ABG stands for arterial blood gas — a sample of arterial blood that reports pH, PaCO₂, PaO₂, HCO₃⁻, and oxygen saturation to show how well a patient is ventilating, oxygenating, and buffering acid. Normal values are pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L, PaO₂ 80–100 mmHg. Interpret in four steps: read the pH (acidosis or alkalosis), read the PaCO₂ (respiratory driver), read the HCO₃⁻ (metabolic driver), then decide whether compensation is absent, partial, or full.
What ABGs are and why they are drawn
ABG is the abbreviation for arterial blood gas. The sample is taken from an artery — usually the radial artery after a modified Allen test, or from an arterial line — because arterial blood reflects gas exchange after it has passed through the lungs.
Clinicians order ABGs to answer three separate questions:
- Is the patient ventilating? PaCO₂ answers this directly, because CO₂ elimination depends on alveolar ventilation.
- Is the patient oxygenating? PaO₂ and SaO₂ answer this, and can be combined with FiO₂ into a P/F ratio.
- Is the acid-base balance intact? pH plus HCO₃⁻ answer this and reveal metabolic contributions such as lactic acidosis or vomiting-induced alkalosis.
Normal ABG values to memorize
| Value | Normal range | What it reflects |
|---|---|---|
| pH | 7.35 – 7.45 | Net acid-base status of the blood |
| PaCO₂ | 35 – 45 mmHg | Respiratory component (alveolar ventilation) |
| HCO₃⁻ (bicarbonate) | 22 – 26 mEq/L | Metabolic/renal buffering component |
| PaO₂ | 80 – 100 mmHg | Oxygenation — dissolved oxygen tension |
| SaO₂ | 95 – 100% | Hemoglobin oxygen saturation |
| Base excess | −2 to +2 mEq/L | Metabolic acid or base surplus |
| Anion gap | 8 – 12 mEq/L | Unmeasured anions in metabolic acidosis |
A useful mnemonic for direction is ROME: Respiratory Opposite (pH and PaCO₂ move in opposite directions), Metabolic Equal (pH and HCO₃⁻ move in the same direction).
The four-step interpretation method
- pH. <7.35 = acidosis. >7.45 = alkalosis. Inside the range = normal or fully compensated.
- PaCO₂. >45 = respiratory acidosis. <35 = respiratory alkalosis.
- HCO₃⁻. <22 = metabolic acidosis. >26 = metabolic alkalosis.
- Compensation. If only one system is abnormal, the disorder is uncompensated. If both are abnormal but the pH is still outside range, it is partially compensated. If both are abnormal and the pH is back inside the range, it is fully compensated.
Compensation directions
The lungs compensate quickly (minutes) by changing the respiratory rate; the kidneys compensate slowly (hours to days) by retaining or excreting bicarbonate. So a metabolic acidosis with a low PaCO₂ means the lungs are already blowing off CO₂ to help, and a chronic respiratory acidosis with a high HCO₃⁻ means the kidneys have had days to adapt — a classic chronic COPD picture.
Causes of each acid-base disorder
| Disorder | Common causes |
|---|---|
| Respiratory acidosis (↑PaCO₂) | Hypoventilation: opioid or sedative overdose, COPD exacerbation, neuromuscular weakness, chest trauma, obstructive sleep apnea, inadequate ventilator settings |
| Respiratory alkalosis (↓PaCO₂) | Hyperventilation: anxiety and panic, pain, fever, sepsis, pulmonary embolism, high altitude, over-ventilation |
| Metabolic acidosis (↓HCO₃⁻) | High anion gap: lactic acidosis, diabetic ketoacidosis, renal failure, toxic alcohols, salicylates. Normal gap: diarrhea, renal tubular acidosis, large-volume saline |
| Metabolic alkalosis (↑HCO₃⁻) | Prolonged vomiting or nasogastric suction, loop or thiazide diuretics, hypokalemia, excess bicarbonate or antacid intake, hyperaldosteronism |
Ten worked practice ABGs
| # | Values | Interpretation |
|---|---|---|
| 1 | pH 7.28, PaCO₂ 58, HCO₃⁻ 25 | Uncompensated respiratory acidosis — acute hypoventilation |
| 2 | pH 7.32, PaCO₂ 60, HCO₃⁻ 32 | Partially compensated respiratory acidosis — chronic CO₂ retention |
| 3 | pH 7.38, PaCO₂ 62, HCO₃⁻ 36 | Fully compensated respiratory acidosis — chronic COPD |
| 4 | pH 7.52, PaCO₂ 28, HCO₃⁻ 24 | Uncompensated respiratory alkalosis — acute hyperventilation |
| 5 | pH 7.21, PaCO₂ 36, HCO₃⁻ 14 | Uncompensated metabolic acidosis — check anion gap and lactate |
| 6 | pH 7.25, PaCO₂ 28, HCO₃⁻ 12 | Partially compensated metabolic acidosis — Kussmaul respirations, consider DKA |
| 7 | pH 7.56, PaCO₂ 44, HCO₃⁻ 34 | Uncompensated metabolic alkalosis — vomiting or NG suction |
| 8 | pH 7.47, PaCO₂ 50, HCO₃⁻ 34 | Partially compensated metabolic alkalosis — diuretic therapy |
| 9 | pH 7.18, PaCO₂ 55, HCO₃⁻ 15 | Mixed respiratory and metabolic acidosis — cardiac arrest picture |
| 10 | pH 7.40, PaCO₂ 38, HCO₃⁻ 24, PaO₂ 52 on 60% FiO₂ | Normal acid-base with severe hypoxemia — oxygenation failure, not a ventilation problem |
Case 10 is the one students miss most: the acid-base panel is perfect while the patient is in respiratory failure. Always read oxygenation as a separate question.
Nursing priorities after an abnormal ABG
- Respiratory acidosis: improve ventilation — reposition to high Fowler's, encourage deep breathing, reverse sedation if indicated, escalate to non-invasive or invasive ventilation.
- Respiratory alkalosis: treat the cause — pain control, anxiety reduction, coached breathing; rule out pulmonary embolism and sepsis before calling it anxiety.
- Metabolic acidosis: restore perfusion and treat the source — fluids, insulin and fluids for DKA, dialysis for renal failure; monitor potassium closely during correction.
- Metabolic alkalosis: replace volume, chloride, and potassium; review diuretics and NG losses.
- Always: repeat the gas after an intervention, and document the value, the action, and the response in your assessment note.
Frequently Asked Questions
Sources & Further Reading
- Acid-base physiology. Hall JE & Hall ME. Guyton and Hall Textbook of Medical Physiology, chapter on acid-base regulation.
- Arterial blood gas analysis. Castro D, Patil SM, Keenaghan M. Arterial Blood Gas. StatPearls.