Forearm blood flow by Doppler ultrasound during test and exercise: tests of day-to-day repeatability

Data analysis technique influences blood flow kinetics parameter estimates for moderate- and heavy-intensity exercise transitions

NEW FINDINGS

What is the central question of this study? During exercise, there are fluctuations in conduit artery blood flow (BF) caused by both cardiac and muscle contraction-relaxation cycles. What is the optimal method to process Doppler ultrasound-measured BF for the purpose of characterizing the dynamic response of BF during step-transitions in exercise? What is the main finding and its importance? Continuous BF data were processed in relation to either cardiac or muscle contraction-relaxation cycles and computed based on 'binned' or 'rolling' averages over one, two or five consecutive cycles. Kinetics characterization revealed no data processing technique-specific differences in steady-state BF, but variability in the rapidity at which BF attained steady-state (i.e., mean response time) was observed.

ABSTRACT

The overall rate of blood flow (BF) adjustment (i.e., kinetics) from the onset of an exercise transition can be quantified by the mean response time (MRT). However, the BF response profile can be distorted during rhythmic, dynamic exercise consequent to variations caused by the cardiac cycle (HR) and the muscle contraction-relaxation (CR) cycle. We examined the extent to which distortions imposed by HR and CR cycles affected BF kinetics. Eight healthy, young men (27 (4) years; mean (SD)) performed transitions of alternate-leg knee-extension exercise from 3 W to either a moderate- (MOD) or heavy-intensity (HVY) power output. Femoral artery BF was continuously measured by Doppler ultrasound and averaged over one, two or five 'binned' (e.g., HR2b, etc.) or 'rolling' (e.g., CR5r, etc.) HR and CR cycles. Among analysis techniques, there were no differences for steady-state BF values at the 3 W baseline. In MOD, MRT using contraction-relaxation cycle (CR1) was smaller than most other analysis techniques. For both MOD and HVY, the 95% confidence interval for MRT was generally larger when using HR- compared to CR-related methods, and monoexponential fits based on 'rolling' averages (HR2r, HR5r, CR2r, CR5r) had a poorer ability to estimate the true end-exercise BF in HVY than in MOD. When modelling BF kinetics, we conclude that the CR1 method is a good option because of its ability to accurately estimate the 'data-determined' end-exercise BF value from the 'model-derived' response, maintain a relatively high density of data points during the transition and yield a relatively small 95% CI.

Intra-rater reliability of leg blood flow during dynamic exercise using Doppler ultrasound

Developing an exercise model that resembles a traditional form of aerobic exercise and facilitates a complete simultaneous assessment of multiple parameters within the oxygen cascade is critically for understanding exercise intolerances in diseased populations. Measurement of muscle blood flow is a crucial component of such a model and previous studies have used invasive procedures to determine blood flow kinetics; however, this may not be appropriate in certain populations. Furthermore, current models utilizing Doppler ultrasound use isolated limb exercise and while these studies have provided useful data, the exercise model does not mimic the whole-body physiological response to continuous dynamic exercise. Therefore, we aimed to measure common femoral artery blood flow using Doppler ultrasound during continuous dynamic stepping exercise performed at three independent workloads to assess the within day and between-day reliability for such an exercise modality. We report a within-session coefficient of variation of 5.8% from three combined workloads and a between-day coefficient of variation of 12.7%. These values demonstrate acceptable measurement accuracy and support our intention of utilizing this noninvasive exercise model for an integrative assessment of the whole-body physiological response to exercise in a range of populations.

Assessment of resistance vessel function in human skeletal muscle: guidelines for experimental design, Doppler ultrasound, and pharmacology

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions.

Habitual exercise training in older adults offsets the age-related prolongation in leg vasodilator kinetics during single-limb lower body exercise

We tested the hypothesis that aging is associated with prolonged leg vasodilator kinetics and habitual exercise training in older adults improves these responses relative to untrained older adults. Additionally, we examined the relationship between contraction-induced rapid onset vasodilation (ROV) and vasodilator kinetics. Young ( n = 10), older untrained ( n = 13), and older trained ( n = 14) adults performed single and rhythmic knee-extension contractions at 20% and 40% work-rate maximum (WR_max). Femoral artery diameter and mean blood velocity were measured by Doppler ultrasound. Vascular conductance (VC; ml·min^-1·mmHg^-1) was calculated using blood flow (ml/min) and mean arterial pressure (mmHg). The primary outcome was the kinetic response (mean response time; MRT), modeled using an exponential model, expressed as the number of duty cycles to change 63% of the steady-state amplitude. There were no age- or training-related differences in VC MRT between the groups at 20% WR_max. Older untrained adults exhibited prolonged VC MRT at 40% WR_max relative to young (37 ± 16 vs. 24 ± 10 duty-cycles; P < 0.05) and older trained adults (37 ± 16 vs. 23 ± 14 duty-cycles; P < 0.05). There were no differences in VC MRT between young and older trained adults at 40% WR_max ( P = 0.96). There were no associations between peak ROV and VC MRT at 20% or 40% WR_max ( r = -0.08 and 0.22; P = 0.67 and 0.20, respectively) in the group as a whole. Our data suggest 1) advancing age prolongs leg vasodilator kinetics; 2) habitual exercise training in older adults offsets this age-related prolongation; and 3) contraction-induced ROV is not related to vasodilator kinetics within a group of young and older adults. NEW & NOTEWORTHY Aging is associated with reductions in exercise hyperemia and vasodilation at the onset of exercise, as well as during steady-state exercise. Habitual endurance exercise training offsets these age-related reductions. We found that aging prolongs vasodilator kinetics in the leg of older untrained but not older trained adults. Finally, our results demonstrate that contraction-induced rapid vasodilation is not associated with vasodilator kinetics within the leg of young and older adults.

Methods for the determination of skeletal muscle blood flow: development, strengths and limitations

Since the first measurements of limb blood flow at rest and during nerve stimulation were conducted in the late 1800s, a number of methods have been developed for the determination of limb and skeletal muscle blood flow in humans. The methods, which have been applied in the study of aspects such as blood flow regulation, oxygen uptake and metabolism, differ in terms of strengths and degree of limitations but most have advantages for specific settings. The purpose of this review is to describe the origin and the basic principles of the methods, important aspects and requirements of the procedures. One of the earliest methods, venous occlusion plethysmography, is a noninvasive method which still is extensively used and which provides similar values as other more direct blood flow methods such as ultrasound Doppler. The constant infusion thermodilution method remains the most appropriate for the determination of blood flow during maximal exercise. For resting blood flow and light-to-moderate exercise, the non-invasive ultrasound Doppler methodology, if handled by a skilled operator, is recommendable. Positron emission tomography with radiolabeled water is an advanced method which requires highly sophisticated equipment and allows for the determination of muscle-specific blood flow, regional blood flows and estimate of blood flow heterogeneity within a muscle. Finally, the contrast-enhanced ultrasound method holds promise for assessment of muscle-specific blood flow, but the interpretation of the data obtained remains uncertain. Currently lacking is high-resolution methods for continuous visualization and monitoring of the skeletal muscle microcirculation in humans.

Reliability of forearm oxygen uptake during handgrip exercise: assessment by ultrasonography and venous blood gas

Assessment of forearm oxygen uptake (V˙O2 ) during handgrip exercise is a keenly investigated concept for observing small muscle mass metabolism. Although a combination of Doppler ultrasound measurements of brachial artery blood flow (Q˙) and blood gas drawn from a deep forearm vein has been utilized to calculate forearm V˙O2 for more than two decades, the applicability of this experimental design may benefit from a thorough evaluation of its reliability during graded exercise. Therefore, we evaluated the reliability of this technique during incremental handgrip exercise in ten healthy young (24 ± 3(SD) years.) males. V˙O2 and work rate (WR) exhibited a linear relationship (1.0 W: 43.8 ± 10.1 mL·min-1 ; 1.5 W: 53.8 ± 14.1 mL·min-1 ; 2.0 W: 63.4 ± 16.3 mL·min-1 ; 2.5 W: 72.2 ± 17.6 mL·min-1 ; 3.0 W: 79.2 ± 18.6 mL·min-1 ; r = 0.65, P < 0.01). In turn, V˙O2 was strongly associated with Q˙ (1.0 W: 359 ± 86 mL·min-1 ; 1.5 W: 431 ± 112 mL·min-1 ; 2.0 W: 490 ± 123 mL·min-1 ; 2.5 W: 556 ± 112 mL·min-1 ; 3.0 W: 622 ± 131 mL·min-1 ; r = 0.96; P < 0.01), whereas arteriovenous oxygen difference (a-vO2diff ) remained constant following all WRs (123 ± 11-130 ± 10 mL·L-1 ). Average V˙O2 test-retest difference was -0.4 mL·min-1 with ±2SD limits of agreement (LOA) of 8.4 and -9.2 mL·min-1 , respectively, whereas coefficients of variation (CVs) ranged from 4-7%. Accordingly, test-retest Q˙ difference was 11.9 mL·min-1 (LOA: 84.1 mL·min-1 ; -60.4 mL·min-1 ) with CVs between 4 and 7%. Test-retest difference for a-vO2diff was -0.28 mL·dL-1 (LOA: 1.26mL·dL-1 ; -1.82 mL·dL-1 ) with 3-5% CVs. In conclusion, our results revealed that forearm V˙O2 determination by Doppler ultrasound and direct venous sampling is linearly related to WR, and a reliable experimental design across a range of exercise intensities.

Blood flow in humans following low-load exercise with and without blood flow restriction

Blood flow restriction (BFR) in combination with exercise has been used to increase muscle size and strength using relatively low loads (20%–30% 1-repetition maximum (1RM)). In research, the range of applied pressures based on a percentage of arterial occlusion pressure (AOP), is wide. The purpose of the study is to measure the blood flow response before exercise, following each set of exercise, and postexercise to low-load elbow flexion combined with no restriction (NOBFR), 40% of AOP (40BFR), and 80% of AOP (80BFR). One hundred and fifty-two participants volunteered; 140 completed the protocol (women = 75, men = 65). Participants were counter-balanced into 1 of 3 conditions. Following AOP and 1RM measurement, ultrasound was used to measure standing blood flow at rest in the right brachial artery. Participants performed 4 sets of elbow flexion at 30% 1RM. Blood flow was measured between sets and at 1 and 5 min postexercise. Blood flow decreased following inflation, with no difference between conditions (p < 0.001). Men had greater blood flow than women in all conditions at all time points (p < 0.001). Resting hyperemia decreased with pressure (NOBFR > 40BFR > 80BFR, p < 0.001). Blood flow increased from rest to after set 1 regardless of condition. Following cuff deflation, blood flow increased in both the 80BFR and 40BFR conditions. The reduction in hyperemia during BFR is pressure-dependent. Contrary to previous investigations, blood flow was increased above baseline following exercise.

Blackcurrant Alters Physiological Responses and Femoral Artery Diameter during Sustained Isometric Contraction

Blackcurrant is rich in anthocyanins that may affect exercise-induced physiological responses. We examined tissue oxygen saturation, muscle activity, cardiovascular responses and femoral artery diameter during a submaximal sustained isometric contraction. In a randomised, double-blind, crossover design, healthy men (n = 13, age: 25 ± 4 years, BMI: 25 ± 3 kg·m^-2, mean ± SD) ingested New Zealand blackcurrant (NZBC) extract (600 mg∙day^-1 CurraNZ™) or placebo (PL) for 7-days separated by 14-days washout. Participants produced isometric maximal voluntary contractions (iMVC) and a 120-s 30%iMVC of the quadriceps with electromyography (EMG), near-infrared spectroscopy, hemodynamic and ultrasound recordings. There was no effect of NZBC extract on iMVC (NZBC: 654 ± 73, PL: 650 ± 78 N). During the 30%iMVC with NZBC extract, total peripheral resistance, systolic, diastolic, and mean arterial pressure were lower with increased cardiac output and stroke volume. With NZBC extract, EMG root mean square of the vastusmedialis and muscle oxygen saturation were lower with higher total haemoglobin. During the 30%iMVC, femoral artery diameter was increased with NZBC extract at 30 (6.9%), 60 (8.2%), 90 (7.7%) and 120 s (6.0%). Intake of NZBC extract for 7-days altered cardiovascular responses, muscle oxygen saturation, muscle activity and femoral artery diameter during a 120-s 30%iMVC of the quadriceps. The present study provides insight into the potential mechanisms for enhanced exercise performance with intake of blackcurrant.

Repeatability of popliteal blood flow and lower limb vascular conductance at rest and exercise during body tilt using Doppler ultrasound

We tested the data repeatability for popliteal blood flow velocity (PBV), popliteal arterial diameter (AD(pop)), popliteal blood flow (PBF) and lower limb vascular conductance (VC) at rest and exercise in three body positions, two work rates and two inspired oxygen fractions. Fifteen, eleven and ten healthy volunteers participated in the three phases of the studies. Resting protocols were performed in horizontal (HOR), 35° head-down tilt (HDT) and 45° head-up tilt (HUT) for 5 min in each body position. Participants also exercised at lower and higher power outputs (repeated plantar flexion contractions at 20% and 30% maximal voluntary contraction, respectively) in HOR, HDT and HUT and in normoxia (21%O2) and hypoxia (14%O2) with the same work rates and body positions. PBV and AD(pop) were measured by ultrasound to determine PBF, and VC was estimated by dividing PBF by muscle perfusion pressure (MPP). PBV, AD(pop), PBF and VC were not different, demonstrated good agreement and consistency between the two days of testing during both rest and exercise conditions regardless of body position. Therefore, these data support the utilization of Doppler and echo Doppler ultrasound as a reproducible method to measure PBV and AD(pop) and consequently estimate PBF and VC responses in such conditions.

Effects of the intensity of leg isometric training on the vasculature of trained and untrained limbs and resting blood pressure in middle-aged men

The purpose of this study was to establish whether changes in resting blood pressure and the vasculature of trained and untrained limbs are dependent on training intensity, following isometric-leg training. Thirty middle-aged males undertook an 8 week training programme (4 × 2 min bilateral-leg isometric contractions 3 times per week). Two groups trained at either high (HI; 14%MVC) or low (LO; 8%MVC) intensity a third group (CON) acted as controls. All parameters were measured at baseline, 4-weeks and post-training. Resting SBP (−10.8 ± 7.9 mmHg), MAP (−4.7 ± 6.8 mmHg) and HR (−4.8 ± 5.9 b·min⁻¹) fell significantly in the HI group post-training with concomitant significant increases in resting femoral mean artery diameter (FMAD; 1.0 ± 0.4 mm), femoral mean blood velocity (FMBV; 0.68 ± 0.83 cm·s⁻¹), resting femoral artery blood flow (FABF; 82.06 ± 31.92 ml·min⁻¹) and resting femoral vascular conductance (FVC, 45%). No significant changes occurred in any brachial artery measure nor in any parameters measured in the LO or CON groups. These findings show that training-induced reductions in resting blood pressure after isometric-leg training in healthy middle-aged men are associated with concomitant adaptations in the local vasculature, that appear to be dependent on training intensity and take place in the later stages of training.

Age- and limb-related differences in the vasoconstrictor response to limb dependency are not mediated by a sympathetic mechanism in humans

AIMS

We tested the hypotheses that vasoconstrictor responses to limb dependency are: (i) greater in the leg than the arm, (ii) impaired with age and (iii) not sympathetically mediated.

METHODS

Vascular responses to limb dependency (i.e. lowering the limb from heart level to 30 cm below heart level) were determined in 17 young and 17 older adults. Indices of blood flow were obtained in the brachial and popliteal arteries (Doppler ultrasound) as well as in the cutaneous circulation (forearm and calf using laser-Doppler flowmetry). Vasoconstriction was quantified by calculating the indices of vascular resistance as height corrected mean arterial pressure/limb blood velocity or skin flux. A second group of subjects repeated the limb dependency trials after acute systemic sympathetic blockade.

RESULTS

Limb dependency increased vascular resistance index in the brachial artery (∆59 ± 8%; P<0.05) and popliteal artery (∆99 ± 10%; P<0.05 for change in heart level and brachial vs. popliteal) of young and older adults (∆60 + 9% brachial and ∆61 ± 7% popliteal arteries; P<0.05 for change in heart level and response in popliteal young vs. older adults). In contrast, cutaneous vasoconstrictor responses to limb dependency were similar in the forearm (∆218 ± 29% and ∆200 ± 29% for young and older adults, respectively) and calf (∆257 ± 32% and ∆236 ± 29%; all P<0.05 from heart level) of young and older adults. Vasoconstrictor responses to limb dependency were not affected by sympathetic blockade in young or older adults.

CONCLUSION

These findings indicate that age-, limb-, and tissue-related differences may exist in the vasoconstrictor response to limb dependency in healthy humans, which are not sympathetically mediated.

Validation of a power law model in upper extremity vessels: potential application in ultrasound bleed detection

Vascular ultrasound can provide quick and reliable diagnosis of arterial bleeding but it requires trained and experienced personnel. Development of automated sonographic bleed detection methods would potentially be valuable for trauma management in the field. We propose a detection method that (1) measures blood flow in a trauma victim, (2) determines the victim's expected normal limb arterial flow using a power law biofluid model where flow is proportional to the vessel diameter taken to a power of k and (3) quantifies the difference between measured and expected flow with a novel metric, flow split deviation (FSD). FSD was devised to give a quantitative value for the likelihood of arterial bleeding and validated in human upper extremities. We used ultrasound to demonstrate that the power law with k = 2.75 appropriately described the normal brachial artery bifurcation geometry and adequately determined the expected normal flows. Our metric was then applied to three-dimensional (3-D) computational models of forearm bleeding and on dialysis patients undergoing surgical construction of wrist arteriovenous fistulas. Computational models showed that larger sized arterial defects produced larger flow deviations. FSD values were statistically higher (paired t-test) for arms with fistulas than those without, with average FSDs of 0.41 ± 0.12 and 0.047 ± 0.021 (mean ± SD), respectively. The average of the differences was 0.36 ± 0.12 (mean ± SD).

Nitric oxide and prostaglandin inhibition during acetylcholine-mediated cutaneous vasodilation in humans

Acetylcholine-induced endothelium-dependent vasodilation in conduit arteries primarily depends on nitric oxide (NO). However, the biochemical mediators in the microvasculature remain less well defined. We tested whether prostaglandins and NO are responsible for cutaneous acetylcholine-mediated vasodilation and if they interact to modulate vasodilation. We measured skin blood flow (SBF) using laser Doppler flow (LDF) with intradermal microdialysis in the calves of 23 healthy volunteers. We examined the response of SBF to different doses of acetylcholine (0.01-100 mM), the nonisoform-specific NO synthase inhibitor, nitro-L-arginine (NLA, 10 mM), the nonspecific cyclo-oxygenase (COX) inhibitor, ketorolac (Keto, 10 mM), and combined NLA + Keto. NLA had no effect on baseline SBF, while Keto increased baseline SBF by approximately 150%. The increase was blunted with combined NLA + Keto. SBF increased by approximately 700% with the highest acetylcholine concentration and reduced by approximately 60% by NLA. Ketorolac alone also reduced the response to acetylcholine, although the reduction varied between 10 and 20% at differing acetylcholine doses. NLA plus ketorolac reduced the responses to different doses of acetylcholine by some 30%, which was intermediate to NOS or COX inhibition alone. These data suggest that cutaneous acetylcholine-mediated endothelium-dependent vasodilation is highly NO-dependent and is also strongly related to the interactions of NO with prostaglandins.

Sympathetic cholinergic nerve contributes to increased muscle blood flow at the onset of voluntary static exercise in conscious cats

We examined whether a sympathetic cholinergic mechanism contributed to increased blood flow of the exercising muscle at the onset of voluntary static exercise in conscious cats. After six cats were operantly conditioned to perform static bar press exercise with a forelimb while maintaining a sitting posture, a Transonic or pulsed Doppler flow probe was implanted on the brachial artery of the exercising forelimb, and catheters were inserted into the left carotid artery and jugular vein. After the baseline brachial blood flow and vascular conductance decreased and became stable in progress of postoperative recovery, the static exercise experiments were started. Brachial blood flow and vascular conductance began to increase simultaneously with the onset of exercise. Their initial increases reached 52 +/- 8% and 40 +/- 6% at 3 s from the exercise onset, respectively. Both a sympathetic ganglionic blocker (hexamethonium bromide) and atropine sulfate or methyl nitrate blunted the increase in brachial vascular conductance at the onset of static exercise, whereas an inhibitor of nitric oxide synthesis (N(omega)-nitro-l-arginine methyl ester) did not alter the increase in brachial vascular resistance. Brachial blood flow and vascular conductance increased during natural grooming behavior with the forelimb in which the flow probe was implanted, whereas they decreased during grooming with the opposite forelimb and during eating behavior. Thus it is likely that the sympathetic cholinergic mechanism is capable of evoking muscle vasodilatation at the onset of voluntary static exercise in conscious cats.

Assessment of endothelial function in complex regional pain syndrome type I

The objective of this study is to investigate the endothelial function in complex regional pain syndrome. A total of 21 patients with CRPS and 15 healthy controls were enrolled. The mean age of patients was 22 +/- 4.9 years and the mean duration of the disease was 5.9 +/- 2.5 months. Flow-mediated vasodilatation (FMD) technique was performed for evaluating the endothelial function. Parameters were the waveforms obtained and the increase in diameter and blood flow of the brachial artery, which were expressed as the percent change from baseline. The differences of the waveforms obtained in the affected limbs in comparison to non-affected limbs and to controls were statistically significant. We observed a trend of greater percent dilating responses in the affected limbs; however, the differences showed no statistical significance. In macrovascular evaluation, the endothelial function seems to be impaired in the earlier stage of the disease.

Forearm blood flow responses to fatiguing isometric contractions in women and men

Previous studies suggest that women experience less vascular occlusion than men when generating the same relative contractile force. This study examined forearm blood flow (FBF) in women and men during isometric handgrip exercise requiring the same relative force. Thirty-eight subjects [20 women and 18 men, 22.8 ± 0.6 yrs old (means ± SE)] performed low- and moderate-force handgrip exercise on two occasions. Subjects performed five maximum voluntary contractions (MVC) before exercise to determine 20% and 50% MVC target forces. Time to task failure (TTF) was determined when the subject could not maintain force within 5% of the target force. Mean blood velocity was measured in the brachial artery with the use of Doppler ultrasonography. Arterial diameter was measured at rest and used to calculate absolute FBF (FBFa; ml/min) and relative FBF (FBFr; ml·min−1·100 ml−1). Women generated less ( P < 0.05) absolute maximal force (208 ± 10 N) than men (357 ± 17 N). The TTF was longer ( P < 0.05) at 20% MVC for women (349 ± 32 s) than for men (230 ± 23 s), but no difference between the sexes was observed at 50% MVC (women: 69 ± 5 s; men: 71 ± 8 s). FBFa and FBFr increased ( P < 0.05) from rest to TTF in both women and men during 20% and 50% MVC trials. FBFr was greater in women than in men at ≥30% TTF during 50% MVC. At exercise durations ≥60% of TTF, FBFa was lower ( P < 0.05) in women than in men during handgrip at 20% MVC. Despite the longer exercise duration for women at the lower contraction intensity, FBFr was similar between the sexes, suggesting that muscle perfusion is matched to the exercising muscle mass independent of sex.

Muscle blood-flow dynamics at exercise onset: do the limbs differ?

Common approaches to understanding control of muscle blood flow in exercise focus on the contributions of various putative vasoregulatory mechanisms to the magnitude of the steady-state response. The application of systems-control principles offers a unique approach to characterizing and quantifying the non-steady-state adaptation of muscle blood flow with exercise onset. Information gained from this approach provides novel insight into the nature of control mechanisms governing physiological responses to exercise. This review is intended to provide the reader with an understanding of 1) exercise models, methodology for measuring muscle blood flow, and analysis approaches for quantifying muscle blood-flow dynamics; 2) what is currently known about the dynamic response of muscle blood-flow control mechanisms in humans; and 3) the similarities and differences in exercising muscle blood-flow control in the upper versus the lower limbs in humans.

Isometric handgrip training improves local flow-mediated dilation in medicated hypertensives

Bilateral isometric handgrip (IHG) training lowers resting arterial blood pressure (BP) in medicated hypertensives. Numerous mechanisms have been suggested, but have yet to be investigated. One such mechanism is that of improved systemic endothelial-dependent vasodilation. The purpose of this investigation was twofold: (1) to determine if bilateral IHG training had any beneficial effects on endothelial-dependent vasodilation, and (2) to see if improved systemic endothelial-dependent vasodilation was responsible for lowering BP. Sixteen participants performed four, 2 min IHG contractions at 30% of their maximal voluntary effort, using either a bilateral (n = 7) or a unilateral IHG protocol (n = 9), three times per week for 8 weeks. Brachial artery (BA) flow-mediated dilation (FMD, an index of endothelial-dependent vasodilation, measured in both arms) was assessed pre- and post-training. Following bilateral IHG training, BA FMD improved in both arms (normalized to peak shear rate 0.005 +/- 0.001 to 0.02 +/- 0.002 s(-1), P < 0.01). Following unilateral IHG training, BA FMD improved in the trained arm only (normalized 0.009 +/- 0.002 to 0.02 +/- 0.005 s(-1), P < 0.01). These findings suggest that although IHG training improves endothelial-dependent vasodilation, the improvements only occur locally in the trained limbs. This suggests that enhanced systemic endothelial-dependent vasodilation is not the mechanism responsible for the observed post-IHG training reductions in BP in medicated hypertensives.

Isometric handgrip training improves local flow-mediated dilation in medicated hypertensives

Bilateral isometric handgrip (IHG) training lowers resting arterial blood pressure (BP) in medicated hypertensives. Numerous mechanisms have been suggested, but have yet to be investigated. One such mechanism is that of improved systemic endothelial-dependent vasodilation. The purpose of this investigation was twofold: (1) to determine if Bilateral IHG training had any beneficial effects on endothelial-dependent vasodilation, and (2) to see if improved systemic endothelial-dependent vasodilation was responsible for lowering BP. Sixteen participants performed four, 2 min IHG contractions at 30% of their maximal voluntary effort, using either a Bilateral (n = 7) or a Unilateral IHG protocol (n = 9), three times per week for 8 weeks. Brachial artery (BA) flow-mediated dilation (FMD, an index of endothelial-dependent vasodilation, measured in both arms) was assessed pre-and post-training. Following Bilateral IHG training, BA FMD improved in both arms (normalized to peak shear rate, 0.005 +/- 0.001 to 0.02 +/- 0.002 s(-1), P < 0.01). Following Unilateral IHG training, BA FMD improved in the trained arm only (normalized: 0.009 +/- 0.002 to 0.02 +/- 0.005 s(-1), P < 0.01). These findings suggest that although IHG training improves endothelial-dependent vasodilation, the improvements occur only locally in the trained limbs. This suggests that enhanced systemic endothelial-dependent vasodilation is not the mechanism responsible for the observed post-IHG training reductions in BP in medicated hypertensives.

Femoral and Axillary Ultrasound Blood Flow during Exercise

PURPOSE

To use Doppler ultrasound 1) to assess the relationship between exercise intensity and changes in femoral and axillary artery diameter, 2) to determine whether volume blood flow (BF) measured during early recovery accurately reflects exercise BF, and 3) to assess the influence of artery caliber and/or site as well as exercise intensity on BF measurement reproducibility.

METHODS

Thirteen healthy subjects (mean age 25.9+/-7.7 yr) performed progressive and maximal leg-extension (LE) and elbow-flexion (EF) exercises in the supine position. The duration of each stage was 150 s, followed by a 30-s recovery period. Arterial diameter and blood flow velocity were recorded simultaneously and continuously during the last 30 s of exercise as well as 30 s into recovery.

RESULTS

Arterial dilation was 3.5 and 6.5% at maximal effort in femoral and axillary arteries, respectively. A significant increase was observed for both arteries from workload 2 to peak exercise when arterial cross-sectional area was calculated. Blood flow velocity during the recovery period was significantly different from end-exercise values, depending on time and workload. The coefficients of variation of BF measurement during exercise were 7.1-12.1% and 6.4-9.5% in LE and EF, respectively.

CONCLUSION

This study showed that BF measurement with Doppler ultrasound during exercise is reproducible but requires measurement of arterial diameter at each workload. Measurements performed immediately after exercise cannot be used as a surrogate for blood flow velocity during exercise.

Disturbed endothelial function of the internal thoracic artery in patients with coronary artery disease

OBJECTIVES AND BACKGROUND

The internal thoracic artery is an established arterial graft for myocardial revascularization. It never had been investigated, whether there are functional differences in this vessel between patients with or without coronary artery disease.

METHODS

We investigated the left internal thoracic artery of 28 patients (15 with and 13 without coronary artery disease) with a duplex-system at rest and with a handgrip exercise.

RESULTS

Concerning the measured flow velocities at rest there was only a significant difference between the diastolic mean and peak velocity between the two groups, the other investigated parameters demonstrate no significant difference. The peak diastolic and the mean diastolic velocity was less in patients with coronary artery disease during the handgrip-test. The flow reserve was decreased in patients with coronary artery disease (12.6+/-24.0% vs. 32.3+/-30.9%, P < 0.05).

CONCLUSIONS

We demonstrated, that patients with coronary artery disease have a higher peripheral resistance and a lower diastolic velocity of the internal thoracic artery during stress testing. This corresponds to a disturbed vasomotion and may be an early marker of arteriosclerosis.

Effect of acute sympathetic nervous system activation on flow-mediated dilation of brachial artery

We tested the hypothesis that flow-mediated dilation (FMD) of the brachial artery would be impaired by acute increases in sympathetic nervous system activity (SNA) in models where similar peak shear stress stimulus was achieved by varying the duration of forearm muscle ischemia. Eleven healthy young men were studied under four different conditions, each with its own control: lower body suction (LBS), cold pressor test (CPT), mental arithmetic task (MAT), and activation of muscle chemoreflex (MCR). The duration of ischemia before observation of FMD by ultrasound imaging was 5 min each for control, LBS, and CPT; 3 min for MAT; and 2-min for MCR. Peak shear rate was not different between control and any of the SNA conditions, although total shear in the first minute was reduced in MAT. MCR was the only condition in which brachial artery vasoconstriction was observed before forearm occlusion [4.38 (SD 0.53) vs. control 4.60 (SD 0.53) mm, P < 0.05]; however, diameter increased to the same absolute value as that of the control, so the percent FMD was greater for MCR [9.85 (SD 2.33) vs. control 5.29 (SD 1.50)%]. Blunting of the FMD response occurred only in the CPT model [1.51 (SD 1.20)%]. During SNA, the increase in plasma cortisol from baseline was significant only for MCR; the increase in plasma norepinephrine was significant for MCR, LBS, and CPT; and the increase in epinephrine was significant only for MCR. These results showed that the four models employed to achieve increases in SNA had different effects on baseline brachial artery diameter and that blunted FMD is not a general response to increased SNA.

Frequency-domain characteristics and filtering of blood flow following the onset of exercise: implications for kinetics analysis

We examined the validity and usefulness of a low-pass filter (LPFILTER) to reduce point-to-point variability and enhance parameter estimation of the kinetics of blood flow (BF). Computer simulations were used to determine the power spectrum of simulated responses. Moreover, we studied the leg BF response to a single transition in four subjects during supine knee-extension exercise using three methods of data processing [beat-by-beat, average of 3 cardiac cycles (AVG3 BEATS), and LPFILTER]. The power spectrum of BF containing the kinetics information (≤0.2 Hz) did not overlap with the oscillations due to muscle contraction and cardiac cycle (simulations and Doppler measurements). There were no significant differences between the parameter estimates for a two-exponential model using Beat-by-Beat, AVG3 BEATS, and LPFILTER ( P > 0.05; n = 4). However, LPFILTER (cutoff = 0.2 Hz) resulted in a significantly lower standard error of the estimate for all parameters ( P < 0.05). The means ± SD for the standard error of the estimate for Beat-by-Beat, AVG3 BEATS, and LPFILTER were, respectively, time constant- phase 1 = 5.0 ± 1.1 s, 4.5 ± 2.1 s, and 0.3 ± 0.2 s; time delay- phase 2 = 17.8 ± 7.9 s, 12.8 ± 7.5 s, and 1.4 ± 1.4 s; time constant- phase 2 = 15.8 ± 4.6 s, 9.9 ± 2.9 s, and 1.1 ± 0.5 s. In conclusion, LPFILTER appeared to be a valid procedure providing a high signal-to-noise ratio and data density and thus LPFILTER resulted in the smallest confidence interval for parameter estimates of BF kinetics.

The effects of body-weight supported treadmill training on cardiovascular regulation in individuals with motor-complete SCI

STUDY DESIGN

Four-month longitudinal within-subject exercise training study.

OBJECTIVE

Although body-weight supported treadmill training (BWSTT) has not shown promise as a means of improving ambulation in individuals with motor-complete spinal cord injury (SCI), it may still improve cardiovascular health and function in this population. The purpose of this study was to (i) investigate the effects of BWSTT on peripheral muscular and elastic artery dimension and function and measures of heart rate variability (HRV) and blood pressure variability (BPV) in individuals with motor-complete SCI, and (ii) to make a preliminary examination of what factors may predict favourable cardiovascular outcomes following BWSTT in this population.

SETTING

Centre for Health Promotion and Rehabilitation, McMaster University, Hamilton, Ontario, Canada.

METHODS

Six individuals (four male, two female; age 37.7+/-15.4 years) with chronic SCI (C4-T12; ASIA A-B; 7.6+/-9.4 years post-injury) were included in the present investigation. Doppler ultrasound was used to determine femoral (exercising; muscular), carotid (elastic) and brachial (non-exercising control; muscular) artery dimension and function before and after 4 months of BWSTT. Continuous heart rate and blood pressure were also recorded before and after 4-months of BWSTT to determine frequency domain measures of HRV and BPV; clinically valuable indices of neurocardiac and neurovascular control, respectively.

RESULTS

Two-way ANOVA (vessel x time) revealed no exercise-induced change in femoral or carotid artery cross-sectional area, blood flow or resistance and no change in carotid artery compliance following the 4 months of BWSTT compared to the non-exercising control brachial artery. However, there was a significant exercise-induced increase in femoral artery compliance. There were no exercise-induced changes in HRV or BPV when all participants were considered together. However, the results suggest that the subgroup of individuals who had a substantial heart rate response to BWSTT (n=3), experienced exercise-training induced changes in HRV reflective of a relative shift toward cardiac vagal predominance and reductions in BPV.

CONCLUSIONS

BWSTT may cause an increase in femoral artery compliance in individuals with motor-complete SCI and therefore, should be encouraged as a means of improving cardiovascular health in this population. BWSTT may also cause modest improvements in measures of HRV and BPV in a select subgroup of individuals who respond to ambulation with moderate to large increases in HR. In the present study, factors associated with a substantial HR response to BWSTT were a propensity to orthostatic intolerance and muscular spasticity.

Assessing Muscle Vasodilation Using Near-Infrared Spectroscopy in Cardiac Patients

BACKGROUND

In patients with chronic heart failure, an inadequate increase in muscle blood flow resulting from impaired vasodilation plays a key role in their exercise intolerance. However, no non-invasive methods to assess muscle vasodilation during dynamic exercise were available. We investigated whether the changes in tissue hemoglobin and myoglobin content (total-Hb + Mb) determined by non-invasive measurement using near-infrared spectroscopy (NIRS) reflect vessel conductance of working muscle during exercise.

METHODS AND RESULTS

Sixteen patients (10 patients with normal cardiac systolic function, 6 with cardiac dysfunction) performed incremental bicycle exercise testing. Total-Hb + Mb from the right vastus lateralis muscle was monitored using NIRS. Leg blood flow (LBF) in the right femoral vein was measured using a thermodilution technique every 30-60 s. Leg vessel conductance was calculated as LBF/mean arterial pressure at each time of the measurement. In all cases except 1, the levels of total-Hb + Mb showed significant correlation with the leg vessel conductance (r=0.792 to 0.980). Intra-subject reproducibility of the NIRS measurement was also confirmed in 6 patients.

CONCLUSIONS

Total-Hb + Mb from NIRS reflected muscle vasodilation during sub-maximal dynamic exercise in patients with and without cardiac dysfunction, indicating that NIRS provides a valuable method to assess the working muscle vasodilation.

Dynamic response characteristics of local muscle blood flow regulatory mechanisms in human forearm exercise

We sought to understand the nature of control mechanisms involved in the adaptation of exercising muscle hyperemia. Seven subjects performed rhythmic dynamic forearm exercise under two exercise conditions: small step 1 [step increase from rest to 40% peak forearm vascular conductance (FVC), in ml.min(-1).100 mmHg-1] for 5 min followed by small step 2 (further increase to 80% peak FVC for 5 min), and large step (step increase from rest to 80% peak FVC for 5 min). FVC data were fit with a two- (small step 1) and three-component (small step 2, large step) exponential as appropriate. For the rapid phase I response, FVC dynamic response characteristics (time delay, time constant) were not affected by the magnitude of the work intensity increase when the transition began from rest, but were slower in the 40-80% transition. Rest-80% gain was greater than either rest-40% or 40-80% transitions but represented the same proportion of the phase I + phase II gain across all transitions (57 vs. 56 vs. 57%, respectively, P = 0.975). For the slower phase II response, dynamic response characteristics were not affected by the magnitude of the work intensity increase when initiated from rest. The time constant was not altered when the transition began from exercise vs. rest. We conclude that 1) dynamic response characteristics of exercise hyperemia control mechanisms are not affected by the magnitude of work rate increase when forearm exercise is initiated from rest, 2) phase I but not phase II dynamic response characteristics are sensitive to baseline exercise intensity, and 3) the mechanisms contributing to phase I result in the same relative response magnitude, regardless of the size of the step increase in exercise intensity or the baseline from which it is initiated.

Effects of age on brachial artery myogenic responses in humans

The myogenic response, the inherent ability of blood vessels to rapidly respond to changes in transmural pressure, is involved in local blood flow autoregulation. Animal studies suggest that aging impairs the myogenic response. The purpose of this study was to compare the effects of changes in transmural pressure on mean blood velocity (MBV, cm/s) in young and older subjects. Twelve younger men and women (25 ± 1 yr) were gender and body composition matched to twelve older men and women (65 ± 1 yr). A specially designed tank raised or lowered forearm pressure by 50 mmHg within 0.2 s. Brachial artery MBV was measured directly above the site of forearm pressure change using Doppler methods. In response to increasing transmural pressure (i.e., release of +50 mmHg), older subjects compared with younger subjects had significantly lower peak MBV (Δ 12.43 ± 1.16 vs. Δ 17.97 ± 2.01 cm/s; P < 0.05), reduced rates in the dynamic fall of MBV after peak values were achieved (vasoconstriction) (−1.88 ± 0.17 vs. −2.90 ± 0.28 cm·s−1·s−1; P < 0.05), and lower MBV values with sustained suction. In response to decreasing transmural pressure (i.e., change to +50 mmHg), there was a significantly greater increase in MBV (Δ peak flow from trough 7.71 ± 1.32 vs. 4.38 ± 0.71 cm/s; P < 0.05) and a trend toward a greater rate of rise in MBV (vasodilation; 1.61 ± 0.29 vs. 0.96 ± 0.21 cm·s−1·s−1; P = 0.08) in the older subjects. Older subjects compared with the younger subjects exhibited decreased dynamic vasoconstriction, enhanced steady-state constriction, as well as evidence for enhanced dynamic vasodilation responses to sustained alterations in forearm transmural pressure.

Immediate exercise hyperemia: contributions of the muscle pump vs. rapid vasodilation

A striking characteristic of the blood flow adaptation at exercise onset is the immediate and substantial increase in the first few (0–5 s) seconds of exercise. The purpose of this mini-review is to put into context the present evidence regarding mechanisms responsible for this phase of exercise hyperemia. One potential mechanism that has received much attention is the mechanical effect of muscle contraction (the muscle pump). The rapid vasodilatory mechanism(s) is another possible mechanism that has recently been shown to exist. This review will provide the reader with 1) an understanding of the basic physics of blood flow and the theories of muscle pump function, 2) a critical examination of evidence both for and against the contribution of the muscle pump or rapid vasodilatory mechanisms, and 3) an awareness of the limitations and impact of experimental models and exercise modes on the contribution of each of these mechanisms to the immediate exercise hyperemia. The inability to measure microvenular pressure continues to limit investigators to indirect assessments of the muscle pump vs. vasodilatory mechanism contributions to immediate exercise hyperemia in vivo. Future research directions should include examination of muscle-contraction-induced resistance vessel distortion as a trigger for rapid smooth muscle relaxation and further investigation into the exercise mode dependency of muscle pump vs. rapid vasodilatory contributions to immediate exercise hyperemia.

Impact of controlling shear rate on flow-mediated dilation responses in the brachial artery of humans

The reactive hyperemia test (RHtest) evokes a transient increase in shear stress as a stimulus for endothelial-dependent flow-mediated vasodilation (EDFMD). We developed a noninvasive method to create controlled elevations in brachial artery (BA) shear rate (SR, estimate of shear stress), controlled hyperemia test (CHtest), and assessed the impact of this vs. the RHtest approach on EDFMD. Eight healthy subjects participated in two trials of each test on 3 separate days. For the CHtest, SR was step increased from 8 to 50 s−1, created by controlled release of BA compression during forearm heating. For the RHtest, transient increases in SR were achieved after 5 min of forearm occlusion. BA diameter and blood flow velocity (ultrasound) were measured upstream of compression and occlusion sites. Both tests elicited significant dilation (RHtest: 6.33 ± 3.12%; CHtest: 3.00 ± 1.05%). The CHtest resulted in 1) reduced between-subject SR and EDFMD variability vs. the RHtest [SR coefficient of variation (CV): 4.9% vs. 36.6%; EDFMD CV: 36.16% vs. 51.80%] and 2) virtual elimination of the impact of BA diameter on the peak EDFMD response (peak EDFMD vs. baseline diameter for RHtest, r2 = 0.64, P < 0.01, vs. CHtest, r2 = 0.14, P < 0.01). Normalization of the RHtest EDFMD response to the magnitude of the SR stimulus eliminated test differences in between-subject response variability. Reductions in trial-to-trial and day-to-day SR variability with the CHtest did not reduce EDFMD variability. Between-subject SR variability contributes to EDFMD variability with the RHtest. SR controls with the CHtest or RHtest response normalization are essential for examining EDFMD between groups differing in baseline arterial diameter.

Muscle Contraction-Induced Limb Blood Flow Variability during Dynamic Knee Extensor

PURPOSE

To evaluate whether muscle contraction-induced variability of limb femoral arterial blood flow (FABF) can be reduced with longer sampling durations. This was assessed in relation to muscle contraction-relaxation cycles (CRcycles) during steady-state, one-legged, dynamic knee-extensor exercise (KEE) at varying "exercise intensities" and "contraction frequencies."

METHODS

Eleven male subjects performed steady-state KEE at 10-40 W at 30 and 60 contractions per minute (cpm). FABF (Doppler ultrasound) and contraction-relaxation-induced variability in FABF was determined for 1-, 2-, 5-, 10-, 15-, 20-, and 30-CRcycles during approximately 4-min steady-state KEE. Variability was determined as coefficients of variation (CV).

RESULTS

During KEE at 30 and 60 cpm CVFABF was significantly higher for 1-CRcycles (12.3% and 15.5%) and 2-CRcycles (9.6% and 11.8%) than for 30-CRcycles (4.0% and 5.2%), but similar for 10-CRcycles to 30-CRcycles at all work rates and contraction frequencies. The CVFABF between work rates at 30 and 60 cpm did not statistically differ (P = NS) for any of the CRcycle measurements. However, the single CRcycles-induced CVFABF at 60 cpm was significantly higher (P < 0.05) than that at 30 cpm at the lower exercise intensities of 10 and 20 W, but with no significant difference at 30 and 40W.

CONCLUSION

Limb blood flow variability was markedly reduced with a longer sampling measurement of at least 10-CRcycles, which had a CVFABF of approximately 5%. Furthermore, the 1-CRcycle-induced FABF variability was similar at each exercise intensity, but significant variations were seen between contraction frequencies at lower exercise intensities. It is speculated the difference between the contraction frequencies at lower exercise intensities may be due to the muscle contraction-relaxation-induced variations in muscle force (intramuscular pressure), along with the superimposed blood pressure waves.

Forearm vascular control during acute hyperglycemia in healthy humans

The vascular endothelium is a site of pathological changes in patients with diabetes mellitus that may be related to severe chronic hyperglycemia. However, it is unclear whether transient hyperglycemia alters vascular function in an otherwise healthy human forearm. To test the hypothesis that acute, moderate hyperglycemia impairs endothelium-dependent forearm vasodilation, we measured vasodilator responses in 25 healthy volunteers (11 F, 14 M) assigned to one of three protocols. In protocol 1, glucose was varied to mimic a postprandial pattern (i.e., peak glucose approximately 11.1 mmol/l) commonly observed in individuals with impaired glucose tolerance. Protocol 2 involved 6 h of mild hyperglycemia (approximately 7 mmol/l). Protocol 3 involved 6 h of euglycemia. Glucose concentration was maintained with a variable systemic glucose infusion. Insulin concentrations were maintained at approximately 65 pmol/l by means of a somatostatin and "basal" insulin infusion. Glucagon and growth hormone were replaced at basal concentrations. Forearm blood flow (FBF) was calculated from Doppler ultrasound measurements at the brachial artery. In each protocol, FBF dose responses to intrabrachial acetylcholine (ACh) and sodium nitroprusside (NTP) were assessed at baseline and at 60, 180, and 360 min of glucose infusion. Peak endothelium-dependent vasodilator responses to ACh were not diminished by hyperglycemia in any trial. For example, peak responses to ACh during protocol 2 were 307 +/- 47 ml/min at euglycemic baseline and 325 +/- 52, 353 +/- 65, and 370 +/- 70 ml/min during three subsequent hyperglycemic trials (P = 0.46). Peak endothelium-independent responses to NTP infusion were also unaffected. We conclude that acute, moderate hyperglycemia does not cause short-term impairment of endothelial function in the healthy human forearm.

Flow-mediated dilation in human brachial artery after different circulatory occlusion conditions

Different magnitudes and durations of postocclusion reactive hyperemia were achieved by occluding different volumes of tissue with and without ischemic exercise to test the hypotheses that flow-mediated dilation (FMD) of the brachial artery would depend on the increase in peak flow rate or shear stress and that the position of the occlusion cuff would affect the response. The brachial artery FMD response was observed by high-frequency ultrasound imaging with curve fitting to minimize the effects of random measurement error in eight healthy, young, nonsmoking men. Reactive hyperemia was graded by 5-min occlusion distal to the measurement site at the wrist and the forearm and proximal to the site in the upper arm. Flow was further increased by exercise during occlusion at the wrist and forearm positions. For the two wrist occlusion conditions, flow increased eightfold and FMD was only 1 to 2% (P > 0.05). After the forearm and upper arm occlusions, blood flow was almost identical but FMD after forearm occlusions was 3.4% (P < 0.05), whereas it was significantly greater (6.6%, P < 0.05) and more prolonged after proximal occlusion. Forearm occlusion plus exercise caused a greater and more prolonged increase in blood flow, yet FMD (7.0%) was qualitatively and quantitatively similar to that after proximal occlusion. Overall, the magnitude of FMD was significantly correlated with peak forearm blood flow (r = 0.59, P < 0.001), peak shear rate (r = 0.49, P < 0.002), and total 5-min reactive hyperemia (r = 0.52, P < 0.001). The prolonged FMD after upper arm occlusion suggests that the mechanism for FMD differs with occlusion cuff position.

Blood flow response to electrically induced twitch and tetanic lower-limb muscle contractions

OBJECTIVES

To compare the effect of electric stimulation (ES)-induced twitch with tetanic leg muscle contractions on blood flow responses and to assess blood flow responses in the contralateral inactive leg.

DESIGN

Intervention with within-subject comparisons.

SETTING

University research laboratory.

PARTICIPANTS

A volunteer sample of 12 healthy men (mean age, 25.1+/-3.0y).

INTERVENTION

ES was applied at 1 and 3Hz to induce twitch contractions and at 35Hz to induce tetanic contractions of the lower- and upper-leg muscles. Exercise periods consisted of ES/rest cycles (6s/20s) for 5 minutes.

MAIN OUTCOME MEASURES

Blood flow velocity changes measured by echo Doppler ultrasonography at rest and during the first 2 subsequent ES cycles.

RESULTS

Blood flow significantly increased from resting values for the tetanic 2-leg, tetanic 1-leg, and 3-Hz conditions, but not for the 1-Hz twitch condition or in the inactive leg.

CONCLUSIONS

Both tetanic and 3-Hz twitch contractions, but not 1-Hz twitch contractions, increased leg blood flow in humans. Because blood flow elevations induced by the 3-Hz contractions did not differ statistically from those induced by the tetanic contractions but were realized with less discomfort, this mode is preferable for therapeutic interventions. Because stimulation of the ipsilateral leg muscles did not change blood flow in the contralateral inactive leg, the muscles in the area of desired effect must be stimulated.

Rapid blunting of sympathetic vasoconstriction in the human forearm at the onset of exercise

The purpose of this study was to test the hypothesis that sympathetic vasoconstriction is rapidly blunted at the onset of forearm exercise. Nine healthy subjects performed 5 min of moderate dynamic forearm handgrip exercise during -60 mmHg lower body negative pressure (LBNP) vs. without (control). Beat-by-beat forearm blood flow (Doppler ultrasound), arterial blood pressure (finger photoplethysmograph), and heart rate were collected. LBNP elevated resting heart rate by approximately 45%. Mean arterial blood pressure was not significantly changed (P = 0.196), but diastolic blood pressure was elevated by approximately 10% and pulse pressure was reduced by approximately 20%. At rest, there was a 30% reduction in forearm vascular conductance (FVC) during LBNP (P = 0.004). The initial rapid increase in FVC with exercise onset reached a plateau between 10 and 20 s of 126.6 +/- 4.1 ml. min(-1). 100 mmHg(-1) in control vs. only 101.6 +/- 4.1 ml. min(-1). 100 mmHg(-1) in LBNP (main effect of condition, P = 0.003). This difference was quickly abolished during the second, slower phase of adaptation in forearm vascular tone to steady state. These data are consistent with a rapid onset of functional sympatholysis, in which local substances released with the onset of muscle contractions impair sympathetic neural vasoconstrictor effectiveness.

Effects of transmural pressure on brachial artery mean blood velocity dynamics in humans

The effects of changes in transmural pressure on brachial artery mean blood velocity (MBV) were examined in humans. Transmural pressure was altered by using a specially designed pressure tank that raised or lowered forearm pressure by 50 mmHg within 0.2 s. Brachial MBV was measured with Doppler directly above the site of forearm pressure change. Pressure changes were evoked during resting conditions and after a 5-s handgrip contraction at 25% maximal voluntary contraction. The handgrip protocol selected was sufficiently vigorous to limit flow and sufficiently brief to prevent autonomic engagement. Changes in transmural pressure evoked directionally similar changes in MBV within 2 s. This was followed by large and rapid adjustments [-2.14 +/- 0.24 cm/s (vasoconstriction) during negative pressure and +2.14 +/- 0.45 cm/s (vasodilatation) during positive pressure]. These adjustments served to return MBV to resting levels. This regulatory influence remained operative after 5-s static handgrip contractions. Of note, changes in transmural pressure were capable of altering the timing of the peak MBV response (5 +/- 0, 2 +/- 0, 6 +/- 1 s ambient, negative, and positive pressure, respectively) as well as the speed of MBV adjustment (-2.03 +/- 0.18, -2.48 +/- 0.15, -0.84 +/- 0.19 cm x s(-1) x s(-1) ambient, negative, and positive pressure, respectively) after handgrip contractions. Vascular responses, seen with changes in transmural pressure, provide evidence that the myogenic response is normally operative in the limb circulation of humans.

Ultrasound recorded axillary artery blood flow during elbow-flexion exercise

PURPOSE

To characterize and evaluate the repeatability of ultrasound recorded blood flow in the axillary artery during one-arm dynamic elbow flexion.

METHOD

11 healthy women (23 +/- 0.9 yr, 168 +/- 1.7 cm, 63 +/- 1.1 kg) performed 90 degrees elbow flexion in supine position. Mean maximum blood velocity was recorded by ultrasound during and after two submaximal loads, representing 16 and 24% of maximal voluntary contraction, and after exhaustion (.VO(2peak))Axillary artery diameter was measured after each workload. Each subject was examined two times 8 wk apart. Oxygen uptake was measured during exercise. Values are mean +/- SE.

RESULTS

Flow was not different in the two tests. Flow was 25% higher immediately after than during exercise at the submaximal workloads. Flow immediately after .VO(2peak) was 1420 +/- 79 mL.min(-1). Axillary artery diameter after .VO(2peak) was 8% larger than at rest. Coefficient of variation ranged from 11 to 23%, coefficient of repeatability ranged from 230 mL.min(-1) at rest to 940 mL.min(-1) after exercise. Coefficient of repeatability for artery diameter was 0.09 cm; coefficient of variation for diameter was about 4% at all recording times.

CONCLUSION

The present study showed that ultrasound recordings of blood flow during dynamic elbow-flexion exercise were reproducible. The method can be used to study training-induced flow changes and can detect differences of about 130 mL.min(-1). Artery diameter seemed to increase as flow and load increased.

Assessment of brachial artery blood flow across the cardiac cycle: retrograde flows during cycle ergometry

We describe a novel software system that utilizes automated algorithms to perform edge detection and wall tracking of high-resolution B-mode arterial ultrasound images, combined with synchronized Doppler waveform envelope analysis, to calculate conduit arterial blood flow (BF) across the cardiac cycle. Furthermore, we describe changes in brachial arterial BF to the resting forearm during incremental cycle ergometry in eight subjects. During exercise, peak BF during the cardiac cycle increased at each workload (P < 0.001), because of increased velocity in the presence of unaltered cross-sectional area. In contrast, mean BF calculated across each cardiac cycle decreased at lower workloads before increasing at 100 and 160 W (P < 0.001). Differences in the pattern of peak and mean cardiac cycle flows were due to the influence of retrograde diastolic flow, which had a larger impact on mean flows at lower workloads. In conclusion, BF can be measured with high temporal resolution across the cardiac cycle in humans. Resting brachial arterial flow, including retrograde flow, increases during lower limb exercise.

Muscle chemoreflex elevates muscle blood flow and O2 uptake at exercise onset in nonischemic human forearm

We tested the hypothesis that increases in forearm blood flow (FBF) during the adaptive phase at the onset of moderate exercise would allow a more rapid increase in muscle O2 uptake (VO2 mus). Fifteen subjects completed forearm exercise in control (Con) and leg occlusion (Occ) conditions. In Occ, exercise of ischemic calf muscles was performed before the onset of forearm exercise to activate the muscle chemoreflex evoking a 25-mmHg increase in mean arterial pressure that was sustained during forearm exercise. Eight subjects who increased FBF during Occ compared with Con in the adaptation phase by >30 ml/min were considered "responders." For the responders, a higher VO2 mus accompanied the higher FBF only during the adaptive phase of the Occ tests, whereas there was no difference in the baseline or steady-state FBF or VO2 mus between Occ and Con. Supplying more blood flow at the onset of exercise allowed a more rapid increase in VO2 mus supporting our hypothesis that, at least for this type of exercise, O2 supply might be limiting.

Blood flow and muscle oxygen uptake at the onset and end of moderate and heavy dynamic forearm exercise

We hypothesized that forearm blood flow (FBF) during moderate intensity dynamic exercise would meet the demands of the exercise and that postexercise FBF would quickly recover. In contrast, during heavy exercise, FBF would be inadequate causing a marked postexercise hyperemia and sustained increase in muscle oxygen uptake (VO(2musc)). Six subjects did forearm exercise (1-s contraction/relaxation, 1-s pause) for 5 min at 25 and 75% of peak workload. FBF was determined by Doppler ultrasound, and O(2) extraction was estimated from venous blood samples. In moderate exercise, FBF and VO(2musc) increased within 2 min to steady state. Rapid recovery to baseline suggested adequate O(2) supply during moderate exercise. In contrast, FBF was not adequate during heavy dynamic exercise. Immediately postexercise, there was an approximately 50% increase in FBF. Furthermore, we observed for the first time in the recovery period an increase in VO(2musc) above end-exercise values. During moderate exercise, O(2) supply met requirements, but with heavy forearm exercise, inadequate O(2) supply during exercise caused accumulation of a large O(2) deficit that was repaid during recovery.

Peripheral circulatory factors limit rate of increase in muscle O(2) uptake at onset of heavy exercise

We used an exercise paradigm with repeated bouts of heavy forearm exercise to test the hypothesis that alterations in local acid-base environment that remain after the first exercise result in greater blood flow and O(2) delivery at the onset of the second bout of exercise. Two bouts of handgrip exercise at 75% peak workload were performed for 5 min, separated by 5 min of recovery. We continuously measured blood flow using Doppler ultrasound and sampled venous blood for O(2) content, PCO(2), pH, and lactate and potassium concentrations, and we calculated muscle O(2) uptake (VO(2)). Forearm blood flow was elevated before the second exercise compared with the first and remained higher during the first 30 s of exercise (234 +/- 18 vs. 187 +/- 4 ml/min, P < 0.05). Flow was not different at 5 min. Arteriovenous O(2) content difference was lower before the second bout (4.6 +/- 0.9 vs. 7.2 +/- 0.7 ml O(2)/dl) and higher by 30 s of exercise (11.2 +/- 0.7 vs. 10.8 +/- 0.7 ml O(2)/dl, P < 0. 05). Muscle VO(2) was unchanged before the start of exercise but was elevated during the first 30 s of the transition to the second exercise bout (26.0 +/- 2.1 vs. 20.0 +/- 0.9 ml/min, P < 0.05). Changes in venous blood PCO(2), pH, and lactate concentration were consistent with reduced reliance on anaerobic glycolysis at the onset of the second exercise bout. These data show that limitations of muscle blood flow can restrict the adaptation of oxidative metabolism at the onset of heavy muscular exertion.

Venous emptying mediates a transient vasodilation in the human forearm

We tested the hypothesis that venous emptying serves as a stimulus for vasodilation in the human forearm. We compared the forearm blood flow (FBF; pulsed Doppler mean blood velocity and echo Doppler brachial artery diameter) response to temporary elevation of a resting forearm from below to above heart level when venous volume was allowed to drain versus when venous drainage was prevented by inflation of an upper arm cuff to approximately 30 mmHg. Arm elevation resulted in a rapid reduction in venous volume and pressure. Cuff inflation just before elevation effectively prevented these changes. FBF was briefly reduced by approximately 16% following arm elevation. A transient (86%) increase in blood flow began by approximately 5 s of arm elevation and peaked by 8 s, indicating a vasodilation. This response was completely abolished by preventing venous emptying. Arterial inflow below heart level was markedly elevated by 343% following brief (4 s) forearm elevation. This hyperemia was minor when venous emptying during forearm elevation had been prevented. We conclude that venous emptying serves as a stimulus for a transient (within 10 s) vasodilation in vivo. This vasodilation can substantially elevate arterial inflow.

Forearm vascular responses during orthostatic stress in control subjects and patients with posturally related syncope

The aim of this study was to compare the changes in forearm vascular resistance that occurred during orthostatic stress in asymptomatic volunteer subjects with those in patients with posturally related syncope. The authors hoped firstly that it would indicate the importance of vasoconstriction in the maintenance of blood pressure, and secondly that it might have diagnostic value if there were differences between symptomatic patients and asymptomatic volunteers. Twelve volunteers and 67 patients with unexplained syncope were classified as early or late fainters, based on their endurance of a test of combined head-up tilting and lower-body suction. Responses of vascular resistance were assessed from the ratio of arterial blood pressure (Finapres) to brachial artery blood velocity (Doppler). Changes in vascular resistance were greater in volunteers at all stages of the procedure than in patients. There was, however, no significant difference between the responses of early and late-fainting volunteers. These results demonstrate the importance of vasoconstriction in the resistance to posturally related syncope, and they indicate that assessments of responses of vascular resistance may improve the accuracy of the diagnosis.

Limb and skeletal muscle blood flow measurements at rest and during exercise in human subjects

The aim of the present review is to present techniques used for measuring blood flow in human subjects and advice as to when they may be applicable. Since blood flow is required to estimate substrate fluxes, energy turnover and metabolic rate of skeletal muscle, accurate measurements of blood flow are of extreme importance. Several techniques have therefore been developed to enable estimates to be made of the arterial inflow to, venous outflow from, or local blood flow within the muscle. Regional measurements have been performed using electromagnetic flow meters, plethysmography, indicator methods (e.g. thermodilution and indo-cyanine green dye infusion), ultrasound Doppler, and magnetic resonance velocity imaging. Local estimates have been made using 133Xe clearance, microdialysis, near i.r. spectroscopy, positron emission tomography and laser Doppler. In principle, the aim of the study, the type of interventions and the limitations of each technique determine which method may be most appropriate. Ultrasound Doppler and continuous indo-cyanine green dye infusion gives the most accurate limb blood flow measurements at rest. Moreover, the ultrasound Doppler is unique, as it does not demand a steady-state, and because its high temporal resolution allows detection of normal physiological variations as well as continuous measurements during transitional states such as at onset of and in recovery from exercise. During steady-state exercise thermodilution can be used in addition to indo-cyanine green dye infusion and ultrasound Doppler, where the latter is restricted to exercise modes with a fixed vessel position. Magnetic resonance velocity imaging may in addition be used to determine blood flow within deep single vessels. Positron emission tomography seems to be the most promising tool for local skeletal muscle blood-flow measurements in relation to metabolic activity, although the mode and intensity of exercise will be restricted by the apparatus design.

Contributions of MSNA and stroke volume to orthostatic intolerance following bed rest

We examined whether the altered orthostatic tolerance following 14 days of head-down tilt bed rest (HDBR) was related to inadequate sympathetic outflow or to excessive reductions in cardiac output during a 10- to 15-min head-up tilt (HUT) test. Heart rate, blood pressure (BP, Finapres), muscle sympathetic nerve activity (MSNA, microneurography), and stroke volume blood velocity (SVV, Doppler ultrasound) were assessed during supine 30 degrees (5 min) and 60 degrees (5-10 min) HUT positions in 15 individuals who successfully completed the pre-HDBR test without evidence of orthostatic intolerance. Subjects were classified as being orthostatically tolerant (OT, n = 9) or intolerant (OI, n = 6) following the post-HDBR test. MSNA, BP, and SVV during supine and HUT postures were not altered in the OT group. Hypotension during 60 degrees HUT in the post-bed rest test for the OI group (P < 0.05) was associated with a blunted increase in MSNA (P < 0.05). SVV was reduced following HDBR in the OI group (main effect of HDBR, P < 0.02). The data support the hypothesis that bed rest-induced orthostatic intolerance is related to an inadequate increase in sympathetic discharge that cannot compensate for a greater postural reduction in stroke volume.

Ischemic muscle chemoreflex response elevates blood flow in nonischemic exercising human forearm muscle

We tested the hypothesis that forearm blood flow (FBF) might be reduced during forearm exercise when a vasoconstrictor response was evoked by calf exercise during calf ischemia (CE + I). In nine healthy subjects, brachial artery FBF and finger-cuff mean arterial pressure (MAP) were measured beat by beat during rest and forearm exercise. CE + I initiated before 5 min of forearm exercise (condition A) increased MAP by 24% and reduced resting forearm vascular conductance (FVC) by 24% such that FBF remained at the same level as without CE + I (control, condition C). With the onset of forearm exercise, the difference in FVC between condition A and condition C was abolished; consequently, the FBF adaptation to exercise was greater after 3 min of exercise in condition A (247.0 +/- 14.8 ml/min) than in condition C (197.1 +/- 9.4 ml/min, P < 0. 05) because of the elevated MAP. Gradual stimulation of the chemoreflex by the addition of CE + I at 3 min of a 9-min bout of forearm exercise (condition B) did not affect FVC such that progressive elevations in MAP resulted in proportional increases in FBF. We concluded that chemoreflex-mediated increases in systemic sympathetic nervous activity appear to affect resting FVC. Evidence from this study suggests that local factors responsible for initiating and maintaining vasodilation during moderate, small-muscle mass exercise can quickly override this vasoconstrictor influence such that FBF is elevated during exercise in direct proportion to the elevation in MAP.