Device-detected atrial tachyarrhythmias: what do we know?

This editorial refers to ‘Detection of atrial high-rate events by continuous Home Monitoring: clinical significance in the heart failure–cardiac resynchronization therapy population’ by N. Shanmugam et al., on page 230 Depending on one's perspective, modern pacemakers and defibrillators provide physicians with a wealth or burden of diagnostic information. Currently, there is great interest in the ability of these devices to monitor thoracic impedance, ST-segments, and other physiological parameters.1,2 However, the ability to document and characterize otherwise undetected atrial tachyarrhythmias, referred to as atrial high-rate episodes (AHRE), has been available for >10 years.3 Despite this familiarity with AHRE, there remains a formidable knowledge gap which stands between physicians and the optimal use of these data for patient care. As paroxysmal atrial fibrillation appears to pose the same risk of stroke as sustained episodes,4 it is tempting to simply assume that AHRE should be managed like conventionally diagnosed atrial fibrillation. However, these otherwise silent atrial tachyarrhythmias may have a significantly different prognosis and the impact of prophylactic oral anticoagulation may be quite different in this population. The ASSERT (A Symptomatic Stroke and atrial fibrillation Evaluation in pacemakeR patients Trial) study did suggest that the absolute CHADS2-specific stroke rates for patients with AHRE are lower than for conventional diagnosed atrial fibrillation, although important differences exist between study populations.5,6 At present, <25% of patients with isolated AHRE and additional risk factors for stroke receive treatment with oral anticoagulants,7 suggesting that physicians are seeking a better understanding of the risk, before routinely advocating treatment. In their landmark 2003 publication from the MOde Selection Trial (MOST), Glotzer et al.3 observed that AHRE > 5 min in duration were detected in 51% of patients during a mean of 27 months following implantation of a dual-chamber pacemaker for sinus node dysfunction. The presence of these AHRE was associated with a 2.5-fold increase in the risk of stroke or embolism.3 However, this was a retrospective analysis, including only 312 patients and more than one-third of those with AHRE already had a clinical diagnosis of atrial fibrillation.3 Next, the TRENDS (The Relationship Between Daily Atrial Tachyarrhythmia Burden From Implantable Device Diagnostics and Stroke Risks Study) enrolled 2486 patients, of whom 24% developed AHRE over a mean follow-up of 1.4 years.8 Patients with maximum daily burden of >5.5 h during the preceding 30 days had a hazard ratio of 2.2 for the development of stroke or transient ischaemic attack.8 Although TRENDS prospectively validated the hypothesis from MOST in a much larger cohort, it was still limited by a relatively short follow-up and a small number of outcome events. Finally, the ASSERT trial prospectively followed 2580 patients over the age of 65 years who had a history of hypertension and received a dual-chamber pacemaker.5,9 In this study, which excluded all patients with a history of atrial tachyarrhythmias, the presence of any AHRE longer than 6 min in duration within 3 months of enrolment was associated with a 2.5-fold increase in the risk of stroke or embolism, which was independent of other clinical predictors of stroke. As well, this effect persisted after censoring patients who developed clinical atrial fibrillation. Together, the data from MOST, TRENDS, and ASSERT strongly support a link between AHRE detected only by a pacemaker and the occurrence of stroke or systemic embolism. However, the temporal association between AHRE and stroke is unknown and it remains unclear as to whether there is a minimum burden of AHRE which is necessary to confer this increased in risk. The current study from Shanmugam et al.10 helps further our understanding of the relationship between AHRE and stroke. In this trial, 560 patients with cardiac resynchronization devices were prospectively followed for a mean of 370 days, using remote monitoring technology. The study included patients with and without a prior history of atrial fibrillation. The investigators did not adjudicate individual episodes of AHRE, but defined them as significant if they were documented for at least 1% of any day (14 min). Thrombo-embolic complications developed in 2% of patients overall and were nine-times more likely to develop among patients who had at least 3.8 h of AHRE detected during any day. However, patients with AHRE, but not >3.8 h per day, still had a trend towards an increased risk of thrombo-embolism, with a hazard ratio of 4.3, P= 0.11. Despite showing a convincing association between AHRE and thrombo-embolic events, this study did not demonstrate a strong temporal relationship between the two. The median interval between the most recent prior AHRE and the thrombo-embolic complication was 47 days and only 27% of patients with significant AHRE who suffered a thrombo-embolic complication were in atrial fibrillation at the time of that event. This is in keeping with the recently published report from the TRENDS trial which found that in patients with AHRE and a thrombo-embolic event, only 30% were in atrial fibrillation at the time of the event, and in the remaining patients, the most recent AHRE was an average of 168 ± 199 days earlier. Together, these two studies challenge our traditional understanding of the relationship between atrial fibrillation and stroke. Despite the existing data, including this report,10 physicians still face challenges when confronted with a patient experiencing AHRE. Although there appears to be a robust link between AHRE and stroke, given the risks and inconvenience of oral anticoagulation,11 more data are needed before routine treatment with oral anticoagulation can be recommended. The results of the present study10 and the ASSERT trial5 raise doubts as to whether a critical AHRE burden threshold exists and suggesting instead that traditional clinical risk factors may be more important.12 Although the recent introduction of direct thrombin inhibitors13 and factor Xa inhibitors14 may lead to an increased rate of treatment of AHRE, randomized trials of anticoagulation in this population are still desired.15 Not only will such studies help define clinical practice, but they will help to further elucidate the precise relationship between atrial tachyarrhythmias and stroke. Conflict of interest: J.H. has received research grants from St Jude Medical and Boston Scientific.

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