When people go into paroxysmal supraventricular tachycardia, the first line of diagnosis and treatment is carotid sinus massage (don’t try this at home, folks!)
Eerrrmmmm..... no. Don't try this at home, I absolutely agree with. But...
For those who don't do this for a living, CSM has the potential to convert some abnormal but typically fairly stable rhythms back to a normal rhythm. It can also cause such fun things as a stroke. Or conversion to a lethal rhythm such as Ventricular Fibrillation.
The use of CSM as a first line intervention is vanishingly rare, at least in the US. Although various Vagal Maneuvers are taught to patients and pre-hospital personnel, CSM is not one of them. The various ways of performing the Valsalva Maneuver are the go-to choice, in hospital or out.
Conversion rates for the various Vagal Maneuvers are, across the board, virtually identical. They're all in the 20% range. Given that they're all equally efficacious, and that CSM exposes the patient to (admittedly rare, but real) potentially devastating injuries, any reasonable risk/benefit analysis is going to come down against the use of CSM, except in a few very rare cases.
As a diagnostic tool... EKG/rhythm strips are too readily available and far more accurate. I have little confidence that anyone can differentiate between PSVT, A-Fib with RVR, 2:1 A-Flutter, V-Tach, or even a fast Ventricular Bigeminy by touch. Diagnostically, I think it is most useful for assessing for Carotid Sinus Hypersensitivity as a cause of syncope. And even for that there are more definitive tests and making that diagnosis based on CSM results would be imprudent.
I have seen it suggested that CSM can be used to help differentiate PSVT with aberrant conduction from V-Tach, but I think in most cases there are more reliable and safer options. If they're unstable, you'd go straight to electrical cardioversion, of course. If they're stable, giving them a dose of adenosine can be a good choice. If it's PSVT, it has a better than 90% chance to convert the rhythm. If it's A-Fib with RVR or A-Flutter with high conduction, it will slow the Ventricular Response (albeit briefly) allowing you to see the atrial activity. If it's Ventricular, it won't do anything.
There are plenty of other, safer vagal maneuvers they can try outside the hospital. Or in the hospital. One of my favorites is to hand them a 10ml syringe and have them try to push the plunger back by blowing into the needle end (before anyone freaks, there's no needle on it). And I can't think of anyone who routinely uses it diagnostically. EKG/rhythm strips are too readily available. And far more accurate, diagnostically.