Explaining trends in coronary heart disease hospitalisations in New Zealand: trend for admissions and incidence can be in opposite directions

Background: A recent increase in the absolute number of hospitalisations for acute myocardial infarction (AMI) in New Zealand may signal a new epidemic of coronary heart disease (CHD). Objective: To quantify the impact of factors other than incidence of disease on these national hospitalisation trends. Methods: A total of 324 663 electronic records of New Zealand public CHD hospitalisations from 1993 to 2005 were examined. Repeat admissions were identified by record linkage using a unique national health identifier for each patient. Results: Hospitalisations for AMI increased by about 8% a year throughout the 13-year study period. Interhospital transfers increased by 117% over the study period, while readmissions increased by 42%. By 2005 over 60% of all admissions for CHD were readmissions. After accounting for readmissions, hospital transfers and population changes, the age-standardised first AMI hospitalisation rate peaked in 1995 and has since declined by 15%. Reciprocal trends in AMI and angina hospitalisations were seen, indicating changing diagnostic criteria. Overall hospitalisation rates for first CHD events remained relatively steady at about 216.4 events per 100 000 between 1993 and 2000 and subsequently declined by 25% to 162.2 events per 100 000 in 2005. Conclusion: Recent trends in hospitalisation rates for AMI are significantly influenced by factors other than underlying changes in CHD incidence. Increasing absolute numbers of admissions coded as AMI in New Zealand between 1993 and 2005 can be accounted for by increases in readmissions, increases in interhospital transfers, changes in diagnostic criteria for AMI and in demography.