ABSTRACT
Exercise limitation can arise from a numbers of sources, including uncommon conditions apart from the more common maladies of the heart or lungs. Muscle, which is an essential element in generating movement, can also have conditions that present with impaired exercise performance or dyspnea. This chapter reviews myopathies, which have impairment of cytoplasmic fuel utilization or mitochondrial dysfunction. McArdle's syndrome is an example of the former, in which the muscle cannot mobilize muscle glycogen with exercise and thus can not supply additional pyruvate needed for exercise. Hence, for these patients, ATP production primarily relies on free fatty acids for energy generation. There is a myriad of mitochondrial myopathies, some of which present in adulthood in which ATP generation is markedly impaired and subjects experience an early lactate threshold and decreased exercise performance. Further testing is necessary to establish these diagnoses, but the use of CPET is essential to raise suspicion for these myopathies.
