Abstract
A siphon is suggested to support cerebral blood flow but appears not to be established because internal jugular venous (IJV) pressure is close to zero in upright humans. Thus, in eleven young healthy males, IJV pressure was 9 ± 1 mmHg (mean ± SE) when supine and fell to 3 ± 1 mmHg when seated, and middle cerebral artery mean blood velocity (MCA Vmean; P < 0.007) and the near-infrared spectroscopy-determined frontal lobe oxygenation (ScO2; P = 0.028) also decreased. Another subject, however, developed (pre)syncopal symptoms while seated and his IJV pressure decreased to −17 mmHg. Furthermore, his MCA Vmean decreased and yet within the time of observation ScO2 was not necessarily affected. These findings support the hypothesis that a negative IJV pressure that is a prerequisite for creation of a siphon provokes venous collapse inside the dura, and thereby limits rather than supports CBF.
Highlights
Regulation of cerebral blood flow (CBF) is viewed from perspective of the arterial inflow: it is stabilized by “autoregulation” maintaining CBF despite changes in mean arterial pressure (MAP), responds to the arterial carbon dioxide tension (PaCO2), and might be modulated by sympathetic activity (Paulson et al, 1990)
This study presents a subject who developed a marked decrease in internal jugular venous (IJV) pressure when seated which is a prerequisite if a siphon should be established but since he developedsyncopal symptoms, the observation indicates that a negative IJV pressure limits rather than supports CBF
The supine MAP was 85 ± 2 mmHg, increasing to 91 ± 3 mmHg when the subjects were seated and likewise heart rate (HR) increased from 61 ± 4 to 83 ± 4 beats/min and both variables increased further with exercise intensity (Figure 1)
Summary
Regulation of cerebral blood flow (CBF) is viewed from perspective of the arterial inflow: it is stabilized by “autoregulation” maintaining CBF despite changes in mean arterial pressure (MAP), responds to the arterial carbon dioxide tension (PaCO2), and might be modulated by sympathetic activity (Paulson et al, 1990). The arterial and venous system could represent the respective ascending and descending limbs of a siphon. In this inverted U-tube arrangement the heart does not need to overcome the hydrostatic influence on blood to serve the brain but needs only to overcome vascular resistance represented largely by the small vessels within the brain, even in an animal with a long neck such as the giraffe. If a siphon is established to support CBF, venous pressure should be negative (by ∼ −100 mmHg for the giraffe, depending of the length of its neck) and in humans for whom the distance from the right atrium to the cerebrum is ∼25 cm, venous pressure at the base of the brain should be ∼ −19 mmHg
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