VQ – Dead space

Master answer

Dead space

• Ventilated lung volume in which no gas exchange occurs
• Anatomical dead space
  • Dead space volume of conducting airways
    • Oropharynx, nasopharynx and first 16 divisions of the tracheobronchial tree
    • Approx 2ml/kg
  • Measured by fowlers method (N₂ washout)
• Alveolar dead space
  • Dead space volume of parts of the respiratory airways that are ventilated but not perfused
    • West Zone 1 (P_A > P_a)
      • Low cardiac output
      • Positive pressure ventilation
      • Posture
    • Disease states
      • Pulmonary Embolism
  • Measured by subtracting anatomical deadspace from physiological dead space
• Physiological dead space
  • Alveolar + anatomical dead space
  • Calculated from Bohr equation
  • Normal arterial to end-tidal CO₂ gradient is approx 5mmHg
    • Increases if increased dead space
• Apparatus dead space
  • Dead space contributed by artificial airway devices (endotracheal tubes, LMA, HME filters)
  • Usually reduces anatomical deadspace (due to bypass of oropharynx)

Calculation / Measurement of Dead Space

Bohr’s Method

• Bohr’s Equation:

• Based on the principle that all CO₂ exhaled must come from ventilated alveoli

• PĒ_CO2 is mixed-expired CO2 in an expired tidal breath
• Alveolar PCO2 is difficult to measure, so the Enghoff modification is used
  • which assumes P_ACO₂ = P_aCO₂

Fowler’s Method

Single-breath nitrogen washout test

• Single Vital Capacity Breath – 100% O2 → Exhales to Residual volume
• Expired Nitrogen concentration and volume is measured
• Plot of concentration by volume generated
  • Phase I: Pure dead space. No Nitrogen present.
  • Phase II: Midpoint is volume of Anatomical dead space.
  • Phase III: Expired N2 plateaus
  • Phase IV: Closing capacity. Sudden Increase in Nitrogen.

Consequences of increasing dead space

• Alveolar ventilation
  • RR x (tidal volume – dead space)
  • Therefore, as dead space approaches tidal volume, alveolar ventilation approaches zero, and cannot be compensated by increasing respiratory rate
• P_aCO₂ is inversely proportional to alveolar ventilation → P_aCO₂ increases as alveolar ventilation decreases
  • Decreased pH
  • Increased central and peripheral stimulation of ventilation → Reduction in P_aCO₂
    • Central chemoreceptors activate medullary vasomotor centre → increased sympathetic discharge
  • If P_aCO₂ increases, alveolar CO₂ in non-dead space alveoli will increase
    • P_AO₂ will decrease according to alveolar gas equation

Sakurai / JC 2016

Answer by JC, Sakurai