Journal of Veterinary Cardiology (2019) 22,20e39 www.elsevier.com/locate/jvc Review Indications for permanent pacing in dogs and cats R.A. Santilli, Dr Med Vet, PhD a,b,*, F. Giacomazzi, Dr Med Vet a, D.M. Porteiro Va´zquez, DVM Dr Med Vet a, M. Perego, Dr Med Vet a a Clinica Veterinaria Malpensa, Viale Marconi 27, 21017 Samarate, Varese, Italy b Department of Clinical Sciences, Cornell University, Ithaca, NY, USA Received 28 August 2018; received in revised form 14 December 2018; accepted 14 December 2018 KEYWORDS Abstract Pacemaker implantation is considered as a standard procedure for treat- Cardiac pacing; ment of symptomatic bradycardia in both dogs and cats. Advanced second-degree Atrioventricular block; and third-degree atrioventricular blocks, sick sinus syndrome, persistent atrial Sick sinus syndrome; standstill, and vasovagal syncope are the most common rhythm disturbances that Atrial standstill; require pacing to either alleviate clinical signs or prolong survival. Most pacemakers Syncope are implanted transvenously, using endocardial leads, but rarely epicardial leads may be necessary. To decide whether a patient is a candidate for pacing, as well as which pacing modality should be used, the clinician must have a clear under- standing of the etiology, the pathophysiology, and the natural history of the most common bradyarrhythmias, as well as what result can be achieved by pacing pa- tients with different rhythm disturbances. The goal of this review was, therefore, to describe the indications for pacing by evaluating the available evidence in both human and veterinary medicine. We described the etiology of bradyarrhythmias, clinical signs and electrocardiographic abnormalities, and the choice of pacing modality, taking into account how different choices may have different physiologi- cal consequences to selected patients. It is expected that this review will assist ve- terinarians in recognizing arrhythmias that may require permanent pacing and the risk-benefit of each pacing modality and its impact on outcome. ª 2019 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: [email protected] (R.A. Santilli). https://doi.org/10.1016/j.jvc.2018.12.003 1760-2734/ª 2019 Elsevier B.V. All rights reserved. Indications for pacing 21 management strategy for individual patients, tak- Abbreviations ing into account the impact on outcome and the riskebenefit ratio of particular diagnostic or ther- AAI (R) atrium-paced, atrium-sensed inhib- apeutic methods. ition of pacemaker in case of sensed intrinsic activity rate modulation Etiology, natural history, and role of pacing AF atrial fibrillation AV atrioventricular Atrioventricular conduction disturbances AVB atrioventricular block AVB is a disturbance of impulse conduction through cTnI cardiac troponin I the atrioventricular (AV) node and/or the ECG electrocardiogram HisePurkinje system that can be permanent or DDD(R) dual-paced, dual-sensed, dual mode transient [18]. Atrial impulses can be conducted of response rate modulation with delay or be completely blocked depending on PAS persistent atrial standstill anatomical or functional impairment along the PM pacemaker conduction system [18]. Atrioventricular dis- SA sinoatrial turbances can occur as a result of a primary dis- SND sinus node dysfunction order or secondary to acute or chronic myocardial SSS sick sinus syndrome abnormalities [19]. In human medicine, two VDD ventricle-paced, dual-sensed, dual degenerative diseases (Lev and Lenegre diseases) mode of response have been described as idiopathic degeneration VVS vasovagal syncope with fibrosis replacement of the intraventricular VVI (R) ventricle-paced, ventricle-sensed conduction system, with or without changes in the inhibition of pacemaker in case of cardiac skeleton [19]. Similar degenerative sensed intrinsic activity rate changes have been reported in veterinary medi- modulation cine in dogs with chronic mitral valve disease [20]. Any primary or secondary cardiac disease that affects the AV conduction system (congenital, Introduction acquired, or secondary to traumatic, neoplastic, infiltrative, inflammatory, and systemic disorders) In recent years, implantation of artificial cardiac may result in complete AVB. More rarely, con- pacemakers (PMs) has become a standard proce- genital forms or functional disorders (hyper- dure for dogs with symptomatic bradycardia, such kalemia, toxicity, antiarrhythmic drugs, high vagal as advanced second-degree and third-degree tone, and hyperthyroidism) have also been atrioventricular block (AVB), sinus node disease, described [21e31]. In human beings, the most persistent atrial standstill (PAS), and vasovagal common cause of transient AVB is acute myo- syncope (VVS) [1e15]. In human medicine, the first carditis [32,33]. The pathogenesis of transient AVB reported PM implantation dates from 1952 [16], secondary to acute myocarditis implies the pres- whereas, in veterinary medicine, the first case was ence of myocardial interstitial edema, and a cor- reported in 1967 [17]. In dogs, most PMs are relation between increased thickness of the implanted transvenously using endocardial leads, edematous cardiac walls and severity of AVB has whereas, in cats and in particular circumstances been described [34]. The same mechanism has also in dogs, the use of epicardial leads via thor- been suspected in dogs, with unknown prevalence, acotomy or laparotomy may be necessary [7,8]. as it has been documented that 13% of dogs pre- Modern PMs consist of a sealed pulse generator sented with transient AV conduction disturbances with a lithiumeiodine battery that generates [35]. Both experimental models and human electrical impulses and senses intrinsic cardiac patients with spontaneously occurring acute lym- rhythm via a unipolar or a bipolar lead attached to phocytic myocarditis very often present AVB that the endocardial or epicardial surface of the heart lasts a few days or, less frequently, 2e3 weeks, [7]. Artificial pacing rate, voltage, current, pulse and then regresses completely during the con- width, and sensitivity can be programmed by a valescent stage of 2e4 weeks in 67%e96% of the telemetry system, allowing intraoperative and patients [32e34,36,37]. In addition, one study postoperative adjusting [7]. reported regression of AVB with improvement This review describes the indications for pacing recorded within one month from PM implantation and evaluates the available evidence with the aim in 13% of dogs [35]. In human beings, it has been of assisting veterinarians in selecting the best reported that the prevalence of persistent AVB 22 R.A. Santilli et al. after acute myocarditis is of approximately 4% [34] reported and consistently observed in the and permanent PM implantation is required in branching portions of the left and right bundle these patients. In children, the occurrence of branches in association with extensive fibrosis of persistent AVB after resolution of acute myo- the central fibrous body and endocardial and carditis is higher (28%) [33]. Chronic myocarditis as myocardial fibrosis at the upper border of the a cause of longstanding AVB in human beings is ventricular septum [51]. It would seem possible considered somewhat uncommon [38]. In dogs, the that the pathological process is fundamentally prevalence of myocarditis-related AVB is unknown, related to the normal aging phenomenon and is and a few studies have described acute myo- probably exaggerated or accelerated by the carditis in dogs with AVB based on postmortem abnormal mechanical forces created by myo- examination [22,23,30] or using endomyocardial cardial hypertrophy [52]. Interestingly, in human biopsy [39]. This latter study defined enteric beings with hypertrophic cardiomyopathy and coronavirus as the pathogen that most commonly subvalvular aortic stenosis, the AV conduction causes acute myocarditis and AVB in dogs. In one system is extensively impaired by mechanical of these case reports, a very high serum concen- factors caused by hypertrophy of the ventricular tration of cardiac troponin I (cTnI) was docu- septum [52]. mented and the authors suggested that this Previously published data have demonstrated biomarker might help detect dogs with AVB sec- that dogs affected by advanced second-degree and ondary to active myocarditis [22]. Another study third-degree AVB are larger, older, and more fre- proved that cTnI is high at the time of presentation quently intact female [9] and breeds such as Lab- in dogs with permanent AVB but showed a sig- rador retrievers, Cocker spaniels, German nificant reduction over time after PM implantation shepherd, Dachshund, and English bulldogs are [40,41]. Lead type and arrhythmia severity did not predisposed [2,7e13]. show significant correlation with cTnI [40].In Cats affected by third-degree AVB are usually human medicine, endomyocardial biopsy is the middle-aged to older and have concurrent sys- gold standard for in vivo diagnosis of myocarditis temic diseases that might account for many of the and increased serum concentrations of cTnI and clinical signs. In one case series, the presence of cardiac troponin T are considered reliable to con- congestive heart failure or identifiable structural firm the diagnosis [42e44]. heart disease at diagnosis did not affect median In cats, transient AV conduction disturbances survival times [50]. (trifascicular block) have been reported associated Survival data suggest that high grade AVB is with concurrent elevations of cTnI concentrations associated with a guarded prognosis