Is there such a thing as too much rescue breathing? There could be, judging from the results of a study published in the April 27 edition of Circulation, the journal of the American Heart Association.
In that study, a team led by Tom Aufderheide, MD, professor of emergency medicine at the Medical College of Wisconsin, found that rescuers often provided cardiac-arrest victims more than the AHA-recommended 12–15 CPR breaths per minute. In animal models, the researchers discovered, “similar excessive ventilation rates resulted in significantly increased intrathoracic pressure and markedly decreased coronary perfusion pressures and survival rates.”
“I think the fundamental physiologic principle here—that excessive ventilation rates and high mean intrathoracic pressure reduce forward-flow effectiveness of CPR and survival rates—has been definitively established with this study,” Aufderheide says.
In the first seven cardiac-arrest cases Aufderheide’s group studied, the average ventilation rate was over 30 and reached as high as 49. Even after retraining of those delivering the CPR, the average for the next six was still 22. None of these 13 patients survived. These results were then compared to tests done on pigs. Nine arresting pigs were ventilated with either 12, 20 or 30 breaths per minute. In these animals, the mean intrathoracic pressure increased (7.1, 11.6 and 17.5 mmHg/min., respectively) and coronary perfusion pressure decreased (23.4, 19.5, 16.9 mmHg) when more breaths were provided.
What happens with excessive breathing is that it increases intrathoracic pressure, which reduces coronary perfusion because blood can’t flow back into the heart. “It reduces venous blood return to the heart, and reduced blood return means reduced blood outflow from the heart,” says Aufderheide.
The researchers also studied survival rates in groups of seven cardiac-arrest pigs given 12 breaths per minute of 100% O2, 30 breaths per minute of 100% O2, or 30 breaths per minute of 95% O2/5% CO2. In the first group, six of seven survived; in the latter two groups, only one of seven did.
It is worth noting that this study did not address how widespread CPR hyperventilation might be or the reasons why rescuers might provide too many breaths (though other research has documented the problem in hospitals as well). For those reasons, additional research is needed.
“We’re hoping that other centers will monitor their ventilation rates and report them, so we have an idea of how widespread the practice is,” says Aufderheide. “Locally, we’ve worked very hard to reduce the ventilation rates delivered at the scene to 12–15 breaths per minute, and we’ve succeeded in that. Having done that, we’ll be looking at the impact on survival.”
Another solution may be to employ a sort of timer or pacing device in the field to help those performing CPR stick to AHA breaths-per-minute recommendations. Says Aufderheide: “A light that flashes for one second 12 times a minute could greatly assist rescuers in providing the correct ventilation rate during cardiac arrests.”
Aufderheide TP, Sigurdsson G, et al. Hyperventilation-induced hypotension during cardiopulmonary resuscitation. Circulation 109(16): 1,960–1,965, April 27, 2004.