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8.5.3 Pacemaker implantation following septal myectomy and alcohol septal ablation In a study involving 2482 patients with obstructive HCM, 2.3% developed AVB after septal myectomy588 (only 0.6% in those with normal baseline conduction vs. 34.8% in patients with pre-existing RBBB). Alcohol septal ablation causes AVB in 720% of patients;576 those with pre-existing conduction defects, mainly LBBB, are at highest risk.585

8.5.4 Cardiac resynchronization therapy in end-stage hypertrophic cardiomyopathy Based on the findings of a small cohort study,589 CRT was given both a class IIa and a class IIb recommendation in previous guidelines for patients with HCM, HF, LBBB, and LVEF <50%.576,590 More recent studies did not demonstrate sustained efficacy of this therapy.591593 Until further evidence becomes available, standard criteria for CRT are recommended in patients with HCM (section 6).

8.6 Pacing in rare diseases

8.6.1 Long QT syndrome There are multiple inter-relationships between the different forms of long QT syndrome (LQTS) and bradycardia: LQTS can be associated with sinus bradycardia; very long ventricular myocardial refractory periods can cause 2:1 AVB; sudden rate changes can trigger torsadesde-pointes tachycardia; and treatment with beta-blockers to suppress sympathetic triggers of torsades-de-pointes may cause bradycardia. As current ICDs provide all pacemaker functions, a standalone pacemaker is rarely indicated in LQTS today. However, in individual patients with LQTS and catecholamine-induced torsades-depointes, shock therapy may be disadvantageous or even fatal; in these cases, pacing and beta-blocker therapy alone without an ICD may be used. Pacemaker instead of ICD implantation represents a treatment option in neonates and small infants with LQTS,594 and an alternative in LQTS patients with symptomatic bradycardia (spontaneous or due to beta-blockers) if ventricular tachyarrhythmias are unlikely or if ICD implantation is not desired (e.g. patient preference).

An indication for a pacemaker in LQTS exists in neonates and infants with a 2:1 AVB due to excessive corrected QT prolongation with long myocardial refractory periods.595 Temporary pacing at an increased rate (usually 90- 120 b.p.m.) is an important treatment in LQTS patients with electrical storm, because an increase in the basic heart rate shortens the window of vulnerability for reinduction of torsade de pointes ventricular tachycardia.

8.6.2 Neuromuscular diseases Neuromuscular diseases are a group of heterogeneous inherited disorders affecting the skeletal muscle and frequently also involve the heart (for extended literature on conduction disorders in neuromuscular disease, see the supplementary literature on pacing in rare disease and Supplementary Table 17). The cardiac phenotype variably includes all types of cardiomyopathies, conduction defects with or without cardiomyopathies, and supraventricular and ventricular tachyarrhythmias.596598 Duchenne, Becker, and limb-girdle types 2C, 2F, and 2I are muscular dystrophies in which the development of dilated cardiomyopathy is common and usually the predominant feature. Arrhythmias and conduction disease can be associated with the development of cardiomyopathy.596598 Such patients are considered for pacemakers or ICDs on the basis of guidelines used for other non-ischaemic cardiomyopathies.242 Myotonic dystrophy types 1 and 2, EmeryDreifuss, and limb-girdle type 1B often present with conduction disease and associated arrhythmias, and variably with cardiomyopathy.596,597 The recommendations present guidance in the instances where the recommendations for cardiac pacing differ from those used for other patients with bradycardia.

Recommendations for pacing in hypertrophic obstructive cardiomyopathy

Recommendations Classa Levelb

AV sequential pacing with short AV delay may be considered in patients in SR who have other pacing or ICD indications if drug-refractory symptoms or baseline or provocable LV outflow tract gradients >_50 mmHg are present.576581,584 IIb B AV sequential pacing with short AV delay may be considered in selected adults with drug-refractory symptoms, >_50 mmHg baseline or provocable LV outflow tract gradient, in SR, who are unsuitable for or unwilling to consider other invasive septal reduction therapies.576581,584 IIb B AV sequential pacing with short AV delay may be considered in selected patients with drugrefractory symptoms, >_50 mmHg baseline or provocable LV outflow tract gradients, in SR, at high risk of developing AVB during septal ablation.585,586 IIb C

AV = atrioventricular; AVB = atrioventricular block; ICD = implantable cardioverter-defibrillator; LV = left ventricular; SR = sinus rhythm. aClass of recommendation. bLevel of evidence. Pacing parameters should be optimized to achieve maximum pre-excitation of the RV apex with minimal compromise of LV filling (typically achieved with a resting sensed AV interval of 100 ± 30 ms).587 ESC 2021

Recommendations for cardiac pacing in rare diseases

Recommendations Classa Levelb

In patients with neuromuscular diseases such as myotonic dystrophy type 1 and any second- or third-degree AVB or HV >_70 ms, with or without symptoms, permanent pacing is indicated.c 599602 I C In patients with neuromuscular disease such as myotonic dystrophy type 1 with PR >_240 ms or QRS duration >_120 ms, permanent pacemaker implantation may be considered.c 600,603,604 IIb C

AVB = atrioventricular block; CRT = cardiac resynchronization therapy; HV = Hisventricular interval; ICD = implantable cardioverter-defibrillator. aClass of recommendation. bLevel of evidence. cWhenever pacing is indicated in neuromuscular disease, CRT or an ICD should be considered according to relevant guidelines. ESC 2021

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