What the numbers show
Researchers at the University of Chicago analysed records from 10 US health systems in the Common Longitudinal ICU Format Consortium, a network that standardises hospital data for research. They matched each day's local temperature to admissions for out-of-hospital cardiac arrest, the sudden loss of heart function outside a hospital, and adjusted for humidity and air pollution.
The comparison point was days reaching 85F. At 91F, cardiac arrest admissions to intensive care were 45% higher. At 98F they were 102% higher, roughly double. Women and older adults showed the largest increases.
Why the intensive care unit matters
About 90% of people who suffer an out-of-hospital cardiac arrest do not survive. Those who are resuscitated usually arrive unconscious and unable to breathe on their own, so they go straight to an ICU and may need advanced life support for days or weeks.
That is where heat becomes a hospital problem as well as a street problem. ICU beds, ventilators and trained staff are fixed in the short run. A hot week that adds critically ill patients can push a hospital past its capacity even if total admissions rise only modestly.
Peter Graffy, a postdoctoral fellow in pulmonary and critical care medicine at Chicago and the study's presenter, said heat waves are becoming more frequent and intense, adding strain on emergency services and on hospitals that must absorb the influx of climate-related illness.
An outcome that surprised the authors
Among patients who reached the ICU, survival and neurological recovery during heat waves looked much the same as at other times. The researchers also checked whether heat changed how many patients needed organ support, and found no clear difference in outcomes.
That is a preliminary finding from an abstract, not a full peer-reviewed paper, and it covers only patients sick enough to be admitted. It does not capture people who died before reaching hospital, where most cardiac arrest deaths occur. The work will be presented at the American Heart Association's Resuscitation Science Symposium in Chicago on 7-8 November.
The practical message is about preparation: cooling centres, heat alerts and staffing plans for emergency and intensive care departments in the weeks when forecasts turn hot.