Chapter 15. Blood gas analysis and pulse oximetry

Key Components of Arterial Blood Gas (ABG) Analysis

Blood gas analysis provides five key measurements:

1. pH – Indicates the acid-base balance. Normal range: 7.35-7.45.
2. PaCO₂ (Partial Pressure of Carbon Dioxide in Arterial Blood) – Assesses ventilation. Normal range: 4.7–6.0 kPa.
3. Bicarbonate (HCO₃⁻) – A key buffer in acid-base balance. Normal range: 22–26 mmol/L.
4. Base Excess – Reflects metabolic component of acid-base balance. Normal range: -2 to +2 mmol/L.
5. PaO₂ (Partial Pressure of Oxygen in Arterial Blood) – Assesses oxygenation. Normal range: 10-13 kPa (on room air).

These values help distinguish between respiratory and metabolic acid-base disturbances.

Interpreting ABG Results: The 5-Step Approach

 Assess the patient clinically – Consider the history, symptoms, and treatment before interpreting results.
 Check for hypoxaemia – Compare PaO₂ with the FiO₂ the patient is receiving. PaO₂ should be 10 kPa lower than inspired O₂%.
 Assess acid-base balance – Identify acidaemia (pH <7.35) or alkalaemia (pH >7.45).
 Determine if it is respiratory or metabolic:

• Respiratory acidosis: ↑PaCO₂ >6.0 kPa (e.g., COPD, hypoventilation).
• Respiratory alkalosis: ↓PaCO₂ <4.7 kPa (e.g., hyperventilation, pain, anxiety).
• Metabolic acidosis: ↓HCO₃⁻ <22 mmol/L (e.g., sepsis, lactic acidosis, DKA).
• Metabolic alkalosis: ↑HCO₃⁻ >26 mmol/L (e.g., vomiting, diuretics).
   Assess compensation – The body tries to normalise pH via respiratory or metabolic compensation.

 Example Case:

• pH = 7.26, PaCO₂ = 8.2 kPa, HCO₃⁻ = 26 mmol/L
• Interpretation: Respiratory acidosis (high PaCO₂), likely due to hypoventilation (e.g., opioid overdose).

Venous Blood Gas (VBG) vs. ABG

• VBGs are useful for assessing metabolic disturbances (e.g., diabetic ketoacidosis) but do not reliably measure oxygenation (PaO₂).
• VBGs from central veins provide a better reflection of tissue acid-base balance during cardiac arrest, as ABGs may not accurately represent metabolic status.
• Typical VBG in cardiac arrest:
  • pH <7.35, base deficit < -2 mmol/L, bicarbonate <22 mmol/L.

 Pulse Oximetry

Principles of Pulse Oximetry

• Uses light absorption at two wavelengths to determine oxygen saturation (SpO₂).
• A normal SpO₂ range is 94-98% in most patients and 88-92% in COPD patients at risk of hypercapnia.
• Pulse oximetry does not measure PaCO₂, so it cannot assess ventilation.

Limitations of Pulse Oximetry

• Can be affected by:
  • Carboxyhaemoglobin (CO poisoning) – falsely high SpO₂ readings.
  • Methaemoglobinaemia (drug-induced) – unreliable readings.
  • Surgical dyes (e.g., methylene blue) – falsely low SpO₂.
  • Nail varnish, poor circulation, hypothermia – inaccurate readings.
• Does not provide reliable readings during CPR.

 Targeted Oxygen Therapy

General Guidelines

• Give 100% O₂ during cardiac arrest.
• After ROSC, adjust FiO₂ to maintain SpO₂ 94-98% (to avoid hyperoxia).
• In COPD patients, target SpO₂ 88-92% to avoid worsening hypercapnia.
• For acute coronary syndrome (ACS), stroke, and head injury, avoid excess O₂ – keep SpO₂ 94-98%.

 Summary

✔ Use a structured 5-step approach to interpret blood gas results.
✔ Distinguish between respiratory and metabolic causes of acidosis/alkalosis.
✔ VBGs are useful in cardiac arrest but do not provide reliable oxygenation assessment.
✔ Pulse oximetry is useful for monitoring SpO₂ but does not measure ventilation (PaCO₂).
✔ Targeted oxygen therapy should be tailored to individual patients (e.g., COPD, ACS, ROSC).