If at any point during monitoring you experience consistent chest pain, and/or an inability to catch your breath, go to the hospital ASAP, those are both signs of severe cardiac compromise and the need for immediate medical attention.
A cheap, easy, and simple way to monitor yourself for potential myocarditis. A finger digital pulse-oximeter! You can get them for as little as $10, and they are foolproof. They all typically display two things, your pulse rate, and your oxygen saturation.
Average typical pulse rate for most people is right around 70-80. Yours could be much lower, it depends on how much of an athlete you are. If you are super-athletic, you could even fall into what is generally considered the bradycardic range, which is a pulse rate less than 60. That is nothing to be concerned about. A pulse rate of less than 60 being bradycardia is only for normies. If you are a normie and your pulse rate is less than 60, consider talking to your primary care provider about a referral to a cardiologist, since that is not normal. Some people have a typical pulse rate around 90-100. Again, nothing to be concerned about. If you pulse rate exceeds 100 it is considered tachycardia. If your pulse rate exceeds 100, at rest, and you aren't consuming caffeine, or other stimulants or stimulant medication, again, consider talking to your primary care provider about a cardiology referral as tachycardia at rest is also not normal. If you have a known medical condition that causes your heart rate to be below 60 or above 100, knowing what your expected average typical rate is usually is all that is important.
So, before you get the vaccine, determine what your average pulse rate is according to the pulse-oximeter. Consider talking it in the morning, afternoon, evening, and before you go to bed. This will give you an idea of the typical range for your pulse rate, and yes, it is normal for it to vary throughout the day. Try to take it when you are able to sit and rest for a few minutes, and you haven't consumed any stimulants recently. If you take stimulant medication, don't worry about it, since you will probably be taking it in the day after the vaccination, it will be accounted for in your baseline pulse rate.
The second number on the pulse-oximeter is the oxygen saturation % of your blood. A average typical range for a healthy person is 100 to 93. If your pulseoximeter consistently shows your oxygen saturation below 92, consider that your pulse-oximeter is not working properly. If you measure it with another one, and it says the same thing, make an appointment with your primary care physician for an evaluation because that isn't normal. Note when you take your pulse what the oxygen saturation is as well. You will notice this can also change throughout the day by a few points, which is totally normal. If you have a known medical condition that causes you to have a oxygen saturation rate below 92, knowing what your expected average typical oxygen saturation rate is usually is all that is important.
Once you know your baseline, you can get your vaccine and you can monitor yourself for potential issues using the pulse-oximeter. Generally you would want to use your pulse-oximeter to check your pulse and oxygen saturation every 4-6 hours, depending on how you are feeling. Getting a vaccine is not terribly different for your body than being mildly ill. As a result, just as with illness, you may see your pulse increase. Seeing a pulse increase of upwards of 20, at most, would be considered something to be expected. This tolerance is lower for people that have higher average pulse rates. If your average pulse rate is 70, then a pulse rate of 90 would be ok. If your average pulse rate is 95, anything above 108-110 is pushing it. If your pulse rate is usually 80, and it spikes to 115, that would be something to be concerned about. If you see your pulse rate spike, and it remains that way, consider taking readings every hour from that point forward to track your pulse rate and oxygen saturation. It could just be that your body is really reacting to the vaccine, or it could be the very first sign of a potential problem.
Even though a vaccine is like a minor illness, it shouldn't have much of any effect on your oxygen saturation. Generally your oxygen saturation should track within a point or two of what you saw when you established your baseline. If you see more than a consistent 5 point discrepancy, along with a significant increase in heart rate, start monitoring your heart rate and oxygen saturation rate every hour. If you become tachycardic and your oxygen saturation dips to 92, consider going to the hospital, or you can continue to very closely monitor yourself taking measurements at least every hour if not more frequently. If you are tachycardic and your oxygen saturation is consistently below 92, go to the hospital immediately, as those two signs are a strong indicator that your hearts ability to pump oxygenated blood throughout your body has become compromised. If your oxygen saturation rate is typically lower than the average range, your lower bound would be 90, as anyone who has a oxygen saturation below 90 who does not have established severe pulmonary (lung) issues and is not already on oxygen, should never see a reading below 90.
So, why have I mentioned all of this? The reason is that in some of the cases of fulminant (severe) myocarditis related to COVID vaccination the people didn't seek help until they were severely out of breath or had severe chest pain. At that point, they were in very bad shape, they were extremely tachycardic with oxygen saturation in the 80's with what is known as ventricular dysfunction and a left ejection fraction of 15% or less. That means their heart was compromised to the point where they were just barely alive. Some of these people recovered, but some of them now have permanently reduced cardiac function. That isn't something anyone should have to live with just for getting a vaccine.
Okay, the discussion from here on out is assuming the absolute worst, you are actually one of the people that has been impacted by myocarditis. This is what you will likely experience and what you need to ask the medical staff taking care of you to do. Why am I including this section? Because people die in hospitals everyday because they fall through the cracks, the healthcare providers don't take their symptoms seriously, etc... You and anyone with you, need to be your best patient advocates, as sometimes that can be the difference between life and death or serious permanent injury.
You arrive at the hospital, your heart rate is tachycardic and your oxygen situation is just barely 92, you are experiencing mild chest pain. Tell them you have been vaccinated recently, that you have chest pain and shortness of breath, and that your oxygen saturation is at 92 and dropping. Once they hear this, they should move their asses like no tomorrow. The first thing they need to do is hook you up to a monitor and start an IV, directly after that they need to do an EKG, preferably what is known as a 12-lead EKG. A 12-lead EKG is the most accurate kind that can pick up even small deviations from the typical. The EKG, if it is myocarditis, will probably show diffuse ST-segment changes, usually elevation, but decreases have been seen, as well as non-specific T-wave changes. At this point they will probably want to draw blood for laboratory evaluation, they want to run Troponin-T, and CK-MB (which is a certain for of an enzyme known as creatinine kinase), both of which are likely to be highly elevated. They may also do an echocardiogram, which will provide functional imaging of the heart and it will demonstrate any overt dysfunction. The best treatment known at this time are high dose corticosteroids, such as Prednisone. It should stop the immune reaction and allow your heart to heal. It is also possible they may want to start you on a diuretic (water-pill) and/or an ACE or ARB (angiotensin converting enzyme inhibitor or angiotensin receptor blocker) to temporarily decrease the load on your heart so it can heal more easily. If you get treatment soon enough, and the reaction wasn't too severe, there is no reason to think you won't make a full recovery.
One aspect of recovery you need to take absolutely seriously is the restriction on physical activity. Overwork a recovering heart, and you may end up dead, or with permanent damage. The general guideline is 3-6 months restriction to light physical activity, and your physician should define "light" for you, so there is no misunderstanding. Typically before giving you the go ahead to resume normal physical activity will involve an echocardiogram to know if there is any residual dysfunction of the ventricles.