Hyperkinetic circulation during exercise in neuromuscular disease

Increased oxygen transport by the circulation is normally tightly coupled to increased oxygen uptake (VO₂)during exercise; cardiac output (Q) increases 5 to 6 liters for every liter of increased oxygen utilization(ΔQ/ΔVO₂≃5). We measured cardiac output and oxygen uptake at rest and during bicycle exercise in 7 patients with myalgia without evident muscle disease and 15 patients with myopathies. Resting circulation was normal in all patients, and during exercise the increase in cardiac output relative to oxygen uptake was normal in all myalgia and most myopathy patients. However, in four patients (with dermatomyositis, phosphorylase deficiency, carnitine deficiency, and ocular myopathy with “ragged-red fibers”) exercise cardiac output was excessive and ΔQ/ΔVO₂ high, resulting in an abnormally high level of cardiac work for a given level of exercise. This hyperkinetic response may represent aberrant regulation of the circulation by skeletal muscle as a consequence of some myopathies.