Outline the abnormalities in pulmonary function testing of a person with severe obstructive lung disease (40% marks). Describe the physiological changes that explain these abnormalities (60% marks). 

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College Answer

The majority of candidates were able to state that obstructive lung disease resulted in a reduction 
in the FEV1 and the FVC. Very few gave any indication of how much these parameters might be 
reduced in severe obstructive lung disease or that FVC is usually only slightly reduced, whereas 
the FEV1 is markedly reduced and therefore so is the FEV1/FVC ratio. Candidates were also 
expected to indicate that peak expiratory flow rate is also reduced (by 50% or more in severe 
disease), that carbon monoxide diffusing capacity is also reduced, and residual volume and total 
lung capacity are increased. A reasonable proportion of candidates commented that the forced 
expiratory flow (FEF) 25-50% being reduced and this results in a ‘scalloped’ flow volume loop. 
Accurately reproducing a normal flow volume loop and a flow volume loop of a patient with 
severe obstructive lung disease on the same graph could have depicted most of the above points 
and scored the majority of marks available. 
The second part of this question was poorly answered. Whilst the majority correctly stated that an 
increase in airways resistance was present, few stated what changes gave rise to this. Dynamic 
airway compression, was mentioned by many, but why this occurred was explained poorly. The 
effect of closing capacity rising above the FRC was seldom mentioned and why the FRC 
increased again was seldom mentioned or explained. A detailed knowledge of what is a core 
subject in intensive care medicine was expected.

Discussion

This is one of those things where one's answer would probably benefit from being in a table, with a column for "abnormalities alongside a column for "physiological changes which explain them".

Changes in Pulmonary Function in Obstructive Lung Disease
Variable Abnormality Physiological change which explains it
Spirometry findings
FVC Slightly reduced Increased RV due to gas trapping and dynamic hyperinflation
FEV1 Markedly reduced

High airflow resistance in the airways due to bronchoconstriction leads to slower rates of flow. 

FEV1/FVC Reduced ratio
PEF (FEF) Reduced peak flow
Lung volumes
Closing capacity Increased Increased muscularity and mucus content of peripheral airways makes them more narrow and therefore makes their collapse earlier, at higher lung volumes. This produces dynamic airway compression and contributes to hyperinflation
TLC Increased In chronic obstructive lung disease, chronic hyperinflation of the lungs gives rise to gradual remodelling of the chest, with an increase in the TLC (mainly due to increased RV)
FRC, RV, ERV Increased Because of increased closing capacity, flow from some of the lung units stops before they have emptied (as airways collapse dynamically upon expiration). The remaining trapped gas volume contributes to an increase in the RV, and therefore also FRC and ERV. 
IC, IRV Decreased Diaphragmatic flattening, hyperinflation of the lungs producing an increased RV limits further chest expansion and therefore decreases inspiratory volumes.
Diffusion parameters
DLCO Reduced Carbon monoxide diffusion from the lung is decreased because the inspiratory lung volume is decreased; there may also be concomitant interstitial lung disease, pulmonary hypertension and chronic carbon monoxide toxicity, all of which can lower the uncorrected DLCO.
Or potentially, increased DLCO may also be increased in COPD by the coexisting polycythaemia 

The college also alluded to a picture which might be worth one thousand marks. In fact the examiners were weirdly specific in spelling out this request. They wanted "a normal flow volume loop and a flow volume loop of a patient with severe obstructive lung disease on the same graph", depicting the spirometry changes of COPD. Such a graph would probably look something like this:

Normal and COPD flow volume loop

References

References

Pellegrino, Riccardo, et al. "Interpretative strategies for lung function tests." European respiratory journal 26.5 (2005): 948-968.

Johnson, Jeremy D., and Wesley M. Theurer. "A stepwise approach to the interpretation of pulmonary function tests." American family physician 89.5 (2014): 359-366.

Miller, Martin R., et al. "Standardisation of spirometry." European respiratory journal 26.2 (2005): 319-338.

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

Graham, Brian L., et al. "2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung." European Respiratory Journal 49.1 (2017): 1600016.

González, P., et al. "Experience with Guillain-Barré syndrome in a neurological intensive care unit." Neurología (English Edition) 31.6 (2016): 389-394.

Quanjer, PhH, et al. "Peak expiratory flow: conclusions and recommendations of a Working Party of the European Respiratory Society." European respiratory journal 10.24 (1997): 2s.

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