A team of Emory researchers recently found scientific evidence that the well-known health risks of extreme heat can have especially severe consequences for patients with pre-existing cardiovascular disease.
They made their discovery by correlating the National Weather Service's Historical HeatRisk data, a scoring system based on daily temperatures and mortality-linked temperature thresholds developed for each of the weather stations in the US, with ten years of cardiovascular encounters over the warm weather months at three Emory hospitals. They found people exposed to one or more days of extreme HeatRisk (level 4) in the previous five days were 35.6% more likely to require admission to the Cardiac ICU. For people who had 6 consecutive days of HeatRisk level 3 ("major risk") or greater, Cardiac ICU admissions nearly doubled from 18.1% to 32.5%.
National Oceanic and Atmospheric Administration's heat risk index. The higher the value, the greater the level of heat concern would be for that location and the higher the risk of heat-related impacts.
The findings confirmed what investigator Charles Searles, MD, and his colleagues had suspected for a long time. "Physiologic response to heat requires higher cardiac output, primarily through increased heart rate," says Searles, professor in the Emory School of Medicine. "It dilates blood vessels in the skin and increases skin blood flow. It also causes loss of water from the body, which alters blood electrolyte levels and makes patients more susceptible to different arrhythmias, including atrial fibrillation and ventricular tachycardia. A lot of our patients with pre-existing cardiovascular disease are walking a fine line for these issues, so monitoring or modulating their heat exposure is crucial."
Noting the limited availability of clinical guidelines for heart patients and hot weather, the team is in the process of incorporating the HeatRisk Index tool, developed by the National Oceanic and Atmospheric Administration and Centers for Disease Control and Prevention, into their study. This HeatRisk Index tool assesses not simply temperature, but the likelihood of increasing temperature to cause physical stress and injury. Different levels, from 0 to 4, indicate increasingly severe impacts on people. "This modeling tool incorporates heat and health data, humidity, time of year and acclimatization," says Searles' collaborator Laura Seeff, MD, who works at the CDC. "You can live somewhere in May and have a certain response to temperatures that will look different in that same place in September. High humidity, low humidity will have different impacts on your heat exposure. This is a tool that combines all those things together. to forecast the impact of heat on health at a very local level."
The team wants to extend its research by bringing air quality into the cardiac risk equation as well. But team member Priya Kohli, MD, an assistant professor of cardiology in the School of Medicine, believes even these findings based on heat alone can be useful. "If we know that heat's going to be a problem going forward, it can help you plan what resources you have available in a hospital. How do communities plan for what are probably going to be sicker patients going forward?"
Seeff thinks there's a big educational component in that preparedness. "It starts with awareness," she says. "In the hot months, if people don't have air conditioning at home, they need to know about the cooling centers in their neighborhood. There should also be a doctor-patient conversation before the heat season. For example, a patient with heart failure who's on a diuretic and outside sweating is going to be losing more water and then trying to take in more water because of the heat. A discussion before the heat season between an individual like this and their doctor about how to handle diuretic and fluid on high HeatRisk days could be helpful."
After showing that heat can be a cardiovascular risk factor, the team now wants to look at air quality, which frequently worsens as temperatures rise and has been shown to trigger cardiovascular events. They'd also like to know which cardiac care resources such as ICU beds or special medications are most strained by hotter weather. But Searles thinks their findings could be useful even now. "If you look at the guidelines for the American College of Cardiology, the American Heart Association, they address blood pressure, diabetes, cholesterol, obesity, smoking, exercise, sleep, diet," he says. "Maybe they should be addressing heat as one of these modifiable risk factors that would improve patients' outcomes and health."
