St Peter's Community Hospital serves all heart patients in Helena and the surrounding area, with a total population of 68 140. It is nearly 100 km to the next nearest hospital with cardiology services.
About 90% of the population of Helena live in the 59601 zip code. The 10% remaining live in the 59602 zip code, which includes a residential area immediately adjacent to Helena. Many people who live there work in Helena. We surveyed 500 consecutive patients aged over 18 years old who resided in the 59602 zip code and were admitted to the hospital for all causes. Of the 213 of these patients who were employed outside the home, 192 worked in the 59601 zip code area, so we considered 59602 part of Helena for our analyses. We also included zip codes 59604 and 59624 (post office boxes in Helena). All other 596xx zip codes, 59713, and 59728 were considered “not Helena.”
Selection of patients
St Peter's Hospital uses a combination of paper and electronic medical records and computerised billing. We reviewed charts for the months of June to November (the months the ban was in effect) for each year from 1998 to 2003 for patients with a primary or secondary diagnosis of acute myocardial infarction (ICD-9 (international classification of diseases, ninth revision) codes 410.xx). During these months, there were 10 497 admissions for all causes (including acute myocardial infarction) from Helena and 3367 from outside Helena. The attending physician made the diagnosis at the time of discharge, and the hospital billing staff assigned the codes. (Two of the authors (RPS and RMS) were attending physicians for 18 of the 304 admissions included in this study and so assigned the diagnosis. All but three of these patients were treated before we thought of doing this study. These three patients were also seen by a cardiologist and thus had independent blinded corroboration of the diagnosis.) Data were sorted by primary and secondary diagnoses and by zip code to compare the incidence of acute myocardial infarction in residents with zip codes for the city of Helena and residents of the surrounding areas, where there was no ban.
We studied patients' charts if there was a primary or secondary discharge or emergency room diagnosis of acute myocardial infarction. Acute myocardial infarction was the primary diagnosis for 283 cases. Selection criteria were onset of symptoms in the study area, a primary diagnosis of acute myocardial infarction, and no recent procedure that could have precipitated acute myocardial infarction. We excluded eight cases because onset of symptoms occurred outside the study area and one because the patient died in the emergency room three days after angioplasty. The charts of three patients were reviewed because of multiple admissions in any 60 day period. Of a total of five such admissions, one was excluded because there was no chemical evidence (raised troponin I concentrations or creatine phosphokinase activity) for a new event. We therefore included 274 admissions with a primary diagnosis in the analysis.
We reviewed 71 cases with a secondary diagnosis of acute myocardial infarction. To be included, patients had to have chemical evidence (raised troponin I concentrations or creatine phosphokinase activity) at the time of admission or within the first 24 hours, onset of symptoms inside the study area, and no recent procedure that could have precipitated acute myocardial infarction. In the analysis we included 30 admissions with a secondary diagnosis and excluded 41.
In all cases, we accepted the attending physician's diagnosis of acute myocardial infarction, and all attending physicians (other than the authors) were blinded to the study. In the three cases included after the study was started a consulting cardiologist, who was blinded to the study, confirmed the diagnosis, according to the medical record. We did not change any diagnosis. We excluded or included cases according to the criteria noted above.
We reviewed charts of patients from outside the study area to determine whether onset of symptoms was in or out of the study area and included them if the patient's symptoms started in the study area. Twenty six patients in the primary acute myocardial infarction group had out of area zip codes; 14 were included. Eight patients with a secondary diagnosis of acute myocardial infarction had zip codes out of the area. We included three patients with a diagnosis of primary myocardial infarction (for example, primary diagnosis of cardiogenic shock with secondary diagnosis of acute myocardial infarction) whose symptoms had started in the study area.
Overall we selected 354 admissions for review, and 304 met the inclusion criteria.
Statistical methods
We tested the hypothesis that the law was associated with changes in the total number of admissions for acute myocardial infarction in the six months of June to November (when the law was in effect). We compared the number of admissions during the six months the law was in effect (in 2002) with the average number of admissions during the same six months in the years before (1998-2001) and after (2003) the law.
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