Clinical Assessments of Cerebrovascular Health: Is there a neED(D) for Cerebral Shear Mediated Dilation?

Introduction:

Cerebrovascular diseases, such as stroke, in conjunction with Alzheimer’s Disease and related dementias, are among the leading causes of death worldwide, and their prevalence is projected to rapidly rise in coming years as a result of the aging population. One major contributor to increased risk of these diseases is vascular endothelial dysfunction, which is characterized, in part, by reductions in bioavailability of the key vasodilatory, anti-inflammatory, and athero-protective molecule, nitric oxide (NO). Endothelial function can be assessed by measuring endothelium-dependent dilation (EDD) in response to either chemical (i.e., acetylcholine) or mechanical (i.e., blood-flow related shear stress) stimuli. In the peripheral vasculature, brachial artery flow-mediated dilation (FMD_BA) in response to an acute increase in intravascular shear stress is largely NO-mediated and considered the gold-standard, non-invasive measure of macrovascular EDD. Furthermore, FMD_BA serves as an independent predictor of future cardiovascular disease risk in a variety of populations (Ras et al., 2013). Currently, there is no universally accepted analog of FMD_BA specifically for cerebrovascular function due, in part, to the complex and incompletely understood mechanisms mediating cerebrovascular regulation. However, NO is thought to be one such mechanism; thus, studies have aimed to elucidate the contribution of NO to assessments of cerebrovascular function, such as the commonly utilized steady-state cerebrovascular CO₂ reactivity (CVR) test, though with mixed results (Hoiland et al., 2021). Recently, an increasing number of laboratories have assessed an alternative measure of cerebrovascular function by utilizing a shear-mediated test of cerebral vascular function (cSMD), but whether cSMD could serve as a bioassay of NO-mediated cerebrovascular endothelial function is unknown.

The recent work by Hoiland et al. (2021) published in The Journal of Physiology addressed these gaps in the literature. The mechanistic studies performed by this group are necessary to extend our understanding of cerebrovascular function and establish sensitive and reliable methods to assess the integrity of cerebrovascular mechanisms that reflect concomitant changes in cerebrovascular endothelial function. Such efforts are crucial to improve cerebrovascular health and reduce disease burden.

Results:

The authors (Hoiland et al., 2021) investigated the effects of NO synthase inhibition during CVR and cSMD to characterize the contribution of NO within these measures of cerebrovascular function. In a single-blinded, counterbalanced experiment, steady-state (CVR) and transient (cSMD) CO₂ tests were conducted in 11 healthy young (age 26±5 years) men during intravenous infusion of the non-selective NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA), or volume-matched saline (placebo). During the steady-state test, the partial pressure of end tidal CO₂ (P_ETCO₂) was increased in a stepwise fashion to +4.5 and +9 mmHg above baseline values, and measurements were taken near the end of each 5-minute CO₂ stage. In a randomized order, subjects also underwent the novel transient CO₂ test which utilized brief exposure to elevated CO₂ to increase P_ETCO₂ +9 mmHg and evoke an increase in internal carotid artery (ICA) shear stress. Measures of cSMD were taken at 60 and 120 seconds following the return to baseline P_ETCO₂ to eliminate direct effects of CO_2. In both tests, diameter and blood velocity of the ICA and vertebral artery were measured using a duplex ultrasound. During the CVR test, blood velocity was measured in the middle and posterior cerebral arteries using a transcranial Doppler ultrasound. Blood samples were collected to assess arterial blood gases and measure NO₂ ⁻ exchange across the cerebral vascular bed using paired radial artery and jugular vein sampling.

In both tests, the hypercapnic shear stimulus was unaltered between saline and L-NMMA conditions. Furthermore, there was a significant shift towards trans-cerebral NO₂ ⁻ release in the saline trial from baseline to the +9 mmHg hypercapnic stimulus; during L-NMMA infusion, the significant shift towards NO₂ ⁻ release was abolished. This suggests significant cerebral endothelial NO production during both tests and successful reduction of NO bioavailability via NO synthase inhibition. Interestingly, there was no observed difference in the magnitude of ICA vasodilation between the two conditions in response to the steady-state CO₂ stimulus (Figure 6 from Hoiland et al. 2021). However, during the transient test, cSMD in the ICA was reduced by 37% following L-NMMA infusion (Figure 6). These data suggest that NO does not play a requisite role in CVR, as NO synthase inhibition did not alter ICA dilation or reactivity during the steady-state test. The lack of NO specificity demonstrated by the CVR test may be due to redundant pathways existing in the cerebral vascular bed, however more research is required to investigate this observation. Importantly, this study provides novel evidence that cSMD of the ICA following a transient CO₂ stimulus is largely NO dependent.

Significance:

Cerebrovascular dysfunction is a major risk factor for cerebral and neurological impairments and an emerging risk factor for Alzheimer’s disease and related dementias. Therefore, it is imperative to develop and refine assessments of the cerebrovascular endothelium that provide insight into the health and function of this vascular bed. The most notable finding by Hoiland et al. (2021) is the presence of a 37% reduction in shear-mediated dilation of the ICA following global NO synthase inhibition during the transient CO₂ test, while reductions in ICA dilation and CVR were absent during the steady-state CO₂ test. As the authors suggest, these findings highlight the transient CO₂ test of cSMD as a measure of NO-dependent cerebrovascular endothelial function. Moreover, this test may represent the cerebrovascular analog to the peripheral FMD_BA as an independent predictor of future cerebrovascular disease risk and possibly Alzheimer’s disease and related dementias. Future studies are needed to establish cSMD in this capacity.

Establishing cSMD as clinically relevant:

This study presents novel evidence that the transient CO₂ test of cSMD offers an NO-dependent measure of cerebrovascular endothelial function in healthy young men (Hoiland et al., 2021). To further clinical relevance, steps must be taken to apply transient cSMD to other subject groups, such as healthy and clinical populations of both sexes. Existing literature has examined cSMD in subject populations other than healthy young men, however, these methods utilized steady-state CO₂ tests, which are now known to not specifically assess NO-dependent cerebral vasodilation (Hoiland et al., 2021). For example, it was found that aging attenuated shear-mediated dilation of the ICA in healthy older adults (Iwamoto et al., 2018). Another study interrogated the effects of changes in estrogen (during the menstrual cycle and menopausal transition) on shear-mediated ICA dilation. It was found that cSMD increased from low to high estradiol phases in naturally cycling premenopausal women and was reduced with advancing menopause stage. The results suggest that declines in cerebrovascular function with aging may be partially mediated by reduced levels of circulating estradiol (Iwamoto et al., 2021). Whether the transient CO₂ test is reflective of changes in cerebrovascular NO-mediated EDD with aging, exercise, and across the menstrual cycle or menopausal transition requires further investigation.

Another critical component of establishing efficacy of the transient CO₂ test of cSMD is to determine if it predicts 'hard' cerebrovascular outcomes such as stroke, cognitive decline, and/or the onset of Alzheimer’s disease and related dementias. The potential predictive ability of cSMD is of high importance as the literature generally reports a mixed ability of CVR to predict stroke (Hoiland et al., 2021). To assess the predictive value of cSMD, longitudinal studies must be performed that measure cSMD at baseline and across a multitude of ages with incremental follow-up assessments and cerebrovascular event tracking across the cohort’s lifetime. The goal of this longitudinal cohort study would be to investigate changes in cSMD with aging and determine whether baseline cSMD values could predict future risk of cerebrovascular morbidity and mortality. This design has the potential to establish cSMD as a clinically relevant measure, similar to how lower FMD_BA, mediated by either age or disease, is associated with increased risk of future cardiovascular disease (Ras et al., 2013). As mentioned above, FMD_BA is the gold-standard, non-invasive measure of peripheral macrovascular function and holds clinical relevance to cardiovascular disease risk. However, FMD_BA may not be appropriate to characterize cerebrovascular endothelial function; a test that is specific to the cerebrovascular bed, such as cSMD, is needed (Carr et al., 2020).

Finally, randomized controlled trials should be performed to examine whether pharmacological or lifestyle interventions can improve cSMD and consequently lower risk of cerebrovascular morbidity and mortality. Important subject populations to consider for this type of study design include both healthy midlife/older adults without signs of cognitive impairment, as well as subjects with clinical presentation of disease. Efficacious interventions could be used to prevent or delay the onset of future cerebrovascular events and/or slow or reverse already-declining cerebrovascular health.

Overall, the main findings from (Hoiland et al., 2021) highlight the transient CO₂ test of cSMD as a non-invasive, functional bioassay of NO-mediated cerebrovascular function. Such a test possesses important implications for improving our understanding and assessment of clinical cerebrovascular health and disease. Furthermore, with future studies, cSMD could be established as a therapeutic target, with the ultimate goal of reducing the risk and burden of future disease. As such, many exciting research questions emerge to establish this promising technique as a valid clinical measure of cerebrovascular endothelial function and as an independent predictor of future cerebrovascular disease, as well as Alzheimer’s Disease and related dementias risk.