Increased acute myocardial infarction mortality following the 1995 Great Hanshin-Awaji earthquake in Japan

Keiko Ogawaa, Ichiro Tsujia, Keishi Shionob and Shigeru Hisamichia

a Department of Public Health, Tohoku University School of Medicine, Sendai, Japan.
b Department of Civil Engineering, Nagaoka College of Technology, Nagaoka, Japan.

Reprint requests to: Keiko Ogawa, Department of Public Health, Tohoku University School of Medicine, 2–1, Seiryo-machi, Aoba-ku, Sendai, Miyagi, 980–8575, Japan. E-mail: keikoo{at}mail.cc.tohoku.ac.jp


    Abstract
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Background This study examined the factors affecting mortality from acute myocardial infarction (AMI) following the 1995 Great Hanshin-Awaji earthquake.

Methods We examined the death certificates of all decedents between January 1994 and December 1996 in 16 municipalities, which covered most of the area affected by the 1995 Great Hanshin-Awaji earthquake. We analysed the extent and duration of the increased mortality from AMI. The standardized mortality ratio (SMR) of AMI was calculated weekly after the earthquake, taking the number of AMI deaths during the same period in 1994 as a reference. The main outcome measures were the number of deaths from AMI (ICD-9 410; ICD-10 I21, I22) in the study area before and after the earthquake, and the weekly SMR after the earthquake.

Results A significant increase in mortality from AMI in the study area as a whole continued for about 8 weeks after the earthquake. There was wide variation amongst the regions with respect to the extent and duration of the increased mortality from AMI. The SMR of AMI showed a positive relationship with the percentage of houses which were completely destroyed, and was almost significant (r = 0.530, P = 0.062).

Conclusions The duration of increased cardiac mortality after the 1995 Great Hanshin-Awaji earthquake was longer than seen with previous earthquakes. Further analysis to identify the factors affecting cardiac mortality is needed so that we may reduce adverse health effects during the recovery stage following natural disaster.

Keywords Acute myocardial infarction, 1995 Great Hanshin-Awaji earthquake, natural disaster

Accepted 17 December 1999


    Introduction
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
An increase in mortality from cardiac disease following earthquakes has been reported in several countries; namely Greece,1,2 Italy,3,4 Australia,5 USA6–9 and Japan.10,11 For example, there was a significant increase in cardiac mortality on the day of the Northridge earthquake, which hit the Los Angeles area on 17 January 1994.7 In most studies, increased cardiac mortality was sustained for a maximum of a few days. However, in the case of the 1995 Great Hanshin-Awaji earthquake, increased cardiac mortality was reported over a number of weeks.10,11 These reports were limited in two respects; firstly, they examined cardiac events in an area of Awaji island only and did not include Kobe City, where the earthquake damage was more serious. Secondly, they followed the cardiac events only for a few months after the earthquake. Therefore, the extent and duration of the increase in mortality from cardiac disease following this earthquake has not been fully investigated.

In order to analyse the characteristics of increased cardiac mortality following earthquakes, we need to observe long-term trends over one year in the affected area. Identification of the factors affecting cardiac mortality may lead us to the provision of effective prevention of the indirect long-term health effects of earthquakes.

The objectives of our study are to characterize increased cardiac mortality following the 1995 Great Hanshin-Awaji earthquake in comparison with the baseline value and to identify the factors related to increased cardiac mortality. For this purpose, we analysed the extent and the duration of increased mortality due to acute myocardial infarction (AMI) in 16 municipalities, which covered most of the area affected by the earthquake.


    Subjects and Methods
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
The 1995 Great Hanshin-Awaji Earthquake hit the southern part of Hyogo Prefecture on 17 January 1995. The epicentre was located on the northern tip of Awaji island. The magnitude was given as 7.2 by the Japanese Meteorological Agency.12

The study area comprised 16 municipalities; the nine districts consisted of Kobe City, five other cities (Amagasaki, Ashiya, Itami, Nishinomiya, Takarazuka) in the Hyogo prefecture and two municipalities (Sumoto and Tsuna) on Awaji island.

Table 1Go shows the number of human casualties, the crude death rate, and the percentage of completely destroyed houses in each of the 16 municipalities. The total number of human casualties in the study area included more than 95% of the total number of human casualties from the 1995 Great Hanshin-Awaji Earthquake (5470/5480).


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Table 1 The number of human casualties, the crude death rates and the percentages of houses which were completely destroyed houses in 16 municipalities
 
We examined the death certificates of all the decedents in the above 16 municipalities between January 1994 and December 1996. These data were provided by the Ministry of Health and Welfare in Japan with official permission (No. 792), after each person's name or other identification had been deleted. The data included date of death, age at death, underlying cause of death, sex and the area code for each decedent. The cause of death was coded according to the International Classification of Diseases, Ninth Revision (ICD-9) for 1994 and Tenth Revision (ICD-10) for 1995 and 1996.

We counted the number of deaths from AMI (ICD-9 410; ICD-10 I21, I22) in the study area at weekly intervals. We compared the values before and after the earthquake by calculating the weekly standardized mortality ratio (SMR) after the earthquake, taking the number of AMI deaths during the same period in 1994 as a reference.

In order to calculate the death rates for 1995, we obtained the population from the 1995 Population Census of Japan. Since the population census is conducted every 5 years in Japan, the population in 1994 was extrapolated from the 1985 and 1990 censuses to reduce the influence of the earthquake in 1995. The population in 1996 was calculated by modifying the method applied in Population Projections for Japan.13

The statistical significance of the SMR was analysed using the {chi}2 test. The impact of seismic force on the human casualties and excess mortality from AMI was tested using the Pearson product-moment correlation coefficient (r) on the variables and percentage of houses that were completely destroyed. In all statistical analyses, P < 0.05 was considered statistically significant.


    Results
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 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
The total number of AMI deaths in 1995 amounted to 546, more than twice the number in 1994 (266). Figure 2Go shows the number of deaths from AMI in the study area (16 municipalities) from 2 August 1994 to 31 July 1995.



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Figure 2 The number of deaths from acute myocardial infarction (AMI) in the study area from August 1994 to July 1995

 
Taking into consideration the seasonal variation in AMI deaths, we calculated the SMR of AMI weekly after the earthquake, using the number of AMI deaths during the same period in 1994 as a reference. The number of deaths rose sharply after the earthquake. During the first week from 17–23 January, there were 68 deaths and the SMR was 8.4 (P < 0.05) in the study area. A significant increase in the SMR of AMI deaths continued for about 8 weeks until 13 March. The number of AMI deaths (N) and the SMR after the first week were as follows; N = 71, SMR = 3.2 (P < 0.05) for 24–30 January; N = 59, SMR = 2.7 (P < 0.05) for 31 January–6 February; N = 37, SMR = 3.0 (P < 0.05) for February 7–13; N = 38, SMR = 1.3 (P = 0.09) for February 14–20; N = 30, SMR = 1.5 (P < 0.05) for 21–27 February; N = 34, SMR = 1.6 (P < 0.05) for 28 February–6 March; N = 39, SMR = 1.9 (P < 0.05) 7–13 March.

There was wide variation amongst the regions with respect to the temporary trend in increased mortality from AMI. Figures 3–5GoGoGo show three examples; the number of deaths from AMI every 2 weeks in the Higashi-Nada, Nada and Nagata districts, Kobe City, respectively between August 1994 and July 1995. We compared the values before and after the earthquake by calculating the biweekly SMR following the disaster, taking the mean number of AMI deaths during the same period in 1994 as a reference. In Higashi-Nada (Figure 3Go), sharply increased mortality was observed during the first 4 weeks following the earthquake. In Nada (Figure 4Go), a significant increase continued for 8 weeks. In Nagata (Figure 5Go), AMI mortality increased beyond the pre-disaster level during the first 2 weeks only.



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Figure 3 The number of deaths from acute myocardial infarction (AMI) in Higashi-Nada from August 1994 to July 1995

 


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Figure 4 The number of deaths from acute myocardial infarction (AMI) in Nada from August 1994 to July 1995

 


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Figure 5 The number of deaths from acute myocardial infarction (AMI) in Nagata from August 1994 to July 1995

 
These regional variations in the temporary trend of increased AMI mortality were observed despite the fact that there were no regional differences in either the crude death rate of human casualties (per 100 000) or the percentage of houses that were completely destroyed (Table 1Go; 849 per 100 000 and 22.9%, 879 per 100 000 and 18.9%, 788 per 100 000 and 21.0% at Higashi-Nada, Nada, and Nagata, respectively.

Table 2Go shows the absolute number of AMI deaths between January and March in 1994, 1995 and 1996, and the SMR and P-values for these 3 months. A statistically-significant increase (P < 0.05) in AMI was observed in 1995 in Higashi-Nada, Nada, Hyogo, Nagata, Suma, Tarumi, Chuou, Amagasaki, Nishinomiya, Ashiya, Itami, Takarazuka and Tsuna. Nada showed the largest ratio, followed by Tsuna. There was almost no change in the number of AMI deaths in Kita between 1994 and 1995.


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Table 2 The absolute number of acute myocardial infarction (AMI) deaths between January and March in 1994, 1995 and 1996 and the standardized mortality ratio (SMR) and P-values for these 3 months
 
A significant positive correlation (r = 0.897, P = 0.001) was observed between the crude death rate of human casualties and the percentage of houses which were completely destroyed (Figure 6aGo). The SMR of AMI showed a positive relationship with the percentage of houses which were completely destroyed (Figure 6bGo), and was almost significant (r = 0.530, P = 0.062); r being smaller than in the cases of human casualties. There was no significant relation between the duration of increased AMI mortality and the SMR of AMI.



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Figure 6 (a)Correlation between crude death rate of human casualties and percent of completely destroyed houses

(b)Correlation between SMR and AMI from January to March 1995 and percent of completely destroyed houses

 

    Discussion
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Both direct and indirect health effects are observed after an earthquake. Direct health effects, such as casualties trapped by collapsed buildings, occur in the impact phase. Indirect health effects are defined as any that are not directly caused by seismic force. Cardiac events would be one example. Indirect health effects may be sustained for a few months as described in our study. This classification of health effects is rather conceptual, but explains well the different aetiology and suggests the necessity for different approaches to preventative measures.

In this study, we observed the cardiac events for 3 years including the pre- and post-earthquake phases and covered most of the regions that received structural damage following the 1995 Great Hanshin-Awaji earthquake. More prolonged duration of increased cardiac mortality in comparison with past earthquakes was observed in the study area. A significant increase in mortality from AMI in the study area as a whole continued for about 8 weeks after the earthquake. The duration of increased mortality from AMI varied with municipality; it was 2 weeks at Nagata, 4 weeks at Higashi-Nada, 8 weeks at Nada, and about 1 year at Chuou, Amagasaki, and Itami. There was no significant relation between the duration of increased AMI mortality and the SMR of AMI. The SMR of AMI showed a positive relationship with the percentage of completely destroyed houses, and was almost significant.

There were considerable differences in the duration of cardiac mortality between the 1995 Great Hanshin-Awaji earthquake and previous earthquakes. The prolonged increase in AMI mortality can be explained by a sustained increase in the incidence of AMI. Most previous studies concluded that psychological or emotional stress at the onset of the earthquake accounted for the increased cardiac mortality. The 1995 Great Hanshin-Awaji earthquake caused a far greater number of deaths in comparison with those seen in previous earthquakes. For example, the Northridge earthquake killed 61 people, injured 7000 and left 50 000 homeless.14 The 1995 Great Hanshin-Awaji earthquake killed approximately 6000 people, injured 37 000 and left 310 000 homeless. Such a large-scale disaster is likely to cause chronic stress amongst the population during the recovery and reconstruction stage, resulting in prolonged cardiac events, although there has been no data of AMI incidence before and after the earthquake in the study area. Alternation in cardiovascular risk factors after the 1995 Great Hanshin-Awaji earthquake has been reported.15,16

Another explanation for the increased AMI mortality is the worsening of prognosis in AMI cases due to the loss or damage of hospital function. Baba reported on the loss of or damage to hospital function due to disruption of the lifeline following the 1995 Great Hanshin-Awaji earthquake.17 It was reported that in Kobe City, 103 of the 112 hospitals and 763 of the 1363 clinics suffered some damage.14 Quantitative analysis will be required to investigate the possibility that loss of hospital function caused by structural and non-structural damage after the earthquake is related to the worsening of prognosis in AMI.

Some other factors might be responsible for the increase in AMI mortality after the 1995 Great Hanshin-Awaji earthquake. For instance, socioeconomic status might influence the degree of structural damage, recovery process and access to medical care; these factors might confound the present results. Our study was solely based upon review of death certificates, in most of which verification by autopsy was not made. We therefore must admit the possibility of misclassification of cause of death during the period following the earthquake. The factors responsible for indirect health effects due to the 1995 Great Hanshin-Awaji earthquake are yet to be clarified.

In this paper, we presented the characteristics of the increased mortality from AMI following the 1995 Great Hanshin-Awaji earthquake as an indirect health effect. Unlike the direct health effects of disaster, indirect health effects are predictable and preventable. Knowledge on the factors causing indirect health effects can help us to plan appropriate resource mobilization during the recovery phase following natural disaster. Further investigation is required to clarify the factors affecting regional variation with respect to the extent and duration of increased cardiac mortality.



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Figure 1 Map of the study area (16 municipalities) and the epicenter of the 1995 Great Hanshin-Awaji Earthquake

 

    Acknowledgments
 
This study was supported in part by grant-in-aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture of Japan (Grant No. 09234102). The authors would like to thank Dr Eric K Noji, Department of Emergency and Humanitarian Action, the World Health Organization, for his valuable comments on the manuscript.


    References
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 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
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