Percutaneous method for multiple epicardial monophasic action potential recordings during magnetocardiographic mapping in intact rats

A surgical approach for subdiaphragmatic-programmed electrical stimulation (PES) in mice has been reported by Gutstein et al. [D.E. Gutstein, S.B. Danik, J.B. Sereysky, G.E. Morley, G.I. Fishman, Subdiaphragmatic murine electrophysiological studies: sequential determination of ventricular refractoriness and arrhythmia induction, Am J Physiol Heart Circ Physiol. 285 (3) (2003) H1091-96.]. We developed a percutaneous method to drive a patented magnetocardiography-compatible amagnetic catheter (AC), specifically designed for multiple monophasic action potentials (Multi-MAP) recording, at the epicardium of spontaneously breathing small animals. The aim of this study was to test the feasibility of simultaneous magnetocardiographic (MCG) estimate of ventricular repolarization (VR) and Multi-MAP recording in 5 rats. Under fluoroscopic control, the AC was introduced with a sub-xyphoid puncture, in all animals, and moved at several epicardial sites until 4 stable MAPs were recorded, with a fixed inter-electrode distance of 1 mm. An unshielded 36-channel DC-SQUID system (sensitivity 20 fT/Hz 1 / 2) was used to record the MCG. MAP signals, differentially amplified and filtered at DC-500 Hz, were digitized at 1 kHz. The same AC was used for programmed PES at pacing cycles between 200 and 250 ms. Ventricular effective refractory period (VERP) was evaluated with the accuracy of 2 ms. MAP duration (MAPd) was measured, at 90% levels of repolarization. Simultaneous MCG and MAP recordings were successful in all animals. One rat died for respiratory arrest. Four animals tolerated well the procedure and survived. In WRs, MAPd 90% and VERP were 67 ± 9.4 ms and 69.6 ± 5.6 ms, respectively. This minimally invasive method is well tolerated, avoids animal sacrifice, can be used to validate the accuracy of surface MCG estimate of dispersion of VR and for assessment of arrhythmogenic potential of new drugs. Keywords Magnetocardiography Cardiac mapping Monophasic action potential Wistar rat 1 Introduction Gutstein et al [1] have reported a surgical approach for subdiaphragmatic-programmed cardiac pacing (PES) in mice, that implies assisted respiration, incompatible with magnetocardiographic mapping (MCG), which is a contactless method for non-invasive multisite cardiac mapping of small experimental animals. Monophasic action potential (MAP) recording has been used to interpret the electrogenesis of murine ECG [1] . On the contrary, an experimental assessment of the accuracy of MCG in evaluating cardiac electrophysiology by comparison with simultaneous invasive MAP recordings, is still lacking. We have developed a novel percutaneous method to drive a patented MCG-compatible amagnetic catheter (AC), specifically designed for multiple MAP (Multi-MAP) recording, at the epicardium of spontaneously breathing intact Wistar rats (WR) [2] . The technique allows simultaneous MCG and Multi-MAP recordings with a single amagnetic catheter, in SR and under local pacing. This study was aimed: 1) to test the feasibility of simultaneous MCG estimate of ventricular repolarization (VR) and Multi-MAP recording; 2) to evaluate the relationship between MCG and Multi-MAP estimate of de-repolarization; 3) to test 3D MCG localization accuracy for non-fluoroscopic imaging of the Multi-MAP catheter. 2 Methods A 36-channel DC-SQUID MCG system ( CardioMag Imaging Inc ) was used for MCG (intrinsic sensitivity of 20 fT/Hz 1 / 2) ( Fig. 1 A ). Signal acquisition and post processing were carried out as detailed elsewhere [2] . A portable digital fluoroscopy system was used to acquire fluoroscopic images for off-line reconstruction of the animals' 3-D heart model ( Fig. 1 B and C). The amagnetic catheter, specifically designed for Multi-MAP recording, is shown in Fig. 1 D [3] . Signal analysis was carried out with the Windows NT-based CardioMag and the UNIX-based Neuromag software packages [2,4] . 5 WRs [age: 14–15 months (weight: 596 ± 90 g)] were studied anesthetized (Ketamine: 50 mg/kg b.w.; Diazepam: 2 mg/kg b.w.), in sinus rhythm and spontaneously breathing. The investigation was approved by the Catholic University Ethical Committee and conforms to the Guide for the Care and Use of Laboratory Animals (NIH Publication # 85-23, revised 1996). MCG lasted typically 300 s. Under fluoroscopic control, the AC was introduced with a sub-xyphoid puncture, and moved at several epicardial sites until 4 stable MAPs were recorded, with a fixed inter-electrode distance of 1 mm. MAP signals, differentially amplified and filtered at DC-500 Hz, were digitized at 1 kHz. The same AC was used for programmed PES at pacing cycles between 200 and 250 ms. Ventricular effective refractory period (VERP) was evaluated with the accuracy of 2 ms. MAP duration (MAPd) was measured, at 90% levels of repolarization. Data are reported as mean ± standard deviation. 3 Results Ventricular Multi-MAP recordings were successful in all animals ( Fig. 2 A ). Local pacing was successful using two MAP electrodes for pacing and the other two for MAP recording ( Fig. 2 B). Good quality right atrial MAP ( Fig. 2 C) was also recordable in two animals. One WR died at the end of the procedure, for respiratory arrest due to pneumo-thorax. Four animals recovered well after the procedure and were followed up. One of them was restudied after 6 months and sacrificed after the second procedure (no cardiac lesion was found). Average sinus cycle length was 296 ± 35.7 ms; average MAP amplitude was 17.4 ± 9.8 mV; average MAPd 90% and VERP were 69.6 ± 10.26 ms and 70 ± 5.7 ms, respectively. Simultaneous MCG and MAP recordings were successful in all animals. Examples of the relationship between MAP recordings and simultaneous estimate of cardiac magnetic field distribution are shown in Fig. 3 . MCG localization accuracy of the AC was 2 ± 0.53 mm. 4 Discussion The percutaneous single catheter Multi-MAP recording and pacing method is minimally invasive (avoids cut-down), is well tolerated and MCG-compatible. Multi-MAP epicardial recording in combination with contactless multisite MCG provides a novel approach for a complete electrophysiological study of small experimental animals, and might be useful for a comprehensive preclinical assessment of electrophysiological effects of new drugs. It provides information about local arrhythmogenic mechanisms (i.e. MAPd/ERP ratio, VR inhomogeneity, local block), with high spatial resolution, and can be used for correlative studies between surface estimate of T-wave abnormalities, QT dispersion and MAP/ERP duration. After the “learning curve”, it avoids animal sacrifice, thus might be useful for longitudinal studies. References [1] D.E. Gutstein Subdiaphragmatic murine electrophysiological studies: sequential determination of ventricular refractoriness and arrhythmia induction Am. J. Physiol, Heart Circ. Physiol. 285 3 2003 H1091 H1096 [2] D. Brisinda M.E. Caristo R. Fenici Contactless magnetocardiographic mapping in anaesthetized Wistar rats: evidence of age-related changes of cardiac electrical activity Am. J. Physiol, Heart Circ. Physiol. 291 1 2005 H368 H378 [3] Fenici R. (Rome, IT) Consiglio Nazionale delle Ricerche (Rome, IT) Catheter guidance by magnetocardiographic mapping. United States Patent 6,527,724, March 4, 2003. [4] J. Nenonen Magnetocardiography J. Clarke A. Braginski SQUID Handbook 2005 Whiley-VCH Berlin