I'm no expert, opiestrer, but my reading says that SVT is an arrhythmia. While it might have the same intervals from beat to rapid beat on an ECG, the beats are not consonant with the corresponding beats of the ventricle below the atrium. The normal rhythm is for the atrium to contract, forcing blood through the mitral valve and filling/pressurizing the left ventricle...which then pumps almost immediately and forces blood up the arching aorta, through the aortic valve. If there are three or more beats in the atrium while the ventricle only beats once, that's an arrythmia!

Yes, we often use shorthand, forgetting that not everyone knows what the initialisms mean. Sorry...NSR does mean 'normal sinus rhythm'. However, I did possibly mislead about the conversion. Atrial fibrillation is what I was thinking (very technically, AF is still an SVT: https://www.mayoclinic.org/diseases-conditions/supraventricular-tachycardia/symptoms-causes/syc-20355243 , but the medical community uses SVT for a non-synchronous rapid rhythm....which fibrillation is not. Fibrillation is irregularly irregular in beating, and one cannot call it a rhythm, whereas SVT is just a very rapid beat with the same intervals between them...again....unlike fibrillation where the intervals are all over the place. SVT is rhythmic from beat to beat, but out of synch with the ventricle. Whereas AF with what is known as 'rapid ventricular response' imposes a danger because the ventricle wants to beat along with the atrium's rapid and irregular rate (meaning ventricular tachycardia (which can quickly be lethal), SVT doesn't normally involve the ventricle, which continues to beat at its sino-atrial node rate. This, again, is based on my learning, but I'm not a physician. I hope that makes sense.

The cold dunk method can work for any arrythmia, ostensibly. It changes the tone of the Vagus nerve, charges you with epinephrine, increases muscle tone and lung volume, not to mention blood pressure, and it can bring on NSR. Again, you have nothing to lose.

All arrythmias are electrically disordered conditions. All of them can damage myocytes over time, meaning cardiomyopathy, enlargement, and heart failure. All of them create fibrotic tissue in the endothelium if left too long, which is inelastic and forces the heart muscle to grow more to overcome the resistance imposed on the inner surfaces of the affected chamber. Growing heart tissue means thickening, or enlargement, and this often/usually means heart failure if not treated.