Local perfusion and metabolic demand during exercise: a noninvasive MRI method of assessment

Limb, sex, but not acute dietary capsaicin, modulate the near-infrared spectroscopy-vascular occlusion test estimate of muscle metabolism

The downward slope during the near-infrared spectroscopy (NIRS)-vascular occlusion test (NIRS-VOT) is purported as a simplified estimate of metabolism. Whether or not the NIRS-VOT exhibits sex- or limb-specificity or may be acutely altered remains to be elucidated. Thus, we investigated if there is limb- or sex specificity in tissue desaturation rates (DeO2) during a NIRS-VOT, and if acute dietary capsaicin may alter this estimate of muscle metabolism. Young healthy men (n = 25, 21 ± 4 years) and women (n = 20, 20 ± 1 years) ingested either placebo or capsaicin, in a counterbalanced, single-blind, crossover design after which a simplified NIRS-VOT was conducted to determine the DeO2 (%/s), as an estimate of oxidative muscle metabolism, in both the forearm (flexors) and thigh (vastus lateralis). There was a significant limb effect with the quadriceps having a greater DeO2 than the forearm (-2.31 ± 1.34 vs. -1.78 ± 1.22%/s, p = 0.007, ηp 2 = 0.19). There was a significant effect of sex on DeO2 (p = 0.005, ηp 2 = 0.203) with men exhibiting a lesser DeO2 than women (-1.73 ± 1.03 vs. -2.36 ± 1.32%/s, respectively). This manifested in significant interactions of limb*capsaicin (p = 0.001, ηp 2 = 0.26) as well as limb*capsaicin*sex on DeO2 (p = 0.013, ηp 2 = 0.16) being observed. Capsaicin does not clearly alter O2-dependent muscle metabolism, but there was apparent limb and sex specificity, interacting with capsaicin in this NIRS-derived assessment.

Non-invasive MR imaging techniques for measuring femoral arterial flow in a pediatric and adolescent cohort

Magnetic Resonance Imaging (MRI) is well‐suited for imaging peripheral blood flow due to its non‐invasive nature and excellent spatial resolution. Although MRI is routinely used in adults to assess physiological changes in chronic diseases, there are currently no MRI‐based data quantifying arterial flow in pediatric or adolescent populations during exercise. Therefore the current research sought to document femoral arterial blood flow at rest and following exercise in a pediatric‐adolescent population using phase contrast MRI, and to present test‐retest reliability data for this method. Ten healthy children and adolescents (4 male; mean age 14.8 ± 2.4 years) completed bloodwork and resting and exercise MRI. Baseline images consisted of PC‐MRI of the femoral artery at rest and following a 5 × 30 s of in‐magnet exercise. To evaluate test‐retest reliability, five participants returned for repeat testing. All participants successfully completed exercise testing in the MRI. Baseline flow demonstrated excellent reliability (ICC = 0.93, p = 0.006), and peak exercise and delta rest‐peak flow demonstrated good reliability (peak exercise ICC = 0.89, p = 0.002, delta rest‐peak ICC = 0.87, p = 0.003) between‐visits. All three flow measurements demonstrated excellent reliability when assessed with coefficients of variance (CV’s) (rest: CV = 6.2%; peak exercise: CV = 7.3%; delta rest‐peak: CV = 7.1%). The mean bias was small for femoral arterial flow. There was no significant mean bias between femoral artery flow visits 1 and 2 at peak exercise. There were no correlations between age or height and any of the flow measurements. There were no significant differences between male and female participants for any of the flow measurements. The current study determined that peripheral arterial blood flow in children and adolescents can be evaluated using non‐invasive phase contrast MRI. The MRI‐based techniques that were used in the current study for measuring arterial flow in pediatric and adolescent patients demonstrated acceptable test‐retest reliability both at rest and immediately post‐exercise.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Effect of differential muscle activation patterns on muscle deoxygenation and microvascular haemoglobin regulation

NEW FINDINGS

What is the central question of this study? Does the presence and extent of heterogeneity in the ratio of O2 delivery to uptake across human muscles relate specifically to different muscle activation patterns? What is the main finding and its importance? During ramp incremental knee-extension and cycling exercise, the profiles of muscle deoxygenation (deoxy[haemoglobin + myoglobin]) and diffusive O2 potential (total[haemoglobin + myoglobin]) in the vastus lateralis corresponded to different muscle activation strategies. However, this was not the case for the rectus femoris, where muscle activation and deoxygenation profiles were dissociated and might therefore be determined by other structural and/or functional attributes (e.g. arteriolar vascular regulation and control of red blood cell flux).

ABSTRACT

Near-infrared spectroscopy has revealed considerable heterogeneity in the ratio of O2 delivery to uptake as identified by disparate deoxygenation {deoxy[haemoglobin + myoglobin] (deoxy[Hb + Mb])} values in the exercising quadriceps. However, whether this represents a recruitment phenomenon or contrasting vascular and metabolic control, as seen among fibre types, has not been established. We used knee-extension (KE) and cycling (CE) incremental exercise protocols to examine whether differential muscle activation profiles could account for the heterogeneity of deoxy[Hb + Mb] and microvascular haemoconcentration (i.e. total[Hb + Mb]). Using time-resolved near-infrared spectroscopy for the quadriceps femoris (vastus lateralis and rectus femoris) during exhaustive ramp exercise in eight participants, we tested the following hypotheses: (i) the deoxy[Hb + Mb] (i.e. fractional O2 extraction) would relate to muscle activation levels across exercise protocols; and (ii) KE would induce greater total[Hb + Mb] (i.e. diffusive O2 potential) at task failure (i.e. peak O2 uptake) than CE irrespective of muscle site. At a given level of muscle activation, as assessed by the relative integrated EMG normalized to maximal voluntary contraction (%iEMGmax ), the vastus lateralis deoxy[Hb + Mb] profile was not different between exercise protocols. However, at peak O2 uptake and until 20% iEMGmax for CE, rectus femoris exhibited a lower deoxy[Hb + Mb] (83.2 ± 15.5 versus 98.2 ± 19.4 μm) for KE than for CE (P < 0.05). The total[Hb + Mb] at peak O2 uptake was not different between exercise protocols for either muscle site. These data support the hypothesis that the contrasting patterns of convective and diffusive O2 transport correspond to different muscle activation patterns in vastus lateralis but not rectus femoris. Thus, the differential deoxygenation profiles for rectus femoris across exercise protocols might be dependent upon specific facets of muscle architecture and functional haemodynamic events.

Muscle oxygenation during dynamic plantar flexion exercise: combining BOLD MRI with traditional physiological measurements

Blood-oxygen-level-dependent magnetic resonance imaging (BOLD MRI) has the potential to quantify skeletal muscle oxygenation with high temporal and high spatial resolution. The purpose of this study was to characterize skeletal muscle BOLD responses during steady-state plantar flexion exercise (i.e., during the brief rest periods between muscle contraction). We used three different imaging modalities (ultrasound of the popliteal artery, BOLD MRI, and near-infrared spectroscopy [NIRS]) and two different exercise intensities (2 and 6 kg). Six healthy men underwent three separate protocols of dynamic plantar flexion exercise on separate days and acute physiological responses were measured. Ultrasound studies showed the percent change in popliteal velocity from baseline to the end of exercise was 151 ± 24% during 2 kg and 589 ± 145% during 6 kg. MRI studies showed an abrupt decrease in BOLD signal intensity at the onset of 2 kg exercise, indicating deoxygenation. The BOLD signal was further reduced during 6 kg exercise (compared to 2 kg) at 1 min (-4.3 ± 0.7 vs. -1.2 ± 0.4%, P < 0.001). Similarly, the change in the NIRS muscle oxygen saturation in the medial gastrocnemius was -11 ± 4% at 2 kg and -38 ± 11% with 6 kg (P = 0.041). In conclusion, we demonstrate that BOLD signal intensity decreases during plantar flexion and this effect is augmented at higher exercise workloads.

Heterogeneity of Muscle Blood Flow and Metabolism: Influence of Exercise, Aging, and Disease States

The systematic increase in V˙O2 uptake and O2 extraction with increasing work rates conceals a substantial heterogeneity of O2 delivery (Q˙O2)-to- V˙O2 matching across and within muscles and other organs. We hypothesize that whether increased/decreased Q˙O2/V˙O2 heterogeneity can be judged as "good" or "bad," for example, after exercise training or in aged individuals or with disease (heart failure, diabetes) depends on the resultant effects on O2 transport and contractile performance.

Advanced Noncontrast MR Imaging in Musculoskeletal Radiology

Multiple nonmorphologic magnetic resonance sequences are available in musculoskeletal imaging that can provide additional information to better characterize and diagnose musculoskeletal disorders and diseases. These sequences include blood-oxygen-level-dependent (BOLD), arterial spin labeling (ASL), diffusion-weighted imaging (DWI), and diffusion-tensor imaging (DTI). BOLD and ASL provide different methods to evaluate skeletal muscle microperfusion. The BOLD signal reflects the ratio between oxyhemoglobin and deoxyhemoglobin. ASL uses selective tagging of inflowing blood spins in a specific region for calculating local perfusion. DWI and DTI provide information about the structural integrity of soft tissue including muscles and fibers as well as pathologies.

Matching of postcontraction perfusion to oxygen consumption across submaximal contraction intensities in exercising humans

Studying the magnitude and kinetics of blood flow, oxygen extraction, and oxygen consumption at exercise onset and during the recovery from exercise can lead to insights into both the normal control of metabolism and blood flow and the disturbances to these processes in metabolic and cardiovascular diseases. The purpose of this study was to examine the on- and off-kinetics for oxygen delivery, extraction, and consumption as functions of submaximal contraction intensity. Eight healthy subjects performed four 1-min isometric dorsiflexion contractions, with two at 20% MVC and two at 40% MVC. During one contraction at each intensity, relative perfusion changes were measured by using arterial spin labeling, and the deoxyhemoglobin percentage (%HHb) was estimated using the spin- and gradient-echo sequence and a previously published empirical calibration. For the whole group, the mean perfusion did not increase during contraction. The %HHb increased from ∼28 to 38% during contractions of each intensity, with kinetics well described by an exponential function and mean response times (MRTs) of 22.7 and 21.6 s for 20 and 40% MVC, respectively. Following contraction, perfusion increased ∼2.5-fold. The %HHb, oxygen consumption, and perfusion returned to precontraction levels with MRTs of 27.5, 46.4, and 50.0 s, respectively (20% MVC), and 29.2, 75.3, and 86.0 s, respectively (40% MVC). These data demonstrate in human subjects the varied recovery rates of perfusion and oxygen consumption, along with the similar rates of %HHb recovery, across these exercise intensities.

Validation of a high-power, time-resolved, near-infrared spectroscopy system for measurement of superficial and deep muscle deoxygenation during exercise

Near-infrared assessment of skeletal muscle is restricted to superficial tissues due to power limitations of spectroscopic systems. We reasoned that understanding of muscle deoxygenation may be improved by simultaneously interrogating deeper tissues. To achieve this, we modified a high-power (∼8 mW), time-resolved, near-infrared spectroscopy system to increase depth penetration. Precision was first validated using a homogenous optical phantom over a range of inter-optode spacings (OS). Coefficients of variation from 10 measurements were minimal (0.5-1.9%) for absorption (μa), reduced scattering, simulated total hemoglobin, and simulated O2 saturation. Second, a dual-layer phantom was constructed to assess depth sensitivity, and the thickness of the superficial layer was varied. With a superficial layer thickness of 1, 2, 3, and 4 cm (μa = 0.149 cm(-1)), the proportional contribution of the deep layer (μa = 0.250 cm(-1)) to total μa was 80.1, 26.9, 3.7, and 0.0%, respectively (at 6-cm OS), validating penetration to ∼3 cm. Implementation of an additional superficial phantom to simulate adipose tissue further reduced depth sensitivity. Finally, superficial and deep muscle spectroscopy was performed in six participants during heavy-intensity cycle exercise. Compared with the superficial rectus femoris, peak deoxygenation of the deep rectus femoris (including the superficial intermedius in some) was not significantly different (deoxyhemoglobin and deoxymyoglobin concentration: 81.3 ± 20.8 vs. 78.3 ± 13.6 μM, P > 0.05), but deoxygenation kinetics were significantly slower (mean response time: 37 ± 10 vs. 65 ± 9 s, P ≤ 0.05). These data validate a high-power, time-resolved, near-infrared spectroscopy system with large OS for measuring the deoxygenation of deep tissues and reveal temporal and spatial disparities in muscle deoxygenation responses to exercise.

Effects of lung ventilation-perfusion and muscle metabolism-perfusion heterogeneities on maximal O2 transport and utilization

KEY POINTS

We expanded a prior model of whole-body O2 transport and utilization based on diffusive O2 exchange in the lungs and tissues to additionally allow for both lung ventilation-perfusion and tissue metabolism-perfusion heterogeneities, in order to estimate V̇O2 and mitochondrial PO2 (PmO2) during maximal exercise. Simulations were performed using data from (a) healthy fit subjects exercising at sea level and at altitudes up to the equivalent of Mount Everest and (b) patients with mild and severe chronic obstructive pulmonary disease (COPD) exercising at sea level. Heterogeneity in skeletal muscle may affect maximal O2 availability more than heterogeneity in lung, especially if mitochondrial metabolic capacity (V̇ MAX ) is only slightly higher than the potential to deliver O2 , but when V̇ MAX is substantially higher than O2 delivery, the effect of muscle heterogeneity is comparable to that of lung heterogeneity. Skeletal muscle heterogeneity may result in a wide range of potential mitochondrial PO 2 values, a range that becomes narrower as V̇ MAX increases; in regions with a low ratio of metabolic capacity to blood flow, PmO2 can exceed that of mixed muscle venous blood. The combined effects of lung and peripheral heterogeneities on the resistance to O2 flow in health decreases with altitude.

ABSTRACT

Previous models of O2 transport and utilization in health considered diffusive exchange of O2 in lung and muscle, but, reasonably, neglected functional heterogeneities in these tissues. However, in disease, disregarding such heterogeneities would not be justified. Here, pulmonary ventilation-perfusion and skeletal muscle metabolism-perfusion mismatching were added to a prior model of only diffusive exchange. Previously ignored O2 exchange in non-exercising tissues was also included. We simulated maximal exercise in (a) healthy subjects at sea level and altitude, and (b) COPD patients at sea level, to assess the separate and combined effects of pulmonary and peripheral functional heterogeneities on overall muscle O2 uptake (V̇O2) and on mitochondrial PO2 (PmO2). In healthy subjects at maximal exercise, the combined effects of pulmonary and peripheral heterogeneities reduced arterial PO2 (PaO2) at sea level by 32 mmHg, but muscle V̇O2 by only 122 ml min(-1) (-3.5%). At the altitude of Mt Everest, lung and tissue heterogeneity together reduced PaO2 by less than 1 mmHg and V̇O2 by 32 ml min(-1) (-2.4%). Skeletal muscle heterogeneity led to a wide range of potential PmO2 among muscle regions, a range that becomes narrower asV̇ MAX increases, and in regions with a low ratio of metabolic capacity to blood flow, PmO2 can exceed that of mixed muscle venous blood. For patients with severe COPD, peak V̇O2 was insensitive to substantial changes in the mitochondrial characteristics for O2 consumption or the extent of muscle heterogeneity. This integrative computational model of O2 transport and utilization offers the potential for estimating profiles of PmO2 both in health and in diseases such as COPD if the extent for both lung ventilation-perfusion and tissue metabolism-perfusion heterogeneity is known.

A method for assessing heterogeneity of blood flow and metabolism in exercising normal human muscle by near-infrared spectroscopy

Heterogeneity in the distribution of both blood flow (Q̇) and O2 consumption (V̇O2) has not been assessed by near-infrared spectroscopy in exercising normal human muscle. We used near-infrared spectroscopy to measure the regional distribution of Q̇ and V̇O2 in six trained cyclists at rest and during constant-load exercise (unloaded pedaling, 20%, 50%, and 80% of peak Watts) in both normoxia and hypoxia (inspired O2 fraction = 0.12). Over six optodes over the upper, middle, and lower vastus lateralis, we recorded 1) indocyanine green dye inflow after intravenous injection to measure Q̇; and 2) fractional tissue O2 saturation (StiO2) to estimate local V̇O2-to-Q̇ ratios (V̇o2/Q̇). Varying both exercise intensity and inspired O2 fraction provided a (directly measured) femoral venous O2 saturation range from about 10 to 70%, and a correspondingly wide range in StiO2. Mean Q̇-weighted StiO2 over the six optodes related linearly to femoral venous O2 saturation in each subject. We used this relationship to compute local muscle venous blood O2 saturation from StiO2 recorded at each optode, from which local V̇O2/Q̇ could be calculated by the Fick principle. Multiplying regional V̇O2/Q̇ by Q̇ yielded the corresponding local V̇O2. While six optodes along only in one muscle may not fully capture the extent of heterogeneity, relative dispersion of both Q̇ and V̇O2 was ∼0.4 under all conditions, while that for V̇O2/Q̇ was minimal (only ∼0.1), indicating in fit young subjects 1) a strong capacity to regulate Q̇ according to regional metabolic need; and 2) a likely minimal impact of heterogeneity on muscle O2 availability.

Reliability of arterial spin labelling measurements of perfusion within the quadriceps during steady-state exercise

Arterial spin labelling (ASL) provides a potential method to non-invasively determine muscle blood flow and examine the impact of interventions such as supplementation and training. However, it's a method with intrinsically low signal, leading to limitations in accuracy and temporal resolution. To examine these limitations, the current study measured perfusion via ASL on three occasions in the rectus femoris of 10 healthy adults, during light and moderate exercise, over three different exercise durations. For data sampled over 9 min, light intensity exercise gave an average perfusion of 35.0 ± 5.1 ml/min.100g(-1) with a coefficient of variation (COV) of 16% and single intraclass correlation coefficient (ICC) of 0.67. For the moderate bout, perfusion was 51.3 ± 5.6 ml/min.100g(-1) (COV 10%, ICC 0.82). When the same data were analyzed over 5 min 24 s, perfusion was 37.8 ± 11.13 (COV 30%, ICC 0.13) during light and 49.5 ± 8.8 ml/min.100g(-1) (COV 18%, ICC 0.52) during moderate exercise. When sampling was reduced to 1 min 48 s, perfusion was 41.2 ± 13.7 (COV 33%, ICC 0.26) during light and 49.5 ± 13.6 ml/min.100g(-1) (COV 28%, ICC 0.04) during moderate exercise. For 9 min a significant perfusion difference was found between the exercise intensities; however, this was not the case for sampling over 5 min 24 s or 1 min 48 s. Such findings illustrate the potential of ASL to non-invasively monitor muscle perfusion under steady-state conditions, but highlight that extended exercise protocols are necessary in order to generate date of sufficient reliability to be able to discriminate intervention dependent perfusion differences.

Dynamic heterogeneity of exercising muscle blood flow and O2 utilization

Resolving the bases for different physiological functioning or exercise performance within a population is dependent on our understanding of control mechanisms. For example, when most young healthy individuals run or cycle at moderate intensities, oxygen uptake (VO2) kinetics are rapid and the amplitude of the VO2 response is not constrained by O2 delivery. For this to occur, muscle O2 delivery (i.e., blood flow × arterial O2 concentration) must be coordinated superbly with muscle O2 requirements (VO2), the efficacy of which may differ among muscles and distinct fiber types. When the O2 transport system succumbs to the predations of aging or disease (emphysema, heart failure, and type 2 diabetes), muscle O2 delivery and O2 delivery-VO2 matching and, therefore, muscle contractile function become impaired. This forces greater influence of the upstream O2 transport pathway on muscle aerobic energy production, and the O2 delivery-VO2 relationship(s) assumes increased importance. This review is the first of its kind to bring a broad range of available techniques, mostly state of the art, including computer modeling, radiolabeled microspheres, positron emission tomography, magnetic resonance imaging, near-infrared spectroscopy, and phosphorescence quenching to resolve the O2 delivery-VO2 relationships and inherent heterogeneities at the whole body, interorgan, muscular, intramuscular, and microvascular/myocyte levels. Emphasis is placed on the following: 1) intact humans and animals as these provide the platform essential for framing and interpreting subsequent investigations, 2) contemporary findings using novel technological approaches to elucidate O2 delivery-VO2 heterogeneities in humans, and 3) future directions for investigating how normal physiological responses can be explained by O2 delivery-VO2 heterogeneities and the impact of aging/disease on these processes.

Evidence of Preserved Oxidative Capacity and Oxygen Delivery in the Plantar Flexor Muscles With Age

Studies examining the effect of aging on skeletal muscle oxidative capacity have yielded equivocal results; however, these investigations may have been confounded by differences in oxygen (O(2)) delivery, physical activity, and small numbers of participants. Therefore, we evaluated skeletal muscle oxidative capacity and O(2) delivery in a relatively large group (N = 40) of young (22 ± 2 years) and old (73 ± 7 years) participants matched for physical activity. After submaximal dynamic plantar flexion exercise, phosphocreatine (PCr) resynthesis ((31)P magnetic resonance spectroscopy), muscle reoxygenation (near-infrared spectroscopy), and popliteal artery blood flow (Doppler ultrasound) were measured. The phosphocreatine recovery time constant (Tau) (young: 33 ± 16; old: 30 ± 11 seconds), maximal rate of adenosine triphosphate (ATP) synthesis (young: 25 ± 9; old: 27 ± 8 mM/min), and muscle reoxygenation rates determined by the deoxyhemoglobin/myoglobin recovery Tau (young: 48 ± 5; old: 47 ± 9 seconds) were similar between groups. Similarly, although tending to be higher in the old, there were no significant age-related differences in postexercise popliteal blood flow (area under the curve: young: 1,665 ± 227 vs old: 2,404 ± 357 mL, p = .06) and convective O(2) delivery (young: 293 ± 146 vs old: 404 ± 191 mL, p = .07). In conclusion, when physical activity and O(2) delivery are similar, oxidative capacity in the plantar flexors is not affected by aging. These findings reveal that diminished skeletal muscle oxidative capacity is not an obligatory accompaniment to the aging process.

Detailed cross-sectional study of 60 superficial femoral artery occlusions: morphological quantitative analysis can lead to a new classification

OBJECTIVE

Current clinical classification of superficial femoral artery (SFA) occlusions as defined by TASC II guidelines is limited to length and calcifications analysis on 2D angiograms, while state-of-the-art cross-sectional imaging like computed tomography angiography (CTA) and magnetic resonance angiography (MRA) provides much more detailed anatomical information than traditional invasive angiography: quantitative morphological analysis of these advanced imaging techniques could therefore be the basis of a refined classification.

METHODS AND RESULTS

Forty-six patients (65% men, 68±11.6 years) that underwent lower limb CTA were retrospectively included, totalizing 60 SFA occlusions. Lesions were classified as TASC II stage A in 3% of cases, stage B in 20%, stage C in 2% and stage D in 75%. For each pathological artery, curved multiplanar reconstructions following the occluded SFA course were used to measure the total length and the mean diameter of the occluded segment. Color-coded map provided an accurate estimation of calcifications' volume. Thirty-nine percent of the occlusions were total. Mean occluded segment length was 219±107 mm (range, 14-530 mm); mean occluded segment diameter was 6.1±1.6 mm (range, 3.4-10 mm); mean calcifications' volume in the occluded segment was 1,265±1,893 mm(3) (range, 0-8,815 mm(3)), corresponding to a percentage of 17.4%±20% (range, 0-88.7%). Shrinked occluded occlusions were defined by a mean diameter under 5 mm and heavily calcified occlusions by a mean percentage of calcifications above 4%. Use of these thresholds allowed the distinction of four groups of patients: heavily calcified occlusions with preserved caliber (56%), non-calcified occlusions with preserved caliber (19%), non-calcified occlusions with small caliber (15%) and heavily calcified occlusions with small caliber (10%).

CONCLUSIONS

SFA OCCLUSIONS ARE DISPARATE: this simple morphological study points out TASC II classification weaknesses for SFA occlusions, as quantitative cross-sectional imaging analysis with measurement of mean occluded diameter and percentage of calcifications can refine it. This could be particularly useful in the management of TASC II type D lesions, for which new endovascular revascularization techniques are arising, and where a CTA or MRA-based morphological classification could provide support in choosing between them.

KEYWORDS

Computer-assisted image processing; femoral artery; multidetector computed tomography; magnetic resonance angiography (MRA); peripheral arterial disease.

The role of muscle mass in exercise-induced hyperemia

Exercise-induced hyperemia is often normalized for muscle mass, and this value is sometimes evaluated at relative exercise intensities to take muscle recruitment into account. Therefore, this study sought to better understand the impact of muscle mass on leg blood flow (LBF) during exercise. LBF was assessed by Doppler ultrasound in 27 young healthy male subjects performing knee-extensor (KE) exercise at three absolute (5, 15, and 25 W) and three relative [20, 40, and 60% of maximum KE (KEmax)] workloads. Thigh muscle mass (5.2-8.1 kg) and LBF were significantly correlated at rest (r = 0.54; P = 0.004). Exercise-induced hyperemia was linearly related to absolute workload, but revealed substantial between-subject variability, documented by the coefficient of variation (5 W: 17%; 15 W: 16%; 25 W: 16%). Quadriceps muscle mass (1.5-2.7 kg) and LBF were not correlated at 5, 15, or 25 W (r = 0.09-0.01; P = 0.7-0.9). Normalizing blood flow for quadriceps muscle mass did not improve the coefficient of variation at each absolute workload (5 W: 21%; 15 W: 21%; 25 W: 22%), while the additional evaluation at relative exercise intensities resulted in even greater variance (20% KEmax: 29%; 40% KEmax: 29%; 60% KEmax: 27%). Similar findings were documented when subjects were parsed into high and low aerobic capacity. Thus, in contrast to rest, blood flow during exercise is unrelated to muscle mass, and simply normalizing for muscle mass or comparing normalized blood flow at a given relative exercise intensity has no effect on the inherent blood flow variability. Therefore, during exercise, muscle mass does not appear to be a determinant of the hyperemic response.

Muscle metabolism and activation heterogeneity by combined 31P chemical shift and T2 imaging, and pulmonary O2 uptake during incremental knee-extensor exercise

The integration of skeletal muscle substrate depletion, metabolite accumulation, and fatigue during large muscle-mass exercise is not well understood. Measurement of intramuscular energy store degradation and metabolite accumulation is confounded by muscle heterogeneity. Therefore, to characterize regional metabolic distribution in the locomotor muscles, we combined 31P magnetic resonance spectroscopy, chemical shift imaging, and T2-weighted imaging with pulmonary oxygen uptake during bilateral knee-extension exercise to intolerance. Six men completed incremental tests for the following: (1) unlocalized 31P magnetic resonance spectroscopy; and (2) spatial determination of 31P metabolism and activation. The relationship of pulmonary oxygen uptake to whole quadriceps phosphocreatine concentration ([PCr]) was inversely linear, and three of four knee-extensor muscles showed activation as assessed by change in T2. The largest changes in [PCr], [inorganic phosphate] ([Pi]) and pH occurred in rectus femoris, but no voxel (72 cm3) showed complete PCr depletion at exercise cessation. The most metabolically active voxel reached 11 ± 9 mM [PCr] (resting, 29 ± 1 mM), 23 ± 11 mM [Pi] (resting, 7 ± 1 mM), and a pH of 6.64 ± 0.29 (resting, 7.08 ± 0.03). However, the distribution of 31P metabolites and pH varied widely between voxels, and the intervoxel coefficient of variation increased between rest (∼10%) and exercise intolerance (∼30-60%). Therefore, the limit of tolerance was attained with wide heterogeneity in substrate depletion and fatigue-related metabolite accumulation, with extreme metabolic perturbation isolated to only a small volume of active muscle (<5%). Regional intramuscular disturbances are thus likely an important requisite for exercise intolerance. How these signals integrate to limit muscle power production, while regional "recruitable muscle" energy stores are presumably still available, remains uncertain.

Methodological validation of the dynamic heterogeneity of muscle deoxygenation within the quadriceps during cycle exercise

The conventional continuous wave near-infrared spectroscopy (CW-NIRS) has enabled identification of regional differences in muscle deoxygenation following onset of exercise. However, assumptions of constant optical factors (e.g., path length) used to convert the relative changes in CW-NIRS signal intensity to values of relative concentration, bring the validity of such measurements into question. Furthermore, to justify comparisons among sites and subjects, it is essential to correct the amplitude of deoxygenated hemoglobin plus myoglobin [deoxy(Hb+Mb)] for the adipose tissue thickness (ATT). We used two time-resolved NIRS systems to measure the distribution of the optical factors directly, thereby enabling the determination of the absolute concentrations of deoxy(Hb+Mb) simultaneously at the distal and proximal sites within the vastus lateralis (VL) and the rectus femoris muscles. Eight subjects performed cycle exercise transitions from unloaded to heavy work rates (>gas exchange threshold). Following exercise onset, the ATT-corrected amplitudes (Ap), time delay (TDp), and time constant (τp) of the primary component kinetics in muscle deoxy(Hb + Mb) were spatially heterogeneous (intersite coefficient of variation range for the subjects: 10–50 for Ap, 16–58 for TDp, 14–108% for τp). The absolute and relative amplitudes of the deoxy(Hb+Mb) responses were highly dependent on ATT, both within subjects and between measurement sites. The present results suggest that regional heterogeneity in the magnitude and temporal profile of muscle deoxygenation is a consequence of differential matching of O2 delivery and O2 utilization, not an artifact caused by changes in optical properties of the tissue during exercise or variability in the overlying adipose tissue.

Muscle intracellular oxygenation during exercise: optimization for oxygen transport, metabolism, and adaptive change

Exercise is the example par excellence of the body functioning as a physiological system. Conventionally we think of the O(2) transport process as a major manifestation of that system linking and integrating pulmonary, cardiovascular, hematological and skeletal muscular contributions to the task of getting O(2) from the air to the mitochondria, and this process has been well described. However, exercise invokes system responses at levels additional to those of macroscopic O(2) transport. One such set of responses appears to center on muscle intracellular PO(2), which falls dramatically from rest to exercise. At rest, it approximates 4 kPa, but during heavy endurance exercise it falls to about 0.4-0.5 kPa, an amazingly low value for a tissue absolutely dependent on the continual supply of O(2) to meet very high energy demands. One wonders why intracellular PO(2) is allowed to fall to such levels. The proposed answer, to be presented in the review, is that a low intramyocyte PO(2) is pivotal in: (a) optimizing oxygen's own physiological transport, and (b) stimulating adaptive gene expression that, after translation, enables greater exercise capacity-all the while maintaining PO(2) at levels sufficient to allow oxidative phosphorylation to operate sufficiently fast enough to support intense muscle contraction. Thus, during exercise, reductions of intracellular PO(2) to less than 1% of that in the atmosphere enables an integrated response that fundamentally and simultaneously optimizes physiological, biochemical and molecular events that support not only the exercise as it happens but the adaptive changes to increase exercise capacity over the longer term.

Heterogeneity of muscle deoxygenation kinetics during two bouts of repeated heavy exercises

This study examines the effect of prior heavy exercise on the spatial distribution of muscle deoxygenation kinetics at the onset of heavy-intensity cycling exercise. Young untrained male adults (n = 16) performed two consecutive bouts of 6 min of high intensity cycle exercise separated by 6 min at 35 W. Muscle deoxygenation (HHb) was monitored continuously by near-infrared spectroscopy at eight sites in the quadriceps. Prior heavy exercise reduced the delay before the increase in HHb (9 +/- 2 vs. 5 +/- 2 s; P < 0.001). The standard deviation of TD HHb of the eight sites was decreased by the performance of prior exercise (1.1 +/- 0.5 vs. 0.8 +/- 0.4 s; P < 0.05). The transient decrease in HHb during the first 10 s of exercise was less during the second bout than during the first bout (0.6 +/- 0.6 vs. 0.3 +/- 0.3 A.U.; P < 0.01). The standard deviation of this decrease was also reduced by prior exercise (0.5 +/- 0.3 vs. 0.3 +/- 0.2 A.U.; P < 0.01). Lastly, prior exercise decreased significantly the standard deviation of the HHb rise during the time period corresponding to the pulmonary VO(2) slow component. These results indicate that prior heavy exercise reduced the spatial heterogeneity of muscle deoxygenation kinetics at the early onset of heavy exercise and during the development of the pulmonary VO(2) slow component. It indicates that the distribution of the VO(2)/O(2) delivery ratio within muscle was improved by the performance of a prior exercise.

Influence of priming exercise on pulmonary O2 uptake kinetics during transitions to high-intensity exercise at extreme pedal rates

We investigated the pedal rate dependency of the effect of priming exercise on pulmonary oxygen uptake (Vo(2)) kinetics. Seven healthy men completed two, 6-min bouts of high-intensity cycle exercise (separated by 6 min of rest) using different combinations of extreme pedal rates for the priming and criterion exercise bouts (i.e., 35-->35, 35-->115, 115-->35, and 115-->115 rev/min). Pulmonary gas exchange and heart rate were measured breath-by-breath, and muscle oxygenation was assessed using near-infrared spectroscopy. When the priming bout was performed at 35 rev/min (35-->35 and 35-->115 conditions), the phase II Vo(2) time constant (tau) was not significantly altered (bout 1: 31 +/- 7 vs. bout 2: 30 +/- 5 s and bout 1: 48 +/- 16 vs. bout 2: 46 +/- 21 s, respectively). However, when the priming bout was performed at 115 rev/min (115-->35 and 115-->115 conditions), the phase II tau was significantly reduced (bout 1: 31 +/- 7 vs. bout 2: 26 +/- 5 s and bout 1: 48 +/- 16 vs. bout 2: 39 +/- 9 s, respectively, P < 0.05). Muscle oxygenation was significantly higher after priming exercise in all four conditions, but significant effects on Vo(2) kinetics were only evident when muscle O(2) extraction (measured as Delta[deoxyhemoglobin]/DeltaVo(2)) was elevated in the fundamental response phase. These data indicate that prior high-intensity exercise at a high pedal rate can speed Vo(2) kinetics during subsequent high-intensity exercise, presumably through specific priming effects on type II muscle fibers.

Muscle deoxygenation of upper-limb muscles during progressive arm-cranking exercise

The purpose of this study was to determine which upper-limb muscle exhibits the greatest change in muscle deoxygenation during arm-cranking exercise (ACE). We hypothesized that the biceps brachii (BB) would show the greatest change in muscle deoxygenation during progressive ACE to exhaustion relative to triceps brachii (TR), brachioradialis (BR), and anterior deltoid (AD). Healthy young men (n = 11; age = 27 +/- 1 y; mean +/- SEM) performed an incremental ACE test to exhaustion. Near-infrared spectroscopy (NIRS) was used to monitor the relative concentration changes in oxy- (O2Hb), deoxy- (HHb), and total hemoglobin (Hbtot), as well as tissue oxygenation index (TOI) in each of the 4 muscles. During submaximal arm exercise, we found that changes to NIRS-derived measurements were not different between the 4 muscles studied (p > 0.05). At maximal exercise HHb was significantly higher in the BB compared with AD (p < 0.05). Relative to the other 3 muscles, BB exhibited the greatest decrease in O2Hb and TOI (p < 0.05). Our investigation provides two new and important findings: (i) during submaximal ACE the BB, TR, BR, and AD exhibit similar changes in muscle deoxygenation and (ii) during maximal ACE the BB exhibits the greatest change in intramuscular O2 status.

Spatial heterogeneity in the muscle functional MRI signal intensity time course: effect of exercise intensity

It has previously been observed that during isometric dorsiflexion exercise, the time course of T2-weighted signal intensity (SI) changes is spatially heterogeneous. The purpose of this study was to test the hypothesis that this spatial heterogeneity would increase at higher contraction intensities. Eight subjects performed 90-s isometric dorsiflexion contractions at 30% and 60% of maximum voluntary contraction (MVC) while T2-weighted (repetition time/echo time=4000/35 ms) images were acquired. SI was measured before, during and after the contractions in regions of interest (ROIs) in the extensor digitorum longus (EDL) muscle and the deep and superficial compartments of the tibialis anterior (D-TA and S-TA, respectively). For all ROIs at 30% MVC, SI changes were similar. The maximum postcontraction SI was greater than the SI during exercise. At 60% MVC, SI changes during contraction were greater in the S-TA than in the D-TA and EDL. For the EDL and D-TA, the maximum postcontraction SI was greater than those during exercise. For the S-TA, the maximum postcontraction change was greater than the changes at t=8, 20 and 56 s but not the end-exercise value. We conclude that spatial heterogeneity increases during more intense dorsiflexion contractions, possibly reflecting regional differences in perfusion or neural activation of the muscle.

Testing of a Noninvasive Probe for Measurement of Blood Perfusion

Parameter estimation techniques have been utilized in the development of a methodology to noninvasively measure blood perfusion using a new thermal surface probe. The core of this probe is comprised of a small, lightweight heat flux sensor that is placed in contact with tissue and provides time-resolved signals of heat flux and surface temperature while the probe is cooled by air jets. Parameter estimation techniques were developed that incorporate heat flux and temperature data with calculated data from a biothermal model of the tissue and probe. The technique simultaneously estimates blood perfusion and thermal contact resistance between the probe and tissue. Validation of this concept was carried out by experimentation with controlled flow through nonbiological porous media. Warm water was circulated through a fine pore sponge to provide a phantom model for blood perfusion through biological tissue. The parameter estimation technique was applied to measurements taken over a range of flow rates. Heat flux and temperature measurements and the resulting perfusion estimates correlated well with the experimentally imposed perfusion rate. This research helps establish the validity of using this method to develop a practical, noninvasive probe to clinically measure blood perfusion.

Diffusion weighted imaging in osteoradiology

Diffusion weighted imaging gained attention as an imaging modality, which provides information on the microstructure of a tissue, which can be used for tissue characterization. This is of importance in patients where other diagnostic tools provide equivocal or unspecific information. In addition quantitative diffusion measurements provide objective parameters for unbiased comparison of treatment response, which is mandatory for therapy monitoring. Technical restriction limited the use of Diffusion Weighted Imaging to the brain. However, with the improvement in scanner technology and the availability of new MR sequences investigation of the Muskulo Skeletal System was made possible. We describe the potential of Diffusion Weighted Imaging as a non-invasive technique to evaluate pathological, inflammatory and physiological processes in osteoradiology.

Peripheral arterial disease assessment: wall, perfusion, and spectroscopy

INTRODUCTION

Peripheral arterial disease (PAD) is characterized by lower limb arterial obstruction due to atherosclerosis and is increasingly common. Presently used methods for diagnosis and follow-up as well as for assessment of novel therapies are limited.

MATERIALS AND METHODS

Three distinct magnetic resonance examinations were developed. The first was high-resolution black-blood atherosclerotic plaque imaging of the superficial femoral artery using a surface coil and flow saturation. Second, first-pass contrast-enhanced dual-contrast perfusion imaging of the calf muscle was performed at peak exercise using a magnetic resonance (MR)-compatible pedal ergometer. Lastly, (31)P MR spectroscopy was also performed at peak exercise to measure phosphocreatine (PCr) recovery kinetics.

RESULTS

Seventeen patients (age, 63 +/- 10 yrs) with mild to moderate PAD were studied with black-blood atherosclerotic plaque imaging. Mean atherosclerotic plaque volume measured was 7.27 +/- 3.73 cm(3). Eleven patients (age, 61 +/- 11 yrs) with mild to moderate symptomatic PAD and 22 normal control subjects were studied with first-pass contrast-enhanced perfusion imaging. Perfusion index was stepwise increased from patients to normal subjects with matched workload to normal subjects at maximal exercise. For PCr recovery kinetics, 20 patients with mild to moderate PAD and 14 controls were studied. The median recovery time constant of PCr was 34.7 seconds in the controls and 91.0 seconds in the PAD patients (P < 0.0001).

CONCLUSIONS

Three distinct MR examinations of different aspects of peripheral arterial disease have been developed and tested and shown to differentiate patients with mild to moderate PAD from normal controls. Taken together, these tests are potential quantitative end points for clinical trials of novel therapies in PAD.

Spatial heterogeneity of quadriceps muscle deoxygenation kinetics during cycle exercise

To test the hypothesis that, during exercise, substantial heterogeneity of muscle hemoglobin and myoglobin deoxygenation [deoxy(Hb + Mb)] dynamics exists and to determine whether such heterogeneity is associated with the speed of pulmonary O(2) uptake (pVo(2)) kinetics, we adapted multi-optical fibers near-infrared spectroscopy (NIRS) to characterize the spatial distribution of muscle deoxygenation kinetics at exercise onset. Seven subjects performed cycle exercise transitions from unloaded to moderate [<gas exchange threshold (GET)] and heavy (>GET) work rates and the relative changes in deoxy(Hb + Mb), at 10 sites in the quadriceps, were sampled by NIRS. At exercise onset, the time delays in muscle deoxy(Hb + Mb) were spatially inhomogeneous [intersite coefficient of variation (CV), 3~56% for <GET, 2~21% for >GET]. The primary component kinetics (time constant) of muscle deoxy(Hb + Mb) reflecting increased O(2) extraction were also spatially inhomogeneous (intersite CV, 6~48% for <GET, 7~47% for >GET) and faster (P < 0.05) than those of phase 2 pVo(2). However, the degree of dynamic intersite heterogeneity in muscle deoxygenation did not correlate significantly with phase 2 pVo(2) kinetics. In conclusion, the dynamics of quadriceps microvascular oxygenation demonstrates substantial spatial heterogeneity that must arise from disparities in the relative kinetics of Vo(2) and O(2) delivery increase across the regions sampled.

Time-courses of perfusion and phosphocreatine in rat leg during low-level exercise and recovery

PURPOSE

To develop a noninvasive protocol for measuring local perfusion and metabolic demand in muscle tissue with sufficient sensitivity and time resolution to monitor kinetics at the onset of low-level exercise and during recovery.

MATERIALS AND METHODS

Capillary-level perfusion, the critical factor that determines oxygen and substrate delivery to active muscle, was measured by an arterial spin labeling (ASL) technique optimized for skeletal muscle. Phosphocreatine (PCr) kinetics, which signal the flux of oxidative phosphorylation, were measured by (31)P MR spectroscopy. Perfusion and PCr measurements were made in parallel studies before, during, and after three different intensities of low-level, stimulated exercise in rat hind limb.

RESULTS

The data reveal close coupling between the perfusion response and PCr changes. The onset and recovery time constants for PCr changes were independent of contractile force over the range of forces studied. Perfusion time constants during both onset of exercise and recovery tended to increase with contractile force.

CONCLUSION

These results demonstrate that the protocol implemented can be useful for probing the mechanisms that control skeletal muscle blood flow, the physiological limits to muscle performance, and the causes for the attenuated exercise-induced hyperemia observed in disease states.

Calf muscle perfusion at peak exercise in peripheral arterial disease: measurement by first-pass contrast-enhanced magnetic resonance imaging

PURPOSE

To develop a contrast-enhanced magnetic resonance (MR) technique to measure skeletal muscle perfusion in peripheral arterial disease (PAD).

MATERIALS AND METHODS

A total of 11 patients (age = 61 +/- 11 years) with mild to moderate symptomatic PAD (ankle-brachial index [ABI] = 0.75 +/- 0.08) and 22 normals were studied using an MR-compatible ergometer. PAD and normal(max) (Nl(max); N = 11) exercised to exhaustion. Nl(low) (N = 11) exercised to the same workload achieved by PAD. At peak exercise, 0.1 mm/kg of gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) was infused at 3-4 cm(3)/second followed by a saline flush at the same rate. A dual-contrast gradient echo (GRE) sequence enabled simultaneous acquisition of muscle perfusion and arterial input function (AIF). The perfusion index (PI) was defined as the slope of the time-intensity curve (TIC) in muscle divided by the arterial TIC slope.

RESULTS

Median workload was 120 Joules in PAD, 210 Joules in Nl(low), and 698 Joules in Nl(max) (P < 0.001 vs. Nl(low) and PAD). Median PI was 0.29 in PAD (25th and 75th percentiles [%] = 0.20, 0.40), 0.48 in Nl(low) (25th, 75th % = 0.36, 0.62; P < 0.02 vs. PAD), and 0.69 in Nl(max) (25th, 75th % = 0.5, 0.77; P < 0.001 vs. PAD). Area under the ROC-curve for PI differentiating patients from Nl(max) was 0.95 (95% confidence interval [CI] = 0.77-0.99).

CONCLUSION

Peak-exercise measurement of lower limb perfusion with dual-contrast, first-pass MR distinguishes PAD from normals. This method may be useful in the study of novel therapies for PAD.

Quantitative diffusion tensor MRI-based fiber tracking of human skeletal muscle

Diffusion-tensor magnetic resonance imaging (DT-MRI) offers great potential for understanding structure-function relationships in human skeletal muscles. The purposes of this study were to demonstrate the feasibility of using in vivo human DT-MRI fiber tracking data for making pennation angle measurements and to test the hypothesis that heterogeneity in the orientation of the tibialis anterior (TA) muscle's aponeurosis would lead to heterogeneity in pennation angle. Eight healthy subjects (5 male) were studied. T(1)-weighted anatomical MRI and DT-MRI data were acquired of the TA muscle. Fibers were tracked from the TA's aponeurosis by following the principal eigenvector. The orientations of the aponeurosis and muscle fiber tracts in the laboratory frame of reference and the orientation of the fiber tracts with respect to the aponeurosis [i.e., the pennation angle (theta)] were determined. The muscle fiber orientations, when expressed relative to the laboratory frame of reference, did not change as functions of superior-to-inferior position. The sagittal and coronal orientations of the aponeurosis did not change in practically significant manners either, but the aponeurosis' axial orientation changed by approximately 40 degrees . As a result, the mean value for theta decreased from 16.3 (SD 6.9) to 11.4 degrees (SD 5.0) along the muscle's superior-to-inferior direction. The mean value of theta was greater in the deep than in the superficial compartment. We conclude that pennation angle measurements of human muscle made using DT-MRI muscle fiber tracking are feasible and reveal that in the foot-head direction, there is heterogeneity in the pennation properties of the human TA muscle.

Nonlinear magnetic field gradients can reduce SAR in flow-driven arterial spin labeling measurements

This work describes how custom-built gradient coils, designed to generate magnetic fields with amplitudes that vary nonlinearly with position, can be used to reduce the potential for unsafe tissue heating during flow-driven arterial spin labeling processes. A model was developed to allow detailed analysis of the adiabatic excitation process used for flow-driven arterial water stimulation with elimination of tissue signal (FAWSETS) an arterial spin labeling method developed specifically for use in skeletal muscle. The model predicted that, by adjusting the amplitude of the gradient field, the specific absorption rate could be reduced by more than a factor of 6 while still achieving effective labeling. Flow phantom measurements and in vivo measurements from exercising rat hind limb confirmed the accuracy of the model's predictions. The modeling tools were also applied to the more widely used continuous arterial spin labeling (CASL) method and predicted that specially shaped gradients could allow similar reductions in SAR.

Effects of “priming” exercise on pulmonary O2 uptake and muscle deoxygenation kinetics during heavy-intensity cycle exercise in the supine and upright positions

We hypothesized that the performance of prior heavy exercise would speed the phase 2 oxygen consumption (V̇o2) kinetics during subsequent heavy exercise in the supine position (where perfusion pressure might limit muscle O2 supply) but not in the upright position. Eight healthy men (mean ± SD age 24 ± 7 yr; body mass 75.0 ± 5.8 kg) completed a double-step test protocol involving two bouts of 6 min of heavy cycle exercise, separated by a 10-min recovery period, on two occasions in each of the upright and supine positions. Pulmonary O2 uptake was measured breath by breath and muscle oxygenation was assessed using near-infrared spectroscopy (NIRS). The NIRS data indicated that the performance of prior exercise resulted in hyperemia in both body positions. In the upright position, prior exercise had no significant effect on the time constant (τ) of the V̇o2 response in phase 2 ( bout 1: 29 ± 10 vs. bout 2: 28 ± 4 s; P = 0.91) but reduced the amplitude of the V̇o2 slow component ( bout 1: 0.45 ± 0.16 vs. bout 2: 0.22 ± 0.14 l/min; P = 0.006) during subsequent heavy exercise. In contrast, in the supine position, prior exercise resulted in a significant reduction in the phase 2 τ ( bout 1: 38 ± 18 vs. bout 2: 24 ± 9 s; P = 0.03) but did not alter the amplitude of the V̇o2 slow component ( bout 1: 0.40 ± 0.29 vs. bout 2: 0.41 ± 0.20 l/min; P = 0.86). These results suggest that the performance of prior heavy exercise enables a speeding of phase 2 V̇o2 kinetics during heavy exercise in the supine position, presumably by negating an O2 delivery limitation that was extant in the control condition, but not during upright exercise, where muscle O2 supply was probably not limiting.

Muscle blood flow and oxygenation measured by NMR imaging and spectroscopy

Tissue perfusion and oxygenation in many organs can be evaluated by various NMR techniques. This review focuses on the specificities, limitations and adaptations of the NMR tools available to investigate perfusion and oxygenation in the skeletal muscle of humans and animal models. A description of how they may be used simultaneously is provided as well. 1H NMR spectroscopy of myoglobin (Mb) monitors intramyocytic oxygenation. It measures the level of deoxy-Mb, from which Mb concentration, Mb desaturation/resaturation rates, muscle oxygenation changes and intracellular partial oxygen pressure (pO2) can be calculated. Positive and negative blood oxygen level-dependent (BOLD) contrasts exist in skeletal muscle. BOLD contrasts primarily reflect changes in capillary-venous oxygenation, but are also directly or indirectly dependent on muscle blood volume, perfusion, vascular network architecture and angulation, relative to the main magnetic field. Arterial spin labelling (ASL) techniques, having high spatial and temporal resolution, are the methods of choice to quantify and map skeletal muscle perfusion non-invasively. Limitations of ASL are poor contrast-to-noise ratio and sensitivity to movement; however, with the introduction of specific adaptations, it has been proven possible to measure skeletal muscle perfusion at both rest and during exercise. The possibility of combining these NMR measurements with others into a single dynamic protocol is most interesting. The 'multiparametric functional (mpf) NMR' concept can be extended to include the evaluation of muscle energy metabolism simultaneously with 31P NMR or with lactate double quantum filtered 1H NMR spectroscopy, an approach which would make NMR an exceptional tool for non-invasive investigations of integrative physiology and biochemistry in skeletal muscle in vivo.

Relationship between local perfusion and FFA uptake in human skeletal muscle—no effect of increased physical activity and aerobic fitness

We investigated heredity-independent effects of increased physical activity and aerobic fitness on skeletal muscle free fatty acid (FFA) uptake, perfusion, and their heterogeneity at rest and during exercise. Also, the relationship between local skeletal muscle FFA uptake and perfusion was studied. Nine young adult male monozygotic twin pairs with significant difference in physical activity [229 min (SD 156) average time spent for conditioning exercise per week in more and 98 min (SD 71) in less active twins, P = 0.013] and aerobic fitness [18% (SD 10) difference in maximum O2 uptake] between brothers were studied using positron emission tomography. Submaximal knee-extension exercise increased perfusion, FFA uptake, and oxygen uptake in quadriceps femoris muscles 6–10 times compared with resting values ( P < 0.001). More active twins tended to utilize more oxygen, while no differences were found in muscle perfusion or FFA uptake between groups. Mean perfusion and FFA uptake correlated strongly at a whole muscle level, both at rest ( r = 0.97, P = 0.03 in more and r = 0.98, P = 0.02 in less active twins) and during exercise ( r = 0.99, P = 0.01 and r = 0.94, P = 0.06), but at the voxel level (87 mm3) correlation was only moderate during exercise [ r = 0.73 (SD 0.08) vs. r = 0.74 (SD 0.10), P = 0.92] and weak at rest [ r = 0.28 (SD 0.13) vs. r = 0.33 (SD 0.21), P = 0.58]. Exercise decreased both perfusion and FFA uptake heterogeneity within the muscles ( P < 0.001) similarly in both groups. In conclusion, long-term history of moderately increased physical activity tends to enhance muscle oxidative metabolism, but it does not have any significant influence on the FFA uptake or perfusion rates or their heterogeneity in skeletal muscle. Submaximal knee-extension exercise decreases heterogeneity of muscle FFA uptake and perfusion and improves matching between local muscle perfusion and FFA uptake. Thus it seems that the genetic influence is more important to determine the heterogeneity of perfusion and FFA uptake in skeletal muscle than exercise training.

Skeletal muscle energetics with PNMR: personal views and historic perspectives

This article reviews historical and current NMR approaches to describing in vivo bioenergetics of skeletal muscles in normal and diseased populations. It draws upon the first author's more than 70 years of personal experience in enzyme kinetics and the last author's physiological approaches. The development of in vivo PNMR jointly with researchers around the world is described. It is explained how non-invasive PNMR has advanced human exercise biochemistry, physiology and pathology. Further, after a brief explanation of bioenergetics with PNMR on creatine kinase, anerobic glycolysis and mitochondrial oxidative phosphorylation, some basic and controversial subjects are focused upon, and the authors' view of the subjects are offered, with questions and answers. Some of the research has been introduced in exercise physiology. Future directions of NMR on bioenergetics, as a part of system biological approaches, are indicated.

Muscle Perfusion and Metabolic Heterogeneity

Recent developments in noninvasive imaging techniques have enabled the study of local changes in perfusion and metabolism in skeletal muscle as well as patterns of heterogeneity in these variables in humans. In this review, the principles of these techniques along with some recent findings on functional heterogeneity in human skeletal muscle will be presented.

An automated image-processing strategy to analyze dynamic arterial spin labeling perfusion studies. Application to human skeletal muscle under stress

Arterial spin labeling (ASL) perfusion measurements allow the follow-up of muscle perfusion with high temporal resolution during a stress test. Automated image processing is proposed to estimate perfusion maps from ASL images. It is based on two successive analyses: at first, automated rejection of the image pairs between which a large displacement is detected is performed, followed by factor analysis of the dynamic data and cluster analysis to classify pixels with large signal variation characteristic of vessels. Then, after masking these "vascular" pixels, factor analysis and cluster analysis are further applied to separate the different muscles between low or high perfusion increase, yielding a functional map of the leg. Data from 10 subjects (five normal volunteers and five elite sportsmen) had been analyzed. Resulting time perfusion curves from a region of interest (ROI) in active muscles show a good accordance whether extracted with automated processing or with manual processing. This method of functional segmentation allows automated suppression of vessels and fast visualization of muscles with high, medium or low perfusion, without any a priori knowledge.

Alpha-Adrenergic receptor responsiveness is preserved during prolonged exercise

Our laboratory has previously reported a decline in sympathetic nervous system restraint of skeletal muscle blood flow during prolonged mild-intensity exercise. This decline may be explained by a decrease in alpha(1)- and alpha(2)-adrenergic receptor responsiveness over time. Thus the purpose of the present study was to investigate the effect of exercise duration on alpha(1)- and alpha(2)-adrenergic receptor responsiveness during prolonged constant-load exercise. Mongrel dogs (n = 6) were instrumented chronically with transit-time flow probes on the external iliac arteries and an indwelling catheter in a branch of the femoral artery. On separate days, flow-adjusted doses of selective alpha(1)- (phenylephrine) alpha(2)-adrenergic-receptor (clonidine) agonists, and tyramine (to evoke endogenous norepinephrine release) were infused following 5, 30 and 50 min of mild-intensity treadmill exercise (3 miles/h), with hindlimb blood flow (HBF) and mean arterial pressure (MAP) monitored continuously. Vascular conductance (VC) was calculated as HBF/MAP. While the dogs ran on the treadmill at 3 miles/h, infusion of phenylephrine resulted in similar decreases in VC after 5 [73% (SD 10)], 30 [76% (SD 9)], and 50 [73% (SD 10)] min of exercise. Infusion of the alpha(2)-agonist clonidine also produced similar decreases in VC after 5 [58% (SD 10)], 30 [58% (SD 11)], and 50 [53% (SD 12)] min of exercise. Infusion of tyramine resulted in similar decreases in VC after 5 [55% (SD 15)], 30 [51% (SD 10)], and 50 [50% (SD 7)] min of exercise. These results demonstrate that alpha(1)- and alpha(2)-adrenergic receptor responsiveness to infusion of selective alpha(1)- and alpha(2)-adrenergic-receptor agonists and endogenous norepinephrine release (tyramine) does not decline during prolonged mild-intensity exercise. Thus a decrease in alpha-adrenergic receptor responsiveness over time does not appear to be responsible for the decrease in sympathetic restraint of muscle blood flow during prolonged exercise.

Nitric oxide and prostaglandins influence local skeletal muscle blood flow during exercise in humans: coupling between local substrate uptake and blood flow

Synergic action of nitric oxide (NO) and prostaglandins (PG) in the regulation of muscle blood flow during exercise has been demonstrated. In the present study, we investigated whether these vasodilators also regulate local blood flow, flow heterogeneity, and glucose uptake within the exercising skeletal muscle. Skeletal muscle blood flow was measured in seven healthy young men using near-infrared spectroscopy and indocyanine green and muscle glucose uptake using positron emission tomography and 2-fluoro-2-deoxy-d-[18F]glucose without and with local blockade of NO and PG at rest and during one-legged dynamic knee-extension exercise. Local blockade was produced by infusing nitro-l-arginine methyl ester and indomethacin directly in the muscle via a microdialysis catheter. Blood flow and glucose uptake were measured in the region of blockade and in two additional regions of vastus lateralis muscle 1 and 4 cm away from the infusion of blockers. Local blockade during exercise at 25 and 40 watts significantly decreased blood flow in the infusion region and in the region 1 cm away from the site of infusion but not in the region 4 cm away. During exercise, muscle glucose uptake did not show any regional differences in response to blockade. These results show that NO and PG synergistically contribute to the local regulation of blood flow in skeletal muscle independently of muscle glucose uptake in healthy young men. Thus these vasodilators can play a role in regulating microvascular blood flow in localized regions of vastus lateralis muscle but do not influence regional glucose uptake. The findings suggest that local substrate uptake in skeletal muscle can be regulated independently of regional changes in blood flow.

Delayed calf muscle phosphocreatine recovery after exercise identifies peripheral arterial disease

OBJECTIVES

In this study we intend to characterize phosphocreatine (PCr) recovery kinetics with phosphorus-31 ((31)P) magnetic resonance spectroscopy in symptomatic peripheral arterial disease (PAD) patients compared with control subjects and determine the diagnostic value and reproducibility of this parameter.

BACKGROUND

Due to the inconsistent relationship between flow and function in PAD, novel techniques focused on the end-organ are needed to assess disease severity and measure therapeutic response.

METHODS

Fourteen normal subjects (5 men, age 45 +/- 14 years) and 20 patients with mild-to-moderate symptomatic PAD (12 men, age 67 +/- 10 years, mean ankle brachial index 0.62 +/- 0.13) were studied. Subjects exercised one leg to exhaustion while supine in a 1.5-T magnetic resonance scanner using a custom-built plantar flexion device. Surface coil-localized, free induction decay acquisition localized to the mid-calf was used. Each 31P spectrum consisted of 25 signal averages at a repetition time of 550 ms. The PCr recovery time constant was calculated by monoexponential fit of PCr versus time, beginning at exercise completion.

RESULTS

Median exercise time was 195.0 s in normal subjects and 162.5 s in PAD patients (p = 0.06). Despite shorter exercise times in patients, the median recovery time constant of PCr was 34.7 s in normal subjects and 91.0 s in PAD patients. Area under the receiver-operating characteristic curve was 0.925 +/- 0.045. Test-retest reliability was excellent.

CONCLUSIONS

The PCr recovery time constant is prolonged in patients with symptomatic PAD compared with normal subjects. The method is reproducible and may be useful in the identification of disease. Further study of this parameter's ability to track response to therapy as well as its prognostic capability is warranted.

Quantitative ASL muscle perfusion imaging using a FAIR-TrueFISP technique at 3.0 T

The feasibility of muscle perfusion imaging with diagnostic image quality was demonstrated using the FAIR-TrueFISP arterial spin labeling technique on a clinical 3.0 T whole-body scanner. In eight healthy volunteers (24 to 42 years old), quantitative perfusion maps of the forearm musculature were acquired before and after intense exercise. All measurements were carried out in a 3.0 T whole-body MR unit in combination with an eight-channel head coil. Pulsed arterial spin labeling and data recording were performed with an adapted FAIR-TrueFISP technique and quantitative perfusion maps were calculated on a pixel-by-pixel basis by means of the extended Bloch equations. Perfusion images with an in-plane resolution of 1 mm showed no significant distortions or blurring. Perfusion-time curves could be recorded with a temporal resolution of 6.4 s. Maximum perfusion in the musculature was found approximately 2 min after exercise, reaching values of up to 220 mL/min per 100 g of tissue with good delineation between the active muscles and the musculature not involved in the exercise. In conclusion, the TrueFISP pulsed arterial spin labeling technique allows patient-friendly assessment of muscular perfusion in a clinical whole-body scanner.

Effects of prior heavy-intensity exercise during single-leg knee extension on v̇o2 kinetics and limb blood flow

The effects of prior heavy-intensity exercise on O2 uptake (V̇o2) kinetics of a second heavy exercise may be due to vasodilation (associated with metabolic acidosis) and improved muscle blood flow. This study examined the effect of prior heavy-intensity exercise on femoral artery blood flow (Qleg) and its relationship with V̇o2 kinetics. Five young subjects completed five to eight repeats of two 6-min bouts of heavy-intensity one-legged, knee-extension exercise separated by 6 min of loadless exercise. V̇o2 was measured breath by breath. Pulsed-wave Doppler ultrasound was used to measure Qleg. V̇o2 and blood flow velocity data were fit using a monoexponential model to identify phase II and phase III time periods and estimate the response amplitudes and time constants (τ). Phase II V̇o2 kinetics was speeded on the second heavy-intensity exercise [mean τ (SD), 29 ( 10 ) s to 24 ( 10 ) s, P < 0.05] with no change in the phase II (or phase III) amplitude. Qleg was elevated before the second exercise [1.55 (0.34) l/min to 1.90 (0.25) l/min, P < 0.05], but the amplitude and time course [τ, 25 ( 13 ) s to 35 ( 13 ) s] were not changed, such that throughout the transient the Qleg (and ΔQleg/ΔV̇o2) did not differ from the prior heavy exercise. Thus V̇o2 kinetics were accelerated on the second exercise, but the faster kinetics were not associated with changes in Qleg. Thus limb blood flow appears not to limit V̇o2 kinetics during single-leg heavy-intensity exercise nor to be the mechanism of the altered V̇o2 response after heavy-intensity prior exercise.