Background: Acute myocardial infarction (AMI) is a major health problem worldwide and in Vietnam. In addition to the list of recognised genetic, behavioural and environmental risk factors for AMI, there is increasing concern about the ambient temperature effects associated with global climate change on cardiovascular diseases in general, and AMI in particular. Vietnam is one of the countries that has been forecast to suffer more disadvantageous weather and natural disasters due to human-induced global warming. However, very few studies on temperature-related health effects have been conducted in Vietnam. In particular, to date, no study has focused on possible links between air temperature extremes and AMI, one of the most important causes of morbidity and mortality in the country. The present study in the central coast region of Vietnam investigated ambient temperature effects and hospital admissions for AMI. This study aimed to: 1) explore the pre-hospital delay period and its associated factors among AMI patients living in the Central Coast region of Vietnam from 2008 to 2015, 2) examine the long-term trends and seasonality of AMI hospital admissions, 3) estimate the short-term effects of ambient temperature on daily adult AMI hospital admissions , and 4) evaluate the added effects of extreme temperature conditions (heat waves and cold spells) on daily adult AMI hospital admissions during this period. 

Methods: A retrospective ecological study design was used. Data were collected from a total of 3,328 hard-copy medical records of AMI patients hospitalised in three highest-level hospitals in the Central Coast region of Vietnam from 2008 to 2015.  Information on weather and influenza circulation was obtained from the Vietnamese National Hydrometeorology and Environment Network Centre (National Hydrometeorological and Environment Network Center, 2016) and Statistical Yearbooks of Infectious Diseases (Department of Preventive Medicine-Vietnamese Ministry of Health 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016). A time-series analytic approach with a generalised linear model /distributed lag non-linear model, and negative binomial regression were used to examine the pre-hospital delay time period, seasonality of AMI hospital admissions, and impacts of ambient temperature variations/extremes on local AMI hospital admissions. Long-term trends, the seasonality of AMI hospital admissions, humidity, wind speed, air pressure, influenza like illness counts, and weekend days and holidays were also controlled for when estimating the effects of temperature variations/extremes.

Results: The results showed that 46.1% of local AMI patients experienced delay in their first presentation to a medical centre by 12 or more hours after the disease occurrence. This put the patients at higher risk of not receiving reperfusion therapy, an effective AMI treatment. The groups likely to have longer pre-hospital delay time were women, the elderly, patients hospitalised at low-level medical centres, those who had less severe health conditions at the onset, those who had non-ST elevated myocardial infarction, and individuals with at least one comorbidity. 

AMI hospital admission rates were significantly higher in winter compared to those in summer. January-February was the peak time for AMI hospitalisations, while the hospitalisation rates reached their trough in July-August. Interestingly, there were significantly more daily hospital admissions due to AMI when the ambient temperature was high in the South Central Coast region (savanna tropical climate); while in the North Central Coast region (monsoon tropical climate), a significantly higher rate of AMI hospitalisation was found for lower temperatures. Moreover, on days of heat wave exposure, the rate of daily hospital admissions for AMI was 22% (95% CI: 4%–44%) higher than those not exposed heat waves on the South Central Coast. However, on days of cold spell exposure, the rate of daily hospital admissions for AMI was 35% (95% CI: 3%–76%) higher than those not exposed cold spells on the North Central Coast. Males, younger age groups, those with ST-segment elevation myocardial infarction, and patients without comorbidities showed significant increases in risk of AMI admissions associated with heat waves in the South Central Coast region. In contrast, being male and elderly were found to be significant risk factors for AMI admissions in relation to cold spells in the North Central Coast region.

Conclusion and Discussion: The effect of seasonality and ambient temperature extremes on hospitalisation due to AMI differed between the North Central Coast and South-Central Coast regions, as well as among different population subgroups. Among studies that explore the associations between ambient temperature and AMI morbidity worldwide, there are inconsistent  findings  cross regions, countries, latitudes and climates (Bijelović et al., 2017; Fernández-García et al.; Honda et al., 2016; Kwon et al., 2015; Ravljen et al., 2018; Tian et al., 2016; Loughnan et al., 2014; Mohammadi et al., 2018; Morabito et al., 2006; Wijnbergen et al., 2012; Yamaji et al., 2017). The current study adds to the global evidence regarding sub-regional variation. While the effects were not large, they were significant and importantly, the study shows they are detectable even across sub-regions of a primarily tropical-climate country.

The study could contribute to the development of targeted public health strategies to reduce pre-hospital delay such as introducing education programs about early signs, symptoms and the importance of early hospitalisation after initial signs of the onset of AMI. It is necessary to raise the awareness of the population, especially vulnerable groups (such as the elderly, outside workers, and poorer people) about the risk of AMI from exposure to temperature extremes. We recommend the Vietnamese Government should incorporate health messages relating to AMI (and other serious environment-related health conditions) in the form of warnings during the weather forecasts in terms of extreme weather, as happens in many countries worldwide Further, the government should incorporate recognition of the population health impacts of extremes of temperature into legislation and national targets CVDs prevention and climate change adaptation.