Time-course analysis of cerebral circulation and cardiorespiratory responses to acute central blood volume reduction in healthy young males.

Central blood volume (CBV) reduction challenges circulatory and respiratory homeostasis, particularly during the initial compensatory phase (0-2 min), when rapid physiological adaptations occur. In this study, we examined dynamic cardiorespiratory responses to CBV reduction using lower-body negative pressure (LBNP) in 11 healthy young males. Participants completed three standardized 2 min LBNP trials at -45 mmHg, with respiratory variables assessed via flow measurement and breath-by-breath gas analysis, while cardiovascular parameters and cerebral blood flow were monitored using ECG, blood pressure and transcranial Doppler ultrasonography. During LBNP exposure, thoracic admittance, an indicator of CBV reduction, decreased by 13.4% (p < 0.001), indicating significant CBV reduction. Following rigorous statistical correction for multiple comparisons, time-course analysis revealed that mean blood pressure decreased temporarily during the initial phase (0-30 s), whereas heart rate increased progressively (16.4%, p < 0.001). End-tidal $P_{C{O}_2}$ showed a consistent reduction (5.9%, p < 0.001), whereas minute ventilation and middle cerebral artery mean blood velocity showed no significant changes after statistical correction (-9.3% and -5.0%, respectively, p > 0.05). Exploratory correlation analysis revealed a significant negative correlation between mean blood pressure and tidal volume during the initial phase only (r = -0.78, p = 0.004). Cross-correlation analysis suggested temporal patterns between respiratory and cerebrovascular responses, with respiratory changes preceding cerebrovascular adjustments by 10-20 s. These findings, along with individual variability, suggest rapid cardiorespiratory and cerebrovascular interactions during orthostatic stress, demonstrating dynamic cardiovascular and respiratory responses with distinct temporal patterns that provide insights into physiological mechanisms maintaining homeostasis during gravitational stress.