Key Factors to Consider in the Design and Operation of Electrocardiograph

There are various factors that must be considered in the design and application of the electrocardiograph. These key factors must be considered by the biomedical engineer as well as the electrocardiograph operator and the physician who interprets the recorded information.

We discuss these factors as follows:

Frequency distortion

The electrocardiograph does not always meet the frequency response standards. When this happens, the frequency distortion is observed in the ECG. The high frequency distortion rounds off the sharp corners of the waveforms and diminishes the amplitude of the QRS complex.

An instrument that has a frequency response of 1 to 150 Hz shows low-frequency distortion. The baseline is no longer horizontal especially immediately following an event in the tracing. Monophasic waves in the ECG appear to be biphasic.

Related: Electrocardiogram (ECG)

Saturated or cut off distortion

High offset voltages at the electrodes or improperly adjusted amplifiers in the electrocardiograph can produce saturation or cut off distortion that can greatly modify the appearance of the ECG. The combination of input-signal amplitude and offset voltage drives the amplifier into saturation during a portion of the QRS complex. The peaks of the QRS complex are cut off because the output of the amplifier cannot exceed the saturation voltage. In a similar occurrence, the lower portions of the ECG are cut off. This can result from negative saturation of the amplifier. In this instance, only a portion of the S wave may be cut off. In extreme cases of this type of distortion even the P and T waves may be below the cut off level such that only R wave appears.

Open lead wires

Often one of the wires connecting a biopotential electrode to the electrocardiograph becomes disconnected from its electrodes or breaks as a result of excessively rough handling, in which case the electrode is no longer connected to the electrocardiograph.

Relatively high potentials can often be induced in the open wires as a result of electric fields originating from the power lines or other sources in the vicinity of the machine. This causes a wide, peak-to-peak deflection of the trace on the recorder at the power line frequency and hence a signal loss. Such a condition also arises when an electrode is not making good contact with the patient.

Ground loops

Patients who are having their ECGs taken on either a clinical electrocardiograph or continuously on a cardiac monitor are often connected to other pieces of electric apparatus. Each electric device has its own ground connection either through the power line or in some cases through a heavy ground wire attached to a ground point somewhere in the room. A ground loop can exist when two machines are connected to the patient. Both the electrocardiograph and a second machine have a ground electrode attached to the patient. The electrocardiograph is grounded through the power line at a particular socket. The second machine or device is also grounded through the power line, but it is plugged into an entirely different outlet across the room, which has a different ground. If one ground is slightly at higher potential than the other ground, a current from one ground flows through the patient to the ground electrode of the electrocardiograph and along its lead wires to the other ground. Besides this current presenting a safety problem, it can elevate the patient’s body potential to some voltage above the lowest ground to which the instrumentation system is attached. This produces common-mode voltages on the electrocardiograph that, if it has a poor common mode rejection ratio (CMRR), can increase the amount of interference observed.

Interferences from electric devices

A key source of interference when one is recording or monitoring the ECG is the electric-power system. Besides, powering the electrocardiograph, power lines are connected to other pieces of equipment and appliances in the typical hospital room or office. There are also power lines in the walls, floor, and ceiling running past the room to other points in the building. These power lines can affect the recording of the ECG and introduce interference at the line frequency in the recorded trace. Such interference appears on the recordings as a result of two mechanisms each operating individually or in some instances, both operating both.

Electric field coupling between the power lines and the electrocardiograph and or the patient is a result of the electric fields surrounding main power lines and the power codes connecting different pieces of electric apparatus outlets. These fields can be present even when the apparatus is not turned on, since current is not necessary to establish the electric field.

The other source of interference from power lines is magnetic induction. Current in the power lines establishes a magnetic field in the vicinity of the lines. Magnetic fields can also sometimes emanate from transformers and ballasts in fluorescent lights or electric apparatus and appliances.

If such magnetic fields pass through the effective single-turn coil produced by the electrocardiograph, lead wires and the patient, a voltage is induced. This voltage is proportional to the magnetic field strength and the area of the effective single turn-coil.

The magnetic induction interference can be minimized by:

• Twisting the lead wires together and keeping them close to the body in order to subtend a much smaller area.
• Keeping the electrocardiograph and leads away from the potential magnetic field regions.

Related: 3 Sources of Noise in Biomedical Measurement Systems

Artifact from large electric transients

In some cases in which a patient is having an ECG taken, cardiac defibrillation maybe required. In such a circumstance, a high-voltage high current electric pulse is applied to the chest of the patient so that transient potentials can be observed across the electrodes. These potentials can be several orders of magnitude higher than the normal potentials encountered in the ECG. Other electric sources can cause similar transients. When this occurs, it can cause an abrupt deflection in the ECG due to saturation of the amplifiers in the electrocardiograph caused by the relatively high-amplitude pulse or step at its input.

This pulse is sufficiently large to cause the build-up of charge on coupling capacitances in the amplifier, resulting in its remaining saturated for a finite period of time following the pulse and then slowly drifting back to the original baseline with a time constant determined by the low corner frequency of the amplifier.

Related: Cardiac Defibrillators