Describe the physiological consequences of decreasing Functional Residual Capacity (FRC) by
one litre in an adult.

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College Answer

This is core knowledge and it was expected candidates would describe physiological
consequences accurately. Good answers included a definition of FRC and correct value. A number
of candidates omitted this. It was also expected that candidates mention that as FRC falls,
alveolar closure occurs, lung compliance decreases and airway resistance increases work of
breathing increases, pulmonary vascular resistance, and thus right ventricular afterload increases.
Many candidates described alveolar closure as causing increased dead space ventilation rather
than altered V/Q.
Syllabus: B1e
References: Nunn’s respiratory physiology, pages 51-56

Discussion

This question is identical to Question 8 from the first paper of 2017; the second time it came up the pass rate was much better

A short answer would probably have these points:

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

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.