Accidental Hypothermia in Denmark: a Nationwide Cohort Study of Incidence and Outcomes
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Open access Original research BMJ Open: first published as 10.1136/bmjopen-2020-046806 on 31 May 2021. Downloaded from Accidental hypothermia in Denmark: A nationwide cohort study of incidence and outcomes Sebastian Wiberg ,1 Asmus Friborg Mortensen,2 Jesper Kjaergaard,1 Christian Hassager,1,3 Michael Wanscher4 To cite: Wiberg S, ABSTRACT Strengths and limitations of this study Mortensen AF, Kjaergaard J, Objectives To investigate the incidence of accidental et al. Accidental hypothermia hypothermia (AH) in a nationwide registry and the ► Large contemporary study to investigate the inci- in Denmark: A nationwide associated outcomes. cohort study of incidence dence, comorbidity burden and outcome in patients Design Nationwide retrospective cohort study and outcomes. BMJ Open diagnosed with accidental hypothermia. Participants and settings All patients at least 18 years 2021;11:e046806. doi:10.1136/ ► Data originated from a national cohort of patients. old, admitted to hospitals in Denmark with a diagnosis of bmjopen-2020-046806 ► The registry data limited the data granularity to what AH, with an International Classification of Diseases, 10th has been presented in the present paper. ► Prepublication history for edition code of T689, from January 1996 to November ► The presented incidences are likely to be signifi- this paper is available online. 2016. Other recorded diagnoses were included in the To view these files, please visit cantly different in other countries, depending on analyses. the journal online (http:// dx. doi. climate, demography and so on. org/ 10. 1136/ bmjopen- 2020- Primary and secondary outcome measures The 046806). primary outcome was 1- year mortality. Results During the inclusion period, 5242 patients were SW and AFM contributed equally. admitted with a diagnosis of AH, corresponding to a mean in association with acute or chronic illness annual incidence of 4.4±1.2 (range by calendar year: 2.9– (secondary hypothermia).2 The severity of Received 09 November 2020 6.4) per 100 000 inhabitants. A total of 2230 (43%) had AH hypothermia can be graded into three stages Accepted 14 May 2021 recorded as the primary diagnosis without any recorded secondary diagnoses (primary AH), 1336 (25%) had AH based on core temperature: 32°C–35°C (mild recorded as the primary diagnosis with other recorded AH), 28°C–32°C (moderate AH) and <28°C 3–6 secondary diagnoses (AH+2° diagnosis), and 1676 (32%) (severe AH). In- hospital mortality after http://bmjopen.bmj.com/ had AH recorded as a secondary diagnosis with another severe hypothermia has been reported to be recorded primary diagnosis (1° diagnosis+AH). Alcohol as high as 30%. It is estimated that approx- intoxication was the most common diagnosis associated imately 1500 people die from hypothermia with AH. Overall 1- year mortality was 27%. In patients every year in the USA.1 4 Few previous studies with primary AH, 1- year mortality was 22%, compared have investigated the incidence and outcome © Author(s) (or their with 26% in patients with secondary AH type I, and 35% in employer(s)) 2021. Re- use after AH, however, the studies represent patients with secondary AH type II (plog- rank<0.001). permitted under CC BY- NC. No Conclusions The present study investigated the incidence heterogenous populations from different commercial re- use. See rights 7–10 climate zones and different time periods. of AH, associated comorbidities and mortality after AH in on October 1, 2021 by guest. Protected copyright. and permissions. Published by Accordingly, our knowledge of the contempo- BMJ. Denmark from 1995 to 2016. The diagnosis is associated with a high comorbidity burden and a considerable 1- rary incidence, demographics and outcomes 1Department of Cardiology, Rigshospitalet, Copenhagen, year mortality. In the high proportion of patients with after AH remains sparse. Denmark associated comorbidities, establishing whether AH or The aim of this study was to investigate 2Department of Anaesthesiology the comorbidities are the drivers of mortality remains the incidence of AH in a Danish nationwide and Intensive Care, Bispebjerg difficult. This complicates our understanding of AH and registry, and the associated outcomes in adult and Frederiksberg Hospital, makes it difficult to find modifiable factors associated with patients. Copenhagen, Denmark both AH and outcomes. Future prospective studies are 3 Department of Clinical needed elucidate the causal relationship between AH and Medicine, University of associated comorbidities. Geography Copenhagen, Copenhagen, Denmark covers a relatively small area of Denmark 4Department of CardioThoracic 4 300 000 hectares but has a comparatively Anaesthesiology, Rigshospitalet, INTRODUCTION long coastline. The highest point is 173 m Copenhagen, Denmark Accidental hypothermia (AH), defined as an above sea level. Denmark is located on the involuntary drop in core temperature below 55th to 57th parallel north in the temperate Correspondence to Dr Sebastian Wiberg; 35°C, is a serious condition with consider- climate zone. Mean temperatures range from 1 2 Sebastian. christoph. wiberg@ able mortality. AH can result from isolated 0°C in January and February to 15.7°C in regionh. dk cold exposure (primary hypothermia) or August. The population was about 5.8 million Wiberg S, et al. BMJ Open 2021;11:e046806. doi:10.1136/bmjopen-2020-046806 1 Open access as of 1 January 2016. About 1.8 million (about 31% of the diagnosis (AH+2° diagnosis) or whether AH was recorded BMJ Open: first published as 10.1136/bmjopen-2020-046806 on 31 May 2021. Downloaded from population) lived in the capital, Copenhagen. as a secondary diagnosis (1° diagnosis+AH). The annual incidence of AH per calendar year was plotted for the total cohort as well as after stratification by AH type. The mean annual incidence±SD with range was METHODS presented. To assess if the incidence of AH was changing The present study is a nationwide retrospective cohort over time, linear models were applied with calendar year study including all adult patients admitted to hospitals being included as a continuous covariate assuming a in Denmark with a diagnosis of AH from January 1996 linear trend. The absolute change in AH incidence per to November 2016. The primary outcome was 1- year calendar year±SD was presented. The distribution of AH mortality after diagnosis. We chose a priori to include diagnoses per calendar months were displayed along only those cases with a first- time diagnosis of AH. The with mean outside temperatures. The mean annual inci- rationale behind this decision was to ensure that a patient dence±SD of AH after stratification by age group was with multiple hospital admissions for AH only counted presented. once in the analysis. We presented crude 1- year mortality rates for the total All Danish residents are given a personal identification cohort, as well as after stratification by AH type, by sex and number at the time of birth or immigration. This number by age group. We presented 1-year mortality after strat- is used in all contacts with the healthcare system, and ification by sex and age group. Differences in mortality it is used as identifier in national registries. Data were between females and males, stratified by age group, were extracted from the Danish National Patient Register analysed by application of the χ2 test. (NPR) to identify all patients admitted with a discharge For each patient in the cohort, the date of entry (ie, diagnosis of AH, defined by the International Classifica- the date of AH diagnosis) as well as the date of death was tion of Diseases, 10th edition (ICD-10, which has been recorded in the registries. The Kaplan-Meier estimator used in Denmark since 1993) code T689. was applied to visualise time to death within 1 year from Patients were stratified into three groups based on admission, stratified by AH type, sex and age group. The whether AH was recorded as the only diagnosis (primary log- rank test was applied to assess possible differences AH), whether AH was recorded as the primary diagnosis between strata. with other recorded secondary diagnoses (AH+2° diag- To analyse time to death between groups, we applied nosis), or whether AH was recorded as a secondary diag- Cox proportional hazard models adjusting for AH type, nosis (1° diagnosis+AH) for a given admission. sex, age at admission and calendar year. We presented The primary diagnosis is defined as the diagnosis HRs with 95% CIs. A significance level of <0.05 was primarily causing the need for the admission, while the applied throughout. SAS software, V.9.4 (SAS Institute) secondary diagnoses are defined as other conditions that was used for all statistical analysis. http://bmjopen.bmj.com/ required attention during a given admission. Main treat- ment categories, such as ‘intensive care admission’, and ‘respirator treatment’ were also extracted from the NPR. RESULTS The AH diagnosis was linked to survival (ie, the primary Over the inclusion period of almost 22 years, the Danish outcome) using the nationwide Danish Register of Causes Health Care System received a total of 5242 adult patients of Death. admitted with a diagnosis of AH. A total of 2230 (43%) had AH recorded as the final primary diagnosis without any recorded secondary diagnoses (primary AH), 1336 Patient and public involvement on October 1, 2021 by guest. Protected copyright. (25%) had AH recorded as the final primary diagnosis Patients or the public were not involved in the design of with other recorded secondary diagnoses (AH+2° diag- the present study. nosis), and 1676 (32%) had AH recorded as a final secondary diagnosis with another recorded primary diag- Statistical analyses nosis (1° diagnosis+AH). Throughout, categorical variables are presented as A total of 1981 (38%) of the patients admitted with AH counts (%), normally distributed continuous variables were women. The mean age was 61±21 years. Female sex are presented as mean±SD and skewed continuous vari- was slightly less prevalent among patients with primary ables are presented as median (25th percentile to 75th AH compared with patients with AH+2° diagnosis and percentile).