Abstract
Pulsatile blood flow is generally mediated by the compliance of blood vessels whereby they distend locally and momentarily to accommodate the passage of the pressure wave. This freedom of the blood vessels to exercise their compliance may be suppressed within the confines of the rigid skull. The effect of this on the mechanics of pulsatile blood flow within the cerebral circulation is not known, and the situation is compounded by experimental access difficulties. We present an approach which we have developed to overcome these difficulties in a study of the mechanics of pulsatile cerebral blood flow. The main finding is that while the innate compliance of cerebral vessels is indeed suppressed within the confines of the skull, this is compensated somewhat by compliance provided by other “extravascular” elements within the skull. The net result is what we have termed “intracranial compliance,” which we argue is more pertinent to the mechanics of pulsatile cerebral blood flow than is intracranial pressure.
Highlights
Under conditions of pulsatile blood flow, blood vessels exercise their compliance to absorb the rising pressure in systole by distending locally and momentarily to accommodate the passage of the pressure wave (Lighthill, 1975; London and Guerin, 1999; Zamir, 2016)
While Transcranial Doppler Ultrasound (TCD) provides some access to flow measurements, no such access exists for the corresponding measurements of pressure
The lack of access to pulsatile pressure measurements within the cerebral circulation is critical because of the unknown effects of intracranial pressure (ICP) on the mechanics of this circulation
Summary
Under conditions of pulsatile blood flow, blood vessels exercise their compliance to absorb the rising pressure in systole by distending locally and momentarily to accommodate the passage of the pressure wave (Lighthill, 1975; London and Guerin, 1999; Zamir, 2016). The relationship between the oscillatory components of pressure and flow in pulsatile blood flow depends on four main properties of the vascular bed in which pressure and flow are being measured, namely the total resistance R to the steady component of flow in that bed, the collective (“lumped”) compliance C of the vessels comprising the bed, the viscoelastic resistance to stretch K within the vessel walls, and the prevailing inertial effects L of accelerating/decelerating flow within the vascular bed as a whole (Zamir, 2005) These four properties are useful markers of the mechanical/functional health of a vascular bed because they determine the efficiency of oscillatory dynamics of blood flow within the blood vessels comprising that bed. If pressure and flow waveforms are measured, the above relationship between their individual harmonics can be used to compute the values of the properties R, C, K, L.
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