Background Intracranial pressure (ICP) is directly related to cranial dural venous pressure ( P dural ). In the upright posture, P dural is affected by the collapse of the internal jugular veins (IJVs) but this regulation of the venous pressure has not been fully understood. A potential biomechanical description of this regulation involves a transmission of surrounding atmospheric pressure to the internal venous pressure of the collapsed IJVs. This can be accomplished if hydrostatic effects are cancelled by the viscous losses in these collapsed veins, resulting in specific IJV cross-sectional areas that can be predicted from flow velocity and vessel inclination. Methods We evaluated this potential mechanism in vivo by comparing predicted area to measured IJV area in healthy subjects. Seventeen healthy volunteers (age 45?±?9?years) were examined using ultrasound to assess IJV area and flow velocity. Ultrasound measurements were performed in supine and sitting positions. Results IJV area was 94.5?mm2 in supine and decreased to 6.5?±?5.1?mm2 in sitting position, which agreed with the predicted IJV area of 8.7?±?5.2?mm2 (equivalence limit ±5?mm2, one-sided t tests, p?=?0.03, 33 IJVs). Conclusions The agreement between predicted and measured IJV area in sitting supports the occurrence of a hydrostatic-viscous pressure balance in the IJVs, which would result in a constant pressure segment in these collapsed veins, corresponding to a zero transmural pressure. This balance could thus serve as the mechanism by which collapse of the IJVs regulates P dural and consequently ICP in the upright posture.
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