Communications to the editor.

We postulate that as peripheral edema increases the distance between the nerve and the electrodes, the resistance increases and the amount of voltage required for nerve depolarization will also increase according to Ohm's law. This relationship may be similar to that oftransthoracic resistance and defibrillation current. Human studies have shown that transthoracic resistance increases with chest size and decreases with increased contact pressure between paddles and chest wall.. In the patient with COPD, when contact pressure between electrodes and skin was increased, a train-of-four stimulation elicited a twitch response despite massive edema. Edema is a major problem in the ICU and accumulates quickly in critically ill patients secondary to a number of factors, eg, low albumin, increased venous pressure secondary to mechanical yen-tilation, and massive volume resuscitation. Therefore, the absence of a twitch response in patients with new or persistent edema may erroneously suggest overparalysis. Consider the effect of peripheral edema on resistance. Perhaps in patients with peripheral edema the extremity should be elevated to minimize the edema, or alternate monitoring sites, such as the facial nerve, should be considered. sedating drugs and neuromuscular blocking agents in patients requiring mechanical ventilation for respiratory failure. blockade in the intensive care unit.racic resistance in human defibrillation. We think that the observations of Drs. Casale and Siegel are very important. There are many patients in whom it may be very difficult to obtain an accurate response to wrist-nerve stimulation, especially those with massive arm edema (and those with cold extremities). Furthermore, it is a common finding to detect a change in response when pressure is applied to one or both of the electrodes applied to the wrist for nerve stimulation. However, we have some differences in opinion as to the underlying biophysical principles and explanations for their findings regarding edema and electrode pressure. The main resistance between the two stimulating electrodes is determined by the electrode-skin interface (ie, metal anode! cathode, gel, and skin) and the volume ofunderlying subcutaneous tissue (Fig 1 and 2). These two elements are in series. The volume ofunderlying subcutaneous tissue in turn is made up of extracellular fluid, fat, muscle, and nerve-all in parallel. The current that is supplied by the stimulator (of either constant-voltage or constant-current type) is thus divided between the two parallel elements-the nerve and nonnerve subcutaneous compartments (which are in parallel). Ofthe total stimulator current, it is likely that only a small part actually …