What is my heart up to?

September 25, 2006 4:37 AM

Can anyone explain what happens to my heart rate when I visit the facilities?

Having been given a heart rate monitor for my birthday, I've started wearing it for most of the day out of interest. But I've noticed a strange pattern when using the facilities. When engaged in producing a 'number 2', particularly if it is unusually large or solid, my normal heart rate of 70-80bpm drops by 10-20 bpm until the splash, at which point it shoots up to >100bpm for a few seconds, and then back to the normal resting rate. This was noticed during a highly unscientific process of glancing at the watch part every few seconds or so with no recording at all. But it seems to be a reproducible pattern. What's going on?

posted by talitha to health & fitness (5 comments total)

You're inadvertently performing a "Valsalva maneuver" when your bear down to squeeze it out. In rare cases, this pressure drop can trigger a stroke; repeated performance may weaken the large-intestinal wall or the inguinal tissues (this is thought to have killed Elvis Presley).

The Valsalva maneuver works by stimulating the vagus nerve, evolutionarily the most ancient nerve and a major component of the parasympathetic nervous system.
posted by orthogonality at 5:11 AM on September 25, 2006

To expand on orthogonality's answer, not only is your Valsalva maneuver increasing the activity of your vagus nerve, but the stretching of your anal sphincter can also increase vagal activity, so there are two relatively significant stimuli that are triggering a decrease in heart rate and blood pressure. This can progress all the way to loss of consciousness which is known as defecation syncope. Other stimuli that can trigger a vasovagal reaction include pressure on the eyeball (the oculocardiac reflex), stretching or pressure on the abdomen, compression of the carotid bodies in the neck (part of the mechanism of action in a choke hold), and stimulation of the structures of the voice box as when inserting an endotracheal tube. Other fun facts about the vagus nerve include that it is the predominant nerve controlling the heart rate (as eveidenced by the fact that a normal resting heart rate is about 70, but a transplanted heart that has had its nerve connections severed beats at 100 beats per minute) and that it increases gastric acid secretion (severing the vagus branches to the stomach was a fairly common treatment for ulcers in the past).

As to the second part of your question, when your heart rate and blood pressure drop, that stimulates your symapthetic nervous sytem into action, causing the adrenal glands and sympathetic nerves throughtout the body to release epinephrine and norepinephrine to rasie your heart rate and blod pressure. For a few seconds as you describe the sytem may overcompensate before settling back down to normal.
posted by TedW at 5:45 AM on September 25, 2006 [1 favorite]

Wow - the body is a wonderful thing. I wonder how many other interesting tricks it does.
posted by talitha at 6:07 AM on September 25, 2006

Many!
posted by flabdablet at 6:39 AM on September 25, 2006

As an addendum: this AskMe is closely related.
posted by TedW at 7:25 AM on September 25, 2006