Microshock hazards

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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Microshock hazards

Brian A. Hall, MD

Electricity is ubiquitous in the operating room environment; its use poses inherent risks to health care workers and special risks to patients. Isolated power systems were developed several decades ago when ether was used in operating rooms. Line-isolation monitors (LIMs) were originally set to trigger at 2 mA, to decrease the chances of a spark occurring, which, in an ether-enriched environment, could create an explosion or fire. With the elimination of ether in operating rooms, in the late 1980s, the Occupational Health and Safety Administration increased the alarm threshold to 5 mA for LIMs used in operating rooms that had isolated power transformers to decrease the possibility of macroshocks occurring.

The 5-mA level was selected because (1) this level was thought to be potentially injurious to a grounded patient, depending on the site at which the current would contact the patient, thereby creating a path through which current might flow, and (2) the resistance (impedance) to current flow is estimated by convention to be 500 Ω.

A potential current leak to ground can occur and will go undetected by the LIM if the leak is well below the detection threshold. For this reason, the American National Standards Institute mandates that the maximum 60-Hz current leakage from an indwelling device not exceed 10 μA. This level is well below the ventricular fibrillation threshold (∼100 μA).

Independent of which system exists in an operating room, it is always prudent to ensure that, to the extent possible, a patient is never grounded. However, patients are connected to many electrical devices (e.g., via electrodes and electrical leads to monitoring devices). The chassis of the device is grounded, and such devices must be checked by a biomedical engineer no less frequently than every 3 months. All electrical devices have some leakage of current; extracorporeal electrical devices that are connected to patients cannot have more than 300 μA of leakage of current. The goal of isolated power transformers, ground-fault circuit interrupters (GFCIs), and even grounded circuits is to prevent macroshocks (i.e., patients exposed to electrical current flows of >5 mA).

With the advent of intracorporeal placement of electrical devices in or on the heart (e.g., pacemakers, defibrillators, oximetric and pacing pulmonary artery catheters, and cardiac output devices), the concept of microshock was born. The hazard associated with microshock is the low current threshold that may cause serious arrhythmias. A current as low as 100 μA can produce ventricular fibrillation. The threshold for the microshock is therefore in stark contrast with that of macroshock, which is the aforementioned 5 mA (i.e., 5000 μA). The difference in the magnitude of current needed to produce a macroshock versus a microshock is related to current density. When an electrical conductor is in direct contact with the myocardium, current density is several orders of magnitude higher than the current density produced when the conducting electrode is in contact with the outside of the body, such as with the hand.

It is blatantly obvious how a pacemaker with leads embedded in the myocardium could cause patient harm if electricity were conducted through those leads; however, anesthesia providers must also be aware that any catheter they insert into a patient, such as an internal jugular catheter or pulmonary catheter that is filled with conducting solution (e.g., saline or, especially, total parenteral nutrition), has the potential to conduct electricity directly to the heart. Manufacturers of devices used to monitor pressures from a central line go to great efforts to ensure that these devices are well grounded with maximum impedances everywhere possible. However, an anesthesia provider could overcome these safety measures by placing an electrical device with a relatively large leakage current in close proximity to, or touching, one of these central cannulas or catheters.

Care must be taken to avoid this scenario, and care must be used when handling wires that are in direct contact with the heart, such as intravascular guidewires or pacemaker leads. When procedures are performed in which these wires will be manipulated, the health care professional must ensure immediate availabiliy of equipment for direct-current cardioversion should a life-threatening arrhythmia occur. All personnel working in an operating room must be aware of the potential for the occurrence of microshock. Of equal importance is having a skilled biomedical engineer as part of the operating room staff.

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