Periods of extreme heat associated with increased hospitalization risk for older adults

Between 1999 and 2010, periods of extreme heat in the U.S. were associated with an increased risk of hospitalization for older adults for fluid and electrolyte disorders, kidney failure, urinary tract infections, septicemia and heat stroke, according to a study in the December 24/31 issue of JAMA. The authors note that the absolute risk increase was small and of uncertain clinical importance.

Extreme heat is the most common cause of severe weather fatalities in the United States, and these weather-related outcomes are expected to escalate as heat waves become more frequent, more intense, and longer lasting with climate change. Although extreme heat is known to adversely affect multiple physiological processes, previous studies of the health effects have examined only a few major categories of health outcomes, such as cardiovascular and respiratory diseases or well-known heat-related diseases, such as heat stroke or dehydration, according to background information in the article.

Jennifer F. Bobb, Ph.D., of the Harvard School of Public Health, Boston, and colleagues used Medicare inpatient claims data to systematically examine possible ways in which exposure to heat waves might be associated with serious illness requiring hospitalization in older adults. The study included hospital admissions of Medicare enrollees (23.7 million fee-for-service beneficiaries [65 years of age or older] per year; 85 percent of all Medicare enrollees) for the period 1999 to 2010 in 1,943 counties in the United States, along with at least five summers of near-complete (>95 percent) daily temperature data. Heat wave periods, defined as two or more consecutive days with temperatures exceeding the 99th percentile of county-specific daily temperatures, were matched to non-heat wave periods by county and week.

Of 214 disease groups that accounted for 99.9% of hospitalizations, 5 diseases (fluid and electrolyte disorders, renal [kidney] failure, urinary tract infections, heat stroke, and septicemia [body-wide illness with toxicity due to invasion of the bloodstream by bacteria usually coming from a localized site of infection]) had statistically significantly elevated risk of hospitalization during heat wave days. These risks were larger when the heat wave periods were longer and more extreme and were largest on the heat wave day but remained elevated and statistically significant for 1 to 5 subsequent days.

Absolute risk differences were 0.34 excess daily admissions per 100,000 individuals at risk for fluid and electrolyte disorders, 0.25 for renal failure, 0.24 for urinary tract infections, 0.21 for septicemia, and 0.16 for heat stroke.

The researchers write that their analysis of hospitalization rates on days after a heat wave provides 2 additional insights. "For some diseases, risk of hospitalization remained elevated for up to 5 days following a heat wave day. This suggests that prevention and treatment of heat-related illnesses is critical not only during the heat wave itself but also on subsequent days. Additionally, quantifying the extra number of hospital admissions attributable to heat waves without consideration of a delayed effect may underestimate the health care burden of heat."