Signals and Signal Processing for the Electrophysiologist

I n part II of this series, using the background covered in part I on electrogram acquisition and signal processing, we discuss potential errors in mapping because of artifact or inappropriate filtering of signals that may lead to unsuitable selection of ablation sites. Artifact Saturation ArtifactIf a 3-mV peak signal is being amplified at a gain of 5000, the final output will be 15 000 mV or 15 V; however, if the system supply voltage is limited to 10 V, the overall signal amplitude would be limited to about 9.5 V (slightly below the supply voltage).The signal would simply be cut off above 9.5 V, although the amplifiers would attempt to drive themselves up to the required 15 V output unsuccessfully.The amplifiers remain "saturated" during this time, and the top of the signal flattens out and remains flattened until the signal amplitude decreases to below 9.5 V, when it can be tracked with fidelity once again.This is a form of distortion and can only be avoided by reducing the gain of the system (in this numeric example, to less than Ϸ3000) (Figure 1).When amplifiers saturate, they can take several milliseconds to recover, further delaying the time to faithful reproduction of signals.This is exacerbated with extremely large signals, such as pacing signals, which are typically 3 orders of magnitude larger than the electrograms.In this case, the signal recovery time for the paced channel as well as adjacent electrodes can be long enough that the artifact envelops a substantial portion of electrogram information immediately following the pacing spike (Figure 2).This can be a troublesome problem when attempting to measure postpacing intervals after entraining a tachycardia.Despite this artifact, the operator must make every effort to use the paced electrode signals for this measurement, as using more distant electrodes may produce misleading results.Saturation from pacing creates an artifactual difficulty in measuring the postpacing interval on the mapping catheter.