Impact of age on cerebrovascular dilation versus reactivity to hypercapnia.

This study quantified the effect of age on cerebrovascular reactivity and cerebrovascular conductance while accounting for differences in grey matter volume in younger (YA: n = 12; 24 ± 4 years, six females) and older adults (OA: n = 10; 66 ± 7 years; five females). Cerebral blood flow velocity (CBFV; transcranial Doppler) in the middle cerebral artery (MCA), MCA cross-sectional area (CSA), intracranial volumes (magnetic resonance imaging), and mean arterial pressure (MAP; Finometer), were measured under normocapnic and hypercapnic (6% carbon dioxide) conditions. Cerebral blood flow (CBF) was quantified from CBFV and MCA CSA and normalized to grey matter volume. Grey matter volume was 719 ± 98 mL in YA and 622 ± 50 mL in OA (P = 0.009). Cerebrovascular reactivity (%ΔCBF/Δ

Acute changes in forearm vascular compliance during transient sympatho-excitation

The study of vascular regulation often omits important information about the elastic properties of arteries under conditions of pulsatile flow. The purpose of this study was to examine the relationship between muscle sympathetic nerve activity (MSNA), vascular bed compliance, and peripheral blood flow responses in humans. We hypothesized that increases in MSNA would correlate with reductions in vascular compliance, and that changes in compliance would correspond with changes in peripheral blood flow during sympatho-excitation. MSNA (microneurography), blood pressure (Finopres), and brachial artery blood flow (Doppler ultrasound), were monitored in six healthy males at baseline and during the last 15 s of voluntary end-inspiratory, expiratory apneas and 5 min of static handgrip exercise (SHG; 20% maximum voluntary contraction) and 3 min of post-exercise circulatory occlusion (SHG + PECO; measured in the non-exercising arm). A lumped Windkessel model was employed to examine vascular bed compliance. During apnea, indices of MSNA were inversely related with vascular compliance, and reductions in compliance correlated with decreased brachial blood flow rate. During SHG, despite increased MSNA, compliance also increased, but was unrelated to increases in blood flow. Neither during SHG nor PECO did indices of MSNA correlate with forearm vascular compliance nor did vascular compliance correlate with brachial flow. However, during PECO, a linear combination of blood pressure and total MSNA was correlated with vascular compliance. These data indicate the elastic components of the forearm vasculature are regulated by adrenergic and myogenic mechanisms during sympatho-excitation, but in a reflex-dependent manner

Changes in volumetric and metabolic parameters relate to differences in exposure to sub-concussive head impacts

Structural and calibrated magnetic resonance imaging data were acquired on 44 collegiate football players prior to the season (PRE), following the first four weeks in-season (PTC) and one month after the last game (POST). Exposure data collected from g-Force accelerometers mounted to the helmet of each player were used to split participants into HIGH (N = 22) and LOW (N = 22) exposure groups, based on the frequency of impacts sustained by each athlete. Significant decreases in grey-matter volume specific to the HIGH group were documented at POST (P = 0.009), compared to baseline. Changes in resting cerebral blood flow (CBF0), corrected for partial volume effects, were observed within the HIGH group, throughout the season (P < 0.0001), suggesting that alterations in perfusion may follow exposure to sub-concussive collisions. Co-localized significant increases in cerebral metabolic rate of oxygen consumption (CMRO2|0) mid-season were also documented in the HIGH group, with respect to both PRE- and POST values. No physiological changes were observed in the LOW group. Therefore, cerebral metabolic demand may be elevated in players with greater exposure to head impacts. These results provide novel insight into the effects of sub-concussive collisions on brain structure and cerebrovascular physiology and emphasize the importance of multi-modal imaging for a complete characterization of cerebral health

Heterogeneous patterns of vasoreactivity in the middle cerebral and internal carotid arteries

© 2015 the American Physiological Society. This study compared changes in cross-sectional area (CSA) and flow (Q) between the middle cerebral artery (MCA) and the internal carotidartery (ICA) at baseline and during 5 min of hypercapnia (HC; 6% CO2) and hypocapnia (HO; hyperventilation) and quantified how these changes contribute to estimates of cerebrovascular reactivity (CVR). Measures of MCA CSA were made using 3T magnetic resonance imaging. On a separate day, MCA flow velocity was measured with transcranial Doppler ultrasound and ICA diameters and flow velocity were measured with duplex ultrasound. Fourteen subjects (23 ± 3 yr, 7 females) participated, providing data for 11 subjects during HC and 9 subjects during HO. An increase in MCA CSA (P \u3c 0.05) was observed within the first minute of HC. During HO, the decrease in MCA CSA (P \u3c 0.05) was delayed until minute 4. No changes were observed in ICA CSA during HC or HO. The relative changes in QICA and QMCA were similar during HC and HO. Therefore, the MCA, but not ICA, dilates and constricts during 5 min of HC and HO, respectively. The consequent impact on QMCA significantly affects estimates of CVR, and reactivity cannot be attributed solely to changes in smaller arterioles

Resting CMRO2 fluctuations show persistent network hyper-connectivity following exposure to sub-concussive collisions

Exposure to head impacts may alter brain connectivity within cortical hubs such as the default-mode network (DMN). However, studies have yet to consider the confounding effects of altered resting cerebral blood flow (CBF0) and cerebrovascular reactivity (CVR) on changes in connectivity following sub-concussive impacts. Here, 23 Canadian collegiate football players were followed during a season using calibrated resting-state MRI and helmet accelerometers to examine the interplay between the neural and vascular factors that determine functional connectivity (FC). Connectivity-based analyses using blood oxygen level dependent (BOLD) and cerebral metabolic rate of oxygen consumption (CMRO2) mapping were used to study the DMN longitudinally. Network-specific decreases in CBF0 were observed one month following the season, while impaired CVR was documented at both mid-season and one month following the season, compared to pre-season baseline. Alterations in CBF0 and BOLD-based CVR throughout the season suggest that neurophysiological markers may show different susceptibility timelines following head impacts. DMN connectivity was increased throughout the season, independent of changes in cerebrovascular physiology, suggesting that alterations in FC following sub-concussive impacts are robust and independent of changes in brain hemodynamics. No significant correlations between impact kinematics and DMN connectivity changes were documented in this study. Altogether, these findings create a strong paradigm for future studies to examine the underlying neural and vascular mechanisms associated with increases in network connectivity following repeated exposure to sub-concussive collisions, in an effort to improve management of head impacts in contact sports. Keywords: Calibrated resting-state, Cerebrovascular physiology, Functional connectivity, Helmet accelerometers, Sub-concussive collision

Baroreflex sensitivity is reduced in adolescents with probable developmental coordination disorder

Developmental coordination disorder (DCD) is a neurodevelopmental condition characterized by poor motor skills leading to a significant impairment in activities of daily living. Compared to typically developing children, those with DCD are less fit and physically active, and have increased body fat. This is an important consequence as both sedentary lifestyle and obesity are risk factors for cardiovascular disease. One indicator of cardiovascular health is baroreflex sensitivity (BRS), which is a measure of short term blood pressure (BP) regulation and is partly accomplished through changes in heart rate. Diminished BRS is predictive of future cardiovascular morbidity and mortality. The purpose of this study was to compare BRS in typically developing (TD) adolescents with probable DCD (pDCD) or suspect pDCD (spDCD) adolescents (13-14 years of age). Percentile scores on the Movement Assessment Battery for Children, 2nd edition, assessed at two time points were averaged and used to classify participants into the following groups: pDCD≤5th percentile, spDCD>5th percentile and ≤16th percentile and TD>16th percentile. Following 15min of supine rest, 5min of continuous beat-by-beat blood pressure (Finapres) and R-R interval were recorded (standard ECG). Spectral indices were computed using Fast Fourier Transform with transfer function analysis used to compute BRS in the low frequency region (0.04-0.15Hz). BRS was compared between groups with an ANOVA and post hoc Bonferroni correction. BRS was reduced in the pDCD compared to the TD groups. In multivariate regression analyses predicting BRS, when pDCD and spDCD were entered as the only variables, pDCD was found to be a significant predictor of BRS (b=-6.74, p=0.016). However, when sex, VO peak, and percent body fat (PBF) were entered as covariates, pDCD was no longer a predictor, while PBF approached significance (-0.32, p=0.056). Therefore, in this sample, BRS was reduced in adolescents with pDCD principally due to increased PBF

Cerebral blood flow velocity underestimates cerebral blood flow during modest hypercapnia and hypocapnia.

To establish the accuracy of transcranial Doppler ultrasound (TCD) measures of middle cerebral artery (MCA) cerebral blood flow velocity (CBFV) as a surrogate of cerebral blood flow (CBF) during hypercapnia (HC) and hypocapnia (HO), we examined whether the cross-sectional area (CSA) of the MCA changed during HC or HO and whether TCD-based estimates of CBFV were equivalent to estimates from phase contrast (PC) magnetic resonance imaging. MCA CSA was measured from 3T magnetic resonance images during baseline, HO (hyperventilation at 30 breaths/min), and HC (6% carbon dioxide). PC and TCD measures of CBFV were measured during these protocols on separate days. CSA and TCD CBFV were used to calculate CBF. During HC, CSA increased from 5.6 ± 0.8 to 6.5 ± 1.0 mm(2) (P \u3c 0.001, n = 13), while end-tidal carbon dioxide partial pressure (PETCO2) increased from 37 ± 3 to 46 ± 5 Torr (P \u3c 0.001). During HO, CSA decreased from 5.8 ± 0.9 to 5.3 ± 0.9 mm(2) (P \u3c 0.001, n = 15), while PetCO2 decreased from 36 ± 4 to 23 ± 3 Torr (P \u3c 0.001). CBFVs during baseline, HO, and HC were compared between PC and TCD, and the intraclass correlation coefficient was 0.83 (P \u3c 0.001). The relative increase from baseline was 18 ± 8% greater (P \u3c 0.001) for CBF than TCD CBFV during HC, and the relative decrease of CBF during HO was 7 ± 4% greater than the change in TCD CBFV (P \u3c 0.001). These findings challenge the assumption that the CSA of the MCA does not change over modest changes in PETCO2

Longitudinal assessment of left ventricular structure and function in adolescents with developmental coordination disorder

Children with developmental coordination disorder (DCD) are more likely to develop cardiovascular disease (CVD) risk factors such as obesity and reduced cardio-respiratory fitness. It has also been shown that adolescents with probable DCD (p-DCD) have elevated cardiac output (CO) and stroke volume (SV) compared to typically developing (TD) controls, which in turn may heighten their risk of developing elevated left ventricle mass (LVM) or left ventricular hypertrophy (LVH). The purpose of this study was to assess left ventricular structure and function longitudinally in adolescents with and without p-DCD. This three year study included 86 adolescents with significant motor impairment (33) and TD controls (53). Adolescents were 12 years old at the beginning of the study. The Movement ABC test (M-ABC-2) was used to classify children as p-DCD. Cardiac dimensions were measured using ultrasound echocardiography. Body mass, fat mass (FM) and body mass index (BMI) were significantly elevated in the p-DCD group in all three years. Peak aerobic fitness normalized to fat-free mass (peak VO ) was significantly elevated in the TD controls in each year. Heart rate was also increased in the p-DCD group in years one and three. A repeated measures ANCOVA with time-varying covariates was performed for CO and LVM on p-DCD while controlling for peak VO and FFM. CO and LVM were significantly elevated in the p-DCD which remained constant over time. FM completely mediated the association between p-DCD and CO in adolescents. For LVM, both FM and CO accounted for elevated LVM in adolescents with p-DCD. In conclusion, elevated FM in adolescents with p-DCD contributes to a higher CO and LVM over time compared to TD controls. If this persists throughout adolescents and into adulthood, these adolescents may be at risk of developing LVH

The association between baroreflex sensitivity and blood pressure in children

Low baroreflex sensitivity (BRS) and heart rate variability (HRV) are associated with the pathogenesis of adult hypertension. However, limited information exists about the negative consequences of elevated childhood blood pressure (BP) and autonomic regulation. Additionally, there are developmental changes in autonomic regulation throughout puberty, yet studies have not appropriately accounted for this. The objective of this study was to investigate BRS and HRV in children with 2 different BP profiles, while controlling for the effects of maturation, age, sex, and body composition. A sample of 11- to 14-year-old participants were divided into 2 BP groups: high BP (HBP; ≥95th percentile; n = 21) and normal BP (NBP