A hemodynamically unstable patient has the risk of going into general shock. As fluid therapy is the primary treatment for shock, the fluid responsiveness (FR) of the patient should be evaluated before volume expansion. However, conventional methods predict FR by analyzing the variation of blood pressure signal in time domain which is a nonstationary problem and makes it difficult to provide a stable index for FR. Instantaneous pulse rate variability (iPRV) is a cardiovascular assessment in

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... uency domain. Furthermore, iPRV uses ensemble empirical mode decomposition (EEMD), which could solve the nonstationary problem and overcome the frequency limitation in power spectrum of heart rate variability (HRV). iPRV provides a new indication in very high frequency (VHF) range (0.4-0.8Hz) of spectrum for peripheral responses. The aim of this study was to verify the ability of iPRV to indicate VHF for cardiac output assessment. Twenty-six healthy participants participated in this study and the acquired signal was recorded in supine baseline, during head-up tilt (HUT), and passive leg raising (PLR), which induces variation of venous return and helps the quantitative assessment of cardiac output individually. The result showed that the normalized power of VHF in HRV was small and there was no corresponding variation in different postures. In contrast, the normalized power of VHF in iPRV presented relative trend with different venous return changes. Overall, iPRV provides a novel and short-term cardiac output assessment in frequency domain and it has potential to evaluate FR. Keywords-fluid responsiveness (FR); instantaneous pulse rate variability (iPRV); ensemble empirical mode decomposition (EEMD); head-up tilt (HUT); passive leg raising (PLR).