Question 2

College Answer

This is a core topic within respiratory physiology. There was a very low pass rate for this question. Expected components of the answer included: a definition of WOB as a product of pressure and volume or force and distance including the units of measurement; followed by a detailed explanation of the following three broad components – elastic resistance, viscous resistance and airflow resistance. Further marks were awarded to situations where the energy for expiration increases beyond stored potential energy as well as the impact of respiratory rate and tidal volume on different aspects of the WOB. For example, the changes in TV will have relatively greater impact on the elastic component, whereas RR will impact the resistance component. Additional marks were awarded for describing the efficiency of breathing. A common area where candidates missed out on marks was producing a diagram of WOB without a description; many diagrams were often incorrectly drawn or had no axes labelled. There were many incorrect definitions or respiratory equations provided without any link to the written answer. Factual inaccuracy and limited depth of knowledge were also prevalent in poorly performing answers. Marks were not awarded for a description of the control of breathing.

Discussion

• Work is the product of force and distance, and is measured in Joules (1J = 1N per 1m)
• In respiratory physiology, work is the product of pressure and volume
• Several components contribute to the total work of breathing:
  • Elastic work 
    • Work done to overcome elastic recoil of the lung
      • This increases with increasing inspiratory volume
    • Work done to overcome elastic recoil of the chest 
      • subtracted from the work done to overcome the elastic recoil of the lung; i.e. elastic recoil of the chest wall does work to inflate the lung
      • With small volumes, work is actually done to reduce the chest wall volume down to FRC
      • With larger tidal volumes, the energy for expiration increases beyond stored potential energy and work must be done to overcome both the elastic recoil of the lungs and the chest wall
  • Resistive work 
    • Work done to overcome tissue resistance otherwise referred to as viscous resistance:
      • Chest wall resistance
      • Lung resistance
      • Displacement of abdominal organs
      • Compression of mediastinal structures
      • Normally accounts for no more than 10-20% of the total work of breathing
      • Increased with raised intraabdominal pressure, pleural disease, mediastinal masses, etc
    • Work done to overcome airway resistance, which includes:
      • Airway resistance
      • Resistance of airway devices and circuits
      • Usually minimal contribution to the total work of breathing
      • Increased in scenarios where flow rate is increased (eg. with increased respiratory rate) or where airway diameter is decreased (eg. small ETT, bronchospasm)
    • Work done to overcome respiratory inertance
    • Work done to compress intrathoracic gas

For a diagram, they probably would have wanted a Campbell diagram, like this:

Cabello, Belen, and Jordi Mancebo. "Work of breathing." Intensive care medicine 32.9 (2006): 1311-1314.

Banner, Michael J., MARC J. Jaeger, and ROBERT R. Kirby. "Components of the work of breathing and implications for monitoring ventilator-dependent patients." Critical care medicine 22.3 (1994): 515-523.

Otis, Arthur B. "The work of breathing." Physiological reviews 34.3 (1954): 449-458.

Mancebo, J., et al. "Comparative effects of pressure support ventilation and intermittent positive pressure breathing (IPPB) in non-intubated healthy subjects." European Respiratory Journal 8.11 (1995): 1901-1909.

Liljestrand, Q. "Untersuchungen über die Atmungsarbeit 1." Skandinavisches Archiv Für Physiologie 35.1 (1918): 199-293.

Zakynthinos, S., and C. Roussos. "Oxygen Cost of Breathing." Tissue Oxygen Utilization. Springer, Berlin, Heidelberg, 1991. 171-184.

Campbell, E. J. M., E. K. Westlake, and R. M. Cherniack. "Simple methods of estimating oxygen consumption and efficiency of the muscles of breathing." Journal of applied physiology 11.2 (1957): 303-308.

Campbell, Edward James Moran. The respiratory muscles and the mechanics of breathing. Lloyd-Luke, 1958.

Peters, Richard M. "The energy cost (work) of breathing." The Annals of thoracic surgery 7.1 (1969): 51-67.

Jonson, B., and L. G. Olsson. "Measurement of the work of breathing to overcome pulmonary viscous resistance." Scandinavian journal of clinical and laboratory investigation 28.2 (1971): 135-140.

Crosfill, Mt L., and J. G. Widdicombe. "Physical characteristics of the chest and lungs and the work of breathing in different mammalian species." The Journal of physiology 158.1 (1961): 1-14.