Electrostatic Discharge Interference in the Clinical Environment
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Case 1
A neonate was receiving an infusion of 3.7 mL/hour of total parenteral nutrition. During regularly scheduled patient charting, fluid status was checked and the nurse found the general purpose microinfusion pump to be delivering at a rate of 88.8 mL/hour with no alarms indicated. Compensatory therapy was necessary to restore the patient's blood chemistry to normal. Continued infusion at the erroneous rate would have been lethal. I Clinical staff interviewed indicated the belief that no one had manually altered the pump control settings. Consultation with the manufacturer's representatives repealed that when the microprocessor is reset, the rate register defaults to 88.8 mL/hour as a means of providing seven-segment display verification. Such a reset is accompanied by an audible alarm and a halt in fluid delivery.
Laboratory examination of the pump revealed no evidence of any malfunction. Controlled electrostatic discharge of greater than 5 KV to circuit ground or to the IV pole was demonstrated to reset the microprocessor, albeit with coincidental alarm. While it was not possible in the laboratory to recreate the precise consequences observed during the clinical incident, ESD interference is believed to have been a contributing factor.
The temperature outside the hospital was less than 20°F when the incident occurred. While a hygrometer was not in place at the time, humidity levels in the high 20% range were subsequently measured within the neonatal intensive care unit, prior to adjusting the humidification system.
Case 2
While making rounds, a nurse in a postsurgical orthopedic unit measured and documented patient temperature with a battery-powered IR tympanic thermometer. As the nurse inserted the device into the patient's ear, a spark occurred that"shocked" both the patient's ear and one of the nurse's fingers of the hand that was holding the thermometer. Coincidental with the spark, the thermometer sounded an audible alarm that could be terminated only by removal of the batteries. Upon replacement of the batteries, normal function was restored, with no deviation from manufacturer's specifications discernible.
Providing that ESD resulted in a device's malfunction can be very difficult.
Both the nurse and the patient were certain that they had been shocked by the tympanic thermometer. Examination of the device, however, revealed no defect in its plastic insulative housing, nor any operating voltage greater than 15 volts. When interviewed, the nurse agreed that ESD could have occurred between her finger and the patient's earlobe, both of which were in close proximity to the device. The fact that no permanent damage was obvious makes the temporary malfunction appear to have been related to radiofrequency interference of the microprocessor, secondary to the ESD.
Case 3
Portable electrocardiographs function predictably throughout a hospital for nine months out of the year. During the coldest months, they are subject to intermittent unpredictable failure modes. Technicians* attempts to "repair" the devices are frustrated by the lack of any evidence of the malfunction during subsequent laboratory tests. Most cardiograph malfunctions suspected of being attributable to ESD occur while the devices are being used to acquire ECGs on patient floors, as opposed to within the cardiology department. Manufacturer modifications of circuit grounds are found to significantly reduce failure incidence. The authors are aware of other ESD-related malfunctions having occurred as well, with apparatus including cardiopulmonary-bypass pumps, hemodialysis equipment, physiologic monitoring systems, and an assortment of hospital information system devices.
ESD Avoidance
In a facility known to have ESD problems, steps can be taken, prior to acquisition of new apparatus, to reduce their likelihood. Mere reference to high ESD immunity ithin bid specs is one such step. Some manufacturers will provide results of ESD testing. Improvement of device internal circuits grounding may be an option as exemplified in case 3 above. Similarly, adding shielding around, or RF filtering to, a known susceptible circuit may be possible. Often these strategies are not fiscally feasible, however.
For apparatus users, the primary ESD avoidance tactic is maintenance of the clinical working environment at a relative humidity that is high enough to enable adequate static-decay rates through the atmosphere. The American Institute of Architects Committee on Architecture for Health makes recommendations regarding temperature and humidity control.5 These recommendations indicate an appropriate range of 30-60% for critical care areas, and a range of 45-60% for surgical suites and temperatures in the low 70-degree range (Fahrenheit). Their values were formulated based on comfort, asepsis, and odor control. No limits are placed on many other instrumentation intensive clinical areas.
The authors have observed disruptive, unintentional ESD to occur in a room where the relative humidity was being monitored. The hygrometer indicated a relative humidity level of 36% when the discharge was observed (Figure 2). Since control of humidity between possibly 40% and 60% is more costly and difficult to achieve, this single precautionary measure is likely to be inadequate.
A second common tactic employed in the antistatic charge-generation armamentarium
is the use of antistatic flooring.Various synthetic tiles and carpets are
effective in eliminating the floorshoe generator. When both humidity levels
and floor composition vary throughout an institution, however, the possibility
exists that a charge generated in one room or hall will be carried into an
area employing susceptible apparatus. It would also be a mistake to assume
that movements of shoes on floors are the only static generators in hospitals. Contact between plastics and synthetic
clothing, for instance, has also been found to create destructive levels of charge.6
Institutions believed not to have a problem can also benefit from preventive measures.
Localized use of antistatic sprays or liquids is more complicated still.
Different products have varying degrees of effectiveness, and some produce
side effects on humans (e.g.. eye and nose irritation). Also, reliance on such tools
requires frequent use that may be forgotten when no one has been "shocked" recently.
Restriction of access to, or electrical isolation of. a site to which ESD is known to produce undesirable consequences can be an effective means of eliminating a recurring problem. When an IV pole was shown to act as an antenna for an EMI signal generated by ESD to the pole, for example, an insulative pole covering was employed. (Figure 3).
Where not superseded by a product-specific standard, IEC 10004-2 indicates that equipment may display "temporary degradation or loss of function or performance which is self-recoverable,"given air discharge ESD of a potential 4 kV. Between 4 and 8 kV the standard allows "temporary degradation or loss of function or performance which requires operator intervention."* While environmental conditions are obviously critical, the authors have found naturally occurring potentials of between 10 kV and 15 kV to be fairly common within health care institutions.Walking across certain carpeted flooring with relative humidity levels below 20% has been found to generate potentials in the range of 35 kV.
Resource:
NFPA 99: Standard for Health Care Facilities
Institutions believed not to have an ESD problem can also benefit from preventive ESD measures. Where potential exists for even uncommon brief cold snaps or humidification disruptions, previously unforeseen consequences can result.
About the authors
Wallace Elliott, MSEE, CCE, is a clinical engineer with the
Technical Services Program at the University of Vermont,
He holds an undergraduate degree in electrical
engineering from Georgia Institute of Technology, Atlanta,
and a master of science degree in electrical engineering
from the University of New Hampshire, Durham.
Mr. Elliott enjoys volunteering with third world medical
instrumentation support organizations.
Gilbert Gianetti is an electronic engineer wit the Instrumentation and Model Facility of the University of Vermont. Prior to assuming his present biomedical research design responsibilities, Mr. Gianetti was employed by the broadcast and electrical utility industries. Mr. Gianetti is an amateur radio operator.
References
5. The American Institute of Architects Committee on Architecture for Health. Guidelines for Construction and Equipment of Hospital and Medical Facilities- Washington, DC: The American Institute of Architects Press, 1993.
6. Leahy W, Massimino R, Wohlfert W. Intermittent failure of a Stockert/Shiley multiSow roller pump module. J Extra-Corporeal Tech. 1993; 25(3):74-7.
