The Health Service Executive West says tomorrow's industrial action by nurses has forced the cancellation of a total of 21 appointments and surgical procedures at Portiuncula Hospital in Ballinasloe.
The patients were informed tonight that their operations and appointments have been cancelled.
Nurses will stage a one-hour stoppage tomorrow morning at 22 hospitals and health facilities nationwide.
Further two-hour and three-hour stoppages are set to take place next Tuesday, Wednesday and Friday.
The HSE has warned that thousands more patients will be affected by the escalating industrial dispute by nurses seeking a pay rise and a 35-hour working week.
A spokesperson for the HSE said they are particularly concerned about next week's longer stoppages which will take place on top of the ongoing work-to-rule.
The HSE said it is now 'actively considering' cutting nurses pay to offset the estimated €2m a week cost of their work-to-rule.
The HSE has asked patients with appointments scheduled over the coming week to turn up, unless advised otherwise by hospital authorities.
The situation is further complicated by Monday's Bank Holiday - which may make it more difficult to alert patients about cancellations.
The HSE is also asking people not to attend Accident and Emergency units, unless absolutely necessary.
They point out that some of the major hospitals see up to 1,000 out-patients a day - and losing half a day means cancellations for up to 500 patients in each hospital.
Surgical procedures are also facing widespread cancellations.
It is also becoming more difficult to plug the gaps caused by the nurses' work-to-rule, as other grades are too tired to do more overtime.
Nursing unions are to review their campaign on 10 May at a special delegate conference in Dublin, which will consider further escalations, including a ban on overtime.
