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.
Highlights
The tissue hemoglobin index (THI) is a hemoglobin signal strength metric provided on the InSpectraTM StO2 Tissue Oxygenation Monitor, Model 650
In addition to StO2, the InSpectraTM monitor displays the tissue hemoglobin index (THI), a measurement of hemoglobin signal strength useful for determining whether the StO2 sensor is optimally positioned over muscle
More recently the THI has been studied during the clinical assessment of tissue oxygen perfusion status to convert an StO2 downward slope during arterial occlusion to an index of local oxygen consumption [5] and to assess microvascular reactivity when blood flow is re-established after arterial occlusion [6]
Summary
The tissue hemoglobin index (THI) is a hemoglobin signal strength metric provided on the InSpectraTM 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. The InSpectraTM StO2 Tissue Oxygenation Monitor, Model 650 (Hutchinson Technology Inc., Hutchinson, MN, USA) provides continuous non-invasive assessment of tissue hemoglobin oxygen saturation (StO2) in the clinical setting. In addition to StO2, the InSpectraTM monitor displays the tissue hemoglobin index (THI), a measurement of hemoglobin signal strength useful for determining whether the StO2 sensor is optimally positioned over muscle. More recently the THI has been studied during the clinical assessment of tissue oxygen perfusion status to convert an StO2 downward slope during arterial occlusion to an index of local oxygen consumption [5] and to assess microvascular reactivity when blood flow is re-established after arterial occlusion [6]. Since invasive blood draws are not always feasible in patients, researchers have sought to establish a link between non-invasive continuous tissue hemoglobin measurements (THCs) and blood total hemoglobin concentration (Hbt) [7,8,9,10]
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