Letter to the editor regarding cardiac arrest with vagal stimulation during intraoperative nerve monitoring

To the Editor We wish to thank you for publishing this important Case Report on intraoperative neurophysiological monitoring (IONM), which warns us about the risks of cardiac arrest secondary to stimulating the vagus nerve. Two aspects of this Case Report require comment. First, the stimulation parameters are not adequately defined. The authors describe the device and the stimulation parameters as a “. . .3.0 NIM device. . .with a default stimulation voltage (sic) of 1 mA.” The stimulus intensity was actually a current rather than voltage; however, the more important point is that this single parameter does not adequately characterize the stimulus. It is also critical to know the stimulus pulse width because neural tissue, whether peripheral or central, is excited by charge that is the product of pulse amplitude and width (or the integral of the current pulse if the pulse is not uniformly square). It is well known that nerve excitability is captured by strength-duration curves and that excessive charge may overstimulate the nerve producing a volley of efferent action potentials, which, in the case of stimulation of the vagus nerve, may reach the sinoatrial node resulting in the release of acetylcholine that would bind to muscarinic acetylcholine receptors. This binding would inhibit the activity of the sinoatrial node, causing bradycardia and, possibly, asystole. The stimulus frequency is a third defining parameter characterizing the stimulation properties. Too high a stimulus frequency may lead to summation of the IPSPs produced in the region of the cardiac sinus pacemaker and could lead to the described asystole. Each of these 3 stimulation parameters is individually controllable with the NIM 3.0 system and the user should ensure that each is appropriately set. It should be pointed out that the NIM system only displays the stimulus amplitude, not the stimulus pulse width or frequency. This is an easily correctable failure in the design of the user interface of this system, but clearly may lead individuals to believe that only the amplitude is an important parameter. It is quite conceivable that these patients were driven into asystole by an inappropriate set of parameters. Finally, with respect to this first point, the International Standard is silent on these important issues of stimulus pulse width and frequency. This standard only discusses stimulus amplitude. It is likely time to revise this important document. Second, in the discussion, the authors miss the entire goal of IONM; namely to prevent nerve injury rather than merely to document whether it has occurred. We agree that stimulation before beginning the actual operative procedure is important, as is stimulation at completion of the resection. However, most important are the evaluation of irritation activity, injury activity, and evoked activity during the operative procedure. We have monitored over 1200 thyroidectomy procedures over the past 5 years. Our stimulus parameters were usually 0.8 mA/0.2 ms pre-resection, and 0.3 mA/0.2 ms post-resection, all at 5.1 Hz stimulus frequency. In addition, we have both observed on-going electromyography to watch for irritation or injury activity and have encouraged the surgeons to stimulate putative nerves often. The goals of IONM are to both guide the surgeon through difficult resections and, when possible, to reduce morbidity. This requires continuous assessment of realtime activity occurring both with and without stimulation.