INTRODUCTION

The decision to gain epicardial access to eliminate ventricular tachycardia (VT) by means of catheter ablation is based on the suspicion of an epicardial site of origin of the ventricular arrhythmia and/or on the presence of an intramural and epicardial scar that incorporate VT reentrant circuits. A comprehensive analysis of the 12-lead VT-QRS complex will help to elucidate a probable epicardial VT origin (see Chapter 19 in this book). Preprocedural cardiac imaging, usually utilizing MRI techniques, can identify intramural and epicardial scar and helps determine the potential need for epicardial ablation. However, the intramural and epicardial scarring may be subtle, thinly layered, or microscopic, and in this setting, imaging may underestimate the amount of myocardial scarring.

Endocardial unipolar voltage mapping is a highly sensitive technique that helps to identify intramural/epicardial disease, even in the setting of layered scarring. This mapping technique has also the advantage over MRI and CT imaging of characterizing the epicardial VT substrate and helping to define potential epicardial ablation targets on an online basis.

In this chapter, we discuss the usefulness and limitations of endocardial unipolar mapping to identify distant mid-wall/epicardial scarring, in order to determine the need for epicardial mapping/ablation during endocardial mapping and help define potential epicardial ablation targets from the endocardium.

ENDOCARDIAL UNIPOLAR MAPPING TO IDENTIFY THE EPICARDIAL VT ANATOMIC SUBSTRATE

Endocardial Unipolar Mapping: Catheter Configuration and Other Technical Issues

The superiority of unipolar over bipolar endocardial mapping to identify distant myocardial disease derives from the wider field of view of the former technique. The reference electrode for unipolar recordings does not correspond to a proximal pole of the mapping catheter, but to an indifferent electrode, the Wilson Central Terminal, or at a distant intravascular structure (e.g., inferior vena cava).¹ Bipolar electrograms (EGM) are indeed constructed from two unipolar signals obtained from two electrodes separated by only several millimeters, and during the acquisition process, remote electrical activity is effectively substracted out from the final bipolar EGM. As a consequence, bipolar mapping is preferred over unipolar mapping to detect abnormality of the myocardial tissue contiguous to the mapping catheter (either from the endocardium or the epicardium), whereas endocardial unipolar mapping is preferred over endocardial bipolar mapping to identify epicardial scarring from the endocardium.^2,3

The use of multiple, closely spaced, small electrodes improves the spatial resolution of conventional bipolar mapping.^4,5 The value of small unipolar electrodes over conventional unipolar mapping using a standard ablation catheter in their ability to pick up distant myocardial disease is still a matter of debate; a lesser magnitude of the antenna of the small electrodes may be counterbalanced by the fact that the conventional 3.5- to 4-mm tip catheters increase the electrode’s footprint over the endocardium, thus increasing the relative contribution of the near-field to the unipolar EGM signal.^4,6,7

Other factors may also influence information obtained from unipolar recordings.^1,4,6,8–10 Different high-pass filtering settings (from ≤ 0.5–2 Hz or 30–100 Hz) affect the ability of unipolar recordings to pick up low-frequency signals, which are usually of a remote nature and thus can also influence the ability of endocardial unipolar mapping to identify distant myocardial disease.¹ The degree of electrode contact and orientation with respect to the contiguous endocardium are also important for accurate unipolar recording information.

Reference populations of patients with normal hearts served to define the normal endocardial unipolar peak-to-peak voltage for both the right (RV) and left (LV) ventricles.11,12 With this methodology, a statistical definition of normal endocardial unipolar voltages was defined as the amplitude value that included 95% of the recorded signals (Figure 16.1).³ These reference unipolar voltage cutoff values also apply to patients without macroscopic scarring as depicted by gadolinium enhancement on MRI, and can be used to identify microscopic diffuse interstitial fibrosis. A leftward shift in the distribution of LV endo UNIP voltages (using < 8.3 mV value as the reference value) helps to define irreversibility of the myocardial disease even in the absence of gadolinium enhancement on MRI.^14,15 The conventional voltage reference value of 8.3 mV has also been used to predict adverse LV remodeling and a poor clinical outcome in patients with both ischemic and nonischemic cardiomyopathies, and to guide the endomyocardial biopsy to detect abnormal RV or septal cardiomyopathic substrates.^16–19

Additional studies have revisited the proposed voltage cutoffs. These studies have in common their lower values for defining abnormal as compared with the conventional < 8.3 mV (LV), < 5.5 mV (RV free wall), < 7.5 mV (anterior RV septum), and < 6 mV (posterior RV septum) cutoff values.^11,12,20 These reports have suggested unipolar values of < 4 mV in Chagas-related cardiomyopathy,²¹ < 5.64 mV to < 7.5 mV in other nonischemic cardiomyopathies (NICM),^22–24 < 5.1 mV to < 6.52 mV in ischemic cardiomyopathies (ICM),^25–27 and < 3.3 mV to < 4.4 mV in RV cardiomyopathy^7,27 for identifying epicardial scar. The reason for this apparent discrepancy is twofold: In these additional reports, the statistical methodology used to define abnormal unipolar electrogram amplitude was mostly based on Receiver Operating Characteristics (ROC) curves that helped define the unipolar voltage that best matched the epicardial area of bipolar scar, and not on the comparison of the 95th percentile of normality with a reference population of patients with normal hearts. Second, the presence of adjacent endocardial scar or transmural scar in most of the above mentioned reports resulted in a significant decrease in the absolute unipolar voltage required to identify epicardial scar. Unequivocally, adjusting the voltage slider bar to a lower cutoff will improve the specificity of endocardial unipolar recording to identify the presence of an epicardial scar in the setting of abnormal endocardial bipolar scar or significant transmural abnormalities.^21,23–28

Figure 16.1 Definition of endocardial unipolar voltages of reference. Electroanatomic mapping in patients with no structural heart disease served to determine the “normal” endocardial unipolar voltage cutoff. The unipolar voltage amplitude that includes 95% of all unipolar electrograms in the normal heart equaled 5.5 mV for the right ventricle free wall and 8.3 mV for the left ventricle. In patients with modest or no endocardial bipolar scarring, such unipolar voltage cutoffs are useful to identify epicardial scarring, as evidenced by epicardial bipolar mapping. Adapted from Hutchinson et al.¹¹ and Polin et al.¹² Abbreviations: LV, left ventricle; NICM, nonischemic cardiomyopathy; RV, right ventricle; RVCM, right ventricular cardiomyopathy.

The use of less sensitive and lower reference unipolar voltages has the advantage of providing disease-specific values that best depict either the epicardial bipolar scar area or the penumbra areas of intramural/epicardial scarring surrounding an endocardial bipolar scar and densely transmural abnormality, as will be discussed below. Nevertheless, the use of such lower voltage cutoffs may also underestimate discrete (< 3 mm), thinly layered gross scarring recorded through normal or hypertrophied myocardium (Figure 16.2).^{15,19,23,29,30} In patients with myocardial scarring confined to the epicardial or midmyocardial layers (such as those encountered in the initial stages of Chagas disease or cardiac sarcoidosis), the more sensitive 8.3 mV cutoff helps to minimize the number of false-negative cases of supposedly normal endocardial unipolar voltages actually abutting epicardial scar, thus preventing the underestimation of intramural or epicardial scarring. This is especially critical when unipolar mapping is performed within endocardial areas with normal bipolar voltages, in which the endocardial unipolar voltages identifying epicardial scar tend to be higher (Figure 16.3).^19,21

Figure 16.2 Impact of different endocardial unipolar voltage cutoffs in the assessment of modest scarring. Endocardial recordings from patient with ischemic cardiomyopathy and large apical infarctions with septal endocardial extension. In this example, a confluent area of thin and layered subendocardial scarring bridging two dense endocardial scar areas at the left ventricular septum is suggested by endocardial unipolar mapping with the voltage threshold set at < 8.3 mV (upper right panel, solid circle) but the depth of the abnormality cannot be determined. When the slider bar is decreased to < 5.5 mV, the modest superficial endocardial scarring is not recognized (upper left panel, dashed circle). Endocardial bipolar mapping confirms bipolar scarring contiguous to the endocardial surface in this region (lower panel, solid circle). The bipolar electrogram with a more focused and limited field of view is best for confirming the location of the endocardial abnormality. The unipolar electrogram is superior for identifying abnormalities deeper to the endocardium. In the setting of coronary disease, the myocardium on the septum deeper to the endocardium is typically normal and will reflected by the absence of contiguous abnormalities on endocardial unipolar electrograms. Abbreviation: Endo = Endocardial.

Stay updated, free articles. Join our Telegram channel

Join