The ecg doesn’t register timing and strength of the electrical signals anymore. How to solve this?

First force restart it 

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo. 

 Then Force Restart the paired iphone   Force restart iPhone – Apple Support (UK)

 If that is no help 

 Unpair and erase your Apple Watch – Apple Support (UK) 

and then 

Set up your Apple Watch – Apple Support (UK) 

 You may also want ensure Watch and iPhone are up to date.

Update your Apple Watch - Apple Support

Update your iPhone or iPad - Apple Support

 If you've completed that troubleshooting and the issue is still not resolved then Contact Support. 

 Click here > Contact - Official Apple Support

or phone numbers  

Click here > Contact Apple for support and service - Apple Support

or

Genius Bar Reservation and Apple Support Options - Apple