Age Reduces Microvascular Function in the Leg Independent of Physical Activity

Feasibility of Multiparametric Perfusion Assessment in Diabetic Foot Ulcer Using Intravoxel Incoherent Motion and Blood Oxygenation-Level Dependent MRI

BACKGROUND

Patients with type-2 diabetes (T2DM) are at increased risk of developing diabetic foot ulcers (DFU) and experiencing impaired wound healing related to underlying microvascular disease.

PURPOSE

To evaluate the sensitivity of intra-voxel incoherent motion (IVIM) and blood oxygen level dependent (BOLD) MRI to microvascular changes in patients with DFUs.

STUDY TYPE

Case-control.

POPULATION

20 volunteers who were age and body mass index matched, including T2DM patients with DFUs (N = 10, mean age = 57.5 years), T2DM patients with controlled glycemia and without DFUs (DC, N = 5, mean age = 57.4 years) and healthy controls (HC, N = 5, mean age = 52.8 years).

FIELD STRENGTH/SEQUENCE

3T/multi-b-value IVIM and dynamic BOLD.

ASSESSMENT

Resting IVIM parameters were obtained using a multi-b-value diffusion-weighted imaging sequence and two IVIM models were fit to obtain diffusion coefficient (D), pseudo-diffusion coefficient (D*), perfusion fraction (f) and microvascular volume fraction (MVF) parameters. Microvascular reactivity was evaluated by inducing an ischemic state in the foot with a blood pressure cuff during dynamic BOLD imaging. Perfusion indices were assessed in two regions of the foot: the medial plantar (MP) and lateral plantar (LP) regions.

STATISTICAL TESTS

Effect sizes of group mean differences were assessed using Hedge's g adjusted for small sample sizes.

RESULTS

DFU participants exhibited elevated D*, f, and MVF values in both regions (g ≥ 1.10) and increased D (g = 1.07) in the MP region compared to DC participants. DC participants showed reduced f and MVF compared to HC participants in the MP region (g ≥ 1.06). Finally, the DFU group showed reduced tolerance for ischemia in the LP region (g = -1.51) and blunted reperfusion response in both regions (g < -2.32) compared to the DC group during the cuff-occlusion challenge.

DATA CONCLUSION

The combined use of IVIM and BOLD MRI shows promise in differentiating perfusion abnormalities in the feet of diabetic patients and suggests hyperperfusion in DFU patients.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY: Stage 1.

An observational study of microcirculation among healthy individuals by age and sex

OBJECTIVE

This study measured normal ranges of microcirculatory parameters in healthy individuals and investigated differences in parameters by age and sex.

METHODS

Participants were enrolled into three groups with equal numbers of male and female: young (20-39 years), middle-aged (40-59 years), and elderly (60-79 years). Sublingual microcirculation images were obtained using the incident dark field (IDF).

RESULTS

A total of 75 female and 75 male healthy individuals were enrolled. The elderly group had a higher TVD (26.5 [2] vs. 25.2 [1.8]; p = 0.019) and a lower PPV (97 [2] vs. 98 [3]; p = 0.03) than did the young group. In the elderly group, systolic blood pressure (SBP) and mean arterial pressure (MAP) were moderately and positively correlated with MFI score (r = 0.407, p < 0.05, and r = 0.403, p < 0.05, respectively). The female participants had a lower MFI score than did the male participants (2.9 [2.8-3] vs. 3.0 [2.9-3]; p = 0.015).

CONCLUSIONS

This study revealed the range of microcirculatory parameters between different ages and sexes in healthy individuals. We found that blood pressure levels were correlated with microcirculatory parameters, especially in elders and female.

Sex differences in microvascular function across lower leg muscles in humans

Studies have reported sex-based differences in conduit artery function, however little is known about possible sex-based differences in microvascular function, and possible influence of muscle group. Blood-oxygen-level-dependent (BOLD) MR images acquired during ischemia-reperfusion assess the reactive hyperemic response in the microvasculature of skeletal muscle. We tested the hypothesis that women have greater microvascular reactivity, reflected by faster time-to-peak (TTP) and time-to-half-peak (TTHP) of the BOLD response, across all lower leg muscles. In healthy, young men (n = 18) and women (n = 12), BOLD images of both lower legs were acquired continuously during 30 s of rest, 5 min of cuff occlusion and 2 min of reperfusion, in a 3 T MR scanner. Segmentation of tibialis anterior (TA), soleus (SO), gastrocnemius medial (GM), and the peroneal group (PG) were performed using image registration, and TTP and TTHP of the BOLD response were determined for each muscle. Overall, women had faster TTP (p = 0.001) and TTHP (p = 0.01) than men. Specifically, women had shorter TTP and TTHP in TA (27.5-28.4%), PG (33.9-41.6%), SO (14.7-19.7%) and GM (15.4-18.8%). Overall, TTP and TTHP were shorter in TA compared with PG (25.1-31.1%; p ≤ 0.007), SO (14.3-16%; p ≤ 0.03) and GM (15.6-26%; p ≤ 0.01). Intra class correlations analyses showed large variation in absolute agreement (range: 0.10-0.81) of BOLD parameters between legs (within distinct muscles). Faster TTP and TTHP across all lower leg muscles, in women, provide novel evidence of sex-based differences in microvascular function of young adults matched for age, body mass index, and physical activity level.

Four functional magnetic resonance imaging techniques for skeletal muscle exploration, a systematic review

BACKGROUND

The study of muscle health has become more relevant lately, due to global aging and a higher incidence of musculoskeletal pathologies. Current exploration techniques, such as electromyography, do not provide accurate spatial information.

OBJECTIVE

The objective of this work is to perform a systematic review of the literature to synthesize the contributions that can offer functional MRI techniques commonly used in neuroimaging, applied to skeletal muscle: Blood Oxygen Level Dependent (BOLD), IntraVoxel Incoherent Motion (IVIM), Arterial Spin Labeling (ASL) and Dynamic Contrast Enhanced (DCE).

EVIDENCE ACQUISITION

Web of Science and Medline databases were searched, over the last 10 years, focused on the use of BOLD, ASL, IVIM or DCE in skeletal muscle.

EVIDENCE SYNTHESIS

59 articles were included after applying the selection criteria. 37 studies were performed in healthy subjects, and 22 in patients with different pathologies: in peripheral arterial disease, systemic sclerosis, diabetes, osteoporosis, adolescent idiopathic scoliosis, and dermatomyositis. Reference values in healthy subjects still vary in some cases.

CONCLUSION

The studies show the feasibility of implementing functional MRI through BOLD, ASL, IVIM or DCE imaging in several muscles and their possible utility in different pathologies. A synthesis of how to implement such exploration is given here.

CLINICAL IMPACT

These four techniques are based on sequences already present in clinical MRI scanners, therefore, their use for functional muscle exploration does not require additional investment. These techniques allow visualization and quantification of parameters associated with the vascular health of the muscles and represent interesting support for musculoskeletal exploration.

Skeletal muscle microvascular perfusion responses to cuff occlusion and submaximal exercise assessed by contrast-enhanced ultrasound: The effect of age

Impairments in skeletal muscle microvascular function are frequently reported in patients with various cardiometabolic conditions for which older age is a risk factor. Whether aging per se predisposes the skeletal muscle to microvascular dysfunction is unclear. We used contrast‐enhanced ultrasound (CEU) to compare skeletal muscle microvascular perfusion responses to cuff occlusion and leg exercise between healthy young (n = 12, 26 ± 3 years) and older (n = 12, 68 ± 7 years) adults. Test–retest reliability of CEU perfusion parameters was also assessed. Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after: (a) 5‐min of thigh‐cuff occlusion, and (b) 5‐min of submaximal intermittent isometric plantar‐flexion exercise (400 N) using CEU. Whole‐leg blood flow was measured using strain‐gauge plethysmography. Repeated measures were obtained with a 15‐min interval, and averaged responses were used for comparisons between age groups. There were no differences in post‐occlusion whole‐leg blood flow and muscle microvascular perfusion between young and older participants (p > .05). Similarly, total whole‐leg blood flow during exercise and post‐exercise peak muscle microvascular perfusion did not differ between groups (p > .05). The overall level of agreement between the test–retest measures of calf muscle perfusion was excellent for measurements taken at rest (intraclass correlation coefficient [ICC] 0.85), and in response to cuff occlusion (ICC 0.89) and exercise (ICC 0.95). Our findings suggest that healthy aging does not affect muscle perfusion responses to cuff‐occlusion and submaximal leg exercise. CEU muscle perfusion parameters measured in response to these provocation tests are highly reproducible in both young and older adults.

Postcontractile blood oxygenation level-dependent (BOLD) response in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is characterized by a progressive replacement of muscle by fat and fibrous tissue, muscle weakness, and loss of functional abilities. Impaired vasodilatory and blood flow responses to muscle activation have also been observed in DMD and associated with mislocalization of neuronal nitric oxide synthase mu (nNOSμ) from the sarcolemma. The objective of this study was to determine whether the postcontractile blood oxygen level-dependent (BOLD) MRI response is impaired in DMD and correlated with established markers of disease severity in DMD, including MRI muscle fat fraction (FF) and clinical functional measures. Young boys with DMD (n = 16, 5-14 yr) and unaffected controls (n = 16, 5-14 yr) were evaluated using postcontractile BOLD, FF, and functional assessments. The BOLD response was measured following five brief (2 s) maximal voluntary dorsiflexion contractions, each separated by 1 min of rest. FFs from the anterior compartment lower leg muscles were quantified via chemical shift-encoded imaging. Functional abilities were assessed using the 10 m walk/run and the 6-min walk distance (6MWD). The peak BOLD responses in the tibialis anterior and extensor digitorum longus were reduced (P < 0.001) in DMD compared with controls. Furthermore, the anterior compartment peak BOLD response correlated with function (6MWD ρ = 0.87, P < 0.0001; 10 m walk/run time ρ = -0.78, P < 0.001) and FF (ρ = -0.52, P = 0.05). The reduced postcontractile BOLD response in DMD may reflect impaired microvascular function. The relationship observed between the postcontractile peak BOLD response and functional measures and FF suggests that the BOLD response is altered with disease severity in DMD.NEW & NOTEWORTHY This study examined the postcontractile blood oxygen level-dependent (BOLD) response in boys with Duchenne muscular dystrophy (DMD) and unaffected controls, and correlated this measure to markers of disease severity. Our findings indicate that the postcontractile BOLD response is impaired in DMD after brief muscle contractions, is correlated to disease severity, and may be valuable to implement in future studies to evaluate treatments targeting microvascular function in DMD.

Quantitative and Dynamic MRI Measures of Peripheral Vascular Function

The endothelium regulates and mediates vascular homeostasis, allowing for dynamic changes of blood flow in response to mechanical and chemical stimuli. Endothelial dysfunction underlies many diseases and is purported to be the earliest pathologic change in the progression of atherosclerotic disease. Peripheral vascular function can be interrogated by measuring the response kinetics following induced ischemia or exercise. In the presence of endothelial dysfunction, there is a blunting and delay of the hyperemic response, which can be measured non-invasively using a variety of quantitative magnetic resonance imaging (MRI) methods. In this review, we summarize recent developments in non-contrast, proton MRI for dynamic quantification of blood flow and oxygenation. Methodologic description is provided for: blood oxygenation-level dependent (BOLD) signal that reflect combined effect of blood flow and capillary bed oxygen content; arterial spin labeling (ASL) for quantification of regional perfusion; phase contrast (PC) to quantify arterial flow waveforms and macrovascular blood flow velocity and rate; high-resolution MRI for luminal flow-mediated dilation; and dynamic MR oximetry to quantify oxygen saturation. Overall, results suggest that these dynamic and quantitative MRI methods can detect endothelial dysfunction both in the presence of overt cardiovascular disease (such as in patients with peripheral artery disease), as well as in sub-clinical settings (i.e., in chronic smokers, non-smokers exposed to e-cigarette aerosol, and as a function of age). Thus far, these tools have been relegated to the realm of research, used as biomarkers of disease progression and therapeutic response. With proper validation, MRI-measures of vascular function may ultimately be used to complement the standard clinical workup, providing additional insight into the optimal treatment strategy and evaluation of treatment efficacy.

In vivo assessment of mitochondrial capacity using NIRS in locomotor muscles of young and elderly males with similar physical activity levels

Mitochondrial capacity is pivotal to skeletal muscle function and is suggested to decline with age. However, there is large heterogeneity in current data, possibly due to effect modifiers such as physical activity, sex and muscle group. Yet, few studies have compared multiple muscle groups in different age groups with comparable physical activity levels. Here, we newly used near-infrared spectroscopy (NIRS) to characterise mitochondrial capacity in three different locomotor muscles in young (19-25 year) and older (65-71 year), healthy males with similar physical activity levels. Mitochondrial capacity and reperfusion after arterial occlusion was measured in the vastus lateralis (VL), the gastrocnemius (GA) and the tibialis anterior (TA). Physical activity was verified using accelerometry and was not different between the age groups (404.3 ± 214.9 vs 494.9 ± 187.0 activity kcal per day, p = 0.16). Mitochondrial capacity was significantly lower in older males in the GA and VL, but not in the TA (p = 0.048, p = 0.036 and p = 0.64, respectively). Reperfusion rate was not significantly different for the GA (p = 0.55), but was significantly faster in the TA and VL in the young group compared to the older group (p = 0.0094 and p = 0.039, respectively). In conclusion, we identified distinct modes of mitochondrial ageing in different locomotor muscles in a young and older population with similar physical activity patterns. Furthermore, we show that NIRS is suitable for relatively easy application in ageing research and can reveal novel insights into mitochondrial functioning with age.

Aging and Physiological Lessons from Master Athletes

Sedentary aging is often characterized by physical dysfunction and chronic degenerative diseases. In contrast, masters athletes demonstrate markedly greater physiological function and more favorable levels of risk factors for cardiovascular disease, osteoporosis, frailty, and cognitive dysfunction than their sedentary counterparts. In many cases, age-related deteriorations of physiological functions as well as elevations in risk factors that are typically observed in sedentary adults are substantially attenuated or even absent in masters athletes. Older masters athletes possess greater functional capacity at any given age than their sedentary peers. Impressive profiles of older athletes provide insight into what is possible in human aging and place aging back into the domain of "physiology" rather than under the jurisdiction of "clinical medicine." In addition, these exceptional aging athletes can serve as a role model for the promotion of physical activity at all ages. The study of masters athletes has provided useful insight into the positive example of successful aging. To further establish and propagate masters athletics as a role model for our aging society, future research and action are needed. © 2020 American Physiological Society. Compr Physiol 10:261-296, 2020.

Aerobic Exercise Improves Microvascular Function in Older Adults

Microvascular function is reduced with age, disease, and inactivity. Exercise is well known to improve vascular health and has the potential to improve microvascular function in aging and disease.

PURPOSE

The study aimed to assess changes in peripheral microvascular function in sedentary older adults after aerobic exercise training.

METHODS

Twenty-three sedentary older adults (67 ± 5 yr, body mass index = 29 ± 5, mean ± SD) successfully completed a randomized 12-wk graded treadmill walking intervention. The exercise group (EX) performed 40 min of uphill walking 4 d·wk at 70% heart rate reserve. The control group (CON) maintained a sedentary lifestyle for 12 wk. Blood oxygen level-dependent (BOLD) responses of the soleus measured by magnetic resonance imaging were used to evaluate microvascular function; brief (1 s) maximal plantarflexion contractions were performed. Separately, blood flow in the popliteal artery was measured by ultrasound after brief contraction. Phosphorus magnetic resonance spectroscopy of the calf was used to examine muscle oxidative capacity, and whole-body peak oxygen consumption (V˙O2peak) was used to confirm training-induced cardiorespiratory adaptations.

RESULTS

Peak postcontraction BOLD response increased by 33% in EX (PRE, 3.3% ± 1.0%; POST, 4.4% ± 1.4%) compared with CON (PRE, 3.0% ± 1.3%; POST, 3.2% ± 1.5%), P < 0.05. EX with hypertension tended to show a blunted peak BOLD increase (n = 6, 15%) compared with EX normotensive (n = 7, 50%), P = 0.056. Peak postcontraction blood flow increased by 39% in EX (PRE, 217 ± 88 mL·min; POST, 302 ± 167 mL·min) compared with CON (PRE, 188 ± 54 mL·min; POST, 184 ± 44 mL·min), P < 0.05. EX muscle oxidative capacity (kPCr) improved by 40% (PRE, 1.60 ± 0.57 min; POST, 2.25 ± 0.80 min) compared with CON (PRE, 1.69 ± 0.28 min; POST, 1.76 ± 0.52 min), P < 0.05. V˙O2peak increased by 9% for EX (PRE, 19.0 ± 3.1 mL·kg·min; POST, 20.8 ± 2.9 mL·kg·min) compared with a 7% loss in CON (PRE, 21.9 ± 3.6 mL·kg·min; POST, 20.4 ± 3.5 mL·kg·min), P < 0.05.

CONCLUSION

Moderate aerobic exercise significantly improved microvascular function of the leg in older adults.

The Role of Muscle Perfusion in the Age-Associated Decline of Mitochondrial Function in Healthy Individuals

Maximum oxidative capacity of skeletal muscle measured by in vivo phosphorus magnetic resonance spectroscopy (³¹P-MRS) declines with age, and negatively affects whole-body aerobic capacity. However, it remains unclear whether the loss of oxidative capacity is caused by reduced volume and function of mitochondria or limited substrate availability secondary to impaired muscle perfusion. Therefore, we sought to elucidate the role of muscle perfusion on the age-related decline of muscle oxidative capacity and ultimately whole-body aerobic capacity. Muscle oxidative capacity was assessed by ³¹P-MRS post-exercise phosphocreatine recovery time (τ_PCr), with higher τ_PCr reflecting lower oxidative capacity, in 75 healthy participants (48 men, 22–89 years) of the Genetic and Epigenetic Signatures of Translational Aging Laboratory Testing study. Muscle perfusion was characterized as an index of blood volume at rest using a customized diffusion-weighted MRI technique and analysis method developed in our laboratory. Aerobic capacity (peak-VO₂) was also measured during a graded treadmill exercise test in the same visit. Muscle oxidative capacity, peak-VO₂, and resting muscle perfusion were significantly lower at older ages independent of sex, race, and body mass index (BMI). τ_PCr was significantly associated with resting muscle perfusion independent of age, sex, race, and BMI (p-value = 0.004, β = −0.34). τ_PCr was also a significant independent predictor of peak-VO₂ and, in a mediation analysis, significantly attenuated the association between muscle perfusion and peak-VO₂ (34% reduction for β in perfusion). These findings suggest that the age-associated decline in muscle oxidative capacity is partly due to impaired muscle perfusion and not mitochondrial dysfunction alone. Furthermore, our findings show that part of the decline in whole-body aerobic capacity observed with aging is also due to reduced microvascular blood volume at rest, representing a basal capacity of the microvascular system, which is mediated by muscle oxidative capacity. This finding suggests potential benefit of interventions that target an overall increase in muscle perfusion for the restoration of energetic capacity and mitochondrial function with aging.