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Cerebral autoregulation (CA) is a mechanism to maintain cerebral blood flow (CBF) in response to changes in cerebral perfusion pressure (CPP), through active vasoconstriction and vasodilation of arterioles in the brain. Dynamic CA is believed to act as a high-pass filter such that only low frequency changes in pressure are counteracted by an active vasculature response. With high frequency oscillations in pressure, such as those that occur at the heart rate (HR), the effects of dynamic CA are absent and changes in CPP are passively transmitted to CBF based on the cerebrovascular resistance (CVR) and compliance (CVC). These changes in CVR/CVC occur with steady-state changes in CA which can be described by Lassen’s curve. However, it is unclear what drives phase differences between pressure and flow at the respiration rate of around 0.2 Hz (12 breaths per minute). Quantifying phase differences at the physiologic respiration rate could be useful to gain a better understanding of the effects of CA and as a potential clinical monitoring tool. In this work, we looked at phase differences between arterial blood pressure (ABP) and intracranial pressure (ICP) measured with invasive pressure sensors, which serve as surrogates for CPP and CBF, to investigate how Arg(ABP)-Arg(ICP) change at the respiration rate as a function of the CPP. We quantify how Arg(ABP)-Arg(ICP) changes with respect to CPP after low-frequency oscillations, respiratory induced oscillations, and with oscillations driven by the heart rate. In each frequency regime, the trends in phase differences between Arg(ABP)-Arg(ICP) are unique with respect to CPP. At the respiration rate, the trends in Arg(ABP)-Arg(ICP) did not completely follow those predicted by a dynamic CA response or by CVC/CVR, thus we believe that there is a combination of effects influencing the phase difference between Arg(ABP)-Arg(ICP) at the respiration frequency. We also explore whether this response could be monitored completely non-invasively using near infrared spectroscopy (NIRS). We use Arg(ΔHbT)-Arg(ΔHbO) as surrogates for CPP and CBF and see a similar response of phase differences with respect to CPP at the respiration rate.
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