Incomplete ablation of renal nerves owing to the presence of accessory renal arteries that are too small for denervation might partly account for persistent hypertension after renal denervation say researchers. Before denervation, stimulation of renal nerves in the main and accessory renal arteries of 21 patients with resistant hypertension induced substantial increases in systolic blood pressure. After denervation, the blood pressure response to renal nerve stimulation was reduced in the main renal arteries but not in non-denervated accessory arteries, indicating a residual source of renal sympathetic tone.