Electrical Issues

All electrical devices, including EMG machines, require current to operate. Current is delivered from an electrical cord plugged into a wall receptacle (Figure 40–1). A typical electrical receptacle in the United States contains three inputs: a black “hot” lead that carries 120 volts (V) of 60 Hz alternating current, a white “neutral” lead near 0 V, and a green ground lead that is used to dissipate leakage currents. When a circuit is created, current flows from the hot lead to the EMG machine and then returns via the neutral lead, based on the amount of resistance between the two leads as determined by Ohm’s law (see Chapter 39). Every wire, including power cords, has some small resistance; thus, a small voltage develops on the neutral lead, which equals the current flowing multiplied by the resistance in the power cord (Figure 40–2). The voltage increases with the length of the power cord and increases further if extension cords are added to the power cord. In addition, small voltage leaks often are present on the machine chassis, caused by stray capacitance and inductance from internal electronics (Figure 40–3). Thus, leakage currents may be transmitted onto the patient either from stray voltages on the machine chassis or on the neutral (reference) lead. As the ground electrode is close to true electrical neutral, the ground lead allows a pathway for stray current leaks to harmlessly dissipate.

FIGURE 40–1 Standard electrical wall receptacle.

A safe electrical receptacle must contain three inputs: a black “hot” lead that carries 120 V of 60 Hz alternating current, a white “neutral” lead near 0 V, and a green ground lead that is used to dissipate leakage currents. It is essential that all electromyography machines use a three-input receptacle, which includes a ground, for electrical safety.

FIGURE 40–2 Stray leakage current: reference lead.

Because every wire, including power cords, has some small resistance (R_N), a small voltage (V_N) develops on the neutral or reference lead as determined by Ohm’s law (V = I × R, where I is the current). The voltage increases with the length of the power cord and increases further if extension cords are added to the power cord. Thus, the voltage on the reference electrode is not zero and is a potential source for a leakage current transmitted to a patient.

(Adapted from Kimura, J., 1983. Electrodiagnosis in diseases of muscle and nerve. FA Davis, Philadelphia, pp. 615–619, with permission.)

FIGURE 40–3 Stray leakage current: stray voltages from the electromyography machine.

Small voltage leaks often are present on the machine chassis, caused by stray capacitance and inductance from internal electronics. These stray voltages are another potential source for leakage current that can be transmitted to a patient.

The risk of electrical injury depends on the amount of leakage current and whether the circuit passes through the heart. A very small current [e.g., 200 microamperes (µA)] applied directly to the heart can result in ventricular fibrillation and death. However, the normal, healthy individual typically is well protected by two important mechanisms. First, dry and intact skin provides a high resistance. Second, the large volume of soft tissue that surrounds the heart dilutes any current applied to the body (e.g., a current applied from arm to arm degrades to 1/1000 of the original signal when it reaches the heart, due to the dissipation from surrounding tissues).

The risk of electrical injury from leakage current increases in the following situations

The latter two (multiple electrical devices attached to the patient and loss of the body’s normal protective mechanisms) result in the “electrically sensitive” patient, a common situation in the intensive care unit.

To prevent the possibility of an electrical injury during EDX studies, it is essential for equipment to be regularly maintained, to always use a ground electrode, and to follow simple guidelines when using electrical devices attached to the patient (Box 40–1). A wooden examining table is preferable to a metal table, as it does not conduct electricity. Machines should be turned on before attaching electrodes to the patient and turned off after disconnecting the patient, to minimize the risk of power surges. Equipment should be periodically inspected by a biomedical engineer to measure leakage current and verify proper grounding. In general, the maximum amount of acceptable leakage current is 100 µA or less, measured from chassis to ground, and 50 µA or less from any input lead to ground. Extension cords should be avoided to reduce the risk of voltages developing on the reference electrodes. Ground electrodes should always be used to avoid current flows from reaching the patient. The ground needs to be placed on the same limb as the active electrodes so that leakage currents cannot flow in a path through the heart (Figure 40–5A).

Box 40–1

Measures to Ensure Proper Grounding

• Always use a three-hole power receptacle with properly grounded outlet.

• Unnecessary electrical equipment should be kept outside the EMG examining room.

• Suspect improper grounding if

Equipment is wet or has been subjected to spillage of liquids

Equipment has been physically damaged or has loose parts

Equipment gives a tingling sensation when touched

Equipment becomes hot or gives off unusual odor or sound

There is damaged or cracked insulation in the power cable

• Use a wooden examining table if possible (metal conducts electricity) (Figure 40–4).

• Avoid patient contact with any metal objects or any part of the EMG machine.

EMG, electromyography.

From Al-Shekhlee, A., Shapiro, B.E., Preston, D.C., 2003. Iatrogenic complications and risks of nerve conduction studies and needle electromyography. Muscle Nerve 27, 517–526, with permission.

FIGURE 40–4 Wooden bed and electrodiagnostic studies.

Wood does not conduct electricity; therefore, wooden examining tables are preferable to metal tables to ensure safety during electrodiagnostic studies.

FIGURE 40–5 Leakage current and the risk of electrical injury.

A: Correct placement of the ground electrode on the limb being studied. If a stray leakage current develops, the ground allows a pathway for the current to dissipate safely. B: When a patient is connected to two electrical devices, leakage current present on one device potentially can flow to another. In the example shown here, device 1 has a faulty ground electrode. A leakage current from that device creates a circuit by flowing to the ground electrode of the second device. If the pathway traverses the heart and the current is large enough, potentially dangerous arrhythmias may result.

(Adapted from Starmer, C.F., McIntosh, H.D., Whalen, R.E., 1971. Electrical hazards and cardiovascular function. N Engl J Med 284, 181–186, with permission.)

The issue of an intact ground electrode and proper ground placement is most important when a patient is connected to other electrical devices. If the ground from the EMG machine is not functioning (i.e., ground fault), stray current from the EMG machine could flow to a ground electrode from a different electrical device. If the pathway included the heart and the amount of current was large enough, a cardiac arrhythmia could theoretically occur (Figure 40–5B).

Risk of Electrical Injury

Central Lines and Electrical Wires

One of the more common ways a patient can become electrically sensitive is when the normal protective function of the skin is breached by intravenous lines and wires. This danger increases if the lines are actually in contact with or in close proximity to the heart, as occurs in central intravenous catheters (Figure 40–6). Most dangerous is the presence of an external wire near or in the heart, such as occurs with placement of a temporary external pacemaker and during the use of a guidewire while placing or changing a central line. Skin resistance typically is several million Ohms (MΩ). A central catheter traversing the skin reduces this resistance to 300,000 Ohms (kΩ). Any fluid spill where a catheter enters the body decreases the resistance even further. If a catheter has an internal guidewire, the resistance drops to 70 Ohms (Ω). An external pacemaker wire essentially has no resistance. In situations where the resistance is so low, small leakage voltages may result in small leakage currents, known as microcurrents. Whereas microcurrents are completely harmless in a patient with intact skin, they are potentially very dangerous in an electrically sensitive patient (i.e., a patient with a central line, external pacemaker wires, etc.).

FIGURE 40–6 Risk of electrical injury from central lines: stimulation sites to avoid.

One of the more common ways that a patient can become electrically sensitive is when the normal protective function of the skin is breached by intravenous lines and wires that are in contact with or in close proximity to the heart, as occurs in central intravenous catheters. Nerve conduction studies can be performed safely in these patients, provided certain precautions are taken. Proximal stimulation sites should be avoided, most importantly at the axilla and Erb’s point.

Thus, EDX studies should never be performed on patients with external wires in place (i.e., external pacing wire, guidewires, etc.) because the conductive pathway to the heart is so vulnerable. However, studies can be performed on patients with central lines provided certain precautions are followed. Equipment must be maintained. Ground electrodes must always be used. If an upper extremity must be studied, in general it is preferable and safer to study the upper extremity contralateral to the one with the central line. If that is not possible, one should refrain from proximal stimulation sites (i.e., axilla, Erb’s point, and root). Likewise, one should never proceed if there is a fluid spill where the central catheter enters the skin. It is important to note, however, that there is NO contraindication to performing routine nerve conduction studies on patients with peripheral IVs. Studies have been reported that specifically address this question and find that NCSs are completely safe in patients with peripheral IVs, regardless of whether they are infusing saline or any other solution.

Implanted Pacemakers and Cardioverter-Defibrillators

Patients with implantable cardiac pacemakers and cardioverter-defibrillators are at much lower risk from stray current leaks than patients with central lines or external wires in place, because these devices are implanted under the skin, which leaves the normal protective mechanism of the skin intact. Implantable pacemakers and cardioverter-defibrillators both have an electronic sensing as well as an electronic delivery function. Pacemakers are designed to treat bradycardia, as opposed to cardioverter-defibrillators which are primarily for tachyarrhythmias, especially ventricular fibrillation. In theory, stimulation delivered during NCSs might be mistaken as an abnormal cardiac rhythm. If the stimulator has a pulse duration greater than 0.5 ms and a stimulus rate greater than 1 Hz, a demand pacemaker might theoretically confuse such a stimulus with the ECG signal. There is only a single case report of an implantable pacemaker failure thought to be related to peripheral nerve stimulation. Other studies have shown no pacemaker inhibition or dysfunction with NCSs. Less is known about implantable automatic cardioverter-defibrillators (IACDs), which are now common. In theory, IACDs could be triggered by stimulation during NCSs, resulting in subsequent cardiac arrhythmias; however, there is no such reported case. One study directly addressed the safety of nerve conduction studies, including stimulating Erb’s point, in patients with IACDs. Schoeck et al. studied ten patients with pacemakers and five with IACDs. No electrical impulse was detected by either the atrial or ventricular amplifiers of the pacemakers or of the IACDs during median and peroneal nerve conduction studies. These studies included Erb’s point stimulation on the left. The authors emphasized that all modern pacemakers and IACDs use bipolar leads wherein both leads (active and reference for sensing, and cathode and anode for stimulating) are imbedded in the cardiac wall. This is in contradistinction to the pacemakers used 25 years ago wherein a single wire lead was placed in the heart, and the metal body of the pacemaker in the chest served as the reference. In modern pacemakers and IACDs, the bipolar leads are very close together in the heart, and very far away from the surface, making any electrical contamination from NCSs extremely unlikely. Although the number of patients in this study was small, the results are reassuring that NCSs can be safely performed in patients with pacemakers and IACDs.

If NCSs are performed in patients with implantable pacemakers or IACDs, several simple procedures are recommended to be followed in order to preserve safety (Box 40–2). Stimulation should not be performed near the actual implanted device. There should always be a minimum of 6 inches between the implanted device and the stimulator. Just as with NCSs performed in a patient with a central line, it is preferable to use the contralateral arm if possible. High stimulus intensities should be avoided and stimulus pulse duration should be 0.2 ms or less so that the stimulation is not misinterpreted as a QRS complex. Stimulation rates should be no greater than 1 Hz so as to prevent the theoretical risk that the stimulation is misinterpreted as a cardiac rhythm. Thus, the typical repetitive stimulation done during neuromuscular junction testing is best avoided.

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