By John Camm
Atrial fibrillation (AF) is associated with substantial morbidity, which primarily involves troublesome symptoms, heart failure, and thromboembolic events.
Results of any therapy applied are usually judged by the absence of clinical symptoms and documentation of sinus rhythm on available electrocardiogram (ECG) recordings. Routine office ECGs, occasional 24-hour ambulatory ECG and longer term ECG event recording are the standard monitoring strategies.
Whether such surveillance is sufficient to gain reliable information concerning the suppression or cure of atrial fibrillation is debated. However, when AF is deemed to have been completely suppressed or cured, physicians are tempted to stop anticoagulation in an effort to avoid the unnecessary risk of bleeding. The concern is that asymptomatic and/or undetected recurrences of AF could expose patients to increased risk of ischaemic stroke and thromboembolic events.
Implantable loop recorders are largely accepted in clinical practice for diagnosing patients with unexplained syncope (Europace 2009; 11:671–687. EHRA Position Paper). The same technology, implemented with specific software for automatic AF detection, is now available to monitor symptomatic and asymptomatic episodes of atrial fibrillation.
The XPECT study (Reveal XT Performance Trial) is a prospective, multicentre trial, designed to evaluate the performance of Reveal XT, the first MRI approved subcutaneous, leadless, insertable cardiac monitor (ICM) featuring an automatic AF detection algorithm. The first patient was enrolled in September 2007 and the last patient enrolled in July 2008. The first data were presented by Professor Hindricks (Heart Center Leipzig, Germany) at the HRS congress 2009 in Boston, US.
Patients who were likely to present paroxysmal AF were selected. A dedicated external Holter device stored a 46 hour ECG recording of a signal that was provided by the Reveal XT with related markers and two surface ECG leads. The surface ECG recordings were reviewed by a core-lab, against which the automatic arrhythmia classification of the Reveal XT was evaluated.
XPECT Study – The facts
- Analysable Holter recordings: 206 collected from 247 patients.
- 71 contained core-lab-classified AF with an AF burden >1% (i.e. >14.5 min per 24h).
- In all recordings, AF was detected by the ICM (sensitivity=100%).
- There were 76 patients with at least one AF episode and the overall sensitivity to identify patients with AF was 96.1%.
- The automatic algorithm of the device was also effective in correctly classifying patients without AF, with a Negative Predictive Value (NPV)=97.4%.
The major findings of this study are that Reveal XT equipped with a dedicated AF detection algorithm is not only sensitive for the detection of AF episodes, but also accurate for the measurement of AF burden and above all highly reliable for the exclusion of the presence of AF.
The XPECT study evaluated the performance of Reveal XT for automatic AF detection in a technical sense. It demonstrated that subcutaneous monitoring of AF is feasible and can be accurately performed. The positive results of this trial may have relevant clinical implications.
Continuous monitoring of AF as a superior method
The low reliability of symptoms to detect AF and measure its burden is well known and has been assessed in patients with and without implanted devices: the perception of AF may markedly differ among patients and many patients can perceive AF-like symptoms in the absence of arrhythmia.
Given the unpredictable nature of the episodes continuous monitoring of AF is likely to be superior to any other intermittent method based on the ECG or symptoms. The low reliability of symptoms and intermittent monitoring to detect AF makes any decision about stopping/starting anticoagulation unsafe for the patient.
Inability to detect AF episodes and low reliability to exclude AF, may lead to incorrect conclusions regarding the efficacy of the rhythm control strategy and, consequently, to incorrect decisions regarding the administration of the optimal antithrombotic therapy. Thus, a careful and reliable assessment of AF presence/absence, burden and episode duration would considerably benefit any primary or secondary stroke prevention strategy. Continuous monitoring can play an important role even after the discontinuation of anticoagulation therapy in confirmed AF free patients, as anticoagulation can be restarted as soon as clinically relevant relapses are detected, independently of symptoms.
Continuous AF monitoring may also play a role in some patients without a history of AF, such as patients with cryptogenic stroke. Recent scientific reports demonstrated the clinical relevance of more intensive AF monitoring in this patient population immediately after the event (Neurology 2008;71:1696–170. J Stroke Cerebrovasc Dis 2009; 18(3):185-189). Each time AF is detected, the patient can immediately start the appropriate anticoagulation therapy with the objective of preventing a deleterious stroke recurrence. The high sensitivity for identifying patients with AF (96.1%) makes ICMs suitable for clinical practice because the physician can rely on the ability of the device to detect AF episodes. On the other hand, the high NPV=97.4%, which allows patients without AF to be identified, is also highly relevant in clinical practice, because the physician can rely on the device. When no AF episode has been stored the patient is really an AF-free patient.
A detailed analysis of all stored ECGs to verify correct arrhythmia classification can be made by the physician during a follow-up visit or by means of a tele-monitoring system that goes along with the implantable device (e.g. CareLink, Medtronic).
Professor John Camm is head of the Department of Cardiac and Vascular Sciences and chairman of the Discipline of Cardiology at St George’s, University of London, UK.