Abstract Body: BACKGROUND
Out-of-hospital cardiac arrest (OHCA) patients often experience refractory ventricular fibrillation (R-VF), defined as requiring ≥3 shocks during resuscitation. R-VF does not distinguish between incessant R-VF where VF persists despite shock from recurrent R-VF where VF terminates then recurs or mixed R-VF where R-VF is characterized by incessant and recurrent. These R-VF subtypes may represent discrete phenotypes with distinct prognoses benefiting from differential treatment. However, there is not an established definition of R-VF subtypes.

OBJECTIVE
We compared R-VF subtype occurrence using two previously-described subtype definitions and characterized care and outcome.

METHODS
We performed a cohort study of adult VF-OHCA from a metropolitan EMS system between 2006–21. Data were collected by review of digital defibrillator recordings and EMS and hospital records. We tabulated shock-specific occurrence of incessant versus recurrent R-VF subtypes according to two definitions. Method 1 defined recurrent R-VF as the presence of a non-VF rhythm for ≥5 s post-shock versus incessant R-VF as post-shock VF with <5 s of non-VF rhythm. “Snapshot” method 2 defined recurrent R-VF as the absence of VF at a specific time T sec post-shock versus incessant R-VF as the presence of VF at time T. We classified and compared each patient’s overall refractory subtype based on the first two shock responses as recurrent alone, mixed (incessant + recurrent), or incessant alone.

RESULTS
Among 1833 eligible patients, median age was 62, 403 (22%) were female, 1019 (56%) were refractory, and 775 (42%) survived with favorable status. Of the 1019 R-VF patients, 890 had technically sufficient ECG recordings to ascertain R-VF subtype with approximately half classified as recurrent, a third as mixed, and 15% as incessant (Table 1). Method 1 and method 2 (T=8 s post-shock) had excellent agreement (kappa=0.95) compared to T=6 s (kappa=0.84) and T=10 s (kappa=0.89). Although characteristics tended to be comparable, shock count and resuscitation duration increased while clinical outcomes decreased for recurrent to mixed to incessant R-VF regardless of method (p<0.001) (Table 2).

CONCLUSIONS
Two methods to define recurrent vs incessant VF produce similar classification with the T=8 s snapshot providing a simpler approach. Incessant shocks were associated with a “dose-dependent” decrease in outcome, highlighting the opportunity to risk stratify R-VF patients in a more refined manner.