FWIW...I always pre-rinse with chlorhexidine before any dental procedures at the dentist. I would insist on doing this. If you have to, bring your own Listerine to pre rinse with beforehand if the office doesn't have any.In the US, they recommended oral antibiotics for all tooth cleaning if you have a mechanical valve of any type and in any position. I don'take to it too well and about 2 years ago asked if it was necessary. My cardiologist said yes. When I pushed and asked if there was any hard evidence, he emphatically stated that in his personal experience he has had patients get endocarditis from simple dental work.
One person on here had endocarditis and was told by their infectious disease specialist to use an oral disinfectant (e.g. Listerine) before any dental procedure. My dentist is quite happy to let me rinse at the beginning