Atrial tachycardia is a supraventricular tachycardia and is explained here
https://www.mayoclinic.org/diseases-conditions/atrial-tachycardia/cdc-20355258
In very simple terms (from a retired doctor not specialising in cardiology, and other docs here can correct me)….
The atrium fires off off it’s own rhythm (that’s the supra part, above the ventricle), and so we diagnose on the rate (as per Atrial fibrillation), The arrhythmia is the atrium going haywire and suddenly the heart rate can shoot up quickly from normal (say 60-70) to tachycardia (100)and above to 180 (not good, lots of symptoms). In the emergency dept we usually use carotid artery massage to bring it down while we do investigations (via the vagus), ECG bloods etc, you would have had all of that with the cardiologist etc.
So the heart is controlled by the sympathetic nervous system firing it up to prepare for action, and the parasympathetic nervous system fires off to bring abnormal heart rates back to resting and homeostasis (unique to the person).
A rough guide to your resting heart rate/and HRV, is first thing in the morning after an overnight sleep when cortisol levels are starting to rise quickly and triggers waking, starts around 4 am for “normal” people, (of course we have unusual sleep, circadian rhythms and cortisol levels - never really understood….) and so this rise prepares your physiology to function for the day ahead.
You would need to learn to record your resting heart rate via palpating your pulse at wrist or neck and counting the beats for a minute, or use a smart watch, or a BP monitor or a pulse oximeter (all of them measure heart rate).
It does take a while to work out what is your “normal for you” resting heart rate, perhaps do it for two weeks and if you do have a good sleep cycle…but if you record a rough estimate of sleep hours a pattern will emerge. It may also be useful to record events of the day that may increase heart rate like going upstairs, walking faster, activities that make you breathless (likely PEM) and stressful events or emotions that do increase heart rate but once abate your heart rate should go down, sometimes not back to normal but with sleep hopefully.
HRV is also a pattern but is more affected by the normal fluctuations of cortisol levels during the day so big rise in morning, peaking midday dipping a bit in afternoon then rising a bit to a lesser peak and then falling off for bedtime. (ME can change all of that but it will depend on your particular experience/physiology and severity of illness)
If you use a smart watch app like WatchME, you can set limits and get warnings and see what various activities increase your HR (and likely Post exertional symptom exacerbation PESE or PEM (depends on your experience if have never got stability or some recovery of your symptoms . Look for changes in heart rate, especially standing (orthostatic stress) but also when you do relaxing and pleasurable activities, or deep abdominal breathing, mindfulness techniques, breathwork etc, (and meditation which needs a quiet space to learn it so you get the result - over time.) as well as all the visualisation stuff, or what ever people find useful, listening to music, doing repetitive craft movements etc.
You can also use the Welltory app but need to read all the information they provide for “normal” people. There are also Facebook groups that use HR and HRV for pacing with a variety of apps and heart rate recording devices.
I do think PEM is shown in heart rate changes, as that is what I experience, my baseline ( housebound severity) but with orthostatic symptoms but controlling PEM, my resting heart rate is about 65 and can keep within the recommended 15 beats more (Workwell Foundation guidelines - if my recollection is right) so 80 pacing through day but if something stressful happens goes up to say 90 plus but settles back. My HRV is on average around 30 but fluctuates between 11 (full PESE and PEM) but the odd ones above that up to 50 if doing lots of stop, sit, slow down, rest, deep breathing if PESE starts.
Edited abit for typos but PESE starting…
Further edit to add some more stuff on pacing strategies…