Vasovagal Reaction

By Danielle Scheer

In the summer of 2008 I was diagnosed with neurocardiogenic syncope, a condition
described as being “a temporary loss of consciousness associated with a drop in
arterial blood pressure, quickly followed by a slowed heart rate (1). I wanted to know
the physics of what is going on:

Because we are considering a fluid flow, and assuming non-viscous liquid:

Q= πR4 (P1-P2)/8ηL
In a vasovagal response, a person’s veins dilate when they should constrict, so
assuming Q stays constant, ΔP will decrease as radius increases.

P1 + ρgy1 + ½ ρv12 = P2 + ρgy2 + ½ ρv22

If the pressure is decreased, the blood won’t be able to get as high up, which is why
people with vasovagal conditions lose consciousness (lack of blood and oxygen to
the brain).


(1) Grubb, B.P. & McMann, M.C. (2001) The fainting phernomenon: Understanding why
people faint and what can be done about it. New York: Futura Publishing Company,
Inc. p. 133