1. What is the relationship between near infrared spectroscopic regional mixed arterio-venous oxygen saturation and jugular bulb venous saturation during core cooling and bilateral selective antegrade cerebral perfusion? Vamsidhar B Dronavalli, Ashvini Menon, Ishtiaq Rahman, David Green, Timothy Jones, Jorge Mascaro, Robert S Bonser Department of Cardiothoracic Surgery, University Hospital Birmingham NHS Foundation Trust, Queen Elizabeth Hospital, Birmingham, UK. Conflict of interest - none

2. Aim To assess near infrared spectroscopy (NIRS) as a regional index of mixed arterio-venous cortical saturation (rSO2 ) in managing patients during core cooling (CC)

3. Introduction Increasing jugular bulb venous oxygen saturation(SjO2) during CC correlates with reducing brain O2 consumption and guides safe commencement of hypothermic circulatory arrest (HCA). NIRS a regional index of rSO2 has been advocated as a surrogate of SjO2.

4. Methodology 9 patients undergoing CC Utilising NIRS, rSO2 was measured –At 35 o C, 30 o C, 25 o C and 15 o C during CC –At 5, 15,30 and 45 mins of SACP –At 15 o C, 20 o C, 25 o C,30 o C and 35 o C during warming Utilising a jugular bulb line venous oxygen saturation, S j O 2 was measured at similar time points Standardised anaesthetic and bypass protocol

5. The patients 15 patients Procedures 7x Ascending aorta + arch + elephant trunk ± Aortic valve replacement 2x Ascending aorta+ arch replacement 5x Ascending aortic aneurysm and arch aneurysm 2x Mega aortic syndrome 1X chronic type A dissection 1x Pseudo aneurysm of distal arch and proximal descending Diagnosis Mean age=55

6. Results Temp o CrSO2SjO2SjO2 Core cooling3557±13%77±25% 3061±1391±11%, 2559±1497±3 2067±999±0.7 1575±10%99±0.5% SACP at 15 o C5min 78±697±3 15 min72±1397±4 30 min71±1596±3 End Re-warming phase 35 87±1564±12% Both rSO2 and SjO2 increased with cooling p<0.01 and p=0.03 repectively

7. rSO2 and S j O 2 vs temperature change

8. Scatter plot of SjO2% vs rSO2%

9. Conclusion NIRS rSO2 assessment during CC underestimates SjO 2 for reasons that remain unclear. Regional SO 2 is paradoxically lower than SjO 2 during CC and our data raises questions of what each index of oxygen extraction is actually measuring.