Physiological adaptation of the jugular venous pulse during long-duration spaceflight.

The jugular venous pulse (JVP) offers a non-invasive window into right atrial pressure. In microgravity, the serial JVP waveform during long-duration spaceflight had never been characterised. Using ECG-synchronised high-resolution ultrasound and a validated AI model, we quantified internal jugular vein cross-sectional area (IJV-CSA) throughout the cardiac cycle in four astronauts before and after flight, with in-flight measurements in one astronaut. Mean IJV-CSA fell significantly post-flight in three astronauts (ASTRO 0: from 0.723 ± 0.076 to 0.453 ± 0.061 cm2, ASTRO 2: - 46.0%, ASTRO 3: - 48.4%; p < 0.001). The reduction in ASTRO 0 developed between the first and fifth month in orbit. Maximum CSA declined consistently with reduced central venous pressure. Pulsatile excursion, an index never previously reported in spaceflight, increased markedly (ASTRO 0: + 105.0%, p < 0.001; ASTRO 2: + 337.1%, p < 0.001), suggesting enhanced transmission of cardiac pressure waves towards the cerebral venous compartment in the absence of hydrostatic gradients. ASTRO 1 showed the opposite pattern (minimum CSA + 42.5%, pulsatile excursion - 60.3%, p < 0.001), probably due to IJV thrombosis, representing the third documented case in space medicine. The JVP waveform retains terrestrial morphology and ECG timing in microgravity. Whether JVP parameters predict venous thrombosis or relate to glymphatic function in long-term spaceflight warrants further studies.