Formal Pulmonary Function Tests

Question 21.2 from the first  paper of 2014  and the identical Question 9.1 from the first  paper of 2011 asked the candidates to interpret some formal pulmonary function tests, complete with fancy carbon monoxide diffusion values and faintly remembered variables from that annoying respiratory volumes diagram. An excellent overview of this can be found in a 2005 article by Pellegrino et al.

That diagram again:

respiratory volumes

The college presents the candidates with a series of lung function variables, as well as actual and predicted values. The specific table is reproduced below:

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

The meaning of these variables will be briefly discussed:

FEV1 is the forced expired volume over 1 second, and is a measure of maximal air flow. A decreased FEV1 may mean either an obstructive pattern of lung disease, or a diminished expiratory effort (eg. in a patient who has some sort of myopathy or neuropathy).

FVC is the forced vital capacity, from maximal inspiration to maximal expiration. A decreased FVC may reflect poor respiratory effort.

FEV1/FVC ratio is a measure of airway resistance. A FEV1/FVC ratio less than the 5th percentile of predicted suggests obstructive airways disease

PEF is the peak expiratory flow rate. A low PEF suggests obstructive disease.

FRC is the functional residual capacity. A high FRC suggests hyperinflation (eg. in asthma) or large volumes of dead space (eg. emphysema)

RV is the residual volume. As with FRC, a high RV suggests hyperinflation or bullous dead space.

TLC is the total lung capacity. A high TLC may coexist with a very poor FEV1 and FVC in emphysema. A low TLC (below the 5th percentile of predicted) suggests restrictive lung disease, such as pulmonary fibrosis.

DLCO is the diffusing capacity for carbon monoxide, a measure of the efficiency of the lung as a gas exchange surface.  Normal  spirometry and lung volumes associated with decreased DLCO may suggest anaemia, pulmonary vascular disorders, early interstitial pulmonary fibrosis or early emphysema. It is expressed in ml/min/mmHg, and a value below 40% of predicted suggests a severe diffusion defect. DLCO may also be decreased if there is reduced lung expansion (i.e. a reduced TLC).

KCO (DlCO/VA) is the transfer coefficient for carbon monoxide. It is calculated as the DLCO per unit of alveolar volume. As such, the KCO will not be confused by changes in lung volume, and is a more faithful representation of the gas diffusion efficiency.

 

References

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

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