After six months, 31.5% of patients in the H group and 22% of those in the S group had survived with good brain function, a difference that was not statistically significant. As secondary outcomes, Blohlávek and his colleagues looked at the proportion of patients in each group who had recovered neurological and cardiac function at 30 days after the OHCA. In the H group, 34.7% had good neurological function at 30 days, compared with 22.7% in the S group, a statistically significant difference. Recovery of cardiac function was similar in the two groups.

The main benefit of the hyperinvasive approach was evident in a subgroup of patients who were resuscitated for more than 45 minutes; 20 patients in the H group survived, compared with six in the S group. Moreover, four of the six survivors in the S group had been crossed over to the H group and had therefore received hyperinvasive rather than standard treatment.

The study design allowed emergency medical staff at the scene and at the hospital to switch (or "crossover") patients randomly assigned to the S group to the H group or vice versa. Patients assigned to the H group could be switched to the S group if hyperinvasive intervention would likely be futile. The overall crossover was low; of the 256 patients enrolled in the study, 11 (8.3%) randomly assigned to the S group were switched to the H group, while nine (7.2%) randomly assigned to the H group were switched to the S group. However, because the study used a rigorous form of analysis known as "intention to treat," patients who crossed over were still counted as part of the group they had been randomly assigned. According to Blohlávek, in patients who crossed over from the S group to the H group, 40% survived with normal neurological outcomes, while none of the patients survived in a switch from the H group to the S group.

The study's data safety and monitoring board--an independent group of experts whose job was to review the study data at prespecified intervals--decided to stop the study after 256 subjects had been enrolled, when it became clear that survival and neurological recovery were superior in the H group, Blohlávek said. For this reason, the study did not reach its prespecified enrollment target of 570 patients.

Patient survival in the S group, at 22%, was more than twice as high as the researchers had expected. "A byproduct of training the emergency medical teams to conduct the hyperinvasive protocol is that they also achieved extraordinarily good results among patients who received standard [advanced cardiac life support]," Blohlávek said.

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