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Ann Thorac Surg 2007;83:300-302
© 2007 The Society of Thoracic Surgeons
a Department of Cardiology, Long Beach Memorial Hospital, University of California, Irvine, California
b Department of Thoracic Surgery, Long Beach Memorial Hospital, University of California, Irvine, California
Accepted for publication March 22, 2006.
* Address correspondence to Dr Shandling, 3801 Katella Ave, Suite 401, Los Alamitos, CA 90720 (Email: ashandling{at}aol.com).
| Dr Bethencourt discloses a financial relationship with Guidant and Boston Scientific.
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Abstract |
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Inappropriate sinus tachycardia (IST) is characterized by an elevated resting sinus heart rate and is usually accompanied by exaggerated heart rate acceleration with exercise. It is a non-re-entrant arrhythmia originating from the sinus node [1, 2]. Treatment modalities include drugs (ie, beta blockers, calcium channel blockers, and digoxin), atrioventricular nodal ablation with pacemaker implantation, and subtotal right atrial (RA) exclusion using cardiopulmonary bypass [3]. More recently, radiofrequency (RF) catheter ablation has been used to treat drug-refractory IST. The technique involves endocardial RF ablation of the superior-lateral RA areas, as defined by three-dimensional electro-anatomical mapping or intracardiac ultrasound. In most cases, it is necessary to ablate a relatively large volume of atrial tissue, although in some the tachycardia may be more focal. The procedure is complicated by the location of the right phrenic nerve, which runs laterally or posterolaterally down the superior vena cava and then along the lateral RA to the diaphragm. It can be inadvertently damaged at a number of points during the RF ablation. This debilitating complication can usually be avoided by pacing at the ablation catheter tip and looking fluoroscopically for diaphragmatic stimulation. If present, RF energy delivery is avoided at these sites. It is not uncommon for diaphragmatic stimulation to occur over an appreciable area of the lateral RA. Under these circumstances, the endocardial ablation procedure must be abandoned. We report the use of thoracoscopic epicardial microwave ablation for treatment of IST in a patient in whom intracardiac RF ablation could not be performed due to phrenic nerve stimulation.
The patient is a 37-year-old woman with the chief complaint of palpitations. She was disabled from work as a registered nurse because of her cardiac symptoms. Combinations of various chronotropic medications were ineffective for symptom amelioration. Long-term electrocardiographic monitoring revealed heart rates up to 160 beats per minute, and a resting heart rate generally over 110. Heart rate increases were gradual and not abrupt, suggesting sinus tachycardia rather than a focal supraventricular tachycardia. Her physical examination was unremarkable. A work-up for hyperthyroidism, a pheochromocytoma, and pulmonary emboli was negative. She did not appear to have the full gamut of symptoms or signs to diagnose the neuropathic postural orthostatic syndrome [4, 5].
Electrophysiologic testing demonstrated no accessory or dual atrioventricular nodal pathways. No atrial tachycardia or re-entrant sino-atrial tachycardia was induced. The resting heart rate was 110 bpm with a rate of 150 bpm on 2 mcg per minute infusion of isoproterenol. Heart rate response to isoproterenol was brisk. Intracardiac electro-anatomic mapping localized the earliest activation of the sinus tachycardia to the high-lateral RA (Fig 1). Pacing through the 4 mm tip of the ablation catheter demonstrated symptomatic diaphragmatic stimulation over a wide area of the high lateral RA, even with currents as low as 2 milliamps at a 2 ms pulse width. It was considered unwise to proceed with endocardial RF ablation and the procedure was aborted. During the ensuing weeks the patient continued to be very symptomatic.
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The resting heart rate at the start of microwave ablation was 120 bpm. This decreased to 105 bpm after initial superolateral ablation. Isoproterenol was then infused at 2 mcg per minute. After further ablations were made more laterally, superiorly, and inferiorly, the heart rate on isoproterenol dropped to 95 to 100 bpm. A total of six ablations were performed. At this point the procedure was terminated, as there was an approximately 20% reduction in the sinus rate, despite isoproterenol. The pericardium, with the attached phrenic nerve, was left reflected away from the heart. Postoperatively the patient had a period of transient respiratory distress. There was elevation in the right hemidiaphragm, possibly due to atelectasis or traction neurapraxia, or both. At 2 months postoperatively, diaphragmatic excursion was fluoroscopically normal, and the diaphragm was in an essentially normal position on a follow-up chest computed tomographic scan. The patient was discharged after several days in the hospital with a resting heart rate of 90 to 95 bpm, and this rate was maintained at 2 months off the medications.
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The IST occurs predominantly in women and can be profoundly debilitating. No consistently reliable therapy has been devised. Endocardial RF energy is the best current option, with a reported success rate of 76% acutely [6]. Recurrence rates are high at 27%, and long-term success (including repeat ablations) is reported at 66% [6]. A higher long-term success rate of 79% after a single ablation attempt has been claimed with noncontact fluoroscopic mapping [7]. Atrioventricular nodal ablation with pacemaker implantation seems effective [6], but it has the problem of atrioventricular dissociation at higher heart rates, and nonphysiologic ventricular-paced activation. Multiple pacemaker generator changes with time would also be required in this predominantly young group of patients.
Surgical approaches to IST have included sinus node and RA resection and sino-atrial node isolation [3]. Success has been suboptimal, and these outdated procedures require cardiopulmonary bypass. A minimally invasive approach to treatment was recently reported in one patient [8]. The approach is less invasive than earlier surgical approaches in that it is done off bypass with a small thoracotomy, but it still requires a thoracotomy large enough for passage of the RF instrument. More extensive dissection is required to be able to isolate the RA and the sino-atrial node with the clamp-like device. Rather than ablate the culprit tissue, the RF clamp technique attempts to isolate the sino-atrial node locus, but leaves lateral tissue intact for possible recurrence.
The approach we have described requires only three 5-mm punctures and it is applicable with microwave, laser, or high frequency ultrasound wands. Thus, it is a truly endoscopic approach. Microwave ablation does provide advantages to endocardial RF ablation in that larger areas of tissue can be rapidly and contiguously ablated, shortening procedure time and reducing the potential for creating a re-entrant substrate. Another advantage of the current technique is that the phrenic nerve remains posterolaterally reflected with the pericardium permanently separated from the heart. This permits future endocardial RF ablation, if required, and helps to address the lack of a discrete heart rate endpoint for IST ablation (ie, a "hybrid procedure").
A limitation of the approach in this case was that intraoperative activation mapping was not performed. This was because of logistical issues relating to transfer of mapping equipment to the operating room. In some cases this could be important because subsidiary atrial pacemakers exist that are outside the cristal area, but nevertheless may play a role in generating IST [6].
This report demonstrates that thoracoscopic epicardial microwave ablation is feasible for the management of IST and offers a unique option for those cases with IST or other tachycardias with unavoidable phrenic nerve stimulation in the electrophysiology laboratory.
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