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Physical activity during summer and recognition of heatstroke prevention among patients with cardiovascular disease in an urban area of Japan: a descriptive study

BMC Research Notes volume 18, Article number: 42 (2025) Cite this article

Abstract

Objective

Patients with cardiovascular disease are considered a high-risk population for heat-related illnesses. This study aimed to describe the difference in physical activity between summer and fall among patients with cardiovascular disease and their recognition of heatstroke prevention in an urban area with high temperature conditions.

Results

We enrolled 56 outpatients who participated in cardiac rehabilitation in the summer of 2022 (median age, 75 years [interquartile range, 68–80]). Physical activity level for each patient was assessed using the International Physical Activity Questionnaire at baseline in summer (August) and follow-up in fall (October). Changes in exercise habits and frequency of going outside during the study period and the patients’ recognition of heatstroke prevention were assessed using questionnaires. The prevalence of low physical activity was higher in summer than in fall (57.1% vs. 37.5%, p = 0.013). Of the patients involved in self-exercise before summer, 33.9% answered that their exercise habits decreased in summer. Of them, 47.4% felt that their exercise habits remained decreased in fall. Regarding the frequency of going outside, 82.1% went outside less in summer. Most participants were highly interested in heatstroke prevention.

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Introduction

Increasing the level of physical activity is recognized as a core component of primary and secondary prevention of cardiovascular disease (CVD) [1, 2]. Exercise-based cardiac rehabilitation (CR) improves cardiovascular risk factors, quality of life, and prognoses of various CVDs [3]. Due to the dose–effect relationship between physical activity volume and cardioprotective effects [4,5,6], guidelines have recommended performing at least 150 min/week of moderate intensity or 75 min/week of vigorous intensity aerobic physical activity [1, 2]. In general, adults are involved in moderate to vigorous physical activity only for 3–8% of the day [7]. To achieve target physical activity levels, patients should incorporate daily activities alongside facility-based CR [8]. Individualized counseling should ensure safe unsupervised exercise, considering tailored prescriptions based on functional capacity, fall and injury prevention, and a suitable exercise environment [9].

Global warming has increased the risk of heat-related illnesses, including heatstroke, particularly over the past two decades [10]. Risk factors for heatstroke include older age, CVD, medications such as beta-blockers and diuretics, and low physical fitness [11]. Thus, patients with CVD are at high risk of developing heatstroke, and heatstroke prevention should be considered in physical activity counseling during summer. From another perspective, a recent study demonstrated that passive heat exposure increased myocardial blood flow, possibly predisposing myocardial ischemia [12]. Based on the above evidence, understanding the impact of environmental changes on the pathophysiology and lifestyle of patients with CVD is an important research topic. However, there is little research on the influence of the environment on physical activity, and current guidelines lack adaptations for seasonal or climate-specific challenges. In Japan, over 50,000 people are transported annually for heatstroke due to the hot, humid summers and urban heat island effects [13]. Therefore, some patients avoid outdoor activities on hot days, while others remain unaware of heatstroke risks.

Few studies have explored physical activity and recognition of heatstroke prevention in patients with CVD during summer in high-risk areas. This prospective descriptive study examined the potential decline in physical activity during summer and heatstroke prevention recognition in patients with CVD in an urban area of Japan, providing preliminary data for safe and effective summer activity counseling.

Methods

Study design and participants

This was a prospective descriptive study. The study hospital is located in Nagoya City, the capital of Aichi Prefecture, and a metropolitan area in the central part of Japan. A baseline survey was conducted between August 1 and 14 of 2022, and a follow-up survey was conducted between October 1 and 14 of 2022. Figure 1 shows the temperature during the study period, indicating that the baseline survey was conducted during a period of extreme heat, and a follow-up survey was conducted during a milder season.

Fig. 1
figure 1

Temperature around the study period in Nagoya. *Heatstroke alert is a warning system announced by the Japanese Ministry of Environment and Japan Meteorological Agency. The alert is announced per 58 different regions of Japan when the WBGT index is above 33

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Inclusion criteria were as follows: consecutive patients with CVD who participated in outpatient CR during the baseline survey period and agreed to participate in the study. Exclusion criteria were as follows: patients who were not capable of walking outside alone, who were diagnosed with dementia or had clinically probable dementia, those who underwent hemodialysis, or those who had scheduled hospitalization during the survey period.

The participants underwent outpatient CR following Japanese Circulation Society guidelines, including exercise training, education, and counseling [9]. Exercise training comprised preparatory, aerobic, and cool-down exercises. Exercise intensity was set at each patient's anaerobic threshold if cardiopulmonary exercise testing was performed. The anaerobic threshold was identified using the V‐slope method, in which anaerobic threshold was defined the point when the rate of increase in VCO2 relative to VO2 steepened. In case that the patients who did not received cardiopulmonary exercise testing for some reasons, the exercise intensity was set using the estimated heart rate around anaerobic threshold based on the Karvonen formula, or an intensity of 11–13 on the Borg scale [9]. Resistance training is generally added to aerobic exercises. In Japan, health insurance is mandatory, and it has covered CR for heart failure since 2007. CR is covered for 150 days in the Japanese healthcare system, and outpatient CR is covered for up to 60 min per session, three times a week. The CR program was not created specifically for this study project, but rather is provided as usual at the study hospital based on the guidelines [9].

In Japan, the first coronavirus disease 2019 (COVID-19) case was reported on January 16, 2020. During the pandemic, 20–30% of the training facilities for CR continued to provide an outpatient program [14]. Patients with cold symptoms were prohibited, and all wore masks during sessions. We conducted careful outpatient CR, disinfected the ergometer handles and saddles with alcohol before and after each procedure, and completely separated outpatients from inpatients. These prophylactic procedures were conducted according to the guidelines of the European Association of Preventive Cardiology [15] and the Japanese Association of Cardiac Rehabilitation [16].

The study protocol was approved by the Ethics Committee of the School of Health Sciences at Nagoya University (approval number: 22–517) and Nagoya Ekisaikai Hospital Ethical Committee (approval number: 2022–021). This study was performed in accordance with the principles of the 1975 Declaration of Helsinki. All enrolled patients provided written informed consent to participate in the study.

Physical activity

Physical activity of each patient was assessed using questionnaires at the baseline and follow-up surveys. The short form of the International Physical Activity Questionnaire (IPAQ) was administered to measure physical activity over a seven-day period. The IPAQ has been shown to have acceptable reliability and validity [17]. This questionnaire comprises nine items that provide information on the time spent walking, moderate and vigorous intensity activity, and sedentary activity. In this study, we divided the participants’ physical activity levels into categories (low/moderate/high) based on guidelines [18]. The present study included older patients, and 50% were aged 75 years or over. Such patients seemed to have difficulty answering questions about accurate exercise time, even if they did not have obvious cognitive impairment. For these reasons, we used the IPAQ for physical activity screening and divided it into three activity levels but did not analyze the calculated MET-Minutes/week [18].

Additionally, we asked patients about changes in exercise habits for their health and frequency of going outside in daily living using an originally developed questionnaire to assess the seasonal effect (Supplementary Information). In the baseline survey, we assessed whether exercise habit decreased during summer (“Do you feel your exercise habit has decreased this summer?”) and frequency of going outside decreased (“Do you feel the frequency of going outside has decreased this summer?”). If patients answered that their exercise habit or frequency of going outside had decreased during the summer, we asked them if their exercise habit or frequency of going outside returned to the level it was before summer in the follow-up survey conducted in October.

During the study period, physical therapists provided physical activity guidance based on standard guidelines. Participants were advised to (1) perform home exercises including stretching and resistance training, (2) walk outdoors in early morning or evening to avoid heatstroke, and (3) monitor physical activity with a pedometer or smartphone during summer. These instructions were conducted after completing the baseline questionnaire survey. Thus, the results of the questionnaires at baseline in August reflected the participants' usual daily living. As temperatures cooled in September to October, participants were encouraged to gradually increase outdoor physical activity.

Recognition of heatstroke prevention

We assessed patients' recognition of heatstroke prevention using a questionnaire developed by the researchers (Supplementary Information) based on the recommendations for heatstroke prevention by the Ministry of Environment of Japan [19], Ministry of Health, Labour and Welfare of Japan [20], Japan Sport Association [21], and Japanese Association of Cardiac Rehabilitation [22]. The questionnaire consists of 10 items related to heatstroke prevention, including avoiding heat, staying hydrated, and monitoring symptoms. After COVID-19 outbreak, mask wearing was recommended depending on the situation to prevent infection. The Japanese government has recommended removing masks outdoors when individuals are 2 m apart or when not talking at a distance < 2 m [20]. Therefore, in this study, we also asked whether the participants wore a mask during outdoor activities (yes/no). From a prevention perspective, the questionnaire focused on preventive behavior but not care after developing heat-related illness. Recognition of heatstroke prevention for each participant was assessed in the baseline survey, followed by an educational program provided by physical therapists.

Patient characteristics

Patients’ medical records were reviewed to extract data on age, sex, body mass index, principal etiology, comorbidities, prescribed medications, and the need for a walking device. Data on left ventricular ejection fraction and biochemical parameters assessed within one month before and after the baseline survey were collected. Social frailty, which is usually conceptualized as being at risk of losing or having lost sufficient social support, activities, or resources [23], was evaluated using the definition proposed by Makizako et al. [24]. The patient characteristics were measured at baseline in August 2022.

Statistical analysis

Continuous variables are expressed as mean and standard deviation for normally distributed variables and as median with interquartile range for non-normally distributed data. Categorical data are expressed as numbers and percentages.

The proportions of patients with low physical activity were compared between the baseline (summer) and follow-up (fall) surveys using the McNemar test. Since we hypothesized that the prevalence of low physical activity would be high in the summer season, the prevalence of low physical activity was compared between the two seasons.

We conducted an exploratory analysis for possible factors associated with seasonal changes in exercise habits or frequency of going outside. Based on the responses to the questionnaire, we divided participants and compared their characteristics using the t-test, Mann–Whitney U test, or chi-square test, as appropriate.

All statistical analyses were performed using Stata/SE software (version 15.1; StataCorp LP, College Station, TX, USA). Results were considered statistically significant at p < 0.05. This study was an exploratory analysis of the association between personality traits and multidimensional medication adherence; therefore, no adjustment for multiple testing was performed. Considering the potential for type I error, this study should be considered a hypothesis-generating study.

Results

Of the 67 patients who received outpatient CR during the baseline survey period, 60 were enrolled in this study. Finally, we analyzed 56 patients after excluding those who dropped out of the follow-up survey (Fig. 2). The median age was 75 years (interquartile range, 68–80), and 77% were men (Table 1). The median left ventricular ejection fraction was 56% (interquartile range, 44–63), and 61% of patients had a history of heart failure.

Fig. 2
figure 2

Flowchart of patient selection

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Table 1 Characteristics of the study participants
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Figure 3 shows the physical activity levels of the study participants in summer (baseline survey in August) and fall (follow-up measurement in October). The prevalence of the low physical activity category defined by the IPAQ was higher in summer than in fall (57.1% vs. 37.5%, p = 0.013). Figure 4 presents the self-reported changes in physical activity during summer and fall. In the baseline survey, 55.4% of participants exercised for their own health in addition to facility-based exercise training. In summer, 33.9% answered that their exercise habits decreased. Of these, 47.4% felt that their exercise habit remained decreased in the follow-up survey in October. Regarding the frequency of going outside, 82.1% went outside less in summer; most of them had a similar frequency of going outside as before summer during the follow-up period in October.

Fig. 3
figure 3

Comparison of physical activity level between August and October in 2022. The left bar presents physical activity of August in 2022, with 57.1% of patients categorized as low physical activity. The right bar shows data of October in 2022, with 37.5% of patients categorized as low physical activity. The results shows that the prevalence of moderate or high physical activity levels is lower in August than October. *p for the comparison of prevalence of low physical activity between the two seasons

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Fig. 4
figure 4

Change in exercise habit and frequency of going outside

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Based on the responses to the questionnaire, we describe the participant characteristics in Table 2. There were no significant differences in the characteristics between those who maintained their exercise habit and those with decreased exercise habit in summer. Participants with maintained exercise habit tended to have lower body mass index and higher albumin levels. Participants whose frequency of going outside decreased during summer had a low percentage of current employment and tended to have a lower body mass index and higher prevalence of social frailty, compared with those who maintained their frequency of going outside.

Table 2 Participant characteristics according to the patterns of the exercise habit and frequency of going outside
Full size table

During the study period, no participant suffered from a heat-related illness that required clinic visits or help from someone. Figure 5 shows the response to the question regarding recognition of heatstroke prevention. Most participants had interest in heatstroke prevention, with over 85% of positive responses to all basic preventive behaviors item except one: “I do not force myself to exercise or go out when I feel hot.” With regard to wearing a mask, 41.1% did not take off their mask when exercising or walking outside, and 44.6% were worried about what other people thought about them taking off a mask.

Fig. 5
figure 5

Recognition of heatstroke prevention. *Heatstroke alert is a warning system announced by the Japanese Ministry of Environment and Japan Meteorological Agency. The alert is announced per 58 different regions of Japan when the WBGT index is above 33

Full size image

Discussion

This prospective descriptive study examined physical activity changes from summer to fall in patients with CVD. Physical activity was lower in summer than in fall, possibly due to the hot climate in the urban areas of Japan. While most participants regained outdoor activity in fall, some maintained decreased exercise habits. Most participants had high interest in heatstroke prevention; however, it was suggested that wearing a mask outdoors on hot days might be a problem specific to the COVID-19 era.

Given the hot and humid summer climate of the study location, the lower physical activity in summer than in fall among the study participants was a reasonable result. Although a recent systematic review of the seasonality of physical activity suggested that people tend to be active in summer and inactive in winter [25]; these results were mainly derived from Western countries with relatively mild summer temperatures. Indeed, similar results were reported by surveys in areas of Japan with severe winter snowfall [26,27,28]. On the other hand, climate change caused by global warming is increasing the frequency of extreme weather conditions, and heat-related illness has been recognized as a global health issue in summer [10]. The difference in physical activity between summer and fall may have been influenced by CR program. However, the participants felt their exercise habits decreased more in the summer than in the spring, suggesting that the low physical activity level in summer may be due to a hot environment. While this study is based on a single urban area in Japan, it has clinical significance by showing the potential for reduced physical activity in high-temperature regions.

At the baseline survey in August, 33.9% (60% of patients who had exercise habits in addition to CR attendance) answered that their exercise habits declined during summer. Additionally, 82.1% felt that their frequency of going outside decreased during summer. Of the patients whose exercise habits decreased in summer, approximately 50% recognized a continuous decline in their exercise habits in October. These findings differ from a previous study that reported no seasonal differences in physical activity in Japanese patients with CVD [29]. However, the participants of this study were young, with an average age of 56–58 years, and the physical activities of different patients measured in four different seasons were compared [29]. The present study conducted a longitudinal repeated assessment in relatively older patients and showed the possibility of declining physical activity during summer. Social frailty also appeared linked to reduced outdoor activity. Larger studies are needed to examine how age and social frailty influence seasonal activity differences. Additionally, future research should include patients with more frail conditions, as our study focused on outpatients attending CR.

Recent guidelines for CR in Japan have cautioned about the risk of heatstroke in summer for patients with CVD [9], but there has been a lack of research on patients' recognition. As walking outside is a familiar and convenient form of exercise that can be incorporated into everyday life [30], patient education should be provided according to the patient’s understanding of heatstroke. This study focused on heatstroke, but there is insufficient evidence to directly link the decline in physical activity to perceived heatstroke risk. The reduction in activity may have been due to thermal discomfort rather than health concerns. Understanding the underlying causes is crucial for personalized physical activity counseling. If health concerns are the cause, tailored medical advice or education is needed, while thermal discomfort may be addressed through public health measures promoting indoor or climate-controlled activities.

Most patients in this study were aware of heatstroke prevention behaviors. Although selection bias from outpatient CR participants, the patients were attentive to government and media alerts related to heatstroke. In contrast, 40% of participants wore a facemask outside, even on hot days, partly due to fear of social judgment. This may be partly due to the national character of Japanese, despite government recommendations to remove a mask outdoors when individuals are approximately 2 m apart [20]. A previous study reported that wearing a facemask raised cardiorespiratory load during aerobic exercise [31], possibly increasing the risk of heatstroke [10]. Patient-medical staff communication to prevent both infection and heatstroke may be a challenge specific to the COVID-19 era.

Limitations

This study has several limitations. First, its single-center design and small sample size limit the generalizability of the findings. Second, physical activity was measured only during summer and fall, and seasonal effects, particularly the decline from spring to summer, were not fully evaluated, although the questionnaire suggested a hot environment's influence. Third, the study was conducted in a single urban area in Japan, limiting its applicability to other regions and climates. Fourth, the causal relationship between perceived heatstroke risk and reduced physical activity in summer remains unclear, as it may stem from general temperature discomfort. Fifth, physical activity was assessed via the IPAQ questionnaire, which, despite its validation, lacks the accuracy of objective tools like accelerometers. Lastly, the small sample size restricted exploration of patient characteristics associated with seasonal activity changes.

Conclusions

This study found that physical activity in summer may decrease among patients with CVD in high-temperature regions with heat-related illness risks. While most participants practiced heatstroke prevention, wearing masks outdoors on hot days emerged as a COVID-19-specific challenge. The findings highlight the need for guidance on maintaining physical activity during summer and public health strategies supporting patients in hot climates, such as promoting heat action plans, hydration, and indoor exercise. Further research is required to understand the impact of high temperatures on physical activity in patients with CVD and develop effective interventions.

Availability of data and materials

All data generated or analyzed during this study are included in this published article. The data underlying this article cannot be shared publicly due to the privacy of individuals that participated in the study. The data will be shared on reasonable request with the corresponding author.

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Acknowledgements

The authors would like to thank all the participants for their cooperation in this study.

Funding

This work was partly supported by JSPS KAKENHI (Grant Number 21K17511).

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Authors and Affiliations

  1. Department of Integrated Sciences, Nagoya University Graduate School of Medicine, 1-1-20 Daiko-Minami, Higashi-Ku, Nagoya, 461-8673, Japan

    Takuji Adachi

  2. Department of Rehabilitation, Nagoya Ekisaikai Hospital, Nagoya, Japan

    Takuji Adachi & Yuki Tsunekawa

  3. Department of Cardiology, Nagoya Ekisaikai Hospital, Nagoya, Japan

    Daisuke Tanimura

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  1. Takuji Adachi
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  2. Yuki Tsunekawa
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Contributions

T.A.: Conceptualization, Investigation, Methodology, Data curation, Formal analysis, Funding acquisition, Writing – original draft, Writing – review & editing. Y.T.: Investigation, Data curation, Writing – review & editing. D.T.: Supervision, Project administration, Writing – review & editing. All authors critically revised the manuscript, approved the final manuscript, and agreed to be accountable for all aspects of the work, ensuring integrity and accuracy.

Corresponding author

Correspondence to Takuji Adachi.

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Ethics approval and consent to participate

The study protocol was approved by the Ethics Committee of the School of Health Sciences at Nagoya University (Approval number: 22-517) and Nagoya Ekisaikai Hospital Ethical Committee (Approval number: 2022-021). This study was performed in accordance with the principles of the 1975 Declaration of Helsinki. All the enrolled patients provided written informed consent to participate in the study.

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Not applicable.

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The authors declare no competing interests.

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Adachi, T., Tsunekawa, Y. & Tanimura, D. Physical activity during summer and recognition of heatstroke prevention among patients with cardiovascular disease in an urban area of Japan: a descriptive study. BMC Res Notes 18, 42 (2025). https://doi.org/10.1186/s13104-025-07120-7

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