Monday, May 02, 2005

Evidence-Based Medicine Meets CPR

One of my readers sent me the following link and asked if I'd comment on it. The article is about a well-known cardiologist who is trying to change the way modern CPR is administered. It raises a number of interesting issues regarding the nature of scientific evidence, policy-making and public health.

Click here for complete post.
One of my readers sent me the following link and asked if I'd comment on it. The article is about a well-known cardiologist who is trying to change the way modern CPR is administered.

Dr. Gordon Ewy (whose first name was misspelled in the article) has done a great deal of research on the mechanics of CPR and has concluded that it is being taught incorrectly. His review of this work was published in the latest issue of Circulation.

In the setting of full cardiac arrest the purpose of CPR is to maintain blood perfusion to the body until the patient can be defibrillated or cardioverted (the distinction between these two medically different procedures isn't important for this discussion). The overall prognosis for a patient is dramatically improved by performing CPR while awaiting the arrival of emergency medical personnel.

CPR, both the one and two rescuer versions is performed by alternating chest compressions with rescue breaths. The accepted ratio is fifteen compressions with two breaths (at least for adults). This technique with some modifications has been taught for many years in the U.S.

Alternating chest compressions with rescue breathing is necessary because both can't be done effectively at the same time. To do both makes physiologic sense. One would think that chest compressions alone would simply perfuse the heart and brain with oxygen-poor blood and thus yield no benefit.

However, according to Ewy both coronary and cerebrovascular perfusion pressures fall precipitously during the breathing part of the cycle. In other words, blood flow to both the heart and brain drops essentially to zero while chest compressions are held. Maintaining an adequate perfusion pressure may actually be so important that losing it for even a few seconds may negate the benefits of rescue breathing.

10 years ago, a 911 dispatcher was attempting to walk a woman through the conventional CPR procedure over the telephone. Her husband had suffered a cardiac arrest. Ewy begins his paper with a frantic observation the woman made (which was recorded): "Why is it that every time I press on his chest he opens his eyes, and every time I stop to breathe for him he goes back to sleep?"

Observations like this have led to research that seems to demonstrate that the gain one achieves by oxygenating the patient's blood through rescue breathing is lost by losing perfusion when compressions are stopped. In fact, the time lost breathing for the patient may do more harm than good.

Ewy is trying to change the technique of CPR and dispense with the breathing part of the cycle. He advocates chest compressions only at a rate of 100 per minute. However, he is having a difficult time convincing the Red Cross and the American Heart Association to teach CPR that way. The reason is presumably the lack of evidence on human patients.

Most of the data supporting his position is on animals. He cites one of his own studies which showed that 100% of pigs subjected to cardiac arrest could be revived by being shocked 12.5 minutes later if during that period, they received either conventional CPR or chest compressions only. All of these pigs had normal neurological function at 24 hours. In other words both methods worked equally well. (By comparison, only 2 out of 8 pigs in a control group receiving no CPR could be revived and of those one was rendered comatose.)

While this evidence seems very reasonable, pigs are not people. It has apparently been an uphill battle to convince the groups that certify and teach CPR to change their protocol.

Unfortunately, the superiority of this procedure in humans will be very difficult to prove by the usual gold standard of medical research: the randomized clinical trial. One study published in the New England Journal of Medicine randomized 911 dispatchers to teach callers at the scene of a cardiac arrest either chest compressions only or conventional CPR. Of the 1st group, 14.6% of patients survived to hospital discharge. In the conventional CPR group only 10.4% did. The difference was not statistically significant however.

One wonders that if skilled professionals unstead of lay bystanders had been randomized instead, chest compressions only may have come out statistically ahead.

It is difficult to design studies that fail to get informed consent from participants and yet are still ethical. Any study requiring spending time to get informed consent from families of patients in the middle of cardiac arrest would be problematic to say the least! For this reason, further large studies are unlikely to be performed.

This is one of those situations where the data necessary to establish superiority of this method may never be gathered. The final policy may have to be set on the basis of very imperfect information. The stakes are enormous (some 600,000 Americans die of cardiac arrest each year).

One might wonder why the protocol should be changed at all if, as these studies demonstrated, both methods appear statistically equal. Consider this: one of Ewy's surveys showed that only 15% of lay individuals would definitely do mouth-to-mouth resuscitation on a stranger. I have no doubt that this number is in the right ballpark. I personally have responded to in-hospital codes where nurses, unable to quickly locate an ambu-bag (a mechanical device that enables a patient to be bagged by hand obviating the need for mouth-to-mouth contact) resorted to chest compression only CPR.

Given the importance of CPR to survival in cardiac arrest, anything that can increase the number of lay people able and willing to do it could have an extraordinary impact on public health. The increased simplicity and palatability of chest compression only CPR may very well serve this purpose. Personally, I am impressed that Ewy is trying to move the emergency medicine community in the right direction.

This is no mere academic exercise.


Anonymous Juan said...

Apathy strikes again. I think compressions-only CPR has its advantages. Why does breathing stop in cardiac arrest, anyway?

July 14, 2009 10:03 PM  

Post a Comment

<< Home