Question 7(p.2)

College Answer

A definition of dead space incorporating subtypes (anatomical, apparatus, alveolar, 
physiological) was expected. Explanation of measurement methods attracted 
additional marks. Changes to End-tidal CO2 relative to PaCO2 were relevant to the 
question. The Bohr equation was stated and variables defined in better answers. 
Causes of an increased dead space should have been described including 
hypoperfusion and increased alveolar pressure. 
Increased dead space primarily results in CO2 retention, unless minute ventilation is 
increased commensurately. The physiological effects of increased PaCO2, 
increased respiratory rate and work of breathing are central to the question. 
Many candidates predicted severe hypoxemia; however the alveolar gas equation 
was not stated to explain this observation. Hypoxemia is a relatively late effect of 
significant hypo-ventilation, especially if the patient is breathing supplemental O2. 
Syllabus: B1e 2c
Reference: Nunn’s Applied Respiratory Physiology p310-311. Principles of 
Physiology for the Anaesthetist, Power & Kam p84-87

Discussion

The college did not specifically ask for the definition of dead space in the stem of this question, but apparently expected it anyway.

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:
  • Shunt
  • Anatomical dead space
  • Alveolar dead space

• Apparatus dead space is the dead space in an artifical breathing circuit
  • It can increase the total dead space:
    • Mechanical ventilation using a large NIV mask
    • Large circuit components, eg. a big HME
  • It can reduce the total dead space
    • Use of ETT (smaller volume than the upper airway)
    • Tracheostomy (bypass the upper airway altogether)
• Physiological dead space is measured using the Enghoff modification of Bohr's equation, using arterial CO₂ instead of exhaled CO₂, and it is composed of:
  • Alveolar dead space
    • The difference between the physiological dead space and the alveolar dead space
    • This is the volume of gas which fills lung units which are unperfused or poorly perfused
    • Wests' Zone 1 contain alveolar dead space.
    • Under normal circumstances, this volume is minimal. 
  • Anatomical dead space 
    • Measured by Fowler's method
    • Represented by Phase I and half of Phase II in the single-breath nitrogen washout test.
    • Represents the volume of gas in the conducting airways
  • Shunt,  when massive, can produce the illusion of increased Enghoff dead space because it increases arterial CO₂ 

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.