ABG Interpretation in Four Steps

Medical-Surgical · Respiratory / Pulmonology·Updated 2026-10-28

What it is

An arterial blood gas (ABG) measures pH, PaCO2, bicarbonate (HCO3) and PaO2. The lungs control carbonic acid by exhaling carbon dioxide within minutes, while the kidneys control bicarbonate over hours to days. A respiratory problem therefore changes PaCO2 first, a metabolic problem changes HCO3 first, and the other system compensates; the nurse treats the cause, not the number.

What the NCLEX focuses on

Step 1, pH: below 7.35 is acidosis; above 7.45 is alkalosis.
Step 2, PaCO2 (the acid): if it moves opposite to the pH (high PaCO2 with low pH), the problem is respiratory.
Step 3, HCO3 (the base): if it moves in the same direction as the pH (low HCO3 with low pH), the problem is metabolic.
Step 4, compensation: uncompensated when the other value is normal; partially compensated when the other value is abnormal and the pH is still abnormal; fully compensated when the pH has returned to 7.35 to 7.45. Example: pH 7.30, PaCO2 50, HCO3 30 is partially compensated respiratory acidosis.
Respiratory acidosis comes from hypoventilation (COPD, opioid or sedative overdose, neuromuscular weakness, chest trauma); respiratory alkalosis from hyperventilation (anxiety, pain, early asthma attack, pulmonary embolism, fever).
Metabolic acidosis comes from DKA, lactic acidosis in shock, kidney failure and diarrhea, with Kussmaul respirations as compensation; metabolic alkalosis from vomiting, NG suction, loop or thiazide diuretics and excess antacids.
Acidosis shifts potassium out of cells (hyperkalemia); alkalosis shifts it into cells (hypokalemia).

Priority nursing actions

Assess the client first: airway, respiratory rate and depth, level of consciousness and SpO2.
Respiratory acidosis: open and support the airway, raise the head of the bed, give naloxone for opioid-induced hypoventilation, and prepare for noninvasive ventilation or intubation.
Respiratory alkalosis: treat the cause (hypoxemia, pain, anxiety) and coach slow breathing.
Metabolic acidosis: treat the cause (IV fluids and insulin in DKA, perfusion in shock) and monitor potassium and the cardiac rhythm.
Metabolic alkalosis: replace fluid, chloride and potassium as prescribed.
After an arterial puncture, hold firm pressure for at least 5 minutes, longer for a client on anticoagulants.

Numbers to know

Value	Normal	Why it matters
pH	7.35 to 7.45	Sets the direction: acidosis or alkalosis
PaCO2	35 to 45 mm Hg (4.7 to 6.0 kPa)	Respiratory component; high means hypoventilation
HCO3	22 to 26 mEq/L (22 to 26 mmol/L)	Metabolic component; low in DKA, high with vomiting
PaO2	80 to 100 mm Hg (10.7 to 13.3 kPa)	Measures oxygenation, not acid-base balance
SaO2	95% to 100%	Saturation of hemoglobin with oxygen
Potassium	3.5 to 5.0 mEq/L (3.5 to 5.0 mmol/L)	Rises in acidosis, falls in alkalosis

Common exam traps

Choosing metabolic alkalosis for pH 7.30, PaCO2 50, HCO3 30 because the HCO3 is high: wrong because the pH sets the direction; low pH with high PaCO2 is respiratory acidosis with renal compensation.
Choosing to have a hyperventilating client rebreathe into a paper bag: wrong because rebreathing causes hypoxemia; treat the cause and coach slow breathing.
Choosing reassurance when an exhausted asthma client's PaCO2 rises from low to normal: wrong because this signals fatigue and impending respiratory failure.
Choosing sodium bicarbonate as first treatment for DKA: wrong because IV fluids and insulin correct ketoacidosis; bicarbonate is reserved for very severe acidosis.
Choosing to wait for the ABG result before acting on a client who is difficult to arouse with slow respirations: wrong because airway support comes first.

What to teach

Before an arterial puncture, explain the brief sharp pain; afterward, report numbness, coldness or bleeding at the site.
Clients taking opioids at home: never combine them with alcohol or other sedatives, and keep naloxone available.
Clients with ongoing vomiting or diarrhea: replace fluids with oral rehydration solution and report weakness, confusion or very low urine output.

Try 2 NCLEX-style questions on this topic

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Question 1 · QID A52AD150

The nurse compares ABG findings in clients with acute respiratory disorders.

For each assessment cluster, identify the most likely acid-base pattern.

	Respiratory alkalosis	Respiratory acidosis
Chest trauma with respiratory fatigue; pH 7.29; PaCO₂ 57 mm Hg
Early severe asthma; marked hyperventilation; pH 7.47; PaCO₂ 31 mm Hg
COPD exacerbation with somnolence; pH 7.27; PaCO₂ 68 mm Hg
Acute pulmonary embolism; RR 34/min; pH 7.49; PaCO₂ 29 mm Hg

Question 2 · QID 91E68710

Which findings support acute respiratory failure rather than uncomplicated respiratory distress? For each finding, indicate whether it supports acute respiratory failure or supports uncomplicated respiratory distress.

	Supports acute respiratory failure	Supports uncomplicated distress
SpO₂ at the client's baseline with clear speech and stable vital signs
Use of accessory muscles followed by decreasing respiratory effort
PaCO₂ 62 mm Hg with pH 7.25
Dyspnea that resolves after resting for 2 minutes
Progressive confusion and inability to remain awake
PaO₂ 54 mm Hg despite supplemental oxygen

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Sources

Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Fluid, Electrolyte, and Acid-Base Imbalances.
Potter PA, Perry AG, Stockert PA, Hall AM. Fundamentals of Nursing. 10th ed. Elsevier; 2021.

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