RESPIRATORY GAS-EXCHANGE AND METABOLIC RESPONSES DURING EXERCISE IN MCARDLES-DISEASE

During normal progressive exercise, the gas exchange anaerobic thresho ld occurs when CO2 production (Vco2) and ventilation (VE) increase so as to depart from a linear relationship to O2 consumption (Vo2). This is thought to represent a gas exchange response to metabolic acidosis due to lactate accumulation. Patients with McArdle's disease have prev iously been reported to exhibit a steepened ventilatory response relat ive to Vo2, despite an inability to produce lactate. However, the Vco2 response has not been studied. We therefore investigated the Vco2-Vo2 and VE-Vo2 relationships in seven McArdle's disease patients and seve n control subjects during symptom-limited maximal treadmill exercise. Analysis of gas exchange showed that whereas all control subjects had an easily identifiable anaerobic threshold, four of the patients had n one and the other three displayed an attenuated threshold. The occurre nce of the threshold in one patient was associated with a small rise i n lactate and in another patient with an abrupt rise in leg discomfort , suggesting a pain response. Ammonia and the purine metabolite hypoxa nthine were elevated during exercise in all patients, suggesting that ammonia may be a product of adenosine monophosphate degradation. Free fatty acid levels were also elevated, and a shift toward utilization o f lipid may contribute to abnormal gas exchange responses. It is concl uded that lactic acidosis contributes to the gas exchange anaerobic th reshold but that other factors, such as discomfort, may be involved in the excess VE seen during heavy exercise.