Decompression-Induced Microbubble Choking in Blood: Acoustic Softening, Sanal Flow Choking, and Spaceflight Implications

Decompression-induced microbubble nucleation and the resulting multiphase flow instability in blood pose a significant but underrecognized risk to cardiovascular stability during extravehicular activity (EVA), rapid cabin depressurization, and emergency re-pressurization. This study provides experimental evidence that reduced ambient pressure can transform blood—typically considered incompressible under physiological conditions—into a compressible multiphase medium, enabling acoustic softening and Sanal Flow Choking (SFC). This thermo-acoustic, geometry-independent fluid-dynamic phenomenon can generate shock-like pressure waves and obstruct flow even at low physiological velocities. Fresh human venous blood was exposed to controlled vacuum conditions (760→100 mmHg, 37–40 °C). Microbubble nucleation consistently occurred between 650–350 mmHg, followed by vaporization and crust formation. Bubble rupture produced intense acoustic bursts, indicating rapid localized pressure transitions. These data suggest that a rupturing bubble behaves like a transient convergent–divergent nozzle capable of generating supersonic outflow and micro-shock events consistent with SFC onset. Using Wood’s multiphase acoustic model, effective sound speed was predicted to collapse from ~1500 m/s to