Timing of Local Activation

The major component of the unipolar electrogram allows determination of the local activation time, although there are exceptions. The point of maximum amplitude, the zero crossing, the point of maximum slope (maximum first derivative), and the minimum second derivative of the electrogram have been proposed as indicators of underlying myocardial activation (Fig. 5-1). The maximum negative slope (i.e., maximum change in potential, dV/dt) of the signal coincides best with the arrival of the depolarization wavefront directly beneath the electrode because the maximal negative dV/dt corresponds to the maximum sodium channel conductance. Using this fiducial point, errors in determining the local activation time as compared with intracellular recordings have typically been less than 1 millisecond. This is true for filtered and unfiltered unipolar electrograms.^1,2

FIGURE 5-1 Unipolar electrogram activation times. A lead II electrocardiogram and a unipolar electrogram (Egm) from the ventricle of a patient with Wolff-Parkinson-White syndrome are shown. The vertical dashed line denotes the onset of the delta wave; the horizontal dotted line is the baseline of the unipolar recording. Several candidates for the timing of unipolar activation are labeled with corresponding activation times relative to delta wave onset.

Direction of Local Activation

The morphology of the unfiltered unipolar recording indicates the direction of wavefront propagation. By convention, the mapping electrode that is in contact with the myocardium is connected to the positive input of the recording amplifier. In this configuration, positive deflections (R waves) are generated by propagation toward the recording electrode, and negative deflections (QS complexes) are generated by propagation away from the electrode (Figs. 5-2 and 5-3). If a recording electrode is at the source from which all wavefronts propagate (at the site of initial activation), depolarization will produce a wavefront that spreads away from the electrode, thus generating a monophasic QS complex. It is also important to recognize that a QS complex can be recorded when the mapping electrode is not in contact with the myocardium, but is floating in the cavity. In that situation, the initial negative slope of the recording is typically slow, suggesting that the electrogram is a far-field signal, generated by tissue some distance from the recording electrode.¹ Filtering at higher corner frequencies (e.g., 30 Hz) alters the morphology of the signal, so that the morphology of the unipolar electrogram is no longer an indication of the direction of wavefront propagation, and the presence or absence of a QS complex cannot be used to infer proximity to the site of earliest activation (Fig. 5-4).²

FIGURE 5-2 Hypothetical recordings from a multipolar electrode catheter. A, The electrodes are near a point source of activation (red dot in center of concentric rings). Note the timing and shape of the resultant electrogram patterns based on distance from point source, unipolar or bipolar recording, and width of bipole. B, The tip electrode (1) is at the point source of activation. Note the differences in timing and shape of the electrograms compared with A.

FIGURE 5-3 Unipolar and bipolar recordings from a patient with Wolff-Parkinson-White syndrome. The dashed line denotes the onset of the QRS complex (delta wave). Right panel, Recordings at the successful ablation site, characterized by QS in the unipolar recording. The most rapid component precedes the delta wave onset by 22 milliseconds, has the same timing as the peak of the ablation distal electrode recording, and precedes the ablation proximal electrode recording (arrows). Left panel, Recordings from a poorer site, with an rS in the unipolar recording; most of the bipolar recordings are atrial. Abl_dist = distal ablation; Abl_prox = proximal ablation; Abl_uni = unipolar ablation; HRA = high right atrium; RV = right ventricle.

FIGURE 5-4 Effect of filtering on intracardiac recordings. The signal labeled “Bipolar 30-500 Hz” is the same signal as the proximal His bundle signal (Hisprox) above it, displayed at lower gain. All signals beneath this are of the same gain but different filter bandwidths, and they illustrate progressive loss of signal amplitude as the bandwidth is narrowed. Unipolar signals below are of the same gain.

Advantages of Unipolar Recordings

One important value of unipolar recordings is that they provide a more precise measure of local activation. This is true for filtered and unfiltered unipolar electrograms. In addition, unfiltered unipolar recordings provide information about the direction of impulse propagation. Using the unipolar configuration also eliminates a possible anodal contribution to depolarization and allows pacing and recording at the same location. This generally facilitates the use of other mapping modalities, namely pace mapping.

Disadvantages of Unipolar Recordings

The major disadvantage of unipolar recordings is that they have poor signal-to-noise ratio and contain substantial far-field signal generated by depolarization of tissue remote from the recording electrode. Therefore, distant activity can be difficult to separate from local activity. This is especially true when recording from areas of prior myocardial infarction (MI), where the fractionated ventricular potentials are ubiquitous and it is often impossible to select a rapid negative dV/dt when the entire QS potential is slowly inscribed—that is, cavity potential.² Another disadvantage is the inability to record an undisturbed electrogram during or immediately after pacing. This is a significant disadvantage when entrainment mapping is to be performed during activation mapping, because recording of the return tachycardia complex on the pacing electrode immediately after cessation of pacing is required to interpret entrainment mapping results.¹

Timing of Local Activation

Algorithms for detecting local activation time from bipolar electrograms have been more problematic, partly because of generation of the bipolar electrogram by two spatially separated recording poles. In a homogeneous sheet of tissue, the initial peak of a filtered (30 to 300 Hz) bipolar signal, the absolute maximum electrogram amplitude, coincides with depolarization beneath the recording electrode, appears to correlate most consistently with local activation time, and corresponds to the maximal negative dV/dt of the unipolar recording (see Fig. 5-3).² However, in the case of complex multicomponent bipolar electrograms, such as those with marked fractionation and prolonged duration seen in regions with complex conduction patterns (e.g., in regions of slow conduction in macroreentrant atrial tachycardia [AT] or ventricular tachycardia [VT]), determination of local activation time becomes problematic, and the decision about which activation time is most appropriate needs to be made in the context of the particular rhythm being mapped. To acquire true local electrical activity, a bipolar electrogram with an interelectrode distance of less than 1 cm is desirable. Smaller interelectrode distances record increasingly local events (as opposed to far-field). Elimination of far-field noise is usually accomplished by filtering the intracardiac electrograms, typically at 30 to 500 Hz.^1,2

Direction of Local Activation

The morphology and amplitude of bipolar electrograms are influenced by the orientation of the bipolar recording axis to the direction of propagation of the activation wavefront. A wavefront that is propagating in the direction exactly perpendicular to the axis of the recording dipole produces no difference in potential between the electrodes and hence no signal.^1,2 However, the direction of wavefront propagation cannot be reliably inferred from the morphology of the bipolar signal, although a change in morphology can be a useful finding. For example, when recording from the lateral aspect of the cavotricuspid isthmus during pacing from the coronary sinus (CS), a reversal in the bipolar electrogram polarity from positive to negative at the ablation line indicates complete isthmus block. Similarly, if bipolar recordings are obtained with the same catheter orientation parallel to the atrioventricular (AV) annulus during retrograde bypass tract (BT) conduction, an RS configuration electrogram will be present on one side of the BT, where the wavefront is propagating from the distal electrode toward the proximal electrode, and a QR morphology electrogram will be present on the other side, where the wavefront is propagating from the proximal electrode toward the distal electrode (Fig. 5-5).

FIGURE 5-5 Recordings from a patient with a left lateral bypass tract during orthodromic atrioventricular reentrant tachycardia showing electrogram inversion at the middle of the coronary sinus (CS) (arrows), where the earliest retrograde atrial activation (over the bypass tract) occurs (dashed line). CS_dist = distal coronary sinus; CS_prox = proximal coronary sinus; His_dist = distal His bundle; His_mid = middle His bundle; His_prox = proximal His bundle; HRA = high right atrium; RV = right ventricle.

Advantages of Bipolar Recordings

Bipolar recordings provide an improved signal-to-noise ratio. In addition, high-frequency components are more accurately seen, which facilitates identification of local depolarization, especially in abnormal areas of infarction or scar.

Disadvantages of Bipolar Recordings

In contrast to unipolar signals, the direction of wavefront propagation cannot be reliably inferred from the morphology of the bipolar signal. Furthermore, bipolar recordings do not allow simultaneous pacing and recording from the same location. To pace and record simultaneously in bipolar fashion at endocardial sites as close together as possible, electrodes 1 and 3 of the mapping catheter are used for bipolar pacing, and electrodes 2 and 4 are used for recording. The precision of locating the source of a particular electrical signal depends on the distance between the recording electrodes, because the signal of interest can be beneath the distal or proximal electrode (or both) of the recording pair.¹

Prerequisites for Activation Mapping

Several factors are important for the success of activation mapping, including inducibility of tachycardia at the time of EP testing, hemodynamic stability of the tachycardia, and stable tachycardia morphology. In addition, determinations of an electrical reference point, of the mechanism of the tachycardia (focal versus macroreentrant), and, subsequently, of the goal of mapping are essential prerequisites.

Selection of the Electrical Reference Point

Local activation times must be relative to some external and consistent fiducial marker, such as the onset of the P wave or QRS complex on the surface ECG or a reference intracardiac electrode. For VT, the QRS complex onset should be assessed using all surface ECG leads to search for the lead with the earliest QRS onset. This lead should then be used for subsequent activation mapping. Similarly, the P wave during AT should be assessed using multiple ECG leads and choosing the one with the earliest P onset. However, determining the onset of the P wave can be impossible if the preceding T wave or QRS is superimposed. To facilitate visualization of the P wave, a ventricular extrastimulus (VES) or a train of ventricular pacing can be delivered to anticipate ventricular activation and repolarization and permit careful distinction of the P wave onset (Fig. 5-6). After determining P wave onset, a surrogate marker, such as a right atrial (RA) or CS electrogram indexed to the P wave onset, where it is clearly seen, can be used rather than the P wave onset.

FIGURE 5-6 Use of ventricular extrastimulus (VES) to clarify onset of the P wave during atrial tachycardia (AT). A single VES (S) delivered during AT advances the timing of ventricular activation to show the P wave (long arrow) by itself without the overlying ST segment and T wave, which made it difficult to determine P wave onset during ongoing tachycardia. The dashed line denotes onset of the P wave; timing of the reference electrogram (high right atrium [HRA]; short arrow) can thereafter be used as a surrogate for P wave onset. Abl_dist = distal ablation; Abl_prox = proximal ablation; Abl_uni = unipolar ablation; RV = right ventricle.

Defining the Goal of Mapping

Determination of the mechanism of the tachycardia (focal versus macroreentrant) is essential to define the goal of activation mapping. For focal tachycardias, activation mapping entails localizing the site of origin of the tachycardia focus. This is reflected by the earliest presystolic activity that precedes the onset of the P wave (during focal AT) or QRS (during focal VT) by an average of 10 to 40 milliseconds, because only this short amount of time is required after the focus discharges to activate enough myocardium and begin generating a P wave or QRS complex (Fig. 5-7). For mapping macroreentrant tachycardias, the goal of mapping is identification of the critical isthmus of the reentrant circuit, as indicated by finding the site with a continuous activity spanning diastole or with an isolated mid-diastolic potential (see Fig. 5-7).

FIGURE 5-7 Focal versus macroreentrant ventricular tachycardia (VT). Top, ECG and intracardiac electrograms from the mapping (Map) and right ventricle (RV) catheters. Bottom, Depictions of events at sites where mapped electrograms are obtained. Left, VT focus fires and activation spreads to normal myocardium within 30 to 40 milliseconds, generating a QRS complex. Thus, the electrogram at the site of the focus is generally 40 milliseconds or less prior to the QRS onset. Right, In contrast, in a macroreentrant VT, some myocardium is being activated at each instant in the cardiac cycle. During surface ECG diastole, only a few cells are activating (too few to cause surface ECG deflections). The area of a protected diastolic corridor, often cordoned off by scar, contains mid-diastolic recordings and is an attractive ablation site. The 0 isochrone indicates the time at which the QRS complex begins.

Epicardial Versus Endocardial Mapping

Activation mapping is predominantly performed endocardially. Occasionally, epicardial mapping is required because of an inability to ablate some VTs, ATs, or AV BTs by using the endocardial approach. Limited epicardial mapping can be performed with special recording catheters that can be steered in the branches of the CS. This technique has been used for mapping VTs and AV BTs, but its scope is limited by the anatomy of the coronary venous system.

Another epicardial mapping technique utilizes a subxiphoid percutaneous approach for accessing the epicardial surface. This technique has become an important adjunctive strategy to ablate a diverse range of cardiac arrhythmias including cardiomyopathic VT, BTs, atrial fibrillation, and idiopathic VTs. Transthoracic epicardial mapping and ablation are seeing increasingly wider application, especially in patients with scar-related VT, in whom more reentrant circuits with vulnerable isthmuses are on the epicardial surface. The same fundamental principles of activation mapping are used for both endocardial mapping and epicardial mapping.^3–7

Mapping Catheters

The simplest form of mapping is achieved by moving the mapping catheter sequentially to sample various points of interest on the endocardium to measure local activation. The precision of locating the source of a particular electrical signal depends on the distance between the recording electrodes on the mapping catheter. For ablation procedures, recordings between adjacent electrode pairs are commonly used (e.g., between electrodes 1 and 2, 2 and 3, and 3 and 4), with 1- to 5-mm interelectrode spacing. In some studies, wider bipolar recordings (e.g., between electrodes 1 and 3 and 2 and 4) have been used to provide an overlapping field of view. For bipolar recordings, the signal of interest can be beneath the distal or proximal electrode (or both) of the recording pair. As noted, this is germane in that ablation energy can be delivered only from the distal (tip) electrode.