During dialysis, samples of dialysate, from the arterial and venous sides of the dialyzer will be collected at 30-minute intervals during the dialysis period. In addition, the entire dialysate will be collected, its volume recorded, and a sample retained for the measurement of droxidopa and metabolites including but not limited to 3-OM-DOPS, NE, vanillic acid, and protocatechuic acid concentrations.