To the Editor:-Essential anesthesia monitoring equipment causes tremendous frustration of anesthesiologists and technicians when it does not work properly. Sixty-cycle electrical interference is a common problem encountered with electrocardiograph monitors in the operating room. Sixty-cycle noise usually originates from an external source, such as lights, motors, alternating current cables, etc. Noise may also originate from internal resistance within circuits or a broken 60-Hz filter. We had severe 60-Hz interference occurring in several operating rooms not traceable to any external source. All monitors in this operating room suite are older Datascope 2000 series units (Datascope, Paramus, NJ). Exhaustive evaluation of the monitors and cables could not locate the problem. Curiously, when inducing 60-cycle interference with a simulator, some monitors blocked the noise, whereas other did not. By systematically swapping circuit boards from a good unit, we isolated the problem to the "Refresh Memory' board, not the electrocardiograph circuit board. After closer examination, we found the problematic circuit boards had a switch not described in the service manual. On some units, the switch ("S1") was in the IN position, which filters noise, and on other units, it was in the OUT position. Other units did not have this switch on the circuit board.

Our query prompted further research at Datascope that discovered this switch was incorporated into units originally intended for international shipment. Some countries have line voltage frequency 50 Hz or 60 Hz. Under the 50-Hz conditions, the frequency response to the display must be changed from 40 Hz to 20 Hz. The internal switch limits the frequency response of beam deflection on the screen. Because the electronic filter controlled by this switch also filters 60-Hz noise, the manufacturer decided the switch should be in the "IN" position. Circuit boards labeled "K" (the company's designation) or higher have this switch.

In this cost-conscious era, we use monitors longer than ever before and frequently use older units in nonoperating room anesthesia sites. When anesthesia equipment malfunctions, our predictable response is to seek the purchase of new ones. Hopefully, this simple discovery will ease some of those 60-cycle headaches and add years of service to valuable equipment. In addition, with the replacement of an inexpensive picture tube and basic servicing, these popular and user friendly monitors can be brought back to crisp, "like new" condition.

Gary R. Haynes, Ph.D., M.D., Associate Professor; Andrew Lisicki, B.A., Biomedical Engineering, Department of Anesthesia and Perioperative Medicine, Medical University of South Carolina, 171 Ashley Avenue, Charleston, South Carolina 29425-2207.

(Accepted for publication August 19, 1996.)