Microcirculatory response to lower body negative pressure and the association to large vessel function

Vital signs reflect circulatory function and hence hemodynamics on a macroscopic scale and are often unreliable or late indicators of hemodynamic instability. Previous studies support that alterations in the microcirculation may provide early indicators of deterioration and impending shock. Microcirculation is also restored late in the recovery process. Hence, monitoring microcirculation is important since treatments based on normalizing classical vital signs will not always restore microvascular hemodynamics and the microcirculation may remain in shock although e.g., blood pressure seems normal. The aim of this study was to investigate alterations in skin microcirculation dynamics during lower body negative pressure as a model of shock and central hypovolemia. By using spatial frequency domain imaging (SFDI) and polarized reflectance imaging, we investigated the association between micro- and macrovascular function during these conditions. Furthermore, we evaluated microvascular reactivity using the capillary refill test. A cohort of 9 subjects were subjected to a progressive lower body negative pressure (LBNP) protocol. At baseline and at LBNP = -20mmHg, -30mmHg and -40mmHg, SFDI images were acquired and analyzed for tissue hemoglobin content and oxygenation. Superficial hemoglobin content was estimated by polarized reflectance imaging. We found that microcirculatory reactivity was prolonged during LBNP, but recovered after end of the protocol. These results indicate a correlation between negative pressure and microcirculatory function and that may provide a basis for early detection of shock in emergency care settings.