A diagnostic cycle test for McArdle's disease.

We investigated whether the second wind phenomenon (ie, a decrease in heart rate and perceived exertion during exercise) is pathognomonic for McArdle's disease. Twenty-four patients with McArdle's disease, 17 healthy subjects, and 25 patients with other inborn errors of muscle metabolism cycled a constant workload for 15 minutes. In McArdle's disease patients, heart rate consistently decreased by 35 +/- 3 beats per minute from the 7(th) to the 15(th) minute of exercise, whereas heart rate increased progressively with exercise in all 42 control subjects. The findings indicate that cycling at a moderate, constant workload provides a specific, sensitive, and simple diagnostic test for McArdle's disease.