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Sloping ABR baselines and the ECG myogenic artefact
1a ERA Training & Consultancy Ltd , Wirral , UK.
Objective:
To investigate the possibility that a sloping baseline in an ABR recording has its origins in cardiac activity and if so, identify how it is expressed.
Design:
The effect of ECG removal on the averaged ABR was investigated at two artefact rejection levels.
Study Sample:
Ten 1-minute records of raw EEG containing ABR responses but contaminated with cardiac activity were recorded from babies under 12 weeks of age and re-averaged using two artefact rejection levels. The slope of the ABR recording was measured. The measurements were repeated after removing effectively the cardiac activity from the records.
Results:
A sloping baseline was observed at one or both artefact rejection levels in all records. The slope varied as the artefact rejection level was changed, suggesting this may be implicated in slope generation. The slope effectively disappeared when the cardiac activity was removed from the record.
Conclusions:
Cardiac activity has the potential to cause a sloping ABR baseline. A possible explanation for this effect is offered, together with suggestions for tester strategy when a sloping ABR baseline is seen in a clinical setting.