It is hard for me to know if it is life threatening, but since you black out from the pain, it could be very dangerous if it occurs at the wrong place/time. These SVTs are usually responsive to medications which can decrease the frequency and severity of events, and are also usually eminently treatable by electrophysiologist who can ablate the focus of the tachycardia so that it does not recur at all.
What doctor are you seeing about this? a pulse of 260 is extremely fast, I would definitely think you need to see an Electrophysiologist immediately
What is an elctrophysiologist? I have seen a lot of cardiologists. I've had all the standard tests, EKG, ECG, Holter monitor. I'm not seeing anyone right now. I was getting pretty discouraged by the "you're imagining it" line -- it's been recorded by the holter monitor, so I'm sure I''m not imagining it.
An electrophysiologist is the heart doctor who specialises in the electrics of the heart. One of my cardio nurses likened cardiology to some building tradespeople... the "electrician" deals with the rhythms and electrical messages of the heart. The "plumber" deals with the valves and blood vessels. The builder deals with the structure, the muscle itself. For me, I have arrhymias caused by the muscle structure being affected by a virus, aso I see primarily an "electrician"(electrophysiologist), and increasingly, the "builder" (transplant team)... I hope my memory hasnt made my understanding and description here inaccurate, and that my layman's explanation is helpful.
I think you need to see an EP and start taking some kind of meds to control your heart rate. Before my ablation, I suffered from a-fib, a-flutter and v-tach. Whenever my heart rate went up to 250-300 I would get crushing chest pains and pass out. I was put on event monitors to capture these events. My EP insisted on an ablation. The meds were no longer controlling my heart rate. I was told that it was dangerous for me to have such a high rate and passing out by several different EP's. You need to get this under control.
Good Luck. Wishing you well and wishing you enough...