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CPR quality...this what matters

There are many studies that address the quality of CPR.  Here, we discuss a multicenter, observational study of in-hospital and out-of-hospital cardiac arrests.  While we prefer randomized trial data, it would not have been ethical to conduct the following study as a trial.
Edelson et al. analyzed 60 VF arrests to identify predictors of successful shocks.  A shock was deemed successful if VF was terminated for at least 5 seconds (a common definition in the literature).
Using logistic regression, the study reported that successful defibrillation was associated with deeper compressions and shorter pre-shock pauses.  Each 5 mm increase in compression depth and each 5 second decrease in pre-shock pause was associated with an approximate two-fold increase in the likelihood of shock success. Note, however, that the study was not powered to predict “harder” outcomes, such as survival or neurological recovery.
Predictors of successful shocks
Below is another way to look at the data. After only 10 seconds of pausing compressions, the likelihood of a successful shock dropped by 22%.  That’s a big drop. While the sample size was small and no error bars were reported, the trend was statistically significant.
Predictors of successful shocks chart 2
In the figure below, compression depth is categorized in 0.5 inch or 11 mm intervals. The trend was that the deeper the compressions, the better the shock success. Although the sample size was small (n=5), the shock success rate was 100% with 5 cm of compression. Based on this data, guidelines for compressions for adults have changed from 3.8-5 cm (1.5-2 inches) to > 5 cm (2 inches). A remaining question is whether or not an even greater compression depth would be better.
Predictors of successful shocks chart 3
Below is an example of a successful shock preceded by an 8 second pre-shock pause and deep compressions.
Successful shock diagram
Below is an example of an unsuccessful shock preceded by a 16 second pre-shock pause and shallow compressions.
Unsuccessful shock diagram

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