Alterations in the Blood Velocity Profile Influence the Blood Flow Response during Muscle Contractions and Relaxations

The Contributions of Arterial Cross-Sectional Area and Time-Averaged Flow Velocity to Arterial Blood Flow

Background:

Ultrasound has been used for noninvasive assessments of endothelial function in both clinical and athletic settings and to identify changes in muscle blood flow in response to exercise, nutritional supplementation, and occlusion. The purposes of the present study were to examine the reliability and relative contributions of arterial cross-sectional area and time-averaged flow velocity to predict muscle blood flow as a result of fatiguing exercise in men and women.

Methods:

Eighteen healthy men and 18 healthy women performed 50 consecutive eccentric repetitions of the elbow flexors at 60% of their pretest eccentric peak torque at a velocity of 180° s⁻¹. Test-retest reliability and stepwise linear regression analyses were performed to determine the ability of arterial cross-sectional area and time-averaged flow velocity to predict brachial artery muscle blood flow for the men, women, and combined sample.

Results:

There was no systematic test versus retest mean differences (P > 0.05) for any of the ultrasound determined variables. The two-variable regression models significantly improved the ability to predict muscle blood flow and were associated with smaller standard error of the estimates (3.7%–10.1% vs. 16.8%–37.0% of the mean baseline muscle blood flow values) compared to the one-variable models.

Conclusions:

The findings of the present study supported the use of ultrasound for reliable assessments of arterial diameter, arterial cross-sectional area, time-averaged flow velocity, and muscle blood flow from the brachial artery in men and women. Furthermore, time-averaged flow velocity was a more powerful predictor of muscle blood flow than arterial cross-sectional area.

Mechanical compression during repeated sustained isometric muscle contractions and hyperemic recovery in healthy young males

BACKGROUND

An elevated intramuscular pressure during a single forearm isometric muscle contraction may restrict muscle hyperemia. However, during repeated isometric exercise, it is unclear to what extent mechanical compression and muscle vasodilatation contribute to the magnitude and time course of beat-to-beat limb hemodynamics, due to alterations in leg vascular conductance (LVC).

METHODS

In eight healthy male subjects, the time course of both beat-to-beat leg blood flow (LBF) and LVC in the femoral artery was determined between repeated 10-s isometric thigh muscle contractions and 10-s muscle relaxation (a duty cycle of 20 s) for steady-state 120 s at five target workloads (10, 30, 50, 70, and 90% of maximum voluntary contraction (MVC)). The ratio of restricted LBF due to mechanical compression across workloads was determined by the formula (relaxation LBF--contraction LBF)/relaxation LBF (%).

RESULTS

The exercise protocol was performed completely by all subjects (≤ 50% MVC), seven subjects (≤ 70% MVC), and two subjects (≤ 90% MVC). During a 10-s isometric muscle contraction, the time course in both beat-to-beat LBF and LVC displayed a fitting curve with an exponential increase (P < 0.001, r (2) ≥ 0.956) at each workload but no significant difference in mean LBF across workloads and pre-exercise. During a 10-s muscle relaxation, the time course in both beat-to-beat LBF and LVC increased as a function of workload, followed by a linear decline (P < 0.001, r (2) ≥ 0.889), that was workload-dependent, resulting in mean LBF increasing linearly across workloads (P < 0.01, r (2) = 0.984). The ratio of restricted LBF can be described as a single exponential decay with an increase in workload, which has inflection point distinctions between 30 and 50% MVC.

CONCLUSIONS

In a 20-s duty cycle of steady-state repeated isometric muscle contractions, the post-contraction hyperemia (magnitude of both LBF and LVC) during muscle relaxation was in proportion to the workload, which is in agreement with previous findings. Furthermore, time-dependent beat-to-beat muscle vasodilatation was seen, but not restricted, during isometric muscle contractions through all target workloads. Additionally, the relative contribution of mechanical obstruction and vasodilatation to the hyperemia observed in the repeated isometric exercise protocol was non-linear with regard to workload. In combination with repeated isometric exercise, the findings could potentially prove to be useful indicators of circulatory adjustment by mechanical compression for muscle-related disease.

Thin-beam ultrasound overestimation of blood flow: how wide is your beam?

It has been predicted that the development of thin-beam ultrasound could lead to an overestimation of mean blood velocity by up to 33% as beam width approaches 0% of vessel diameter. If both beam and vessel widths are known, in theory, this overestimation may be correctable. Therefore, we updated a method for determining the beam width of a Doppler ultrasound system, tested the utility of this technique and the information it provides to reliably correct for the error in velocity measurements, and explored how error-corrected velocity estimates impact the interpretation of in vivo data. Using a string phantom, we found the average beam width of four different probes varied across probes from 2.93 ± 0.05 to 4.41 ± 0.06 mm (mean ± SD) and with depth of insonation. Using this information, we tested the validity of a calculated correction factor to minimize the thin-beam error in mean velocity observed in a flow phantom with known diameter. Use of a correction factor reduced the overestimation from 39 ± 11 to 7 ± 9% (P < 0.05). Lastly, in vivo we explored how knowledge of beam width improves understanding of physiological flow conditions. In vivo, use of a correction factor reduced the overestimation of mean velocity from 23 ± 11 to -4 ± 9% (P < 0.05). Thus this large source of error is real, has been largely ignored by the early adaptors of Doppler ultrasound for vascular physiology studies in humans, and is correctable by the described techniques.

Change in the lower limb deep venous flow in peripheral atherosclerotic arterial disease

Aim

This prospective study was undertaken to determine how peripheral atherosclerotic disease influences the flow in the deep veins of the leg.

Material and method

Thirty one subjects with peripheral atherosclerotic disease and 23 age matched control subjects were studied. The popliteal vein flow velocity was measured at rest and during reactive hyperemia by means of color duplex ultrasound scanning. Patient age, ankle-brachial index (ABI) and the presence of risk factors for venous thrombosis were also recorded.

Results

There was a negative correlation between the ankle-brachial index and venous flow velocity among subjects with peripheral arterial disease (p=0.001). There was a negative correlation between dyslipidemia and resting venous flow velocity (p=0.049). During reactive hyperemia, venous flow velocity increased less in subjects with peripheral arterial disease than it did in control subjects (p=0.007). The subjects with dyslipidemia showed no changes in venous flow velocity in reactive hyperemia measurements (p=0.908).

Conclusion

Increasing the venous flow velocity in peripheral arterial disease, may confer some protection against the deep venous thrombosis.

Anterograde and retrograde blood velocity profiles in the intact human cardiovascular system

Current assessments of the effects of shear patterns on vascular function assume that a parabolic velocity profile is always present. Any substantial deviation in the profile away from this may result in misinterpretation of the importance that shear patterns have on vascular function. The present investigation tested the hypothesis that anterograde and retrograde blood flow would have a parabolic velocity profile at rest, during cold pressor test and exercise. Eight healthy subjects completed a cold pressor test and a graded knee-extension exercise test. Doppler ultrasound was used to determine time-averaged mean velocity (V(mean)) and time-averaged peak velocity (V(peak)) for both anterograde and retrograde flow in the femoral artery (FA) and brachial artery (BA). The V(mean)/V(peak) ratio was used to interpret the shape of the blood velocity profile (parabolic, V(mean)/V(peak) = 0.5; plug-like, V(mean)/V(peak) = 1.0). At rest, BA and FA V(mean)/V(peak) ratios of anterograde and retrograde flow were not significantly different from 0.5. During cold pressor test, anterograde V(mean)/V(peak) in the BA (0.56 ± 0.02) and FA (0.58 ± 0.03) were significantly greater than 0.5. During peak exercise, the V(mean)/V(peak) ratio of anterograde flow in the FA (0.53 ± 0.04) was not significantly different from 0.5. In all conditions, the retrograde V(mean)/V(peak) ratio was lower than anterograde. These data demonstrate that blood flow through two different conduit arteries during two different physiological stressors maintains a velocity profile that resembles a slightly blunted parabolic velocity profile.

Physiological aspects of the determination of comprehensive arterial inflows in the lower abdomen assessed by Doppler ultrasound

Non-invasive measurement of splanchnic hemodynamics has been utilized in the clinical setting for diagnosis of gastro-intestinal disease, and for determining reserve blood flow (BF) distribution. However, previous studies that measured BF in a "single vessel with small size volume", such as the superior mesenteric and coeliac arteries, were concerned solely with the target organ in the gastrointestinal area, and therefore evaluation of alterations in these single arterial BFs under various states was sometimes limited to "small blood volumes", even though there was a relatively large change in flow. BF in the lower abdomen (BF(Ab)) is potentially a useful indicator of the influence of comprehensive BF redistribution in cardiovascular and hepato-gastrointestinal disease, in the postprandial period, and in relation to physical exercise. BF(Ab) can be determined theoretically using Doppler ultrasound by subtracting BF in the bilateral proximal femoral arteries (FAs) from BF in the upper abdominal aorta (Ao) above the coeliac trunk. Prior to acceptance of this method of determining a true BF(Ab) value, it is necessary to obtain validated normal physiological data that represent the hemodynamic relationship between the three arteries. In determining BF(Ab), relative reliability was acceptably high (range in intra-class correlation coefficient: 0.85-0.97) for three arterial hemodynamic parameters (blood velocity, vessel diameter, and BF) in three repeated measurements obtained over three different days. Bland-Altman analysis of the three repeated measurements revealed that day-to-day physiological variation (potentially including measurement error) was within the acceptable minimum range (95% of confidence interval), calculated as the difference in hemodynamics between two measurements. Mean BF (ml/min) was 2951 ± 767 in Ao, 316 ± 97 in left FA, 313 ± 83 in right FA, and 2323 ± 703 in BF(Ab), which is in agreement with a previous study that measured the sum of BF in the major part of the coeliac, mesenteric, and renal arteries. This review presents the methodological concept that underlies BF(Ab), and aspects of its day-to-day relative reliability in terms of the hemodynamics of the three target arteries, relationship with body surface area, respiratory effects, and potential clinical usefulness and application, in relation to data previously reported in original dedicated research.

Determination of comprehensive arterial blood inflow in abdominal-pelvic organs: impact of respiration and posture on organ perfusion

Background

Arterial blood flow (BF) to all abdominal-pelvic organs (AP) shows potential for an indicator of comprehensive splanchnic organ circulation (reservoir of blood supply for redistribution) in cardiovascular disease, hepato-gastrointestinal disease or hemodynamic disorders. Our previous assessment of splanchnic hemodynamics, as magnitude of BF_AP [measuring by subtracting BF in both femoral arteries (FAs) from the upper abdominal aorta (Ao) above the celiac trunk] using Doppler ultrasound, was reported as the relationship between Ao and FAs, day-to-day variability and response to exercise. For accurate determination of BF_AP, it is important to consider the various factors that potentially influence BF_AP. However, little information exists regarding the influence of respiration (interplay between inspiration and expiration) and posture on BF_AP.

Material/Methods

Ten healthy males were evaluated in sitting/supine positions following a 12 hr fast. Magnitude of BF_AP was determined as measurement of Ao and FAs hemodynamics (blood velocity and vessel diameter) using pulsed Doppler with spectral analysis during spontaneous 4-sec inspiration/4-sec expiration phases.

Results

BF/blood velocity in the Ao and FAs showed significant lower in inspiration than expiration. BF_AP showed a significant (P<0.005) reduction of ~20% in inspiratory phase (sitting, 2213±222 ml/min; supine, 2059±215 ml/min) compared with expiratory phase (sitting, 2765±303 ml/min; supine, 2539±253 ml/min), with no difference between sitting and supine.

Conclusions

Respiratory-related to alterations in BF_AP were observed. It may be speculated that changes in intra-abdominal pressure during breathing (thoracic-abdominal movement) is possibly reflecting transient changes in blood velocity in the Ao and FAs. Respiratory effects should be taken into account for evaluation of BF_AP.

Femoral artery blood flow and its relationship to spontaneous fluctuations in rhythmic thigh muscle workload

BACKGROUND AND AIM

Limb femoral arterial blood flow (LBF) is known to increase linearly with increasing workload under steady-state conditions, suggesting a close link between LBF and metabolic activity. We, however, hypothesized that sudden physiological and spontaneous changes in exercise rhythm, and consequently workload temporarily alter blood flow to the working muscle. LBF and its relation to fluctuations in the contraction rhythm and workload were therefore investigated.

METHODS

LBF, measured by Doppler ultrasound, and the achieved workload, were continuously measured in nine subjects, aiming to perform steady-state, one-legged, dynamic knee-extensor exercise at 30 and 60 contractions per minute (cpm), at incremental target workloads of 10, 20, 30 and 40 W.

RESULTS

In agreement with previous findings, LBF increased positively and linearly (P<0.05) with increasing target workload. However, LBF was inversely and linearly related (P<0.05) to the actually achieved workload, when measured over 60 consecutive contraction-relaxation cycle bouts, for each target intensity at 30 and 60 cpm respectively. Thus any sudden spontaneous increase or decrease in the achieved workload transiently altered the relationship between LBF and the achieved workload. The influence upon the magnitude of LBF, due to fluctuations in the achieved workload from the target workload, was furthermore similar between target workload sessions at 30 and 60 cpm respectively. LBF was, however, not associated with variations in the contraction frequencies.

CONCLUSIONS

These findings indicate that a transient sudden increase in the workload more rapidly impedes LBF and that vasodilatation may be elicited to restore the intensity related steady-state LBF response in relation to the average metabolic activity.

Arterial blood flow of all abdominal-pelvic organs using Doppler ultrasound: range, variability and physiological impact

The pulsed Doppler method theoretically enables human arterial blood flow (BF) to be determined in all of the abdominal-pelvic organs (BF(AP)) by subtracting the bilateral proximal femoral arterial BF from the upper abdominal aorta BF above the coeliac trunk. Evaluation of BF(AP) is a potentially useful indicator of exercise or food intake related flow distribution to organs; however, there is a lack of information regarding the physiological significance of BF(AP), and the measurements are yet to be validated. The aims of the present study are to examine the range in BF(AP) among subjects, monitor physiological day-to-day variability in BF(AP) over three different days and then determine whether mean BF(AP) (averaged over the three different measurement days) is related to body surface area (BSA). Forty healthy males (19-39 years) with a wide range of body weights (51-89 kg) were evaluated in a sitting position following a 12 h fast. The above-mentioned three conduit arteries were measured to determine BF(AP) using pulsed Doppler with spectral analysis. The mean BF(AP) was 2078 +/- 495 ml min(-1) (mean +/- SD) (range, 1153-3285 ml min(-1)), which is in agreement with a previous study that measured the sum of BF in the major part of the coeliac, mesenteric and renal arteries. The physiological day-to-day variability (mean coefficient of variation) was 14.5 +/- 10.0%. Significant (p < 0.05) positive linear relationships were observed between BF(AP) and BSA as well as body weight, which is in good agreement with the results of a previous study. The present data suggest that BF(AP) determined by three-conduit arterial hemodynamics may be a valid measurement that encompasses physiologic flow to multiple abdominal-pelvic organ systems.