People in cardiac arrest may do worse long-term if they’re given a shot of adrenaline, also known as epinephrine, on the way to the hospital, according to a Japanese study.

Researchers writing in the Journal of the American Medical Association found that people whose hearts suddenly stopped beating had circulation return more often in the ambulance if they were given adrenaline - but those same patients were less likely to be alive and without brain damage a month later.

“This finding implies that epinephrine administration might save the heart but not the brain,” wrote Akihito Hagihara, from Kyushu University Graduate School of Medicine, and colleagues.

For their study, the team looked at about 417,000 cases of cardiac arrest in which patients were treated by emergency medical services (EMS) and taken to the hospital between 2005 and 2008.

In the United States, more than 380,000 sudden cardiac arrests happen outside of the hospital annually, the American Medical Association says. In addition to CPR and sometimes Electrical shock, most of those patients are treated with adrenaline, which causes blood vessels not going to the heart to constrict, shunting as much blood as possible to the heart.

In mid-2006, Japanese laws changed to allow medics to give patients a shot of adrenaline if other methods, including CPR, failed to restart their hearts.

Just over 15,000 patients in cardiac arrest were given adrenaline during the study period. According to EMS records, close to 19 percent of those patients had their circulation come back in the ambulance, compared to only 6 percent of those who weren’t given adrenaline.

However, longer-term outlooks weren’t as good.

About five percent of patients survived a month after cardiac arrest, whether or not they were given adrenaline. But when the researchers considered how long it took EMS to get patients to the hospital, whether they were given CPR by a bystander and other differences between patients, they found that people given adrenaline were less than half as likely to survive the arrest.

What’s more, only about one-quarter of patients who were given an adrenaline shot and survived were still able to function pretty well on their own with limited neurological problems a month later, compared to close to half of those not treated with adrenaline in the ambulance.

“The real thing that patients care about is not just getting to the emergency department with a heartbeat, but getting home intact,” said Clifton Callaway, from the University of Pittsburgh, who wrote a commentary published with the study.

“You have greater success in getting the heart to start if you’re giving epinephrine, but we’re always concerned ... there might be adverse consequences in other organ systems.”

The researchers noted that they couldn’t account for differences in how patients were treated once they arrived at the hospital, and it’s possible some of those who weren’t given adrenaline by EMS got a shot later, which other doctors said could be an important factor to consider.

“There’s so much more that happens, and I think it would be a little naive to think that a one-time dose of epinephrine given during the resuscitation attempt could have that much of an impact on patient outcomes at one month,” said Comilla Sasson, an emergency medicine doctor who has studied cardiac arrest at the University of Colorado School of Medicine.

She also pointed out that the findings can’t necessarily be extrapolated to patients in other countries. Japanese patients typically get a single shot of adrenaline, whereas in the United States it’s standard for EMS to give doses every three to five minutes when trying to get a pulse back.

Both Callaway and Sasson agreed that the findings call for a more rigorous study in which cardiac arrest patients are randomly assigned to get adrenaline or not, then tracked for long-term outcomes.

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