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I just took a CPR course and was instructed to perform 5 cycles of compressions after an AED shock (after which you wait for the AED to do another analysis). My question is: if the shock was successful and the heart started beating again, wouldn't compressions mess with the heart starting up its rhythm?

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The only thing that keeps a patient in cardiac arrest alive is constant, high-quality chest compression.

Cardioversion ("shocking") of a patient aims to return the heart to normal (sinus) rhythm in the case that a cardiac arrest is due to a dysrhythmia. If it works, great - return of spontaneous circulation (ROSC) will be achieved and there will be signs of this; coughing, spluttering, pulse etc.

We assume that it will not be successful and immediately resume compressions because even a couple of seconds delay in chest compressions dramatically reduces organ perfusion and worsens outcome.

A couple of chest compressions in a patient who has been successfully cardioverted will not do any particular harm and the downsides of waiting to see if it has worked are huge.

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    A couple of chest compressions in a patient who has been successfully cardioverted will not do any particular harm and the downsides of waiting to see if it has worked are huge. This. – Narusan Jun 20 '18 at 15:27
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There are several studies that emphasize the importance of providing rapid and deep compressions and that CPR should resume immediately after the shock given by the AED, without the delay entailed in checking for pulse or rhythm conversion.

Here is an excerpt from one of the studies:

"After a successful shock, the rescuer was expected to assess the patient for a return of pulse after conversion of the ventricular rhythm. However, the delivery of repeat shocks (if necessary) and checking of the patient's pulse were found to delay the resumption of CPR for 60 seconds or more. Also, even after conversion, the ventricle was often stunned, so that its effective mechanical function did not return with the resumption of sinus rhythm. These issues, along with the finding that the ventricular rhythm was converted to sinus rhythm with the first shock in 85% of cases (13) led the AHA to modify the algorithm so as to specify only a single shock before immediate resumption of CPR, with a check of the patient's cardiac rhythm and pulse only after 3 minutes of continued compressions (14). This change in the resuscitation guideline, like the elimination of rescue breathing, maximizes the time during resuscitation for chest compressions and thus optimizes efforts to provide tissue perfusion."

Sources:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3116356/

https://www.aedusa.com/training

https://www.ahajournals.org/doi/full/10.1161/circulationaha.105.165674

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