Purpose of the study: 
There is limited data about relationship between duration of cardiopulmonary resuscitation (CPR) in out-of-hospital cardiac arrest (OHCA) and favourable outcome and also about a maximum time-period for successful resuscitation at all[1]. We investigated the influence of CPR-duration and favourable outcome in patients admitted to hospital with return of spontaneous circulation (ROSC).
Materials and methods:
Review of the CPR-database of a county with 252,000 inhabitants from 2014-2016. Data included both medical records and data of the internal ECG-storage. The duration of CPR performed by EMS until time of first ROSC was determined and compared to hospital discharge and neurological favourable outcome (Cerebral Performance Category CPC 1/2. Analysis with U-test, results as median (25%/75%/99% percentiles).
Results: 
CPR-attempts recorded in total: n=647. Complete data available in n=422 cases (100%). Admission with ROSC: n=171 (40.5%). Discharged alive: n=66 (15.6%), discharged with CPC 1/2: n=48 (11.3%). Duration of CPR by EMS until first ROSC (all initial rhythms): 12:47min(07:28/18:50/44:47min). Initial rhythm shockable (n=71) vs non-shockable (n=100): 10:20min(04:19/20:17/44:36min) vs 13:21min(08:48/18:37/42:52min), p=0.165. Discharged alive shockable rhythm (n=42) vs non-shockable (n=22): 07:46min(03:45/15:01/40:09min) vs 08:53min(05:05/11:29/20:56min), p=0.841. Duration of CPR by EMS until first ROSC, CPC 1/2 group (n=48) vs death-in-hospital group (n=107) (all rhythms): 06:53min(03:03/11:22/32:47min) vs 14:59min(09:36/21:44/49:19min), p=<0.00001.
Conclusion:
CPR in OHCA might be associated with survival and favourable outcome even when CPR was performed for more than 40 minutes by EMS. If the time from collapse to arrival of EMS is added, there really seems to be a “golden hour of resuscitation” with any chance of survival. Therefore we recommend not to stop CPR earlier.
References:
[1] L.L. Bossaert et al. / Resuscitation 95 (2015): 302–311

2 - When to stop CPR: Is there a golden hour of resuscitation?