Adenosine administered intravenously slows heart rate, and over the last decade, the drug has been used in many centres in the diagnosis of unexplained syncope (blackouts). However, there is no clear consensus on which underlying disorder is being uncovered. Various researchers propose heart conducting tissue disease (sick sinus syndrome [SSS] and [AVB]) and neurally mediated syncope (carotid sinus syndrome [CSS] and vasovagal syncope [VVS - an exaggerated version of the common faint]) as the conditions diagnosed by adenosine. We propose the first ever study to tackle these issues comprehensively and systematically. The principal research objective will be to compare the heart rate response to adenosine with the underlying diagnosis in patients with SSS, AVB, CSS and VVS and healthy controls.