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.
