Makindo Medical Notes

ABG sampling, particularly from the radial artery, can be extremely painful. It is advisable to inject 1 ml of 1% lidocaine at the needle insertion site to reduce pain prior to sampling.

Indications

• Assessment of respiratory failure and directing patient care.
• Post-cardiac arrest management and determining baseline status.

What ABG Provides

• pH of arterial blood (acid-base balance).
• Partial pressure of oxygen (PaO₂) and carbon dioxide (PaCO₂).
• Oxygen saturation (SaO₂).
• Bicarbonate (HCO₃) levels, essential in acid-base balance interpretation.

Preparation

• If possible, wait at least 20 minutes after any change in oxygen therapy before sampling to achieve a steady-state reading.
• Explain the procedure to the patient, including the reason for the test, the process, and potential complications (e.g., bleeding, bruising, arterial thrombosis, infection, and pain).
• Obtain informed consent to proceed.

Contraindications

• Poor collateral circulation (especially if radial artery is used).
• Coagulopathy or bleeding disorders.
• Severe peripheral vascular disease.

Required Equipment

• 2 ml heparinized syringe with cap.
• 20-22G needle (blue needle). A green needle may be more appropriate for femoral artery sampling.
• Alcohol gel or antiseptic for skin preparation.
• Sharps disposal container, gauze, and sterile gloves.
• Plastic bung or seal for the syringe after sampling.

Procedure

• Wash hands thoroughly with soap and water, and wear gloves.
• Identify the artery (radial, femoral, or brachial). The radial artery is usually preferred using the non-dominant arm.
• Perform a modified Allen test to assess collateral circulation in the hand by compressing both the radial and ulnar arteries until the hand turns pale. Release pressure on the ulnar artery and observe for reperfusion.
• Position the patient's hand palm-side up, extending the wrist by 20-30 degrees to access the radial artery more easily. Palpate the radial pulse and choose a prominent site.
• Clean the sampling site with an alcohol wipe, expel excess heparin from the syringe, and insert the needle at a 30-45 degree angle towards the patient, keeping the bevel up.
• Advance the needle gently to avoid causing too much pain or inducing arterial spasm. Blood will pulsate into the syringe under arterial pressure. Obtain at least 3 ml of arterial blood before withdrawing the needle.

Arterial vs. Venous Blood

• Venous blood is under lower pressure and will not usually self-fill the syringe.
• Venous blood is darker and has a lower oxygen saturation compared to arterial blood.

Post-Procedure

• After obtaining the sample, remove the needle and apply firm, direct pressure to the puncture site for at least 5 minutes or until the bleeding has stopped.
• Dispose of all sharps and contaminated materials appropriately.
• Remove any air bubbles from the sample, and if necessary, place the sample in ice to preserve it for analysis. The sample should be analyzed as soon as possible.
• Document and record the patient's oxygen therapy (FiO₂) at the time of sampling.

ABG Interpretation

Normal Ranges

• pH: 7.35 - 7.45
• PaO₂: 11-13 kPa (82.5 - 97.5 mmHg)
• PaCO₂: 4.7-6.0 kPa (35-45 mmHg)
• HCO₃: 22-26 mmol/L
• Base Excess (BE): -2 to +2 mmol/L
• Anion Gap: 2-10 mmol/L

Step-by-Step Interpretation

• Step 1: Check pH
  • pH < 7.35 indicates acidosis.
  • pH > 7.45 indicates alkalosis.
• Step 2: Assess PaCO₂
  • PaCO₂ > 6.0 kPa (45 mmHg) suggests respiratory acidosis or compensation for metabolic alkalosis.
  • PaCO₂ < 4.5 kPa (35 mmHg) suggests respiratory alkalosis or compensation for metabolic acidosis.
• Step 3: Assess HCO₃
  • HCO₃ < 22 mmol/L suggests metabolic acidosis or renal compensation for respiratory alkalosis.
  • HCO₃ > 26 mmol/L suggests metabolic alkalosis or renal compensation for respiratory acidosis.
• Step 4: Base Excess
  • BE > +2 mmol/L indicates a metabolic alkalosis or compensation for respiratory acidosis.
  • BE < -2 mmol/L indicates a metabolic acidosis or compensation for respiratory alkalosis.
• Step 5: Assess Oxygenation (PaO₂)
  • Normal PaO₂: 11-13 kPa (82.5 - 97.5 mmHg).
  • PaO₂ < 8 kPa (60 mmHg): Indicates hypoxemia and potential respiratory failure.
  • Type 1 Respiratory Failure: PaO₂ < 8 kPa with normal or low PaCO₂ (≤ 6.0 kPa).
  • Type 2 Respiratory Failure: PaO₂ < 8 kPa with elevated PaCO₂ (> 6.0 kPa), indicating ventilatory failure.

Common Acid-Base Disorders

Metabolic Acidosis with Raised Anion Gap

• Diabetic ketoacidosis, alcohol ketoacidosis, methanol or ethylene glycol poisoning, lactic acidosis, and renal failure.

Metabolic Acidosis with Normal Anion Gap

• Diarrhea, renal tubular acidosis, ureteroenterostomy, and rapid saline infusion.

Metabolic Alkalosis

• Vomiting, nasogastric suction, diuretic use, and hyperaldosteronism.

Respiratory Acidosis (CO₂ Retention)

• Opiate overdose, asthma, chronic obstructive pulmonary disease (COPD), and neuromuscular disorders (e.g., Guillain-Barré syndrome).

Respiratory Alkalosis

• Hyperventilation, pulmonary embolism, and anxiety-induced tachypnea.

Related Subjects: | Metabolic Acidosis | Arterial Blood Gas Analysis