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Sources of Artifact Affecting the Interpretation of the Electrocardiogram
Equipment Inducing Movement Artifacts.
Motion may generate electrocardiographic artifact. Both movement of the electrocardiographic lead on the left leg by an intraaortic balloon pump catheter and electrode movement due to a pneumatically driven pump have simulated conduction disturbance and arrhythmia. Similarly high-frequency oscillatory ventilation (frequency between 4–15 Hz) caused high-frequency artifacts due to skin/electrode movements. It appeared as atrial flutter/fibrillation in a neonate and ventricular tachycardia in an adult on both oscilloscope and lead electrocardiograph.
Current Leakage, Grounding Failure, and Interference by Capacitance.
Depending on the type of equipment used, electrical current may leak and pass through a patient. Apart from posing a serious electrical hazard, it can cause artifacts on the electrocardiogram. Electrical current leakage from both intravenous fluid infusion controllers and the light intensity controls of a fiberoptic bronchoscope caused interference with electrocardiographic monitoring. Stray current from loose electrical wiring of a microdebrider for sinus surgery has led to artifact resembling ventricular tachycardia, whereas electrical noise from a pressure controlled irrigation pump, during shoulder arthroscopy, generated pseudo–atrial flutter or fibrillation changes on the monitor. Chase and Brady reported wide QRS complex tachycardia due to placement of new electrical lines near a monitor whose internal insulation was broken. Capacitive coupling between metal plates of a faultily grounded fluid warmer and the insulated plastic fluid package led to pacemaker spikes appearing, coincident with use of the warmer. Several cases of electrocardiographic artifacts mimicking atrial flutter or ventricular tachycardia occurred with the use of a urine output/core temperature monitor and cardiograph, where a signal generated in the isolation circuitry of the urine output monitor was conducted to the patient and then to the cardiograph. Signals unintentionally produced in one device have the potential to be transmitted to another, by capacitive coupling through the patient.
Static and Piezoelectric Effects
Piezoelectricity is an electrical charge generated by the mechanical deformation of polymeric materials. Plastics also generate static electricity when constantly rubbed and separated by dissimilar nonconductive surface materials. Static charge and subsequent artifact generation can result due to droplet formation associated with the use of infusion pumps, and a blood/fluid warming set. The amplitude and frequency of the artifacts were inversely related to the drop rate. Khambatta et al. found an incidence of 68% in a study of electrocardiographic artifacts during cardiopulmonary bypass. They identified static electricity generation between polyvinyl chloride tubing and the pump roller head as source of artifactual arrhythmia. In another case, a loose electrode connection accentuated the contribution of static electricity toward a reproducible artifactual atrial flutter during cardiopulmonary bypass. Piezoelectric or static electrical signals, in the presence of poor electrode contact, lessen the common-mode rejection capabilities of the differential amplifiers, and set the stage for the recording of spurious signals. In the ICU, artifacts in the form of pseudo–atrial flutterfibrillation have been described in patients receiving dialysis due to the generation of static and piezoelectric currents in pumps rotating between 50 and 600 rpm. Such currents flow into the patient via fluid in the tubing and are detected by the electrocardiographic electrodes. Development of a periodic electrostatic charge could also cause a periodic variation in the body surface potential measured during the electrocardiographic monitoring and recording, by either inductive or capacitive interference. Static electricity can be generated with the use of synthetic clothing and carpeting in dry atmosphere. A life-threatening electrocardiographic artifact was described by Schiller, where static electricity (generated as a technician slapped a panel of styrofoam against his thigh) was detected as an R wave, and fired an electrocardiographic-triggered lithotripter asynchronously. The currents associated with piezoelectric or static electricity pose no electrical safety risk in themselves.
Electromagnetic and Radiofrequency Interference
Electromagnetic interference resulting in artifacts on the electrocardiogram has been described with the use of cell phones. The effect depends on distance, frequency over which they operate, and technology used for communication. It also depends on the ability of medical devices to resist electromagnetic interference. Walkie-talkies used by maintenance and security personnel cause more interference than cell phones because they operate at lower frequency and have a higher power output. In the modern neurosurgical operating room, highfield magnetic resonance imaging (MRI) is used to facilitate surgery. Electrocardiographic artifacts can arise from either static or pulsed (dynamic) magnetic fields. In the presence of a static magnetic field, electrical voltages may be generated within the body due to flow potentials, which are the result of blood moving within the patient. In a dynamic magnetic field, rapidly switched magnetic gradients can induce electrical interference in monitoring leads. In both of these circumstances, artifacts may vary in their frequency and time of occurrence, even mimicking atrial flutter and ventricular tachycardia or fibrillation. Radiofrequency pulses can also cause problems if not properly removed from the amplification system. Electrocautery-induced electrical interference on the electrocardiogram is mainly due to very high-frequency currents (radiofrequency range) of 800,000 to 1 million Hz (800–1,000 KHz). Other contributing factors are power line (50/60 Hz) and low-frequency noise (0.1–10 Hz) from intermittent contact of the electrosurgical units with the patient’s tissues.
Other sources of environmental electrical interference are radiofrequency transmitters. Such interference is picked up by electrical conduction or induction. Surrounding power cords, electrical instruments, and transformers all radiate electrical energy, which becomes electrically or magnetically coupled to the body. Personal pagers, handheld two-way radios, and cell phones are also radiofrequency transmitters and generate interference in the environment. A patient with a permanent pacemaker in postoperative care experienced high-frequency pseudopacer spikes on his electrocardiographic trace. Despite intensive investigation, no cause could be found other than radiofrequency emission.
