Spectral analysis of arterial blood pressure and cerebral blood flow velocity during supine rest and orthostasis

This study evaluates the effect of orthostasis on the low frequency (LF, 0. 04 to 0.15 Hz) fluctuations in the blood flow velocity of the middle cerebr al artery (MCAN) in relation to its arterial blood pressure (ABP) equivalen t to further define and quantify this relationship in cerebrovascular regul ation. Spectral analysis was performed on 22 healthy subjects during supine rest and head-up tilt. The power in the LF range can be used to quantify t he LF fluctuations, and four types of LF power data could be obtained for e ach individual: LF power of supine MCAN, LF power of supine ABP, LF power o f tilt MCAFV, and LF power of tilt ABP. By comparing LF power of MCAFV with LF power of ABP, two power ratios could be generated to describe the flow- pressure relationship during supine rest and head-up tilt. respectively, su pine power ratio (LF power of supine MCAFV/ LF power of supine ABP) and til t power ratio (LF power of tilt MCAFV LF power of tilt ABP). In addition, a n index for dynamic autoregulation in response to orthostasis can be calcul ated from these two power ratios (tilt power ratio/supine power ratio). The authors found that this index was dependent on the extent of orthostatic M CAN changes, and the dependency could be mathematically expressed (r = 0.61 , P =.0001), suggesting its involvement in cerebrovascular regulation. More over, these data further support the previous observation that the LF fluct uations of MCAFV might result from modulation of its ABP equivalent, and th e modulation effect could be quantified as the power ratio (LF power of MCA FV/ LF power of ABP). These observations could be an important step toward further insight into cerebrovascular regulation, which warrants more resear ch in the future.