Baroreflex sensitivity at rest and during stress in individuals with a history of vasovagal syncope.

The baroreflex sensitivity of healthy young adult males who reported some history of vasovagal syncope during emotionally stressful situations (e.g. blood donation) was compared to a control group of individuals who reported no such history. The baroreflex, which induces compensatory bradycardia and vasodilation in response to acute elevations of blood pressure, was examined as a possible mechanism underlying predisposition to vasovagal reactions. Changes in heart rate and vasoconstriction in response to baroreflex stimulation (negative external cervical pressure) at rest and during administration of a mental arithmetic task and a constrictive pain stimulus were assessed. Individuals with a history of vasovagal reactions displayed greater baroreflex sensitivity during the pain stimulus and at rest, but not during mental arithmetic. These findings suggest one mechanism of risk for syncope reactions, particularly in situations involving the experience of pain.