how come when a child takes a deep breath and hold it, she passes out and heart rate increases?

Question

It is said that this results from baroreceptor-mediated sympathetic activation causing heart rate to increase, but what does taking a deep breath have to do with blood pressure? Why is this increase in heart rate NOT due to chemoreceptors stimulated but BARORECEPTOR?

Answer 1

Because it has nothing to do with oxygen levels in your blood and everything to do with the preload going to the heart.

The child is performing a valsalva maneuver. By taking in a deep breath and holding, she is increasing the pressure within the chest. As a result, less blood flows back into the heart (less preload). Less preload--> less contractility, the less the heart is pumping forward, leading to a drop in blood pressure (it can be enough to make someone pass out). Because your blood pressure has been acutely dropped, the body's baroreceptors speed up the heart to help overcome the drop in pressure.

While the above poster is right about sinus arrhythmia and how you get small changes in heart rate with breathing, it is not enough typically to make someone pass out. You can breathe deeply all you want, your heart rate may change a little. It's when you hold the breath in and bear down that the preload drops enough to drop your blood pressure.

Answer 2

Sinus arrhythmia is a physiological heart rate change during breathing.

The rate of heart usually increases with inspiration and decreases with expiration.

This rhythm is most commonly seen with breathing due to fluctuations in parasympathetic vagal tone. During inspiration stretch receptors in the lungs stimulate the cardioinhibitory centers in the medulla via fibers in the vagus nerve.