Abstracts

Rationale: Blackouts, better termed Transient Loss of Consciousness (TLoC) can be due to syncope, epilepsy or psychogenic causes (Diagnosis and management of patients with blackouts. Fitzpatrick AP, Cooper PN. Heart 2006; 92: 559-568) Most patients with TLoC are unable to describe their blackout, and often no collateral history is available from a witness. Investigations may be unhelpful. Implantable ECG monitoring, including the use of devices such as Reveal allows recording throughout a TLoC, but despite high rates of symptom/ECG correlation, the plain ECG may often not establish the diagnosis. Our hypothesis was that analysis of sympathovagal balance would define distinct 'signatures' which may help define the cause of TLoC.Methods: 20 patients with TLoC underwent prolonged EEG recording; including cases of reflex syncope, epileptic and non-epileptic seizures, and arrhythmias. Beat-to-beat RR intervals were extracted from the ECG signals. Marginal RR intervals (reflecting cardiac hyper-excitability) were analyzed using a statistical assessment of the percentage of RR values outside a confidence interval (window size of 40 beats). Heart rate variability (HRV) was assessed with a parametric spectral estimation using a sliding analysis window of 60-240s, shifted with 5-20s increments (Virag N, Sutton R, Vetter R, Markowitz T, Erickson M. Prediction of vasovagal syncope from heart rate and blood pressure trend and variability: Experience in 1,155 patients. Heart Rhythm 2007; 4:1375-1382). The sympatho-vagal balance was computed as the ratio of the low frequency components (LF: 0.04 Hz-0.15Hz, mainly sympathetic activity) versus the high frequency components (HF: 0.15-0.4 Hz, parasympathetic activity). Results: Under normal conditions the amount of marginal RR intervals is very low, and prior to vasovagal syncope remains less than 4%. Much larger values are found prior to epileptic seizures. Events involving tachycardia tend to show changes in HRV (mostly sympathetic) but a low marginality, whereas events involving an irregular rhythm tend to have a high marginality with irregular HRV patterns (high sympathetic and parasympathetic). Patients with non-epileptic attacks show high marginality and high sympathetic and parasympathetic activation, with a background high sympatho-vagal balance. Conclusions: The pattern of sympatho-vagal balance observed during reflex syncope, in tachycardia, and in other arrhythmias is very different from that seen prior to epileptic seizures. Distinct patterns may also help distinguish epilepsy from psychogenic non-epileptic seizures, but this requires clarification on a larger group of cases. We believe that analysis of sympatho-vagal balance may therefore provide a novel means to help determine the cause of an undiagnosed blackout (TLoC).