Muscle pump-dependent self-perfusion mechanism in legs in normal subjects and patients with heart failure

Cardiovascular reflex contributions to sympathetic inhibition during low intensity dynamic leg exercise in healthy middle-age

Aging augments resting muscle sympathetic nerve activity (MSNA) and sympatho-inhibition during mild dynamic 1-leg exercise. To elucidate which reflexes elicit exercise-induced inhibition, we recruited 19 (9 men) healthy volunteers (mean age 56 ± 9 SD years), assessed their peak oxygen uptake (VO2peak ), and, on another day, measured heart rate (HR), blood pressure (BP) and MSNA (microneurography) at rest and during 1-leg cycling (2 min each at 0 load and 30%-40% VO2peak ), 3 times: (1) seated +2 min of postexercise circulatory occlusion (PECO) (elicit muscle metaboreflex); (2) supine (stimulate cardiopulmonary baroreflexes);and (3) seated, breathing 32% oxygen (suppress peripheral chemoreceptor reflex). While seated, MSNA decreased similarly during mild and moderate exercise (p < 0.001) with no increase during PECO (p = 0.44). Supine posture lowered resting MSNA (main effect p = 0.01) BP and HR. MSNA fell further (p = 0.04) along with diastolic BP and HR during mild, not moderate, supine cycling. Hyperoxia attenuated resting (main effect p = 0.01), but not exercise MSNA. In healthy middle-age, the cardiopulmonary baroreflex and arterial chemoreflex modulate resting MSNA, but contrary to previous observations in young subjects, without counter-regulatory offset by the sympatho-excitatory metaboreflex, resulting in an augmented sympatho-inhibitory response to mild dynamic leg exercise.

Calf muscle pump pressure-flow cycle during ambulation

OBJECTIVE

Calf muscle pump (CMP) failure contributes to the severity and progression of chronic venous disease. Attempts to improve CMP function through resistance exercise have failed to improve chronic venous disease severity or quality of life, partially because the selection of the type of exercise was based on the assumption that the CMP ejects blood from the intramuscular venous sinuses (VSs), which has never been tested in humans. In the present study, we investigated the real-time changes in the pressure and size of the VS during the entire gait cycle of ambulation.

METHODS

We studied 12 lower extremities of nine healthy volunteers at rest and while walking on a treadmill at three different speeds (60, 90, and 120 steps/min). The changes in the VS cross-sectional area (CSA) and pressure were measured. Myography of the gastrocnemius muscle (GCM) and anterior tibial muscle (ATM) was used to register muscle activity. The relationship between the phases of the gait cycle and the measured parameters was analyzed using video records of all experiments.

RESULTS

The observed timing of events was consistent among all limbs studied. At rest, with the participants standing still, the VS pressure and CSA was 70.3 ± 4.2 mm Hg and 23.3 ± 14.6 mm2, respectively. During ambulation, at the first half of the stance, the GCM and ATM eccentrically contract, and the pressure is low (17 ± 8 mm Hg, 20 ± 12 mm Hg, and 29 ± 13 mm Hg at 1, 1.5, and 2 Hz, respectively), and the VS is collapsed. When the heel starts rising (the second half of the stance), the GCM concentrically contracts, the pressure increases, reaching its maximum value (143 ± 37, 134 ± 46, and 128 ± 41 mm Hg), and the VS opens, reaching its maximal size (1.8 ± 1.4 and 2.3 ± 2.2 mm2 at 1 and 1.5 Hz, respectively), followed by collapse of the VS. During the swing phase, the GCM relaxes, and the ATM concentrically contracts, resulting in a rapid decrease in pressure (2.6 ± 4.7, 1.1 ± 6.2, and -4.7 ± 3.2 mm Hg). The VS CSA remained negligible.

CONCLUSIONS

The GCM concentric contraction was associated with a simultaneous increase in VS pressure and CSA. GCM relaxation with ATM concentric contraction coincided with a decrease in VS pressure to negative values. The VSs do not fill but remain empty during the swing phase of ambulation, acting, not as a reservoir, but as a conduit, transferring blood from the network of intramuscular veins to the axial deep veins.

Influence of simulated hypogravity on oxygen uptake during treadmill running

Prolonged exposure to microgravity during spaceflights leads to severe deterioration in the physical performance of astronauts. To understand the effectiveness of existing in‐flight daily countermeasures and to plan exercise onboard the International Space Station, we compared supine treadmill running to traditional upright treadmill running on earth. Specifically, we assessed the cardiorespiratory responses to conventional upright running to the responses to supine treadmill running under 0.3 g, 0.6 g, and 1 g of body weight in younger (20–30 years, n = 14, 8 females) and older healthy adults (50–60 years, n = 12, 6 females). Maximal cardiorespiratory capacity was additionally evaluated by performing an incremental running protocol on each treadmill. Maximum speed was greater for 0.3 g and 0.6 g in supine than for upright running (18.5 km/h (1.1) and 15.9 (3.1) vs 13.2 (2.4) p < 0.001). In contrast, maximum oxygen uptake (V˙O2max) and maximum heart rate (HR_max) were greater in upright running than in all supine conditions (Upright treadmill running vs S1.0G vs S0.6G vs S0.3G, 41.7 ml kg⁻¹ min⁻¹ (7.2) vs 30.5 (6.6) vs 32.9 (7.0) vs 30.9 (5.2), p < 0.001 and 171 beats min⁻¹ (14) vs 152 (24) vs 155 (20) vs 152 (18), p < 0.001, respectively). The reduction in V˙O2max was remarkably similar across all three supine conditions, could not be increased by higher running speeds and can be well explained by reduced ground reaction forces (GRF). Thus, although a gravity‐related restriction of pulmonary gas exchange or perfusion of the legs when exercising in the supine position can be suspected, findings are also explicable on grounds of the vertical treadmill mechanics. Reduced loading will constitute a substantial limitation to V˙O2 in space with implications for crew health and the physical deterioration of astronauts.

Skeletal Muscle Fatigability in Heart Failure

Evidence suggests that heart failure (HF) patients experience skeletal muscle fatigability in the lower extremity during single-limb tasks. The contribution of skeletal muscle fatigability to symptoms of exercise intolerance (perceived fatigue and dyspnea) is relatively unclear. Symptomatic or ‘perceived’ fatigue is defined by the sensations of exhaustion or tiredness that patients experience either at rest or while performing a motor task. Although factors that contribute to symptoms of fatigue in patients with HF are multifactorial; the skeletal muscle likely plays a major role. Skeletal muscle fatigability, as opposed to symptomatic fatigue, is an objective measure of a reduction in muscle force or power or reduced ability of the muscles to perform over time. Indeed, evidence suggests that patients with HF experience greater skeletal muscle fatigability which may contribute to a diminution in motor performance and the overall symptomatology that is hallmark of exercise intolerance in HF. This review will discuss (1) skeletal muscle fatigability in patients with HF, (2) the mechanisms contributing to locomotor skeletal muscle fatigability in HF and (3) the relationship of fatigability to symptoms of perceived fatigue and exercise intolerance in HF patients. Evidence suggests that cardiac dysfunction alone does not contribute to exercise intolerance. Therefore, mechanisms of skeletal muscle fatigability and their contribution to symptoms of fatigue and exercise intolerance, is an increasingly important consideration as we develop rehabilitative strategies for improving motor performance and functional capacity in patients with HF.

Effects of walking in deep venous thrombosis: a new integrated solid and fluid mechanics model

Deep venous thrombosis (DVT) is a common disease. Large thrombi in venous vessels cause bad blood circulation and pain; and when a blood clot detaches from a vein wall, it causes an embolism whose consequences range from mild to fatal. Walking is recommended to DVT patients as a therapeutical complement. In this study the mechanical effects of walking on a specific patient of DVT were simulated by means of an unprecedented integration of 3 elements: a real geometry, a biomechanical model of body tissues, and a computational fluid dynamics study. A set of computed tomography images of a patient's leg with a thrombus in the popliteal vein was employed to reconstruct a geometry model. Then a biomechanical model was used to compute the new deformed geometry of the vein as a function of the fiber stretch level of the semimembranosus muscle. Finally, a computational fluid dynamics study was performed to compute the blood flow and the wall shear stress (WSS) at the vein and thrombus walls. Calculations showed that either a lengthening or shortening of the semimembranosus muscle led to a decrease of WSS levels up to 10%. Notwithstanding, changes in blood viscosity properties or blood flow rate may easily have a greater impact in WSS.

Skeletal muscle inflammation and atrophy in heart failure

Heart failure represents a systemic disease with profound effects on multiple peripheral tissues including skeletal muscle. Within the context of heart failure, perturbations in skeletal muscle physiology, structure, and function strongly contribute to exercise intolerance and the morbidity of this devastating disease. There is growing evidence that chronic heart failure imparts specific pathological changes within skeletal muscle beds resulting in muscle dysfunction and tissue atrophy. Mechanistically, systemic and local inflammatory responses drive critical aspects of this pathology. In this review, we will discuss pathological mechanisms that drive skeletal muscle inflammation and highlight emerging roles for distinct innate immune subsets that reside within damage muscle tissue focusing on the recently described embryonic and monocyte-derived macrophage lineages. Within this context, we will discuss how immune mechanisms can be differentially targeted to stimulate skeletal muscle inflammation, catabolism, fiber atrophy, and regeneration.

Effect of sodium nitrite on local control of contracting skeletal muscle microvascular oxygen pressure in healthy rats

Exercise intolerance characteristic of diseases such as chronic heart failure (CHF) and diabetes is associated with reduced nitric oxide (NO) bioavailability from nitric oxide synthase (NOS), resulting in an impaired microvascular O₂ driving pressure (Po₂mv; O₂ delivery/O₂ utilization) and metabolic control. Infusions of the potent NO donor sodium nitroprusside augment NO bioavailability yet decrease mean arterial pressure (MAP) thereby reducing its potential efficacy for patient populations. To eliminate or reduce hypotensive sequelae, [Formula: see text] was superfused onto the spinotrapezius muscle. It was hypothesized that local [Formula: see text] administration would elevate resting Po₂mv and slow Po₂mv kinetics [increased time constant (τ) and mean response time (MRT)] following the onset of muscle contractions without decreasing MAP. In 12 anesthetized male Sprague-Dawley rats, Po₂mv of the circulation-intact spinotrapezius muscle was measured by phosphorescence quenching during 180 s of electrically induced twitch contractions (1 Hz) before and after superfusion of sodium nitrite (NaNO₂ 30 mM). [Formula: see text] superfusion elevated resting Po₂mv (control: 28.4 ± 1.1 vs. [Formula: see text]: 31.6 ± 1.2 mmHg; P ≤ 0.05), τ (control: 12.3 ± 1.2 vs. [Formula: see text]: 19.7 ± 2.2 s; P ≤ 0.05), and MRT (control: 19.3 ± 1.9 vs. [Formula: see text]: 25.6 ± 3.3 s; P ≤ 0.05). Importantly, these effects occurred in the absence of any reduction in MAP (103 ± 4 vs. 105 ± 4 mmHg, pre- and postsuperfusion respectively; P > 0.05). These results indicate that [Formula: see text] supplementation delivered to the muscle directly through [Formula: see text] superfusion enhances the blood-myocyte oxygen driving pressure without compromising MAP at rest and following the onset of muscle contraction. This strategy has substantial clinical utility for a range of ischemic conditions.

NEW & NOTEWORTHY

Ischemic conditions as diverse as chronic heart failure (CHF) and frostbite inflict tissue damage via inadequate O₂ delivery. Herein we demonstrate that direct application of sodium nitrite enhances the O₂ supply-O₂ demand relationship, raising microvascular O₂ pressure in healthy skeletal muscle. This therapeutic action of nitrite-derived nitric oxide occurred without inducing systemic hypotension and has the potential to relieve focal ischemia and preserve tissue vitality by enhancing O₂ delivery.

Influence of menopause status and age on integrated central and peripheral hemodynamic responses to subsystolic cuffing during submaximal exercise

Although pathophysiological links between postmenopause and healthy aging remain unclear, both factors are associated with increased blood pressure and sympathetic nerve activity (SNA) in women. Activation of polymodal musculoskeletal neural afferents originating within adventia of venules modulates SNA and blood pressure control during exercise in healthy adults. We hypothesized transient subsystolic regional circulatory occlusion (RCO) during exercise sensitizes these afferents leading to augmented systemic vascular resistance (SVR)-mediated increased mean arterial pressure (MAP) in postmenopause vs. premenopause. Normotensive women in premenopause or postmenopause (n = 14 and 14; ages: 30 ± 9 and 55 ± 7 yr, respectively; P < 0.01) performed: 1) peak exercise testing and 2) fixed-load cycling at 30% peak workload (48 ± 11 and 38 ± 6 W, respectively; P < 0.01), whereby the initial 3 min were control exercise without RCO (CTL), thereafter including 2 min of bilateral-thigh RCO to 20, 40, 60, 80, or 100 mmHg (randomized), with 2 min deflation between RCO. Both MAP (17 ± 4 vs. 4 ± 4%, P = 0.02) and SVR (16 ± 8 vs. -3 ± 8%, P = 0.04) increased at 80 mmHg from CTL in postmenopause vs. premenopause, respectively. However, cardiac index was similar in postmenopause vs. premenopause at 80 mmHg from CTL (1 ± 6 vs. 7 ± 6%, respectively; P = 0.15). There was no continuous effect of aging in MAP (P = 0.12), SVR (P = 0.07), or cardiac index (P = 0.18) models. These data suggest transient locomotor subsystolic RCO sensitizes musculoskeletal afferents, which provoke increased SVR to generate augmented MAP during exercise in postmenopause. These observations provide a novel approach for understanding the age-independent variability in exercise blood pressure control across the normotensive adult pre- to postmenopause spectrum.

Exercise training in chronic heart failure: improving skeletal muscle O2 transport and utilization

Chronic heart failure (CHF) impairs critical structural and functional components of the O2 transport pathway resulting in exercise intolerance and, consequently, reduced quality of life. In contrast, exercise training is capable of combating many of the CHF-induced impairments and enhancing the matching between skeletal muscle O2 delivery and utilization (Q̇mO2 and V̇mO2 , respectively). The Q̇mO2 /V̇mO2 ratio determines the microvascular O2 partial pressure (PmvO2 ), which represents the ultimate force driving blood-myocyte O2 flux (see Fig. 1). Improvements in perfusive and diffusive O2 conductances are essential to support faster rates of oxidative phosphorylation (reflected as faster V̇mO2 kinetics during transitions in metabolic demand) and reduce the reliance on anaerobic glycolysis and utilization of finite energy sources (thus lowering the magnitude of the O2 deficit) in trained CHF muscle. These adaptations contribute to attenuated muscle metabolic perturbations (e.g., changes in [PCr], [Cr], [ADP], and pH) and improved physical capacity (i.e., elevated critical power and maximal V̇mO2 ). Preservation of such plasticity in response to exercise training is crucial considering the dominant role of skeletal muscle dysfunction in the pathophysiology and increased morbidity/mortality of the CHF patient. This brief review focuses on the mechanistic bases for improved Q̇mO2 /V̇mO2 matching (and enhanced PmvO2 ) with exercise training in CHF with both preserved and reduced ejection fraction (HFpEF and HFrEF, respectively). Specifically, O2 convection within the skeletal muscle microcirculation, O2 diffusion from the red blood cell to the mitochondria, and muscle metabolic control are particularly susceptive to exercise training adaptations in CHF. Alternatives to traditional whole body endurance exercise training programs such as small muscle mass and inspiratory muscle training, pharmacological treatment (e.g., sildenafil and pentoxifylline), and dietary nitrate supplementation are also presented in light of their therapeutic potential. Adaptations within the skeletal muscle O2 transport and utilization system underlie improvements in physical capacity and quality of life in CHF and thus take center stage in the therapeutic management of these patients.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

Rapid onset vasodilatation is blunted in obese humans

AIM

Conduit artery function in obese humans is frequently assessed at rest, but very little is known about resistance artery function in response to muscle contraction. We tested the hypothesis that obese adults will exhibit reduced contraction-induced rapid onset vasodilatation. Single and brief forearm contractions were used to isolate the local effects of muscle contraction on the forearm vasodilatory response, independent of systemic haemodynamic and sympathetic neural influence.

METHODS

We measured forearm blood flow (Doppler ultrasound), blood pressure (finger photoplethysmography) and heart rate (electrocardiogram) on a beat-by-beat basis in 14 obese (body mass index = 36.2 ± 1.7 kg m(-2)) and 14 lean (body mass index = 21.6 ± 0.7 kg m(-2)) young (18-40 years) adults. Percent changes from baseline in forearm vascular conductance (FVC(%) ) were calculated in response to single, brief forearm contractions performed in random order at 15, 20, 25, 30, 40 and 50% of maximal voluntary contraction (MVC).

RESULTS

In both groups, each single contraction evoked a significant (P < 0.05), immediate (within one cardiac cycle) and graded FVC(%) increase from one up to six cardiac cycles post-contraction. Immediate (20-50% MVC), peak (15-50% MVC) and total (area under the curve, 20-50% MVC) vasodilatory responses were reduced with obesity. The degree of impaired vasodilatation increased with increasing workloads.

CONCLUSIONS

These novel findings demonstrate a blunted contraction-induced rapid onset vasodilatation with obesity that is exercise intensity dependent. Impaired rapid onset vasodilatation may negatively impact haemodynamic responses to everyday intermittent activities performed by obese humans.

Muscle oxygen transport and utilization in heart failure: implications for exercise (in)tolerance

The defining characteristic of chronic heart failure (CHF) is an exercise intolerance that is inextricably linked to structural and functional aberrations in the O(2) transport pathway. CHF reduces muscle O(2) supply while simultaneously increasing O(2) demands. CHF severity varies from moderate to severe and is assessed commonly in terms of the maximum O(2) uptake, which relates closely to patient morbidity and mortality in CHF and forms the basis for Weber and colleagues' (167) classifications of heart failure, speed of the O(2) uptake kinetics following exercise onset and during recovery, and the capacity to perform submaximal exercise. As the heart fails, cardiovascular regulation shifts from controlling cardiac output as a means for supplying the oxidative energetic needs of exercising skeletal muscle and other organs to preventing catastrophic swings in blood pressure. This shift is mediated by a complex array of events that include altered reflex and humoral control of the circulation, required to prevent the skeletal muscle "sleeping giant" from outstripping the pathologically limited cardiac output and secondarily impacts lung (and respiratory muscle), vascular, and locomotory muscle function. Recently, interest has also focused on the dysregulation of inflammatory mediators including tumor necrosis factor-α and interleukin-1β as well as reactive oxygen species as mediators of systemic and muscle dysfunction. This brief review focuses on skeletal muscle to address the mechanistic bases for the reduced maximum O(2) uptake, slowed O(2) uptake kinetics, and exercise intolerance in CHF. Experimental evidence in humans and animal models of CHF unveils the microvascular cause(s) and consequences of the O(2) supply (decreased)/O(2) demand (increased) imbalance emblematic of CHF. Therapeutic strategies to improve muscle microvascular and oxidative function (e.g., exercise training and anti-inflammatory, antioxidant strategies, in particular) and hence patient exercise tolerance and quality of life are presented within their appropriate context of the O(2) transport pathway.

Skeletal muscle arteriolar function following myocardial infarction: Analysis of branch-order effects

Diminished bioavailability of nitric oxide (NO) may impair skeletal muscle arteriolar function after myocardial infarction (MI). We tested the hypotheses that chronic MI induced would diminish 1) endothelial function in large (resting diameter ~75μm) feed arterioles, and 2) functional dilation in feed arterioles, but not smaller arcade (~25μm) or transverse (~15μm) arterioles, in the spinotrapezius muscle of female Sprague-Dawley rats. Additionally, we hypothesized that blockade of NO production with N(G)-nitro-l-arginine methyl ester (l-NAME; 30mg/kg i.v.) would have a greater blunting effect on control rats than MI rats. Endothelial function of the feed arterioles was assessed with an infusion of acetylcholine (1.5μg i.v.) after pretreatment with indomethacin (5mg/kgi.p.). MI blunted the response to acetylcholine in feed arterioles (p=0.037), but did not affect resting or post-contraction diameter at any branching order. l-NAME had similar effects on MI and SHAM rats; the response to acetylcholine was blunted in feed arterioles (p=0.003), resting diameter was diminished in arcade arterioles (p=0.003), and post-contraction diameter was diminished in both arcade arterioles (p=0.03) and transverse arterioles (p=0.05). In conclusion, despite endothelial dysfunction in feed arterioles, functional dilation was not affected by MI in any branching order studied. l-NAME had similar effects on MI and SHAM rats that were branch order-dependent. These branch-order effects should be considered in future studies of the control of blood flow.

Role of mechanical factors in governing muscle blood flow

This study evaluates how mechanical factors impact cardiac output at the systemic level, and how mechanical factors influence muscle blood flow at the local level. Importantly, the two are intertwined; events that work locally in the periphery to augment muscle blood flow can also act centrally to contribute to the development of a high cardiac output.

Exercise performance and VO2 kinetics during upright and recumbent high-intensity cycling exercise

This study investigated cycling performance and oxygen uptake V(O)₂kinetics between upright and two commonly used recumbent (R) postures, 65 degrees R and 30 degrees R. On three occasions, ten young active males performed three bouts of high-intensity constant-load (85% peak-workload achieved during a graded test) cycling in one of the three randomly assigned postures (upright, 65 degrees R or 30 degrees R). The first bout was performed to fatigue and second and third bouts were limited to 7 min. A subset of seven subjects performed a final constant-load test to failure in the supine posture. Exercise time to failure was not altered when the body inclination was lowered from the upright (13.1 +/- 4.5 min) to 65 degrees R (10.5 +/- 2.7 min) and 30 degrees R (11.5 +/- 4.6 min) postures; but it was significantly shorter in the supine posture (5.8 +/- 2.1 min) when compared with the three inclined postures. Resulting kinetic parameters from a tri-exponential analysis of breath-by-breath V(O)₂data during the first 7 min of exercise were also not different between the three inclined postures. However, inert gas rebreathing analysis of cardiac output revealed a greater cardiac output and stroke volume in both recumbent postures compared with the upright posture at 30 s into the exercise. These data suggest that increased cardiac function may counteract the reduction of hydrostatic pressure from upright approximately 25 mmHg; to 65 degrees R approximately 22 mmHg; and 30 degrees R approximately 18 mmHg such that perfusion of active muscle presumably remains largely unchanged, and also therefore, V(O)₂kinetics and performance during high-intensity cycling.

The impact of intermittent pneumatic compression devices on deep venous flow velocity in patients with congestive heart failure

BACKGROUND

Intermittent pneumatic compression (IPC) has been used to prevent deep venous thrombosis (DVT), but the effects of IPC on the hemodynamics of popliteal and soleal veins, especially in patients with congestive heart failure (CHF) have not been evaluated. The aim of this study was to evaluate the effects of IPC on the flow velocity of deep veins in the lower extremities and to compare the efficacy of two different types of IPC in deep venous flow enhancement in patients with CHF.

METHODS

Flow velocities of popliteal and soleal veins were recorded in 19 patients with CHF and in 19 control subjects using a high-resolution linear probe. Peak and mean flow velocities were measured (1) at rest, (2) with sequential foot and calf IPC (SFC-IPC) which consists of an electrically driven air compressor and four air chambers, and (3) with impulse foot IPC (IF-IPC) which consists of a pneumatic impulse generator operated at an applied pressure of 130 mmHg.

RESULTS

In the resting condition, popliteal venous flow velocity in the CHF group was attenuated (12.8+/-4.7 cm/s vs. 21.1+/-13.5 cm/s; p<0.05). Both SFC-IPC and IF-IPC increased venous velocity, but the increase with IF-IPC in CHF patients was lower than that in control subjects. In the soleal veins, after applying SFC-IPC, the peak and mean velocity in CHF increased to the same extent as in the control group. IF-IPC increased soleal venous velocity in control subjects, but there was no increase in CHF patients.

CONCLUSION

Two-dimensional Doppler scanning revealed a significant increase in the mean and peak velocities in the soleal and popliteal veins with SFC-IPC but not with IF-IPC in patients with CHF. These results indicate that SFC-IPC could have favorable effects in preventing DVT in patients with CHF.

Effect of change in blood volume in skin plus active muscle on heart rate drift during submaximal exercise

The purpose of the present study was to examine the effect of change in blood volume in skin plus active muscle on heart rate drift during moderate exercise and heavy exercise for 30 min. Total hemoglobin concentration (Total Hb) in the vastus lateralis muscle plus its skin was determined by near-infrared spectroscopy. Total Hb significantly increased and remained stable from 20 min in moderate exercise and from 10 min in heavy exercise. Heart rate (HR) rapidly increased until 3 min and showed a steady state in moderate exercise. HR at 30 min was significantly higher than that at 3 min in moderate exercise. HR rapidly increased until 3 min and then gradually but significantly increased in heavy exercise. Increase in total Hb was not significantly related with HR after 3 min of exercise when HR was around 120 beats per min in moderate exercise. Increase in total Hb was significantly related with HR from 3 min to 10 min in the heavy exercise (correlation coefficients ranged from 0.959 to 0.702). It is concluded that an increase in the blood volume in skin plus active muscle is not simply associated with HR drift.

Measuring muscle blood flow: a key link between systemic and regional metabolism

PURPOSE OF REVIEW

To provide a brief overview of the main techniques to measure muscle blood flow in humans and highlight some of the strengths and weaknesses associated with each technique.

RECENT FINDINGS

Peak muscle blood flow values of 300 ml/min per 100 g are possible in humans during heavy exercise performed with small muscle mass. This value is far higher than that which appears in most textbooks. Accurate and reliable techniques are therefore essential in measuring muscle blood flow. Current invasive techniques commonly used include indicator dilution (thermodilution and dye dilution) and radiolabel tracer washout (e.g. 133Xe washout) methods. Although invasive techniques have provided valuable insight into tissue blood flow, noninvasive techniques such as venous occlusion plethysmography and Doppler ultrasound are frequently used and provide accurate measurements of blood flow. Newer imaging techniques (MRI, positron emission tomography, and contrast-enhanced ultrasonography) promise increased resolution of measurements of local blood flow, including in discrete tissues in which more classical techniques are not able to be used.

SUMMARY

Muscle blood flow is a key link in the interplay and regulation of systemic and local muscle metabolism. Recognizing the advantages and limitations of each technique is essential to translational researchers studying the effects of nutrition and metabolism on muscle blood flow.

Haemodynamic responses to exercise, ATP infusion and thigh compression in humans: insight into the role of muscle mechanisms on cardiovascular function

The muscle pump and muscle vasodilatory mechanism are thought to play important roles in increasing and maintaining muscle perfusion and cardiac output ((.)Q) during exercise, but their actual contributions remain uncertain. To evaluate the role of the skeletal muscle pump and vasodilatation on cardiovascular function during exercise, we determined leg and systemic haemodynamic responses in healthy men during (1) incremental one-legged knee-extensor exercise, (2) step-wise femoral artery ATP infusion at rest, (3) passive exercise (n=10), (4)femoral vein or artery ATP infusion (n=6), and (5) cyclic thigh compressions at rest and during passive and voluntary exercise (n=7). Incremental exercise resulted in progressive increases in leg blood flow (DeltaLBF 7.4 +/- 0.7 l min(-1)), cardiac output (Delta (.)Q 8.7 +/- 0.7 l min(-1)), mean arterial pressure (DeltaMAP 51 +/- 5 mmHg), and leg and systemic oxygen delivery and (.)VO2 . Arterial ATP infusion resulted in similar increases in (.)Q , LBF, and systemic and leg oxygen delivery, but central venous pressure and muscle metabolism remained unchanged and MAP was reduced. In contrast,femoral vein ATP infusion did not alter LBF, (.)Q or MAP. Passive exercise also increased blood flow (DeltaLBF 0.7 +/- 0.1 l min(-1)), yet the increase in muscle and systemic perfusion, unrelated to elevations in aerobic metabolism, accounted only for approximately 5% of peak exercise hyperaemia.Likewise, thigh compressions alone or in combination with passive exercise increased blood flow (DeltaLBF 0.5-0.7 l min(-1)) without altering (.)Q, MAP or (.)VO2. These findings suggest that the skeletal muscle pump is not obligatory for sustaining venous return, central venous pressure,stroke volume and (.)Q or maintaining muscle blood flow during one-legged exercise in humans.Further, its contribution to muscle and systemic peak exercise hyperaemia appears to be minimal in comparison to the effects of muscle vasodilatation.

The final frontier: oxygen flux into muscle at exercise onset

In humans at exercise onset, intramuscular phosphocreatine decreases immediately, whereas muscle oxygen (O2) uptake seems to rise after a delay of up to 15 s which is inconsistent with models of metabolic control. Novel microcirculatory investigations reveal that elevated capillary-to-myocyte O2 flux in rat muscle is, in fact, initiated simultaneously with contractions.

Muscle microvascular oxygenation in chronic heart failure: role of nitric oxide availability

AIM

To test the hypothesis that diminished vascular nitric oxide availability might explain the inability of individuals with chronic heart failure (CHF) to maintain the microvascular PO(2)'s (PO(2mv) proportional, variant O(2) delivery-to-uptake ratio) seen in healthy animals.

METHODS

We superfused sodium nitroprusside (SNP; 300 microm), Krebs-Henseleit (control, CON) and L-nitro arginine methyl ester (L-NAME; 1.5 mM) onto the spinotrapezius muscle and measured PO(2mv) by phosphorescence quenching in female Sprague-Dawley rats (n = 26) at rest and during twitch contractions (1 Hz). Seven rats served as controls (Sham) while CHF was induced by myocardial infarction. CHF rats were grouped as moderate (MOD; n = 15) and severe CHF (SEV; n = 4) according to morphological data and baseline PO(2mv).

RESULTS

In contrast to Sham and MOD, L-NAME did not affect the PO(2mv) response (dynamics and steady-state) of SEV when compared with CON. SNP restored the PO(2mv) profile of SEV to that seen in Sham animals during CON. Specifically, the effect of L-NAME expressed as Delta(L-NAME - CON) were: Baseline PO(2mv) [in mmHg, DeltaSham = -7.0 +/- 1.6 (P < 0.05); DeltaSEV =-1.2 +/- 2.1], end-contractions PO(2mv) [in mmHg, DeltaSham = -5.0 +/- 1.0 (P < 0.05); DeltaSEV = -2.5 +/- 0.5] and time constant of PO(2mv) decrease [in s, DeltaSham = -6.5 +/- 3.0 (P < 0.05); DeltaSEV = -3.2 +/- 1.8].

CONCLUSION

These data provide the first direct evidence that the pathological profiles of PO(2mv) associated with severe CHF can be explained, in part, by a diminished vascular NO availability.

Temporal profile of rat skeletal muscle capillary haemodynamics during recovery from contractions

In skeletal muscle capillaries, red blood cell (RBC) flux (F(RBC)), velocity (V(RBC)) and haematocrit (Hct(CAP)) are key determinants of microvascular O2 exchange. However, the mechanisms leading to the changes in F(RBC), V(RBC) and Hct(CAP) during muscle contractions and recovery thereafter are not fully understood. To address this issue we used intravital microscopy to investigate the temporal profile of the rat spinotrapezius muscle (n = 5) capillary haemodynamics during recovery from 3 min of twitch muscle contractions (1 Hz, 4-6 V). Specifically, we hypothesized that (1) during early recovery F(RBC) and V(RBC) would decrease rapidly and F(RBC) would display a biphasic response (consistent with a muscle pump effect on capillary haemodynamics), and (2) there would be a dynamic relationship between changes (Delta) in V(RBC) and Hct(CAP). The values at rest (R) and end-recovery (ER) were significantly lower (P < 0.05) than at end-contraction (EC) for F(RBC) (in cells s(-1), R = 30.1 +/- 7.8, EC = 46.2 +/- 7.3 and ER = 26.0 +/- 6.1), V(RBC) (in microm s(-1), R = 368 +/- 83, EC = 497 +/- 62 and ER = 334 +/- 59) and Hct(CAP) (R = 0.193 +/- 0.016, EC = 0.214 +/- 0.023 and ER = 0.185 +/- 0.019). The first data point where a significant decrease in F(RBC), Hct(CAP) and V(RBC) occurred was at 5, 5 and 20 s post-contraction, respectively. The decrease in F(RBC) approximated a monoexponential response (half-time of approximately 26 s). The relationship between DeltaV(RBC) and DeltaHct(CAP) was not significant (P > 0.05). Based on the early decrease in F(RBC) (within 5 s), overall dynamic profile of F(RBC) and the approximately 20 s 'delay' to the decrease in V(RBC) we conclude that the muscle pump does not appear to contribute substantially to the steady-state capillary haemodynamics in the contracting rat spinotrapezius muscle. Moreover, our findings suggest that alterations in V(RBC) do not obligate proportional changes in Hct(CAP) within individual capillaries following muscle contractions.

Kinetics of estimated human muscle capillary blood flow during recovery from exercise

The kinetic characteristics of muscle capillary blood flow (Qcap) during recovery from exercise are controversial (e.g. one versus two phases). Furthermore, it is not clear how the overall Qcap kinetics are temporally associated with muscle oxygen uptake (VO2m) kinetics. To address these issues, we examined the kinetics of Qcap estimated from the rearrangement of the Fick equation (Qcap=VO2m/C(a-v)O2) using the kinetics of pulmonary VO2 (VO2p, primary component) and deoxy-haemoglobin concentration ([HHb]) as indices of VO2m and C(a - v)O2 (arterio-venous oxygen difference) kinetics, respectively. VO2p (l min-1) was measured breath by breath and [HHb] (microm) was measured by near infrared spectroscopy during moderate (M; below lactate threshold, LT) and heavy exercise (H, above LT) in nine subjects. The kinetics of Qcap were biphasic, with an initial fast phase (tauI; M=9.3+/-4.9 s and H=6.0+/-3.8 s) followed by a slower phase 2 (tauP; M=29.9+/-8.6 s and H=47.7+/-26.0 s). For moderate exercise, the overall kinetics of Qcap (mean response time [MRT], 36.1+/-8.6 s) were significantly slower than the kinetics of VO2p (tauP; 27.8+/-5.3 s) and [HHb] (MRT for [HHb]; 16.2+/-6.3 s). However, for heavy exercise, there was no significant difference between MRT-[HHb] (34.7+/-10.4 s) and tauP for VO2p (32.3+/-6.7 s), while MRT for Qcap (48.7+/-21.8 s) was significantly slower than MRT for [HHb] and tauP for VO2p. In conclusion, during recovery from exercise the estimated Qcap kinetics were biphasic, showing an early rapid decrease in blood flow. In addition, the overall kinetics of Qcap were slower than the estimated VO2m kinetics.

Muscle blood flow–O2 uptake interaction and their relation to on-exercise dynamics of O2 exchange

A computer model was developed to provide a theoretical framework for interpreting the dynamics of muscle capillary O(2) exchange in health and disease. We examined the effects of different muscle oxygen uptake (V O(2m)) and CvO(2) profiles on muscle blood flow (Q (m)) kinetics (Q (m)=V O(2m)/[CaO(2)-CvO(2)]). Further, we simulated V O(2m) and Q (m) responses to predict the CvO(2) profile and the underlying dynamics of capillary O(2) exchange (CvO(2)=CaO(2)-V O(2m)/Q (m)). Exponential equations describing V O(2m), CvO(2) and Q (m) responses in vivo were used in the simulations. The results indicated that Q (m) kinetics were relatively insensitive to CvO(2) parameters, but directly associated with V O(2m) kinetics. The biphasic Q (m) response produced a substantial fall in CvO(2) within the first 15-20s of the exercise transition (phase 1 of Q (m)). These results revealed that the main determinant of CvO(2) (or O(2) extraction) kinetics was the dynamic interaction of Q (m) and V O(2m) kinetics during phase 1 of Q (m).

Effect of body tilt on calf muscle performance and blood flow in humans

To explore the effect of posture on muscle performance, we tested the effects of body tilt angle on the strength, endurance, and fatigue of, and blood flow into, the plantar flexors. Human subjects were fixed to a tilt table that could tilt them from the horizontal (0 degrees ) to upright (90 degrees ) position and enabled force to be applied to a footplate through isometric action of the right calf muscle. In experiment 1, six subjects performed a strength test and graded test (intermittent contractions) to the point of failure at three tilt angles (0, 47, and 90 degrees ). In Experiment 2, seven subjects performed a strength test and constant-force test [70% maximum force (F(max)); intermittent contractions] to the point of failure in the horizontal and three inclined positions (32, 47, and 67 degrees ). In experiment 3, leg blood flow was assessed during constant-force exercise at two intensities (30 and 70% F(max)) and two tilt angles (0 and 67 degrees ) in six subjects. Strength was not affected (P > 0.05) by tilt angle. Time to failure during the graded test was significantly higher at 47 degrees (25.9 +/- 2.0 min) and 90 degrees (25.1 +/- 3.0 min) than 0 degrees (22.2 +/- 2.6 min). Time to failure during the constant-force test was also significantly higher at 32 degrees (7.1 +/- 3.6 min), 47 degrees (8.0 +/- 5.2 min), and 67 degrees (8.6 +/- 5.6 min) compared with 0 degrees (4.0 +/- 2.6 min). When graded or constant-force exercise was performed with arterial flow to the leg eliminated, there were no differences in exercise time between the horizontal and an inclined position. During nonischemic exercise, leg blood flow was significantly higher during exercise in the inclined position. These results demonstrate that head-up tilt improves endurance of the plantar flexors, that this effect occurs in the absence of an effect on strength, and that it depends on an intact peripheral circulation. Moreover, the postural effect on muscle endurance appears to be due to a greater blood flow into the leg, an effect that is established during the initial contractions.

Muscle contraction-blood flow interactions during upright knee extension exercise in humans

To test for evidence of a muscle pump effect during steady-state upright submaximal knee extension exercise, seven male subjects performed seven discontinuous, incremental exercise stages (3 min/stage) at 40 contractions/min, at work rates ranging to 60-75% peak aerobic work rate. Cardiac cycle-averaged muscle blood flow (MBF) responses and contraction-averaged blood flow responses were calculated from continuous Doppler sonography of the femoral artery. Net contribution of the muscle pump was estimated by the difference between mean exercise blood flow (MBFM) and early recovery blood flow (MBFR). MBFM rose in proportion with increases in power output with no significant difference between the two methods of calculating MBF. For stages 1 and 5, MBFM was greater than MBFR; for all others, MBFM was similar to MBFR. For the lighter work rates (stages 1-4), there was no significant difference between exercise and early recovery mean arterial pressure (MAP). During stages 5-7, MAP was significantly higher during exercise and fell significantly early in recovery. From these results we conclude that 1) at the lightest work rate, the muscle pump had a net positive effect on MBFM, 2) during steady-state moderate exercise (stages 2-4) the net effect of rhythmic muscle contraction was neutral (i.e., the impedance due to muscle contraction was exactly offset by the potential enhancement during relaxation), and 3) at the three higher work rates tested (stages 5-7), any enhancement to flow during relaxation was insufficient to fully compensate for the contraction-induced impedance to muscle perfusion. This necessitated a higher MAP to achieve the MBFM.

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.

Immediate exercise hyperemia in humans is contraction intensity dependent: evidence for rapid vasodilation

We tested the hypothesis that rapid vasodilation proportional to contraction intensity contributes to the immediate (first cardiac cycle after initial contraction) exercise hyperemia. Ten healthy subjects performed single 1-s isometric forearm contractions at 5, 10, 15, 20, 30, 50, and 70% maximal voluntary contraction intensity (MVC) in arm above heart (AH) and below heart (BH) positions. Forearm blood flow (FBF; brachial artery mean blood velocity, Doppler ultrasound), mean arterial pressure (arterial tonometry), and heart rate (electrocardiogram) were measured beat by beat. Venous emptying (measured with a forearm strain gauge) was already maximized at 5% MVC, indicating that increases in contraction intensity did not further empty the forearm veins. Immediate increases in FBF were linearly proportional to contraction intensity from 5 to 70% MVC in AH (slope = 4.4 ± 0.5%ΔFBF/%MVC). In BH, the immediate increase in FBF demonstrated a curvilinear relationship with increasing contraction intensity and was greater than AH at 15, 20, 30, and 50% MVC ( P < 0.05). Peak changes in FBF were greater in BH vs. AH from 10 to 50% MVC, even when venous refilling was complete ( P < 0.05). These data support the existence of a rapid-acting vasodilatory mechanism(s) at the onset of human forearm exercise.

Effects of chronic heart failure on skeletal muscle capillary hemodynamics at rest and during contractions

Chronic heart failure (CHF) reduces muscle blood flow at rest and during exercise and impairs muscle function. Using intravital microscopy techniques, we tested the hypothesis that the speed and amplitude of the capillary red blood cell (RBC) velocity (VRBC) and flux (FRBC) response to contractions would be reduced in CHF compared with control (C) spinotrapezius muscle. The proportion of capillaries supporting continuous RBC flow was less (P < 0.05) in CHF (0.66 +/- 0.04) compared with C (0.84 +/- 0.01) muscle at rest and was not significantly altered with contractions. At rest, VRBC (C, 270 +/- 62; CHF, 179 +/- 14 microm/s) and FRBC (C, 22.4 +/- 5.5 vs. CHF, 15.2 +/- 1.2 RBCs/s) were reduced (both P < 0.05) in CHF vs. C muscle. Contractions significantly (both P < 0.05) elevated VRBC (C, 428 +/- 47 vs. CHF, 222 +/- 15 microm/s) and FRBC (C, 44.3 +/- 5.5 vs. CHF, 24.0 +/- 1.2 RBCs/s) in C and CHF muscle; however, both remained significantly lower in CHF than C. The time to 50% of the final response was slowed (both P < 0.05) in CHF compared with C for both VRBC (C, 8 +/- 4; CHF, 56 +/- 11 s) and FRBC (C, 11 +/- 3; CHF, 65 +/- 11 s). Capillary hematocrit increased with contractions in C and CHF muscle but was not different (P > 0.05) between CHF and C. Thus CHF impairs diffusive and conductive O2 delivery across the rest-to-contractions transition in rat skeletal muscle, which may help explain the slowed O2 uptake on-kinetics manifested in CHF patients at exercise onset.

Effect of rhythmic tetanic skeletal muscle contractions on peak muscle perfusion

The purpose of this investigation was to examine the effect of rhythmic tetanic skeletal muscle contractions on peak muscle perfusion by using spontaneously perfused canine gastrocnemii in situ. Simultaneous pulsatile blood pressures were measured by means of transducers placed in the popliteal artery and vein, and pulsatile flow was measured with a flow-through-type transit-time ultrasound probe placed in the venous return line. Two series of experiments were performed. In series 1, maximal vasodilation of the muscles' vascular beds was elicited by infusing a normal saline solution containing adenosine (29.3 mg/min) and sodium nitroprusside (180 microg/min) for 15 s and then simultaneously occluding both the popliteal artery and vein for 5 min. The release of occlusion initiated a maximal hyperemic response, during which time four tetanic contractions were induced with supramaximal voltage (6-8 V, 0.2-ms stimuli for 200-ms duration at 50 Hz, 1/s). In series 2, the muscles were stimulated for 3 min before the muscle contractions were stopped for a period of 3 s; stimulation was then resumed. The results of series 1 indicate that, although contractions lowered venous pressure, muscle blood flow was significantly reduced from 2,056 +/- 246 to 1,738 +/- 225 ml x kg(-1) x min(-1) when contractions were initiated and then increased significantly to 1,925 +/- 225 ml x kg(-1) x min(-1) during the first 5 s after contractions were stopped. In series 2, blood flow after 3 min of contractions averaged 1,454 +/- 149 ml x kg(-1) x min(-1). Stopping the contractions for 3 s caused blood flow to increase significantly to 1,874 +/- 172 ml x kg(-1) x min(-1); blood flow declined significantly to 1,458 +/- 139 ml x kg(-1) x min(-1) when contractions were resumed. We conclude that the mechanical action of rhythmic, synchronous, maximal isometric tetanic skeletal muscle contractions inhibits peak muscle perfusion during maximal and near-maximal vasodilation of the muscle's vascular bed. This argues against a primary role for the muscle pump in achieving peak skeletal muscle blood flow.