Browsing by Subject "central hypovolemia"
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Item Pulsatile Perfusion Therapy: A Novel Approach for Improving Cerebral Blood Flow and Oxygenation Under Simulated Hemorrhagic Stress(2018-05) Anderson, Garen K.; Rickards, Caroline A.; Goulopoulou, Styliani; Mallet, Robert T.; Jones, Harlan P.Introduction: Tolerance to both actual and simulated hemorrhage varies between individuals. Low frequency (~0.1 Hz) oscillations in mean arterial pressure (MAP) and brain blood flow (indexed via middle cerebral artery velocity, MCAv), may play a role in tolerance to reduced central blood volume; subjects with high tolerance to simulated hemorrhage induced via application of lower body negative pressure (LBNP) exhibit greater low frequency power in MAP and MCAv compared to low tolerant subjects. The mechanism for this association has not been explored. We hypothesized that inducing low frequency oscillations in arterial pressure and cerebral blood flow would attenuate reductions in cerebral blood flow and oxygenation during simulated hemorrhage. Methods: 14 subjects (11M/3F) were exposed to oscillatory (0.1 Hz, 0.05 Hz) and non-oscillatory (0 Hz) LBNP profiles with an average chamber pressure of -60 mmHg. Each profile was separated by a 5-min recovery. Measurements included arterial pressure and stroke volume via finger photoplethysmography, MCAv via transcranial Doppler ultrasound, and cerebral oxygenation of the frontal lobe (ScO2) via near infrared spectroscopy. Results: No differences were observed between profiles for reductions in MAP (P=0.60) and MCAv (P=0.90). The reduction in ScO2, however, was attenuated (P=0.04) during the oscillatory profiles compared to the 0 Hz profile. A similar attenuation was observed in stroke volume (P [less than] 0.001). Importantly, tolerance was higher during the oscillatory profiles (P=0.03). Discussion: In partial support of our hypothesis, cerebral oxygenation was protected during the oscillatory profiles. While MCAv was similar between conditions, the oscillatory pattern of cerebral blood flow may elicit a shear-stress induced vasodilation, so assessment of velocity may mask an increase in flow. Importantly, more subjects were able to tolerate the oscillatory profiles compared to the static 0 Hz profile, despite similar arterial pressure responses. These findings emphasize the potential importance of hemodynamic oscillations in maintaining perfusion and oxygenation of cerebral tissue during hemorrhagic stress.Item THE ROLE OF CEREBRAL OXYGENATION ON TOLERANCE TO CENTRAL HYPOVOLEMIA(2014-03) Kay, Victoria; Rickards, CarolineThis study will assess how the human body responds to a decrease in blood volume entering the heart (central hypovolemia). Central hypovolemia leads to a decrease in blood flow to the brain, which can occur under various clinical conditions including hemorrhage and stroke. People have different tolerance to a decrease in central blood volume, and we want to determine the physiological mechanisms responsible for making some people more tolerant than others, with a particular focus on the role of blood flow and oxygen supply to the brain. We will also determine the potential mechanisms responsible for improved tolerance to central hypovolemia with a special breathing technique called inspiratory resistance breathing. Our goal is to determine how the body responds during decreased central blood volume, and if these mechanisms could be augmented to save lives during hemorrhage. These experiments may provide the clinical community with data to support the implementation of a diagnostic tool for assessing the level of bleeding by measuring brain oxygen levels. Moreover, by providing data to support the use of inspiratory resistance as a therapeutic intervention in the field, this technique could potentially save many lives by giving patients more time to reach a hospital where more advanced medical treatment can take place. Purpose (a): Tolerance to central hypovolemia varies between individuals, and recent studies have shown that protection of absolute cerebral blood flow is not an underlying mechanism. We hypothesized that subjects with high tolerance (HT) to central hypovolemia maintain cerebral oxygenation (ScO2) at higher levels of lower body negative pressure (LBNP) compared to their low tolerant (LT) counterparts, despite similar reductions in absolute flow. Methods (b): 15 healthy human subjects (10 male; 5 female) were instrumented for assessment of ScO2 (via near-infrared spectroscopy, NIRS) and mean middle cerebral artery velocity (MCAv; via transcranial Doppler, TCD). All subjects completed a presyncopal-limited lower body negative pressure (LBNP) protocol with an onset rate of 3 mmHg/min. Subjects who made it to ≥80mmHg LBNP were classified as HT, and subjects who made it to ≤70 mmHg LBNP were classified as LT. Results (c): The minimum difference in LBNP tolerance between the HT (N=6) and LT (N=9) group was 206 s (LT=1400±104 s vs. HT=2080±65 s; P=0.0003). Up to -45 mmHg LBNP, ScO2 was maintained in HT subjects (P≥0.538), while the LT (N=9) subjects had a progressive decrease in ScO2 (P≤0.016) from baseline. MCAv decreased from baseline in both HT and LT subjects (P≤0.022). There was a strong linear relationship between %∆ MCAv and %∆ ScO2 within the LT group (R2=0.98; P=0.013), whereas a weaker association between perfusion and oxygenation (R2=0.53; P=0.271) was observed in the HT group. Conclusions (d): In support of our hypothesis, higher tolerance to progressive central hypovolemia was associated with the protection of ScO2, despite an early and significant reduction in cerebral blood flow. This may have important clinical implications for the monitoring of cerebral perfusion and oxygenation in trauma patients.