• Welcome to the DeeperBlue.com Forums, the largest online community dedicated to Freediving, Scuba Diving and Spearfishing. To gain full access to the DeeperBlue.com Forums you must register for a free account. As a registered member you will be able to:

    • Join over 44,280+ fellow diving enthusiasts from around the world on this forum
    • Participate in and browse from over 516,210+ posts.
    • Communicate privately with other divers from around the world.
    • Post your own photos or view from 7,441+ user submitted images.
    • All this and much more...

    You can gain access to all this absolutely free when you register for an account, so sign up today!

Carotid Sinus - Bradycardia

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.


Working Class Spearo
Mar 17, 2002
was answering another post and later took out one of my old Masterdiver manuals and read the section on Carotid Sinus Refex. It states as follows:

"The principal arteries supplying blood to your brain, known as the Carotid Arteries, have a sinus at a bifurcation of the artery in the neck. The purpose of the carotid sinus is to control your heart rate according to your blood pressure...

...Pressure applied to the carotid sinuses produces bradycardia..."

taken from Advanced Diving: Technology and Techniques

Is it just me or does this have applications for apnea?

fainting spell?

Well, yes, if your suit is too tight, you could produce a bradycardia.

However, since your force it, and your body isn't prepared to it, it could cause fainting, and I don't believe we want that during our dive...

It is better to get bradycardia by being relaxed than by forcing it...

K. Feenstra
Last edited:
Absolute versus relative pressure

Whenever you get in the water - you apply greater absolute pressure to the carotid sinus, but relative pressure remains the same (after a period of adaptation). The question you raise needs to be put in perspective. If you apply pressure to this node whilst on land, and get a drop in heart rate, that drop will be transient as the blood pressure will drop soon afterwards, thereby increasing the heart rate again.

One of the major problems when considering the effects of pressure are teasing apart the differences between reltive pressure changes and absolute pressure changes. To illustrate:

Say the internal pressure of a box is 1 atm in air. the internal pressure is equlivalent to the external pressure. Take that box down to 10m and the external pressure is 2 atm. Now-

if the box is incompressible the internal pressure is still 1 atm of absolute pressure; thereby having a huge pressure differential between inside and outside.

if the box was 100% compressible then the box would compress to half its original size and the air inside is 2 atm - which is higher absolute pressure, but the pressure differential is zero again. The same as it was at the surface.

the third scenario is what happens when the box is 50% compressible. Then at 10m the intenal pressure is say - 1.5 Atm absolute, but there is a 0.5 atm pressure differential.

What relevance is this?

The human body is full of 'components' that have differring compresibilities, and some 'components' compressibility also alters with absolute and differential pressure. This problem is one of the greatest causes of error in diving theories.

Back to your original point: Your carotid sinus will react to both absolute and relative pressure changes. You need to consider carefully how it will react under different conditions.

When you hold your breath with full lungs you immediately bump up the blood pressure to the brain, because the heart is in the thoracic cavity as is put under pressure. The overall is effect is a transient bradycardia. This increased thoracic pressure also immediately lowers the flow of blood into the heart, which further decreases heart rate. The body then starts to equilibrate, and the heart rate goes back up, but to a level which was slightly slower than the original heart rate.

Give it a go. Get your pulse steady with shallow breathing. Take a big breath and hold it. You don't need packing. Your heart will drop instantaneously. Then pick up over 10 seconds or so to a rate slightly slower than original.

This is all changes in relative pressure - include absolute pressure changes in a dive, and the effects are slightly different, because that high pressure in the thoracic cavity is gone after a few kicks away from the surface.
DeeperBlue.com - The Worlds Largest Community Dedicated To Freediving, Scuba Diving and Spearfishing


ISSN 1469-865X | Copyright © 1996 - 2024 deeperblue.net limited.

DeeperBlue.com is the World's Largest Community dedicated to Freediving, Scuba Diving, Ocean Advocacy and Diving Travel.

We've been dedicated to bringing you the freshest news, features and discussions from around the underwater world since 1996.