Abstract Body (Do not enter title and authors here): BACKGROUND: Reduced heart rate variability (HRV) reflects sympatho-vagal imbalance and is associated with cardiovascular mortality in adults. Literature is limited in pediatrics, but reduced HRV has been observed in congenital heart disease (CHD) and is associated with prolonged postoperative hospitalization.

OBJECTIVE: To investigate the effect of HRV metrics on outcomes after surgery for CHD.

METHODS: This single-center retrospective cohort study included children aged 0-18 years who underwent surgery for CHD (2010-2018). Preoperative 12-lead ECGs (within 10 days) and the first postoperative ECGs were analyzed. ECGs with atrial or ventricular pacing were excluded. Ultra-short term HRV metrics were calculated, including standard deviation of interbeat intervals (SDNN) and the root mean square of successive RR interval differences (RMSSD). SNDD and RMSSD were classified as low if <25^th percentile. Post-operative complication was defined as CPR, unplanned open chest, unplanned reoperation, low cardiac output, bleeding, extracorporal membrane oxygenation (ECMO) or death prior to hospital discharge. Major adverse cardiac event (MACE) was defined as CPR, ECMO and/or death. All analysis were adjusted for age and gender.

RESULTS: Among 7,192 children undergoing surgery (55% male, age 0.9 [0.2, 4.5] years), 2,413 (34%) experienced postop complications, 344 (5%) MACE, and 270 (4%) died prior to hospital discharge. HRV metrics could be calculated in 3347 (47%) patients pre-operatively and 3392 (47%) post-operatively. SDNN and RMSS were 0.012 [0.005, 0.028] ms and 0.010 [IQR 0.005, 0.029] ms pre-operatively, and 0.002 [0.002, 0.005] ms and 0.003 [IQR 0.002, 0.005] ms post-operatively. Pre-operative low SDNN and RMSSD were associated with increased risk of post-operative complications, MACE, and death prior to hospital discharge (Figure). Low post-operative RMSS was associated with MACE and death prior to hospital discharge. Low pre- and post-operative SDNN and RMSSD were associated with prolonged CICU and hospital length of stay.

CONCLUSION: Low HRV metrics were associated with worse post-operative outcomes. Pre-operative HRV metrics were most predictive of post-operative complications and mortality.