Artifacts

Patients

The electrical signal generated by the heart and monitored by the ECG is very weak, amounting to only 0.5 to 2 mV at the skin surface. It is therefore imperative that the skin be prepared optimally to avoid signal loss at the skin-electrode interface. Hair should be removed from the electrode sites with scissors and shavers. The skin should be cleaned with alcohol and should be free of any dirt. It is best to abrade the skin lightly to remove part of the stratum corneum, which can be a source of high resistance to the measured voltages. To avoid the problem of muscle artifact, electrodes should be placed over bony prominences, whenever possible. Muscle movement, in the form of shivering, can produce significant electrocardiographic artifact.

Electrodes and Leads

Loose electrodes and broken leads may produce a variety of artifacts that may simulate arrhythmias, Q waves, or inverted T waves. Pregelled, disposable silver/silver chloride electrodes are generally used in the operating room. The technical standards for such electrodes have been published by the American Association for the Advancement of Medical Instrumentation.^[26] It is important that all the electrodes be moist, uniform, and not out of date. Needle electrodes should be avoided because of the risk of thermal injury. Some electrocardiographic monitors have built-in cable testers that enable a lead to be tested by connecting the cable's distal end into the monitor. A high resistance causes a large voltage drop, indicating that the lead is faulty. The main source of artifact from electrocardiographic leads is loss of the integrity of the lead insulation. This subsequently leads to pickup of other electric fields in the operating room, such as the 60-Hz alternating current (AC) from lights and currents from the electrocautery device. Any damaged electrocardiographic lead should be discarded for this reason. Lead movement can also lead to artifact.

Operating Room Environment

Many pieces of equipment found in the operating room emit electric fields that can interfere with the ECG: 60-Hz power lines for lights, electrosurgical equipment, cardiopulmonary equipment, and defibrillator discharges. Most of this interference can be minimized by proper shielding of the cables and leads of the ECG, although the interference created by the electrosurgical equipment cannot be reliably filtered without distortion of the ECG. Electrocautery is the most important source of interference on the ECG in the operating room; it usually completely obliterates the electrocardiographic tracing. Analysis of the electrocautery has identified three component frequencies.

1397

Other causes of electrocardiographic artifacts in the operating room environment have been reported. Intraoperative monitoring of somatosensory-evoked potentials has been known to simulate pacemaker spikes.^[27] They are caused by incorporation in certain monitors of a "pacer enhancement circuit." This problem can be eliminated by disabling the circuit. Artifactual spikes have been found to coincide with the drip rate in the drip chamber of a warming unit. ^[28] The spikes were probably related to the generation of static electricity from water droplets. The use of an automated percutaneous lumbar diskectomy nucleotome has been reported to simulate supraventricular tachycardia (SVT), related to a mechanical interference.^[29]

Monitoring System

All electrocardiographic monitors use filters to narrow the bandwidth in an attempt to reduce environmental artifacts. The high-frequency filters reduce distortions from muscle movement, 60-Hz electrical current, and electromagnetic interference from other electrical equipment.^[30] The low-frequency filters ensure a more stable baseline by reducing respiratory and body movement artifacts and those resulting from poor electrode contact. The AHA recommends that a flat frequency response be obtained at a bandwidth of 0.05 to 100 Hz.^[25] The 100-Hz high-frequency limit ensures that tracings are of sufficient fidelity to assess QRS morphology and to evaluate rapid rhythms such as atrial flutter accurately. The low-frequency limit of 0.05 Hz allows accurate representation of slower events such as P-wave and T-wave morphology and ST-segment excursion.

Most modern electrocardiographic monitors allow the operator a choice among several bandwidths. The actual filter frequencies tend to vary among manufacturers. One manufacturer (Hewlett Packard, Andover, MA) allows a choice among a diagnostic mode with a bandwidth of 0.05 to 130 Hz for adults and 0.5 to 130 Hz for neonates, a monitoring mode with a bandwidth of 0.5 to 40 Hz for adults and 0.5 to 60 Hz for neonates, and a filter mode with a bandwidth of 0.5 to 20 Hz. The importance of bandwidth selection on the detection of perioperative myocardial ischemia was evaluated by Slogoff and coworkers.^[31] These investigators simultaneously used five electrocardiographic systems: a Spacelabs (Redmond, WA) Alpha 14 model series 3200 Cardule with bandwidths of 0.05 to 125 Hz or 0.5 to 30 Hz, a Marquette Electronics (Milwaukee, WI) MAC II ECG with 0.05 to 40 Hz or 0.05 to 100 Hz, and a Del Mar (Cincinatti, OH) Holter recorder at 0.1 to 100 Hz. The ST-segment positions with the three systems using the lower filter limit (0.05 Hz) recommended by the AHA were similar, whereas on the Spacelabs device (0.5 to 30 Hz), they were consistently more negative. The ST-segment displacement on the Holter recorder was consistently less negative and less positive. In at least one automatic ST-segment analysis system (Hewlett Packard), the lower frequency filter (0.05 Hz) is automatically activated when the ST analyzer is in use.