Tissue oxygenation index measured using spatially resolved spectroscopy correlates with changes in cerebral blood flow in newborn lambs

Background/Objective  Abnormal cerebral haemodynamics in very preterm infants undergoing neonatal intensive care have been associated with adverse outcome, but cerebral blood flow (CBF) is difficult to assess at the cotside. Spatially resolved spectroscopy (SRS) continuously measures cerebral tissue oxygen saturation expressed as tissue oxygenation index (TOI, %), and TOI would reflect changes in CBF with constant cerebral metabolic demand. We aimed to evaluate the relationship between simultaneous measurements of CBF (Transonic System) and TOI (Hamamatsu NIRO-200) in newborn lambs (n = 8). We hypothesised that alterations in CBF of different magnitudes and frequencies would be reflected as changes of TOI in both time domain and frequency domain analyses. Methods  A silicon cuff positioned around the common brachiocephalic artery was inflated to induce 10–30% reductions in cerebral perfusion pressure so as to produce mild-to-moderate variations of CBF. Relationships between changes in CBF and TOI were evaluated in the time domain (Pearson correlation) and frequency domain (Coherence). Results  In time domain analysis, there was significant correlation between ΔTOI (%) and ΔCBF (%) (R ² = 0.69, P < 0.001). In frequency domain analysis, CBF-TOI coherence was ≥0.5 at frequencies below 0.1 Hz, but <0.5 at higher frequencies. Conclusions  We conclude that overall TOI changes are concordant with CBF variations, when arterial oxygen saturation and cerebral oxygen consumption are constant. While TOI reflects CBF, it is more sensitive to variations of CBF of low frequency (<0.1 Hz) than to more rapid, higher frequency changes.