Cerebral Autoregulation of Blood Velocity and Volumetric Flow During Steady-State Changes in Arterial Pressure

Abstract

C erebral autoregulation (CA) is essential to maintain a constant cerebral blood flow (CBF) in the context of changes in cerebral perfusion pressure.1 Assessment of CA reflects cerebrovascular function and has been used widely in hypertension studies and other clinical settings. 2,3 Quantitative assessment of CA is challenged by the methods used for CBF measurement. Modern imaging modalities such as single-photon emission computed tomography, positron emission tomography, perfusion computed tomography, and MRI are difficult to be applied for CBF measurement in clinical studies of CA because of the cumbersome experimental conditions, the limitations of using radioactive isotopes (single-photon emission computed tomography and positron emission tomography), or other imaging contrast agents (computed tomography and MRI) for repeated measurements. 4 Recently, transcranial Doppler (TCD) has been used to assess CA because of its bedside availability, noninvasiveness, and high temporal resolution in measuring changes in cerebral blood flow velocity (CBFV) in the basal cerebral arteries. 5 However, because the diameter of the insonated vessels cannot be measured directly using TCD, the validity of using this technique to assess CA is based on a fundamental assumption that changes in CBFV represent changes in volumetric CBF, that is, by assuming that the diameter of basal cerebral arteries does not change significantly in the face of changes in blood pressure. 5 For its importance, this assumption has been evaluated repeatedly by using a variety of imaging modalities to measure CBF and to compare with TCD measurement of CBFV during changes in arterial pressure. 6,7 However, the findings so far are inconsistent. [8][9][10] One of the major limitations of these studies is that CBF and CBFV often were not measured simultaneously or were measured with different temporal and spatial resolutions, thus making it difficult or even impossible for direct comparisons between CBF and CBFV Abstract-The validity of using transcranial Doppler measurement of cerebral blood flow velocity (CBFV) to assess cerebral autoregulation (CA) still is a concern. This study measured CBFV in the middle cerebral artery using transcranial Doppler and volumetric cerebral blood flow (CBF) in the internal carotid artery (ICA) using color-coded duplex ultrasonography to assess CA during steady-state changes in mean arterial pressure (MAP 7 However, this study was conducted only in 7 patients with cerebrovascular diseases under surgical conditions, and these observations need to be confirmed.Color-coded duplex ultrasonography (CDUS) is a noninvasively bedside available technology that has been used to measure volumetric CBF in the ICA.12,13 Similar to TCD, CDUS also has high temporal resolution.14 In addition, changes in CBF in the ICA most likely reflect those in the MCA and anterior cerebral artery, which are the major branches of ICA.In this study, we simultaneously measured changes in CBFV in the MCA and ICA and CBF in the ICA to assess CA during stepw...

Assessing cerebrovascular autoregulation from critical closing pressure and resistance area product during upright posture in aging and hypertension

“…The consistency of these findings is in agreement with the notion that intact sCA does not connote constant CBF, but rather an attenuation of the impact of changes in blood pressure on cerebral perfusion. In older adults with and without hypertension, the gain of the resistance model (i.e., BP MCA ¡ CVRi) was ϳ0.7, which agrees with prior work in similar populations in which hypotensive stimuli were induced by lower body negative pressure (12) and sodium nitroprusside infusion (26). The drop in BP MCA induced by orthostatic stress in the present study was in excess of 20 mmHg, providing important evidence that the window of autoregulatory control encompasses blood pressure deviations common to stresses of daily living, such as posture change, in older adults with controlled and uncontrolled hypertension.…”

“…The ICA is a major conduit artery, supplying ϳ75% of CBF bilaterally (43), and has recently received attention as a relevant extracranial site for assessment of both dynamic (40) and static CA (26). Examination of the ICA offers the benefits of ameliorated ultrasound access to the vessel with certain knowledge of Doppler angle and vessel diameter, allowing for more exact quantification of cerebral hemodynamics.…”

“…Hence, a lower resting CBF, as observed in aging and hypertension (6), increases the risk for transient hypoperfusion in upright postures and emphasizes the need for effective cerebrovascular control. Cerebral autoregulation (CA) characterizes the efficiency in maintaining CBF despite changes in arterial blood pressure (ABP) (48) and is commonly assessed by the change in cerebrovascular resistance (CVR) for a given change in pressure (21,26,48). When considering CVR as the solitary regulator (i.e., CVR ϭ ABP/CBF), CA has proven to be a robust physiological trait, resistant to aging (12,14,26,34) and hypertensive pathology (12,45,53).…”

“…Cerebral autoregulation (CA) characterizes the efficiency in maintaining CBF despite changes in arterial blood pressure (ABP) (48) and is commonly assessed by the change in cerebrovascular resistance (CVR) for a given change in pressure (21,26,48). When considering CVR as the solitary regulator (i.e., CVR ϭ ABP/CBF), CA has proven to be a robust physiological trait, resistant to aging (12,14,26,34) and hypertensive pathology (12,45,53).The definition of CA has been dichotomized as "dynamic," involving rapid changes in pressure (25, 54), and "static," involving steady-state changes over the course of minutes or longer (48). Both definitions of CA, however, are characterized by the relationship between mean flow and pressure, averaged across one or more cardiac cycles.…”

Central Hemodynamics and Target Organ Damage in Hypertension

“…Analogous with the kidney, the cerebral microvasculature receives high and pulsatile blood flow owing to its low resistance and impedance, as manifest by the unidirectional flow features (without reverse flow) of the cerebral arteries (Liu et al 2013). Structurally, the relationship between the large middle cerebral arteries and small perforating arteries of the brain resembles the relationship between the arcuate arteries and juxtamedullary afferent arterioles of the kidney in that the tiny arterioles directly branch off from the large arteries within a short distance (Ito 2012).…”