(Chest. 2004;125:1830-1836.)
© 2004
American College of Chest Physicians
Use of Novel Nonfluoroscopic Three-Dimensional Electroanatomic Mapping System To Monitor and Analyze Heart Surgery in Animal Models*
Gil Bolotin, MD, PhD;
Frederik H. van der Veen, PhD;
Tamir Wolf, PhD;
Rona Shofti, DVM;
Roberto Lorusso, MD, PhD;
Shlomo A. Ben-Haim, MD, PhD and
Gideon Uretzky, MD
* From the Department of Cardiothoracic Surgery (Drs. Bolotin, Wolf, and Uretzky), Tel Aviv Sourasky Medical Center, Tel Aviv, Israel; the Rappaport Institute of Research in the Medical Sciences (Drs. Shofti and Ben-Haim), Department of Biomedical Engineering, Technion-Israel Institute of Technology, Haifa, Israel; and the Department of Cardiothoracic Surgery (Drs. Lorusso and van der Veen), Academic Hospital Maastricht, the Netherlands.
Correspondence to: Gil Bolotin, MD, PhD, Section of Cardiac and Thoracic Surgery, University of Chicago, Room E500, MC5040, 5841 S. Maryland Ave, Chicago, IL 60637-1470; e-mail: gbolotin{at}surgery.bsd.uchicago.edu
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Abstract
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Background: The new method of three-dimensional (3D) electroanatomic mapping was presented as an important tool for cardiac imaging and intervention. We present herein the first use of this technology for the monitoring, analysis, and development of cardiac surgery at the preclinical stage.
Methods: The method is based on utilizing a locatable catheter connected to an endocardial mapping and navigating system, to accurately establish the location and orientation of the tip of the mapping catheter and simultaneously record its local electrogram. The 3D geometry of the beating cardiac chamber is reconstructed in real time. The system was tested on six goats that underwent dynamic cardiomyoplasty. Two maps of each animal were performed: preoperative and postoperative during the stimulation protocol of the skeletal muscle.
Results: The electroanatomic mapping system provided detailed maps of the left ventricle during the stimulation protocol, which demonstrated a striking geometric difference between the assisted and the unassisted beats. These geometric changes are best described by referring to left ventricular long-axis movements (22.3 ± 3.8° vs 3.4 ± 1.6°, p < 0.001), center-of-mass movements (10.4 ± 3.0 mm vs 3.9 ± 1.6 mm, p < 0.005), and the changes in upward movement viewed along the base (7.9 ± 1.9 mm vs 3.6 ± 1.7 mm, p < 0.01), middle (13.8 ± 4.0 mm vs 7.3 ± 1.8 mm, p < 0.005), and the apex of the heart (28.1 ± 4.5 vs 5.3 ± 2.3 mm, p < 0.001) [mean ± SD].
Conclusions: The 3D electroanatomic mapping system allows detailed reconstruction of the left ventricular geometry and a clear view of the difference between the assisted and the unassisted beats. This novel monitoring system may serve as an important tool for the analysis and development of new techniques in cardiac surgery.
Key Words: heart failure surgery • hemodynamics • imaging • mapping
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Introduction
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The novel nonfluoroscopic, three-dimensional (3D), electroanatomic mapping system has been described for the evaluation of left ventricular function.123 The technique is capable of reconstructing the 3D beating-heart chamber with a superimposed color-coded map of the endocardial ECG, based on information gathered from sensors at the catheter tip. The electroanatomic mapping system was proven to be highly accurate and reproducible both in vitro and in vivo.23 This mapping system was successfully compared to several other imaging modalities, such as echocardiography, radionuclide perfusion imaging, and computerized left ventricular angiography, both in human and in animal models.4567
The system is currently used clinically for several cardiologic imaging and intervention procedures, including hemodynamic evaluation, distinction between infarcted and healthy myocardial tissue, ablation, and direct myocardial revascularization.3891011 The importance of imaging and monitoring techniques for the analysis of preclinical cardiac surgery cannot be overestimated. We hypothesize that implementation of this new technique at the preclinical evaluation phase may reveal new information about the 3D geometrical and hemodynamic effects of cardiac surgery. The cardiomyoplasty procedure was chosen as our model since it has already been subjected to broad preclinical and clinical evaluations using a wide range of monitoring and imaging modalities; therefore, we chose to assess the significance of this novel technique by applying it for further preclinical evaluation of this procedure. We present herein the first use of this technology for the analysis of cardiac surgery.
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Materials and Methods
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Basic Concepts of the Electroanatomic Mapping Technique
The system is based on the physics principle that a metal coil placed in a magnetic field can generate a current. The size of the current is proportional to the strength of the magnetic field, the location of the coil, and the orientation of the coil in the field.1 The electroanatomic mapping technique was proven to be highly accurate and reproducible both in vitro and in vivo.23
Mapping System Components
The mapping and navigator system (Biosense, J&J; Haifa, Israel) uses an ultra-low electromagnetic field generated from a triangular location pad. A 7F deflectable-tip catheter, containing a miniature location sensor, provides real-time 3D electrical and anatomic maps of the endocardial surface. The system is comprised of five basic components: (1) a triangular location pad with three coils generating an ultra-low magnetic field (5 x 106 to 5 x 105 T); (2) a stationary reference catheter with a miniature magnetic field sensor, located either in the right heart or externally on the body surface; (3) a 7F navigation mapping catheter, with a deflectable tip and electrodes that provide unipolar or bipolar endocardial signals when inserted into the left ventricle; (4) a miniature passive location sensor within the mapping catheter; and (5) the NOGA workstation (Biosense, J&J; Haifa, Israel) processing the information obtained from the mapping catheter and constructing the 3D left ventricular geometry.
The electromagnetic field generated by the system contains the information necessary to resolve the location and orientation of the sensor in six degrees of freedom. The sensor measures the magnetic field to determine the distance from each of its sources, and to enable the determination of the location and the orientation of the sensor in the space.
Mapping Procedure
The mapping catheter is introduced through a femoral or carotid sheath and placed in the left ventricle. The reference catheter is placed in the right ventricle. The location of the mapping catheter, relative to the reference catheter, is recorded continuously; thus, the general movements of the animal as well as breathing movements are corrected automatically and are not presented in the final map of the beating left ventricle. The tip of the mapping catheter is subsequently moved to multiple left ventricular endocardial sites, and the NOGA processor uses a triangular algorithm to reconstruct the left ventricular anatomy. Local activation time and unipolar or bipolar intracardiac electrical recordings are acquired simultaneously. The 3D reconstruction, using a triangular algorithm that connects the mapped points, results in the beating endocardial anatomic map with the local activation time color coding superimposed on it. The number of endocardial points needed to be mapped depends on the animal model, the accuracy required, and the experience of the examiner. In our study the mean number of points for each map was 40 ± 5 (± SD), and the duration of each mapping procedure was 20 to 30 min.
Software Developed for the Current Study
The need to reconstruct maps that include a beat before and after the assisted beat was solved with the elongation of the sampling time to 3 s and the tagging of the system to the stimulator of the skeletal muscle. Since the stimulator was also tagged to the spontaneous heartbeats, the result was a synchronous beating 3D reconstruction of at least three heartbeats, including the preassisted and postassisted beats. For the investigation of the different areas of the left ventricle participating in the hemodynamic work, three inner cross-sections were plotted on the left ventricular endocardium: at the apex, mid, and base of the heart (Table 1
). The three cross-sections were plotted at 25%, 50%, and 75% of the left ventricular long axis, respectively (Fig 1
). The area of these three cross-sections was recorded continuously through a normal cardiac cycle as well as during the assisted beats.
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Table 1. Cross-Section Areas of Base, Mid, and Apex of the Left Ventricle Systole and Diastole of Preassisted, Assisted, and Postassisted Beats*
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Figure 1. The three cross-sections, plotted at 25%, 50%, and 75% of the long axis of the left ventricle. The area of these three cross-sections was recorded continuously through a normal cardiac cycle as well as during the assisted beats.
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Surgical Procedure
Six goats underwent dynamic cardiomyoplasty. General anesthesia was induced by IV sodium thiopental, 15 mg/kg, and maintained after endotracheal intubation with O2:NO (1:2) and 1.5% fluothane. Throughout the experiments, lung ventilation was achieved using a positive-pressure respirator (Harvard Apparatus; South Natick, MA). Body temperature was kept constant using a heating mattress. Before surgery, a single dose of 5,000 U of IV heparin was administered.
A left-sided midaxillary incision was performed above the latissimus dorsi (LD) muscle, and all collateral blood vessels to the distal part of the muscle were coagulated. All attachments of the muscle, except for the axillary pedicle, were disconnected to keep the thoracodorsal artery, vein, and nerve intact. Two IM electrodes (Medtronic SP 5590 stimulation leads; Medtronic; Kerkrade, the Netherlands) were implanted in the upper part of the LD muscle flap, perpendicular to the main branches of the thoracodorsal nerve, as described previously by Chachques and coworkers.12 To ensure proper positioning of the electrodes, satisfactory threshold (0.3 to 0.6 V) and total recruitment (1.0 to 2.5 V) values were obtained after connection of the electrodes to the stimulator system (Medtronic Cardio-Myo Stimulator SP 3076; Medtronic). A 5-cm segment of the anterior portion of the second rib, including the periosteum, was then resected to allow transposition of the LD muscle flap into the thorax. The muscle was inserted into the chest cavity, its tendon cut and sutured to the periosteum of the third rib, before closing the thoracic window. The thorax was then opened at the fourth left intercostal space, and the pericardium was cut open. A sensing electrode (Medtronic 6500 sensing lead; Medtronic) was implanted in the right ventricular wall (adjacent to the septum), and the sensing threshold (4.5 to 16.4 mV) was measured. The left LD muscle flap was wrapped in a counterclockwise fashion around both ventricles. The muscle was first positioned around the right ventricle and fixed with interrupted sutures near the atrioventricular groove at the base of the heart. Subsequently, the remaining part of the muscle was wrapped around the left ventricle. The distal portion was sutured to the proximal part of the muscle (Fig 2
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Mapping and Statistical Analysis
The mapping catheter was introduced into the left ventricle at the beginning of the experiment and remained inside throughout the entire procedure. Two maps were obtained from each animal: one preoperatively and another postoperatively, after the chest was closed, during the stimulation protocol of the skeletal muscle, approximately 45 min after accomplishing the wrapping. In case of premature ventricular beat during point recording, the point was rerecorded in sinus rhythm. The cardiomyoplasty-assisted beats were compared to the previous unassisted beats, using Student paired t test. Results are expressed as mean ± SD. Differences were accepted to be significant at a p value < 0.05.
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Results
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The 3D reconstructions of the beating left ventricle before and after the operation were accurate, as were the detailed maps obtained during the stimulation protocol. A significant geometric difference was observed between the assisted and the unassisted beats. There were no differences between the superimposed color coding of the endocardial activation propagation mapped either at the prewrapping or postwrapping stages or during stimulation of the skeletal muscle wrapped around the heart.
Comparison of Assisted Beats and Unassisted Beats
The striking difference between the assisted and unassisted beats is best understood when studying the 3D left ventricular beating maps reconstructed in all the experiments. A two-dimensional reconstruction of these changes is shown in Figure 3
, while the same map was superimposed twice at different points in the cardiac cycle: at normal systole and during the assisted systole thereafter.
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Figure 3. A left anterior oblique 3D reconstruction of the left ventricle of the goat, superimposed at two time points: unassisted systole in grid reconstruction and assisted systole in full-color reconstruction.
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The left ventricular long axis was found to move 3.4 ± 1.6° during normal systole and 22.3 ± 3.8° during the assisted beat systole (p < 0.001). The center of mass, which was found to move 3.9 ± 1.6 mm between normal diastole and systole, moved a mean of 10.4 ± 3.0 mm between the assisted systole and the previous diastole (p < 0.005).
To study the regional effects of the assisted left ventricle, different areas in the heart were evaluated by means of mean movement direction of points in the apex, mid, and base of the heart. There was a difference in the number of point movements during assisted systole as compared to unassisted systole in the three regions: base (7.9 ± 1.9 mm vs 3.6 ± 1.7 mm, p < 0.01), middle (13.8 ± 4.0 mm vs 7.3 ± 1.8 mm, p < 0.005), and apex of the heart (28.1 ± 4.5 mm vs 5.3 ± 2.3 mm, p < 0.001). The other difference between the regions was in the direction of the movements of the point during the assisted beats: the apex moved upward 71 ± 7.5° and 
– 30 ± 13°, the mid part of the heart moved upward 64 ± 16° and – 14 ± 25°, and the base moved upward 11 ± 21° and – 32 ± 26°. Although the SD was found to be relatively high, the difference in point movement direction was statistically significant between the three areas of the heart (p < 0.001). These relatively complex regional changes can be observed in Figure 4
, where the direction of each point movement is indicated. The ejection fraction of the assisted beats was statistically higher (39 ± 7% vs 28 ± 7% in the unassisted beats, p < 0.01), and the main change was found to be in the left ventricular end-systolic volumes (40 ± 6 mL vs 46 ± 8 mL in the unassisted beats, p < 0.05), while no significant difference was found in the left ventricular end-diastolic volumes (66 ± 9 mL vs 64 ± 10 mL in the unassisted beats, p = 0.11). To evaluate the relative contribution of the different heart segments during the assisted systole, the area of three inner cross-sections around the apex, mid, and base of the heart was studied during normal and assisted systole. It was found that only the mid part of the heart contributes to the higher ejection fraction in the assisted beats (cross-section area of 412 ± 123 mm2 vs 471 ± 129 mm2 in the previous unassisted beats, p = 0.0004). An example of changes in the cross-section area at the mid part of the heart is presented in Figure 5
. While the cross-section area of the base did not change, the cross-section area of the apex was paradoxically higher in the assisted beats (276 ± 104 mm2 vs 249 ± 85 mm2 in the previous unassisted beats, p = 0.047). In contrast, the postassisted diastole cross-section area was not significantly changed in the mid part of the heart, while end-diastolic cross-section areas were found to be high in the postassisted diastole, both in the apex (cross-section area of 439 ± 87 mm2 vs 389 ± 113 mm2 in the preassisted diastole, p = 0.0053), and base (cross-section area of 1,026 ± 232 mm2 vs 991 ± 214 mm2 in the previous unassisted beats, p = 0.017).
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Figure 4. A map of local trajectories showing the motion of each mapped endocardial point during the unassisted and assisted cardiac cycles. Note the horizontal minor movements in the base of the heart as compared to the large upward movement around the apex.
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Discussion
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Several methods are currently in use for the monitoring and analysis of experimental cardiac surgery at the animal preclinical stage, including the Swan- Ganz catheter, BP curves, blood flow measurements, echocardiography, and conductance catheters.131415 The results in the current study suggest that the electroanatomic mapping technique is capable of demonstrating more data than can be gathered using only one of the other imaging modalities, as well as data that are achieved only by the electroanatomic mapping technique.
The ability of the system to be tagged and to reconstruct the beating left ventricle in three dimensions is an important tool for the understanding and analysis of cardiac interventions, in particular if the intervention does not affect every spontaneous beat, as is the case with the intra-aortic balloon pump, several assist devices, cardiac pacing, and interventions such as aortomyoplasty and cardiomyoplasty. Findings from the present study show that the change in the long-axis direction of the left ventricle in the assisted beat during cardiomyoplasty can be clearly observed with this technique. These results are in accordance with the results of Blom and coworkers16 using 3D MRI reconstruction. These changes of the left ventricle long axis may appear as an artifact in other monitoring techniques such as echocardiography and conductance catheter. In chronic situations, adhesions around the skeletal muscle may reduce these long-axis movements; however, the current data suggest that this assumption would be best analyzed by the electroanatomic mapping technique. In the current study, movement of points in several areas of the heart was analyzed during the normal systole and the assisted systole. Figure 4 includes all data describing each point movement during a normal systole and the local movements in different parts of the heart as a result of contraction of the skeletal muscle around the left ventricle. Echocardiography is capable of generating 3D geometric information of the heart, and the improvements in 3D as compared to two-dimensional echocardiography are important in this respect.1718 However, although several methods were developed to facilitate automatic border detection, the main drawback of echocardiography for experimental use is the subjective bias of the device operator and the interpreter of the echocardiography data.19202122 Moreover, the accuracy of echocardiography in assessing the left ventricular volumes was found acceptable only when the volumes of the left ventricle were at a steady state, but insufficient when examining the cardiac chambers at more extreme preload situations.2324 Obviously the main advantages of the echocardiography technique over the electroanatomic mapping system are that it is noninvasive and allows for much faster data acquisition, advantages that are not always significant at the preclinical experimental stage.25 In the present study, the information from the three cross-section areas of the left ventricle during the normal cardiac cycle and the assisted beats supplies us with new data regarding the squeezing of the left ventricle by the skeletal muscle. The reduction visible in the cross area of the mid part of the heart during the assisted systole leads us to conclude that this is the part of the heart most affected by cardiomyoplasty. Moreover, since this data can be generated in real-time, the system can serve as a method for improving the wrapping technique. The other significant data gathered from the cross-section areas is the increase in the next beat end-diastolic cross-section area at the apex and base, data that can be useful when adjustments are made to find the ideal wrapping configuration. It can be hypothesized that the ideal wrapping configuration and stimulation setting should reduce the three cross-section areas during the assisted systole and increase the end-diastolic volumes at the next diastole.
The conductance catheter technology was used for monitoring the real-time effect of cardiomyoplasty in experimental and clinical studies.1526 The conductance catheter and the electroanatomic mapping techniques are both invasive and were both compared to other modalities and found to be highly accurate.27 However, each technique has its own advantages: while the conductance catheter technique is a real-time, beat-to-beat technique, the electroanatomic mapping technique is capable of producing a beat-to-beat analysis of a mean map gathered during a period of 20 to 30 min. This makes the latter technique more vulnerable to animal or patient hemodynamic instability. Moreover, the time needed to accomplish an electroanatomic map is one of the main drawbacks in the use of this technique in the clinical setting, especially in cardiac surgery, since the hemodynamic changes are more frequent and acute and should be evaluated more closely than with a mean map of every 20 min. However, with the electroanatomic mapping technique, more data than pressure volume loops are generated, such as the 3D reconstruction, the data about the movement of each point, and the hemodynamic analysis of the different heart segments with the cross-section areas. Changes in general and regional left ventricular mechanics were studied by other authors by means of cine-MRI.28 The major advantage of cine-MRI is the noninvasiveness of the procedure, while possible advantages of the electroanatomic mapping technique are the endocardial activation propagation data and lower costs.
In the current study, no difference was found in the endocardial activation propagation between the prewrapping and postwrapping maps, nor during the activation protocol; however, these data may be very important for surgery that interferes with the endocardial integrity, such as the Batista operation and aneurysmectomy procedure.
In conclusion, the 3D electroanatomic mapping system allows detailed reconstruction of the left ventricular geometry and a clear view of the difference between assisted and unassisted beats. This novel monitoring system may serve as an important tool for the analysis and development of new techniques in cardiac surgery.
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Footnotes
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Abbreviations: LD = latissimus dorsi; 3D = three dimensional
Dr. Ben-Haim is an employee of J&J, which own Biosense.
Received for publication April 11, 2003.
Accepted for publication October 9, 2003.
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References
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Abstract
Introduction
