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improved patient selection has shown S-ICDs are non-inferior to transvenous ICDs in this regard.199202 They may be the preferred option for patients with difficult venous access or those who require ICD explantation due to infection. Patients must be carefully selected, as S-ICDs cannot treat bradyarrhythmia (except post-shock pacing) and cannot deliver either anti-tachycardia pacing or CRT. Substantial RCTs with these devices and more long-term data on safety and efficacy are awaited. A wearable cardioverter-defibrillator that is able to recognize and treat ventricular arrhythmias may be considered for a limited period of time in selected patients with HF who are at high risk for sudden death but otherwise are not suitable for ICD implantation.162,175,176,203 However, the large VEST trial failed to show that the wearable cardioverter-defibrillator reduced arrhythmic death in patients with an LVEF <_35% following a recent acute MI.204 For more detailed recommendations on the use/indications of ICD we refer the reader to the ESC/European Heart Rhythm Association (EHRA) Guidelines on ventricular tachyarrhythmias and sudden cardiac death.201

6.2 Cardiac resynchronization therapy

In appropriately selected individuals, CRT reduces morbidity and mortality.211 Furthermore, CRT improves cardiac function, and enhances QOL.209,225

Whilst the CARE-HF206,208 and COMPANION210 trials compared the effect of CRT with medical therapy (MT), the majority of CRT studies have compared CRT-D with ICD, and a few have compared CRT-P with backup pacing. The prevention of fatal bradycardia might be an important mechanism of benefit shared by all pacing devices. In CARE-HF, at baseline, 25% of patients had a resting heart rate of <_60 b.p.m.206,208,209 If prevention of bradycardia is important, the effect of CRT will appear greater in trials where there is no device in the control group. However, in MADIT-II, 35% of those who died with an ICD did so suddenly even though they were protected from both brady- and tachyarrhythmia.226 Most studies of CRT have specified that the LVEF should be <_35%, but RAFT212 and MADIT-CRT213,214 specified an LVEF <_30%, while REVERSE207,215,227 specified <_40% and BLOCK-HF216 <_50%. Relatively few patients with an LVEF of 3540% have been randomized, but an IPD meta-analysis suggests no diminution of the effect of CRT in this group.211 Assessing the ‘response’ to CRT is challenging. Indeed, many who do not appear to ‘respond’ favourably in terms of their symptoms or LV function may well have experienced the mortality benefit. Several characteristics predict improvement in morbidity and mortality. The extent of reverse remodelling is one of the most important mechanisms of action of CRT. Patients with HFrEF of an ischaemic aetiology have less improvement in LV function due to myocardial scar tissue, which is less likely to undergo favourable remodelling.228 Conversely, women may be more likely to respond than men, possibly due to smaller body and heart size.220,224,229 QRS width predicts CRT response and was the inclusion criterion in all randomized trials,211 but QRS morphology has also been related to a beneficial response to CRT. Several studies have shown that patients with left bundle branch block (LBBB) morphology are more likely to respond favourably to CRT, whereas there is less certainty about patients with non- LBBB morphology.230 This latter group is also underrepresented in the large CRT trials.206,210,213 However, patients with LBBB

Recommendations for cardiac resynchronization therapy implantation in patients with heart failure

Recommendations Classa Levelb

CRT is recommended for symptomatic patients with HF in SR with a QRS duration >_150 ms and LBBB QRS morphology and with LVEF <_35% despite OMT in order to improve symptoms and reduce morbidity and mortality.205215 I A CRT rather than RV pacing is recommended for patients with HFrEF regardless of NYHA class or QRS width who have an indication for ventricular pacing for high degree AV block in order to reduce morbidity. This includes patients with AF.216219 I A CRT should be considered for symptomatic patients with HF in SR with a QRS duration >_150 ms and non-LBBB QRS morphology and with LVEF <_35% despite OMT in order to improve symptoms and reduce morbidity and mortality.205215 IIa B CRT should be considered for symptomatic patients with HF in SR with a QRS duration of 130149 ms and LBBB QRS morphology and with LVEF <_35% despite OMT in order to improve symptoms and reduce morbidity and mortality.211,220 IIa B Patients with an LVEF <_35% who have received a conventional pacemaker or an ICD and subsequently develop worsening HF despite OMT and who have a significant proportion of RV pacing should be considered for ‘upgrade’ to CRT.221 IIa B

Continued CRT may be considered for symptomatic

patients with HF in SR with a QRS duration of 130149 ms and non-LBBB QRS morphology and with LVEF <_35% despite OMT in order to improve symptoms and reduce morbidity and mortality.208,213 IIb B CRT is not recommended in patients with a QRS duration <130 ms who do not have an indication for pacing due to high degree AV block.222224 III A

AF = atrial fibrillation; AV = atrio-ventricular; CRT = cardiac resynchronization therapy; HF = heart failure; HFrEF = heart failure with reduced ejection fraction; ICD = implantable cardioverter-defibrillator; LBBB = left bundle branch block; LVEF = left ventricular ejection fraction; NYHA = New York Heart Association; OMT = optimal medical therapy (class I recommended medical therapies for at least 3 months); QRS = Q, R, and S waves of an ECG; RV = right ventricular; SR = sinus rhythm. aClass of recommendation. bLevel of evidence. ESC 2021

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