Three EMCCs, located at Haugesund, Stavanger and Innlandet hospitals, were involved in the study, with the three corresponding districts covering 816 000 inhabitants (18% of the total Norwegian population). Data were collected prospectively from October 1 to December 31 2007.
All 19 EMCCs in Norway use a software system called Acute Medical Information System (AMIS) to record all incoming cases. Usage of the AMIS results in an electronic form with registration of each incident (not the individual patient). The AMIS form contains information about the incident, the patient (or patients, if more than one patient is involved in the incident) and all available logistics, including date, time of day, and to where the patients are transported ("left at scene", home, casualty clinic, hospital). Prehospital response time is also registered, defined as the time period from when the caller calls 113 until the nearest available ambulance reaches the patient [9, 10].
Based on the immediate available information, the EMCC operator (usually a specially trained nurse) gives the incident one clinical criteria code and one response level according to the Index . The Index is based on ideas from the Criteria Based Dispatch system in the US , and was first published in 1994. It categorises clinical symptoms, findings and incidents into 39 chapters, and each chapter is subdivided into a red, yellow and green criteria based section, correlating to the appropriate level of response. Red colour is defined as an "acute" response, with the highest priority, and will trigger the transmission of a radio alarm to both the primary care doctor on-call and the ambulance service. Yellow colour is defined as an "urgent" response, with a high, but lower priority, where the patient should be examined as soon as the doctor-on-call is available. Green colour is defined as a "non-urgent" response, with the lowest priority. Chapter 10 in the Index covers the symptom "Chest pain", and usage of the red response section will result in the code A10 - Chest pain (A for "acute"). An example of a criterion leading to a red response will be "chest pain with breathing difficulties", while "pain not particular strong, and the patient feels fine" is defined as a yellow criterion, leading to an urgent response, but with lower priority than red response.
Copies of all AMIS forms involving incidents classified as red response were sent to the project manager every other week throughout the study. The EMCCs also sent copies of ambulance records from all red responses which involved ground or boat ambulances. In cases where doctors on-call, casualty clinics, primary care doctors or air ambulances had been involved, copies of medical records were requested and collected separately. This collection of medical records continued also after the study period, until October 2008. To secure a uniform use of the variables in the AMIS program, a meeting was held between the persons in charge of the participating EMCCs.
The severity of the medical problem was classified using The National Committee on Aeronautics (NACA) Score System based on all available information . In the NACA system, the patient's status is classified from 0 to 7, zero indicating no disease or injury, while seven indicates the patient being dead. NACA score was categorised in the analyses as NACA 0-1 (patient with either no symptoms/injuries or in no need of medical treatment), NACA 2-3 (patient in need of medical help, where value 3 indicates need of hospitalisation, but still not a life-threatening situation), NACA 4-6 (4 is a potentially, and 5 and 6 are definitely, life-threatening medical situations) and NACA 7 (dead person).
Based on information from all available forms and medical records the cases were also classified into symptom groups according to the International Classification of Primary Care - 2 (ICPC - 2) . The analyses presented in the results-section are based on the patients who were given the code A10 - Chest pain. Results on all the clinical categories and symptom groups, are published in a previous article .
The statistical analyses were performed using Statistical Package for the Social Sciences (SPSS version 15). Standard univariate statistics, including median and percentiles, were used to characterise the sample. Median, with 25th-75th percentiles, was used to analyse data where normal distribution was not present. Rates are presented as numbers of red responses per 1 000 inhabitants per year with a 95%-confidence interval (CI). Mann-Whitney U test was used for comparing age between males and females, for other comparisons the Pearson Chi-Square test was used. A P-value of < 0.05 was considered statistically significant.
Ethics and approvals
Approval of the study was given by the Privacy Ombudsman for Research, Regional Committee for Medical Research Ethics, and the Norwegian Directorate of Health.