Re: pulse in carotid artery

Dear Rhonda, 

many thanks for your question regarding the carotid pulse. Indeed, if you
feel a carotid pulse, you should do it on one side only.

There are two reasons to be careful. The first reason goes – basically –
for all people, the other one is important if you feel the pulse in people
(usually the elderly) who might have arteriosclerosis of the carotid arteries. 

Reason no. 1: The main carotid artery branches into two large arteries just
below the place where the jawbone joins the rest of the skull (the external
and internal carotid artery; the former is feeding the face and other
outlying portions of the skull, while the internal artery is entirely
reserved for feeding the brain). At this point, there is a widening of the
artery, the so-called carotid bulb (bulb is from Latin „bulbus“, meaning
onion). This bulb contains a nervous organ, the glomus caroticum (carotid
„clew“ or carotid body – a fuzzy patch of nerve tissue inside the artery
wall). This organ is a kind of sensor, measuring, among other things,
oxygen content of the blood and blood pressure. Now if you put pressure on
these organs on both sides, they will report a much, much elevated blood
pressure to the brain, and the brain will act accordingly: Namely by
slowing down the heartbeat and open up peripheral blood vessels to lower
blood pressure – and since the blood pressure is in reality not elevated
but normal, you will get a severe lowering of blood pressure. Perhaps, if
the reaction is vigorous, there might even be a cardiac arrest for a few
seconds. And, oops, there goes your victim fainting and falling to the
ground. Unpleasant. Although the reaction can occur in anybody, elderly
people are especially at risk – Tight collars or neckties have been known
to cause this kind of fainting („syncope“ in the medical language) in
sensitive people, and in these even exerting pressure on only one of the
two sinus can cause a severe reaction – the condition is called
hypersensitive carotid sinus or carotid sinus syndrome.

By the way, rubbing one (one! carefully!) of the carotid sinus can cause a
beneficial reaction, too: In people with certain disturbances of the heart
rhythm where the heart beats very fast, carotid sinus massage can stop the
arrhythmia entirely. I did this once in my hospital’s emergency room, and,
wow, it saved my day: There was this patient sweating and puffing, having
an absolutely racing heartbeat. I touched him once by the neck, and – bang
– there he was in a normal rhythm, breathing easily, suddenly feeling
perfectly well. He looked as if I had worked some magic touch on him, when
it was really just a bit of physiology...

But sorry for bragging – here’s your  reason No. 2: You probably know that
arteriosclerosis of the neck arteries can cause strokes, by narrowing the
arteries and thus cutting off blood supply to the brain. Now the good thing
about the brain’s blood supply is that it is very flexible: There is one
(smaller) artery getting to the brain through the spinal column, and there
are the two larger internal carotid arteries (plus a very small amount of
blood bridging over from the external). They all meet at the base of the
brain in a kind of roundabout, the circle of Willis. This roundabout makes
sure that even if the blood flow one or two of the arteries is cut off, the
third one can usually (but barely) do the job. So in a young and healthy
person, even cutting off the flow from both carotid arteries will not
always result in a stroke or brain damage (DO! NOT! TRY! THIS!). But
imagine someone with a reduced blood supply to the brain, i.e. an elderly
person with arteriosclerosis: Even exerting mild pressure to the carotid
(when feeling the pulse with both hands) might shut off the little blood
supply there is and your victim might suffer a transient paralysis in the
limb, unpleasant tickling or numbness, impaired vision  or speech
disturbances – and if it is a really bad day, even a permanent stroke
(rare, but again you do not want to try!).

In this second case, you can even do damage with only one hand – probably
not by just feeling a pulse, but by carotid sinus massage – especially if
you massage the „better“ of the two carotid arteries. If the other one
happens to be completely obstructed by arteriosclerosis, supplies can run
very low very fast. This occasionally in clinical practice: Carotid massage
can be applied to test for the presence of the hypersensitive carotid sinus
syndrome (see reason no. 1): If someone’s blood pressure goes down heavily
upon massage, that might explain why this someone has been having a lot of
falls recently . And there is a study showing that transient disturbances
of the brain aren’t all that rare after this kind of maneuver.

So, to summarize – you will probably not hurt yourself or your schoolmates
by just feeling a pulse on both sides, but you will have a lot fewer nasty
surprises if you do it on one side only. And with elderly, frail people,
take special care (as always!). By the way, in most people it is possible
to feel the pulse by a very light touch to the neck, if you know where to
put your hands. With this kind of approach, you will not hurt anybody, ever. 

So I hope this helps – 
Yours truly
Jens Peter Bork

References: 
 A description of carotid sinus massage: http://www.chclibrary.org/micromed/00041610.html
 A short, technical description of carotid sinus syndrome: http://www.5mcc.com/Assets/SUMMARY/TP0163.html
 A study showing that carotid sinus massage is useful for detecting people
with hypersensitive carotid sinus: http://ageing.oupjournals.org/cgi/content/abstract/32/6/666
 The study showing that neurological damage can occur after carotid sinus
massage:  http://ageing.oupjournals.org/cgi/content/abstract/29/5/413
 A nice depiction of the circle of Willis, plus an article about what
happens in a stroke: http://www.umm.edu/patiented/articles/what_a_stroke_000045_1.htm
 And finally, a picture about the reflexes that steer the reaction of the
carotid sinus plus a picture of the sinus itself: http://cmbi.bjmu.edu.cn/hyper-book/ch02/ch02-8.htm