Kwong Wah Hospital on Thursday said it has launched an investigation after a patient was found unconscious and with no heartbeat after his ventilator tube came loose.

Staff managed to revive the patient, a 51-year-old man with tuberculosis, after they noticed what had happened to him on Monday afternoon.

He remains in critical condition.

The hospital said an initial assessment showed that the tube became loose because of the man's movements, but a monitoring device didn't alert staff to the problem.

"At this moment, we have the information that the central monitoring doesn't have the signal coming from that particular monitoring device during those periods. And why is that the case? We are still under close investigation why that information was not found in the central nursing monitoring station," said the hospital's chief executive, Tang Kam-shing.

Tang said the monitoring device did issue an alarm in the ward itself, but it couldn't be heard outside the room because it was an isolation ward and its doors were closed.

No systemic error has been found after checks on other such monitoring devices, he added.

The hospital said it has set up a panel to look into the incident.