During liver transplantation, the thermodilution cardiac output (CO) technique cannot respond to sudden hemodynamic changes associated with postreperfusion syndrome. Photoplethysmography (PPG) can reflect changes in intravascular volume and thus can be used to assess vasomotor tone and arterial stiffness on the pressure-volume relation. We investigated whether a beat-to-beat analysis of the arterial pressure-PPG relationship can estimate dynamic changes in vascular characteristics immediately after liver graft reperfusion.In 10 recipients, arterial blood pressure and PPG waveforms recorded simultaneously were analyzed from the beginning of fall to nadir in systolic blood pressure immediately after reperfusion. On a beat-to-beat basis, we compared the ratio of the amplitude of PPG to arterial pulse pressure (PPGamp/PP, as relative vascular compliance) to total peripheral resistance (TPR) and Windkessel compliance (Cwk) obtained from the Modelflow CO algorithm.Following graft reperfusion, PPGamp/PP and Cwk increased (median 41.5%; P = .005 and 42.0%; P < .001, respectively), whereas TPR decreased (median -46.4%; P < .001). Beat-to-beat PPGamp/PP was negatively correlated with TPR (median r = -0.80 [95% CI -0.85 to -0.76] on linear regression and r = 0.84 [95% CI 0.73-0.92] on curvilinear regression), and was positively correlated with Cwk (median r = 0.86 [95% CI 0.81-0.91] on linear regression and r = 0.88 [95% CI 0.75-0.96] on curvilinear regression).Our results suggest that relative compliance, obtained from beat-to-beat analysis of PPG and arterial pressure waveforms, can track abrupt changes in vascular characteristics associated with postreperfusion syndrome. This simple index would contribute to differential diagnoses of sudden hypotension.