Out-of-Hospital Cardiac Arrests Spike 14% Amid Heat Waves

Pam Harrison

July 13, 2016

SEONGNAM, REPUBLIC of KOREA — Heat waves, defined by temperatures above the 98th percentile lasting at least 2 days, increase the risk of out-of-hospital cardiac arrest (OHCA) by 14%, according to a multicity Korean study[1].

The risk of OHCA rises to 16% for heat waves with daily temperatures of at least 35°C (95°F) for at least 2 consecutive days.

Although the risk further increased over time up to 4 consecutive days, a novel finding of the study was that the added effects were relatively small, Dr Si-Hyuck Kang (Seoul National University College of Medicine, Seongnam, Republic of Korea) and colleagues report July 5, 2016 in the International Journal of Cardiology.

Prior studies have shown that compared with cardiovascular mortality, hospital admissions for cardiovascular causes such as MI, cardiac dysrhythmia, and congestive heart failure were unaffected by heat waves. "The results of this study could explain these apparently paradoxical findings," they write.

The investigators point out that the impact of extreme heat on medical crises like OHCA is garnering greater and greater attention as concerns about climate change soar.

Like other countries, Korea is seeing an increase in the annual incidence rate of OHCA, rising from 37.5 per 100,000 persons in 2006 to 46.8 per 100,000 in 2010.

This is particularly compelling considering that the case fatality rate for OHCA is 97% in Korea.

Temperatures Rising

Although studies have consistently shown that cardiac arrest occurs more frequently at low ambient temperature, Kang and colleagues write that studies are limited on the association of OHCA and high temperature.

To evaluate this, the nationwide emergency medical service database was used to identify 50,318 cases of OHCA from January 2006 to December 2013 in seven Korean cities with more than one million residents. Mean age of the cohort was 66.1 years and approximately two-thirds were male.

The investigators tested several definitions of heat wave including a daily mean temperature above the 98th percentile of the yearly distribution for at least 2 consecutive days throughout the study period.

"Using this definition, a 14% increase in the risk of OHCA was observed with heat waves for both mean and maximal temperatures," Kang observes.

Not surprising, the peak time for OHCA was during the late afternoon from 3 to 5 pm, when outdoor temperatures are highest.

While a V-shaped relationship has been reported between mean temperature and OHCA occurrence, the investigators observe that the relationship between ambient temperature and OHCA was nonlinear and J-shaped.

The risk of OHCA was minimal at a mean daily temperature of 22.5oC (72.5oF), but the lowest risk of having a cardiac arrest was found when the daily maximal temperature was 28oC (82.4oF).

"I think it's less about 28oC being protective and more about the fact that cardiac stress climbs as temperature climbs," Dr Benjamin Abella (University of Pennsylvania, PA) told heartwire from Medscape.

"Statistically, there is also a high incidence of cardiac arrest in colder temperatures as well," he added.

"So it's not so much that 28oC is protective but rather that being at a very low temperature or a very high temperature is a mitigating factor for cardiac arrest, and the key take-home message is, as the temperature climbs above 28oC, which it does in many countries, we really have an increased risk of OHCA."

In subgroup analyses, adults 65 years or older were significantly more likely to experience OHCA during a heat wave (P=0.039), whereas cardiac arrests with ventricular tachycardia or fibrillation were unaffected by heat waves.

It is well-known that the elderly do not react to signs and symptoms of dehydration as well as younger patients and are thus typically at greater risk for ill effects from the heat.

The Korean group also report that the effect of a heat wave on the incidence of OHCA did not change significantly after adjustment for air pollution.

The study was supported by the National Emergency Management Agency of Korea and the Korean Centers for Disease Control and Prevention (CDC) and funded by the Seoul Metropolitan City Government, the Korean CDC, and the Korea Meteorological Administration Research and Development Program. The authors and Dr Abella reported no relevant financial relationships.

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