Why does breathing stop during cardiac arrest?

Question

I've been reading about resuscitation, and found several suggestions that people don't continue breathing after effective heart function stops because the diaphragm quickly runs out of oxygen. Is this correct?

As best I can tell, current conventions are to continue to attempt resuscitation so long as brain stem cells are likely to stay alive, which, at normal body temperature, seems to be somewhere around 5 minutes after loss of pulse.

But while the brain stem is alive, does the respiration center not continue to (try to) breathe? If so, does the diaphragm really use all of its blood's reserve oxygen in just a few breaths, and simply not have energy to breathe? If not, is there some other effect that impairs breathing when the heart is not pumping effectively? I suspect it's the latter, since otherwise CPR would never include respiration, and effective CPR would be evidenced by the subject's breathing.

Answer 1

During approximately 50% of cardiac arrests, the patient continues to breathe for a time. However, this breathing is known as agonal respiration and is essentially gasping for air. This gasping is actually beneficial if CPR can be started while it is still occurring, it is believed that this may increase the chances of survival during a cardiac arrest incident and is better than any form of artificial respiration. This information was found on Sarver Heart Center's website.

The most interesting and, I hope helpful, site I found was PubMed.gov which discussed this exact issue. In the article, they mention that it is not known for sure why cardiac arrest often leads very quickly (if not instantly) to apnea as the O2 levels in the brain stem don't drop immediately.

If you have any additional questions about cardiac arrest itself or it's treatment, I highly recommend paying the Merck Manuals website a visit.

To sum things up a bit, it isn't known for sure why respiratory arrest often occurs at the same time as cardiac arrest, nor why cardiac arrest simply leads to agonal breathing initially in 50% of cases.