Timely detection of AF is of high clinical importance in patients at a high risk of stroke, who will benefit most from anticoagulation therapy. In addition, detection of AF in ICD patients may prevent worsening of heart failure and the occurrence of inappropriate shocks. Dual and triple chamber ICDs and recently single chamber ICDs are able to detect SCAF by detecting atrial high rates (AHRE) from the atrial electrogram or by RR interval based algorithms. A similar algorithm is in use in the subcutaneous ICD. By estimating the incidence of SCAF in a high-risk population of patients with CAD and a reduced LVEF, the INDICO AF study will advance our understanding of the value of detection of SCAF and its consequences. Subsequently, the INDICO AF study may provide a clinical justification for active monitoring for new onset (subclinical) AF.
Clinical implication of device-detected AF
Of all strokes, 25% are of unknown cause and undiagnosed AF may be the aetiological factor [12
]. Continuous monitoring of AF is superior to conventional follow-up visits or intermittent rhythm monitoring to identify AF after a cryptogenic stroke or detect AF recurrence after therapeutic interventions [9
]. In patients with a pacemaker, the incidence of SCAF is high and associated with an increased risk of stroke [14
]. A relation between the duration of device-detected AF and stroke was demonstrated in a subanalysis of the ASSERT trial, although the event rate was lower than in patients with overt AF [15
]. Not surprisingly, CIEDs with the ability to detect SCAF are of great interest to timely detect AF and prevent AF-related complications, as the population receiving ICD is generally older and has comorbidities increasing the risk for both AF and stroke. Indeed, the incidence of SCAF detected by an implantable loop recorder in patients without a history of AF but with a CHADS score ≥2 was 34% in 1 year [16
Whether or not the implications with regard to oral anticoagulation are the same in patients with device-detected AHRE is subject of two ongoing clinical trials [17
]. There is no consensus either on whether AHRE detected from an atrial electrogram should be valued the same as SCAF detection based on irregularity of the RR intervals, or that stroke risk is merely driven by the duration of SCAF episodes [15
]. Irrespective of whether anticoagulation should be started in all patients, establishing the diagnosis of AF is critical to determine increased stroke risk. This also applies for patients with an embolic stroke of undetermined source, as it was shown that starting rivaroxaban without documentation of AF in those patients was found to be futile compared with standard of care with aspirin [19
CIEDs with an atrial lead, such as conventional dual and triple chamber devices, detect SCAF by recording episodes of AHRE in combination with or without the use of pre-programmed algorithms. The incidence of AHRE is high in patients with CIEDs, and CIED-detected AHRE lasting longer than 6 min is indeed associated with an increased risk of stroke [20
]. However, it is worth noting that not all AHRE equal atrial arrhythmias or atrial fibrillation. In addition, the high rate of SCAF in patients detected by AHRE may be overestimated as the atrial lead needed for the detection of AHRE may cause transient atrial pro-arrhythmia [22
CIEDs without an atrial electrode such as implantable loop recorders, the VISIA AF™ and S‑ICD mostly use RR interval based algorithms for the detection of SCAF [6
]. The AF interval-based algorithm in the VISIA AF™ ICD is based on the same RR interval detection and P wave evidence algorithms that are integrated in implantable loop recorders for the detection of AF (e. g. Reveal XT, Reveal Linq) [7
]. The VISIA AF™ uses an intracardiac ventricular ECG instead of a subcutaneous ECG, and episodes of SCAF are defined as the result of three 2‑min blocks of AF instead of one (Fig. 2
). These extensions have resulted in an increased positive predictive value compared with strategies used in implantable loop recorders. Contrary to AF detection using atrial electrograms, the RR interval based algorithm is AF specific, and will thereby not identify atrial flutter or atrial tachycardia. High-risk patients with RR interval based implantable loop recorders show a substantial incidence of SCAF [10
], but data on the clinical importance of SCAF detected by single chamber ICDs are inconclusive. The INDICO AF study will not only investigate the incidence SCAF in high-risk patients with a single chamber ICD, but will also determine the burden of SCAF. The combination of these device-detected SCAF episodes and the documentation of clinical characteristics, clinical follow-up and plasma biomarkers for fibrosis, inflammation, oxidative stress and heart failure will be introduced in a decision model that may allow preterm identification of those patients who will develop AF or AF-related complications.
Limitations of device-detected SCAF
Advances in CIED technology have made it possible to continuously monitor SCAF in different patient populations, leading to an increasing number of patients diagnosed with SCAF. SCAF is associated with an increased risk of stroke; however, there are no guidelines on oral anticoagulant (OAC) therapy yet as data on the benefit of OAC in this population is lacking. Ongoing trials will evaluate the value of OAC in patients with detected devices [17
]. A subanalysis from the ASSERT trial showed that only patients with SCAF duration longer than 24 h significantly increase the risk of stroke [15
]. It is therefore debatable if—and if so when—SCAF truly represents clinical AF, and whether SCAF only mirrors the high-risk population. Future studies are needed to standardise the definition of SCAF and the considerations in therapy.