Easy way to remember

• Metabolic problems involve bicarbonate (HCO₃⁻), mainly regulated by kidneys and gastrointestinal losses.
• Respiratory problems involve carbon dioxide (CO₂), caused by underbreathing or overbreathing.
• Hypoventilation → CO₂ retention → respiratory acidosis
• Hyperventilation → CO₂ loss → respiratory alkalosis

• Oxygenation: PaO₂
• Ventilation: PaCO₂, especially CO₂ retention
• Acid-base status: pH, HCO₃⁻, base excess
• Often also lactate, electrolytes, and co-oximetry values depending on the analyzer

Common clinical indications

1. Acute respiratory distress or suspected respiratory failure
   • Severe dyspnea, tachypnea, cyanosis, altered sensorium
   • Low or unreliable pulse-oximeter reading
   • Suspected hypoxemia or hypercapnia
2. COPD or other disorders with risk of CO₂ retention
   • Acute COPD exacerbation
   • Obesity hypoventilation syndrome
   • Neuromuscular weakness
   • Severe asthma with fatigue, drowsiness, or a “normalizing” PaCO₂ despite distress
3. Severe lung or chest disease
   • Pneumonia, pulmonary edema, ARDS
   • Pulmonary embolism when assessing gas exchange
   • Severe COVID-like viral pneumonitis or other acute hypoxic lung disease
4. Critically ill patients
   • Sepsis or septic shock
   • Cardiogenic or hypovolemic shock
   • Major trauma, cardiac arrest, severe burns
   • ICU monitoring or rapid clinical deterioration
5. Suspected significant acid-base disorder
   • Diabetic ketoacidosis
   • Severe renal failure or renal tubular acidosis
   • Severe/prolonged diarrhea, vomiting, or diuretic use
   • Lactic acidosis
   • Poisoning, especially salicylates, methanol, ethylene glycol, or carbon monoxide
6. Monitoring respiratory support
   • Before and after starting non-invasive ventilation, such as BiPAP
   • Mechanically ventilated patients, especially after major ventilator adjustments
   • Assessing response to oxygen therapy when the patient remains unwell
7. Preoperative assessment in selected high-risk patients
   • Significant chronic lung disease with suspected hypercapnia or impaired gas exchange. Fishman notes preoperative ABG consideration particularly when raised PaCO₂ would affect postoperative pulmonary risk.

ABG is often not needed when

What is methemoglobin?

Normal value

Causes of high FMetHb

• Nitrates/nitrites, contaminated well water
• Local anesthetics, especially benzocaine and prilocaine
• Dapsone
• Sulfonamides
• Nitrites/nitroprusside
• Aniline dyes and some chemical exposures
• Congenital cytochrome b5 reductase deficiency or abnormal hemoglobin

Clinical relevance

Steps

1. Confirm indication, identity, and oxygen settings
   • Record whether the patient is on room air or oxygen, including the delivery device and FiO₂.
   • Explain the procedure and obtain consent where required.
2. Check the site
   • Use the radial artery, on the thumb side of the wrist, usually the non-dominant hand.
   • Avoid a site with infection, burns, hematoma, vascular graft/fistula, or absent/very weak pulse.
   • Assess collateral hand circulation according to local policy. A modified Allen test may be used, although its routine use varies by institution.
3. Position
   • Patient supine or seated.
   • Rest the wrist on a rolled towel or small support in about 20-30° extension, palm facing upward.
4. Prepare
   • Use gloves and aseptic technique.
   • Palpate the strongest radial pulse with the index and middle fingers.
   • Clean with antiseptic and allow it to dry.
   • Use a pre-heparinized ABG syringe with a fine needle, commonly 22-25 gauge.
5. Puncture
   • Insert the needle just distal to your palpating fingers, bevel up.
   • Enter at about 30-45° to the skin, directed toward the pulse.
   • Advance slowly until bright red, pulsatile arterial blood enters the syringe.
   • Usually, arterial pressure fills the syringe, so do not pull back forcefully on the plunger.
6. Collect and withdraw
   • Obtain the volume required by your analyzer, often about 1 mL.
   • Withdraw needle and immediately apply firm direct pressure with gauze.
7. Apply pressure
   • Compress for at least 5 minutes.
   • Hold longer, often 10 minutes or more, if the patient has coagulopathy, thrombocytopenia, is receiving anticoagulants, or bleeding continues.
   • Check for persistent bleeding, hematoma, pain, pallor, numbness, or reduced distal perfusion.
8. Handle the ABG sample properly
   • Expel any visible air bubbles immediately.
   • Cap the syringe, gently roll/invert to mix with heparin, label at the bedside, and send promptly.
   • Delayed analysis or air exposure can alter PaO₂, PaCO₂, and pH results.

If you miss the artery

• Withdraw slightly and redirect only if safe and within local policy.
• Avoid repeated blind attempts. Ask a more experienced clinician or use ultrasound guidance, especially with a weak/nonpalpable pulse or difficult anatomy.
• Do not repeatedly probe deeply because of hematoma and arterial injury risk.

Main complications