High heart rate notification does not suffice my needs. I want to see apple watch notifying immediately as soon as heart rate is more than set limit .. without any other checks. It should notify me while in workout session also.

I have a better understanding of your needs now. Thanks for the additional information. Apple already has this feature. You can set up notifications for high or low heart rates on your watch. This Apple Support page describes how but I have copied the pertinent information here. I hope This helps.

Heart health notifications on your Apple Watch - Apple Support

High and low heart rate notifications

If your heart rate remains above or below a chosen beats per minute (BPM), your Apple Watch can notify you. These notifications are available only on Apple Watch Series 1 or later for ages 13 and up.

You can turn on heart rate notifications when you first open the Heart Rate app on your Apple Watch, or at any time later from your iPhone:

1. On your iPhone, open the Apple Watch app.
2. Tap the My Watch tab, then tap Heart.
3. Tap High Heart Rate, then choose a BPM. 
4. Tap Low Heart Rate, then choose a BPM.

this is a user helping users with tech questions, we are not apple and apple do not read your posts here

if it was to monitor your heart 24-7 then it might not last long in terms of battery

but you can reach out to them with suggestions

If you wish to reach out to apple then try  Get Support (apple.com),  Contact - Official Apple Support

Or provide them with feedback Feedback - Watch - Apple

This only goes into effect when the heart rate has been high or low for 10 minutes. Is there a way to set a notification for the moment my heart rate exceeds a certain BPM?

It sounds like you need a medical grade device. Given the circumstances, you may want to contact your doctor.

Then use the link provided above to provide your feedback to Apple. No one here works for Apple, nor can any of us give you what you want.