Question 3

College Answer

Some candidates failed to provide a correct definition of dead space. An outline of anatomical,
alveolar and physiological dead space was expected. The Bohr equation was commonly
incorrect, and many did not comment on how to measure the components of the Bohr equation.
Fowler’s method was generally well described though some plotted the axes incorrectly.
The impact of increased dead space was not often well explained. Very few people stated the
major impact of increased dead space is reduced minute ventilation and how this would affect
CO₂.

Discussion

Definition of dead space:

• Dead space is the fraction of tidal volume which does not participate in gas exchange.

Its components:

• Apparatus dead space 
• Physiological dead space, which is composed of:
  • Anatomical dead space
  • Alveolar dead space
  • Shunt, if it is massive

Measurement of dead space:

• Physiological dead space is measured using the Bohr equation:
  
  (the Enghoff modification involves using arterial CO₂ instead of the alveolar)
• Anatomical dead space is measured using Fowler's method:
  • A single breath of 100% oxygen is given to the subject
  • The oxygen replaces nitrogen in the anatomical dead space
  • The exhaled breath has its volume and nitrogen concentration measured
  • The graph of nitrogen concentration over volume can be used to calculate the anatomical dead space:
    
• Alveolar dead space is determined by subtracting the anatomical dead space from the physiological dead space

Effects of increased dead space:

• The effect on gas exchange is the same as the effect of decreasing the tidal volume
  • Decreased CO₂ clearance
  • Decreased oxygenation due to increased alveolar CO₂ 
• The result is a decreased efficiency of ventilation:
  • For any given minute volume, CO₂ clearance will be decreased
  • Thus, there will be increased minute volume requirements
  • Thus, work of breathing is increased

Fowler, Ward S. "Lung function studies. II. The respiratory dead space." American Journal of Physiology-Legacy Content 154.3 (1948): 405-416.

Klocke, Robert A. "Dead space: Simplicity to complexity." Journal of Applied Physiology 100.1 (2006): 1-2.

Hedenstierna, G., and B. Sandhagen. "Assessing dead space. A meaningful variable?." Minerva anestesiologica 72.6 (2006): 521-528.

Robertson, H. Thomas. "Dead space: the physiology of wasted ventilation." European Respiratory Journal 45.6 (2015): 1704-1716.