DiscussionIn an unadjusted analysis of a large prospective study of OHCA, we observed that ALS care was associated with increased survival to hospital discharge compared with BLS only care. In adjusted analyses, this observation was attenuated when the provision of ALS care occurred greater than six minutes after the arrival of BLS providers. Our resultswere consistent across a wide range of secondary outcomes and sensitivity analyses. Our data originated from the largest observational study of OHCA in North America with granular details of the pre hospital clinical care rendered. These observations support the lifesaving role ofALS in pre hospital care of patients with OHCA