ECG settings – Practical importance

ECG settings – Practical importance

Recording paper speed is usually set to 25 millimeters per second. Higher speed recordings may be taken on demand at 50 millimeters per second. Much higher speed settings like 100 millimeters per second or 200 millimeters per second are usual with intracardiac ECG recordings during electrophysiology studies. This is for better delineation of intervals between intracardiac electrograms. Usual ECG recordings are calibrated with a 1 millivolt square wave pulse to yield one centimeter deflection on the Y-axis. Double standardization or double voltage recordings can be made to visualize P wave better in some situations. The interval between various ECG waves and their voltages can be calculated only if the settings are known. Notch filter is meant to neutralize the 50 cycle interference due to the line voltage. In regions where a 60 cycle alternating current is available, the notch filter will be set to 60 Hz. High pass filter allows signals with a frequency higher than the set value to pass through to the recorder and a low pass filter will allow signals below the set value. High pass filter filters out respiratory variations which are of low frequency while low pass filters remove high frequency artefacts like those due to electromyogram interference. Low pass filters may also remove pacemaker (pulse generator) stimulus artefacts. This is why digital ECG may sometimes be inferior to analogue ECG in detecting pacemaker artefacts. Now algorithms are available to specifically detect and mark pacemaker stimulus artefacts. Non optimal filter settings can affect the ECG signal depending on the frequency components of the signals. QRS complexes which have relatively higher frequency components are affected by low settings of low pass filters.