Indoor Versus Outdoor Occurrence in Mortality of Accidental Hypothermia in Japan: The J-Point Registry

Abstract

The impact of the location where accidental hypothermia (AH) occurs has not been fully investigated thus far. This was a multicenter retrospective study of patients with a body temperature ≤35°C obtained at the emergency department of 12 hospitals in Japan between April 2011 and March 2016. We divided the patients into two groups according to the location where AH occurred (indoor group versus outdoor group). The association between each location of the occurrence of AH and in-hospital mortality was evaluated via a multivariable logistic regression analysis. The primary outcome of this study was in-hospital death. The secondary outcomes were the lengths of ICU and hospital stay. A total of 572 patients were enrolled in the hypothermia database, and 537 patients were included in the analyses. A total of 119 and 418 patients experienced hypothermia with outdoor and indoor occurrence, respectively. The indoor group was older and had worse activities of daily living compared with the outdoor group. With regard to the outcome, the proportion of in-hospital death was higher in the indoor group than in the outdoor group (28.2% [118/418] vs. 10.9% [13/119], p < 0.001). The multivariable logistic regression analysis showed that the adjusted odds ratio of the indoor group was 2.48 (95% confidence interval, 1.18–5.17). In terms of secondary outcomes, both the lengths of ICU and hospital stay of the survivors were longer in the indoor group than in the outdoor group. Hypothermia with indoor occurrence accounts for ∼78% of the total AH cases in this study, and the proportion of in-hospital deaths was higher in the indoor group than in the outdoor group. Warnings about the indoor onset of AH must be provided, and countermeasures for the prevention and early recognition of AH with indoor occurrence must be taken.

Introduction

Accidental hypothermia (AH) is defined as an unintentional drop in body temperature to ≤35°C (Zafren et al., 2014). AH is considered an important problem because it accounts for the highest cases of mortality from weather-related diseases. That is, according to a study in the United States, the in-hospital mortality rate of patients with moderate to severe AH was ∼40%, whereas an earlier report from Japan has found that around 30% of all patients who were admitted for AH in the emergency departments (EDs) have died (Vassal et al., 2001; Japanese Association for Acute Medicine, 2013). AH often occurs in militaries, climbers, and individuals who are into outdoor winter sports (Davis and Byers, 2006; Brandstrom et al., 2014). The likelihood of the occurrence of AH is often affected by geographical factors or seasons, and most studies on AH were conducted in regions with cold climates (Zafren et al., 2014). However, a previous study has reported that the elderly population is at a higher risk of hypothermia, and elderly people develop AH despite being adequately dressed with warm clothing, and the mean environmental temperature ranged from 21.4°C to 23.7°C (Kimura-Kataoka et al., 2014). Thus, the occurrence of AH regardless of exposure to cold must be considered (Seman et al., 2010). AH is clinically important in aging countries, such as Japan and other developed countries, irrespective of their temperature.

Recently, in urban areas, the indoor occurrence of AH relating to the use of alcohol and drugs has been reported. In addition, the outdoor occurrence of AH in homeless individuals has been observed (Lim and Duflou, 2008; Zafren et al., 2014). In a Japanese multicenter questionnaire survey on AH for 3 months during winter, the incidence of hypothermia with indoor occurrence was higher than that with outdoor occurrence (Japanese Association for Acute Medicine, 2013). However, the study did not assess the prognosis according to the location where AH occurred (Japanese Association for Acute Medicine, 2013). Therefore, the impact of the location where AH occurs has not been fully investigated thus far.

We performed a multicenter retrospective observational study using the data obtained from the Japanese AH network registry (J-Point registry) (Matsuyama et al., 2018). A total of 537 adult patients with AH were enrolled. Using the data from this registry, we aimed to evaluate the differences between indoor and outdoor occurrence of hypothermia in terms of baseline data, place of occurrence, mortality, and lengths of ICU and hospital stay.

Methods

Study design, patient, and setting

This study retrospectively assessed data from J-Point registry. The study period ranged between April 1, 2011 and March 31, 2016. The present study included hypothermic patients aged ≥18 years and who had information about the location where AH occurred. The exclusion criteria were as follows: those who did not visit the ED of the participating institutions, those whose body temperature was unknown or beyond 35.0°C (Zafren et al., 2014), and those who refused to participate.

J-Point registry

We conducted a multicenter, chart review study of AH using data from the J-Point registry. This study aimed to obtain descriptive information about AH, characterize its management in the EDs of Japan, and improve patient outcome. Details of the study methodology have been described previously (Matsuyama et al., 2018). In brief, the J-Point registry consists of 12 Japanese EDs, including eight critical care medical centers across the Kyoto, Osaka, and Shiga Prefectures in Japan. We retrospectively registered eligible patients using the International Classification of Diseases, Tenth Revision (ICD-10), code for hypothermia (T68). The ethics committee of each participating institution approved this study.

Data collection and quality control

From this registry, data about the demographic characteristics of participants, clinical history, presentation, laboratory findings, and treatments were collected and recorded into a predefined data sheet. All chart abstractors were emergency physicians. Moreover, we were trained for appropriate data abstractions during face-to-face or web meetings. The collected data were checked by the working group and confirmed or returned to each institution in case of discrepancies.

Measurements

The following baseline patient information was collected: sex, age, activities of daily living (ADLs) before being hypothermic (independent, need some assistance, and need total assistance), residence (living alone, not living alone, nursing home, and homelessness), past medical history (cardiovascular diseases [ischemic heart diseases, heart failure, arrhythmia, hypertension, and other cardiovascular diseases], neurological diseases [stroke, epilepsy, Parkinson syndrome, and other neurological diseases], endocrine diseases [diabetes mellitus, thyroid diseases, adrenal insufficiency, and others], psychiatric diseases [chronic alcoholism, depression, schizophrenia, and others], malignant diseases, dementia. We also collected mean outside temperature on the day of arrival at the ED (Japan Meteorological Agency, 2017), season, location (indoor and outdoor), mode of arrival (walk-in and via ambulance), vital signs upon hospital arrival (body temperature, blood pressure, heart rate, and Glasgow Coma Scale [GCS] score), biological data (serum pH, HCO₃ [mEq/L], lactate [mM], sodium [mEq/L], potassium [mEq/L], and glucose [mg/dL] levels), exposure to cold, associated conditions, treatment process, and outcomes. Sequential organ failure assessment score was calculated only for patients admitted to the ICU. Based on previous reports (Brown et al., 2012; Japanese Association for Acute Medicine, 2013), associated conditions were classified into internal diseases (stroke, seizure, Parkinson disease, thyroid diseases, hypoglycemia, infectious diseases, acute pancreatitis, uremia, malignant disease, bowel ischemia, rhabdomyolysis, or others), trauma (fall [injury in the head, spine, and extremity], motor vehicle accident, or others), alcohol intoxication, drowning (indoor or outdoor), self-harm (drug or external), and other factors (iatrogenic, mountain incident, burn, malnutrition/infirmity, or others). Rewarming procedures were divided into active external or minimally invasive rewarming (warm intravenous fluids, warm blanket, forced warm air, heating pads, and warm bath) and active internal rewarming (lavage [stomach, chest, and bladder], intravascular rewarming, hemodialysis, and extracorporeal membrane oxygenation). Other treatment information included tracheal intubation, use of catecholamine, and emergent transvenous cardiac pacing. Data regarding outcomes were in-hospital death, decreased ADL during discharge, and lengths of hospital and ICU stay.

Outcomes

The primary outcome of this study was in-hospital death. The secondary outcomes were lengths of ICU and hospital stay.

Statistical analysis

We divided the patients into two groups according to the location where AH occurred (indoor versus outdoor). Data were compared using the Mann–Whitney U test for continuous variables, and Fisher's exact test for categorical variables. The association between the location where AH occurred and in-hospital death was assessed via univariable and multivariable logistic regression analyses with adjusted odds ratios (AORs) and 95% confidence intervals (CIs) as effect variables. In the multivariable analysis, we adjusted for the following potential confounders that were associated with clinical outcomes: age category (those aged 18–64 years, young-old individuals aged 65–74 years, or old-old individuals aged ≥75 years), sex (male or female), past medical history (none, one, multiple, and unknown), ADLs (independent, need some assistance, need total assistance, or unknown), systolic blood pressure category (cardiac arrest, unmeasurable, 40–90, or <90 mmHg), exposure to cold (yes, no, or unknown), presence of associated internal diseases (yes or no), and active internal rewarming (yes or no). All p-values were two-sided, and a 0.05 significance level was considered. All statistical analyses were carried out using the EZR software (version 1.36).

Results

A total of 572 patients were enrolled in our hypothermia database. After excluding 24 patients whose body temperature was >35.0°C, 3 patients with unknown body temperature, and 8 patients who were under 18 years, the final analysis included 537 hypothermic patients. Moreover, 119 and 418 patients experienced hypothermia with outdoor and indoor occurrence, respectively (Table 1).

Table 1.

Baseline Characteristics

	Outdoor	Indoor	p-Values
	(n = 119)	(n = 418)	p-Values
Men	62 (52.1)	211 (50.5)	0.835
Age, year, median (IQR)	71 (57.5–80)	81.5 (70.3–88)	<0.001
Activities of daily living
Independent	103 (86.6)	270 (64.6)	<0.001
Need some assistance	15 (12.6)	115 (27.5)
Need total assistance	0	32 (7.7)
Unknown	1 (0.8)	1 (0.2)
Residence
Home
Living alone	40 (33.6)	171 (40.9)	<0.001
Not living alone	61 (51.3)	207 (49.5)
Nursing home	2 (1.7)	30 (7.2)
Homelessness	4 (3.4)	2 (0.5)
Unknown	12 (10.1)	8 (1.9)
Season
Spring (3–5 months)	23 (19.3)	73 (17.5)	<0.001
Summer (6–8 months)	11 (9.2)	8 (1.9)
Autumn (9–11 months)	23 (19.3)	48 (11.5)
Winter (12–2 months)	62 (52.1)	289 (69.1)
Mean outside temperature on the day of arrival at the ED, year, median (IQR)	8.3 (4.0–15.2)	5.9 (4.1–10)	0.006
Mode of arrival
Ambulance	117 (98.3)	391 (93.5)	0.04
Walk-in	2 (1.7)	27 (6.5)
Past medical history
Cardiovascular diseases	38 (31.9)	199 (47.6)	0.002
Neurological diseases	9 (7.6)	84 (20.1)	<0.001
Endocrine diseases	19 (16.0)	112 (26.8)	0.016
Psychiatric diseases	39 (32.7)	81 (19.4)	0.003
Malignant diseases	7 (5.9)	47 (11.2)	0.118
Dementia	20 (16.8)	88 (21.1)	0.365
Other	10 (8.4)	84 (20.1)	0.002
Unknown	4 (3.4)	4 (1.0)	0.077

Values are expressed as numbers (percentages) unless indicated otherwise.IQR, interquartile range.

Characteristics of patients and detailed location of AH

The characteristics of patients are described in Table 1. About half in either group were men. The indoor group was older and had worse ADLs compared with the outdoor group. The median outside temperature on the day that AH occurred was colder in the indoor group than in the outdoor group. The prevalence of a past history of internal diseases, such as cardiovascular, neurological, and endocrine diseases, was higher in the indoor group than in the outdoor group, whereas psychiatric diseases were more frequently observed in the outdoor group than in the indoor group.

The location of AH occurrence as reported by our patients is shown in Table 2. For the outdoor group, AH occurred most frequently while in water (39.5%), followed by on the road (30.3%). For the indoor group, most AH cases occurred in the place of residence (97.6%).

Table 2.

Location of Outdoor AH Occurrence

	(n = 119)
Water associated	47 (39.5)
On the road	36 (30.3)
At home	17 (14.3)
Others	9 (7.6)
Unknown	10 (8.4)

Values are expressed as numbers (percentages) unless indicated otherwise.

AH, accidental hypothermia.

In-hospital data

Table 3 shows the in-hospital data. The median body temperature and heart rate were lower for the indoor group than for the outdoor group. The proportion of individuals exposed to cold was higher in the outdoor group than in the indoor group. Regarding associated conditions, internal diseases were more frequently observed in the indoor group than in the outdoor group, whereas trauma, alcohol intoxication, drowning, and self-harm were more frequently noted in the outdoor group. No significant difference was observed between the groups regarding rewarming procedures and other treatments. However, the indoor group showed a higher likelihood of getting admitted to a hospital compared with the outdoor group.

Table 3.

In-Hospital Data

	Outdoor	Indoor	p-Values
	(n = 119)	(n = 418)	p-Values
Body temperature, median (IQR)	31.9 (29.1–33.5)	30.5 (28.1–32.3)	0.002
Systolic blood pressure category			0.055
Cardiac arrest	8 (6.7)	13 (3.1)
Unmeasurable	3 (2.5)	33 (7.9)
40–90 mmHg	22 (18.5)	85 (20.3)
>90 mmHg	86 (72.3)	287 (68.7)
Heart rate, median (IQR)	78 (62.5–92)	61 (45–80)	<0.001
Glasgow Coma Scale, median (IQR)	11 (8.5–14)	11 (8–14)	0.096
Biological data
Serum pH, median (IQR)	7.31 (7.25–7.36)	7.31 (7.23–7.37)	0.911
Serum HCO₃ (mEq/L), median (IQR)	19.1 (15.9–24.1)	21 (15.0–25.7)	0.186
Serum lactate (mM), median (IQR)	4.9 (2.6–8.9)	2.4 (1.3–5.4)	<0.001
Serum sodium (mEq/L), median (IQR)	139 (136–142)	140 (135–144)	0.195
Serum potassium (mEq/L), median (IQR)	4.0 (3.5–4.5)	4.1 (3.6–4.8)	0.048
Serum glucose (mg/dL), median (IQR)	165 (110–233)	123 (84–178)	<0.001
SOFA score^a	3.5 (1–6.25)	5 (2–7)	0.064
Exposure to cold	113 (95.0)	309 (73.9)	<0.001
Associated conditions
Internal diseases	34 (28.6)	326 (78.0)	<0.001
Trauma	30 (25.2)	43 (10.3)	<0.001
Alcohol intoxication	21 (17.6)	29 (6.9)	0.001
Drowning, including immersion	30 (25.2)	2 (0.5)	<0.001
Self-harm	20 (16.8)	14 (3.3)	<0.001
Others	21 (17.6)	116 (27.8)	0.003
Rewarming procedure
Active external or minimally invasive	110 (92.4)	396 (94.7)	0.373
Warm intravenous fluids	89 (74.8)	304 (72.7)	0.725
Warm blanket	78 (65.5)	295 (70.6)	0.311
Forced warm air	15 (12.6)	67 (16.0)	0.39
Heating pads	4 (3.4)	19 (4.5)	0.798
Warm bath	5 (4.2)	11 (2.63)	0.366
Active internal	15 (12.6)	75 (17.9)	0.21
Lavage	6 (5.0)	39 (9.3)	0.188
Intravascular	0 (0)	4 (1.0)	0.58
Hemodialysis	3 (2.5)	23 (5.5)	0.23
ECMO	6 (5.0)	16 (3.8)	0.6
Other treatment
Intubation	10 (8.4)	25 (6.0)	0.399
Catecholamine	15 (12.6)	84 (20.1)	0.08
Emergent transvenous cardiac pacing	0 (0)	7 (1.7)	0.357
Admission ward			0.014
No admission	15 (12.6)	20 (4.8)
General ward	52 (43.7)	190 (45.5)
Intensive care unit	52 (43.7)	208 (49.8)

Values are expressed as numbers (percentages) unless indicated otherwise.

Calculated for patients admitted to the ICU.

ECMO, extracorporeal membrane oxygenation; SOFA, sequential organ failure assessment.

Outcomes

Regarding the primary outcome, the proportion of in-hospital deaths was higher in the indoor group than in the outdoor group (28.2% [118/418] vs. 10.9% [13/119], p < 0.001) (Table 4). The multivariable logistic regression analysis showed that the AOR of the indoor group was 2.48 (95% CI, 1.18–5.17). In terms of the secondary outcomes, both the lengths of ICU and hospital stay of the survivors were longer for the indoor group than for the outdoor group (Table 4).

Table 4.

Outcomes

	Outdoor	Indoor	p-Values
	(n = 119)	(n = 418)	p-Values
Primary outcome
In-hospital death	13 (10.9)	118 (28.2)	<0.001
Adjusted odds ratio (95% CI)	Reference	2.48 (1.18–5.17)
Secondary outcomes
Hospital stay	5 (2–16)	15 (4–34)	<0.001
ICU stay^a	2 (1–4)	3 (2–6)	0.02

Values are expressed as numbers (percentages) unless indicated otherwise.

Calculated for patients admitted to the ICU.

CI, confidence interval.

Discussion

In our multicenter study, more patients from indoor areas suffered from AH compared with those from outdoor areas. Almost all indoor cases were reported to have occurred at the place of residence. Furthermore, the proportion of in-hospital deaths was higher for the indoor group than for the outdoor group, even after adjusting for potential confounders. Our findings provide an important clue to the management of AH in the EDs.

The indoor occurrence of AH accounts for ∼78% of the total cases. Furthermore, 97% of indoor cases occurred in residential places, such as homes and nursing homes. Previously, only few studies have focused on the location where AH has occurred, and the sample size in such studies has been low (Vassal et al., 2001; Lim and Duflou, 2008). One previous study in Sydney has shown that 79% (19/24) of AH cases occurred in indoor settings, such as houses, apartments, and other buildings, suggesting that hypothermia with indoor occurrence is more common than with outdoor occurrence in places having warmer climates (Lim and Duflou, 2008). Indeed, secondary hypothermia can occur in patients with a history of various internal diseases, even if they were not living in a cold environment (Brown et al., 2012). Importantly, among the indoor cases, ∼83.5% of patients were aged ≥65 years. In general, these individuals have a higher risk of developing hypothermia due to limited thermoregulatory capabilities in their body. Moreover, due to a higher number of underlying medical conditions and poor status of immunity in elder individuals, they have a higher likelihood of developing secondary AH (Blatteis, 2012; Stares and Kosatsky, 2015). Previous studies have reported that social isolation, such as homelessness and living alone, is one of the key risk factors of AH occurrence (Centers for Disease Control and Prevention, 2014). In Japan, the number of elderly individuals living alone is higher due to the aging society and the growing number of nuclear families (Japanese Ministry of Health, Labour and Welfare, 2018). Thus, the number of individuals at risk of AH is expected to increase in Japan in the future. Therefore, it is important to pay special attention to the indoor occurrence of AH among elderly individuals.

In addition, the proportion of in-hospital deaths was higher in the indoor group than in the outdoor group. In a previous research, differences in terms of age and underlying medical conditions were correlated to the mortality rate of hypothermia with indoor occurrence (Megarbane et al., 2000; Klein et al., 2017). In our study, the indoor group was older and had a higher number of underlying medical conditions and worse ADLs, which tend to increase the mortality rate. However, the significance and magnitude of the outcomes did not change even after adjusting for the confounding factors. There seems to be less of any biological plausibility for the idea that “indoors” is an independent causative of poor outcomes, and residual confounders might exist, such as time interval from AH occurrence to start of treatment. However, our findings lead to the assumption that those who suffer hypothermia indoors are more likely to have poor outcomes and made little efforts to prevent the indoor occurrence of AH.

Approximately 30% of AH cases in the outdoor settings occurred other than during winter. So far, a higher prevalence of AH has been reported mostly from regions with colder climates (Guillaume Debaty et al., 2015). Similarly, in such regions, most cases of hypothermia occurred due to snowy mountains and drowning (Brandstrom et al., 2014). In our study, hypothermia affecting an individual while in water was common, and is the primary cause of exposure to cold. Therefore, with regard to the outdoor occurrence of AH in Japan, attention should be given to water-related AH as other regions do.

Aging is a major public health burden not only in Japan but also in other industrialized countries. The proportion of individuals aged ≥65 years in Japan was estimated to increase from ∼26.8% in 2015 to 31.6% in 2030 (Ministry of Health, Labour and Welfare, 2018). Along with this, the risk of indoor occurrence of AH will increase in industrialized countries in the near future. Our findings from the world's leading aging country provide a perspective for industrialized countries to consider taking countermeasures against this important health hazard.

Limitations

Our study had some limitations. First, this was a retrospective observational study, and we could not prevent the loss of some data. However, there was about <5% of missing data in most variables other than the GCS score and biological data. The confounding factors did not include the GCS score and biological data. Thus, the missing data could not have significantly affected our results. Second, we did not include residual confounders that affected the in-hospital mortality of AH. Thus, selection bias could not be avoided. For example, there is a possibility that outdoor AH mortality appears to be lower since most of the outdoor AH deaths may have not been reported to the J-Point registry. Third, we recruited study participations based on ICD 10 code, and thus the study has omitted individuals whose body temperature was ≤35°C.

Conclusions

Our multicenter study indicated that the indoor occurrence of hypothermia accounted for ∼78% of the total AH cases, and the proportion of in-hospital deaths was higher for the indoor group than for the outdoor group. Warnings about the onset of indoor AH must be provided, and countermeasures for the prevention and early recognition of AH in indoor locations must be taken.

Footnotes

Acknowledgments

We are deeply indebted to all the members of the J-Point registry for their contribution. We also thank Ms. Marie Hara for her support to this study.

Disclosure Statement

All authors report no conflicts of interest.

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