Describe the physiological consequences of decreasing the functional residual capacity (FRC) in an adult by 1 litre.

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College Answer

High scoring answers began with a definition and normal values, followed by a detailed list of the consequences of decreasing the FRC. Some candidates included descriptions of the normal function of FRC, conditions that decrease FRC and ways of improving reduced FRC. These were not required and did not attract marks. Diagrams require correctly labelled axes, values & units.

Discussion

This question is identical to Question 15 from the second paper of 2010, except this time 70% of trainees passed (instead of 33%), which probably demonstrates the benefit of doing past papers.

In brief:

Effects of decreased FRC on lung mechanics

  • Decreased lung compliance: the decreasing size of alveoli at lower FRCs results in a decreased rate of 
  • Increased airway resistance: because airway resistance is relatively low at FRC, it is going to increase as the FRC decreases. This is due to the fact that collapsing alveoli tend to stop providing the radial traction which keeps the small airways open.
  • Increased work of breathing, owing to the above.
  • Decreased tidal volume and increased respiratory rate, due to decreased lung compliance 
  • Decreased tolerance of position changes, i.e. with a low baseline FRc in the upriht position a patient will not tolerate being supine for very long, as the FRC will drop yet further 

Effect of decreased FRC on gas exchange

  • Decreased oxygen reserves: because the FRC acts as the main oxygen reservoir, the loss of volume here will give rise to an increased fluctuation in the bloodstream oxygen contentbetween breaths, and during episodes of apnoea.
  • Increased atelectasis: Decreasing the FRC to below the closing capacity tends to produce resorption atelectasis, as small airways close in expiration.
  • Increased shunt:  The consequence of abovementioned atelectasis will be shunt, i.e regions of lung which do not participate in gas exchange because they are not ventilated.

Effects of decreased FRC on the pulmonary circulation

  • Increased pulmonary vascular resistance, partly due to the effect of narrowed alveoli on perialveolar vessel caliber and partly owing to the inevitable increase in collapsed hypoxic lung regions which promote hypoxic pulmonary vasoconstriction.
  • Increased right ventricular afterload, which is due to the increase in pulmonary pressure

References

References

Wanger, J., et al. "Standardisation of the measurement of lung volumes." European respiratory journal 26.3 (2005): 511-522.

Lutfi, Mohamed Faisal. "The physiological basis and clinical significance of lung volume measurements." Multidisciplinary respiratory medicine 12.1 (2017): 3.

Hopkins, Erin, and Sandeep Sharma. "Physiology, Functional Residual Capacity." StatPearls [Internet]. StatPearls Publishing, 2019.