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Marco Romanelli
University of Torino
Marco Romanelli, a graduate student at the University of Torino, Italy, explored orthostatic stress triggers multiple compensatory mechanisms, including arteriolar and venous constriction, which are difficult to distinguish. This pilot study used near-infrared spectroscopy (NIRS) during lower body negative pressure (LBNP, –30 mmHg) to separate these responses in the upper limbs. In 4 healthy subjects, experimental conditions (arm elevation and a pressure cuff) were used to isolate vascular contributions to blood volume changes. Results showed that arteriolar constriction accounted for ~19% and venous constriction ~9% of the total blood volume decrease, while the majority (~73%) was due to passive vessel emptying from reduced central venous pressure. These findings suggest that NIRS can non-invasively differentiate vascular responses during orthostatic stress, though further research is needed. Marco will present this research at the 19th Annual Meeting of Young Researchers in Physiology during May in Catania, Italy.
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ABSTRACT
Orthostatic stress presents a complex hemodynamic scenario involving the integration of multiple compensatory pathways which act to contrast both the blood volume shift and the decrease in arterial blood pressure. Among these, are the sympathetically-mediated constrictions of arterioles and veins, which are however difficult to discriminate.
Aim of this work is thus to disentangle venous from arteriolar constriction in response to hypovolemic stimuli produced by lower body negative pressure (LBNP) using the non-invasive Near Infrared Spectroscopy (NIRS) monitoring.
To this aim, blood volume changes were measured in supine subjects by NIRS in the upper arm, without and with a proximal pneumatic cuff (20 mmHg) to prevent direct blood-draining by the LBNP (-30 mmHg) stimulus and with the arm in the vertical position to make the veins collapse and prevent their further reflex constriction. Both pressure cuff inflation and arm raising were performed at least two minutes prior to LBNP for hemodynamic stabilization. Total haemoglobin concentration was continuously monitored in both forearms, to track blood volume changes. Average responses across all subjects were then evaluated.
Preliminary results from 4 healthy subjects indicate that arteriolar constriction, assessed with arms up, accounted for 18.6+9.0 % of the total blood volume decrease, while the venous component, obtained from the with-cuff condition after subtracting the arteriolar component, accounted for 8.7+10 %. Therefore, around 73 % of the blood volume drop during LBNP results from passive vessel emptying, as a consequence of lowered central venous pressure.
This pilot study shows that NIRS can non-invasively identify the magnitude and the time course of the different vasoconstrictive processes that simultaneously occur in the upper limbs during LBNP exposure. Further investigations will be necessary to refine these observations.
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