Table 1: Age and sex distribution of those hospitalized due to cardiovascular and respiratory diseases (group and subgroup)

Respiratory		Cardiovascular		Disease Group Age Group
Female	Male	Female	Male	Disease Group Age Group
647 (16.1%)	927 (20.2%)	37 (0.8%)	31 (0.6%)	0-4
190 (4.7%)	250 (5.5%)	13 (0.3%)	16 (0.3%)	5-14
1679 (41.8%)	1703 (37.2%)	2223 (48.9%)	3117 (62%)	15-64
543 (13.5%)	626 (13.7%)	1106 (13.3%)	915 (17.9%)	65-74
386 (9.6%)	423 (9.2%)	606 (13.3%)	473 (9.3%)	75-79
573 (14.3%)	650 (14.2%)	562 (12.4%)	506 (9.9%)	80 and higher
4018 (100%)	4579 (100%)	4547 (100%)	5112 (100%)	Total

Table 2: Percent change of hospitalization due to cardiovascular and respiratory diseases associated with a 1 °C variation

P	% change (95% CI)
< 0.001	-1.00 (-1.26 to -0.63)	Total Cardiovascular
0.0335	-0.73 (-1.41 to -0.05)	Angina pectoris
0.1128	-0.83 (-1.842 to 0. 20)	Acute myocardial infarction
0.0242	-0.59 (-1.10 to -0.08)	Ischemic heart disease
< 0.001	-2.56 (-3.58 to -1.53)	Heart failure
< 0.001	-4.34 (-4.64 to -4.02)	Total respiratory
< 0.001	-3.78 (-4.93 to -2.63)	Asthma
< 0.001	-5.47 (-5.97 to -4.97)	Influenza and pneumonia

Discussion

Cardiovascular diseases

The obtained results of this research showed that cold weather can cause an increased risk of hospitalization due to cardiovascular diseases in this region. In a study conducted in Copenhagen, Denmark, it was observed that cardiovascular disease is inversely related to temperature (30). In another study conducted in Shanghai, China, similar results were obtained (7). The mechanism of cold temperature in increasing the risk of cardiovascular diseases is still unclear (31). However, several risk factors, such as plasma cholesterol, plasma fibrinogen, blood pressure, and red and white blood cell count, can partially explain this mechanism (30). The study by Michelozzi et al., which examined the relationship between temperature and hospitalizations due to cardiovascular disease in 12 different countries in Europe, showed that temperature has a direct relationship with the hospitalization due to cardiovascular diseases (29). This finding was not in agreement with the results of the present study. Furthermore, the results of this study were consistent with that of the study performed in Beijing (32). Investigation of the relationship between temperature and daily hospitalization rate due to cardiovascular disease in the 4 major cities of South Korea showed that daily ambient temperature increased hospitalizations rate for this group of patients (22). The results of this study are consistent with the results of the present study. The main mechanism of the adverse effects of high temperatures may through the blood circulation system of the human body heat in regulating body temperature, when the layers beneath the skin send blood to the vital organs. It also increases blood viscosity due to diarrhea, excessive sweating, and increased cholesterol levels; other possible mechanisms can also be considered (15, 33).The age-adjusted relationship (one of the most important predictors) between temperature and the mean number of hospitalizations was evaluated in this study. The results of this study suggest that there is no significant difference among different age groups in hospitalization due to cardiovascular disease. A study in Denmark showed no differences among age groups of patients (30). These results were consistent with our findings. The findings of other studies in China, South Korea, and America showed that patients older than 70 years were significantly lower compared with other age groups (22, 32, 34). These findings were consistent with the results of the present study. Elderly patients’ weak physiological systems and inability in environmental adaptation are some of the factors that causes sensitivity to drastic ambient temperature changes and increased hospitalization in this age group (15, 20, 35-39). Gender is another deflator which was examined. The results showed no significant difference between males and females in terms of the mean number of hospitalizations due to cardiovascular disease. These results were consistent with findings from studies in America, England, and France (40-42). However, another study showed higher risk of coronary events in women than in men in hot weather (43). These results are consistent with the findings of the present study.

Many studies have evaluated cardiovascular diseases as their major groups (4). This study also examined the major categories of cardiovascular disease. In this study, the relationship between mean temperature and mean number of hospitalizations due to acute myocardial infarction was not significant. This is consistent with the results found in the literature review. The obtained results from myocardial infarction patients in Melbourne, Australia, showed that the number of hospitalizations due to this disease increases with the decreasing of temperature (44). In this regard, studies in Taiwan, Germany, the UK, and Norway have also been obtained (45-48). The results of another study conducted in the UK, were contradictory to the mentioned studies. In this study, it was shown that hospitalization due to acute myocardial infarction significantly increased as a result of the increase in temperature (49). The heterogeneity in results may be described in any particular area of environment (45).

Another important subgroup of cardiovascular diseases is angina pectoris, which was examined in this study. The results showed that the mean temperature decreases hospitalization. A study in California, USA, showed that the decline in temperature causes an increase in the minimum threshold for hospitalization due to angina pectoris (34). This finding was in agreement with the results of the present study. Ischemic heart disease and other cardiovascular diseases are subgroups of the main studied groups. The results showed a relationship between the average number of patients and mean temperature. This is consistent with the results obtained from literature review. Survey data obtained in California, USA, in terms of hospitalization showed a direct relationship between temperature and mean number of hospitalizations due to strokes (20). The last group of cardiovascular disease that was studied had heart failure results of studies in South Korea and was consistent with another study in America (22, 50). In the present study, mean temperature was found to be inversely related to mean number of hospitalizations due to this disease. The literature review showed that previous study results are consistent with the findings of the present study. A study in the state of New York, USA, showed that there is a direct relationship between temperature and hospitalizations due to heart failure (51). These results are consistent with findings of studies conducted by Green et al. and Ostro et al. (20, 50).

Respiratory disease

The results showed that mean temperature is inversely related to hospitalization due to respiratory diseases. A study conducted in Finland, in 2009, showed that temperature and hospitalization due to respiratory diseases have a significant relationship (52). These results were consistent with the findings of the studies performed in Shanghai, China, and Taiwan (7, 28). A study conducted in 12 cities in Europe yielded mixed results. The study findings highlight the fact that the rate of hospitalization due to respiratory disease increases with the increasing of temperature (29). These results were also reported by Ostro et al., Lin et al., Green et al., and Wang et al. (20, 32, 50, 53). It should be noted that the exact mechanism of hot and cold ambient temperatures on physiological systems remains unknown. These diseases may be directly caused by inflammation of airway responsiveness due to temperature or indirectly by viral infections, pollens, air pollution, mold, number of hours spent outdoors, and activities that aggravate the disease (22, 26, 54). The results for different age groups showed that the rate of hospitalization due to respiratory disease is significantly higher in the 0-4-year age group than other age groups. These results were consistent with the findings of a study conducted in the USA in 2007 (55). In addition, another study in Europe showed that a significantly higher number of patients in the age group of higher than 75 years were hospitalized (29). These findings were consistent with that of the present study.

The results of this study showed no significant difference in the mean number of hospitalizations due to respiratory disease between the sexes. In literature review, no study was found to consider differences among sexes. Asthma is one of the most common types of respiratory diseases that were examined in this study. Results of this study show that with increasing temperature, the mean number of hospitalizations due to this disease significantly reduced. These findings were consistent with the results of studies conducted in Greece and Finland (52, 56). In two studies in the states of North Carolina and New York in the USA, a direct relationship was observed between rising temperatures and increased hospitalization due to this disease (26, 51). These results were consistent with the results of our study. Pneumonia and influenza are other respiratory diseases that are among the major groups studied. The findings of this study showed that the mean number of hospitalizations due to these conditions is related to average temperatures. The results of this study were consistent with that of two studies performed in China and Taiwan (28, 57). These results were also consistent with findings of the present study. Cold weather increases population density and reduces air conditioning and this may cause infectious diseases, and pneumonia and influenza by direct contact transmission (20, 50). The results of studies that examined the relationship between ambient temperature and hospitalizations due to cardiovascular and respiratory diseases with different designs and in different countries and cities were not homogeneous. This may be because people in different parts of the world may not have the same tolerance level to certain climates that have adapted their residences (45). Aside from the mentioned geographic heterogeneity of studies, examining the relationship between temperature and hospitalizations due to cardiovascular and respiratory diseases, there are other factors that can be effective in this heterogeneity. For example, the probability of acceptance for a particular disease may be have different reasons such as the availability of primary care, outpatient services for patients, community care area level socioeconomic deprivation, and community associated clinical management (29).

Conclusions

Our study showed that reduction in mean temperature has been associated with increasing the mean number of hospitalizations due to cardiovascular and respiratory diseases. This association was very strong for respiratory diseases compared to for cardiovascular diseases. According to our findings, people who suffer from cardiovascular and respiratory diseases should be forbidden from exposure to extremely cold temperatures. In case of as a significant reduction in environment temperatures the health system should warn at risk people and children's.

Acknowledgement

We would like thank Rafsanjan University of Medical Sciences for their cooperation in carrying out this research.

Conflict of interests: None declared.

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* Corresponding author: Reza Vazirinejad, Professor, Dept. of Social Medicine, Rafsanjan University of Medical Sciences, Rafsanjan, Iran.

Email Address: rvazirinejad@yahoo.co.uk

Total respiratory		Total cardiovascular
P	% change (95% CI)	P-Value	% change (95% CI)
< 0.001	-9.9 (-10.59 to -9.21)	*	*	0-4
< 0.001	-5.61 (-6.97 to -4.22)	*	*	5-14
< 0.001	-3.10 (-3.60 to -2.59)	< 0.001	-1.03 (-1.44 to -0.6)	15-64
< 0.001	-3.02 (-3.90 to -2.14)	0.1959	-0.46 (-1.16 to 0.24)	65-74
< 0.001	-2.61 (-3.51 to -1.98)	0.0325	-1.02 (-1.94 to -0.08)	75-79
< 0.001	-2.57 (-3.40 to -1.73)	0.0033	-1.42 (-2.35 to -0.47)	80 and higher