Transesophageal low-energy synchronous

cardioversion of atrial flutter/fibrillation in the

dog

The purpose of thls study was to determine the feasibility and efficacy of terminating atrial flutter/fibrillation using low-energy synchronous shocks delivered through a transesophageai catheter in dogs with talc-induced pericarditis. Atrial flutter/fibrillation was induced by employing the pulse train method. The minimum effective cardioversion energy level was compared for three different methods-method A, delivery between a distal esophageal electrode and a proximal esophageal electrode; method B, delivery of shocks through a distal esophageal electrode and a plate placed on the chest; method C, transthoracic cardioversion. The minimum effective cardioversion energy level did not differ significantly between methods A and B (1.30 k 0.46 joules versus 1.29 + 0.35 joules). Transesophageal cardioverslon decreased the defibrillation threshold three- to fourfold from that of conventional transthoracic cardioversion. There were no complications of heart block, ventricular fibrillation, or any pathologic evidence of esophageal injury. Thus transesophageal low-energy synchronous cardioversion Is consldered a feasible and effective method for the treatment of atrial flutter/fibrillation. (AM HEART J 1992;123:417.)

Yoshio Yamanouchi, MD, Koichiro Kumagai, MD, Noritami Tashiro, MD, Tadayuki Hiroki, MD, and Kikuo Arakawa, MD. Fukuoka, Japan

Paroxysmal atrial tachyarrhythmias such as atrial flutter/fibrillation commonly occur in humans, and therapeutic intervention is often necessary.l Direct- current external cardioversion has been recognized as an effective treatment for restoring sinus rhythm.2-5 However, high-energy electrical shocks are traumatic to the body and may result in skin burns and even in myocardial injury. Moreover, heavy sedation is usu- ally required before energy delivery.6y 7 In recent years, transesophageal atria1 pacing has been used to diagnose and treat paroxysmal supraventricular ta- chycardia, since it is associated with less traumatic changes as a result of electrical shock compared with transvenous cardiac pacing.s-10

This study was performed to (1) evaluate the min- imum effective energy level for transesophageal car- dioversion for acute atria1 flutter or fibrillation using an esophageal catheter; (2) compare the results of the

From the Department of Internal Medicine, School of Medicine, Fukuoka University.

Received for publication March 19, 1991; accepted Aug. 2, 1991.

Reprint requests: Yoshio Yamanouchi, MD, Department of Internal Med- icine, School of Medicine, Fukuoka University, 7-45-1, Nanakuma, Jonan- ku, Fukuoka, 814-01, Japan.

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transesophageal catheter technique with those of the conventional transthoracic technique; and (3) assess the effect of the anode position at two different sites (esophageal electrode and external body surface) in transesophageal cardioversion.

METHODS

Twenty adult mongrel dogs weighing between 9 and 20 kg were anesthetized with intravenous pentobarbital so- dium (30 mg/kg body weight). Endotracheal intubation was performed, and the dogs were ventilated using a Har- vard respirator (Harvard Apparatus Inc., S. Natick, Mass.). The chest was opened by right thoracotomy in the fifth in- tercostal space using an aseptic technique. To produce the experimental model of atria1 flutter/fibrillation, sterile talc (20 ml) was injected intrapericardially to induce sterile pericarditis, and the pericardium and chest were closed.

Study preparation. Four days after thoracotomy, the dogs were anesthetized with intravenous pentobarbital so- dium (30 mg/kg), intubated, and mechanically ventilated with room air. A conventional 6F quadripolar catheter was introduced through a surgical cutdown of the femoral vein and was positioned inside the right atrial appendage under fluoroscopic guidance. The distal electrode pair was used for pacing to initiate tachyarrhythmia, and the proximal electrode pair was used to record the atria1 electrogram. An esophageal pacing catheter (10F) was passed pernasally into the distal esophagus. Surface lead II of the electrocar-

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American Heart Journal

Fig. 1. Electrocardiogram (lead II) and intracardiac electrogram showing induction of tachyarrhythmia by the pulse train method. Pulse trains (4 msec, 100 Hz) were administered 50 msec after the onset of the P wave and continued to the end of the S wave. In this case atrial fibrillation was induced when the current was increased to 4 mA.

diogram and an intracardiac electrogram were recorded at a paper speed of 100 mm/set (8M14, San-Ei Co., Ltd., To- kyo, Japan).

Atria1 flutter or fibrillation was induced by a pulse train method that delivered a train of rectangular pulses (4 msec, 100 Hz) during the vulnerable period of the atrium through the distal electrode pair (BC-OPA stimulator, Fukuda Denshi Co., Ltd., Tokyo, Japan). The pulse trains were be- gun 50 msec after the onset of the P wave and were contin- ued to the end of the S wave. The current was increased in 1 mA increments until atria1 flutter or fibrillation occurred (Fig. 1).

A specially designed cardioverter (FC 710, Fukuda Den- shi) was used to deliver a truncated exponential waveform 6 msec in duration at 10 different energy levels (0.1, 0.25, 0.5,0.75,1.0,1.5,2.0,3.0,4.0, and 5.0 joules). All cardiover- sion energy deliveries were synchronized to the QRS com- plex. Starting with 0.1 joule shocks of progressively higher energy were delivered until atrial flutter or fibrillation was terminated (Fig. 2). Atria1 flutter or fibrillation was rein- duced and the same sequence was repeated three to five times to determine the minimum energy required to reproducibly terminate the tachyarrhythmia. A 5-minute interval was allowed between each cardioversion attempt.

Cardloversion protocol. Three methods of cardiover- sion were tested comparing the minimum energy level re- quired to terminate atria1 flutter/fibrillation in each dog- method A, delivery between a distal esophageal electrode (pole 1) as the cathode and a proximal esophageal electrode (pole 2) as the anode; method B, delivery of shocks through a distal esophageal electrode (pole 1) as the cathode and a plate placed on the chest as the anode; and method C, ex- ternal transthoracic cardioversion. Each of the three meth- ods was tested in random order.

In all dogs the reproducibility of the minimum energy level required to initiate atrial flutter or fibrillation was evaluated for each cardioversion series. The reproducibil- ity of the minimum energy required to terminate atria1 flutter/fibrillation using method A was also evaluated.

Esophageal pathology. After 24 hours, gross and histo- logic examination of the esophagus were performed in six dogs for method A and in six dogs for method B.

Definitions. (1) Atrial flutter/fibrillation: Atrial ar- rhythmia with a cycle length of 200 msec or less and last- ing for at least 30 seconds. (2) Successful cardiouersion: Conversion to normal sinus rhythm within 2 seconds after energy delivery. (3) Defibrillation threshold: The mini- mum energy (in joules) required to terminate atrial flutter/ fibrillation.

Statistical analysis. The Student’s t test was used for statistical analysis of the results. All data are expressed as mean + SD.

RESULTS Atrial flutter/fibrillation characteristics. One hun-

dred eighty episodes of atrial flutter or fibrillation (85 of atria1 flutter and 95 of atria1 fibrillation) were ini- tiated by the pulse train method in the 20 dogs. For all tachyarrhythmias, the mean atria1 cycle length ranged from 90 to 160 msec. The mean atrial cycle length of atrial flutter/fibrillation induced in any one dog was generally reproducible, with a standard de- viation of 1.0 to 13.2 msec; the atria1 flutter/fibrilla- tion threshold was also reproducible, with a standard deviation of 2.1 to 11.3 mA.

Cardioversion results. For method A, all 71 cardio- version attempts were successful using shocks of 15

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Number 2 Transesophageal cardioversion of atria1 fibrillation 419

0.05J

Intracardiac

Fig. 2. Electrocardiogram (lead II) and intracardiac electrogram showing termination of the tachyarryth- mia bv svnchronous transesoDhaeeal cardioversion. In this case, the minimum energy required to termi- nate thetachyarrhythmia was 0.05 joule.

joules. Shocks of 11 joule resulted in successful car- dioversion in 45 (63 % ) dogs, and shocks of 10.5 joule resulted in successful cardioversion in 18 (25 %) dogs. For method B, successful cardioversion also occurred in all 68 attempts with shocks of 15 joules. Shocks of I 1 joule resulted in successful cardioversion in 47 (69 % ) dogs, and shocks of <0.5 joule resulted in suc- cessful cardioversion in 30 (44%) dogs. For method C, no successful cardioversion resulted from shocks of 10.5 joule in 41 attempts, and shocks of rl joule resulted in successful cardioversion in only four (10%) dogs.

infiltration of the intima. No serious complications such as ulceration or perforation of the esophagus were observed.

DISCUSSION

Comparison of defibrillation threshold. The mini- mum defibrillation threshold for method B tended to be lower than that of method A, but this difference was not significant (mean 1.30 + 0.46 joules versus 1.29 f 0.35 joules). The threshold for method C was significantly (mean three to four times) higher than that for methods A and B (p < 0.01). The defibrilla- tion threshold for transesophageal cardioversion (method A) was generally reproducible, with a stan- dard deviation of 0.5 to 3.0 joules. For method A, there was no significant difference in the defibrilla- tion threshold of atria1 flutter (n = 32) and atrial fi- brillation (n = 39) (mean 1.27 + 0.29 joules versus 1.32 k 0.41 joules).

Conventional transthoracic cardioversion is ac- cepted as the most effective and safest form of elec- trical therapy for termination of life-threatening ta- chyarrhythmias. However, it has obvious limitations for long-term application. The purpose of this study was to determine the feasibility and efficacy of terminating atria1 flutter or fibrillation using low-en- ergy synchronous direct-current countershock deliv- ered through a transesophageal catheter.

During countershock using conventional surface electrodes, only a fraction of the energy administered to the patient is usefully transmitted to the heart; most of the administered energy is dissipated in ex- tracardiac tissues and wasted, since these tissues do not contribute to depolarization of the heart. The transesophageal and transvenous methods allow elec- trodes to be placed closer to the heart.i1*15 As a result, the transesophageal method can entirely depolarize the heart with delivery of a relatively low-energy level.

Complications. There were no episodes of ventricu- lar fibrillation, atrioventricular (AV) conduction dis- turbance, or delayed sinus recovery time of greater than 1.5 seconds following the shocks.

Esophageal pathology. For methods A and B, none of the 12 dogs showed any histologic evidence of shock-induced injury except for minimal round cell

McNally et al.16 conducted a study on eight anes- thetized and five unanesthetized patients in which one electrode consisting of bars (22 gauge, 7 cm long coil) was placed in the esophagus and the other elec- trode was placed on the precordium. Using this method, the energy required for cardioversion of atria1 fibrillation was reduced to less than half that required using conventional surface electrode place-

420 Yumunouchi et al.

ment. Yunchang et al. l7 first attempted the place- ment of both the anode and the cathode in the esophagus of animals, and showed that transesoph- ageal cardioversion using small amounts of energy can safely and effectively terminate ventricular ta- chyarrhythmias. In their study, the energy levels re- quired for successful cardioversion of ventricular fi- brillation or flutter were decreased to 30 joules or less, and for ventricular tachycardia the levels were de- creased to 5 joules or less. We have recently reported that transvenous cardioversion of experimentally in- duced atria1 flutter/fibrillation is effective and feasi- ble. In this study, transvenous cardioversion de- creased the defibrillation threshold six- to sevenfold from that of transthoracic cardioversion.‘8

In the present study, we utilized an esophageal catheter to perform defibrillation of atria1 flutter/fi- brillation. Transesophageal cardioversion provides a relatively noninvasive method of defibrillation that does not require fluoroscopy, sterile precautions, or cardiac catheterization, and thus appears to have advantages compared with temporary transvenous cardioversion. Transesophageal cardioversion can also be performed expeditiously in ambulatory, bed- side, or emergency situations and can be repeated serially.

This study demonstrated that transesophageal low-energy cardioversion is a feasible and effective method for terminating atria1 flutter/fibrillation in dogs. This technique is a possible alternative to the administration of antiarrhythmic drugs or conven- tional direct-current cardioversion requiring high energies. However, 1 to 2 joules would be painful to humans, and it is likely that higher energies might even be necessary in patients. Therefore further studies are needed to investigate the clinical applica- tion of this method.

Study limitations. Current methodology is inade- quate to permit a complete assessment of the elec- trophysiologic properties of primary atria1 flutter/n- brillation and therefore the similarity of induced atria1 tachyarrhythmias in dogs to those seen in hu- mans is uncertain. However, we believe that the model utilized in this study was useful for the assess- ment of factors determining the success of transe- sophageal cardioversion of atria1 flutter/fibrillation.

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