Why do we send heartbeat messages even if other socket messages going through connection

[CapLek]

Basically, why do we need heartbeats at that time when messages are going back and forth through connection?

Currently, I've got a case when server generates number of messages on one incoming client message. During that response series server fails to send ping message which leads to disconnect after pingTimeout+pingInterval time.

Node.js event loop doesn't seem to be blocked because client can still reconnect and send initial connect message and receive response.

Why heartbeat messages aren't send? Am I missing something?
Thanks in advance.