I do not know if "panic" is the right word: every time I squat, when my stomach get compressed (during the descendent phase), I feel a real uncomfortable feeling. Something like a small panic attack. However, I am not scared of weightlifting and I have this feeling even when I am lifting only 80% of my 5RM. I may be wrong but I have the impression my body is reacting to some kind of compression of the diaphragm. Have you ever experience something like that? If yes, do you know what may be the cause and how to solve such an annoying problem?
There is very likely something going on with your technique. Without more information for your specific problem, all I can do is provide a list of things to look at.
- How big is your gut? You can get too big to squat with proper form, and if you are very obese this can contribute to the problem. In this case losing weight would be the best course of action.
- Are you letting air out while you descend? In order to provide a good rigid platform to support the weight of the bar on your back, you need your diaphragm full of air. Breath into your spine and hold that breath until you are back at the top. Loosing air during the descent will compress your lungs even more adding to the problem.
- How strong is your upper back? If you are constantly folding over into a good morning, then you aren't keeping your chest up enough. That can be due to a weak upper back or shifting your weight on your toes.
- High bar or low bar? One of these will be more comfortable to you due to your physiology. High bar squats allow for a more upright torso, which is just more mechanically sound for keeping your breath. Low bar squats force your body forward more, which might contribute to your problem.
- How well do your brace for the squat? Your whole body should be tight as you descend and go back up. If you have a belt, you should fill that belt with your breath for additional stability. If not, brace your whole body with the same breath described in bullet 2.
- Is your gut hitting your legs? The squat should start with your feet about the same width as you would for a vertical jump. As you descend you need to push your knees out, which provides room for a larger gut and gets your adductors involved in the lift. If your knees come in over your feet on the way up that's fine. If your gut is hitting your legs even with this set up, then see bullet 1.
Hopefully one of these things will help point you in the right direction. While bullet 1 does apply to me, the other bullet points allow me to squat properly to depth without any breathing or panic symptoms.
Disclaimer: I am definitely not a doctor, and have no qualification whatsoever to dispense advice. Everything that follows is a combination of Wikipedia and guesswork.
The vagus nerve passes the diaphragm and in fact has branches attached to the surface of the stomach, among other places. Since the vagus nerve has a number of jobs including involvement in regulation of heart rate, it isn't out of the question that something about your technique or your individual physiology is causing some stimulation of this nerve, which you're feeling as a heart response.
If you're concerned, definitely speak to a professional.