Cerebral arterial and venous contributions to tissue oxygenation index measured using spatially resolved spectroscopy in newborn lambs.
BACKGROUND
Bedside assessments of cerebral oxygenation are sought to monitor cerebral injury in patients undergoing intensive care. Spatially resolved spectroscopy measures tissue oxygenation index (TOI, %) which reflects mixed cerebral arterial and venous oxygenations. We aimed to evaluate arterial and venous components of TOI (cerebral arterial to venous volume ratio [A:V ratio]) in the newborn lamb brain using cerebral arterial and venous blood samples, and to investigate the impact of acute hypoxemia on the A:V ratio and TOI.


METHODS
Nine lambs were ventilated with varied inspired oxygen to generate arterial oxygen saturations between 25% and 100%. Cerebral arterial and venous oxygen saturations analyzed using cooximeter of arterial and superior sagittal sinus blood were used to estimate TOI (TOIcox), assuming cerebral A:V ratio of 25:75. TOIcox was compared with the TOI measured by spatially resolved spectroscopy (TOIsrs). Actual cerebral arterial and venous volume fractions were reestimated using TOIsrs = cerebral arterial volume fraction cerebral arterial oxygen saturation + cerebral venous volume fraction*cerebral venous oxygen saturation.


RESULTS
Median (range) TOIsrs was 48.5% (32.0-64.1%), and TOIcox was 48.4% (13.7-74.4%), and the two were significantly correlated (R = 0.77). The mean difference between TOIsrs and TOIcox was 2.4% (limits of agreement ± 18.1%). The TOIsrs - TOIcox difference varied with oxygen saturations, with TOIsrs higher than TOIcox at low saturations, and lower at high saturations. Cerebral arterial volume fraction was 22.9-27.5% in normoxia and markedly increased in hypoxemia.


CONCLUSIONS
TOI corresponds with cerebral oxygenation. The variable agreement of TOIsrs with TOIcox may reflect changes in cerebral A:V ratio due to arterial oxygenation-related vasoreactivity.
The Ritchie Centre, Monash Institute of Medical Research, Monash University, Melbourne, Victoria, Australia. flora.wong@med.monash.edu.au
Flora Y
FY
Wong
Theodora
T
Alexiou
Thilini
T
Samarasinghe
Vojta
V
Brodecky
Adrian M
AM
Walker
D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001810:Blood Volume / Q000502:physiology; D002536:Cerebral Arteries / Q000502:physiology*; D002550:Cerebral Veins / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology; D000860:Hypoxia / Q000097:blood; D010092:Oximetry; D010100:Oxygen / Q000097:blood; D010101:Oxygen Consumption / Q000502:physiology*; D012756:Sheep; D019265:Spectroscopy, Near-Infrared / Q000379:methods
D010100:Oxygen