An electrocardiogram (ECG) is a fundamental diagnostic tool that captures the electrical signals generated by the heart. This non-invasive test uses electrodes placed on the skin to detect tiny voltage changes, which are then amplified and recorded as a waveform tracing. When this process is interrupted by unwanted electrical noise, the resulting pollution is called artifact. One of the most common types is AC interference, or Alternating Current, which refers to the electricity supplied by standard wall power outlets. Its presence can compromise the accuracy of the heart tracing.
How AC Interference Appears on an ECG
AC interference creates a distinct, recognizable pattern superimposed directly onto the patient’s cardiac waveform. The most immediate sign is a thickening or fuzziness of the baseline, which represents electrical noise overwhelming the subtle heart signals. This noise presents as a series of fine, rapid, uniform spikes.
These regular deflections are generated at the frequency of the power supply in the environment (60 Hertz in North America, 50 Hz elsewhere). This high-frequency oscillation creates a pattern on the tracing that can resemble a continuous, rapid vibration. The high speed and regularity of the spikes distinguish AC interference from other types of electrical noise, such as muscle tremor, which causes a more jagged and irregular baseline. When severe, this interference can completely obscure the heart’s electrical activity, rendering the diagnostic recording unusable.
The Root Causes of Electrical Artifact
The appearance of AC artifact indicates that the sensitive ECG system is picking up electrical energy from the surrounding environment. This energy couples into the system through three primary mechanisms: external sources, grounding issues, and compromised electrode contact. The patient leads and cables function like small antennae, easily picking up radiated electromagnetic fields from nearby electrical devices.
External Electrical Sources
Many devices plugged into the wall outlet radiate an electromagnetic field. If these are too close to the patient or the ECG cables, they can induce a 60 Hz current into the lead wires. Common culprits include laptop power adapters, fluorescent lights, improperly shielded power lines, and other medical equipment like intravenous (IV) pumps. This field coupling is a form of induction, where the alternating current in the source creates a fluctuating magnetic field that induces a current in the ECG’s conductive wires.
Grounding and Shielding Issues
The grounding system provides a safe return path for stray electrical current, and its integrity is paramount for a clean tracing. If the ECG machine is not properly plugged into a grounded outlet, or if the internal shielding of the patient cables is damaged, the system may fail to reject common-mode interference. The right leg (RL) electrode serves as the reference or ground. A poor connection here significantly reduces the machine’s ability to filter out the unwanted AC signal.
Electrode and Lead Contact
The interface between the patient’s skin and the electrode is a frequent point of susceptibility for AC interference. A poor connection increases the electrical resistance, or impedance, between the patient and the recording equipment, making the system highly vulnerable to ambient electrical noise. This high impedance is often caused by inadequate skin preparation, such as not cleaning off oils or lotions, or by the conductive gel on the electrode drying out. Loose electrodes or fractured lead wires also increase this resistance, allowing the 60 Hz current to enter the system more easily than the subtle cardiac signal.
Simple Steps to Eliminate Interference
Resolving AC interference involves a systematic check of the patient, the equipment, and the environment. First, confirm that all electrode patches are firmly adhered to the skin, as loose contact is one of the most common causes of noise pickup. Ensure the skin was properly prepped by wiping the area to remove natural oils or creams, and check that the electrodes are not expired or dried out.
Next, examine the area surrounding the patient and the ECG machine to remove potential sources of electromagnetic radiation. Unplug or move any unnecessary electrical devices, such as cell phones, tablets, or even nearby medical pumps, at least six feet away from the patient and the lead wires. The ECG cable routing should also be checked to ensure it is not lying parallel to a power cord or touching any metal parts of the bed or examination table.
Finally, verify the integrity of the machine’s connection to the power supply. Confirm the ECG machine is plugged into a properly grounded electrical outlet. If the interference persists after these steps, it may indicate a damaged patient cable, which should be replaced, or a deeper issue with the room’s electrical wiring.