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Background: The knowledge of new prognostic factors in out-of-hospital cardiac arrest (OHCA) that can be evaluated since the beginning of cardiopulmonary resuscitation (CPR) manoeuvres could be helpful in the decisionmaking process of prehospital care. We aim to identify metabolic variables at the start of advanced CPR at the scene that may be associated with two main outcomes of CPR (recovery of spontaneous circulation (ROSC) and neurological outcome). Methods: Prospective observational study of all non-traumatic OHCA in patients older than 17 years assisted by emergency medical services (EMS), with doctor and nurse on board, between January 2012 and December 2017. Venous blood gases were sampled upon initially obtaining venous access to determine the initial values of pH, pCO 2 , HCO 3 − , base excess (BE), Na + , K + , Ca 2+ and lactate. ROSC upon arrival at the hospital and neurological status 30 days later (Cerebral Performance Categories (CPC) scale) were recorded. Results: We included 1552 patients with OHCA with blood test data in a 6-year period. ROSC was achieved in 906 cases (58.4%), and good neurological recovery at 30 days (CPC I-II) occurred in 383 cases (24.68%). In multivariate analysis, we found a significant relationship between non-recovery of spontaneous circulation (no-ROSC) and low pH levels (adjusted odds ratio (OR) 0.03 (0.002-0.59), p = 0.020), high pCO 2 levels (adjusted OR 1.03 [1.01-1.05], p = 0.008) and high potassium levels (adjusted OR 2.28 [1.43-3.61], p = 0.008). Poor neurological outcomes were associated with low pH levels (adjusted OR 0.06 [0.02-0.18], p < 0.001), high pCO 2 (adjusted OR 1.05 [1.03-1.08], p < 0.001), low HCO 3 − (adjusted OR 0.97 [0.94-0.999], p = 0.044), low BE (adjusted OR 0.96 [0.93-0.98], p < 0.001) and high potassium levels (adjusted OR 1.37 [1.16-1.60], p < 0.001). Conclusion: There is a significant relationship between severe alterations of venous blood-gas variables and potassium at the start of CPR of non-traumatic OHCA and low-ROSC rate and neurological prognosis.

Therapeutic study, level IV; epidemiologic study, level III.

Epidemiologic description enabled the characteristics of the prehospital assistance procedure to be comprehensively ascertained and quantified. False alarms assume special relevance due to their implications. Emergency medical alerts with a psychosocial component are on the increase, something that must be controlled.

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