Endocardial Electromechanical Mapping in a Porcine Acute Infarct and Reperfusion Model Evaluating the Extent of Myocardial Ische

Catheter-based, left ventricular, electromechanical mapping (EMM) has evolved as a diagnostic tool to characterize ischemic and injured myocardium, and has the potential for direct myocardial interventions.1–15 It is difficult to identify the myocardium at risk and irreversible infarcted areas in the setting of acute myocardial infarction (AMI). Mapping has the potential of improving accuracy since preserved electrical myocardial activity indicates viable myocardium and reduced mechanical activity either can represent infarcted or ischemic myocardium.6 However, the criteria for diagnosis by EMM, of myocardium affected by an evolving infarction, are not well defined. The aim this present study was to, in a longitudinal porcine model with experimental and well-defined AMI, investigate the capacity of separating myocardium with evolving necrosis from viable myocardium. Methods This investigation conformed to the Guide for the Care and Use of Laboratory Animals published by the USA’s National Institute of Health (NIH publication No. 85–23, revised in 1985). Approval from the local animal Ethics Committee was obtained. Experimental MI and reperfusion. Twenty-eight normally fed, female, Swedish landrace pigs with an average weight of 50 kg ± 4 kg were used. Pigs were pre-medicated with an intramuscular injection of midazolam (2 mg/kg), and ketamine (20 mg/kg), and anaesthetized by an IV bolus (3 ml/kg) of a-chloralose, followed by IV infusion of 1.5 ml/kg/h. They were ventilated with 40% oxygen. The pigs were heparinized with a bolus of 300 IE/kg IV, after which 100 IE/kg/hour was given throughout the experiment. After catheterization via the left external carotid, AMI was induced by occluding the left anterior descening coronary artery (LAD), distal to the second diagonal branch, with a balloon catheter for 45 minutes, in a closed-chest model. Reperfusion was verified by coronary angiography. After 4 hours of reperfusion, LAD was ligated at the same location and 40 ml of 2% Evans Blue (EB) was infused. Thereafter, the pigs were sacrificed by inducing ventricular fibrillation with potassium chloride and the hearts were excised. Morphological analysis. The left ventricles were sliced transversely into 10–12 mm sections and then weighted. The area at risk (AAR), identified as the area not stained by EB, was delineated. All slices were then stained with 2, 3, 5-triphenyltetrazolium chloride (TTC) to identify viable myocardium (Figure 1).16 Via planimetry, the AAR and the infarct sizes (IS) were measured and the relative weights of the AAR and the IS were calculated. To compare EMM with morphology, regarding the endocardial extension of infarcted myocardium, the non-TTC stained endocardial surface was measured. In the apical and basal parts the areas were approximated by the equation of the ellipse. Endocardial mapping. The mapping technology, NOGATM (Biosense Webster, Diamond Bar, California), has been documented in various studies.1,2,17 Endocardial three-dimensional (3D) EMM was performed at baseline, with a maximum mapping time of 1 hour. This was repeated 2 hours after reperfusion. The second time point was chosen so as to allow for stabilization after ischemia/reperfusion, and to be able to complete the catheterization in good time before sacrificing the pig. The points were deleted from the map if any of the following criteria, for good catheter-wall contact and stability, were not met: location stability (Electrical maps. At each selected endocardial point, unipolar and bipolar electrograms were recorded (filtered at 0.5–400 Hz and 30–400 Hz, respectively). Local amplitudes were obtained and presented as absolute values in mV, as reconstructed color-coded 3D maps (Figure 2A). Mechanical maps. An algorithm for calculation of global and regional mechanical function is provided by the system. Briefly, local linear shortening (LLS) is obtained by comparing the end-systolic and end-diastolic distances between neighboring endocardial points. The LLSs were presented as reconstructed color-coded 3D maps and absolute values were given in percentages (Figure 2C). Regional parameters. For regional data analysis in between different maps, a fixed polar cylindrical coordinate map was defined with the anatomic apex set as a reference. A long axis, connecting the apex with the "center of the mass" of all endocardial points, was automatically calculated by the system and then divided into 3 sections, consisting of 20, 30 and 40% of its total length. In all, nine segments were created (Figures 2B, 2D). The average value of each segment was calculated and used for comparison. Correlation between EMM and morphology. The most basal areas around the mitral and aortic valves were delineated and excluded from the EMMs, due to known low amplitudes in these regions. Each UPV level, starting at 0 mV and then adding 1 mV until the maximum UPV level was reached, was used as a cut-off level. Regardless of localization, the areas on the EMM, beneath each UPV cut-off level, were delineated and then automatically calculated by the system. The EMMs were then manually analyzed, at each cut-off level; in order to geographically localize areas of significantly decreased activity, see below. The morphological endocardial extension of the infarct was considered as the true extension of the infarct, against which the extension calculated from the EMMs was compared. For sensitivity and specificity calculations, the hearts were considered as in two parts, the infarcted and the non-infarcted region. The true infarcted area was set to the mean area (m) ± 2 standard deviation (SD) of all infarcts. Thus, the true positive infarct region was infarct areas calculated by EMM (IAEMM) of m ± 2 SD of the morphologically calculated infarct areas (IAMORPH). False negative was IAEMM m + 2 SD, if the EMM-defined infarct geographically originated inside the morphologically infarcted region, or > m – 2 SD IAMORPH, if the EMM-defined infarct originated outside the morphologically infarcted region. From the sensitivity and specificity calculations, a Receiver Operating Curve (ROC) was constructed to define the most optimal threshold for infarct detection and localization by means of the UPV EMM. Comparison of EMM after reperfusion with baseline EMM. Average UPV, BPV and LLS values for each of the 9 regional segments in the baseline and reperfusion EMMs were used. An unpaired analysis was performed for all baseline and reperfusion EMMs. In the pigs where both baseline and reperfusion EMMs were completed, a paired analysis was performed. Statistical analysis. Mean values from the regional nine segmental analyses were presented in box plots with 10th, 25th, 50th, 75th and 90th percentiles. A Mann-Whitney U test was used for the unpaired analysis and Wilcoxon signed rank tests for the paired analysis. Friedman’s test, followed by a two-tailed multiple test, was used to compare the activity in the different segments at baseline.18 Weights, areas and number of points are presented as means ± SD; p Morphological analysis. Two pigs with small infarcts, 1.6% and 6.5% respectively of the left ventricle, were excluded. Twelve hearts were available for both morphology and EMM. The average weight of these left ventricles was 127.8 ± 14.5 g (AAR 16.5 ± 3.9% and IS 14.8 ± 2.9%). The IS/AAR ratio was 91.3 ± 13%. The endocardial IS was 10 ± 2.7 cm2. Geographic location of the infarcts was congruent in the apical and anteroseptal midventricular regions. There was a good IS relation between calculated weights of the infarcted areas and the measured infarcted endocardial areas (Figure 3). Three-dimensional endocardial EMM. Baseline EMM was performed in 19 pigs and each was analyzable. Of all 28 pigs, two were excluded as described above, four were mapped only at baseline, and 3 died due to sustained VF. Another 8 pigs developed at least 1 episode of VF that was successfully treated with external defibrillation. Five pigs had VF in conjunction with the EMM procedure. Out of the 19 pigs that were mapped during reperfusion, five were excluded due to frequent supra- and ventricular arrhythmias, thus making the gathering of mapped points with acceptable quality impossible. One pig was excluded due to default values after defibrillation. Thus, 13 pigs had analyzable reperfusion EMMs. Paired data, with both baseline and reperfusion EMMs adhering to the quality criteria described above, were obtained in nine pigs. After online and post-process point filtering, the average number of mapped points was 57 ± 11 and 46 ± 12 at baseline and reperfusion, respectively. Two of 153 segments at baseline and 4/117 segments at reperfusion contained no points. At baseline and reperfusion each of the 9 segments had an average number of 6 ± 4 and 5 ± 3 points, respectively. Correlation between morphology and EMM. Macroscopic morphological infarct localization was congruent with the decreased UPV EMM activity. BPV was not used in this comparison since it did not show any significant decrease in the infarcted segments as compared to baseline. Figure 4 displays the sensitivity and specificity, in a ROC, for different thresholds of UPV EMM for detection and localization of the AMI with the endocardial extension evaluated by morphological analysis as the standard. Thus, a threshold between 6 and 7 mV seemed to be the most accurate. However, the accuracy was still poor. For 6 mV, specificity was 83% while the sensitivity was 50%. Corresponding values for 7 mV were 50 and 83%, respectively. Analyses of segmental activity before and after infarct induction. At baseline, the UPV activity in the lateral basal segment had a significantly lower activity as compared to all the midventricular and apical segments (Figure 5). Furthermore, all the basal segments, except for the septal, had significantly lower activity than the septal and posterior midventricular segments. BPV was significantly lower in the lateral basal segment as compared to the posterior basal segment (Figure 6). UPV, but not BPV, demonstrated a significantly decreased activity in the morphologically infarcted segments after ischemia and reperfusion, as compared to baseline. Also, the thresholds separating normal activity at baseline EMM from activities in infarcted regions differed between the different segments. From this analysis the threshold should be in the order of 9–11 mV for the UPV EMM. At baseline, LLS for the posterior basal segment was significantly lower in comparison with the midventricular septal segment (Figure 7). There was a significant impairment of LLS from baseline to reperfusion in the septal and posterior midventricular segments. Discussion The decreased segmental activity in the EMMs was congruent with the morphological analysis of the infarct location. However, the correlation between the amount of infarcted myocardium, evaluated by morphology and the extent of decreased EMM activity at different UPV levels, was poor. One source of error could be that remote areas with decreased activity, the basal parts excluded, could have influenced the comparison. For LLS, the midventricular posterior segment showed decreased activity not congruent with the morphological infarct location. This could be due to interference with the chordae, papillary muscle and the mitral apparatus. At baseline, a significant difference in activity between the segments was noted and at reperfusion the threshold for infarct detection seemed to vary between the segments, suggesting that a general cut-off value for different segments should not be used. In general, the basal segments had a lower activity in comparison with the other parts. Explanation for this could be that the basal parts of the heart have a lower electrical activity due to the valves. However, when the activities in the separate segments were compared from baseline to reperfusion significant differences between the activities could be noted and local cut-off levels could be suggested. LLS and UPV, but not BPV, showed a significant decrease in activity in the infarcted segments as compared to baseline. The fact that no significant difference occurred regarding BPV was unexpected since others have shown BPV to be more sensitive and accurate than UPV.3–5,19,20 Generally, bipolar electrograms (BPE) are considered to be more accurate in reflecting local changes in electrical activity than unipolar electrograms.20,21 BPE is less likely to be influenced by contact stability, electrode size or “far-field” electrical potentials. However, BPE is more sensitive to electrode tip orientation towards the endocardium.20–22 Figure 6 displays lower BPV in the apical and septal segments at baseline as well as after reperfusion as compared to remote segments. This suggests false low BPV at baseline in the later infarcted segments. This could possibly be overcome by a transseptal atrial approach or by another catheter curve. Conceivable explanation for the diverse finding in our study could be that we compared the same segments before and after reperfusion while others have often compared infarcted regions with remote regions or control subjects or divided the heart into more and smaller segments.3–5,19,22,23 Fewer and thereby lager segments could also have blurred any possible ischemic area. By using the pre-defined segments, an area of decreased activity could be localized in several segments and thereby not detected in the statistical analysis of the segments. The definition of the apical reference point is also crucial for the segmental analysis and for the in-between map comparisons. If the anatomical apex and the base are not well defined the calculated segments will not represent the true parts of the heart. As the EMM is a 3D reconstruction of the heart, different rotations of the heart will interfere with the analysis. This is an issue not easily solved. Others have created radiofrequency ablation marks in the tissue, macroscopically identified the marks and compared them with the EMM and the morphological analysis.4,5,19 Supra- and ventricular arrhythmias during the late EMM made the procedures and analysis more time-consuming and rendered fewer points post-filtering. In 5 pigs, VF was induced during the late EMM by the catheter, thus suggesting caution with EMM in acute myocardial ischemia. VF has been experienced in another acute experimental myocardial infarct study, a canine model, during reperfusion and the late EMM.23 The use of antiarrhythmic drugs could have stabilized the rhythm, but were not used. In the same study,23 they concluded that acute changes, i.e. ischemia and stunning, in UPV, BPV and LLS could be quantified and localized with EMM although no morphological analysis was performed. Hence, various amounts of necrotic myocardium could have been present. Pigs, as opposed to dogs, have poor collateral capacity and thus the ischemia will, with long enough occlusion, lead to infarct of most of the ischemic region. The time delay between the EMM and the morphological analysis may have influenced the results. Thus, it cannot be ruled out that at the time of EMM, the ischemic regions may not yet have been infarcted and there may have been a mix of infarcted and stunned myocardium. In the infarcted, midventricular septal, segment the mean reduction in UPV and LLS, after 2 hours, was approximately 30 and 50% respectively. In another infarction model, pigs undergoing acute LAD occlusion, the reduction in UPV, after 6–10 weeks was 47%.5 Longitudinal canine studies have shown a prompt and severe impairment in LLS with a 70, 64, 88 and 86% reduction after 45 minutes, 90 minutes, 24 hours and 3 weeks, respectively.3,23 Whereas UPV in the immediate acute phase has been relatively preserved with an 11- and 20% reduction after 45 and 90 minutes of ischemia.23 More distinct decreases in UPV were noted after 24 hours and 3 weeks (32 and 66%).3 Similar findings were done in a canine infarct model evaluating endocardial electrical features relevant to local ischemia. UPV correlated with mechanical impairment, assessed by echocardiography, after 3 days but not immediately or 5 hours after LAD occlusion.24 Hence, the gradual decrease in UPV explains some of the discrepancies in our study. A later point in time for the second EMM or an additional EMM would have been preferable. Due to difficulties in keeping the pigs anaesthetized for a long time after an AMI, we did not perform this. Conclusion During evolving MI, endocardial EMM may be unsuitable at this time for hemodynamic and arrhythmogenic reasons. Significant baseline and post-ischemic intersegmental variability in UPV, BPV and LLS activity, demonstrated by the poor correlation between infarct extension evaluated by EMM and morphology, indicate that general cut-off values should not be used. Further characterization of local activities for normal and ischemic myocardium is warranted. A model based upon normal material against which diseased hearts could be evaluated, could possibly deal with this dilemma.

1. Ben-Haim SA, Osadchy D, Schuster I, et al. Nonfluoroscopic, in vivo navigation and mapping technology. Nat Med 1996;2:1393–1395. 2. Gepstein L, Hayam G, Ben-Haim SA. A novel method for nonfluoroscopic catheter-based electroanatomical mapping of the heart. In vitro and in vivo accuracy results. Circulation 1997;95:1611–1622. 3. Kornowski R, Hong MK, Gepstein L, et al. Preliminary animal and clinical experiences using an electromechanical endocardial mapping procedure to distinguish infarcted from healthy myocardium. Circulation 1998;98:1116–1124. 4. Gepstein L, Goldin A, Lessick J, et al. Electromechanical characterization of chronic myocardial infarction in the canine coronary occlusion model. Circulation 1998;98:2055–2064. 5. Callans DJ, Ren JF, Michele J, et al. Electroanatomic left ventricular mapping in the porcine model of healed anterior myocardial infarction. Correlation with intracardiac echocardiography and pathological analysis. Circulation 1999;100:1744–1750. 6. Kornowski R, Hong MK, Leon MB. Comparison between left ventricular electromechanical mapping and radionuclide perfusion imaging for detection of myocardial viability. Circulation 1998;98:1837–1841. 7. Fuchs S, Hendel RC, Baim DS, et al. Comparison of endocardial electromechanical mapping with radionuclide perfusion imaging to assess myocardial viability and severity of myocardial ischemia in angina pectoris. Am J Cardiol 2001;87:874–880. 8. Gyongyosi M, Sochor H, Khorsand A, et al. Online myocardial viability assessment in the catheterization laboratory via NOGA electroanatomic mapping: Quantitative comparison with thallium-201 uptake. Circulation 2001;104:1005–1011. 9. Koch KC, vom Dahl J, Wenderdel M, et al. Myocardial viability assessment by endocardial electroanatomic mapping: Comparison with metabolic imaging and functional recovery after coronary revascularization. J Am Coll Cardiol 2001;38:91–98. 10. Kornowski R, Hong MK, Leon MB. Direct myocardial revascularization in ischemic heart disease. Int J Cardiovasc Intervent 1998:3–9. 11. Kornowski R, Bhargava B, Leon MB. Percutaneous transmyocardial laser revascularization: an overview. Cathet Cardiovasc Intervent 1999;47:354–359. 12. Kornowski R, Hong MK, Haudenschild CC, et al. Feasibility and safety of percutaneous laser revascularization using the Biosense system in porcine hearts. Coron Artery Dis 1998;9:535–540. 13. Kornowski R, Fuchs S, Tio FO, et al. Evaluation of the acute and chronic safety of the Biosense injection catheter system in porcine hearts. Cathet Cardiovasc Intervent 1999;48:447–453; 454–455. 14. Vale PR, Losordo DW, Tkebuchava T, et al. Catheter-based myocardial gene transfer utilizing nonfluoroscopic electromechanical left ventricular mapping. J Am Coll Cardiol 1999;34:246–254. 15. Kornowski R, Leon MB, Fuchs S, et al. Electromagnetic guidance for catheter-based transendocardial injection: A platform for intramyocardial angiogenesis therapy. Results in normal and ischemic porcine models. J Am Coll Cardiol 2000;35:1031–1039. 16. Fishbein MC, Meerbaum S, Rit J, et al. Early phase acute myocardial infarct size quantification: Validation of the triphenyl tetrazolium chloride tissue enzyme staining technique. Am Heart J 1981;101:593–600. 17. Gepstein L, Hayam G, Shpun S, et al. Hemodynamic evaluation of the heart with a nonfluoroscopic electromechanical mapping technique. Circulation 1997;96:3672–3680. 18. Siegel S, Castellan J. Nonparametric statistics for the behavioral sciences. 2nd edition. New York: McGraw-Hill; 1988. 19. Wolf T, Gepstein L, Dror U, et al. Detailed endocardial mapping accurately predicts the transmural extent of myocardial infarction. J Am Coll Cardiol 2001;37:1590–1597. 20. Kimber S, Downar E, Masse S, et al. A comparison of unipolar and bipolar electrodes during cardiac mapping studies. Pacing Clin Electrophysiol 1996;19:1196–1204. 21. Wit A, Janse M. Extracellular electrograms and mode of recording: unipolar, bipolar and composite electrograms. In: The Ventricular Arrhythmias of Ischemia and Infarction: Electrophysiological Mechanisms. Mount Kisco, NY: Futura Publishing. 1993:225–231. 22. Wolf T, Gepstein L, Hayam G, et al. Three-dimensional endocardial impedance mapping: A new approach for myocardial infarction assessment. Am J Physiol Heart Circ Physiol 2001;280:H179–H188. 23. Kornowski R, Hong MK, Shiran A, et al. Electromechanical characterization of acute experimental myocardial infarction. J Invas Cardiol 1999;11:329–336. 24. Schwartzman A, Wolf T, Gepstein L, et al. Characterization of acute myocardial ischemia in a canine model based on principal component analysis of unipolar endocardial electrograms. Med Biol Eng Comput 2001:571–578.