Dead Space

What is dead space

• 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

Fowler's method of measuring anatomic dead space

• 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

Bohr's method of measuring anatomical dead space

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

• 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

• 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

• 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

• Used to calculated the relationship between the fall in P_O2 and the rise in P_CO2 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:

• 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