L.P. Safonova, A.I. Dyachenko, Yu.S. Semenov

Effect of intrathoracic pressure on the hemodynamic system, especially cerebral hemodynamics, is important for general and space medicine. Researchers offered Negative Pressure Breathing (NBP) as an optional method for circulatory dysfunctions correction. But effects of NBP on cerebral hemodynamics remains poor studied. Tissue oximetry, as well as the ultrasound and impedance technologies, is one of the potentially perspective method to study NBP effects. This is a method of the reflective spectrophotometry in the red and near infrared spectral diapasons. Spectrophotometric parameters such as oxy-, desoxy- and total hemoglobin concentrations and tissue oxygen saturation are indirect qualitative parameters of the balance between the tissue metabolic demands and the oxygen delivery. They also reflect adaptative capabilities of the brain circulatory system to optimize blood flow according to changing functional conditions, for instance in conditions of weightlessness or hemorrhage. In the case of weightlessness the blood shift in direction of the chest and the head is observed . The tissue oximetry parameters during the baseline period have been compared to the similar parameters during breathing under the negative inspiratory pressure (inspiratory NBP) produced by a special breathing mask during twenty minutes. Variability of values of the controlled hemodynamic parameters during the baseline period is determined by individual anatomic and functional features and some limitations of the used tissue oximeter. Dynamics of the signals is also characterized by the wide variability on different tissue depth for each of eight volunteers participated in the studies and between the volunteers. Changes of the hemoglobin concentration and tissue saturation of the investigated tissue volume are connected with the cardio-respiratory system processes, with changes of respiratory and circulatory parameters such as ventilation, blood carbon dioxide and oxygen tensions, changes in the arterial pressure and the venous pressure, a vascular tone. Prevalence of the total hemoglobin concentration decrease which reflects the decrease of the blood volume in the studied group of the volunteers during breathing under the negative inspiratory pressure of -10, -15 and -20 cm of water allows to assume that in the majority of the measurements the decrease of the venous pressure could be the main reason of the observed results. The statistically significant increases in the respiratory and the pulse components of the cerebral slow hemodynamic oscillations were obtained. The research results have demonstrated the efficiency of the tissue oximeter technique for estimation of the cerebral hemodynamic changes due to NBP. To clarify mechanisms of different effects of NPB on the cerebral hemodynamics, the tissue oximetry parameters have to be measured simultaneously with the respiratory system parameters and the central hemodynamic parameters.