Question 9.1
A 52 year old female with systemic lupus erythematosus (SLE) and worsening dyspnoea on exertion presents with the following respiratory function tests:
|
Test |
Actual |
Predicted |
|
FEV1 |
1.96 litres |
2.66 litres |
|
FVC |
2.52 litres |
3.11 litres |
|
FEV1/FVC |
78% |
85% |
|
PEF |
7.50 L/sec |
6.47 L/sec |
|
FRC |
2.18 litres |
2.77 litres |
|
RV |
1.08 litres |
1.84 litres |
|
TLC |
3.64 litres |
5.17 litres |
|
DLco |
10.4 ml/min/mmHg |
24.7 ml/min/mmHg |
|
KCO (DlCO/VA) |
2.85 ml/min/mmHg |
4.77 1/min/mmHg |
Describe and explain the results of the respiratory function tests.
Suggest 1 possible diagnosis.
College Answer
Describe and explain the results of the respiratory function tests.
Suggest 1 possible diagnosis.
• Moderate restrictive defect
• High peak expiratory flow; due to fibrotic lung stretching airways open on full inspiration
• Small residual volume; due to cellular infiltration / fibrosis resulting in reduced lung compliance
• Reduced DLco (impaired gas transfer) due to both; A) Reduced lung expansion (restriction) and B) Damage to the lung parenchyma
Diagnosis: Pulmonary fibrosis.
Discussion
This question closely resembles Question 21.2 from the second 2013 paper.
However, this incarnation of the question also includes a nice diagram of the flow/volume curve.
This was abandoned in the 2013 paper probably because it is superfluous.
This question relies on some understanding of formal lung function tests. An excellent overview of this can be found in a 2005 article by Pellegrino et al.
The reduced DLCO and KCO (transfer coefficient for carbon monoxide) strongly suggests that there is a diffusion defect.
This would suggest pulmonary fibrosis.
The college also report that there is a moderate restrictive defect.
According to the 2005 ATS guidelines, a restrictive defect is defined as a TLC below the 5th percentile of the predictive, with a normal FEV1/FVC. This patient has an FEV1/FVC which is still above the 70% cut-off, and is therefore classified as "normal". The TLC percentile bands are not offered by the college, but one can see that it is well below the predicted value ( 3.64 L instead of 5.17 L).
According to the 2010 GOLD guidelines, a restrictive defect is characterised by a normal (or mildly reduced) FEV1, an FVC below 80% of predicted, and a normal (>70%) FEV1/FVC ratio. In the data above, the FVC is 81% of the predicted value, so by this standard the patient is just outside the borders of being classified as restrictive lung disease.
The major diference in this system of classification is the use of a fixed lower limit of normal (LLN), which is easier to remember and more convenient for resource-poor environments (where COPD is prevalent). Unfortunately it seems the use of such fixed cutoffs tends to incorrectly classify up to 20% of patients, particularly at the extremes of age (Miller et al, 2011). The glorious oracle of UpToDate recommends the use of computerised algorithms for predictive values, wherever they are available (i.e. using as the cutoff value the fifth percentile of the predicted FEV1/FVC ratio).
References
Pellegrino, Riccardo, et al. "Interpretative strategies for lung function tests."European Respiratory Journal 26.5 (2005): 948-968.
Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines - 2010
American Thoracic Society (ATS) guidelines for pulmonary function testing
Miller, Martin R., et al. "Interpreting lung function data using 80% predicted and fixed thresholds misclassifies more than 20% of patients." CHEST Journal 139.1 (2011): 52-59.