Question 21.2

Created on Tue, 05/12/2015 - 06:03
Last updated on Fri, 04/28/2017 - 15:59
Pass rate: ?
Highest mark: ?

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A 54-year-old female with scleroderma 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

a) Describe and explain the results of the respiratory function tests.

b) Suggest a possible cause.

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College Answer

a)

  • 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:
    • Reduced lung expansion (restriction) and
    • Damage to the lung parenchyma

b)

Pulmonary fibrosis

Discussion

a)

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).