As anyone who has heard me teach on lung function testing will know, I think there are three steps to interpreting spirometry:
- Is the forced expiratory ratio (FEV1/FVC) reduced (ie is the spirometry obstructive)? In this case it is not.
- Is the FVC reduced (ie is there evidence of restriction)? In this case it is.
- Is there a response to inhaled bronchodilator? A signficant response requires an improvement of 12% and 200ml from baseline in either FEV1 or FVC. In this case there is no such response.
Interpreting the spirometry alone, one could say that this is a restrictive pattern. However, one of the early signs of restriction on spirometry is an elevation of the forced expiratory ratio (FER). In this instance, the FER is tending down (62% at baseline). This raises the possibility that there is underlying obstruction with air trapping, leading to an appearance of restriction on spirometry.
Here is where lung volumes help. Since getting the 'body box' we are now able to do lung volume testing on every new patient. Lung volume measures here confirm that the apparent restriction is marginal - with a total lung capacity / TLC at 85% predicted around the lower limit of normal. Residual volume / RV is plumb average, and the ratio of RV/TLC tending up - suggesting a degree of air trapping.
Is this a true 'mixed defect'? That unusual combination of restriction and obstruction in the one patient? I think it is. Most commonly we see this appearance in older women with COPD and osteoporotic thoracic kyphosis. In this particular case the chest xray appearance is as follows:
Note the perfectly adequately expanded (in fact marginally hyperexpanded) right lung and the elevated hemidiaphragm on the left. This patient has airways disease with signficant obstruction, and in addition restriction due to a paralysed left hemidiaphragm.
A beautiful example!
Andrew
No comments:
Post a Comment