F6viii: Outline methods used to measure V/Q mismatch

Perfusion Scan

  • Radiolabelled tracer
  • ƴ camera
    • Perfusion: radiolabelled dye (Tc99m)
    • Ventilation: radioactive gas (Xe)
  • Comparison images show defective V or Q

Bedside

  • A – a gradient = PAO2 – PaO2
  • PAO2 from alveolar gas equation:
Alveolar Gas Equation
    • FiO2 = patient FiO2
    • PATM = 760mmHg
    • PSVP = 97mmHg
    • PACO2 = from ABG
    • R = 0.8
  • PaO2 taken from ABG
  • A – a gradient normally <15mmHg (non-smoker)