Disturbance of consciousness in syncope is due to failure of blood supply to the brain, due in part to a fall in cardiac output. Cardiac output may also be less than normal if the rhythm of the heart is abnormal. Both very slow and very fast heart rates diminish cardiac output.
The distinction of a disturbance of consciousness due to an abnormality of cardiac rhythm from a seizure is not easy. Occasionally, though, a bystander will note that someone is pulseless or has a very irregular pulse during the attack, and sometimes the sufferer himself notices palpitations before disturbance of consciousness. Cardiac rhythm is easily monitored by electrocardiography. The changes in voltage associated with contraction of the different chambers of the heart are of sufficient amplitude that they can easily be recorded on a cassette recorder for periods of 24 hours, and their occurrence in relation to symptoms analysed. A cardiac cause for disturbance of consciousness has been found in up to one quarter of cases first presenting to neurological clinics with blackouts.