Some operations have been delayed for a week in Northern Ireland because of the flu outbreak and other winter illnesses.
Extra healthcare staff were drafted in to intensive or high dependancy care at hospitals.
The Public Health Agency said the number of swine flu cases is rapidly increasing, although we have not yet reached pandemic levels.
Health board chief executive John Compton said: "What we are doing is carefully adjusting the planned intake of patients, so that intensive care and high dependency units maintain their capacity to cater for those people who are already seriously ill, or come into hospital as new emergency cases.
"This is a practical response to changing demand and we very much welcome the understanding and assistance of patients, particularly those whose appointments will be delayed."
Steps taken to free staff include the postponement for one week of some elective surgery to enable theatre staff with the necessary skills and expertise to work in the intensive care environment. Patients affected will be contacted directly by their hospital.
Mr Compton added: "It is very much regretted that this will cause inconvenience and concern to individuals, but is essential to support the maintenance of critical care services to those patients who need it most.
"The position will be closely monitored and this decision will be reviewed next week."