Compressions

Cardiac output = stroke volume x heart rate
A high cardiac output means that the organs that need blood are getting perfused, so improving the cardiac output in a patient in arrest (output = zero) is the ultimate goal of CPR. We can’t magically change output, but we have control over the heart rate by changing the number of compressions per minute, and we change stroke volume by adjusting the depth of chest compressions.


Cardiac output = stroke volume x heart rate
The limiting factor in changing the stroke volume is that the heart is protected from direct access by the chest wall. Instead of just pumping the heart, we have to pump the entire chest. The chest is compressed, and the pressure in the thorax ejects blood from the ventricle. The ribcage provides recoil, and the negative pressure produced by the recoil pulls blood from the peripheral circulation into the atrium and ventricles so that the next compression actually has blood to eject back into circulation. Not pushing deep enough means stroke volume is decreased, and that lowers the cardiac output. However, deep compressions break ribs and that means a loss of recoil. Contrary to popular belief, breaking someone’s ribs does NOT mean you are doing good CPR.

The balance of adequate stroke volume and appropriate recoil in a good chest compression depresses the sternum 2 inches, or 1/3 the depth of the chest. Ribs may break, and if it’s because you are compressing the chest to the correct depth, that’s ok, but don’t make it your goal.

There are lots of devices coming onto the market to measure chest compressions.  Small sensors ride between patients' chests and rescuers' hands and give continuous feedback.  Many hospital systems are even looking to review this data after-the-fact to evaluate the quality of CPR.


Cardiac output = stroke volume x heart rate
If increasing the stroke volume is so difficult with CPR, the only other option we have to improve cardiac output is to maintain a high heart rate. In CPR, this means lots of compressions, 100 compressions, each minute. Slower than 100, and cardiac output drops to a point where CPR is not doing any good. Compressions faster than 100, and the heart does not have enough time to refill with blood before the next compression.