Question 26.3

The following question is based on the shown pulmonary function tests (PFTs). Assume in each case that the test result is adequate and reproducible.

Key:

FVC	L	Forced Vital Capacity
FEV1	L	Forced Expiratory volume in 1 second
FEV1/FVC	%	Ratio of the above
RV	L	Residual volume at end expiration
TLC	L	Total Lung Capacity
DLCO corr	ml/min/mmHg	Diffusing capacity for carbon monoxide, corrected for Hb

A 46-year-old female has presented with several months of progressive shortness of breath and lethargy compromising her previously active lifestyle. She is markedly hypoxic, with a resting SpO2 of 88% in air. She has had PFTs performed recently as an outpatient

	Predicted	Actual	% Predicted	Post Bronchodilator	% Change
FVC (L)	3.56	3.35	94	2.77	-6
FEV1 (L)	2.88	2.70	93	2.31	-4
FEV1/FVC (%)	81	82		83
RV (L)	1.90	2.03	107
TLC (L)	5.22	5.11	98
DLCO corr (ml/min/mmHg)	23.25	7.96	34

What pattern of abnormality is shown?

List two differential diagnoses. (20% marks)

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College answer

Normal lung function, markedly impaired diffusion of gases
Problem is not in the lungs but with the blood flow i.e. pulmonary vascular disease/pulmonary hypertension

Any 2 of:
idiopathic or familial PAH
cardiac disease - L sided
connective tissue disease /SLE
drug induced
chronic thromboembolic disease (2 marks)

Discussion

Formal pulmonary function tests have also been asked about in Question 21.2 from the first paper of 2014 and the identical Question 9.1 from the first paper of 2011.

To approach this systematically:

FVC is the forced vital capacity, and here it is normal.
FEV1 is the forced expired volume over 1 second, and is a measure of maximal air flow. Here, it is normal.
FEV1/FVC ratio is a measure of airway resistance which incorporates both the abovementioned metrics.
RV is the residual volume. A high RV suggests end-expiratory gas trapping or bullous dead space. This patient's RV is normal.
TLC is the total lung capacity. In this case, the TLC is essentially normal.
DLCO is the diffusing capacity for carbon monoxide, a measure of the efficiency of the lung as a gas exchange surface. In this case, the DLCOis 34% of predicted, demonstrating a significant diffusion defect (which is probably not a measurement error, given the normal TLC). The "corr" in the SAQ refers to the correction of the DLCO for the patient's haemoglobin value, which we are not supplied with. Normal spirometry and lung volumes associated with a decreased haemoglobin-corrected DLCO suggest that something is wrong with the pulmonary vasculature and blood flow.

An isolated defect in the diffusing capacity for carbon monoxide could be anaemia, but the "corr" in the SAQ refers to the correction of the DLCO for the patient's haemoglobin value, which excludes this as a differential. Other possible causes (according to UpToDate) include:

Pulmonary hypertension:
- Chronic thromboembolic
- Primary
- Secondary to left-sided cardiac disease (eg.. MR)
- Secondary to vasculitis, pulmonary fibrosis, etc
Hepatopulmonary syndrome
High carboxyhaemoglobin level
Early interstitial lung disease (i.e. fiborisis is already occurring, but the the TLC and FVC have not had time to change)

References

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

The American Thoracic Society has a page which features an excellent bibliography of the articles which support their interpretation standards.

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