Severity of classic heat stroke recognized

Classic heat stroke during Chicago 1995 heat wave

ICU patients from 1995 Chicago heat wave suffered long-term consequences

August 1, 1998

A study of patients admitted to intensive care units for heat stroke during the Chicago heat wave of July 1995 indicates that even the extraordinary level of weather-related mortality--more than 600 excess deaths in nine days--radically underestimates the real consequences of that episode.

In addition to the pre-hospital deaths, nearly half of the patients admitted to Chicago-area ICUs for heat stroke died within a year--21 percent before discharge and another 28 percent after release from the hospital. Many of the survivors suffered permanent loss of independent function; one-third had severe functional impairment at discharge, and none of them had improved after one year.

The findings are reported in the August 1, 1998 issue of the Annals of Internal Medicine.

"Unlike previous descriptions of classic heat stroke, most of the patients in this study suffered from multi-system organ dysfunction, including neurologic impairment" said Maurice Ndukwu, MD, assistant professor of medicine at the University of Chicago Medical Center and lead author of the study. "For many patients, that dysfunction persisted long after discharge."

Physicians classify heat stroke as: "exertional," triggered in healthy people by intense activity in very hot and humid weather; or "classic," which usually strikes the inactive ill or elderly during prolonged heat. In this study, most patients with classic heat stroke suffered the more profound health consequences associated primarily with exertional heat stroke. They were more likely to suffer brain, kidney, and cardiovascular damage--previously considered rare in classic heat stroke.

Another troubling finding was that because of the overwhelming number of patients that inundated the city's emergency rooms, few patients received optimal treatment: immersion in cool or ice water within 30 minutes. Only one of the 58 patients in this study--which followed every patient at 12 of the area's 24 hospitals with classic heat stroke severe enough to require ICU care--was properly cooled within the recommended time.

"Because they presented with altered mental status, many of these patients were sent for head CT scans, delaying aggressive cooling measures," explained Ndukwu, "but the information gained from these scans did not contribute to patient management."

Unlike previous studies of classic heat stroke, Ndukwu's team found that the majority (66 percent) of patients developed kidney dysfunction--and 45 percent abnormalities in blood clotting, which could contribute to subsequent liver and lung damage.

They also found an unexpectedly high rate (57 percent) of pre-existing infections in patients requiring ICU care, placing these patients at additional risk.

"In this unusual episode, we saw sicker patients with more severe disease than is customary. It taught us that classic heat stroke is a deadly disorder, more complex, more often fatal, and more permanently disabling than the literature on this disorder would predict. And it drove home the crucial importance of prevention and rapid diagnosis and treatment."

Additional authors of the paper include Jane Dematte, from Michael Reese Hospital; Karen O'Mara, Jennifer Buescher, Sean Forsythe, Raghavendra Adiga and Turi McNamee from the University of Chicago; and Cynthia Whitney, Centers for Disease Control and Prevention.

The research was supported by the Park Ridge Health Foundation and the University of Chicago Clinical Research Center.


Classic heat-stroke patients who required intensive care during the 1995 Chicago heat wave often had complications rarely described in previous studies. Half of those patients died within a year and many survivors suffered permanent loss of independent function. The focus only on immediate deaths underestimates the consequences of that episode.