Cerebrospinal fluid pulse waveform as an indicator of cerebral autoregulation

Abstract

Systems analysis of the systemic arterial (SAPW), cerebrospinal fluid (CSFPW), and sagittal sinus (SSPW) pulse waves was carried out in 13 dogs during hypercapnia (5% CO2), intracranial normotension (inhalation of 100% O2), and intracranial hypertension (inhalation of 100% O2 plus an intraventricular infusion). Power amplitude and phase spectra were determined for each wave, and the power amplitude and phase transfer functions calculated between the cerebrospinal fluid (CSF) pressure and systemic arterial pressures, and between the sagittal sinus pressure and CSF pressure. The study indicates that the CSFPW and SSPW were virtually identical when impedance between the cerebral veins and sagittal sinus was minimal, which argues that the CSF pulse was derived from the cerebral venous bed. During inhalation of 100% O2, transmission of the SAPW across the precapillary resistance vessels into the cerebral venous pulse (as represented by the CSFPW) was nonlinear, while transmission across the lateral lacunae into the sagittal sinus was linear. During intracranial hypertension, wave transmission across the precapillary resistance vessels was linear, and across the lateral lacunae was nonlinear. During hypercapnia, wave transmission across the precapillary resistance vessels and the lateral lacunae was linear. When the wave transmission was nonlinear, there was also suppression in transmission of the lower harmonics, particularly the fundamental frequency, and a more positive phase transfer function, suggesting an inertial effect or decrease in acceleration of the pulse. Conversion from a nonlinear to linear transmission across the precapillary resistance vessels is evidence of loss of vasomotor tone, and is accompanied by rounding of the CSFPW. A vascular model which encompasses the above data and is based on flow in collapsible tubes and changes in vasomotor tone is posited to explain control of pulsatile flow and pulse waveform changes in the cerebrovascular bed. The model helps to clarify the strong interrelationship between intracranial pressure, cerebral blood flow, and cerebral autoregulation.

Intracranial pressure waves: characterization of a pulsation absorber with notch filter properties using systems analysis

“…Under elevated ICP conditions, the gain significantly increased (the notch disappeared) and the sharp phase shift present under normal ICP conditions was not observed. This is consistent with earlier observations in other high ICP conditions-such as hypertension and hypercapnia in animal experiments 9,37,39,40 and hypertension in human patients 36 -such that transfer functions became flatter with higher ICP caused by artificial disturbances or pathological conditions. The new computational techniques make the notch easier to see and study, R. Zou et al allowing future application to animal models of pathophysiological conditions and human clinical data.…”

“…In earlier studies of system analysis applied to pressure pulse waves, 9,[35][36][37][38][39][40]44 transfer functions were calculated based on the FFT of input (ABP) and output (ICP) waveforms. Using spectral analysis, one fundamental frequency and multiple harmonic components are detected and transfer functions are evaluated at those discrete frequencies only.…”

“…9,18,19,39,40,45 Clinical studies have shown that the pulse amplitude of the ICP waveform can be used to predict treatment response in patients with hydrocephalus. [20][21][22][23] Therefore, our findings may provide a computational perspective for understanding mechanisms underlying the link between pulsations and hydrocephalus and other pathophysiological conditions involving cerebral blood flow dynamics.…”

“…A number of studies have used transfer function analysis to understand the relationship between the ICP and ABP. 9,10,35,36,[38][39][40] The transfer function may be thought of as reporting a gain, or ratio of amplitudes of the ICP to that of the ABP, and phase shift, calculated at each individual frequency, over the appropriate spectrum. The gain of the transfer function-a measure in the frequency domain describing how much of the ABP fluctuation is transmitted at a given frequency to the ICPis calculated for these discrete frequency components.…”

Continuous monitoring of cerebrovascular pressure reactivity in patients with head injury

“…7,24 Some authors have suggested that cerebrovascular pressure reactivity could be derived from the characteristic pulse waveform from ABP, 26,27 although this has never been demonstrated to work in clinical practice. Perhaps changes in ABP are too fast (a fraction of a second) to mobilize an active vasoregulatory response.…”