Question

Power line interference affects the ECG signal recording and one way to minimize this interference is by

• A. Shielding and grounding each lead at the ECG machine
• B. Raising the skin-electrode impedances
• C. Reducing the magnetic field strength in the vicinity of the ECG machine
• D. Shielding the ECG machine

Hint:

Power line noise couples into ECG circuits via capacitive and inductive loops. While standard cable shielding blocks electric fields, reducing the surrounding magnetic field strength is necessary to minimize inductive coupling noise.

Answer 1

The Correct Option is C

Concept: Power line interference (50/60 Hz noise) is a common source of noise in ECG signal recordings. This interference couples into the patient and the measurement leads through two primary mechanisms: electric field coupling (capacitive coupling) and magnetic field coupling (inductive coupling).

Step 1: Analyzing magnetic field coupling mechanisms.
Stray magnetic fields are generated by alternating current flowing through nearby power cords, isolation transformers, and building wiring. When these magnetic flux lines pass through the loop area formed by the patient leads and the body, they induce an unwanted 50/60 Hz noise voltage directly into the signal path according to Faraday's law of induction: \[ V_{\text{noise}} = -\frac{d\Phi_B}{dt} = -A \cdot \frac{dB}{dt} \] Where $A$ is the loop area and $B$ is the magnetic flux density.

Step 2: Evaluating methods to minimize interference.

• Standard electrostatic shielding (such as copper braids around cables) blocks electric fields but cannot stop magnetic flux lines.
• To minimize inductively coupled noise, you must either decrease the loop area (by twisting the lead wires together) or reduce the magnetic field strength in the vicinity of the ECG machine. This can be achieved by relocating power cords, moving power-hungry appliances away from the patient bed, or using high-permeability magnetic shielding materials. This confirms Option (C) as an effective mitigation strategy.