Temporal trends in myocardial infarction case fatality: methodological challenges and prevention targets
Authors: Nedkoff L , Wright FL , Sanfilippo FM , Briffa TG 
Published: May 2022

Abstract

Funding Acknowledgements
Type of funding sources: Public grant(s) – National budget only. Main funding source(s): National Health and Medical Research Council of AustraliaHealy Medical Research Foundation

Background
Case fatality is an important indicator of severity and quality of care for myocardial infarction (MI). Most studies focus on hospitalised case fatality, which does not capture the total burden of MI deaths. Additionally, changes in diagnostic criteria and acute clinical care, and evidence that identifying all coronary heart disease (CHD) events better captures this patient group,1 have led to suggestions that a different definition of case fatality is required.

Purpose
The aim of the study was to determine the impact of different definitions of case fatality on the composition of fatal cases, and to measure trends in case fatality across a range of case definitions.

Methods
A whole-state linked hospital/death dataset was used to identify all MI, acute coronary syndromes (ACS) and CHD events (fatal + nonfatal) from 1997-2015. The traditional MI case fatality definition included all MI deaths as the numerator, stratified as MI hospitalisation with death ≤28 days, non-MI hospitalisation with MI death ≤28 days, or pre-hospital MI deaths. The denominator was all MIs (fatal plus nonfatal MI). ACS and a broader CHD definition were trialled, with ACS or CHD deaths as the numerator respectively, stratified in a similar manner as MI. Case fatality was age-standardised by 5-year age group using the internal age distribution of each definition as the standard.

Results
From 1997 to 2015, there were 76,928 MI events, 126,470 ACS events, and 235,100 CHD events. Of the MI cohort, 64.1% were men, and 13.0% had a prior MI, with a similar pattern in the ACS and CHD cohorts. For the traditional definition of MI case fatality, 10,819 deaths (53.9%) occurred pre-hospital, 4990 deaths in those hospitalised for MI and dying ≤28 days (24.8%), and 4271 MI deaths (21.3%) ≤28 days of a non-MI hospitalisation (Figure). Using the broadest CHD definition of case fatality, there was a similar proportion of pre-hospital deaths, but a higher proportion of CHD deaths in those with a non-CHD hospitalisation.
In men, age-standardised MI case fatality declined from 40.0% in 1997 to 17.3% by 2015; in women, the decline was from 41.9% to 18.2%. In contrast, using the ACS and broad CHD definitions, age-standardised case fatality was lower than for MI throughout the study period, with a smaller temporal decline (ACS: men 22.2% to 13.1%, women 20.5% to 13.0%; CHD: men 20.6% to 12.4%, women 19.3% to 12.3%).

Conclusion
Despite substantial falls in MI case fatality, the fatal burden remains high. Regardless of the case fatality definition, pre-hospital deaths from acute or all CHD have remained high over time, highlighting the need to target the pre-hospital setting. Caution is needed when using different definitions of case fatality to ensure relevant statistics are used, particularly for temporal trends.
Figure Legend. Proportion of MI deaths occurring ≤28 days after MI hospitalisation (2), ≤28 days after non-MI hospitalisation (3), and pre-hospital (4).