As you breath more slowly, heart rate oscillations (i.e. the variability) increase, which is a reflection of the parasympathetic nervous system activating (the "rest and digest" response). This can help to mediate the stress response, which from the sympathetic nervous system or the fight-or-flight response.

It is interesting that I, and many others, experience our digestive cycle as anxiety.

At certain points in my digestion, my HR spikes, anxiety spikes, impending doom sensations occur.

These are also sensations that happen with pre-stroke SNS responses.

Rest and digest, indeed!

I get something similar in that I start to feel a tiredness come on about 2-5 mins before I start to get gas pains. If I don't help that gas along (by moving or pushing on my stomach) in short order, it turns into me needing to run for the toilet. It's as if the trapped gas, left untreated, triggers a rapid digestive emptying. Or, the cases where I can receive the gas, there's no other contents in the way. The cases where I can't relieve it, well, there's lots of stuff in the way.

Anyways the thing is that I get super tired before the pain in my stomach is even noticed. I've become attuned to it where it now gives me anxiety when it comes on.

Your hyper sensitivity to the gas’s might actually cause the stress, which in turn causes more symptoms. Find a way to relax and let the gas be. Find what works for you: mindfulness, sports, hypnosis, medication… However, try first to focus on normalising your perception of the gas.

This is called IBS, and is caused by bacteria digesting certain carbs very rapidly. Identify the foods with a low Fodmap diet, figure out the quantity that doesnt trigger issues, and live mostly normally. I cant eat fructose at all, and only small amounts of garlic or onions.

I think the "hypersensitivity" line is BS, or at least it's not a complete explanation. In my and others' cases, the "psychiatric" GI symptoms precede the physiological ones. The only time I ever got a straight answer is when I ended up at the hospital because I was passing out, and they said it was likely some combination of low blood pressure and vagus nerve overstimulation.

Hmm interesting! Maybe I can try measuring my blood pressure during these times of tiredness.

It is well established that vagus disruption during digestion has all sorts of side effects.

Including anxiety.

Right, point being that's not a psychological hypersensitivity, it's a lower level subconscious physiological process.

Could this be some kind of food intolerance?

Sounds like it. I found I was gluten and dairy intolerant and removing those foods helped.

I don't interpret that heart rate increase as anxiety, but as the extra workload of digesting food.

It isn't about assigning meaning, it's about your nervous system having an involuntary anxiety reaction to various physiological changes during food digestion.

If you insist. I'm saying I don't have any of the other indicators mentioned -- just the elevated heart rate.

This phenomenon is known as “respiratory sinus arrhythmia” (RSA).

The term is “cardio respiratory coupling”. Basically inspiration increases intrathoracic pressure which triggers the CV control mechanism to think BP is higher via the baroreflex and therefore lower HR. If you take an FFT/PSD of pulse waveforms you will actually see a distinct peak at the breathing frequency. This is how a lot of the finger sleep measurement devices work to track breathing without a chest strap.

This is awesome. I love when people can find a signal in ostensibly disconnected data. This finally answers the nagging question I've had of how my watch can measure my rate of breathing. Thanks for sharing!

> inspiration increases intrathoracic pressure which triggers the CV control mechanism to think BP is higher via the baroreflex and therefore lower HR

But HR is higher during inspiration.

> inspiration increases intrathoracic pressure which triggers the CV control mechanism to think BP is higher via the baroreflex and therefore lower HR

It seems CRC raises HR during inspiration.

From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4052709/ "The increase in HR during inspiration is referred to as respiratory sinus arrhythmia (RSA) and the increase in BP as Traube–Hering waves."