or accumulating metabolic acids. If the pH is alkalotic and the PaCO2 is low (below 35), the primary problem is respiratory alkalosis — the lungs are blowing off too much CO2. If the pH is alkalotic and the HCO3 is high (above 26), the primary problem is metabolic alkalosis — the body has excess bicarbonate. Step 3: Check for compensation. Look at the value you did NOT use in Step 2. Is it abnormal in the opposite direction, trying to bring the pH back to normal? If yes, the body is compensating. If the non-primary value is still within normal range, there is no compensation yet (uncompensated). If the pH has returned to normal range, the compensation is complete (fully compensated).

Respiratory Acidosis (pH down, PaCO2 up): The lungs are not removing enough CO2. Causes include COPD, asthma exacerbation, pneumonia, drug overdose (opioids, sedatives that suppress respiratory drive), neuromuscular diseases (Guillain-Barré, myasthenia gravis), and any condition that reduces ventilation. Think: the patient is not breathing well enough. Respiratory Alkalosis (pH up, PaCO2 down): The lungs are removing too much CO2. The most common cause is hyperventilation — from anxiety, pain, fever, sepsis (early), high altitude, or mechanical ventilation with settings that are too aggressive. Think: the patient is breathing too fast or too deeply. Metabolic Acidosis (pH down, HCO3 down): The body has either accumulated excess acid or lost bicarbonate. High anion gap metabolic acidosis (MUDPILES mnemonic: Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates) involves accumulation of unmeasured acids. Normal anion gap metabolic acidosis involves bicarbonate loss — most commonly from diarrhea (GI loss of bicarbonate) or renal tubular acidosis (kidney failure to reabsorb bicarbonate). Metabolic Alkalosis (pH up, HCO3 up): The body has excess bicarbonate. The most common causes are prolonged vomiting (loss of hydrochloric acid from the stomach increases relative bicarbonate), nasogastric suction, excessive antacid use, diuretic therapy (loop and thiazide diuretics cause loss of H+ and Cl-), and hypokalemia (the kidneys excrete H+ to retain K+, raising bicarbonate).

The body has two compensatory mechanisms for acid-base disturbances: the lungs (which adjust PaCO2 within minutes to hours) and the kidneys (which adjust HCO3 over hours to days). The lungs compensate for metabolic disorders, and the kidneys compensate for respiratory disorders. In metabolic acidosis, the lungs compensate by increasing ventilation (Kussmaul breathing) to blow off CO2, bringing PaCO2 down. You will see: pH low, HCO3 low, PaCO2 low. In metabolic alkalosis, the lungs compensate by decreasing ventilation to retain CO2, bringing PaCO2 up. You will see: pH high, HCO3 high, PaCO2 high. In respiratory acidosis, the kidneys compensate by retaining bicarbonate, bringing HCO3 up. This takes 24-48 hours to become significant. You will see: pH low, PaCO2 high, HCO3 high. In respiratory alkalosis, the kidneys compensate by excreting bicarbonate, bringing HCO3 down. You will see: pH high, PaCO2 low, HCO3 low. Key principle: compensation moves the non-primary value in the same direction as the primary value. Both PaCO2 and HCO3 will be abnormal in the same direction (both high or both low). The pH tells you which disorder is primary — the pH always moves toward the primary disorder's direction. Compensation never overcorrects — if the pH is on the opposite side of normal from what you would expect, there may be a mixed disorder (two simultaneous acid-base disturbances).

Respiratory Acidosis: The priority is improving ventilation. Position the patient upright (high Fowler's) to maximize lung expansion. Administer bronchodilators if ordered (for COPD/asthma). Assist with non-invasive positive pressure ventilation (BiPAP) or prepare for intubation if the patient is deteriorating. If opioid overdose, administer naloxone (Narcan). Monitor respiratory rate, depth, and oxygen saturation continuously. Respiratory Alkalosis: The priority is reducing the respiratory rate. If anxiety-driven, coaching the patient through slow, controlled breathing is first-line. Address the underlying cause — treat pain, reduce fever, adjust ventilator settings if mechanically ventilated. Rebreathing into a paper bag is an outdated practice that is no longer recommended due to hypoxia risk. Metabolic Acidosis: Treat the underlying cause. For DKA: insulin, IV fluids, and potassium replacement (insulin drives K+ into cells). For lactic acidosis: treat the source of poor perfusion (sepsis, hemorrhage, heart failure). Sodium bicarbonate is reserved for severe acidosis (pH below 7.1) because it can overcorrect and cause rebound alkalosis. Monitor potassium closely — correcting acidosis shifts K+ into cells and can cause dangerous hypokalemia. Metabolic Alkalosis: Replace chloride and volume (usually normal saline IV). If caused by NG suction or vomiting, replace fluids and electrolytes. If caused by diuretics, potassium replacement is critical — hypokalemia maintains the alkalosis. Monitor for signs of hypokalemia (weakness, dysrhythmias, flat T waves). NurseIQ provides ABG practice questions in NCLEX format so you can build speed and confidence with interpretation before your exam.