Tissue hemoglobin index: a non-invasive optical measure of total tissue hemoglobin

Abstract

IntroductionThe tissue hemoglobin index (THI) is a hemoglobin signal strength metric provided on the InSpectra™ StO2 Tissue Oxygenation Monitor, Model 650. There is growing interest regarding the physiologic meaning of THI and whether a clinically useful correlation between THI and blood hemoglobin concentration exists. A series of in vitro and in vivo experiments was performed to evaluate whether THI has potential utility beyond its primary purpose of helping InSpectra™ device users optimally position a StO2 sensor over muscle tissue.MethodsThe THI and tissue hemoglobin oxygen saturation (StO2) were measured using the InSpectra™ StO2 Tissue Oxygenation Monitor, Model 650, with a 15 mm optical sensor. A THI normal reference range was established in the thenar eminence (hand) for 434 nonhospitalized human volunteers. In 30 subjects, the thenar THI was also evaluated during 5-minute arterial and venous blood flow occlusions, and with blood volume exsanguination in the hand induced with an Esmarch bandage. In addition, correlation of the THI to blood total hemoglobin concentration (Hbt) was studied in five pigs whose Hbt was isovolumetrically diluted from 13 to 4 g/dl systemically and 0.5 g/dl locally in the hind limb. The sensitivity and specificity of the THI to measure tissue hemoglobin concentration (THC) were characterized in vitro using isolated blood tissue phantoms.ResultsIn human thenar tissue, the average THI was 14.1 ± 1.6 (mean ± standard deviation). The THI extrapolated to 100% blood volume exsanguination was 3.7 ± 2.0 units presumably from myoglobin. On average, the THI increased 1.5 ± 1.0 units with venous occlusion and decreased 4.0 ± 2.0 units with arterial occlusion. In porcine hind limbs, the THI weakly correlated with Hbt (r2 = 0.26) while ΔTHI during venous occlusion had a stronger correlation (r2 = 0.62). In vitro tests indicated that THI strongly correlated (r2 > 0.99) to phantom THC and was insensitive to StO2 changes.ConclusionsSteady-state THI values do not reliably indicate Hbt. The THI is a reproducible quantitative index for THC, and THI trends can discriminate between arterial or venous blood flow occlusions. The THI magnitude permits the estimation of myoglobin's contribution to StO2.

Changes of Oxygenation and Hemoglobin-Concentration in Lower Extremity Free Flaps during Dangling

“…[9][10][11] In a large series of breast reconstruction, the use of tissue oximetry monitoring led to a reduction of the flap loss rate because of its ability to detect impeding vascular compromise before it became clinically evident. 12 Both the transcutaneous oxygen saturation (StO 2 ) and the THI were measured in a continuous fashion before, during, and after dangling.…”

Near-Infrared Spectroscopy in the Monitoring of Adult Traumatic Brain Injury: A Review

“…Importantly, this investigation made significant progress in resolving component contributions from the external carotid artery (ECA) and internal carotid (ICA), minimizing the often confounding surface effect of skin and other extracranial tissue perfusions that have been described. 20,22,24 The THI is an NIRS output parameter derived largely from a depth resolved combined tissue Hb signal strength 20 and has been utilized primarily to provide a surrogate for total tissue Hb content within the observed tissue. Intuitively, this is not entirely accurate, as the exact total volume of tissue under observation through the absorption of the emitted light is not known (it is therefore expressed as an arbitrary unit), although educated estimates have been formulated through novel computational modelling processes.…”

Knee area tissue oxygen saturation is predictive of 14-day mortality in septic shock

“…We used a probe with spacing of 15 mm between the zones transmitting and receiving thus allowing a measurement of StO 2 at 14 mm of depth. After at least 1 min of signal stabilization, the StO 2 was measured and was recorded only if the total hemoglobin index (THI) was higher than 5 [16], indicating a reliable signal. StO 2 was measured simultaneously at two sites: thenar eminence and knee area (close to the knee at the junction between kneecap and the vastus medialis of the quadriceps).…”