Dead Space

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What is dead space[edit]

  • 150 mls of each inhaled tidal volume remains behind in anatomical dead space
  • Alveolar ventilation is therefore (500 - 150) x 15 = 5250ml/min
  • This is the amount of air available for gas exchange
  • Total ventilation is easy to measure, however alveolar ventilation is more difficult
  • Alveolar ventilation equation method:
  • No CO2 is in anatomical dead space, therefore all exhaled CO2 can be assumed to have come from alveolar gas
  • Therefore from a known concentration of alveolar CO2, a known expired volume of CO2 and a total expired volume, alveolar ventilation can be calculated

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Fowler's method of measuring anatomic dead space[edit]

  • Subject inhales a single breath of 100% O2, then expires with a nitrogen analyzer sampling gas at the lips
  • Initially no nitrogen, then a rise and plateau as pure alveolar gas is being sampled
  • The volume of dead space is determined by plotting N2 concentration against expired volume, then drawing a vertical line at the midpoint of the curve
  • This measures the volume output down to midpoint of the transition form dead space to alveolar gas

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Bohr's method of measuring anatomical dead space[edit]

  • All expired CO2 comes from alveolar gas and none from dead space

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  • Fowler's method measures the volume of the conducting airways down to where rapid dilution of inspired gas occurs - the anatomical dead space
  • Bohr's method measures physiologic dead space, as it measures the volume of the lung that does not eliminate CO2
  • Anatomic dead space + Alveolar dead space = Physiologic dead space

Factors Affecting Anatomical Dead Space[edit]

  • Increases with body size, age, lung volume, erect posture, bronchodilation, emphysema
  • Decreases with supine posture, bronchoconstriction, lung volume loss, intubation

Factors Affecting Alveolar Dead Space[edit]

  • Increases with age, decreasing pulmonary artery pressure (due to hypoperfusion), IPPV - via hydrostatic failure of perfusion, increasing tidal volumes, anaesthetic gases, lung disease

Alveolar Gas Equation[edit]

  • Used to calculated the relationship between the fall in PO2 and the rise in PCO2 that occurs in hypoventilation
  • R = the respiratory exchange ratio or respiratory quotient - the ratio of CO2 production to O2 consumption
  • R is determined by the metabolism of tissues in a steady state


  • This formula assumes that alveolar dead space is negligible
  • Also ignores the bulk flow of oxygen generated from the pressure gradient of more O2 being taken up than CO2 excreted in the alveoli, drawing oxygen from the anatomical dead space
  • This effect is included in the following version of the alveolar gas equation:

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  • The alveolar gas equation assumes:
  • Inspired and alveolar gases obey ideal gas laws (Boyle's law and others)
  • There is no water or CO2 in inspired gas
  • Nitrogen in inspired gas is in equilibrium with dissolved nitrogen in blood
  • Alveolar gases are saturated with water