Hypercapnia may be the main cause of cardiovascular instability, which seems to be the major trigger for a decrease in brain oxygenation in triple H syndrome despite severe hypothermia 1).

1)
Strapazzon G, Putzer G, Dal Cappello T, Falla M, Braun P, Falk M, Glodny B, Pinggera D, Helbok R, Brugger H. Effects of hypothermia, hypoxia and hypercapnia on brain oxygenation and haemodynamic parameters during simulated avalanche burial - a porcine study. J Appl Physiol (1985). 2020 Nov 5. doi: 10.1152/japplphysiol.00498.2020. Epub ahead of print. PMID: 33151777.