FIG. 21:  Another important concept in pulmonary rehabilitation is the interaction between the leg muscles and the lungs.  Pharmacologic bronchodilators will cause a reduction in static and dynamic hyperinflation of the lungs in patients with COPD, and indirectly pulmonary rehabilitation also does that.  A higher inspiratory capacity means a lower end-expiratory lung volume; when inspiratory capacity decreases with exercise, then assuming total lung capacity does not change, this means the person is hyperinflating during exercise. 

The data in this Figure, from Casaburi’s group (now in California), show inspiratory capacity before and after pulmonary rehabilitation.^[18]  Before training, inspiratory capacity decreased from almost 2.5 L on average to about 1.5 L, a fall of about 1 L, and exercise was terminated at 5 min.  After exercise training, inspiratory capacity also decreased but by less than 1 L, and maximal exercise duration persisted beyond 15 minutes.  Thus, pulmonary rehabilitation increased exercise duration to a maximum and also reduced hyperinflation, and the reported mechanism for the increased exercise duration was a reduction in dynamic hyperinflation. 

How does this mechanism work?  Several weeks of intensive exercise training in the pulmonary rehabilitation center will mean that patients will have less lactate produced at isowork or isotime during exercise.  This means they no longer have to buffer the lactate with ventilation, and because they do not have to buffer lactate and are more efficient, they have a lower respiratory rate at isotime, more time to exhale, and less dynamic hyperinflation.  Thus pulmonary rehabilitation, through its effect on leg muscles and ambulation, reduces the ventilatory demands at each particular work level, allowing the patients to exhale more readily, with less dynamic hyperinflation. Dynamic hyperinflation is perhaps the most important variable in dyspnea in patients with COPD.  Other lung deflators might include bronchodilators, supplemental oxygen, and lung volume reduction surgery.