Cerebral vasomotor reactivity: steady-state versus transient changes in carbon dioxide tension

Role of pterygopalatine ganglion in regulating isoflurane-induced cerebral hyper-perfusion

Cerebral perfusion is functionally regulated by neural mechanisms in addition to the systemic hemodynamic variation, vascular reactivity and cerebral metabolism. Although anesthesia is generally esteemed to suppress the overall brain neural activity and metabolism, a few inhalation anesthetics, such as isoflurane, can increase cerebral perfusion, thus heightening the risks of higher intracranial pressure, bleeding, and brain edema during surgery. With the aid of laser speckle contrast imaging, we observed a transient yet limited effect of cerebral perfusion enhancement in mice from awake to anesthetized conditions with different concentration of isoflurane. Retrograde and antegrade tracing revealed a higher proportion of parasympathetic control more than sympathetic innervation for the blood vessels. Surprisingly, isoflurane directly activated pterygopalatine ganglion (PPG) explants and induced FOS expression in the cholinergic neurons. Chemogenetic activation of cholinergic PPG neurons reduced isoflurane-related cerebral perfusion. On the contrary, ablation of the cholinergic PPG neurons resulted in further enhancement of cerebral perfusion induced by isoflurane. While blocking muscarinic cholinergic receptors resulted in the overall reduction upon isoflurane stimulation, the blockage of nicotinic cholinergic receptors enhanced the isoflurane-induced cerebral perfusion only when PPG neurons exist. Collectively, these results suggest that PPG play important roles in regulating cerebral perfusion under isoflurane inhalation.

Arterial Aging and Cerebrovascular Function: Impact of Aerobic Exercise Training in Older Adults

Advanced age is the major risk factor for dementia including Alzheimer's disease. The clinical effects of recently developed anti-amyloid therapy for Alzheimer's disease were modest and the long-term outcome is unknown. Thus, an in-depth understanding of the mechanisms of brain aging is essential to develop preventive interventions to maintain cognitive health in late life. Mounting evidence suggests that arterial aging manifested as increases in central arterial stiffness is associated closely with cerebrovascular dysfunction and brain aging while improvement of cerebrovascular function with aerobic exercise training contributes to brain health in older adults. We summarized evidence in this brief review that 1) increases in central arterial stiffness and arterial pulsation with age are associated with increases in cerebrovascular resistance, reduction in cerebral blood flow, and cerebrovascular dysfunction, 2) aerobic exercise training improves cerebral blood flow by modifying arterial aging as indicated by reductions in cerebrovascular resistance, central arterial stiffness, arterial pulsation, and improvement in cerebrovascular function, and 3) improvement in cerebral blood flow and cerebrovascular function with aerobic exercise training may lead to improvement in cognitive function. These findings highlight the associations between arterial aging and cerebrovascular function and the importance of aerobic exercise in maintaining brain health in older adults.

Depression of dynamic cerebral autoregulation during neural activation: The role of responders and non-responders

Neurovascular coupling (NVC) interaction with dynamic cerebral autoregulation (dCA) remains unclear. We investigated the effect of task complexity and duration on the interaction with dCA. Sixteen healthy participants (31.6 ± 11.6 years) performed verbal fluency (naming-words (NW)) and serial subtraction (SS) paradigms, of varying complexity, at durations of 05, 30 and 60 s. The autoregulation index (ARI), was estimated from the bilateral middle cerebral artery blood velocity (MCAv) step response, calculated by transfer function analysis (TFA), for each paradigm during unstimulated (2 min) and neuroactivated (1 min) segments. Intraclass correlation (ICC) and coefficient of variation (CV) determined reproducibility for two visits and objective criteria were applied to classify responders (R) and non-responders (NoR) to task-induced MCAv increase. ICC values demonstrated fair reproducibility in all tasks. ARI decreased in right (RH) and left (LH) hemispheres, irrespective of paradigm complexity and duration (p < 0.0001). Bilateral ARI estimates were significantly decreased during NW for the R group only (p < 0.0001) but were reduced in both R (p < 0.0001) and NoR (p = 0.03) groups for SS tasks compared with baseline. The reproducible attenuation of dCA efficiency due to paradigm-induced NVC response, its interaction, and different behaviour in R and NoR, warrant further research in different physiological and clinical conditions.

Dynamic cerebral autoregulation is preserved during orthostasis and intrathoracic pressure regulation in healthy subjects: A pilot study

Resistance breathing may restore cardiac output (CO) and cerebral blood flow (CBF) during hypovolemia. We assessed CBF and cerebral autoregulation (CA) during tilt, resistance breathing, and paced breathing in 10 healthy subjects. Blood velocities in the internal carotid artery (ICA), middle cerebral arteries (MCA, four subjects), and aorta were measured by Doppler ultrasound in 30° and 60° semi-recumbent positions. ICA blood flow and CO were calculated. Arterial blood pressure (ABP, Finometer), and end-tidal CO2 (ETCO2) were recorded. ICA blood flow response was assessed by mixed-models regression analysis. The synchronization index (SI) for the variable pairs ABP-ICA blood velocity, ABP-MCA velocities in 0.005-0.08 Hz frequency interval was calculated as a measure of CA. Passive tilting from 30° to 60° resulted in 12% decrease in CO (p = 0.001); ICA blood flow tended to fall (p = 0.04); Resistance breathing restored CO and ICA blood flow despite a 10% ETCO2 drop. ETCO2 and CO contributed to ICA blood flow variance (adjusted R2: 0.9, p < 0.0001). The median SI was low (<0.2) indicating intact CA, confirmed by surrogate date testing. The peak SI was transiently elevated during resistance breathing in the 60° position. Resistance breathing may transiently reduce CA efficiency. Paced breathing did not restore CO or ICA blood flow.

Exposure to passive heat and cold stress differentially modulates cerebrovascular-CO2 responsiveness

Heat and cold stress influence cerebral blood flow (CBF) regulatory factors (e.g., arterial CO2 partial pressure). However, it is unclear whether the CBF response to a CO2 stimulus (i.e., cerebrovascular-CO2 responsiveness) is maintained under different thermal conditions. This study aimed to compare cerebrovascular-CO2 responsiveness between normothermia, passive heat, and cold stress conditions. Sixteen participants (8 females; 25 ± 7 yr) completed two experimental sessions (randomized) comprising normothermic and either passive heat or cold stress conditions. Middle and posterior cerebral artery velocity (MCAv, PCAv) were measured during rest, hypercapnia (5% CO2 inhalation), and hypocapnia (voluntary hyperventilation to an end-tidal CO2 of 30 mmHg). The linear slope of the cerebral blood velocity (CBv) response to changing end-tidal CO2 was calculated to measure cerebrovascular-CO2 responsiveness, and cerebrovascular conductance (CVC) was used to examine responsiveness independent of blood pressure. CBv-CVC-CO2 responsiveness to hypocapnia was greater during heat stress compared with cold stress (MCA: +0.05 ± 0.08 cm/s/mmHg/mmHg, P = 0.04; PCA: +0.02 ± 0.02 cm/s/mmHg/mmHg, P = 0.002). CBv-CO2 responsiveness to hypercapnia decreased during heat stress (MCA: -0.67 ± 0.89 cm/s/mmHg, P = 0.02; PCA: -0.64 ± 0.62 cm/s/mmHg; P = 0.01) and increased during cold stress (MCA: +0.98 ± 1.33 cm/s/mmHg, P = 0.03; PCA: +1.00 ± 0.82 cm/s/mmHg; P = 0.01) compared with normothermia. However, CBv-CVC-CO2 responsiveness to hypercapnia was not different between thermal conditions (P > 0.08). Overall, passive heat, but not cold, stress challenges the maintenance of cerebral perfusion. A greater cerebrovascular responsiveness to hypocapnia during heat stress likely reduces an already impaired cerebrovascular reserve capacity and may contribute to adverse events (e.g., syncope).NEW & NOTEWORTHY This study demonstrates that thermoregulatory-driven perfusion pressure changes, from either cold or heat stress, impact cerebrovascular responsiveness to hypercapnia. Compared with cold stress, heat stress poses a greater challenge to the maintenance of cerebral perfusion during hypocapnia, challenging cerebrovascular reserve capacity while increasing cerebrovascular-CO2 responsiveness. This likely exacerbates cerebral hypoperfusion during heat stress since hyperthermia-induced hyperventilation results in hypocapnia. No regional differences in middle and posterior cerebral artery responsiveness were found with thermal stress.

Cerebral blood flow and cerebrovascular reactivity are modified by maturational stage and exercise training status during youth

NEW FINDINGS

What is the central question of this study? Gonadal hormones modulate cerebrovascular function while insulin-like growth factor 1 (IGF-1) facilitates exercise-mediated cerebral angiogenesis; puberty is a critical period of neurodevelopment alongside elevated gonadal hormone and IGF-1 activity: but whether exercise training across puberty enhances cerebrovascular function is unkown. What is the main finding and its importance? Cerebral blood flow is elevated in endurance trained adolescent males when compared to untrained counterparts. However, cerebrovascular reactivity to hypercapnia is faster in trained vs. untrained children, but not adolescents. Exercise-induced improvements in cerebrovascular function are attainable as early as the first decade of life.

ABSTRACT

Global cerebral blood flow (gCBF) and cerebrovascular reactivity to hypercapnia (CV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) are modulated by gonadal hormone activity, while insulin-like growth factor 1 facilitates exercise-mediated cerebral angiogenesis in adults. Whether critical periods of heightened hormonal and neural development during puberty represent an opportunity to further enhance gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ is currently unknown. Therefore, we used duplex ultrasound to assess gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ in n = 128 adolescents characterised as endurance-exercise trained (males: n = 30, females: n = 36) or untrained (males: n = 29, females: n = 33). Participants were further categorised as pre- (males: n = 35, females: n = 33) or post- (males: n = 24, females: n = 36) peak height velocity (PHV) to determine pubertal or 'maturity' status. Three-factor ANOVA was used to identify main and interaction effects of maturity status, biological sex and training status on gCBF andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Data are reported as group means (SD). Pre-PHV youth demonstrated elevated gCBF and slowerCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response times than post-PHV counterparts (both: P ≤ 0.001). gCBF was only elevated in post-PHV trained males when compared to untrained counterparts (634 (43) vs. 578 (46) ml min-1 ; P = 0.007). However,CV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time was faster in pre- (72 (20) vs. 95 (29) s; P ≤ 0.001), but not post-PHV (P = 0.721) trained youth when compared to untrained counterparts. Cardiorespiratory fitness was associated with gCBF in post-PHV youth (r2  = 0.19; P ≤ 0.001) andCV R C O 2 ${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time in pre-PHV youth (r2  = 0.13; P = 0.014). Higher cardiorespiratory fitness during adolescence can elevate gCBF while exercise training during childhood primes the development of cerebrovascular function, highlighting the importance of exercise training during the early stages of life in shaping the cerebrovascular phenotype.

Menstrual phase influences cerebrovascular responsiveness in females but may not affect sex differences

Background and aims: Sex differences in the rate and occurrence of cerebrovascular diseases (e.g., stroke) indicate a role for female sex hormones (i.e., oestrogen and progesterone) in cerebrovascular function and regulation. However, it remains unclear how cerebrovascular function differs between the sexes, and between distinct phases of the menstrual cycle. This study aimed to compare cerebrovascular-CO₂ responsiveness in 1) females during the early follicular (EF), ovulatory (O) and mid-luteal (ML) phases of their menstrual cycle; and 2) males compared to females during phases of lower oestrogen (EF) and higher oestrogen (O). Methods: Eleven females (25 ± 5 years) complete experimental sessions in the EF (n = 11), O (n = 9) and ML (n = 11) phases of the menstrual cycle. Nine males (22 ± 3 years) completed two experimental sessions, approximately 2 weeks apart for comparison to females. Middle and posterior cerebral artery velocity (MCAv, PCAv) was measured at rest, during two stages of hypercapnia (2% and 5% CO₂ inhalation) and hypocapnia (voluntary hyperventilation to an end-tidal CO₂ of 30 and 24 mmHg). The linear slope of the cerebral blood velocity response to changes in end-tidal CO₂ was calculated to measure cerebrovascular-CO₂ responsiveness.. Results: In females, MCAv-CO₂ responsiveness to hypocapnia was lower during EF (-.78 ± .45 cm/s/mmHg) when compared to the O phase (-1.17 ± .52 cm/s/mmHg; p < .05) and the ML phase (-1.30 ± .82; p < .05). MCAv-CO₂ responsiveness to hypercapnia and hypo-to-hypercapnia, and PCAv-CO₂ responsiveness across the CO₂ range were similar between menstrual phases (p ≥ .20). MCAv-CO₂ responsiveness to hypo-to hypercapnia was greater in females compared to males (3.12 ± .91 cm/s/mmHg vs. 2.31 ± .46 cm/s/mmHg; p = .03), irrespective of menstrual phase (EF or O). Conclusion: Females during O and ML phases have an enhanced vasoconstrictive capacity of the MCA compared to the EF phase. Additionally, biological sex differences can influence cerebrovascular-CO₂ responsiveness, dependent on the insonated vessel.

Six nights of sleep extension increases regional cerebral oxygenation without modifying cognitive performance at rest or following acute aerobic exercise

Long sleep durations (≥540 min) are associated with poor cognitive performance in ageing adults, but the underlying cause is unclear. The aim of this study was to investigate the effect of extended sleep on cognitive performance and cerebral vascular function before and then after aerobic exercise. In all, 12 adults completed 6 nights of 8- (control) and 10+-h (sleep extension) time in bed in a randomised, crossover experiment. Sleep was measured using wrist actigraphy. On the last day of each time in bed protocol, participants performed three bouts of brisk walking. Sustained attention, spatial rotation ability, mental flexibility, and working memory were assessed, while prefrontal oxygen saturation index (ΔTSI) was measured using near-infrared spectroscopy. A two-way repeated-measures analysis of variance (time in bed × before/after exercise) was used for statistical analysis. Average sleep duration was longer following sleep extension as compared to control, at a mean (SD) of 551 (16) versus 428 (20) min (p < 0.001). Sleep extension did not alter cognitive performance as compared to control, but increased ΔTSI during tests of spatial rotation ability (main effect for time in bed, p = 0.03), mental flexibility (p = 0.04), and working memory (p < 0.01). Cognitive performance was improved (main effect for exercise, p < 0.05) following brisk walking for all cognitive domains except sustained attention with no interaction with time in bed. In summary, 6 nights of extended time in bed accompanied by long sleep durations does not impair cognitive performance at rest or alter the positive effect of acute aerobic exercise on cognition but may increase frontal cerebral oxygenation during cognitive functioning.

On the use and misuse of cerebral hemodynamics terminology using Transcranial Doppler ultrasound: a call for standardization

Cerebral hemodynamics (e.g., cerebral blood flow) can be measured and quantified using many different methods, with Transcranial Doppler ultrasound (TCD) being one of the most commonly utilized approaches. In human physiology, the terminology used to describe metrics of cerebral hemodynamics are inconsistent, and in some instances technically inaccurate; this is especially true when evaluating, reporting, and interpreting measures from TCD. Therefore, this perspectives article presents recommended terminology when reporting cerebral hemodynamic data. We discuss the current use and misuse of the terminology in the context of using TCD to measure and quantify cerebral hemodynamics and present our rationale and consensus on the terminology that we recommend moving forward. For example, one recommendation is to discontinue use of the term "cerebral blood flow velocity" in favor of "cerebral blood velocity" with precise indication of the vessel of interest. We also recommend clarity when differentiating between discrete cerebrovascular regulatory mechanisms, namely cerebral autoregulation, neurovascular coupling, and cerebrovascular reactivity. This will be a useful guide for investigators in the field of cerebral hemodynamics research.

Hot head-out water immersion does not acutely alter dynamic cerebral autoregulation or cerebrovascular reactivity to hypercapnia

Recurring hot head-out water immersion (HOWI) enhances peripheral vascular function and cerebral blood velocity during non-immersion conditions. However, it is unknown if an acute bout of hot HOWI alters cerebrovascular function. Using two experimental studies, we tested the hypotheses that dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity (CVR) are improved during an acute bout of hot (HOT; 39 °C) vs. thermoneutral (TN; 35 °C) HOWI. Eighteen healthy participants (eight females) completed the dCA study, and 14 participants (6 females) completed the CVR study. Both studies consisted of two randomized (TNdCA vs. HOTdCA; TNCVR vs. HOTCVR) 45minute HOWI visits. Middle cerebral artery blood velocity (MCAvmean) was continuously recorded. dCA was assessed using a respiratory impedance device and analyzed via transfer gain and phase in the low-frequency band. CVR was assessed using stepped hypercapnia. Assessments were completed PRE and 30 minutes into HOWI. Values are reported as a change (Δ) from PRE (mean ± SD). There were no differences at PRE for either study. ΔMCAvmean was greater in TNdCA (TNdCA: 4 ± 4 vs. HOTdCA: -3 ± 5 cm/s; P < 0.01) and TNCVR (TNCVR: 5 ± 4 vs. HOTCVR: -1 ± 6 cm/s; P < 0.01) during HOWI. ΔGain was greater in HOTdCA during HOWI (TNdCA: -0.09 ± 0.15 vs. HOTdCA: 0.10 ± 0.17 cm/s/mmHg; P = 0.04). ΔPhase (P > 0.84) and ΔCVR (P > 0.94) were not different between conditions. These data indicate that hot and thermoneutral water immersion do not acutely alter cerebrovascular function in healthy, young adults.

Cerebral Vasomotor Reactivity in Amnestic Mild Cognitive Impairment

BACKGROUND

Cerebral blood flow (CBF) is sensitive to changes in arterial CO2, referred to as cerebral vasomotor reactivity (CVMR). Whether CVMR is altered in patients with amnestic mild cognitive impairment (aMCI), a prodromal stage of Alzheimer disease (AD), is unclear.

OBJECTIVE

To determine whether CVMR is altered in aMCI and is associated with cognitive performance.

METHODS

Fifty-three aMCI patients aged 55 to 80 and 22 cognitively normal subjects (CN) of similar age, sex, and education underwent measurements of CBF velocity (CBFV) with transcranial Doppler and end-tidal CO2 (EtCO2) with capnography during hypocapnia (hyperventilation) and hypercapnia (rebreathing). Arterial pressure (BP) was measured to calculate cerebrovascular conductance (CVCi) to normalize the effect of changes in BP on CVMR assessment. Cognitive function was assessed with Mini-Mental State Examination (MMSE) and neuropsychological tests focused on memory (Logical Memory, California Verbal Learning Test) and executive function (Delis-Kaplan Executive Function Scale; DKEFS).

RESULTS

At rest, CBFV and MMSE did not differ between groups. CVMR was reduced by 13% in CBFV% and 21% in CVCi% during hypocapnia and increased by 22% in CBFV% and 20% in CVCi% during hypercapnia in aMCI when compared to CN (all p < 0.05). Logical Memory recall scores were positively correlated with hypocapnia (r = 0.283, r = 0.322, p < 0.05) and negatively correlated with hypercapnic CVMR measured in CVCi% (r = -0.347, r = -0.446, p < 0.01). Similar correlations were observed in D-KEFS Trail Making scores.

CONCLUSION

Altered CVMR in aMCI and its associations with cognitive performance suggests the presence of cerebrovascular dysfunction in older adults who have high risks for AD.

One-year aerobic exercise altered cerebral vasomotor reactivity in mild cognitive impairment

The purpose of this study was to test the hypothesis that changes in cerebral vasomotor reactivity (CVMR) after 1-yr aerobic exercise training (AET) are associated with cognitive performances in individuals with amnestic mild cognitive impairment (MCI). Seventy sedentary patients with amnestic MCI were randomized to 1-yr moderate-to-vigorous intensity AET or stretching and toning (SAT) interventions. Cerebral blood flow velocity (CBFV) with transcranial Doppler, mean arterial pressure (MAP) with finapres plethysmograph, and EtCO2 with capnography were measured during hyperventilation (hypocapnia) and a modified rebreathing protocol (hypercapnia) to assess CVMR. Cerebrovascular conductance index (CVCi) was calculated by CBFV/MAP, and CVMR by ΔCBFV/ΔEtCO2 and ΔCVCi/ΔEtCO2. Episodic memory and executive function were assessed using standard neuropsychological tests (CVLT-II and D-KEFS). Cardiorespiratory fitness was assessed by peak oxygen uptake (V̇o2peak). A total of 37 patients (19 in SAT and 18 in AET) completed 1-yr interventions and CVMR assessments. AET improved V̇o2peak, increased hypocapnic CVMR, but decreased hypercapnic CVMR. The effects of AET on cognitive performance were minimal when compared with SAT. Across both groups, there was a negative correlation between changes in hypocapnic and hypercapnic CVMRs in CBFV% and CVCi% (r = -0.741, r = -0.725, P < 0.001). Attenuated hypercapnic CVMR, but not increased hypocapnic CVMR, was associated with improved cognitive test scores in the AET group. In conclusion, 1-yr AET increased hypocapnic CVMR and attenuated hypercapnic CVMR which is associated cognitive performance in patients with amnestic MCI.NEW & NOTEWORTHY One-year moderate-to-vigorous intensity aerobic exercise training (AET) improved cardiorespiratory fitness (V̇o2peak), increased hypocapnic cerebral vasomotor reactivity (CVMR), whereas it decreased hypercapnic CVMR when compared with stretching and toning in patients with amnestic mild cognitive impairment (MCI). Furthermore, changes in hypercapnic CVMR with AET were correlated with improved memory and executive function. These findings indicate that AET has an impact on cerebrovascular function which may benefit cognitive performance in older adults who have high risk of Alzheimer's disease.

Protocol-dependence of middle cerebral artery dilation to modest hypercapnia

There is a need for improved understanding of how different cerebrovascular reactivity (CVR) protocols affect vascular cross-sectional area (CSA) to reduce error in CVR calculations when measures of vascular CSA are not feasible. In human participants, we delivered ∼±4 mm Hg end-tidal partial pressure of CO₂ (P_ETCO₂) relative to baseline through controlled delivery, and measured changes in middle cerebral artery (MCA) CSA (7 Tesla magnetic resonance imaging (MRI)), blood velocity (transcranial Doppler and Phase contrast MRI), and calculated CVR based on a 3-minute steady-state (+4 mm Hg P_ETCO₂) and a ramp (-3 to +4 mm Hg of P_ETCO₂). We observed that (1) the MCA did not dilate during the ramp protocol (slope for CSA across time P > 0.05; R² = 0.006), but did dilate by ∼7% during steady-state hypercapnia (P < 0.05); and (2) MCA blood velocity CVR was not different between ramp and steady-state hypercapnia protocols (ramp: 3.8 ± 1.7 vs. steady-state: 4.0 ± 1.6 cm/s/mm Hg), although calculated MCA blood flow CVR was ∼40% greater during steady-state hypercapnia than during ramp (P < 0.05) with the discrepancy due to MCA CSA changes during steady-state hypercapnia. We propose that a ramp model, across a delta of -3 to +4 mm Hg P_ETCO₂, may provide an alternative approach to collecting CVR measures in young adults with transcranial Doppler when CSA measures are not feasible. Novelty: We optimized a magnetic resonance imaging sequence to measure dynamic middle cerebral artery (MCA) cross-sectional area (CSA). A ramp model of hypercapnia elicited similar MCA blood velocity reactivity as the steady-state model while maintaining MCA CSA.

Effect of 3% CO2 inhalation on respiratory exchange ratio and cardiac output during constant work-rate exercise

BACKGROUND

The aim of this study was to examine whether the decrease in respiratory exchange ratio (RER) during constant work-rate exercise (CWE) with 3% carbon dioxide (CO<inf>2</inf>) inhalation could be caused by the combination of the decrease in CO<inf>2</inf> output (V̇CO<inf>2</inf>) and the increase in oxygen uptake (V̇O<inf>2</inf>). In addition, we investigated the effect of 3% CO<inf>2</inf> inhalation on cardiac output (Q̇) during CWE.

METHODS

Seven males (V̇O<inf>2max</inf>: 44.1±6.4 mL/min/kg) carried out transitions from low-load cycling (baseline; 40w) to light intensity exercise (45% V̇O<inf>2 max</inf>; 89.3±12.5 W) and heavy intensity exercise (80% V̇O<inf>2max</inf>; 186.5±20.2 W) while inhaling normal air (Air) or an enriched CO<inf>2</inf> gas (3% CO<inf>2</inf>, 21% O<inf>2</inf>, balance N<inf>2</inf>). Each exercise session was 6 min, and respiratory responses by Douglas bag technique and cardiac responses by thoracic bio-impedance method were measured during the experiment.

RESULTS

Ventilation for 3% CO<inf>2</inf> was higher than for air through the experiment (P<0.05). Steady and non-steady state RER and V̇CO<inf>2</inf> for 3% CO<inf>2</inf> were less than for air in both light and heavy intensities (P<0.05), but V̇O<inf>2</inf> and Q̇ did not differ between the two conditions.

CONCLUSIONS

3% CO<inf>2</inf> inhalation induced the decrease in RER during CWE at light and heavy intensities, which was due to the decrease in V̇CO<inf>2</inf>. The promoted ventilation with 3% CO<inf>2</inf> did not lead to the increase in V̇O<inf>2</inf>. Moreover, 3% CO<inf>2</inf> inhalation did not affect Q̇ during CWE at light and heavy intensities.

The Valsalva maneuver: an indispensable physiological tool to differentiate intra versus extracranial near-infrared signal

Developing near-infrared spectroscopy (NIRS) parameter recovery techniques to more specifically resolve brain physiology from that of the overlying tissue is an important part of improving the clinical utility of the technology. The Valsalva maneuver (VM) involves forced expiration against a closed glottis causing widespread venous congestion within the context of a fall in cardiac output. Due to the specific anatomical confines and metabolic demands of the brain we believe a properly executed VM has the ability to separate haemodynamic activity of brain tissue from that of the overlying scalp as observed by NIRS, and confirmed by functional magnetic resonance imaging (fMRI). Healthy individuals performed a series of standing maximum effort VMs under separate observation by frequency domain near-infrared spectroscopy (FD-NIRS) and fMRI. Nine individuals completed the clinical protocol (6 males, age 21-40). During the VMs, brain and extracranial tissue targeted signal were significantly different (opposite direction of change) in both fMRI and NIRS (p=0.00025 and 0.00115 respectively), with robust cross correlation of parameters between modalities. Four of these individuals performed further VMs after infiltrating 2% xylocaine/1:100,000 epinephrine (vasoconstrictor) into scalp tissue beneath the probes. No significant difference in the cerebrally derived parameters was observed. The maximum effort VM has the ability to separate NIRS observable physiology of the brain from the overlying extracranial tissue. Observations made by this FD cerebral NIRS device are comparable with fMRI in this context.

The CO2 stimulus duration and steady-state time point used for data extraction alters the cerebrovascular reactivity outcome measure

NEW FINDINGS

What is the central question of this study? Cerebrovascular reactivity (CVR) is a common functional test to assess brain health, and impaired CVR has been associated with all-cause cardiovascular mortality: does the duration of the CO₂ stimulus and the time point used for data extraction alter the CVR outcome measure? What is the main finding and its importance? This study demonstrated CVR measures calculated from 1 and 2 min CO₂ stimulus durations were significantly higher than CVR calculated from a 4 min CO₂ stimulus. CVRs calculated from the first 2 min of the CO₂ stimulus were significantly higher than CVR values calculated from the final minute if the duration was ≥4 min. This study highlights the need for consistent methodological approaches.

ABSTRACT

Cerebrovascular reactivity to carbon dioxide (CVR) is a common functional test to assess brain vascular health, though conflicting age and fitness effects have been reported. Studies have used different CO₂ stimulus durations to induce CVR and extracted data from different time points for analysis. Therefore, this study examined whether these differences alter CVR and explain conflicting findings. Eighteen healthy volunteers (24 ± 5 years) inhaled CO₂ for four stimulus durations (1, 2, 4 and 5 min) of 5% CO₂ (in air) via the open-circuit Douglas bag method, in a randomized order. CVR data were derived from transcranial Doppler (TCD) measures of middle cerebral artery blood velocity (MCAv), with concurrent ventilatory sensitivity to the CO₂ stimulus ( V ̇ E , C O 2 ). Repeated measures ANOVAs compared CVR and V ̇ E , C O 2 measures between stimulus durations and steady-state time points. An effect of stimulus duration was observed (P = 0.002, η² = 0.140), with 1 min (P = 0.010) and 2 min (P < 0.001) differing from 4 min, and 2 min differing from 5 min (P = 0.019) durations. V ̇ E , C O 2 sensitivity increased ∼3-fold from 1 min to 4 and 5 min durations (P < 0.001, η² = 0.485). CVRs calculated from different steady-state time points within each stimulus duration were different (P < 0.001, η² = 0.454), specifically for 4 min (P = 0.001) and 5 min (P < 0.001), but not 2 min stimulus durations (P = 0.273). These findings demonstrate that methodological differences alter the CVR measure.

Cerebral vasomotor reactivity during hypo- and hypercapnia across the adult lifespan

Age is the strongest risk factor for cerebrovascular disease; however, age-related changes in cerebrovascular function are still not well understood. The objective of this study was to measure cerebral vasomotor reactivity (CVMR) during hypo- and hypercapnia across the adult lifespan. One hundred fifty-three healthy participants (21-80 years) underwent measurements of cerebral blood flow velocity (CBFV) via transcranial Doppler, mean arterial pressure (MAP) via plethysmograph, and end-tidal CO₂ (EtCO₂) via capnography during hyperventilation (hypocapnia) and a modified rebreathing protocol (hypercapnia). Cerebrovascular conductance (CVCi) and resistance (CVRi) indices were calculated from the ratios of CBFV and MAP. CVMRs were assessed by the slopes of CBFV and CVCi in response to changes in EtCO₂. The baseline CBFV and CVCi decreased and CVRi increased with age. Advanced age was associated with progressive declines in CVMR during hypocapnia indicating reduced cerebral vasoconstriction, but increases in CVMR during hypercapnia indicating increased vasodilation. A negative correlation between hypo- and hypercapnic CVMRs was observed across all subjects (CBFV%/ EtCO₂: r = -0.419, CVCi%/ EtCO₂: r = -0.442, P < 0.0001). Collectively, these findings suggest that aging is associated with decreases in CBFV, increases in cerebrovascular resistance, reduced vasoconstriction during hypocapnia, but increased vasodilatory responsiveness during hypercapnia.

Internal Carotid Artery Blood Flow Response to Anesthesia, Pneumoperitoneum, and Head-up Tilt during Laparoscopic Cholecystectomy

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Little is known about how implementation of pneumoperitoneum and head-up tilt position contributes to general anesthesia-induced decrease in cerebral blood flow in humans. We investigated this question in patients undergoing laparoscopic cholecystectomy, hypothesizing that cardiorespiratory changes during this procedure would reduce cerebral perfusion.

Methods

In a nonrandomized, observational study of 16 patients (American Society of Anesthesiologists physical status I or II) undergoing laparoscopic cholecystectomy, internal carotid artery blood velocity was measured by Doppler ultrasound at four time points: awake, after anesthesia induction, after induction of pneumoperitoneum, and after head-up tilt. Vessel diameter was obtained each time, and internal carotid artery blood flow, the main outcome variable, was calculated. The authors recorded pulse contour estimated mean arterial blood pressure (MAP), heart rate (HR), stroke volume (SV) index, cardiac index, end-tidal carbon dioxide (ETco2), bispectral index, and ventilator settings. Results are medians (95% CI).

Results

Internal carotid artery blood flow decreased upon anesthesia induction from 350 ml/min (273 to 410) to 213 ml/min (175 to 249; −37%, P &lt; 0.001), and tended to decrease further with pneumoperitoneum (178 ml/min [127 to 208], −15%, P = 0.026). Tilt induced no further change (171 ml/min [134 to 205]). ETco2 and bispectral index were unchanged after induction. MAP decreased with anesthesia, from 102 (91 to 108) to 72 (65 to 76) mmHg, and then remained unchanged (Pneumoperitoneum: 70 [63 to 75]; Tilt: 74 [66 to 78]). Cardiac index decreased with anesthesia and with pneumoperitoneum (overall from 3.2 [2.7 to 3.5] to 2.3 [1.9 to 2.5] l · min−1 · m−2); tilt induced no further change (2.1 [1.8 to 2.3]). Multiple regression analysis attributed the fall in internal carotid artery blood flow to reduced cardiac index (both HR and SV index contributing) and MAP (P &lt; 0.001). Vessel diameter also declined (P &lt; 0.01).

Conclusions

During laparoscopic cholecystectomy, internal carotid artery blood flow declined with anesthesia and with pneumoperitoneum, in close association with reductions in cardiac index and MAP. Head-up tilt caused no further reduction. Cardiac output independently affects human cerebral blood flow.

Effects of Resistance Exercise and Nutritional Supplementation on Dynamic Cerebral Autoregulation in Head-Down Bed Rest

Head-down bed rest (HDBR) is commonly considered as ground-based analog to spaceflight and simulates the headward fluid shift and cardiovascular deconditioning associated with spaceflight. We investigated in healthy volunteers whether HDBR, with or without countermeasures, affect cerebral autoregulation (CA). Twelve men (at selection: 34 ± 7 years; 176 ± 7 cm; 70 ± 7 kg) underwent three interventions of a 21-day HDBR: a control condition without countermeasure (CON), a condition with resistance vibration exercise (RVE) comprising of squats, single leg heel, and bilateral heel raises and a condition using also RVE associated with nutritional supplementation (NeX). Cerebral blood flow velocity was assessed using transcranial Doppler ultrasonography. CA was evaluated by transfer function analysis and by the autoregulatory index (Mxa) in order to determine the relationship between mean cerebral blood flow velocity and mean arterial blood pressure. In RVE condition, coherence was increased after HDBR. In CON condition, Mxa index was significantly reduced after HDBR. In contrast, in RVE and NeX conditions, Mxa were increased after HBDR. Our results indicate that HDBR without countermeasures may improve dynamic CA, but this adaptation may be dampened with RVE. Furthermore, nutritional supplementation did not enhance or worsen the negative effects of RVE. These findings should be carefully considered and could not be applied in spaceflight. Indeed, the subjects spent their time in supine position during bed rest, unlike the astronauts who perform normal daily activities.

Racial disparities in cardiovascular disease risk: mechanisms of vascular dysfunction

Cardiovascular disease (CVD) accounts for a third of all deaths in the United States making it the leading cause of morbidity and mortality. Although CVD affects individuals of all races/ethnicities, the prevalence of CVD is highest in non-Hispanic black (BL) individuals relative to other populations. The mechanism(s) responsible for elevated CVD risk in the BL population remains incompletely understood. However, impaired vascular vasodilator capacity and exaggerated vascular vasoconstrictor responsiveness are likely contributing factors, both of which are present even in young, otherwise healthy BL individuals. Within this review, we highlight some historical and recent data, collected from our laboratories, of impaired vascular function, in terms of reduced vasodilator capacity and heightened vasoconstrictor responsiveness, in the peripheral and cerebral circulations in BL individuals. We provide data that such impairments may be related to elevated oxidative stress and subsequent reduction in nitric oxide bioavailability. In addition, divergent mechanisms of impaired vasodilatory capacity between BL men and women are discussed. Finally, we propose several directions where future research is needed to fill in knowledge gaps, which will allow for better understanding of the mechanisms contributing to impaired vascular function in this population. Ultimately, this information will allow for better lifestyle and therapeutic approaches to be implemented in an effort to minimize the increased CVD burden in the BL population.

Reactivity of larger intracranial arteries using 7 T MRI in young adults

The larger intracranial conduit vessels contribute to the total cerebral vascular resistance, and understanding their vasoreactivity to physiological stimuli is required when attempting to understand regional brain perfusion. Reactivity of the larger cerebral conduit arteries remains understudied due to a need for improved imaging methods to simultaneously assess these vessels in a single stimulus. We characterized reactivity of basal intracranial conduit arteries (basilar, right and left posterior, middle and anterior cerebral arteries) and the right and left internal carotid arteries, to manipulations in end-tidal CO₂ (PetCO₂). Cross-sectional area changes (%CSA) were evaluated from high-resolution (0.5 mm isotropic) images collected at 7 T using a T1-weighted 3D SPACE pulse sequence, providing high contrast between vessel lumen and surrounding tissue. Cerebrovascular reactivity was calculated as %CSA/ΔPetCO₂ in eight healthy individuals (18-23 years) during normocapnia (41 ± 4 mmHg), hypercapnia (48 ± 4 mmHg; breathing 5% CO₂, balance oxygen), and hypocapnia (31 ± 8 mmHg; via hyperventilation). Reactivity to hypercapnia ranged from 0.8%/mmHg in the right internal carotid artery to 2.7%/mmHg in the left anterior cerebral artery. During hypocapnia, vasoconstriction ranged from 0.9%/mmHg in the basilar artery to 2.6%/mmHg in the right posterior cerebral artery. Heterogeneous cerebrovascular reactivity to hypercapnia and hypocapnia was characterized across basal intracranial conduit and internal carotid arteries.

Fast-food meal reduces peripheral artery endothelial function but not cerebral vascular hypercapnic reactivity in healthy young men

Consumption of a representative fast-food meal (FFMeal) acutely impairs peripheral conduit artery vascular function; however, the effect on cerebral vascular function remains unknown. This study tested the hypothesis that a FFMeal would impair cerebral vascular function as indexed by an attenuated increase in cerebral vascular conductance (CVCI) in the middle cerebral artery (MCA) during a hypercapnic challenge. Ten healthy men (age: 24 ± 3 years, BMI: 24.3 ± 3.8 kg/m2 ) were studied under two conditions; a standardized FFMeal (990 kcals, 50% fat, 36% carbohydrate, 14% protein, and 2120 mg sodium) and a fasting control condition. Basal hemodynamics, cerebral vasomotor reactivity (CVMR), and brachial artery flow-mediated dilation (BA FMD) were completed after an overnight fast (Pre) and again 2 h and 4 h later both days. To assess CVMR, subjects rebreathed from a 5-L bag while MCA velocity (MCAVmean ) was measured using transcranial Doppler (TCD) ultrasound and converted into CVCI (MCAVmean /mean arterial pressure). Peripheral artery endothelial function was assessed via BA FMD following a standard 5-min occlusion protocol. As expected, BA FMD was reduced at 2 h (Pre: 6.6 ± 1.7% vs. 5.2 ± 1.8%, P = 0.01). However, despite significant impairment in BA FMD, neither peak CVCI%baseline nor CVMR was affected by the FFMeal (Control-Pre: 1.9 ± 1.1, 2 h: 2.1 ± 1.1, 4 h: 1.7 ± 1.1 ∆CVCI%·∆PET CO2-1 vs. FFMeal-Pre: 2.1 ± 1.1, 2 h: 2.2 ± 0.7, 4 h: 1.9 ± 0.9 ∆CVCI%·∆PET CO2-1 , time × condition P = 0.88). These results suggest that cerebral vascular reactivity to hypercapnia in healthy young men is not altered by an acute FFMeal.

Dynamic cerebral autoregulation is preserved during isometric handgrip and head-down tilt in healthy volunteers

In healthy humans, cerebral blood flow (CBF) is autoregulated against changes in arterial blood pressure. Spontaneous fluctuations in mean arterial pressure (MAP) and CBF can be used to assess cerebral autoregulation. We hypothesized that dynamic cerebral autoregulation is affected by changes in autonomic activity, MAP, and cardiac output (CO) induced by handgrip (HG), head‐down tilt (HDT), and their combination. In thirteen healthy volunteers, we recorded blood velocity by ultrasound in the internal carotid artery (ICA), HR, MAP and CO‐estimates from continuous finger blood pressure, and end‐tidal CO₂. Instantaneous ICA beat volume (ICABV, mL) and ICA blood flow (ICABF, mL/min) were calculated. Wavelet synchronization index γ (0–1) was calculated for the pairs: MAP–ICABF, CO–ICABF and HR–ICABV in the low (0.05–0.15 Hz; LF) and high (0.15–0.4 Hz; HF) frequency bands. ICABF did not change between experimental states. MAP and CO were increased during HG (+16% and +15%, respectively, P < 0.001) and during HDT + HG (+12% and +23%, respectively, P < 0.001). In the LF interval, median γ for the MAP–ICABF pair (baseline: 0.23 [0.12–0.28]) and the CO–ICABF pair (baseline: 0.22 [0.15–0.28]) did not change with HG, HDT, or their combination. High γ was observed for the HR–ICABV pair at the respiratory frequency, the oscillations in these variables being in inverse phase. The unaltered ICABF and the low synchronization between MAP and ICABF in the LF interval suggest intact dynamic cerebral autoregulation during HG, HDT, and their combination.

Association between aerobic fitness and cerebrovascular function with neurocognitive functions in healthy, young adults

Studies of the effects of physical activity on cognition suggest that aerobic fitness can improve cognitive abilities. However, the physiological mechanisms for the cognitive benefit of aerobic fitness are less well understood. We examined the association between aerobic fitness and cerebrovascular function with neurocognitive functions in healthy, young adults. Participants aged 18-29 years underwent measurements of cerebral vasomotor reactivity (CVMR) in response to rebreathing-induced hypercapnia, maximal oxygen uptake (VO₂max) during cycle ergometry to voluntary exhaustion, and simple- and complex-neurocognitive assessments at rest. Ten subjects were identified as having low-aerobic fitness (LF < 15th fitness percentile), and twelve subjects were identified as having high-aerobic fitness (HF > 80th fitness percentile). There were no LF versus HF group differences in cerebrovascular hemodynamics during the baseline condition. Changes in middle cerebral artery blood velocity and CVMR during hypercapnia were elevated more in the HF than the LF group. Compared to the LF, the HF performed better on a complex-cognitive task assessing fluid reasoning, but not on simple attentional abilities. Statistical modeling showed that measures of VO₂max, CVMR, and fluid reasoning were positively inter-correlated. The relationship between VO₂max and fluid reasoning, however, did not appear to be reliably mediated by CVMR. In conclusion, a high capacity for maximal oxygen uptake among healthy, young adults was associated with greater CVMR and better fluid reasoning, implying that high-aerobic fitness may promote cerebrovascular and cognitive functioning abilities.

Carbon dioxide-mediated vasomotion of extra-cranial cerebral arteries in humans: a role for prostaglandins?

KEY POINTS

Cerebral blood flow increases during hypercapnia and decreases during hypocapnia; it is unknown if vasomotion of the internal carotid artery is implicated in these responses. Indomethacin, a non-selective cyclooxygenase inhibitor (used to inhibit prostaglandin synthesis), has a unique ability to blunt cerebrovascular carbon dioxide reactivity, while other cyclooxygenase inhibitors have no effect. We show significant dilatation and constriction of the internal carotid artery during hypercapnia and hypocapnia, respectively. Indomethacin, but not ketorolac or naproxen, reduced the dilatatory response of the internal carotid artery to hypercapnia The differential effect of indomethacin compared to ketorolac and naproxen suggests that indomethacin inhibits vasomotion of the internal carotid artery independent of prostaglandin synthesis inhibition.

ABSTRACT

Extra-cranial cerebral blood vessels are implicated in the regulation of cerebral blood flow during changes in arterial CO2 ; however, the mechanisms governing CO2 -mediated vasomotion of these vessels in humans remain unclear. We determined if cyclooxygenase inhibition with indomethacin (INDO) reduces the vasomotor response of the internal carotid artery (ICA) to changes in end-tidal CO2 (P ETC O2). Using a randomized single-blinded placebo-controlled study, participants (n = 10) were tested on two occasions, before and 90 min following oral INDO (1.2 mg kg(-1) ) or placebo. Concurrent measurements of beat-by-beat velocity, diameter and blood flow of the ICA were made at rest and during steady-state stages (4 min) of iso-oxic hypercapnia (+3, +6, +9 mmHg P ETC O2) and hypocapnia (-3, -6, -9 mmHg P ETC O2). To examine if INDO affects ICA vasomotion independent of cyclooxygenase inhibition, two participant subsets (each n = 5) were tested before and following oral ketorolac (post 45 min, 0.25 mg kg(-1) ) or naproxen (post 90 min, 4.2 mg kg(-1) ). During pre-drug testing in the INDO trial, the ICA dilatated during hypercapnia at +6 mmHg (4.72 ± 0.45 vs. 4.95 ± 0.51 mm; P < 0.001) and +9 mmHg (4.72 ± 0.45 mm vs. 5.12 ± 0.47 mm; P < 0.001), and constricted during hypocapnia at -6 mmHg (4.95 ± 0.33 vs. 4.88 ± 0.27 mm; P < 0.05) and -9 mmHg (4.95 ± 0.33 vs. 4.82 ± 0.27 mm; P < 0.001). Following INDO, vasomotor responsiveness of the ICA to hypercapnia was reduced by 67 ± 28% (0.045 ± 0.015 vs. 0.015 ± 0.012 mm mmHg P ETC O2(-1) ). There was no effect of the drug in the ketorolac and naproxen trials. We conclude that: (1) INDO markedly reduces the vasomotor response of the ICA to changes in P ETC O2; and (2) INDO may be reducing CO2 -mediated vasomotion via a mechanism(s) independent of cyclooxygenase inhibition.