I was reviewing a PFT recently and noticed that the FEV1 was severely reduced and that the FRC and RV were both elevated. This is a pattern we associate with obstructive gas trapping but I’ve also been reviewing textbooks on pulmonary function interpretation and have found that there isn’t any kind of a universal definition for this.
Hyperinflation and gas trapping are used somewhat interchangeably but the distinction is that gas trapping causes hyperinflation. Gas trapping occurs to some extent in everybody but usually at lung volumes below FRC. The lung volume at which gas trapping occurs rises with age and with obstructive lung disease. Hyperinflation is usually considered to be an increase in FRC but FRC is a dynamic lung volume and there is a range in the response to increased gas trapping. The normal progression from mild to very severe COPD goes something like this:
| FEV1: | FVC: | FRC: | RV: | TLC: | RV/TLC: | |
| Mild | ↓ | – | – | – | – | – |
| Moderate | ↓↓ | ↓ | – | ↑ | – | ↑ |
| Severe | ↓↓↓ | ↓↓ | ↑ | ↑↑ | – | ↑↑ |
| Very Severe | ↓↓↓↓ | ↓↓↓ | ↑↑ | ↑↑↑ | ↑ | ↑↑↑ |
Gas trapping and hyperinflation have significant consequences for an individual’s exercise capacity and level of dyspnea. It is an important clinical finding but from a PFT point of view when is it clearly present?