Objectives: The hypothermic to ischemic ratio (HI Ratio) has been previously identified as a potential predictor of survival from out-of-hospital cardiac arrest (OHCA). We investigated its association with functional outcome in the Resuscitation Outcomes Consortium (ROC) Amiodarone, Lidocaine, or Placebo Study (ALPS).
Methods: We included adult, non-traumatic, witnessed, OHCA patients screened for ROC-ALPS who survived to hospital between May 2012 and October 2015. Structured data were collected, including all Utstein variables and relevant time intervals to determine initiation and duration of hypothermia and rewarming. HI Ratio is defined as total time TTM applied (initiation through re-warming) divided by time from arrest to ROSC. Good functional status (mRS 0-3) at hospital discharge was the primary outcome of interest. We fit a logistic regression model to estimate the association between HI ratio and the primary outcome, adjusting for Utstein variables and ROC enrolling site.
Results: A total of 2,214 patients were included, of whom 36.2% were discharged with good functional outcome. Patients had a mean age of 62.8 years (SD 15.2), with 72.0% male, and 59.6% receiving bystander CPR. Median time to ROSC was 0.34 hours (IQR 0.26, 0.44) and median duration of hypothermia was 32.7 hours (IQR 23.9, 37.7). The figure shown plots patients according to ischemia and hypothermia durations, showing a higher proportion of good functional outcomes at a higher HI Ratio. After adjustment, a larger HI Ratio (per 10 unit change: OR 1.127, 95% CI 1.126, 1.128) was associated with good functional outcome.
Conclusions: Larger HI Ratio was associated with good functional outcome after OHCA. Our results provide further rationale for prospective investigation of individualized TTM management for those resuscitated from OHCA.