Conventional intracardiac mapping techniques

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. 6.1 ). The maximum downslope (i.e., maximum change in potential, dV / dt ) of the unipolar electrogram coincides best with the arrival of the depolarization wavefront directly beneath the electrode and is now considered the most accurate marker of local tissue activation. 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.

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. 6.2 and 6.3 ). 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 important to recognize that a QS complex can be recorded when the mapping electrode is floating in the cavity and not in contact with the myocardium. 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 at some distance from the recording electrode.

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 indicative 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. 6.4 ).

Advantages of unipolar recordings

One important value of unipolar recordings is that they provide a more precise measure of the timing of local activation. In addition, unfiltered unipolar recordings provide information about the direction of impulse propagation. Furthermore, unipolar recordings allow pacing and recording at the same location while eliminating a possible anodal contribution to depolarization that is sometimes seen with bipolar pacing at high output. 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 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. Furthermore, a QS electrogram configuration has a low spatial resolution and relatively low specificity, being attainable in an area of more than 1 cm in diameter from the real site of origin of the arrhythmia or when the exploring electrode has poor contact with the myocardium.

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. There is also some baseline drift of the unipolar recording in some recording systems that makes interpretation difficult.

Timing of local activation

A bipolar electrogram is the sum of two unipolar electrograms with a time delay due to the interelectrode distance. When a bipolar electrogram with a small interelectrode distance is recorded in a homogeneous sheet of tissue, activation time delay between the two poles is usually minimal. In the setting of inhomogeneous conduction, however, a significant time delay can exist between two unipolar electrograms, resulting in a fractionated electrogram when converted to a bipolar electrogram ( Figs. 6.5 and 6.6 ).

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.

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 (30–300 Hz) bipolar signal coincides with depolarization beneath the recording electrode and appears to correlate most consistently with local activation time, corresponding to the maximal negative dV / dt of the unipolar recording ( Fig. 6.3 ). However, in the setting 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), determination of local activation time becomes challenging, and the decision about which activation time is most appropriate needs to be made in the context of the particular rhythm being mapped. Therefore, during mapping procedures, the onset (rather than the peak or nadir) of high-frequency components of a local bipolar electrogram is often used because it is easier to determine reproducibly, especially when measuring heavily fractionated, low-amplitude local electrograms. The onset of the bipolar electrogram likely precedes the maximal − dV / dt in unipolar electrogram by 15 to 30 milliseconds.

Direction of local activation

The morphology and amplitude of bipolar electrograms are influenced by many factors including (1) the orientation of the bipolar recording axis to the direction of propagation of the activation wavefront; (2) electrode size; (3) interelectrode distance; (4) electrode-tissue contact (i.e., the distance between the source of the potential and the recording electrode); (5) anisotropic conduction; and (6) intrinsic characteristics of the recorded medium (e.g., normal myocardial tissue versus scar). 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.

Although the direction of wavefront propagation cannot be reliably inferred from the morphology of the bipolar signal, a change in morphology can provide important clues about the activation pattern of the propagating wavefront. The change in polarity in bipolar electrograms recorded across an ablation line is consistent with complete line conduction block. 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 annulus during retrograde bypass tract conduction, an RS configuration electrogram will be present on one side of the bypass tract insertion site, 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 ( eFig. 6.1 ).

Furthermore, the “bipolar” electrogram can be considered as a “mini-unipolar” recording with the proximal electrode simulating the indifferent remote electrode in the conventional unipolar electrogram configuration, and thus the bipolar electrogram morphology may also be helpful in localizing the site of origin of focal arrhythmias. Hence, when the distal electrode of the mapping catheter is positioned at the site of origin of a focal tachycardia (e.g., PVCs), an early negative deflection in the initial component of the bipolar electrogram reflects the wavefront propagation opposite to the recording dipole vector (i.e., depolarization reaches electrode 1 before electrode 2). A recent report found that concordance of negativity in the initial forces of both unipolar and bipolar electrograms (in addition to the electrogram temporal relationship assessment) may be considered a reliable criterion to identify the site of origin of focal ventricular arrhythmias.

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 a single bipolar signal; however, with two adjacent bipolar recordings, the recording that occurs first is closer to the wavefront source. 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.

Technique of activation mapping of focal tachycardias

Initially, one should seek the general region of the origin of the tachycardia as indicated by the surface ECG. In the EP laboratory, additional data can be obtained by placing a limited number of catheters within the heart in addition to the mapping catheter or catheters; these catheters are frequently placed at the right ventricular (RV) apex, His bundle region, high RA, and CS. During initial arrhythmia evaluation, recording from this limited number of sites allows a rough estimation of the site of interest. Mapping simultaneously from as many sites as possible greatly enhances the precision, detail, and speed of identifying regions of interest.

Subsequently, a single mapping catheter is moved over the endocardium of the chamber of interest to sample bipolar signals. Using standard equipment, mapping a tachycardia requires recording and mapping performed at several sites, based on the ability of the investigator to recognize the mapping sites of interest from the morphology of the tachycardia on the surface ECG and baseline intracardiac recordings.

Local activation time is then determined from the filtered (30–300 Hz) bipolar signal recorded from the distal electrode pair on the mapping catheter; this time is determined and compared with the timing reference (fiducial point). Activation times are generally measured from the onset of the first rapid deflection of the bipolar electrogram to the onset of the tachycardia complex on the surface ECG or surrogate marker ( eFig. 6.2 ). Using the onset (rather than the peak or nadir) of the high-frequency component of a local bipolar electrogram is often preferable because it is easier to determine reproducibly, especially when measuring heavily fractionated, low-amplitude local electrograms.

Once an area of relatively early local activation is found, small movements of the catheter tip in the general target region are undertaken until the site is identified with the earliest possible local activation relative to the tachycardia complex. Recording from multiple bipolar pairs from a multipolar electrode catheter is helpful in that if the proximal pair has a more attractive electrogram than the distal, the catheter may be withdrawn slightly to achieve the same position as the distal electrode.

Once the site with the earliest bipolar signal is identified, the unipolar signal from the distal ablation electrode should be used to supplement bipolar mapping. The unfiltered (0.05–300 Hz) unipolar signal morphology should show a monophasic QS complex with a rapid negative deflection if the site was at the origin of impulse formation ( Figs. 6.9 and 6.10 ). However, the size of the area from which a QS complex can be larger than the tachycardia focus, exceeding 1 cm in diameter. Thus, a QS complex should not be the only mapping finding used to guide ablation. Successful ablation is unusual, however, at sites with an RS complex on the unipolar recording, because these are generally distant from the focus ( Fig. 6.2 ). Concordance of the timing of the onset of the bipolar electrogram with that of the filtered or unfiltered unipolar electrogram (with the rapid downslope of the S wave of the unipolar QS complex coinciding with the initial peak of the bipolar signal) helps ensure that the tip electrode, which is the ablation electrode, is responsible for the early component of the bipolar electrogram. Measuring a tissue contact force of 10 to 20 gm, as well as the presence of slight ST elevation on the unipolar recording and the ability to capture the site with unipolar pacing, is used to indicate good electrode-tissue contact.

Furthermore, a negative concordance of the vector of the initial forces (first 20 milliseconds) of both unipolar and bipolar electrograms (both being negative deflections), in conjunction with electrogram temporal relationship assessment, further improves the accuracy of conventional mapping to localize the site of origin of PVCs ( Fig. 6.11 ). In a recent report, the presence of this criterion (“negative concordance“) at sites fulfilling other conventional criteria used to guide focal premature ventricular complex (PVC) ablation highly predicted the acute success rate of RF ablation with a sensitivity and specificity of 94% and 95%, respectively. Furthermore, its positive predictive value was significantly superior to those of all other conventional criteria.

Continuous electrical activity

Theoretically, if reentry were the mechanism of the tachycardia, electrical activity should occur throughout the tachycardia cycle. For example, in macroreentrant AT, the recorded electrical activity at different locations in the atrium should span the TCL ( Fig. 6.12 ).

For macroreentrant VT, conduction during diastole is extremely slow and is in a small area so that it is not recorded on the surface ECG. The QRS complex is caused by propagation of the wavefront from the exit of the circuit from that isthmus to the surrounding myocardium. After leaving the exit of the isthmus, the circulating wavefront propagates through a broad path (loop) along the border of the scar, back to the entrance of the isthmus ( Fig. 6.7 ). Continuous diastolic activity is likely to be recorded only if the bipolar pair records from a small circuit; if a large circuit is present (i.e., the reentrant circuit is larger than the recording area of the catheter, or the catheter is not covering the entire circuit), nonholodiastolic activity will be recorded. In such circuits, repositioning of the catheter to other sites may allow visualization of what is termed bridging of diastole ; electrical activity in these adjacent sites spans diastole.

All areas from which diastolic activity is recorded are not necessarily part of the reentrant circuit. Such sites can reflect late activation of sites that may not be related to the tachycardia circuit. Analysis of the response of these electrograms to spontaneous or induced changes in TCL is critical in deciding their relationship to the tachycardia mechanism. Additionally, electrical signals that come and go throughout diastole should not be considered continuous ( eFig. 6.3 ). For continuous activity to be consistent with reentry, it must be demonstrated that such electrical activity is required for initiation and maintenance of the tachycardia, so that termination of the continuous activity, either spontaneously or following stimulation, without affecting the tachycardia, would exclude such continuous activity as requisite for sustaining the tachycardia. It is also important to verify that an electrogram that extends throughout diastole is not just a broad electrogram whose duration equals the TCL. This can be achieved by analyzing the local electrogram during pacing at a pacing cycle length (PCL) comparable to TCL; if pacing produces continuous diastolic activity in the absence of tachycardia, the continuous electrogram may have no mechanistic significance. Furthermore, the continuous activity should be recorded from a circumscribed area, and motion artifact should be excluded.

Middiastolic activity

An isolated middiastolic potential is defined as a low-amplitude, high-frequency diastolic potential separated from the preceding and subsequent electrograms by an isoelectric segment ( Fig. 6.14 ). Sometimes, these discrete potentials provide information that defines a diastolic pathway, which is believed to be generated from a narrow isthmus of conduction critical to the reentrant circuit. Localization of this pathway is critical for guiding catheter-based ablation.

Detailed mapping usually reveals more than one site of presystolic activity, and middiastolic potentials can be recorded from bystander sites attached to the isthmus. Therefore, regardless of where in diastole the presystolic electrogram occurs (early, middle, or late), its timing and appearance on initiation of the tachycardia, although necessary, does not confirm its relevance to the tachycardia mechanism. One must always confirm that the electrogram is required to maintain, and cannot be dissociated from, the tachycardia. Thus, during spontaneous changes in the TCL or those produced by programmed stimulation, the electrogram, regardless of its timing in diastole, should show a fixed relationship with the subsequent tachycardia complex (and not the preceding one). Very early diastolic potentials (in the first half of diastole) can represent an area of slow conduction at the entrance of a protected isthmus. These potentials remain fixed to the prior tachycardia complex (exit site from the isthmus), and a delay between this complex and the subsequent tachycardia complex would reflect delay in entering or propagating through the protected diastolic pathway. While such potentials that are related to the prior QRS complex can in fact be integral to the tachycardia (for example, if the variability in cycle length [CL] is due to varying delay in the diastolic corridor “downstream” from the recording site), this is not guaranteed as it is when the electrogram is tightly related to the subsequent QRS complex.

If, after very detailed mapping, the earliest recorded site is not at least 50 milliseconds presystolic, this suggests that the map is inadequate (most common), the mechanism of tachycardia is not macroreentry, or the diastolic corridor is deeper than the subendocardium (in the midmyocardium or subepicardium).