 The principle of the nerve stimulation technique consists of using a battery powered peripheral nerve stimulator to generate electrical stimuli through an insulated needle at appropriate anatomical locations. The electrical impulses then trigger depolarization as insulated needle approaches the target nerve bundles. Braun Stimuplex, a common nerve stimulator used for peripheral nerve localization, is currently being used in the United States and other countries. Previous studies have suggested it to be a reliable stimulator for the purpose of peripheral nerve blocks. The operational manual however indicated that this device should not be used in patients with cardiac pacemaker because malfunction of the pacemaker may occur. Literature search on the topic of pacemaker interference from peripheral nerve stimulator showed that there was one case of pacemaker interference caused by activation of a nerve stimulator, resulting in cardiac arrest in a patient with a fixed-rate ventricular pacemaker. Studies and case reports suggest that unipolar electrode seems to be most susceptible to interferences from electrosurgical equipment, TENS unit, spinal cord stimulators. Perhaps this is the reason why the manufacturer of the peripheral nerve stimulator recommends against the use of peripheral nerve stimulator in patients with pacemakers. With the advancements of modern pacemakers technology, it is our experience; interscalene nerve blocks and other peripheral nerve blocks using the nerve stimulator can be performed in patients with pacemakers without notable interferences with pacemaker functions and perhaps the initial recommendation by the peripheral nerve stimulator manufacturer can be modified. In this prospective pilot study, we proposed to evaluate the effects of left interscalene stimulation most likely scenario to interfere with pacemaker function by varying the configurations of the pacemaker leads unipolar vs. bipolar pacer lead configuration and by varying the positions of the positive electrode of the stimulator. 