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

  1. What pattern of abnormality is shown?
  1. 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 DLCO is 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

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.