The VMware - Virtual Machine Heartbeat alarm shows the status of communication between the VMware VirtualCenter and the VMware Tools on the virtual machine.
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When an Information alarm is raised

VMware Tools are not installed or are not running.

When a Low severity alarm is raised

There is an intermittent heartbeat. This may be due to guest load.

When a Medium severity alarm is raised

There is no heartbeat. The guest operating system may have stopped responding.