ECG Simplified – Part 3

ECG Simplified – Part 3

Recording a good ECG is the first step in getting a good diagnosis using it. If the recording is technically incorrect, interpretation can go wrong. First and foremost is to place the electrodes correctly in the recommended positions.

Avoiding interferences from nearby electrically operated devices is often a challenge while recording ECG in the intensive care setting. ECG monitoring leads attached to the patient may have to be temporarily removed to prevent artefacts due to alternating current picked up from these leads.

Power cords of electrically operated beds, pneumatic compression devices, infusion pumps and other electrically operated devices in close vicinity may also have to be removed from power sockets to reduce electromagnetic interference from AC line.

AC interference is seen as a symmetrical sinewave pattern in the baseline, at the frequency of the line voltage in the vicinity. It can be either 50 Hz or 60 Hz, depending on the line voltage frequency in the locality. In most modern ECG machines, notch filters are used to suppress AC interference. In spite of this, AC interference may still appear in the recorded tracings if the interference is strong. In this ECG, AC interference is best noted in leads I and III.

Equally important is avoiding interference from muscle activity – electromyogram or EMG artefacts. If patient is restless or anxious, a good explanation of the procedure and pacification often helps.

Warming the patient with a warmer or a blanket may be needed if shivering due to the cold atmosphere is noted. Sometimes we may have to switch off the air conditioner temporarily to make the person comfortable, especially in the intensive care unit or post-operative ward.

In this ECG, V1 shows multiple small artefacts almost totally obscuring the small QRS complexes. Close scrutiny with comparison with other leads enables recognition of QRS complexes within the artefacts by their timing with other QRS complexes simultaneously recorded in leads like V3, where the amplitude of the artefacts is much lower than that of the QRS complexes. This also illustrates the advantage of monitoring multiple leads simultaneously while observing for heart rhythm abnormalities.

This ECG shows artefacts due to tremor resembling a ventricular tachycardia, a fast rhythm from the lower chambers of the heart. The upper panel shows the artefacts resembling a wide QRS tachycardia and the lower panel shows the ECG with same leads when the tremor was not severe. The spikes of the QRS complexes marked by blue arrows can be seen at regular intervals even when the tremor artefacts are strong.

ECG machines have a high pass filter which passes frequencies above it and a low pass filter which passes frequencies below it to the ECG amplifier. Low pass filter is meant for filtering out high frequency interferences like muscle artefacts and high pass filter for fluctuation in baseline during breathing. Usually for reducing artefacts, default setting of filters is 0.08 – 40 Hz.

When a permanent pacemaker has been implanted, too low setting of low pass filter can make the pacing artefacts almost invisible. Low pass filter has to be kept above 100 Hz, typically 150 Hz, to make the pacing spikes evident, as shown here.

Pacemaker spikes which were not visible when the low pass filter was set at 40 Hz, are visible when low pass filter was set at 150 Hz. Pacemaker spikes are electrical signals picked up directly from the pacemaker on the body surface before it activates the heart muscle.