The Assumptions of Isochronal Cardiac Mapping

Abstract

Isochronal maps of cardiac activation are commonly used to study the mechanisms and to guide the ablative therapies of arrhythmias. Little has been written about the assumptions implicit in the construction and use of isochronal cardiac maps. These assumptions include the following: (1) the location of the recording electrodes is known with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy; (2) a single, discrete activation time can be assigned to each recording electrode location; (3) the presence or absence of activation at an electrode site can be reliable ascertained, and when activation is present, the time of activation can be determined with sufficient accuracy to specify the mechanism of an arrhythmia or to guide therapy; and (4) the recording electrodes are close enough together that the activation sequence can be estimated with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy. The manuscript reviews evidence that these assumptions may not always be true, and when they are not, the isochronal map may be misleading.

Optical Imaging of the Heart

“…The generally used methods to map activation and repolarization are based on surface unipolar and bipolar electrograms measured with arrays of electrodes. 29,30 Although surface electrodes can describe the spread of excitation and repolarization, interpretation of data in some cases is uncertain. 29,31 For instance, activation sequences were difficult to interpret during rapid synchronous depolarization, as after electric shock application, and during slowly changing, low-level depolarization, as in ischemia.…”

Signals and Signal Processing for the Electrophysiologist

“…Reducing the distance between the 2 electrodes of the bipole can further reduce the sensitivity to distant activation events. 10 The amplitude of these signals in healthy ventricular myocardium (measured with a 10-mm electrode spacing) ranges from 3 mV to 10 mV with durations of less than 70 m. 11 These amplitudes are reduced substantially with smaller spacing. Also, a lot of the far-field information present in a unipolar signal is removed from the bipolar signal because each electrode of the bipolar pair "sees" a similar (or identical) far-field voltage, and these similar signals are canceled out by the instrumentation amplifier that subtracts 1 electrode's voltage from the other electrode's voltage.…”

“…In the extreme case, this could be a disadvantage since a wave of depolarization that is perpendicular to the 2 electrodes would be canceled. 10 Conversely, a wave of depolarization that is parallel to the 2 electrodes would be picked up sequentially by both, resulting in maximum amplitude. The time of peak amplitude in a bipolar signal is the equivalent of the fastest negative downslope of the unipolar signal.…”

Organization of Myocardial Activation During Ventricular Fibrillation After Myocardial Infarction

“…18,26,27 Determining the time of local activation during ventricular fibrillation with unipolar electrograms may be problematic because electrotonic interactions between the recording area and adjacent myocardium may confound the usual relationship between transmembrane and extracellular voltage changes. 28,29 Furthermore, the relationship between these electrotonic influences and local activation is complex because of the nonconcordance of activations in regions located very close. As a result, the signals were more likely to be influenced by activation of neighboring myocardium.…”