Understanding Blood Gas Interpretation

Sep 13, 2024

Interpreting Blood Gases

Introduction

  • Blood Gases Importance: Give information on gas exchange status, acid-base status, and preliminary indications of other values.
  • ABG vs VBG: Focus on Arterial Blood Gases (ABG) but also touch on Venous Blood Gases (VBG).

Initial Steps

  • Confirm Patient: Ensure the ABG corresponds to the correct patient, verifying date and time.
  • Clinical Context: Consider the patient’s clinical state and record the oxygen flow rate/device used.

Partial Pressures of Oxygen and Carbon Dioxide

  • PO2 (Partial Pressure of Oxygen):
    • Normal: >10 kPa on room air.
    • With oxygen: Should be 10 less than the fraction of inspired oxygen.
    • <8 kPa indicates respiratory failure.

Types of Respiratory Failure

  • Type 1: Low oxygen, normal CO2.
    • Causes: Ventilation-perfusion mismatch (e.g., pulmonary edema, embolism).
  • Type 2: High CO2 (hypercapnia).
    • Causes: Reduced ventilation.
    • Normal range: 4.5 - 6 kPa.

Hemoglobin Variants and pH

  • Hemoglobin Variants:
    • Carboxyhemoglobin: Carbon monoxide binding.
      • Normal: <3% in non-smokers, up to 15% in smokers.
  • pH Levels:
    • Normal: 7.35 - 7.45.
    • <7.35: Acidemia.

    • 7.45: Alkalemia.

Acid-Base Disturbances

  • Components on ABG:
    • Partial pressure of CO2 (respiratory).
    • Bicarbonate concentration (metabolic).
  • Acidosis & Alkalosis:
    • Respiratory: Due to CO2 retention.
    • Metabolic: Due to acid ingestion/production, bicarbonate loss.

Anion Gap

  • Normal: 4 - 12 mmol/L.
  • High gap suggests unmeasured anions (e.g., ketoacids in diabetic ketoacidosis).
  • Mnemonic for Causes:
    • High Anion Gap: MUD PILES.
    • Normal Anion Gap: HARD UP.

Compensation Mechanisms

  • Respiratory vs Metabolic: Body compensates by adjusting bicarbonate or ventilation.
  • Speed of Compensation: Ventilation adjustments happen faster than bicarbonate changes.

Lactate and Electrolytes

  • Lactate: Marker of anaerobic metabolism; elevated in inadequate oxygen delivery.
    • Causes: Sepsis, hypoxemia, anemia, organ failure, drugs.
  • Electrolytes: Sodium and potassium; ABG can indicate significant electrolyte imbalances.

Practical Tips and Considerations

  • Venous Blood Gases: Easier to obtain, closely match ABG for pH, lactate, bicarbonate.
  • Air in Syringe: Must be removed to prevent gas diffusion and invalid results.
  • Asthma Attack Indicator: Normal/high CO2 in asthma may suggest impending respiratory arrest.