Dead Spaces and Ventilation Rates

Dead Spaces

• Anatomic Dead Space
  • volume of conducting airways that does not participate in gas exchange
• Physiologic Dead Space (V_D) 
  • volume of airway and lungs that does not participate in gas exchange
    • anatomic dead space of conducting airways
    • functional dead space in alveoli
      • “ventilated alveoli that do not participate in gas exchange”
        • e.g., ventilation/perfusion (V/Q) mismatch
      • apex of lung is largest contributor of functional dead space
  • in normal lungs, physiologic dead space ≈ anatomic dead space
    • V_D= physiologic dead space (mL)
    • TV = tidal volume (mL)
    • PaCO₂ = PCO₂ of arterial blood (mm Hg)
    • PECO₂ = PCO₂ of mixed expired air (mm Hg)

Ventilation Rates

• Minute Ventilation
  • minute ventilation = TV x RR
    • TV = tidal volume (mL)
    • RR = respiratory rate (breaths/min)
• Alveolar Ventilation 
  • minute ventilation corrected for physiologic dead space
  • V_A = (TV – V_D) x RR