New figures on 999 ambulance response times for life-threatening cardiac or respiratory cases show that just 45% of incidents in the west of Ireland are responded to within 19 minutes.
The 19-minute target for 85% of cases is a Health Information and Quality Authority standard for Clinical Status 1 ECHO calls.
These calls mean there should be a patient-carrying vehicle at the scene of the incident within 18 minutes 59 seconds.
The figures were released by the Health Service Executive to Independent TD for Roscommon-South Leitrim Denis Naughten following a parliamentary question.
They show that on average 76% of ECHO 1 status incidents in nine HSE areas were responded to within the target time in June.
While the worst response time was in the West with 45% of incidents responded to within 19 minutes, the best performing area was the Midlands, which exceeded the 85% target.
Percentage of 1 ECHO incidents responded to by a patient-carrying vehicle within target:
Dublin Fire Brigade 64%
North West 75%
Mid West 79%
South East 81%
North East 92%
The figures for 999 calls for Clinical Status 1 DELTA calls show an even lower response rate.
These emergency calls are for life-threatening or potentially life-threatening cases other than of cardiac or respiratory origin.
Across all nine HSE areas, the average percentage of cases responded to within target for this category is just over 64%.
The percentage of incidents responded to within 19 minutes was:
South East 58%
North West 62%
Dublin Fire Brigade 65%
North East 73%
Mid West 74%
In its reply to Mr Naughten, the HSE said that response times around the country vary significantly, based on the nature of the area covered and the rural-urban mix.
It said that response times are lower in remote areas of the country and the challenges are reliance on on-call arrangements, geography, road network, resourcing and the use of emergency ambulances for inter hospital transfers.
The HSE said that a performance action plan is in place, aimed at improving response times, with faster mobilisation times for crews, improved processes around call-taking and dispatch and developing community first responder schemes.
The HSE's National Ambulance Service said it is focused on improving call-taking and dispatch functions through rationalising the number of ambulance control rooms across the country from the current nine sites to one system across two sites in Tallaght in Dublin and Ballyshannon in Donegal.
The HSE has set its own lower than HIQA response time targets for this year for 1 ECHO calls with a response target of 70% of cases within 19 minutes and a response target of 68% for 1 DELTA incidents.
It has exceeded its 1 ECHO target, but not met its own 1 DELTA target.