F6v: Explain the concept of shunt, its physiological effects, and its measurement

Definition

  • Shunt = blood that enters arterial circulation without passing through ventilated lung

Causes of Shunt

Physiological

  • Bronchial circulation → adds deoxygenated blood to pulm v (not mixed venous blood)
  • Thebesian veins → empty directly into LV cavity (also not true mixed venous blood)

Pathological

  • Non-ventilated alveoli → V/Q = 0 (true mixed venous blood)
  • Cardiac defects → R – L shunt (true mixed venous blood)

NOTE: Although bronchial & thebesian veins do not contain true mixed venous blood (&∴ strictly can’t be calculate with SHUNT EQUATION), we do use shunt equation to quantify them

Quantifying Shunt = Shunt Equation

Quantifying Shunt = Shunt Equation
Quantifying Shunt = Shunt Equation
Qt
  • Normally expressed as % → 2 – 3%
    • If Cc’O2 = 20mL/dL and CvO2 = 10mL/dL:

∴50% shunting will give CaO2 = 15mL/dL

∴25% shunting will give CaO2 = 17.5mL/dL

Effect of Shunt on PO2

  • Shunt ↓O2 content of arterial blood
  • This does not respond to ↑FiO2
  • Because no matter how much ↑FiO2 you give, the shunted blood is never exposed to it
  • ∴giving 100% FiO2 is a useful diagnostic test → there’s a huge A – a gradient
  • The larger the shunt, the smaller the ↑PaO2 for given ↑FiO2
  • Sigmoid ODC:
    • Blood passing past high FiO2 units is already fully saturated its Hb, so any additional O2 is only dissolved, which is small
    • Blood coming out of these units is not enough to close the shunt gap

Effect of Shunt on PCO2

  • Shunted blood does not ↑PaCO2
  • 2 reasons:
  • CO2 curve is linear → regardless if PO2 is high/low
  • Respiratory compensation
    • Any ↑PaCO2 stimulates chemoR → ↑MV
    • Keeps PaCO2 normal range
    • This ↑MV does not connect hypoxaemia
    • But hypoxaemia can ↑MV further (so PaCO2 may even be low)

HALLMARK of SHUNT → ↓PaO2 + low/normal PaCO2