Characterization of internal jugular vein region-specific distension and flow patterns during progressive volume shifting.

Blood volume shifts during postural adjustment lead to irregular distension of the internal jugular vein (IJV). In microgravity, distension may contribute to flow stasis and thromboembolism, though the regional implications and associated risk remain unexplored. We characterized regional differences in IJV volume distension and flow complexity during progressive head-down tilt (HDT) (0°, -6°, -15°, -30°) using conventional ultrasound and vector flow imaging. We also evaluated low-pressure thigh cuffs (40 mmHg) as a fluid shifting countermeasure during -6° HDT. Total IJV volume expanded 139 ± 95% from supine position (4.6 ± 2.7 mL) to -30° HDT (10.3 ± 5.0 mL). Blood flow profiles had greater vector uniformity at the cranial IJV region ( < 0.01) and became more dispersed with increasing tilt ( < 0.01). Qualitatively, flow was more uniform throughout the IJV during its early flow cycle phase and more disorganized during late flow phase. This disorganized flow was accentuated closer to the vessel wall, near the caudal region, and during greater HDT. Low-pressure thigh cuffs during -6° HDT decreased IJV volume at the cranial region (-12 ± 15%; < 0.01) but not the caudal region ( = 0.20), although flow uniformity was unchanged (both regions, > 0.25). We describe a distensible IJV accommodating large volume shifts along its length. Prominent flow dispersion was primarily found at the caudal region, suggesting multidirectional blood flow. Thigh cuffs appear effective for decreasing IJV volume but effects on flow complexity are minor. Flow complexity along the vessel length is likely related to IJV distension during chronic volume shifting and may be a precipitating factor for flow stasis and future thromboembolism risk. The internal jugular vein (IJV) facilitates cerebral outflow and is sensitive to volume shifts. Concerns about IJV expansion and fluid flow behavior in astronauts have surfaced following thromboembolism reports. Our study explored regional volume distension and blood flow complexity in the IJV during progressive volume shifting. We observed stepwise volume distension and increasing flow dispersion with head-down tilting across all regions. Flow dispersion may pose a risk of future thromboembolism during prolonged volume shifts.