Is exercise tolerance limited by the heart or the lungs?

This 55-year-old man with known rheumatic mitral valve disease is modestly disabled achieving a VO2max of 72% and a maximal power output of 76% of the predicted normal. His capacity to exercise is limited by dyspnea due to a reduction in his capacity to breathe. Ipratropium bromide was initiated to maximize his expiratory flow and improve his ventilatory capacity. A trial of inhaled steroids produced no improvement. He was referred for rehabilitation and smoking cessation. A decision was made to continue surveillance, postponing mitral valve replacement.