Chloe E. Taylor

Post-exercise blood pressure reduction is greater following intermittent than continuous exercise and is influenced less by diurnal variation

Recently, we reported that circadian variation exists in the response of blood pressure (BP) following a bout of uninterrupted exercise. The usual phenomenon of post‐exercise hypotension was absent or reversed when such exercise was performed between 04:00–08:00 h. Nevertheless, research examining BP changes following bouts of intermittent exercise at different times of the day is scarce, even though this type of activity is probably more popular. Therefore, we aimed to compare post‐exercise BP reductions of continuous (CONT) and intermittent (INT) exercise protocols performed at 08:00 h and 16:00 h. At both of these times of day, eight normotensive males completed 30 min of continuous cycling in the CONT and three 10 min bouts of cycling separated by 10 min of rest in the INT protocol. The exercise intensity was set at 70% V˙O2peak during both protocols. Heart rate, systolic (S) and diastolic (D) BP, and mean arterial pressure (MAP) were measured 5 min before and 20 min after exercise. Changes from pre‐exercise baseline were analyzed using linear mixed modeling. MAP was 8±1 mm Hg lower following INT compared with CONT exercise (p<0.05). SBP and DBP were also significantly lower following INT compared with CONT exercise (p<0.05). Diurnal variation in MAP was evident, with attenuated hypotension being observed after morning exercise (p<0.05), although this diurnal variation was less marked following INT compared with CONT exercise (p<0.05). We conclude that intermittent exercise mediates greater post‐exercise hypotension compared with a single continuous bout of equivalent work and that this protocol‐dependent difference is greatest in the afternoon. Therefore, a bout of afternoon exercise that is occasionally interrupted with short rest periods is recommended for lowering BP acutely.

Contribution of arterial Windkessel in low-frequency cerebral hemodynamics during transient changes in blood pressure

The Windkessel properties of the vasculature are known to play a significant role in buffering arterial pulsations, but their potential importance in dampening low-frequency fluctuations in cerebral blood flow has not been clearly examined. In this study, we quantitatively assessed the contribution of arterial Windkessel (peripheral compliance and resistance) in the dynamic cerebral blood flow response to relatively large and acute changes in blood pressure. Middle cerebral artery flow velocity (MCA(V); transcranial Doppler) and arterial blood pressure were recorded from 14 healthy subjects. Low-pass-filtered pressure-flow responses (<0.15 Hz) during transient hypertension (intravenous phenylephrine) and hypotension (intravenous sodium nitroprusside) were fitted to a two-element Windkessel model. The Windkessel model was found to provide a superior goodness of fit to the MCA(V) responses during both hypertension and hypotension (R² = 0.89 ± 0.03 and 0.85 ± 0.05, respectively), with a significant improvement in adjusted coefficients of determination (P < 0.005) compared with the single-resistance model (R² = 0.62 ± 0.06 and 0.61 ± 0.08, respectively). No differences were found between the two interventions in the Windkessel capacitive and resistive gains, suggesting similar vascular properties during pressure rise and fall episodes. The results highlight that low-frequency cerebral hemodynamic responses to transient hypertension and hypotension may include a significant contribution from the mechanical properties of vasculature and, thus, cannot solely be attributed to the active control of vascular tone by cerebral autoregulation. The arterial Windkessel should be regarded as an important element of dynamic cerebral blood flow modulation during large and acute blood pressure perturbation.

Diurnal Variation in the Mechanical and Neural Components of the Baroreflex

Diminished baroreflex sensitivity in the morning negatively influences morning coronary blood flow and blood pressure control in hypertensive patients. Our aim was to determine the contribution of the mechanical and neural components of the cardiac baroreflex to diurnal variation in blood pressure control. In 12 healthy participants, we used the modified Oxford method to quantify baroreflex sensitivity for rising (Gup) and falling (Gdown) pressures in the morning (7:00 AM) and afternoon (4:00 PM). Beat-to-beat blood pressure, R-R intervals, and carotid artery diameter measurements were recorded. Integrated sensitivity was determined by plotting R-R intervals against systolic blood pressure. The mechanical component was carotid artery diameter plotted against systolic blood pressure, and the neural component was R-R intervals plotted against carotid artery diameter. Linear mixed models were used to compare the integrated, mechanical, and neural sensitivities between morning and afternoon. We found significant diurnal variation in integrated sensitivity, with an attenuated response in the morning (Gup=13.0±0.6; Gdown=6.3±0.4 ms/mm Hg) when compared with the afternoon (Gup=15.1±0.6; Gdown=12.6±0.4 ms/mm Hg). For rising pressures, the diminished integrated sensitivity in the morning was caused by a reduction in mechanical sensitivity, whereas for falling pressures it was caused by a reduction in neural sensitivity. Our findings explicate the mechanisms underlying diurnal variation in baroreflex function. Pharmacological and lifestyle interventions targeted specifically at the diminished component of the cardiac baroreflex in the morning may lead to better management of hypertension.

Blood pressure status and post-exercise hypotension: an example of a spurious correlation in hypertension research?

A single bout of exercise lowers blood pressure (BP) for up to 24 h afterwards. The magnitude of this post-exercise hypotension (PEH) has been reported to be correlated most strongly to pre-exercise BP, and this apparent relationship has influenced position statements about the value of exercise in arterial hypertension. Nevertheless, this correlation could be adversely affected by mathematical coupling and regression-to-the-mean artefacts. Therefore, we aimed to examine the degree to which BP status moderates PEH while, for the first time, controlling for these statistical artefacts. A total of 32 participants, with pre-exercise mean arterial pressures of 65–110 mm Hg, cycled for 30 min at 70% peak oxygen uptake. Systolic BP and diastolic BP were measured (Portapres) before exercise and for 20 min after exercise. Changes in BP were regressed against pre-exercise values, and against the mean of pre- and post-exercise BP, among other indices that are also known not to be prone to artefacts. Correlations between pre-exercise BP and the exercise-mediated reductions were typical of those previously reported (r=0.37–0.62, P<0.05), but not large enough to rule out spuriousness (P>0.05). Artefact-free indices of BP status (pre- and post-exercise mean as well as an earlier independent measurement) did not correlate with reductions in BP (P>0.05), which were moderated more by peak oxygen uptake and time of day (P<0.05). These data indicate that, if statistical artefacts are not controlled for, the influence of BP status on the degree of PEH can be spuriously exaggerated to the extent that other more important moderators of BP change are masked.

Appropriate within-subjects statistical models for the analysis of baroreflex sensitivity

An individual’s baroreflex sensitivity is typically described by the relationship between serial manipulations in systolic blood pressure and the changes in pulse interval. Although this experimental approach is essentially within‐subjects in nature, least squares regression (LSR) analysis is typically employed by researchers to derive sensitivity slopes (gains) for individual subjects. These individual gains are then pooled as summary measures for various samples or experimental conditions. We highlight that the underlying assumption for LSR of case independence is violated with such an approach, resulting in possible estimation biases and compromised statistical power. Using a typical data set, we introduce more appropriate analyses based on the linear mixed model, which takes into account the correlated nature of the data at the individual subject level. We encourage researchers to consider the linear mixed model approach because it is more efficient, in that the whole data set is analysed in one step, is associated with less bias and results in greater statistical power compared with conventional analyses of baroreflex sensitivity for samples of subjects.

Pre-Race Dietary Carbohydrate Intake Can Independently Influence Sub-Elite Marathon Running Performance

We examined whether selected anthropometric and nutritional factors influenced field-based marathon running performance. An internet-based data collection tool allowed competitors in the 2009 London Marathon (n=257, mean ± SD age: 39 ± 8 years, finish time: 273.8 ± 59.5 min) to record a range of anthropometric, training and nutritional predictors. Multivariate statistical methods were used to quantify the change in running speed mediated by a unit change in each predictor via the 95% confidence interval for each covariate-controlled regression slope ( B). Gender ( B=1.22 to 1.95 km/h), body mass index ( B=-0.14 to -0.27 km/h), training distance ( B=0.01 to 0.04 km/h) and the amount of carbohydrate consumed the day before the race ( B=0.08 to 0.26 km/h) were significant predictors, collectively accounting for 56% of the inter-individual variability in running speed (P<0.0005). Further covariate-adjusted analysis revealed that those competitors who consumed carbohydrate the day before the race at a quantity of >7 g/kg body mass had significantly faster overall race speeds (P=0.01) and maintained their running speed during the race to a greater extent than with those who consumed <7 g/kg body mass (P=0.02). We conclude that, in addition to gender, body size and training, pre-race day carbohydrate intake can significantly and independently influence marathon running performance.

Relationship between spontaneous sympathetic baroreflex sensitivity and cardiac baroreflex sensitivity in healthy young individuals

Low baroreflex sensitivity (BRS) is associated with elevated cardiovascular risk. However, the evidence is based primarily on measurements of cardiac BRS. It cannot be assumed that cardiac or sympathetic BRS alone represent a true reflection of baroreflex control of blood pressure. The aim of this study was to examine the relationship between spontaneous sympathetic and cardiac BRS in healthy, young individuals. Continuous measurements of blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) were made under resting conditions in 50 healthy individuals (18–28 years). Sympathetic BRS was quantified by plotting MSNA burst incidence against diastolic pressure (sympathetic BRSinc), and by plotting total MSNA against diastolic pressure (sympathetic BRStotal). Cardiac BRS was quantified by plotting R‐R interval against systolic pressure using the sequence method. Significant sympathetic BRSinc and cardiac BRS slopes were obtained for 42 participants. A significant positive correlation was found between sympathetic BRSinc and cardiac BRS (r = 0.31, P = 0.049). Among this group, significant sympathetic baroreflex slopes were obtained for 39 participants when plotting total MSNA against diastolic pressure. In this subset, a significant positive correlation was observed between sympathetic BRStotal and cardiac BRS (r = 0.40, P = 0.012). When males and females were assessed separately, these modest relationships only remained significant in females. Analysis by gender revealed correlations in the females between sympathetic BRSinc and cardiac BRS (r = 0.49, P = 0.062), and between sympathetic BRStotal and cardiac BRS (r = 0.57, P = 0.025). These findings suggest that gender interactions exist in baroreflex control of blood pressure, and that cardiac BRS is not appropriate for estimating overall baroreflex function in healthy, young populations. This relationship warrants investigation in aging and clinical populations.

Postural influences on the mechanical and neural components of the cardiovagal baroreflex

The ability to maintain arterial blood pressure when faced with a postural challenge has implications for the occurrence of syncope and falls. It has been suggested that posture‐induced declines in the mechanical component of the baroreflex response drive reductions in cardiovagal baroreflex sensitivity associated with postural stress. However, these conclusions are largely based upon spontaneous methods of baroreflex assessment, the accuracy of which has been questioned. Therefore, the aim was to engage a partially open‐loop approach to explore the influence of posture on the mechanical and neural components of the baroreflex.

Rate of rise in diastolic blood pressure influences vascular sympathetic response to mental stress

Research indicates that individuals may experience a rise (positive responders) or fall (negative responders) in muscle sympathetic nerve activity (MSNA) during mental stress. In this study, we examined the early blood pressure responses (including the peak, time of peak and rate of rise in blood pressure) to mental stress in positive and negative responders. Negative MSNA responders to mental stress exhibit a more rapid rise in diastolic pressure at the onset of the stressor, suggesting a baroreflex‐mediated suppression of MSNA. In positive responders there is a more sluggish rise in blood pressure during mental stress, which appears to be MSNA‐driven. This study suggests that whether MSNA has a role in the pressor response is dependent upon the reactivity of blood pressure early in the task.

Effect of contraction intensity on sympathetic nerve activity to active human skeletal muscle.

The effect of contraction intensity on muscle sympathetic nerve activity (MSNA) to active human limbs has not been established. To address this, MSNA was recorded from the left peroneal nerve during and after dorsiflexion contractions sustained for 2 min by the left leg at ~10, 25, and 40% MVC. To explore the involvement of the muscle metaboreflex, limb ischemia was imposed midway during three additional contractions and maintained during recovery. Compared with total MSNA at rest (11.5 ± 4.1 mv.min−1), MSNA in the active leg increased significantly at the low (21.9 ± 13.6 mv.min−1), medium (30.5 ± 20.8 mv.min−1), and high (50.0 ± 24.5 mv.min−1) intensities. This intensity-dependent effect was more strongly associated with increases in MSNA burst amplitude than burst frequency. Total MSNA then returned to resting levels within the first minute of recovery. Limb ischemia had no significant influence on the intensity-dependent rise in MSNA or its decline during recovery in the active leg. These findings reveal intensity-dependent increases in total MSNA and burst amplitude to contracting human skeletal muscle that do not appear to involve the muscle metaboreflex.