You should address this at the source:
What client is sending those DNS requests for heartbeat?
What software on that client is issuing those requests?
Chances are that some kind of monitoring software would be involved.
Once you've found out, you'd have a couple of options to limit the seemingly excessive amout of DNS queries hammering your Pi-hole.
a) adjust the heartbeat sending software's time interval to something bigger, i.e. once per minute (or 1,440 requests per 24 hours).
b) make sure the client is caching DNS requests, either again by adjusting the software, or by enabling a caching stub resolver on that client
c) if the client is already caching, try to adjust the heartbeat domain's TTL to a higher value, see e.g. Overriding `local-ttl` in user config files - #2 by Bucking_Horn
d) stop using the software that issues those requests (depending on your personal assessment of its usefulness)