BACKGROUND
Mathematical and mechanical models of cerebral circulation indicate that the resistance of the collapsed internal jugular veins limits cerebral blood flow during high acceleration (+Gz) and that positive pressure breathing (PPB) restores cerebral blood flow by elevating blood pressure and preventing collapse of the vein. The effect of acceleration and PPB on the jugular resistance and flow can be estimated by documenting changes in the lumen area and blood velocity.
METHODS
The right internal jugular vein was imaged with vascular ultrasound in supine and seated human subjects exposed to 0-50 mm Hg of PPB. For each of the PPB posture combinations the vein was imaged at four locations along the length; resistance and flow were calculated using Poiseuille flow approximation.
RESULTS
For the supine subjects, the lumen area, just above the inferior bulb, was 1.0 +/- 0.49 cm2, the estimated resistance was 0.13 +/- 0.07 x 10(-3) mm Hg x cm(-3) x min(-1), and the estimated blood flow was 931 +/- 477 cm3 x min(-1). In the sitting position, the lumen narrowed to 0.11 +/- 0.07 cm2, the resistance increased to 6.3 +/- 4.9 x 10(-3) mm Hg x cm(-3) x min(-1), and the blood flow dropped to 372 +/- 194 cm3 x min(-1). However, the vessel of a sitting subject can be completely reopened with PPB of 30 mm Hg or higher, and the resistance can be brought to supine levels.
CONCLUSIONS
The results demonstrate that the internal jugular vein collapses with transition from supine to sitting position. This implies a significant increase in resistance which is inversely proportional to the square of the lumen area. However, the collapse can be prevented with sufficiently high PPB.