I got a very nice mail with a great critical question on tHb and trend in blood flow. The basic question or positive critic was, whether there are real scientific papers , who would back up our idea, that tHb changes can track blood flow for example from upper body to lower body as we claim is happening with non-involved muscles. . This is an absolutely justified and good question, as it sounds nearly too good to be true that a small equipment like that would allow an incredible feedback on exchange of blood volume and as such as well on energy supply.
Now here one study we may look at in this discussion.
Multi-site and multi-depth near-infrared spectroscopy in a model of simulated (central) hypovolemia: lower body negative pressure.
Bartels SA1, Bezemer R, de Vries FJ, Milstein DM, Lima A, Cherpanath TG, van den Meiracker AH, van Bommel J, Heger M, Karemaker JM, Ince C.
- 1Department of Intensive Care, Erasmus Medical Center, University Medical Center, Rotterdam, The Netherlands. firstname.lastname@example.org
To test the hypothesis that the sensitivity of near-infrared spectroscopy (NIRS) in reflecting the degree of (compensated) hypovolemia would be affected by the application site and probing depth. We simultaneously applied multi-site (thenar and forearm) and multi-depth (15-2.5 and 25-2.5 mm probe distance) NIRS in a model of simulated hypovolemia: lower body negative pressure (LBNP).
The study group comprised 24 healthy male volunteers who were subjected to an LBNP protocol in which a baseline period of 30 min was followed by a step-wise manipulation of negative pressure in the following steps: 0, -20, -40, -60, -80 and -100 mmHg. Stroke volume and heart rate were measured using volume-clamp finger plethysmography. Two multi-depth NIRS devices were used to measure tissue oxygen saturation (StO2) and tissue hemoglobin index (THI) continuously in the thenar and the forearm. To monitor the shift of blood volume towards the lower extremities, calf THI was measured by single-depth NIRS.
The main findings were that the application of LBNP resulted in a significant reduction in stroke volume which was accompanied by a reduction in forearm StO2 and THI.
NIRS can be used to detect changes in StO2 and THI consequent upon central hypovolemia. Forearm NIRS measurements reflect hypovolemia more sensitively than thenar NIRS measurements. The sensitivity of these NIRS measurements does not depend on NIRS probing depth. The LBNP-induced shift in blood volume is reflected by a decreased THI in the forearm and an increased THI in the calf.
This is another reaosn why we like to have SmO2 and tHb wqhen assessing ,as they complement each other in many ways. Using just SmO2 (or in fact HHb and O2Hb ) without at least looking at tHb trends can sometimes give some wrong conclusions.