| Near-infrared spectroscopy (NIRS) is an optically based technique to noninvasively and continuously monitor brain and other tissue oxygenation by determining relative changes in oxyhemoglobin ( D HbO2) and deoxyhemoglobin ( D Hb). The Modified Beer-Lambert Law is used to distinguish D HbO2 from D Hb. The summation of D Hb and D HbO2 gives the relative change in total hemoglobin ( D TotalHb) which is an important hemodynamic parameter that is proportional to changes in blood volume. The NIRS oxygenation indexes {lcub} D HbO2- D Hb{rcub} and D rSO2 ( ≈D HbO2/ D TotalHb) provide information on the relative change in mixed venous oxygen saturation.; By using low power, narrow spectral bandwidth laser diodes to determine the Beer-Lambert parameters, it was possible to construct a small, relatively inexpensive NIRS device. Insights on the clinical interpretation of the NIRS parameters were obtained through different animal and human research studies. Induced physiological changes resulted in predictable changes of the NIRS parameters.; Two major studies are presented in this dissertation. The first study investigated the changes of the NIRS parameters in response to changes in arterial carbon dioxide (PaCO2) levels in piglets. D HbO2 and D TotalHb increased nearly linearly, corresponding to the characteristic increase of the cerebral vascular dilation with increasing depth of hypercapnia, but D Hb remained relatively constant. The expression D TotalHb/ D PaCO2 was also studied because it represents the CO 2-reactivity of the cerebral vasculature. Poor CO2-reactivity indicates possible failure of the cerebral autoregulation system, leaving the brain vulnerable to ischemic insult.; The second study involved the use of NIRS in an U.S. Navy centrifuge. In this unique study, human subjects were exposed to high +Gz acceleration and were induced to lose consciousness. During successive high +Gz pulses of increasing pulse width, the subjects experienced increasing vision loss levels, confusion and disorientation (A-LOC), to loss of consciousness (G-LOC). The NIRS parameters were correlated with the resultant subject +Gz stress symptoms. The results show that the magnitude of the hyperemic response recovery phase after the +Gz pulse, as determined by D HbO2 and D TotalHb, was significantly increased for A-LOC and G-LOC symptoms. |
