Neurosurgical and neurointerventional patients will be enrolled as these patients do rarely have sudden intravascular volume changes. Neurointerventional patients typically have no significant intravascular fluid changes, but do not routinely have intra-arterial catheters. Neurosurgical patients have slow, steady fluid shifts due to mannitol administration. Since most all of these patients have intra-arterial cannulas, it allows arterial waveform data collection which can be compared to the PPG signal.