Medical diagnoses seen upon hospital admission on heat wave days among the elderly in the United States
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BACKGROUND: Exposure to extreme heat and heat waves has detrimental effects on human health, and with increasing predictions of climate change and global warming, it is of the utmost importance to further understand how our bodies will react. There is a high risk of increasing hospital admissions, especially among the elderly, with increasing heat events. Most studies of the health effects of heat waves have focused on a few pre-selected diseases, such as heat stroke and dehydration, and most notably mortality in specified regions and cities. However, the full range of medical diagnoses associated with heat wave exposure has not been identified. OBJECTIVE: This study investigated multiorgan and system consequences of heat waves on the human body. The purpose of this study was to further analyze statistically significant disease categories, and attempt to get a broad spectrum of medical diagnoses and their trends during heat wave events among older adults. METHODS: This study is a follow-up study of a prior investigation that looked broadly at 283 disease categories to determine which diseases had higher rates of hospitalization on heat wave days as compared to matched non-heat wave days. The study population consisted of 23.7 million Medicare enrollees residing in 1,943 counties in the United States, aged 65 years or older. This population represents roughly 85% of all Medicare enrollees. Data were collected between the years of 1999-2010, where data for 5 summers of near-complete daily temperatures were available1. Diagnoses were measured with 6 different heat wave definitions, starting with the baseline definition of a heat wave as a period of at least 2 days of average daily temperature beyond the 99th percentile for that specific county. Of the disease categories that were previously analyzed, 5 showed statistically significantly elevated risk during heat wave days (urinary tract infections, septicemia (except in labor), heat stroke and other external causes, and fluid and electrolyte disorders, and acute and unspecified renal failure), and one was shown to be statistically significantly lower on heat wave days (congestive heart failure; nonhypertensive). The current study evaluated these statistically significant categories to determine which specific medical diagnoses within each were statistically significantly associated with exposure to heat wave days. Using ICD-9-CM codes as diagnosis identification, a log-linear mixed-effects regression model was used to the multisite time series data of hospital admissions (1). RESULTS: Of the 6 disease categories, 12 unique medical diagnoses had statistically significant log relative risks (logRR) on heat wave days as compared to matched non-heat wave days. Seven of the twelve (12) diagnoses came from the disease categories "Other injuries and conditions due to external causes" and "Fluid and electrolyte disorders". Medical diagnoses that had a greater risk of hospital admissions on heat wave days included heat stoke and sunstroke, heat exhaustion (unspecified), heat exhaustion (anhydrotic), hyperosmolality and or/hypernatremia, hypovolemia, other specified septicemias, dehydration, hypoosmolality and/or hyponatremia, acute kidney failure (unspecified), urinary tract infection (site not specified), and unspecified septicemia. One medical diagnosis had a lower risk of hospital admissions, which was unspecified congestive heart failure (logRR: -0.062 (95% CI, -0.085 -- -0.039)). Within the baseline definition of a heat wave, the relative risk values ranged from 22.646 (95% CI, 14.865--34.192) for heat stroke and sunstroke to 1.055 (95% CI, 1.017--1.095) for unspecified septicemias. DISCUSSION: The results from this study indicate a higher risk of hospital admissions among 11 different medical diagnoses during heat wave days and a lower risk of hospital admissions for congestive heart failure. These diagnoses primarily came from two of the broad disease categories: "Other injuries and conditions due to external causes" and "Fluid and electrolyte disorders". It can be concluded from these results that among older adults, there are various specific medical diagnoses that can be associated with heat wave events, leading to a large excess of hospital admissions. However, heat waves may have some potential protective affect on unspecified congestive heart failure. It is important for clinicians and hospitals to be aware and prepared for the predicted increase in reasons for hospital admissions during heat wave events.