Inhibition of nitric oxide synthase does not alter dynamic cerebral autoregulation in humans

The Effect of a Neuronal Nitric Oxide Synthase Inhibitor on Neurovascular Regulation in Humans

BACKGROUND

Neurovascular coupling (NVC) is a key process in cerebral blood flow regulation. NVC ensures adequate brain perfusion to changes in local metabolic demands. Neuronal nitric oxide synthase (nNOS) is suspected to be involved in NVC; however, this has not been tested in humans. Our objective was to investigate the effects of nNOS inhibition on NVC in humans.

METHODS

We performed a 3-visit partially randomized, double-blinded, placebo-controlled, crossover study in 12 healthy subjects. On each visit, subjects received an intravenous infusion of either S-methyl-L-thiocitrulline (a selective nNOS-inhibitor), 0.9% saline (placebo control), or phenylephrine (pressor control). The NVC assessment involved eliciting posterior circulation hyperemia through visual stimulation while measuring posterior and middle cerebral arteries blood velocity.

RESULTS

nNOS inhibition blunted the rapidity of the NVC response versus pressor control, evidenced by a reduced initial rise in mean posterior cerebral artery velocity (-3.3% [-6.5, -0.01], P=0.049), and a reduced rate of increase (ie, acceleration) in posterior cerebral artery velocity (slope reduced -4.3% [-8.5, -0.1], P=0.045). The overall magnitude of posterior cerebral artery response relative to placebo control or pressor control was not affected. Changes in BP parameters were well-matched between the S-methyl-L-thiocitrulline and pressor control arms.

CONCLUSIONS

Neuronal NOS plays a role in dynamic cerebral blood flow control in healthy adults, particularly the rapidity of the NVC response to visual stimulation. This work opens the way to further investigation of the role of nNOS in conditions of impaired NVC, potentially revealing a therapeutic target.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Phase dynamics of cerebral blood flow in subarachnoid haemorrhage in response to sodium nitrite infusion

Aneurysmal subarachnoid haemorrhage (SAH) is a devastating subset of stroke. One of the major determinates of morbidity is the development of delayed cerebral ischemia (DCI). Disruption of the nitric oxide (NO) pathway and consequently the control of cerebral blood flow (CBF), known as cerebral autoregulation, is believed to play a role in its pathophysiology. Through the pharmacological manipulation of in vivo NO levels using an exogenous NO donor we sought to explore this relationship. Phase synchronisation index (PSI), an expression of the interdependence between CBF and arterial blood pressure (ABP) and thus cerebral autoregulation, was calculated before and during sodium nitrite administration in 10 high-grade SAH patients acutely post-rupture. In patients that did not develop DCI, there was a significant increase in PSI around 0.1 Hz during the administration of sodium nitrite (33%; p-value 0.006). In patients that developed DCI, PSI did not change significantly. Synchronisation between ABP and CBF at 0.1 Hz has been proposed as a mechanism by which organ perfusion is maintained, during periods of physiological stress. These findings suggest that functional NO depletion plays a role in impaired cerebral autoregulation following SAH, but the development of DCI may have a distinct pathophysiological aetiology.

Neurovascular coupling and cerebral autoregulation in atrial fibrillation

The risk of cognitive decline and stroke is increased by atrial fibrillation (AF). We sought to determine whether neurovascular coupling and cerebral autoregulation are blunted in people with AF in comparison with age-matched, patients with hypertension and healthy controls. Neurovascular coupling was assessed using five cycles of visual stimulation for 30 s followed by 30 s with both eyes-closed. Cerebral autoregulation was examined using a sit-stand test, and a repeated squat-to-stand (0.1 Hz) manoeuvre with transfer function analysis of mean arterial pressure (MAP; input) and middle cerebral artery mean blood flow velocity (MCA V_m; output) relationships at 0.1 Hz. Visual stimulation increased posterior cerebral artery conductance, but the magnitude of the response was blunted in patients with AF (18 [8] %; mean [SD]) and hypertension (17 [8] %), in comparison with healthy controls (26 [9] %) (P < 0.05). In contrast, transmission of MAP to MCA V_m was greater in AF patients compared to hypertension and healthy controls, indicating diminished cerebral autoregulation. We have shown for the first time that AF patients have impaired neurovascular coupling responses to visual stimulation and diminished cerebral autoregulation. Such deficits in cerebrovascular regulation may contribute to the increased risk of cerebral dysfunction in people with AF.

The short-term effects of sedentary behaviour on cerebral hemodynamics and cognitive performance in older adults: a cross-over design on the potential impact of mental and/or physical activity

BACKGROUND

Sedentary behaviour might be a potential risk factor for cognitive decline. However, the short-term effects of sedentary behaviour on (cerebro) vascular and cognitive performance in older people are unknown.

METHODS

We used a cross-over design with 22 older adults (78 years, 9 females) to assess the short-term hemodynamic and cognitive effects of three hours uninterrupted sitting and explored if these effects can be counteracted with regular (every 30 min) two-minute walking breaks. In addition, we investigated if low versus high mental activity during the three hours of sitting modified these effects. Before and after each condition, alertness, executive functioning, and working memory were assessed with the Test of Attentional Performance battery. Additionally, cerebral blood flow velocity (Transcranial Doppler) and blood pressure (Finapres) were measured in rest, and during sit-to-stand and CO₂ challenges to assess baroreflex sensitivity, cerebral autoregulation, and cerebral vasomotor reactivity.

RESULTS

No short-term differences were observed in cognitive performance, cerebral blood flow velocity, baroreflex sensitivity, cerebral autoregulation, or cerebral vasomotor reactivity across time, or between conditions. Blood pressure and cerebrovascular resistance increased over time (8.6 mmHg (5.0;12.1), p < 0.001), and 0.23 in resistance (0.01;0.45), p = 0.04). However, these effects were not mitigated by mental activity or by short walking breaks to interrupt sitting.

CONCLUSIONS

In older individuals, three hours of sitting did not influence cognitive performance or cerebral perfusion. However, the sitting period increased blood pressure and cerebrovascular resistance, which are known to negatively impact brain health in the long-term. Importantly, we found that these effects in older individuals cannot be mitigated by higher mental activity and/or regular walking breaks.

TRIAL REGISTRATION

Clinical trial registration URL: https://www.toetsingonline.nl/. Unique identifier: NL64309.091.17. Date of registration: 06-02-2018.

Effects of Remote Ischemic Conditioning on Cerebral Hemodynamics in Ischemic Stroke

Ischemic stroke is one of the most common cerebrovascular diseases and is the leading cause of disability all over the world. It is well known that cerebral blood flow (CBF) is disturbed or even disrupted when ischemic stroke happens. The imbalance between demand and shortage of blood supply makes ischemic stroke take place or worsen. The search for treatments that can preserve CBF, especially during the acute phase of ischemic stroke, has become a research hotspot. Animal and clinical experiments have proven that remote ischemic conditioning (RIC) is a beneficial therapeutic strategy for the treatment of ischemic stroke. However, the mechanism by which RIC affects CBF has not been fully understood. This review aims to discuss several possible mechanisms of RIC on the cerebral hemodynamics in ischemic stroke, such as the improvement of cardiac function and collateral circulation of cerebral vessels, the protection of neurovascular units, the formation of gas molecules, the effect on the function of vascular endothelial cells and the nervous system. RIC has the potential to become a therapeutic treatment to improve CBF in ischemic stroke. Future studies are needed to highlight our understanding of RIC as well as accelerate its clinical translation.

Differential vasomotor responses to isocapnic hyperoxia: cerebral versus peripheral circulation

Isocapnic hyperoxia (IH) evokes cerebral and peripheral hypoperfusion via both disturbance of redox homeostasis and reduction in nitric oxide (NO) bioavailability. However, it is not clear whether the magnitude of the vasomotor responses depends on the vessel network exposed to IH. To test the hypothesis that the magnitude of IH-induced reduction in peripheral blood flow (BF) may differ from the hypoperfusion response observed in the cerebral vascular network under oxygen-enriched conditions, nine healthy men (25 ± 3 yr, mean ± SD) underwent 10 min of IH during either saline or vitamin C (3 g) infusion, separately. Femoral artery (FA), internal carotid artery (ICA), and vertebral artery (VA) BF (Doppler ultrasound), as well as arterial oxidant (8-isoprostane), antioxidant [ascorbic acid (AA)], and NO bioavailability (nitrite) markers were simultaneously measured. IH increased 8-isoprostane levels and reduced nitrite levels; these responses were followed by a reduction in both FA BF and ICA BF, whereas VA BF did not change. Absolute and relative reductions in FA BF were greater than IH-induced changes in ICA and VA perfusion. Vitamin C infusion increased arterial AA levels and abolished the IH-induced increase in 8-isoprostane levels and reduction in nitrite levels. Whereas ICA and VA BF did not change during the vitamin C-IH trial, FA perfusion increased and reached similar levels to those observed during normoxia with saline infusion. Therefore, the magnitude of IH-induced reduction in femoral blood flow is greater than that observed in the vessel network of the brain, which might involve the determinant contribution that NO has in the regulation of peripheral vascular perfusion.

Dietary nitrate supplementation enhances cerebrovascular CO2 reactivity in a sex-specific manner

Insufficient nitric oxide (NO) bioavailability plays an important role in endothelial dysfunction, and increased NO has the potential to enhance cerebral blood flow (CBF). Dietary supplementation with sodium nitrate, a precursor of NO, could improve cerebrovascular function, but this has not been investigated. In 17 individuals, we examined the effects of a 7-day supplementation of dietary nitrate (0.1 mmol·kg^-1·day ^-1) on cerebrovascular function using a randomized, single-blinded placebo-controlled crossover design. We hypothesized that 7-day dietary nitrate supplementation increases CBF response to CO₂ (cerebrovascular CO₂ reactivity) and cerebral autoregulation (CA). We assessed middle cerebral artery blood velocity (MCAv) and blood pressure (BP) at rest and during CO₂ breathing. Transfer function analysis was performed on resting beat-to-beat MCAv and BP to determine CA, from which phase, gain, and coherence of the BP-MCAv data were derived. Dietary nitrate elevated plasma nitrate concentration by ~420% (P < 0.001) and lowered gain (d = 1.2, P = 0.025) and phase of the BP-MCAv signal compared with placebo treatment (d = 0.7, P = 0.043), while coherence was unaffected (P = 0.122). Dietary nitrate increased the MCAv-CO₂ slope in a sex-specific manner (interaction: P = 0.016). Dietary nitrate increased the MCAv-CO₂ slope in men (d = 1.0, P = 0.014 vs. placebo), but had no effect in women (P = 0.919). Our data demonstrate that dietary nitrate greatly increased cerebrovascular CO₂ reactivity in healthy individuals, while its effect on CA remains unclear. The selective increase in the MCAv-CO₂ slope observed in men indicates a clear sexual dimorphic role of NO in cerebrovascular function.NEW & NOTEWORTHY We found dietary nitrate supplementation improved the brain blood vessels' response to CO₂, cerebrovascular CO₂ reactivity, without affecting blood pressure in a group of healthy individuals. Meanwhile, the effect of dietary nitrate on the relationship between blood pressure and brain blood flow, cerebral autoregulation, was inconclusive. The improvement in cerebrovascular CO₂ reactivity was only observed in the male participants, alluding to a sex difference in the effect of dietary nitrate on brain blood flow control. Our findings indicate that dietary nitrate could be an effective strategy to enhance cerebrovascular CO₂ reactivity.

Regulation of Regional Cerebral Blood Flow During Graded Reflex-Mediated Sympathetic Activation via Lower Body Negative Pressure

The role of the sympathetic nervous system in cerebral blood flow (CBF) regulation remains unclear. Previous studies have primarily measured middle cerebral artery blood velocity to assess CBF. Recently, there has been a transition towards measuring internal carotid artery (ICA) and vertebral artery (VA) blood flow using duplex Doppler ultrasound. Given that the VA supplies autonomic control centers in the brainstem, we hypothesized that graded sympathetic activation via lower body negative pressure (LBNP) would reduce ICA but not VA blood flow. ICA and VA blood flow were measured during two protocols: Protocol-1, low-to-moderate LBNP (-10, -20, -30, -40 Torr) and Protocol-2, moderate-to-high LBNP (-30, -50, -70 Torr). ICA and VA blood flow, diameter, and blood velocity were unaffected up to -40 LBNP. However, -50 and -70 LBNP evoked reductions in ICA and VA blood flow (e.g., -70 LBNP: %∆VA-baseline= -27.6±3.0) that were mediated by decreases in both diameter and velocity (e.g., -70 LBNP: %∆VA-baseline diameter= -7.5±1.9 and %∆VA-baseline velocity= -13.6±1.7), which were comparable between vessels. Since hyperventilation during -70 LBNP reduced P_ETCO₂, this decrease in P_ETCO₂ was matched via voluntary hyperventilation. Reductions in ICA and VA blood flow during hyperventilation alone were significantly smaller than during -70 LBNP and were primarily mediated by decreases in velocity (%∆VA-baseline velocity= -8.6±2.4; %∆VA-baseline diameter= -0.05±0.56). These data demonstrate that both ICA and VA were unaffected by low-to-moderate sympathetic activation, whereas robust reflex-mediated sympatho-excitation caused similar magnitudes of vasoconstriction in both arteries. Thus, contrary to our hypothesis, the ICA was not preferentially vasoconstricted by sympathetic activation.

Nitric oxide-sensitive guanylyl cyclase signaling affects CO2-dependent but not pressure-dependent regulation of cerebral blood flow

Cerebrovascular CO2 reactivity is affected by nitric oxide (NO). We tested the hypothesis that sildenafil selectively potentiates NO-cGMP signaling, which affects CO2 reactivity. Fourteen healthy males (34 ± 2 yr) were enrolled in the study. Blood pressure (BP), ECG, velocity of cerebral blood flow (CBF; measured by transcranial Doppler), and end-tidal CO2 (EtCO2) were assessed at baseline (CO2 ~39 mmHg), during hyperventilation (CO2 ~24 mmHg), during hypercapnia (CO2 ~46 mmHg), during boluses of phenylephrine (25–200 µg), and during graded head-up tilting (HUT). Measurements were repeated 1 h after 100 mg sildenafil were taken. Results showed that sildenafil did not affect resting BP, heart rate, CBF peak and mean velocities, estimated regional cerebrovascular resistance (eCVR; mean BP/mean CBF), breath/min, and EtCO2: 117 ± 2/67 ± 3 mmHg, 69 ± 3 beats/min, 84 ± 5 and 57 ± 4 cm/s, 1.56 ± 0.1 mmHg·cm−1·s−1, 14 ± 0.5 breaths/min, and 39 ± 0.9 mmHg, respectively. Sildenafil increased and decreased the hypercapnia induced in CBF and eCVR, respectively. Sildenafil also attenuated the decrease in peak velocity of CBF, 25 ± 2 vs. 20 ± 2% ( P < 0.05) and increased the eCVR, 2.5 ± 0.2 vs. 2 ± 0.2% ( P < 0.03) during hyperventilation. Sildenafil did not affect CBF despite significant increases in the eCVRs that were elicited by phenylephrine and HUT. This investigation suggests that sildenafil, which potentiates the NO-cGMP signaling, seems to affect the cerebrovascular CO2 reactivity without affecting the static and dynamic pressure-dependent mechanisms of cerebrovascular autoregulation.

Nitric oxide synthase inhibition restores orthostatic tolerance in young vasovagal syncope patients

OBJECTIVE

Syncope is sudden transient loss of consciousness and postural tone with spontaneous recovery; the most common form is vasovagal syncope (VVS). We previously demonstrated impaired post-synaptic adrenergic responsiveness in young VVS patients was reversed by blocking nitric oxide synthase (NOS). We hypothesised that nitric oxide may account for reduced orthostatic tolerance in young recurrent VVS patients.

METHODS

We recorded haemodynamics in supine VVS and healthy volunteers (aged 15-27 years), challenged with graded lower body negative pressure (LBNP) (-15, -30, -45 mm Hg each for 5 min, then -60 mm Hg for a maximum of 50 min) with and without NOS inhibitor N^G-monomethyl-L-arginine acetate (L-NMMA). Saline plus phenylephrine (Saline+PE) was used as volume and pressor control for L-NMMA.

RESULTS

Controls endured 25.9±4.0 min of LBNP during Saline+PE compared with 11.6±1.4 min for fainters (p<0.001). After L-NMMA, control subjects endured 24.8±3.2 min compared with 22.6±1.6 min for fainters. Mean arterial pressure decreased more in VVS patients during LBNP with Saline+PE (p<0.001) which was reversed by L-NMMA; cardiac output decreased similarly in controls and VVS patients and was unaffected by L-NMMA. Total peripheral resistance increased for controls but decreased for VVS during Saline+PE (p<0.001) but was similar following L-NMMA. Splanchnic vascular resistance increased during LBNP in controls, but decreased in VVS patients following Saline+PE which L-NMMA restored.

CONCLUSIONS

We conclude that arterial vasoconstriction is impaired in young VVS patients, which is corrected by NOS inhibition. The data suggest that both pre- and post-synaptic arterial vasoconstriction may be affected by nitric oxide.

Effects of midodrine and L-NAME on systemic and cerebral hemodynamics during cognitive activation in spinal cord injury and intact controls

We previously showed that increases in mean arterial pressure (MAP) following administration of midodrine hydrochloride (MH) and nitro‐L‐arginine methyl ester (L‐NAME) resulted in increased mean cerebral blood flow velocity (MFV) during head‐up tilt in hypotensive individuals with spinal cord injury (SCI) and question if this same association was evident during cognitive activation. Herein, we report MAP and MFV during two serial subtraction tasks (SSt) given before (predrug) and after (postdrug) administration of MH; (10 mg), L‐NAME (1 mg/kg) or no drug (ND) in 15 subjects with SCI compared to nine able‐bodied (AB) controls. Three‐way factorial analysis of variance (ANOVA) models were used to determine significant main and interaction effects for group (SCI, AB), visit (MH, L‐NAME, ND), and time (predrug, postdrug) for MAP and MFV during the two SSt. The three‐way interaction was significant for MAP (F = 4.262; P = 0.020); both MH (30 ± 26 mmHg; P < 0.05) and L‐NAME (27 ± 22 mmHg; P < 0.01) significantly increased MAP in the SCI group, but not in the AB group. There was a significant visit by time interaction for MFV suggesting an increase from predrug to postdrug following L‐NAME (6 ± 8 cm/sec; P < 0.05) and MH (4 ± 7 cm/sec; P < 0.05), regardless of study group, with little change following ND (3 ± 3 cm/sec). The relationship between change in MAP and MFV was significant in the SCI group following administration of MH (r² = 0.38; P < 0.05) and L‐NAME (r² = 0.32; P < 0.05). These antihypotensive agents, at the doses tested, raised MAP, which was associated with an increase MFV during cognitive activation in hypotensive subjects with SCI.

Changes in cerebral autoregulation in the second half of pregnancy and compared to non-pregnant controls

OBJECTIVE

The mechanism by which pregnancy affects the cerebral circulation is unknown, but it has a central role in the development of neurological complications in preeclampsia, which is believed to be related to impaired autoregulation. We evaluated the cerebral autoregulation in the second half of pregnancy, and compared this with a control group of healthy, fertile non-pregnant women.

METHODS

In a prospective cohort analysis, cerebral blood flow velocity of the middle cerebral artery (determined by transcranial Doppler), blood pressure (noninvasive arterial volume clamping), and end-tidal carbon dioxide (EtCO2) were simultaneously collected for 7min. The autoregulation index (ARI) was calculated. ARI values of 0 and 9 indicated absent and perfect autoregulation, respectively. ANOVA and Pearson's correlation coefficient were used, with p<0.05 considered significant.

RESULTS

A total of 76 pregnant and 18 non-pregnant women were included. The ARI did not change during pregnancy, but pregnant women had a significantly higher ARI than non-pregnant controls (ARI 6.7±0.9 vs. 5.3±1.4, p<0.001). This remained significant after adjusting for EtCO2 (p<0.001).

CONCLUSION

Cerebral autoregulation functionality is enhanced in the second half of pregnancy, when compared to non-pregnant fertile women, even after controlling for EtCO2. The autoregulation does not change with advancing gestational age.

Relative contributions of sympathetic, cholinergic, and myogenic mechanisms to cerebral autoregulation

BACKGROUND AND PURPOSE

Prior work aimed at improving our understanding of human cerebral autoregulation has explored individual physiological mechanisms of autoregulation in isolation, but none has attempted to consolidate the individual roles of these mechanisms into a comprehensive model of the overall cerebral pressure-flow relationship.

METHODS

We retrospectively analyzed this relationship before and after pharmacological blockade of α-adrenergic-, muscarinic-, and calcium channel-mediated mechanisms in 43 healthy volunteers to determine the relative contributions of the sympathetic, cholinergic, and myogenic controllers to cerebral autoregulation. Projection pursuit regression was used to assess the effect of pharmacological blockade on the cerebral pressure-flow relationship. Subsequently, ANCOVA decomposition was used to determine the cumulative effect of these 3 mechanisms on cerebral autoregulation and whether they can fully explain it.

RESULTS

Sympathetic, cholinergic, and myogenic mechanisms together accounted for 62% of the cerebral pressure-flow relationship (P<0.05), with significant and distinct contributions from each of the 3 effectors. ANCOVA decomposition demonstrated that myogenic effectors were the largest determinant of the cerebral pressure-flow relationship, but their effect was outside of the autoregulatory region where neurogenic control appeared prepotent.

CONCLUSIONS

Our results suggest that myogenic effects occur outside the active region of autoregulation, whereas neurogenic influences are largely responsible for cerebral blood flow control within it. However, our model of cerebral autoregulation left 38% of the cerebral pressure-flow relationship unexplained, suggesting that there are other physiological mechanisms that contribute to cerebral autoregulation.

Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome

Cognitive deficits are characteristic of postural tachycardia syndrome (POTS). Intact nitrergic nitric oxide (NO) is important to cerebral blood flow (CBF) regulation, neurovascular coupling, and cognitive efficacy. POTS patients often experience defective NO-mediated vasodilation caused by oxidative stress. We have previously shown dilation of the middle cerebral artery in response to a bolus administration of the NO donor sodium nitroprusside (SNP) in healthy volunteers. In the present study, we hypothesized a blunted middle cerebral artery response to SNP in POTS. We used combined transcranial Doppler-ultrasound to measure CBF velocity and near-infrared spectroscopy to measure cerebral hemoglobin oxygenation while subjects were in the supine position. The responses of 17 POTS patients were compared with 12 healthy control subjects (age: 14-28 yr). CBF velocity in POTS patients and control subjects were not different at baseline (75 ± 3 vs. 71 ± 2 cm/s, P = 0.31) and decreased to a lesser degree with SNP in POTS patients (to 71 ± 3 vs. 62 ± 2 cm/s, P = 0.02). Changes in total and oxygenated hemoglobin (8.83 ± 0.45 and 8.13 ± 0.48 μmol/kg tissue) were markedly reduced in POTS patients compared with control subjects (14.2 ± 1.4 and 13.6 ± 1.6 μmol/kg tissue), primarily due to increased venous efflux. The data indicate reduced cerebral oxygenation, blunting of cerebral arterial vasodilation, and heightened cerebral venodilation. We conclude, based on the present study outcomes, that decreased bioavailability of NO is apparent in the vascular beds, resulting in a downregulation of NO receptor sites, ultimately leading to blunted responses to exogenous NO.

Arterial pressure and cerebral blood flow variability: friend or foe? A review

Variability in arterial pressure and cerebral blood flow has traditionally been interpreted as a marker of cardiovascular decompensation, and has been associated with negative clinical outcomes across varying time scales, from impending orthostatic syncope to an increased risk of stroke. Emerging evidence, however, suggests that increased hemodynamic variability may, in fact, be protective in the face of acute challenges to perfusion, including significant central hypovolemia and hypotension (including hemorrhage), and during cardiac bypass surgery. This review presents the dichotomous views on the role of hemodynamic variability on clinical outcome, including the physiological mechanisms underlying these patterns, and the potential impact of increased and decreased variability on cerebral perfusion and oxygenation. We suggest that reconciliation of these two apparently discrepant views may lie in the time scale of hemodynamic variability; short time scale variability appears to be cerebroprotective, while mid to longer term fluctuations are associated with primary and secondary end-organ dysfunction.

Why is the neural control of cerebral autoregulation so controversial?

Cerebral autoregulation refers to the mechanisms that act to keep cerebral blood flow (CBF) constant during changes in blood pressure. The mechanisms of cerebral autoregulation, especially in humans, are poorly understood but are undoubtedly multifactorial and likely reflect many redundant pathways that potentially differ between species. Whether sympathetic nervous activity influences CBF and/or cerebral autoregulation in humans remains controversial. Following a brief introduction to cerebral autoregulation, this review highlights the likely reasons behind the controversy of the neural control of cerebral autoregulation. Finally, suggestions are provided for further studies to improve the understanding of the neural control of CBF regulation.

Perturbed and spontaneous regional cerebral blood flow responses to changes in blood pressure after high-level spinal cord injury: the effect of midodrine

Individuals with spinal cord injury (SCI) above the T6 spinal segment suffer from orthostatic intolerance. How cerebral blood flow (CBF) responds to orthostatic challenges in SCI is poorly understood. Furthermore, it is unclear how interventions meant to improve orthostatic tolerance in SCI influence CBF. This study aimed to examine 1) the acute regional CBF responses to rapid changes in blood pressure (BP) during orthostatic stress in individuals with SCI and able-bodied (AB) individuals; and 2) the effect of midodrine (alpha1-agonist) on orthostatic tolerance and CBF regulation in SCI. Ten individuals with SCI >T6, and 10 age- and sex-matched AB controls had beat-by-beat BP and middle and posterior cerebral artery blood velocity (MCAv, PCAv, respectively) recorded during a progressive tilt-test to quantify the acute CBF response and orthostatic tolerance. Dynamic MCAv and PCAv to BP relationships were evaluated continuously in the time domain and frequency domain (via transfer function analysis). The SCI group was tested again after administration of 10 mg midodrine to elevate BP. Coherence (i.e., linearity) was elevated in SCI between BP-MCAv and BP-PCAv by 35% and 22%, respectively, compared with AB, whereas SCI BP-PCAv gain (i.e., magnitudinal relationship) was reduced 30% compared with AB (all P < 0.05). The acute (i.e., 0-30 s after tilt) MCAv and PCAv responses were similar between groups. In individuals with SCI, midodrine led to improved PCAv responses 30-60 s following tilt (10 ± 3% vs. 4 ± 2% decline; P < 0.05), and a 59% improvement in orthostatic tolerance (P < 0.01). The vertebrobasilar region may be particularly susceptible to hypoperfusion in SCI, leading to increased orthostatic intolerance.

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

Cerebral autoregulation (CA) is integral to the delicate process of maintaining stable cerebral perfusion and brain tissue oxygenation against changes in arterial blood pressure. The last four decades has seen dramatic advances in understanding CA physiology, and the role that CA might play in the causation and progression of disease processes that affect the cerebral circulation such as stroke. However, the translation of these basic scientific advances into clinical practice has been limited by the maintenance of old constructs and because there are persistent gaps in our understanding of how this vital vascular mechanism should be quantified. In this review, we re-evaluate relevant studies that challenge established paradigms about how the cerebral perfusion pressure and blood flow are related. In the context of blood pressure being a major haemodynamic challenge to the cerebral circulation, we conclude that: (1) the physiological properties of CA remain inconclusive, (2) many extant methods for CA characterisation are based on simplistic assumptions that can give rise to misleading interpretations, and (3) robust evaluation of CA requires thorough consideration not only of active vasomotor function, but also the unique properties of the intracranial environment.

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

Cerebral autoregulation (CA) is integral to the delicate process of maintaining stable cerebral perfusion and brain tissue oxygenation against changes in arterial blood pressure. The last four decades has seen dramatic advances in understanding CA physiology, and the role that CA might play in the causation and progression of disease processes that affect the cerebral circulation such as stroke. However, the translation of these basic scientific advances into clinical practice has been limited by the maintenance of old constructs and because there are persistent gaps in our understanding of how this vital vascular mechanism should be quantified. In this review, we re-evaluate relevant studies that challenge established paradigms about how the cerebral perfusion pressure and blood flow are related. In the context of blood pressure being a major haemodynamic challenge to the cerebral circulation, we conclude that: (1) the physiological properties of CA remain inconclusive, (2) many extant methods for CA characterisation are based on simplistic assumptions that can give rise to misleading interpretations, and (3) robust evaluation of CA requires thorough consideration not only of active vasomotor function, but also the unique properties of the intracranial environment.

Middle cerebral O₂ delivery during the modified Oxford maneuver increases with sodium nitroprusside and decreases during phenylephrine

The modified Oxford maneuver is the reference standard for assessing arterial baroreflex function. The maneuver comprises a systemic bolus injection of 100 μg sodium nitroprusside (SNP) followed by 150 μg phenylephrine (PE). On the one hand, this results in an increase in oxyhemoglobin and total hemoglobin followed by a decrease within the cerebral sample volume illuminated by near-infrared spectroscopy (NIRS). On the other hand, it produces a decrease in cerebral blood flow velocity (CBFv) within the middle cerebral artery (MCA) during SNP and an increase in CBFv during PE as measured by transcranial Doppler ultrasound. To resolve this apparent discrepancy, we hypothesized that SNP dilates, whereas PE constricts, the MCA. We combined transcranial Doppler ultrasound of the right MCA with NIRS illuminating the right frontal cortex in 12 supine healthy subjects 18-24 yr old. Assuming constant O₂ consumption and venous saturation, as estimated by partial venous occlusion plethysmography, we used conservation of mass (continuity) equations to estimate the changes in arterial inflow (ΔQa) and venous outflow (ΔQv) of the NIRS-illuminated area. Oxyhemoglobin and total hemoglobin, respectively, increased by 13.6 ± 1.6 and 15.2 ± 1.4 μmol/kg brain tissue with SNP despite hypotension and decreased by 6 ± 1 and 7 ± 1 μmol/kg with PE despite hypertension. SNP increased ΔQa by 0.36 ± .03 μmol·kg(-1)·s(-1) (21.6 μmol·kg(-1)·min(-1)), whereas CBFv decreased from 71 ± 2 to 62 ± 2 cm/s. PE decreased ΔQa by 0.27 ± .2 μmol·kg(-1)·s(-1) (16.2 μmol·kg(-1)·min(-1)), whereas CBFv increased to 75 ± 3 cm/s. These results are consistent with dilation of the MCA by SNP and constriction by PE.

Defining the characteristic relationship between arterial pressure and cerebral flow

Reliable assessment of cerebrovascular effectiveness in buffering against pressure fluctuations may have important implications for the timing and the outcome of therapy after adverse cerebrovascular events. Although linear approaches may indicate the presence or absence of cerebral autoregulation, they are inadequate to describe its characteristics and its effectiveness. Establishing a simple yet robust methodology to reliably measure the effectiveness of cerebral autoregulation could provide a tool to guide screening and clinical options to characterize and treat adverse cerebrovascular events associated with alterations in cerebral perfusion. To test the utility of one such methodology, an oscillatory lower body negative pressure of 30-40 mmHg was used at six frequencies from 0.03 to 0.08 Hz in 43 healthy volunteers, and the pressure-flow relation and the effectiveness of autoregulation was quantified using projection pursuit regression. Projection pursuit regression explained the majority of the relationship between pressure and cerebral blood flow fluctuations and revealed its nature consistently across individuals and across separate study days. The nature of this relationship entailed an autoregulatory region wherein slow arterial pressure fluctuations are effectively counterregulated and two passive regions wherein pressure fluctuations resulted in parallel changes in flow. The effectiveness of autoregulation was significantly reduced as pressure fluctuations became faster. These results demonstrate the characteristic relationship between arterial pressure and cerebral blood flow. Furthermore, the methodology utilized in this study provides a tool that can provide unique insight to integrated cerebrovascular control and may allow diagnosis of physiological alterations underlying impaired cerebral autoregulation.

Sympathetic control of the cerebral vasculature in humans

BACKGROUND AND PURPOSE

The role of the sympathetic nervous system in cerebral autoregulation remains poorly characterized. We examined cerebral blood flow responses to augmented arterial pressure oscillations with and without sympathetic blockade and compared them with responses in the forearm circulation.

METHODS

An oscillatory lower body negative pressure of 40 mm Hg was used at 6 frequencies from 0.03 to 0.08 Hz in 11 healthy subjects with and without alpha-adrenergic blockade by phentolamine.

RESULTS

Sympathetic blockade resulted in unchanged mean pressure and cerebral flow. The transfer function relationship to arterial pressure at frequencies >0.05 Hz was significantly increased in both the cerebral and brachial circulations, but the coherence of the relation remained weak at the lowest frequencies in the cerebral circulation.

CONCLUSIONS

Our data demonstrate a strong, frequency-dependent role for sympathetic regulation of blood flow in both cerebral and brachial circulations. However, marked differences in the response to blockade suggest the control of the cerebral circulation at longer time scales is characterized by important nonlinearities and relies on regulatory mechanisms other than the sympathetic system.

Decreased upright cerebral blood flow and cerebral autoregulation in normocapnic postural tachycardia syndrome

Postural tachycardia syndrome (POTS), a chronic form of orthostatic intolerance, has signs and symptoms of lightheadedness, loss of vision, headache, fatigue, and neurocognitive deficits consistent with reductions in cerebrovascular perfusion. We hypothesized that young, normocapnic POTS patients exhibit abnormal cerebral autoregulation (CA) that results in decreased static and dynamic cerebral blood flow (CBF) autoregulation. All subjects had continuous recordings of mean arterial pressure (MAP) and CBF velocity (CBFV) using transcranial Doppler sonography in both the supine supine position and during a 70 degrees head-up tilt. During tilt, POTS patients (n = 9) demonstrated a higher heart rate than controls (n = 7) (109 +/- 6 vs. 80 +/- 2 beats/min, P < 0.05), whereas controls demonstrated a higher MAP than POTS (87 +/- 2 vs. 77 +/- 3 mmHg, P < 0.05). Also during tilt, mean CBFV decreased 19.5 +/- 2.6% in POTS patients versus 10.3 +/- 2.0% in controls (P < 0.05). We then used a transfer function analysis of MAP and CFBV in the frequency domain to quantify these changes. The low-frequency (LF; 0.04-0.15 Hz) component of CBFV variability increased during tilt in POTS patients (supine: 3 +/- 0.9 vs. tilt: 9 +/- 2, P < 0.02). In POTS patients, there was an increase in LF and high-frequency coherence between MAP and CBFV, an increase in LF gain, and a lack of significant change in phase. Static CA may be less effective in POTS patients compared with controls, since immediately after tilt CBFV decreased more in POTS patients and was highly oscillatory and autoregulation did not restore CBFV to baseline values until the subjects became supine. Dynamic CA may be less effective in POTS patients because MAP and CBFV during tilt became almost perfectly synchronous. We conclude that dynamic and static autoregulation of CBF are less effective in POTS patients compared with control subjects during orthostatic challenge.

Cerebral blood flow regulation by nitric oxide: recent advances

Nitric oxide (NO) is undoubtedly quite an important intercellular messenger in cerebral and peripheral hemodynamics. This molecule, formed by constitutive isomers of NO synthase, endothelial nitric-oxide synthase, and neuronal nitric-oxide synthase, plays pivotal roles in the regulation of cerebral blood flow and cell viability and in the protection of nerve cells or fibers against pathogenic factors associated with cerebral ischemia, trauma, and hemorrhage. Cerebral blood flow is increased and cerebral vascular resistance is decreased by NO derived from endothelial cells, autonomic nitrergic nerves, or brain neurons under resting and stimulated conditions. Somatosensory stimulation also evokes cerebral vasodilatation mediated by neurogenic NO. Oxygen and carbon dioxide alter cerebral blood flow and vascular tone mainly via constitutively formed NO. Endothelial dysfunction impairs cerebral hemodynamics by reducing the bioavailability of NO and increasing the production of reactive oxygen species (ROS). The NO-ROS interaction is an important issue in discussing blood flow and cell viability in the brain. Recent studies on brain circulation provide quite useful information concerning the physiological roles of NO produced by constitutive isoforms of nitric-oxide synthase and how NO may promote cerebral pathogenesis under certain conditions, including cerebral ischemia/stroke, cerebral vasospasm after subarachnoid hemorrhage, and brain injury. This information would contribute to better understanding of cerebral hemodynamic regulation and its dysfunction and to development of novel therapeutic measures to treat diseases of the central nervous system.

Mechanisms involved in the cerebrovascular dilator effects of cortical spreading depression

Cortical spreading depression (CSD) leads to dramatic changes in cerebral hemodynamics. However, mechanisms involved in promoting and counteracting cerebral vasodilator responses are unclear. Here we review the development and current status of this important field of research especially with respect to the role of perivascular nerves and nitric oxide (NO). It appears that neurotransmitters released from the sensory and the parasympathetic nerves associated with cerebral arteries, and NO released from perivascular nerves and/or parenchyma, promote cerebral hyperemia during CSD. However, the relative contributions of each of these factors vary according to species studied. Related to CSD, axonal and reflex responses involving trigeminal afferents on the pial surface lead to increased blood flow and inflammation of the overlying dura mater. Counteracting the cerebral vascular dilation is the production and release of constrictor prostaglandins, at least in some species, and other possibly yet unknown agents from the vascular wall. The cerebral blood flow response in healthy human cortex has not been determined, and thus it is unclear whether the cerebral oligemia associated with migraines represents the normal physiological response to a CSD-like event or represents a pathological response. In addition to promoting cerebral hyperemia, NO produced during CSD appears to initiate signaling events which lead to protection of the brain against subsequent ischemic insults. In summary, the cerebrovascular response to CSD involves multiple dilator and constrictor factors produced and released by diverse cells within the neurovascular unit, with the contribution of each of these factors varying according to the species examined.

Altered free radical metabolism in acute mountain sickness: implications for dynamic cerebral autoregulation and blood-brain barrier function

We tested the hypothesis that dynamic cerebral autoregulation (CA) and blood-brain barrier (BBB) function would be compromised in acute mountain sickness (AMS) subsequent to a hypoxia-mediated alteration in systemic free radical metabolism. Eighteen male lowlanders were examined in normoxia (21% O(2)) and following 6 h passive exposure to hypoxia (12% O(2)). Blood flow velocity in the middle cerebral artery (MCAv) and mean arterial blood pressure (MAP) were measured for determination of CA following calculation of transfer function analysis and rate of regulation (RoR). Nine subjects developed clinical AMS (AMS+) and were more hypoxaemic relative to subjects without AMS (AMS-). A more marked increase in the venous concentration of the ascorbate radical (A(*-)), lipid hydroperoxides (LOOH) and increased susceptibility of low-density lipoprotein (LDL) to oxidation was observed during hypoxia in AMS+ (P < 0.05 versus AMS-). Despite a general decline in total nitric oxide (NO) in hypoxia (P < 0.05 versus normoxia), the normoxic baseline plasma and red blood cell (RBC) NO metabolite pool was lower in AMS+ with normalization observed during hypoxia (P < 0.05 versus AMS-). CA was selectively impaired in AMS+ as indicated both by an increase in the low-frequency (0.07-0.20 Hz) transfer function gain and decrease in RoR (P < 0.05 versus AMS-). However, there was no evidence for cerebral hyper-perfusion, BBB disruption or neuronal-parenchymal damage as indicated by a lack of change in MCAv, S100beta and neuron-specific enolase. In conclusion, these findings suggest that AMS is associated with altered redox homeostasis and disordered CA independent of barrier disruption.

Central hypervolemia with hemodilution impairs dynamic cerebral autoregulation

BACKGROUND

Frequent changes in the perioperative central blood volume could affect cerebral autoregulation through alterations in sympathetic nerve activity, cardiac output, blood viscosity, and cerebral vasomotor tone. However, the effect of dynamic cerebral autoregulation has not been studied during acute wide-ranging changes in central blood volume, especially with respect to central hypervolemia with hemodilution.

METHODS

We evaluated dynamic cerebral autoregulation during central hypovolemia and central hypervolemia with hemodilution using spectral and transfer function analysis between mean arterial blood pressure (MBP) and cerebral blood flow (CBF) velocity variability in 12 individuals. Rapid changes in central blood volume were achieved using two levels of lower body negative pressure (-15 and -30 mm Hg) and two discrete infusions of normal saline (15 mL/kg and total 30 mL/kg). We then estimated changes in central blood volume as central venous pressure (CVP) and/or cardiac output using impedance cardiography.

RESULTS

Steady-state CBF velocity and cardiac output decreased at -30 mm Hg lower body negative pressure (changes of CVP approximately -4 mm Hg) or were increased by each saline infusion (changes of CVP 4-6 mm Hg), without a significant change in MBP. However, transfer function gain (magnitude of transfer) between MBP and CBF velocity variability significantly increased only after saline infusion, suggesting an increased magnitude of transfer from MBP oscillations to CBF fluctuations during central hypervolemia with hemodilution.

CONCLUSION

Our results suggest that, although steady-state CBF velocity changes under both central hypervolemia and hypovolemia, only hypervolemic hemodilution impairs dynamic cerebral autoregulation.

Early morning impairment in cerebral autoregulation and cerebrovascular CO2reactivity in healthy humans: relation to endothelial function

The reduction in cerebrovascular reactivity to CO(2) and/or endothelial function that occurs in the early hours after waking are potential causes for the increased risk for cardiovascular events at this time point. It is unknown whether cerebral autoregulation is reduced in the morning. We tested the hypothesis that early morning reduction in endothelium-dependent vascular reactivity would be linked to changes in cerebrovascular reactivity to CO(2) and cerebral autoregulation (CA). Overnight changes in a dynamic cerebral autoregulation index (ARI) were determined from continuous recordings of blood flow velocity in the middle cerebral artery (MCAv) and arterial blood pressure (BP) during transiently induced hypotension in 20 individuals. Frontal cortical oxygenation (near infrared spectroscopy) and cerebral haemodynamics were also monitored during hypercapnia and before and during 3 min of active standing. Brachial artery flow-mediated endothelium-dependent vasodilatation (FMD) and endothelium-independent dilatation (NFMD) were also monitored. From evening to morning, there was a significant lowering in ARI (5.3 +/- 0.5 versus 4.7 +/- 0.6 a.u.; P < 0.05), cerebrovascular reactivity to CO(2) (5.3 +/- 0.6 versus 4.6 +/- 1.1% mmHg(-1); P < 0.05) and FMD (7.6 +/- 0.9 versus 6.0 +/- 1.4%; P < 0.05). The lowered FMD was related to the decrease in cerebrovascular reactivity to CO(2) (r = 0.76; P < 0.05). Transient reductions in morning MCAv and cortical oxyhaemoglobin concentrations were observed upon resuming a supine-to-upright position (P < 0.05 versus evening). The early morning reduction in cerebral autoregulation may facilitate the onset of cerebrovascular accidents; this may be of particular relevance to at-risk groups, especially upon resuming the upright position.

IDENTIFICATION OF BLOOD PRESSURE CONTROL MECHANISMS BY POWER SPECTRAL ANALYSIS

1. Blood pressure and organ perfusion are controlled by a variety of cardiovascular control systems, such as the baroreceptor reflex and the renin-angiotensin system (RAS), and by local vascular mechanisms, such as shear stress-induced release of nitric oxide (NO) from the endothelium and the myogenic vascular response. Deviations in arterial blood pressure from its set point activate these mechanisms in an attempt to restore blood pressure and/or secure organ perfusion. However, the response times at which different cardiovascular mechanisms operate differ considerably (e.g. blood pressure control by the RAS is slower than blood pressure control via the baroreceptor reflex). 2. Owing to these different response times, some cardiovascular control systems affect blood pressure more rapidly and others more slowly. Thus, identifying the frequency components of blood pressure variability (BPV) by power spectral analysis can potentially provide important information on individual blood pressure control mechanisms. 3. Evidence is presented that the RAS, catecholamines, endothelial-derived NO and myogenic vascular function affect BPV at very low frequencies (0.02-0.2 Hz) and that low-frequency (LF) BPV (0.2-0.6 Hz) is affected by sympathetic modulation of vascular tone and endothelial-derived NO in rats. In humans, LF BPV (0.075-0.15 Hz) is affected by sympathetic modulation of vascular tone and myogenic vascular function. The impact of the RAS and endothelial-derived NO on BPV in humans requires further investigation. 4. In conclusion, power spectral analysis is a powerful diagnostic tool that allows identification of pathophysiological mechanisms contributing to cardiovascular diseases, such as hypertension, heart failure and stroke, because it can separate slow from fast cardiovascular control mechanisms. The limitation that some cardiovascular control mechanisms affect the same frequency components of BPV requires the combination of blood pressure spectral analysis with other techniques.

Impaired cerebral CO2 vasoreactivity: association with endothelial dysfunction

Conflicting data exist on the role of nitric oxide (NO) in cerebral blood flow (CBF) autoregulation. Previous studies involving human and animal subjects seem to indicate that NO involvement is limited to the CO(2)-dependent mechanism (chemoregulation) and not to the pressure-dependent autoregulation (mechanoregulation). We tested this hypothesis in patients with impaired endothelial function compared with healthy controls. Blood pressure, heart rate, end-tidal Pco(2), CBF velocities (CBFV), forearm blood flow, and reactive hyperemia were assessed in 16 patients with diabetes mellitus and/or hypertension and compared with 12 age- and sex-matched healthy controls. Pressure-dependent autoregulation was determined by escalating doses of phenylephrine. CO(2) vasoreactivity index was extrapolated from individual slopes of mean CBFV during normocapnia, hyperventilation, and CO(2) inhalation. Measurements were repeated after sodium nitroprusside infusion. Indexes of endothelial function, maximal and area under the curve (AUC) of forearm blood flow (FBF) changes, were significantly impaired in patients (maximal flow: 488 +/- 75 vs. 297 +/- 31%; P = 0.01, AUC DeltaFBF: 173 +/- 17 vs. 127 +/- 11; P = 0.03). Patients and controls showed similar changes in cerebrovascular resistance during blood pressure challenges (identical slopes). CO(2) vasoreactivity was impaired in patients compared with controls: 1.19 +/- 0.1 vs. 1.54 +/- 0.1 cm.s(-1).mmHg(-1); P = 0.04. NO donor (sodium nitroprusside) offsets this disparity. These results suggest that patients with endothelial dysfunction have impaired CO(2) vasoreactivity and preserved pressure-dependent autoregulation. This supports our hypothesis that NO is involved in CO(2)-dependent CBF regulation alone. CBFV chemoregulation could therefore be a surrogate of local cerebral endothelial function.

Cerebral blood flow velocity during mental activation: interpretation with different models of the passive pressure-velocity relationship

The passive relationship between arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) has been expressed by a single parameter [cerebrovascular resistance (CVR)] or, alternatively, by a two-parameter model, comprising a resistance element [resistance-area product (RAP)] and a critical closing pressure (CrCP). We tested the hypothesis that the RAP+CrCP model can provide a more consistent interpretation to CBFV responses induced by mental activation tasks than the CVR model. Continuous recordings of CBFV [bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal CO(2) (EtCO(2)) were performed in 13 right-handed healthy subjects (aged 21-43 yr), in the seated position, at rest and during 10 repeated presentations of a word generation and a constructional puzzle paradigm that are known to induce differential cortical activation. Due to its small relative change, the CBFV response can be broken down into standardized subcomponents describing the relative contributions of ABP, CVR, RAP, and CrCP. At rest and during activation, the RAP+CrCP model suggested that RAP might reflect myogenic activity in response to the ABP transient, whereas CrCP was more indicative of metabolic control. These different influences were not reflected by the CVR model, which indicated a predominantly metabolic response. Repeated-measures multi-way ANOVA showed that CrCP (P = 0.025), RAP (P = 0.046), and CVR (P = 0.002) changed significantly during activation. CrCP also had a significant effect of paradigm (P = 0.045) but not hemispheric dominance. Both RAP (P = 0.039) and CVR (P = 0.0008) had significant effects of hemispheric dominance but were not sensitive to the different paradigms. Subcomponent analysis can help with the interpretation of CBFV responses to mental activation, which were found to be dependent on the underlying model of the passive ABP-CBFV relationship.