Hundreds of patients have waited longer than an hour to be taken to an accident and emergency unit.
Some have waited for up to three hours for an ambulance to take them to hospital after a rapid response vehicle arrives at the emergency.
The figures, revealed under a Freedom of Information request, have prompted the chair of the Stormont health committee and a trade union to urge the Northern Ireland Ambulance Service (NIAS) to review its response times in cases where patients waited hours to be taken to hospital.
Rapid response vehicles (RRVs) are medically equipped vehicles manned by paramedics who arrive at the scene quickly and can assess and stabilise the patient. But unlike ambulances, they do not have the capability to transport the patient to hospital.
RRVs cannot leave until an ambulance arrives — which in turn results in the RRV being unable to attend other call outs.
According to figures obtained by the Belfast Telegraph, waiting times for an ambulance to take a patient to hospital after the arrival of an RRV increased between September last year and April this year.
Between April and August last year, no-one waited longer than 90 minutes for an ambulance to arrive to take them to hospital.
But from September the number of patients who waited longer than 90 minutes began to rise — in December 57 people waited longer than 90 minutes and on two occasions they waited between two and three hours.
Earlier this year, it emerged that a dying man lay in the street for an hour before an ambulance crew arrived to take him to hospital. A paramedic was with him in six minutes, but it was 52 minutes before an ambulance arrived.
NIAS statistics show there were 26 cases where patients waited between 45 minutes and an hour for an ambulance to arrive during the month of November 2011.
Sue Ramsey, chair of the Stormont health committee, said: “The committee supports the use of RRVs because they allow a trained paramedic to get to the scene of an accident or an emergency very quickly and provide immediate, sometimes live-saving treatment.
“Looking at these figures, we believe that the Ambulance Service needs to review, on a case by case basis, why some people had to wait for two hours and we will be seeking reassurance on this issue.”
Kevin McAdam, of the union, Unite, said: “We have gone on record in the past to express our concern over the use of RRVs over fully staffed ambulances.
“We believe RRVs are a cost-cutting measure that were introduced to improve targets but these figures show this isn’t happening.”
NIAS said between April 2011 and last month, a vehicle to transport a patient arrived within 15 minutes in 73% of cases and the vehicle was with the patient within 30 minutes on 92% of callouts.
“Further analysis of these 29,470 calls shows that 12,260 were Category A (immediately life threatening) calls and NIAS, following the early arrival of a paramedic, provided a transporting vehicle within 30 minutes on 11,799 (96%) occasions,” he added.
When an RRV is despatched to a life-threatening emergency call, an emergency ambulance is also sent. An emergency ambulance has a stretcher and two crew members — one of whom should be a paramedic. This ambulance contains a variety of essential equipment and supplies. An emergency ambulance can be tasked to emergency calls, undertake patient transfers between hospitals and bring patients home. An RRV is a smaller vehicle carrying all emergency ambulance equipment except for a stretcher and staffed by an experienced paramedic.