Left Ventricular Electromechanical Mapping: A Case Study of Functional Assessment in Coronary Intervention

Abstract

Electromechanical mapping is a new diagnostic tool that can be used to identify viable myocardium. In the case reported here, the technique was used before intervention to map areas of viable myocardium; post-intervention mapping showed improved mechanical function of the revascularized areas. Electromechanical mapping offers the potential of assessing left ventricular function in the cardiac catheterization laboratory before and after interventional procedures.

Electromechanical mapping (EMM) (Biosense-Webster; Diamond Bar, Calif) is a new diagnostic tool that is capable of identifying viable myocardium. ^1–3 It uses an ultra-low, magnetic-field energy source and a location sensor-tipped electrode catheter to accurately construct electrical and mechanical maps of the left ventricle (LV) in 3 dimensions. ^4–6

Left ventricular maps are constructed by processing and displaying multiple data points acquired through the mapping catheter when it is in stable contact with the endocardial surface. Each point consists of endocardial electrical data (unipolar [UP] and bipolar voltages) and point location information. Point location is analyzed by software that compares the average systolic and diastolic distance to neighboring points, thus obtaining a percentage of motion for each point (linear local shortening [LLS]).

Areas of myocardium that have preserved voltage but impaired mechanical activity (LLS) are interpreted as being viable. ¹ Therefore, EMM offers the potential of obtaining online functional assessment of the LV in the cardiac catheterization laboratory. ^7,8 In this report, we demonstrate the capability of EMM in a patient in whom pre-intervention electromechanical mapping suggested extensive areas of viable myocardium, and post-intervention mapping showed improvement in mechanical function of the revascularized areas.

Case Report

In January 2000, a 60-year-old man with known coronary artery disease presented with recurrent angina (Canadian Class III) and new-onset dyspnea following a non-Q-wave myocardial infarction 2 weeks before admission. His risk factors included hypertension, dyslipidemia, and tobacco use. In 1995, the patient had undergone coronary artery bypass graft surgery. A left internal mammary artery (LIMA) graft was placed to the left anterior descending artery (LAD), and saphenous vein grafts (SVG) were placed to the first oblique marginal artery (OM₁) and right coronary artery (RCA).

On the current admission, coronary angiography revealed a severe stenosis (90%) of a heavily calcified left main coronary artery (LMCA), which supplied a large first septal and a large branching diagonal artery (Fig. 1). The LAD was occluded at the diagonal and was supplied by a patent LIMA. The circumflex was occluded at the LMCA. The SVG to the RCA was stenosed to 50% in its proximal segment. The SVG to the OM₁ was widely patent. The left ventricular ejection fraction was severely depressed (20%), with global hypokinesis.

Fig. 1 Right anterior oblique view of left coronary angiogram showing left main stenosis proximal to large septal and diagonal distributions.

An electromechanical map (Biosense-Webster) of the LV was obtained, which suggested myocardial viability in the territory supplied by the LMCA. Rotational atherectomy of the LMCA with abciximab infusion was performed percutaneously with 1.75-mm and 2.0-mm burrs at 175,000 rpm (Boston Scientific Corp.; Maple Grove, Minn). We then deployed a 3.5-mm by 22-mm Crown stent (Cordis Corp.; Miami Lakes, Fla). An excellent angiographic result was obtained after dilation with a noncompliant Ranger balloon (Boston Scientific Corp.; Maple Grove, Minn), inflated to 18 atm (Fig. 2). An EMM was repeated after the interventional procedure. The patient remained clinically stable, and there were no complications.

Fig. 2 Right anterior oblique view of left coronary angiogram after rotablation and stenting of the left main stenosis.

Discussion

The effect of coronary angioplasty on left ventricular function has been previously described. ^9–10 Transient worsening of LV function occurs during balloon inflation. ¹¹ In patients with stable coronary artery disease, successful revascularization reverts both exercise-induced ventricular dysfunction ¹² and resting ischemic systolic dysfunction. ¹³ The impact of coronary intervention on the distal microvascular bed has been demonstrated by devices that assess coronary flow physiology in the catheterization laboratory. ^14,15 These devices, however, do not have the capability of assessing mechanical function of the myocardium. Through detailed segmental evaluation, EMM harbors the potential to integrate mechanical and perfusion data.

In this particular case, EMM was performed before and after intervention. The electrical and mechanical data from these maps were represented in a cylindrical polar reference coordinate map divided into 12 “bull's-eye” segments. Areas of myocardium with preserved voltage (greater than 7.5 mV) ¹ together with severely compromised LLS (less than 6%) ^7,8 are defined as “discordant” and represent segments that are either profoundly ischemic or hibernating. In the first map (Fig. 3), 8 of 12 segments were discordant. After revascularization (Fig. 4), 6 of the 8 previously discordant segments showed increased LLS values, thus revealing an immediate improvement in mechanical function (Fig. 5). Two areas still had discordant values.

Fig. 3 Electromechanical maps before intervention. Antero-posterior view of the unipolar (UP) voltage map (upper left) and linear local shortening (LLS) map (upper right). The respective bull's-eye segmental distributions with UP voltage and LLS values are shown below each map.

Fig. 4 Electromechanical maps after intervention. Antero-posterior view of unipolar (UP) voltage map (upper left) and linear local shortening (LLS) map (upper right). The respective bull's-eye segmental distributions with UP voltage and LLS values are shown below each map. Marked changes in the LLS map and bull's-eye values may be noted.

Fig. 5 Bull's-eye representation of discordance before and after intervention. Grid marks represent normal segments, and black areas represent discordant segments. The white areas in the right bull's-eye represent the segments that were initially discordant but returned to normal after revascularization.

Electromechanical mapping can be used to distinguish between normal and infarcted myocardium. ^16,17 As previously described, areas of infarcted myocardium ¹⁸ have lower LLS values and UP voltage potentials below 7.5 mV. ¹ The combined analysis of UP voltage and LLS values gives this technique the unique potential to integrate resting myocardial viability and mechanical function (a surrogate for adequate perfusion).

The capacity to predict the recovery of LV function after revascularization remains an important issue. Several diagnostic tests have tried in different ways to accurately identify hibernating myocardium. ¹⁹ Electromechanical mapping can provide valuable information to help distinguish those who could benefit the most from revascularization. ^20,21 This is especially true in patients, such as our patient, who exhibit severely depressed LV function, wherein the benefits of an interventional procedure must be clearly defined. Interestingly, in this patient the response to restitution of blood flow is reflected by improvement in mechanical function in the corresponding segments. The improvement is immediate; therefore, this technique may also reveal new insights into the evaluation of results of intervention and into the nature of temporal changes occurring in the myocardium after revascularization.

Analysis of the evaluation of LV function through EMM requires further investigation. As shown in this report, it can be useful in identifying specific targets for intervention and in demonstrating the results of revascularization. In the near future, this mode of cardiac evaluation may play an important role in the functional assessment of the left ventricle in the catheterization laboratory and may contribute to the decision-making process in regard to intervention.