Arterial blood gas (ABG) draw
Formal Definition
A procedure involving percutaneous puncture of a peripheral artery (most commonly radial, sometimes brachial, femoral, or dorsalis pedis as last resort) to obtain arterial blood for laboratory analysis of arterial oxygen and carbon dioxide partial pressures, pH, oxygen saturation, hemoglobin, electrolytes, and lactate; provides rapid, comprehensive acid-base and oxygenation status assessment essential for managing critically ill patients, ventilator adjustments, and resuscitation; performed most often in emergency, ICU, or perioperative settings.
How It's Used on the Ward
"ABG" or "draw an ABG" or "gas" — puncturing an artery (usually the radial) to get direct measurement of oxygen, CO2, pH; gives definitive acid-base and oxygenation status that's more accurate than pulse oximetry or venous blood gas; critical for vent management, septic shock resuscitation, and undifferentiated critically ill patient.
Example
""ICU patient on mechanical ventilation 2 hours after intubation. MAP 28, SpO2 90%. Respiratory therapist performs radial ABG: pH 7.31, PaO2 68, PaCO2 48, HCO3 24, lactate 3.4. Interpretation: respiratory acidosis with mild metabolic compensation, mild hypoxemia, mild lactic acidosis. Ventilator adjustments: increase RR to reduce CO2, increase PEEP to improve oxygenation, recheck gas in 30 minutes.""
Clinical Context
ABG analysis — step-by-step approach: (1) pH (acidemia <7.35, alkalemia >7.45). (2) PaCO2 (acid if elevated, base if low). (3) HCO3 (acid if low, base if elevated). (4) Determine primary disorder: respiratory (PaCO2 abnormal in opposite direction of pH) vs metabolic (HCO3 abnormal in same direction as pH). (5) Compensation: appropriate vs inappropriate — acute respiratory (HCO3 rises 1 mEq/L per 10 mmHg PaCO2 rise); chronic respiratory (3.5 mEq/L per 10 mmHg); metabolic (PaCO2 = 1.5 × HCO3 + 8 ± 2, Winters formula). (6) Calculate anion gap for metabolic acidosis. (7) PaO2/FiO2 ratio for ARDS severity (200-300 mild, 100-200 moderate, <100 severe). Common clinical scenarios: full ventilator assessment, undifferentiated shock (lactate, acid-base), suspected DKA, severe COPD exacerbation, post-cardiac arrest, suspected pulmonary embolism (PaO2 often low). ABG vs VBG (venous blood gas): VBG correlates well with pH and bicarbonate for acid-base assessment, less accurate for PaO2; ABG preferred for precise oxygenation assessment. Risks: arterial puncture bleeding/hematoma, especially with anticoagulation; distal ischemia with femoral or brachial sticks. Modified Allen test before radial stick (debatable utility).