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Acute coronary syndromes
Original article
Trends in hospitalisation for acute myocardial infarction in Ireland, 1997–2008
  1. Siobhan M Jennings1,
  2. Kathleen Bennett2,
  3. Moira Lonergan2,
  4. Emer Shelley1
  1. 1Department of Public Health, HSE, Dublin, Ireland
  2. 2Department of Pharmacology and Therapeutics, St James Hospital, Dublin, Ireland
  1. Correspondence to Dr Siobhan M Jennings, Public Health, Dr Steevens Hospital, Steevens Lane, HSE, Dublin 8, Ireland; siobhan.jennings{at}hse.ie

Abstract

Objective To study the temporal and gender trends in age-standardised hospitalisation rates, in-hospital mortality rates and indicators of health service use for acute myocardial infarction (AMI), and the sub-categories, ST elevation MI (STEMI) and non-ST elevation MI (NSTEMI), in Ireland, 1997–2008.

Design, setting, patients Anonymised data from the hospital inpatient enquiry were studied for the ICD codes covering STEMI and NSTEMI in all 39 acute hospitals in Ireland over a 12-year period. Age standardisation (direct method) was used to study hospitalisation and in-hospital mortality rates. Joinpoint regression analysis was undertaken to identify significant inflection points in hospitalisation trends.

Main outcome measures Age-standardised hospitalisation rates, in-hospital mortality and indicators of health service use (length of stay, bed days) for AMI, STEMI and NSTEMI patients.

Results From 1997 to 2008, hospitalisation rates for AMI decreased by 27%, and by 68% for STEMI patients (test for trend p<0.001), and increased by 122% for NSTEMI, (test for trend p<0.001). The mean age of male STEMI patients decreased (p<0.01), while those for the remaining groupings of AMI and subcategories increased. The proportion of males increased significantly for STEMI and NSTEMI (p<0.001). In-hospital mortality decreased steadily (p=0.01 STEMI, p=0.02 NSTEMI), as did median length of stay.

Conclusions The authors found a steady decrease in hospitalisation rates with AMI, and a shift away from STEMI towards rising rates of NSTEMI patients who are increasingly older. In an ageing population, and with increasing survival rates, surveillance of acute coronary syndrome and allied conditions is necessary to inform clinicians and policy makers.

  • Quality of care and outcomes
  • myocardial ischaemia and infarction (IHD)
  • allied specialities
  • neurology
  • stroke
  • transient ischaemic attack (TIA)
  • psychology/psychiatry
  • aorta
  • great vessels and trauma

https://doi.org/10.1136/heartjnl-2012-301822

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    Introduction

    Mortality from coronary heart disease (CHD) peaked in Ireland in the mid-1980s followed by a decrease over the next two decades with a more rapid decrease from 1997 onwards.1 This pattern is broadly similar to the international trends, with a number of studies explaining the reduction in mortality, in terms of improvement in risk factors, resulting in reduced incidence and improvement in treatment with concomitant reduction in case-fatality.2

    Within the clinical environment, the last 2 decades have seen a number of changes which have had an impact on the pattern of acute myocardial infarction (AMI), such as the availability and use of biomarkers,3 an agreed international definition,4 ,5 coding changes internationally and in Ireland,6 and the evidence base for more effective reperfusion treatments.7 ,8 These changes highlight the need for detailed epidemiological surveillance of incidence and severity of CHD and its impact.9 However, in contrast with the 1970s and 1980s, when the MONICA studies provided considerable data to help understand the evolving CHD epidemic, the last decade has seen major attention dedicated to surveillance of trends in compliance with evidence-based treatments. This has resulted in a dearth of epidemiological studies of AMI, and especially of the sub-categories, ST segment elevation myocardial infarction (STEMI) and non-ST segment elevation myocardial infarction (NSTEMI), which describe disease patterns and can inform policy and practice into the next decade.

    The purpose of this study was to describe the trend in hospitalisations for AMI in Ireland for the period 1997–2008 prior to implementation of a national primary percutaneous coronary intervention programme. Specifically, we aimed to study the trend in age-standardised hospitalisation rates, in-hospital mortality rates and indicators of health service use for AMI and the sub-categories (STEMI and NSTEMI) in Ireland from 1997 to 2008, and compare the epidemiology of Irish hospitalised AMI with international trends.

    Methods

    Cross-sectional data on hospitalisations, including in-hospital deaths (all causes), were extracted from the Economic and Social Research Institute's national Hospital Inpatient Enquiry (HIPE) system which covers all inpatients and day cases receiving curative and rehabilitative care in publicly funded acute hospitals (n=39) in Ireland. All participating hospitals use the same dataset and software, with a range of procedures directed at safeguarding data quality integrated in the software.10 The data coverage in HIPE exceeds 96% in these hospitals. Private hospitals (n=9) do not contribute to HIPE, with one exception. Also, private hospitals, until the late 2000s, lacked emergency departments resulting in few treatments for acute coronary syndrome (B Maurer, personal communication).

    Data extracted covered the years 1997–2008 using ICD-9 codes for data pre-2005 (AMI code 410, STEMI codes 410.0–410.6 and 410.8, NSTEMI codes 410.7 and 410.9) and ICD-10 codes for 2005 onwards (AMI codes I21 and I22, STEMI codes I21.0–I21.3, NSTEMI codes I21.4 and I21.9). No specific validation of clinical diagnosis or coding was undertaken. Comorbidity data were not explored in this study.

    For study of trends in rates of hospitalisation and in-hospital mortality, the unit of analysis was the episode of emergency hospitalisations for patients >25 years of age, excluding transferred patients, to avoid double counting. To accurately reflect hospital activity, the analysis of health service use included patients transferred from other hospitals using specific codes for transfer of patient to home or other hospital/facility.

    Data are reported for number and age-standardised hospitalisation rates for AMI and the sub-types (STEMI, NSTEMI), crude and age-standardised rates for in-hospital mortality, median length of stay (LOS) and total bed days as indicators of health service use. Demographic analysis was undertaken by gender, by mean age, as well as for younger (<65 years) and older (>65 years) age groups.

    Age-standardised rates for both hospitalisations and all-cause in-hospital mortality (standardised to the European Standard population), were calculated using the direct method, and annual population by age and sex made available via the Central Statistics Office (http://www.cso.ie).

    χ2 tests were used to compare proportions over time. Joinpoint regression analysis was used to identify significant inflection points (‘joinpoints’ is where the slope of the trend line significantly changes) in age-standardised rates for STEMI and NSTEMI from 1997 to 2008 by age group (<65, ≥65 years) and gender.11 Data are presented as the annual per cent change (APC) with 95% CIs shown for each distinct period. The analysis was performed using software developed by the Surveillance Research Programme of the US National Cancer Institute (http://surveillance.cancer.gov/joinpoint). Otherwise, SAS V.9.1 was used for analysis and significance at p<0.05 was assumed (http://www.sas.com). Ethical approval was not required as all data were anonymised and released under a confidentiality agreement with the ESRI.10

    The population of Ireland increased during the study period from 3.6 million (Census 1996) to 4.2 million (Census 2006) with the population aged 65 years and over accounting for 11% throughout. All citizens are entitled to acute care in public hospitals, with 47% also paying for private health insurance (http://www.cso.ie).

    Results

    Over the 12 years from 1997 to 2008, there were 58 668 hospitalisations for AMI in patients >25 years of age in all acute hospitals in Ireland. The proportion of AMI hospitalisations coded as STEMI decreased from 78% in 1997 to 33% in 2008, but the reverse trend was observed for NSTEMI.

    Time trend in rates

    For the 12-year period, 1997–2008, the age-standardised hospitalisation rate decreased from 242.4 to 176.2 per 100 000 for AMI, a relative decrease of 27% (figure 1). The pattern for the subcategories diverged with a decrease of 68% in age-standardised rates for STEMI, and an increase of 122% for NSTEMI in the same time period.

    Figure 1
    Figure 1

    Age-standardised rates (95% CI) for AMI, STEMI and NSTEMI, 1997–2008.

    The joinpoint regression analysis found that the decreasing trend in STEMI hospitalisation rate accelerated significantly from 2002 onwards in both the under 65 years and 65-year-plus age groups, with a slowing of pace in the younger age group from 2005 onwards (table 1). Females had a steady, decreasing pattern of STEMI hospitalisation for the 12-year period, whereas, the decrease in males was slower until 2002 when it showed a pace similar to that of females.

    View this table:
    Table 1

    Annual percentage change (APC) and 95% CIs in standardised ST elevation myocardial infarction (STEMI) and non-ST elevation myocardial infarction (NSTEMI) rates, 1997–2008, by age, group and gender*

    The joinpoint regression analysis in NSTEMI hospitalisation rates showed a steady increase over the time period with no points of exceptional change for older or younger populations (table 1). Analysis by gender, however, showed an escalation in the increasing trend in males for the years 2002–2008 with no turning point for females (table 1).

    NSTEMI became the dominant subtype of AMI admission by 2004 in those aged 65 years and over, and by 2006 in the under 65-year-olds.

    Age and gender

    The proportion of males for STEMI increased from 66.0% in 1997 to 68.3% in 2008 (p<0.001), and for NSTEMI from 58.5% to 64.9% (p<0.001), but not significantly for AMI overall (table 2).

    View this table:
    Table 2

    Trend in numbers of hospitalisations, mean age, gender and in-hospital mortality for acute myocardial infarction (AMI), ST elevation myocardial infarction (STEMI) and non- ST elevation myocardial infarction (NSTEMI), 1997–2008

    Females were consistently 7–8 years older than males under all categories of hospitalisation (table 2). Male and female patients with NSTEMI were 3–5 years older than their STEMI counterparts (table 2).

    Mean age rose significantly over time in both male and female AMI patients (table 2), while age significantly decreased in male STEMI patients (p<0.001). There were no significant changes in mean age for STEMI female patients or NSTEMI of either gender (table 2).

    In-hospital mortality

    Crude in-hospital mortality rate (table 2) started to decrease in 2000–2001 with significant reductions for AMI, STEMI and NSTEMI of between 30% and 36% (p<0.001).

    Age-adjusted in-hospital mortality for the two subgroups of STEMI and NSTEMI (table 2) significantly declined over time. This trend is also evident in both, male STEMI and NSTEMIs (p<0.0001 and p=0.005, respectively), and female STEMI and NSTEMIs (p=0.007 and p=0.0076, respectively).

    Health service use

    With inter-hospital transfers included to accurately account for activity, the median LOS for AMI showed a reduction from 8 to 6 days over the 12 years resulting in a reduced bed utilisation of 6576 (11.4%) days in 2008 compared with 1997. The median LOS for STEMI decreased from 8 to 5 days within the same time period. The accompanying reduction in hospitalisation for STEMI resulted in 29 689 (66.7%) less inpatient bed days in 2008 compared with 1997. For NSTEMI, the pattern of a more modest reduction in median LOS from 8 to 7 days, together with an increase in hospitalisation rates, resulted in an increase in bed days utilised of 23 054 (63.6%) days in 2008 compared with 1997.

    Discussion

    From 1997 to 2008, AMI hospitalisation rates decreased, with a notable underlying trend of reduction in STEMI hospitalisations accompanied by a greater increase in NSTEMI. The rate of decrease in STEMI hospitalisations accelerated from 2002 onwards as compared with a steady rate of increase for NSTEMI. With the exception of a decreasing mean age in male STEMI, all other subcategories showed an increase in age at presentation. There was an increase in the proportion of male presentation for both, STEMI and NSTEMI. In-hospital mortality decreased steadily, as did median LOS.

    Similar to other developed countries, CHD mortality in Ireland has decreased, after peaking in the mid-1980s.12–14 From 1997 onwards, the decrease was at a faster rate in Ireland than in Finland and the UK.1 The current study explored the trend in hospitalisation for AMI and found a decrease of 27% in age-standardised rates for the period 1997–2008 in Ireland in spite of changes in clinical practice which would be expected to increase ascertainment of myocardial necrosis. A decrease of similar magnitude was found over this time period in the USA (24%)15 ,16 and in Scotland (30%).17 Smaller decreases have been documented for hospitalisation rates (1994–2004) in Canada (19%),18 for first AMI hospitalisation rates in The Netherlands (19%)19 and in New Zealand (9%),20 with only very modest decreases reported in Australia.21

    The study of AMI subcategories of STEMI and NSTEMI has received limited attention to date, though they represent recognised severity points along a disease continuum, and treatment options differ. One study in the USA15 found a 62% decrease in age-adjusted STEMI rates with an increase and shift in dominance to NSTEMI in early 2000, similar to our study. Two other studies, also from the USA, outline a more modest decrease in STEMI numbers, and either a plateau or very small increase in NSTEMI hospitalisations.22 ,23 European data from the Berlin Registry found a decrease in the percentage of STEMI patients from 1999 to 2004, with STEMI remaining dominant.24

    Explaining the decrease in AMI (and the subcategory of STEMI) hospitalisation is complex. While an increase in out-of-hospital cardiac deaths could offer one explanation, a recent initiative capturing data on out-of-hospital cardiac arrests in Ireland suggests a modest survival rate. http://www.phecit.ie/DesktopDefault.aspx?tabindex=0&tabid=1368 A reduction in the incidence of or readmissions for CHD, or both, have been proposed in international studies.9 ,15 ,25 The current study uses an administrative database, and so, is unable to provide incidence data. However, a modelling study of the decline in CHD deaths in Ireland, 1985–2000,2 attributed equal proportions of the decline to improved access to treatments (case-fatality), and a reduction in risk factors (smoking, population blood pressure and cholesterol) providing some evidence for primary prevention contributing to a reduction in incidence and severity of AMI in Ireland. Further, a recent study in Ireland26 has shown improvement in access to timely treatment and secondary prevention.

    The introduction of troponins as biomarkers, in the 1990s, and the increasing penetration and utilisation in Ireland in 2000s,27 may account for the increasing trend in NSTEMI hospitalisation as proposed in other studies.15 ,25 However, the fact that there is a turning point for male NSTEMI patients in 2002 without any discernable change in pace for females suggests that biomarker change is unlikely to be the sole reason for the observed increase. Further, increased survival, as shown in the reduction in case-fatality for both STEMI and NSTEMI, is likely to be a contributing factor.

    The demographic shift towards older AMI presentation shown in our study has been noted internationally, as has the increasing proportion of males.15 ,28 The exception to this trend was a decrease in mean age in male patients with STEMI. A recent US study found that the proportion of STEMIs in the younger MI patients (<55 years) was much higher than in the older patients, and that higher rates of smoking and obesity in the young was a likely factor.23 Our study shows that Irish female patients are older than their male counterparts in all categories, and that male and female NSTEMI patients are 3–5 years older than STEMI patients.

    Comparison of in-hospital mortality across populations is fraught with problems, as factors such as out-of-hospital survival, the type and timing of reperfusion therapies, comorbidity status, reduced LOS over time, methodologies in data collection, coding and statistical analysis, all confound the assessment. Nonetheless, similar to our study, a relative reduction of approximately 30% in in-hospital mortality rates for AMI has been shown in the USA28 and Canada18 whether crude or age-adjusted rates were studied. Furthermore, similar crude in-hospital mortality rates for STEMI were seen in Ireland (15.0%) in the late 1990s, in the Berlin Registry (13.0%),24 as well as in Roger's study in the USA (11.5%).28 Notably, however, crude in-hospital mortality for STEMI in many European countries in the late 2000s was under 7% compared with our finding of 10.5%.29 Finally, our finding of a greater decrease in NSTEMI in-hospital mortality rates compared with STEMI is reflected in a number of studies.18 ,23 ,24

    The decline to 6 days for median LOS in AMI by 2008 found in our study compared favourably with data in 2000 reported by McIntyre from Scotland.17 Our finding of an LOS for STEMI (5 days) and NSTEMI (7 days) in 2008 was shorter than that reported from the Berlin Registry (both 8 days, 2004).24 It is to be expected that LOS will decrease further with a fully implemented primary percutaneous coronary intervention service in Ireland as median LOS of 3 days are reported for STEMI, and LOS of 4 days for NSTEMI following early angiography.30

    This study of trends over 12 years in AMI hospitalisations in Ireland must be viewed in the context of a number of changes in administrative and clinical practice during the time period. Coding changes from ICD-9 to ICD-10 in Ireland in 2005 may have had an effect, the extent of which has not been determined. The increased use of more sensitive biomarkers, especially from 2000 onwards, would be expected to lead to an increase in AMI hospitalisations contrary to our findings for trends in AMI.

    There are a number of limitations to this study. Though utilising an administrative database with established quality control, the absence of validation of diagnosis and subsequent coding is a weakness. However, the central finding of reduction in AMI hospitalisation rate, and the shift in dominance from STEMI to NSTEMI, echoes other studies.15 ,23 Also, due to the lack of a unique patient identifier, we were unable to link events in different hospitals, with resultant inability to study incidence rates, readmission rates or 30-day mortality. However, by excluding transferred patients in the analysis of trends in hospitalisation, we believe that we have addressed the possibility of multiple counting during the one clinical event.

    Nonetheless, strengths of this study are that it is a population-based examination of data collected consistently over time, it includes data from all the public hospitals in Ireland with few AMI patients going directly to private hospitals prior to 2008 due to lack of Emergency Departments.

    In an era of focus on compliance with evidenced-based care, we present epidemiological surveillance of AMI and its subcategories in Ireland from 1997 to 2008. We draw attention to a steady decrease in numbers hospitalised with AMI, and a shift in trend away from STEMI towards rising numbers of NSTEMI patients who are increasingly older. This is important information for both clinicians and policy makers in an ageing and increasingly obese population. Further detailed epidemiological surveillance of acute coronary syndromes and allied conditions, such as heart failure and dysrhythmias, using various methodologies and lessons learnt from the MONICA era, is needed at international and national levels to guide policy making and service evaluation.

    Key messages

    • We find a steady decrease in hospitalisation rates with acute myocardial infarction, and a shift away from ST elevation myocardial infarction, towards rising rates of non- ST elevated myocardial infarction patients who are increasingly older.

    • In an ageing population, and with increasing survival rates, surveillance of acute myocardial infarction and allied conditions is necessary to inform clinicians and policy makers.

    Acknowledgments

    The Economic and Social Research Institute, in association with the Health Service Executive, manage the Hospital Inpatient Enquiry (HIPE) system.

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    Footnotes

    • Competing interests None.

    • Provenance and peer review Not commissioned; externally peer reviewed.