The subject of EventSentry emails is highly customizable using standard variables (e.g. $HOSTNAME) as well as insertion string variables ($STR1, $STR2, ...).
Since heartbeat alerts include the name of the host for which the status changed, you can determine which insertion to use with the "Event Message Browser".
In the management console, select Tools -> Utilities -> Event Message Browser and select "Application" (for the event log) 'EventSentry' (event source) and '11000' (event ID), you can see that most of the event is converted to %# variables where the # is between 1 & 5 (%1, %2, %3, %4, %5). The numeric variables in the event are converted to $STR# variables which can be used in the subject line.
The example below will show all relevant information in the subject line:
[$STR1]: $STR3 -> $STR4 ($STR5)
[DB3-MYSQL]: OK -> ERROR (100% packets lost)
You can also customize the email sender field to uniquely identify the emails as "heartbeat" emails.
In most cases you will want to create a new email action as well as event log filter which will only route heartbeat-related events to the new customized action.