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

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.

Increased peripheral vascular disease risk progressively constrains perfusion adaptability in the skeletal muscle microcirculation

To determine the impact of progressive elevations in peripheral vascular disease (PVD) risk on microvascular function, we utilized eight rat models spanning "healthy" to "high PVD risk" and used a multiscale approach to interrogate microvascular function and outcomes: healthy: Sprague-Dawley rats (SDR) and lean Zucker rats (LZR); mild risk: SDR on high-salt diet (HSD) and SDR on high-fructose diet (HFD); moderate risk: reduced renal mass-hypertensive rats (RRM) and spontaneously hypertensive rats (SHR); high risk: obese Zucker rats (OZR) and Dahl salt-sensitive rats (DSS). Vascular reactivity and biochemical analyses demonstrated that even mild elevations in PVD risk severely attenuated nitric oxide (NO) bioavailability and caused progressive shifts in arachidonic acid metabolism, increasing thromboxane A2 levels. With the introduction of hypertension, arteriolar myogenic activation and adrenergic constriction were increased. However, while functional hyperemia and fatigue resistance of in situ skeletal muscle were not impacted with mild or moderate PVD risk, blood oxygen handling suggested an increasingly heterogeneous perfusion within resting and contracting skeletal muscle. Analysis of in situ networks demonstrated an increasingly stable and heterogeneous distribution of perfusion at arteriolar bifurcations with elevated PVD risk, a phenomenon that was manifested first in the distal microcirculation and evolved proximally with increasing risk. The increased perfusion distribution heterogeneity and loss of flexibility throughout the microvascular network, the result of the combined effects on NO bioavailability, arachidonic acid metabolism, myogenic activation, and adrenergic constriction, may represent the most accurate predictor of the skeletal muscle microvasculopathy and poor health outcomes associated with chronic elevations in PVD risk.

Determinants of exercise intolerance in patients with heart failure and reduced or preserved ejection fraction

This mini-review summarizes the literature regarding the mechanisms of exercise intolerance in patients with heart failure and reduced or preserved ejection fraction (HFREF and HFPEF, respectively). Evidence to date suggests that the reduced peak pulmonary oxygen uptake (pulm V̇o₂) in patients with HFREF compared with healthy controls is due to both central (reduced convective O₂ transport) and peripheral factors (impaired skeletal muscle blood flow, decreased diffusive O₂ transport coupled with abnormal skeletal morphology, and metabolism). Although central and peripheral impairments also limit peak pulm V̇o₂ in HFPEF patients compared with healthy controls, emerging data suggest that the latter may play a relatively greater role in limiting exercise performance in these patients. Unlike HFREF, currently there is limited evidence-based therapies that improve exercise capacity in HFPEF patients, therefore future studies are required to determine whether interventions targeted to improve peripheral vascular and skeletal muscle function result in favorable improvements in peak pulm and leg V̇o2 and their determinants in HFPEF patients.

Effects of nitrite infusion on skeletal muscle vascular control during exercise in rats with chronic heart failure

Chronic heart failure (CHF) reduces nitric oxide (NO) bioavailability and impairs skeletal muscle vascular control during exercise. Reduction of NO2 (-) to NO may impact exercise-induced hyperemia, particularly in muscles with pathologically reduced O2 delivery. We tested the hypothesis that NO2 (-) infusion would increase exercising skeletal muscle blood flow (BF) and vascular conductance (VC) in CHF rats with a preferential effect in muscles composed primarily of type IIb + IId/x fibers. CHF (coronary artery ligation) was induced in adult male Sprague-Dawley rats. After a >21-day recovery, mean arterial pressure (MAP; carotid artery catheter) and skeletal muscle BF (radiolabeled microspheres) were measured during treadmill exercise (20 m/min, 5% incline) with and without NO2 (-) infusion. The myocardial infarct size (35 ± 3%) indicated moderate CHF. NO2 (-) infusion increased total hindlimb skeletal muscle VC (CHF: 0.85 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1) and CHF + NO2 (-): 0.93 ± 0.09 ml·min(-1)·100 g(-1)·mmHg(-1), P < 0.05) without changing MAP (CHF: 123 ± 4 mmHg and CHF + NO2 (-): 120 ± 4 mmHg, P = 0.17). Total hindlimb skeletal muscle BF was not significantly different (CHF: 102 ± 7 and CHF + NO2 (-): 109 ± 7 ml·min(-1)·100 g(-1) ml·min(-1)·100 g(-1), P > 0.05). BF increased in 6 (∼21%) and VC in 8 (∼29%) of the 28 individual muscles and muscle parts. Muscles and muscle portions exhibiting greater BF and VC after NO2 (-) infusion comprised ≥63% type IIb + IId/x muscle fibers. These data demonstrate that NO2 (-) infusion can augment skeletal muscle vascular control during exercise in CHF rats. Given the targeted effects shown herein, a NO2 (-)-based therapy may provide an attractive "needs-based" approach for treatment of the vascular dysfunction in CHF.

Interval and continuous exercise enhances aerobic capacity and hemodynamic function in CHF rats

OBJECTIVE:

The aim of the present study was to compare the effects of continuous versus interval aerobic exercise training on hemodynamic parameters, cardiac remodeling, and maximal exercise capacity (MEC) in chronic heart failure (CHF) rats.

METHOD:

Twenty-four male Wistar rats were subjected to myocardial infarction (MI) surgery. Five weeks post MI, the animals were assigned to one of three groups: sedentary group (CHF-Sed, n=8), aerobic continuous training group (CHF-ACT, n=8), and aerobic interval training group (CHF-AIT, n=8). Treadmill training was performed five times a week for 8 weeks (ACT: 50 min/day at 15 m/min and AIT: 40 min/day with 8 min of warm-up at 10 m/min and exercise at 15 m/min 4×4 min interspersed with 4×4 min at 23 m/min). MEC was evaluated pre and post exercise program.

RESULTS:

Left ventricular end-diastolic pressure (LVEDP), left ventricular mass/body mass ratio (LVM:BM), and total collagen volume fraction were lower in the trained groups compared with the sedentary group, but no difference was found between the trained groups. Systolic ventricular pressure (SVP) and maximum positive derivative of LV pressure (+dP/dt_max) were higher in the trained groups, but CHF-ACT showed higher +dP/dt_max compared to CHF-AIT. Both training regimens were able to increase MEC. However, the aerobic interval training was superior for improving MEC.

CONCLUSION:

Aerobic training is an important intervention to improve cardiac function and remodeling and physical capacity in CHF rats. Interval training is a potential strategy to maximize the results, but exercise type and intensity are still topics to be explored.

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

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

Skeletal Muscle Vascular Control During Exercise: Impact of Nitrite Infusion During Nitric Oxide Synthase Inhibition in Healthy Rats

The nitric oxide synthase (NOS)-independent pathway of nitric oxide (NO) production in which nitrite (NO2 (-)) is reduced to NO may have therapeutic applications for those with cardiovascular diseases in which the NOS pathway is downregulated. We tested the hypothesis that NO2 (-) infusion would reduce mean arterial pressure (MAP) and increase skeletal muscle blood flow (BF) and vascular conductance (VC) during exercise in the face of NOS blockade via L-NAME. Following infusion of L-NAME (10 mg kg(-1), L-NAME), male Sprague-Dawley rats (3-6 months, n = 8) exercised without N(G)-nitro-L arginine methyl ester (L-NAME) and after infusion of sodium NO2 (-) (7 mg kg(-1); L-NAME + NO2 (-)). MAP and hindlimb skeletal muscle BF (radiolabeled microsphere infusions) were measured during submaximal treadmill running (20 m min(-1), 5% grade). Across group comparisons were made with a published control data set (n = 11). Relative to L-NAME, NO2 (-) infusion significantly reduced MAP (P < 0.03). The lower MAP in L-NAME+NO2 (-) was not different from healthy control animals (control: 137 ± 3 L-NAME: 157 ± 7, L-NAME + NO2 (-): 136 ± 5 mm Hg). Also, NO2 (-) infusion significantly increased VC when compared to L-NAME (P < 0.03), ultimately negating any significant differences from control animals (control: 0.78 ± 0.05, L-NAME: 0.57 ± 0.03, L-NAME + NO2 (-); 0.69 ± 0.04 mL min(-1) 100 g(-1) mm Hg(-1)) with no apparent fiber-type preferential effect. Overall, hindlimb BF was decreased significantly by L-NAME; however, in L-NAME + NO2 (-), BF improved to a level not significantly different from healthy controls (control: 108 ± 8, L-NAME: 88 ± 3, L-NAME + NO2 (-): 94 ± 6 mL min(-1) 100 g(-1), P = 0.38 L-NAME vs L-NAME + NO2 (-)). Individuals with diseases that impair NOS activity, and thus vascular function, may benefit from a NO2 (-)-based therapy in which NO bioavailability is elevated in an NOS-independent manner.

Neural control of circulation and exercise: a translational approach disclosing interactions between central command, arterial baroreflex, and muscle metaboreflex

The last 100 years witnessed a rapid and progressive development of the body of knowledge concerning the neural control of the cardiovascular system in health and disease. The understanding of the complexity and the relevance of the neuroregulatory system continues to evolve and as a result raises new questions. The purpose of this review is to articulate results from studies involving experimental models in animals as well as in humans concerning the interaction between the neural mechanisms mediating the hemodynamic responses during exercise. The review describes the arterial baroreflex, the pivotal mechanism controlling mean arterial blood pressure and its fluctuations along with the two main activation mechanisms to exercise: central command (parallel activation of central somatomotor and autonomic descending pathways) and the muscle metaboreflex, the metabolic component of exercise pressor reflex (feedback from ergoreceptors within contracting skeletal muscles). In addition, the role of the cardiopulmonary baroreceptors in modulating the resetting of arterial baroreflex is identified, and the mechanisms in the central nervous system involved with the resetting of baroreflex function during dynamic exercise are also described. Approaching a very relevant clinical condition, the review also presents the concept that the impaired arterial baroreflex function is an integral component of the metaboreflex-mediated exaggerated sympathetic tone in subjects with heart failure. This increased sympathetic activity has a major role in causing the depressed ventricular function observed during submaximal dynamic exercise in these patients. The potential contribution of a metaboreflex arising from respiratory muscles is also considered.

Effect of endurance and/or strength training on muscle fiber size, oxidative capacity, and capillarity in hemodialysis patients

We previously reported reduced limb muscle fiber succinate dehydrogenase (SDH) activity and capillarity density and increased cross-sectional areas (CSAs) of all fiber types in maintenance hemodialysis (MHD) patients compared with matched controls that may contribute to their effort intolerance and muscle weakness. This study evaluated whether endurance training (ET), strength training (ST), or their combination (EST) alters these metabolic and morphometric aberrations as a mechanism for functional improvement. Five groups were evaluated: 1) controls; 2) MHD/no training; 3) MHD/ET; 4) MHD/ST; and 5) MHD/EST. Training duration was 21.5 ± 0.7 wk. Vastus lateralis muscle biopsies were obtained after HD at baseline and at study end. Muscle fibers were classified immunohistochemically, and fiber CSAs were computed. Individual fiber SDH activity was determined by a microdensitometric assay. Capillaries were identified using antibodies against endothelial cells. Type I and IIA fiber CSAs decreased significantly (10%) with EST. In the ET group, SDH activity increased 16.3% in type IIA and 19.6% in type IIX fibers. Capillary density increased significantly by 28% in the EST group and 14.3% with ET. The number of capillaries surrounding individual fiber type increased significantly in EST and ET groups. Capillary-to-fiber ratio increased significantly by 11 and 9.6% in EST and ET groups, respectively. We conclude that increments in capillarity and possibly SDH activity in part underlie improvements in endurance of MHD patients posttraining. We speculate that improved specific force and/or neural adaptations to exercise underlie improvements in limb muscle strength of MHD patients.

Impact of increased intramuscular perfusion heterogeneity on skeletal muscle microvascular hematocrit in the metabolic syndrome

OBJECTIVE

To determine HMV and PS in skeletal muscle of OZR and evaluate the impact of increased microvascular perfusion heterogeneity on mass transport/exchange.

METHODS

The in situ gastrocnemius muscle from OZR and LZR was examined under control conditions and following pretreatment with TEMPOL (antioxidant)/SQ-29548 (PGH2 /TxA2 receptor antagonist), phentolamine (adrenergic antagonist), or all agents combined. A spike input of a labeled blood tracer cocktail was injected into the perfusing artery. Tracer washout was analyzed using models for HMV and PS. HT was determined in in situ cremaster muscle of OZR and LZR using videomicroscopy.

RESULTS

HMV was decreased in OZR versus LZR. While TEMPOL/SQ-29548 or phentolamine had minor effects, treatment with all three agents improved HMV in OZR. HT was not different between strains, although variability was increased in OZR, and normalized following treatment with all three agents. PS was reduced in OZR and was not impacted by intervention.

CONCLUSIONS

Increased microvascular perfusion heterogeneity in OZR reduces HMV in muscle vascular networks and increases its variability, potentially contributing to premature muscle fatigue. While targeted interventions can ameliorate this, the reduced microvascular surface area is not acutely reversible.

Exercise intolerance in heart failure with preserved ejection fraction: more than a heart problem

Heart failure (HF) with preserved ejection fraction (HFpEF) is the most common form of HF in older adults, and is increasing in prevalence as the population ages. Furthermore, HFpEF is increasing out of proportion to HF with reduced EF (HFrEF), and its prognosis is worsening while that of HFrEF is improving. Despite the importance of HFpEF, our understanding of its pathophysiology is incomplete, and optimal treatment remains largely undefined. A cardinal feature of HFpEF is reduced exercise tolerance, which correlates with symptoms as well as reduced quality of life. The traditional concepts of exercise limitations have focused on central dysfunction related to poor cardiac pump function. However, the mechanisms are not exclusive to the heart and lungs, and the understanding of the pathophysiology of this disease has evolved. Substantial attention has focused on defining the central versus peripheral mechanisms underlying the reduced functional capacity and exercise tolerance among patients with HF. In fact, physical training can improve exercise tolerance via peripheral adaptive mechanisms even in the absence of favorable central hemodynamic function. In addition, the drug trials performed to date in HFpEF that have focused on influencing cardiovascular function have not improved exercise capacity. This suggests that peripheral limitations may play a significant role in HF limiting exercise tolerance, a hallmark feature of HFpEF.

What can we learn about treating heart failure from the heart's response to acute exercise? Focus on the coronary microcirculation

Coronary microvascular function and cardiac function are closely related in that proper cardiac function requires adequate oxygen delivery through the coronary microvasculature. Because of the close proximity of cardiomyocytes and coronary microvascular endothelium, cardiomyocytes not only communicate their metabolic needs to the coronary microvasculature, but endothelium-derived factors also directly modulate cardiac function. This review summarizes evidence that the myocardial oxygen balance is disturbed in the failing heart because of increased extravascular compressive forces and coronary microvascular dysfunction. The perturbations in myocardial oxygen balance are exaggerated during exercise and are due to alterations in neurohumoral influences, endothelial function, and oxidative stress. Although there is some evidence from animal studies that the myocardial oxygen balance can partly be restored by exercise training, it is largely unknown to what extent the beneficial effects of exercise training include improvements in endothelial function and/or oxidative stress in the coronary microvasculature and how these improvements are impacted by risk factors such as diabetes, obesity, and hypercholesterolemia.

Acute inhibition of ATP-sensitive K+ channels impairs skeletal muscle vascular control in rats during treadmill exercise

The ATP-sensitive K(+) (KATP) channel is part of a class of inward rectifier K(+) channels that can link local O2 availability to vasomotor tone across exercise-induced metabolic transients. The present investigation tested the hypothesis that if KATP channels are crucial to exercise hyperemia, then inhibition via glibenclamide (GLI) would lower hindlimb skeletal muscle blood flow (BF) and vascular conductance during treadmill exercise. In 27 adult male Sprague-Dawley rats, mean arterial pressure, blood lactate concentration, and hindlimb muscle BF (radiolabeled microspheres) were determined at rest (n = 6) and during exercise (n = 6-8, 20, 40, and 60 m/min, 5% incline, i.e., ~60-100% maximal O2 uptake) under control and GLI conditions (5 mg/kg intra-arterial). At rest and during exercise, mean arterial pressure was higher (rest: 17 ± 3%, 20 m/min: 5 ± 1%, 40 m/min: 5 ± 2%, and 60 m/min: 5 ± 1%, P < 0.05) with GLI. Hindlimb muscle BF (20 m/min: 16 ± 7%, 40 m/min: 30 ± 9%, and 60 m/min: 20 ± 8%) and vascular conductance (20 m/min: 20 ± 7%, 40 m/min: 33 ± 8%, and 60 m/min: 24 ± 8%) were lower with GLI during exercise at 20, 40, and 60 m/min, respectively (P < 0.05 for all) but not at rest. Within locomotory muscles, there was a greater fractional reduction present in muscles comprised predominantly of type I and type IIa fibers at all exercise speeds (P < 0.05). Additionally, blood lactate concentration was 106 ± 29% and 44 ± 15% higher during exercise with GLI at 20 and 40 m/min, respectively (P < 0.05). That KATP channel inhibition reduces hindlimb muscle BF during exercise in rats supports the obligatory contribution of KATP channels in large muscle mass exercise-induced hyperemia.

Heart failure with preserved ejection fraction in the elderly: scope of the problem

Heart failure with preserved ejection fraction (HFpEF) is the most common form of heart failure (HF) in older adults, particularly women, and is increasing in prevalence as the population ages. With morbidity and mortality on par with HF with reduced ejection fraction, it remains a most challenging clinical syndrome for the practicing clinician and basic research scientist. Originally considered to be predominantly caused by diastolic dysfunction, more recent insights indicate that HFpEF in older persons is typified by a broad range of cardiac and non-cardiac abnormalities and reduced reserve capacity in multiple organ systems. The globally reduced reserve capacity is driven by: 1) inherent age-related changes; 2) multiple, concomitant co-morbidities; 3) HFpEF itself, which is likely a systemic disorder. These insights help explain why: 1) co-morbidities are among the strongest predictors of outcomes; 2) approximately 50% of clinical events in HFpEF patients are non-cardiovascular; 3) clinical drug trials in HFpEF have been negative on their primary outcomes. Embracing HFpEF as a true geriatric syndrome, with complex, multi-factorial pathophysiology and clinical heterogeneity could provide new mechanistic insights and opportunities for progress in management. This article is part of a Special Issue entitled CV Aging.

Incremental large and small muscle mass exercise in patients with heart failure: evidence of preserved peripheral haemodynamics and metabolism

AIM

Doubt still remains as to whether peripheral vascular and skeletal muscle dysfunction accompanies the compromised cardiac function associated with heart failure with reduced ejection fraction (HFrEF). The aim of this study was to examine the effect of HFrEF on the haemodynamic and metabolic responses to exercise with both a large (cycle) and a small [knee extensor (KE)] muscle mass in comparison with well-matched healthy controls (Ctrls).

METHODS

Utilizing blood sampling and thermodilution blood flow measurements, we studied incremental cycle and KE exercise in 12 patients with HFrEF (ejection fraction: 25 ± 3%) and eight Ctrls.

RESULTS

Incremental cycle exercise in both groups [heart failure with reduced ejection fraction (HFrEF): 23 ± 1 to 116 ± 10; Ctrls: 22 ± 1 to 137 ± 5 W] resulted in a similar rise in blood flow (HFrEF: 1525 ± 132 to 4216 ± 408; Ctrls: 1774 ± 161 to 4713 ± 448 mL min(-1)), oxygen uptake (HFrEF: 206 ± 24 to 586 ± 34; Ctrls: 252 ± 21 to 747 ± 89 mL min(-1)) and lactate efflux across the leg (HFrEF: 479 ± 122 to 4929 ± 1255; Ctrls: 537 ± 155 to 5776 ± 1010 mm min(-1)). Vascular resistance fell similarly in both groups with increasing exercise intensity (HFrEF: 66 ± 10 to 24 ± 3; Ctrls: 69 ± 12 to 24 ± 4 mmHg L(-1) min(-1) ). Incremental KE exercise also revealed similar haemodynamic and metabolic responses in both Ctrls and patients.

CONCLUSION

Although assessed in a relatively small cohort, these data reveal that, when compared with well-matched healthy Ctrls, alterations in peripheral haemodynamics and skeletal muscle metabolism during exercise may not be an obligatory accompaniment to HFrEF.

Effects of exercise training on neurovascular control and skeletal myopathy in systolic heart failure

Neurohormonal excitation and dyspnea are the hallmarks of heart failure (HF) and have long been associated with poor prognosis in HF patients. Sympathetic nerve activity (SNA) and ventilatory equivalent of carbon dioxide (VE/VO2) are elevated in moderate HF patients and increased even further in severe HF patients. The increase in SNA in HF patients is present regardless of age, sex, and etiology of systolic dysfunction. Neurohormonal activation is the major mediator of the peripheral vasoconstriction characteristic of HF patients. In addition, reduction in peripheral blood flow increases muscle inflammation, oxidative stress, and protein degradation, which is the essence of the skeletal myopathy and exercise intolerance in HF. Here we discuss the beneficial effects of exercise training on resting SNA in patients with systolic HF and its central and peripheral mechanisms of control. Furthermore, we discuss the exercise-mediated improvement in peripheral vasoconstriction in patients with HF. We will also focus on the effects of exercise training on ventilatory responses. Finally, we review the effects of exercise training on features of the skeletal myopathy in HF. In summary, exercise training plays an important role in HF, working synergistically with pharmacological therapies to ameliorate these abnormalities in clinical practice.

Does impaired O2 delivery during exercise accentuate central and peripheral fatigue in patients with coexistent COPD-CHF?

Impairment in oxygen (O2) delivery to the central nervous system ("brain") and skeletal locomotor muscle during exercise has been associated with central and peripheral neuromuscular fatigue in healthy humans. From a clinical perspective, impaired tissue O2 transport is a key pathophysiological mechanism shared by cardiopulmonary diseases, such as chronic obstructive pulmonary disease (COPD) and chronic heart failure (CHF). In addition to arterial hypoxemic conditions in COPD, there is growing evidence that cerebral and muscle blood flow and oxygenation can be reduced during exercise in both isolated COPD and CHF. Compromised cardiac output due to impaired cardiopulmonary function/interactions and blood flow redistribution to the overloaded respiratory muscles (i.e., ↑work of breathing) may underpin these abnormalities. Unfortunately, COPD and CHF coexist in almost a third of elderly patients making these mechanisms potentially more relevant to exercise intolerance. In this context, it remains unknown whether decreased O2 delivery accentuates neuromuscular manifestations of central and peripheral fatigue in coexistent COPD-CHF. If this holds true, it is conceivable that delivering a low-density gas mixture (heliox) through non-invasive positive pressure ventilation could ameliorate cardiopulmonary function/interactions and reduce the work of breathing during exercise in these patients. The major consequence would be increased O2 delivery to the brain and active muscles with potential benefits to exercise capacity (i.e., ↓central and peripheral neuromuscular fatigue, respectively). We therefore hypothesize that patients with coexistent COPD-CHF stop exercising prematurely due to impaired central motor drive and muscle contractility as the cardiorespiratory system fails to deliver sufficient O2 to simultaneously attend the metabolic demands of the brain and the active limb muscles.

Elevated arterial lactate concentrations early after coronary artery bypass grafting are associated with increased anaerobic metabolism in skeletal muscle

OBJECTIVE

To assess the effect of coronary artery bypass grafting with cardiopulmonary bypass on muscle perfusion, oxygen extraction, and lactate release during postoperative rest and exercise.

DESIGN

Prospective observational study.

SETTING

University hospital.

PARTICIPANTS

Patients undergoing planned coronary artery bypass grafting.

INTERVENTION

Knee-extensor exercise before and after coronary artery bypass grafting.

MEASUREMENTS AND MAIN RESULTS

Femoral artery blood flow was measured with ultrasound. Femoral vein blood and arterial blood were sampled at rest and during light exercise and were analyzed for hemoglobin, lactate, oxygen saturation, and oxygen partial pressure. Fourteen patients were tested before and after surgery. The arterial lactate concentrations were increased after surgery, both at rest and during light exercise. Resting arterial lactate increased from 0.65 (0.5-0.8) to 1.0 (0.9-1.3) mmol/L (p=0.01) (median and interquartile range). Furthermore, lactate was released from the leg even during postoperative rest, and the release of lactate was increased during postoperative exercise. There were no significant differences between the preoperative and postoperative femoral artery blood flow. Femoral vein oxygen partial pressure was reduced significantly after surgery, indicating reduced muscle cell oxygen partial pressure.

CONCLUSIONS

The patients had elevated anaerobic metabolism in skeletal muscle after surgery to compensate for anemia. Lactate was released from the leg into the general circulation during postoperative rest and exercise. The postoperatively reduced hemoglobin concentration of 11.4 mg/dL (10.6-12.3) resulted in increased anaerobic metabolism and release of lactate from skeletal muscle. The authors concluded that coronary artery bypass grafting patients are susceptible to anaerobic metabolism even with maintained peripheral blood flow.

Effect of inorganic nitrate on exercise capacity in heart failure with preserved ejection fraction

BACKGROUND

Inorganic nitrate (NO3(-)), abundant in certain vegetables, is converted to nitrite by bacteria in the oral cavity. Nitrite can be converted to nitric oxide in the setting of hypoxia. We tested the hypothesis that NO3(-) supplementation improves exercise capacity in heart failure with preserved ejection fraction via specific adaptations to exercise.

METHODS AND RESULTS

Seventeen subjects participated in this randomized, double-blind, crossover study comparing a single dose of NO3-rich beetroot juice (NO3(-), 12.9 mmol) with an identical nitrate-depleted placebo. Subjects performed supine-cycle maximal-effort cardiopulmonary exercise tests, with measurements of cardiac output and skeletal muscle oxygenation. We also assessed skeletal muscle oxidative function. Study end points included exercise efficiency (total work/total oxygen consumed), peak VO2, total work performed, vasodilatory reserve, forearm mitochondrial oxidative function, and augmentation index (a marker of arterial wave reflections, measured via radial arterial tonometry). Supplementation increased plasma nitric oxide metabolites (median, 326 versus 10 μmol/L; P=0.0003), peak VO2 (12.6±3.7 versus 11.6±3.1 mL O2·min(-1)·kg(-1); P=0.005), and total work performed (55.6±35.3 versus 49.2±28.9 kJ; P=0.04). However, efficiency was unchanged. NO3(-) led to greater reductions in systemic vascular resistance (-42.4±16.6% versus -31.8±20.3%; P=0.03) and increases in cardiac output (121.2±59.9% versus 88.7±53.3%; P=0.006) with exercise. NO3(-) reduced aortic augmentation index (132.2±16.7% versus 141.4±21.9%; P=0.03) and tended to improve mitochondrial oxidative function.

CONCLUSIONS

NO3(-) increased exercise capacity in heart failure with preserved ejection fraction by targeting peripheral abnormalities. Efficiency did not change as a result of parallel increases in total work and VO2. NO3(-) increased exercise vasodilatory and cardiac output reserves. NO3(-) also reduced arterial wave reflections, which are linked to left ventricular diastolic dysfunction and remodeling.

CLINICAL TRIAL REGISTRATION URL

www.clinicaltrials.gov. Unique identifier: NCT01919177.

Synergy-COPD: a systems approach for understanding and managing chronic diseases

Chronic diseases (CD) are generating a dramatic societal burden worldwide that is expected to persist over the next decades. The challenges posed by the epidemics of CD have triggered a novel health paradigm with major consequences on the traditional concept of disease and with a profound impact on key aspects of healthcare systems. We hypothesized that the development of a systems approach to understand CD together with the generation of an ecosystem to transfer the acquired knowledge into the novel healthcare scenario may contribute to a cost-effective enhancement of health outcomes. To this end, we designed the Synergy-COPD project wherein the heterogeneity of chronic obstructive pulmonary disease (COPD) was addressed as a use case representative of CD. The current manuscript describes main features of the project design and the strategies put in place for its development, as well the expected outcomes during the project life-span. Moreover, the manuscript serves as introductory and unifying chapter of the different papers associated to the Supplement describing the characteristics, tools and the objectives of Synergy-COPD.

Chronic Obstructive Pulmonary Disease heterogeneity: challenges for health risk assessment, stratification and management

Background and hypothesis

Heterogeneity in clinical manifestations and disease progression in Chronic Obstructive Pulmonary Disease (COPD) lead to consequences for patient health risk assessment, stratification and management. Implicit with the classical "spill over" hypothesis is that COPD heterogeneity is driven by the pulmonary events of the disease. Alternatively, we hypothesized that COPD heterogeneities result from the interplay of mechanisms governing three conceptually different phenomena: 1) pulmonary disease, 2) systemic effects of COPD and 3) co-morbidity clustering, each of them with their own dynamics.

Objective and method

To explore the potential of a systems analysis of COPD heterogeneity focused on skeletal muscle dysfunction and on co-morbidity clustering aiming at generating predictive modeling with impact on patient management. To this end, strategies combining deterministic modeling and network medicine analyses of the Biobridge dataset were used to investigate the mechanisms of skeletal muscle dysfunction. An independent data driven analysis of co-morbidity clustering examining associated genes and pathways was performed using a large dataset (ICD9-CM data from Medicare, 13 million people). Finally, a targeted network analysis using the outcomes of the two approaches (skeletal muscle dysfunction and co-morbidity clustering) explored shared pathways between these phenomena.

Results

(1) Evidence of abnormal regulation of skeletal muscle bioenergetics and skeletal muscle remodeling showing a significant association with nitroso-redox disequilibrium was observed in COPD; (2) COPD patients presented higher risk for co-morbidity clustering than non-COPD patients increasing with ageing; and, (3) the on-going targeted network analyses suggests shared pathways between skeletal muscle dysfunction and co-morbidity clustering.

Conclusions

The results indicate the high potential of a systems approach to address COPD heterogeneity. Significant knowledge gaps were identified that are relevant to shape strategies aiming at fostering 4P Medicine for patients with COPD.

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.

Oxygen kinetics during 6-minute walk tests in patients with cardiovascular and pulmonary disease

BACKGROUND

The 6-Minute Walk Test (6MWT) is representative of daily-life activities and reflects the functional capacity of patients. The change of oxygen uptake (VO2) in the initial phase of low-intensity exercise (VO2 kinetics) can be used to assess submaximal exercise performance of patients.The objective of the following study was to analyse VO2 kinetics in patients with different pulmonary and cardiovascular diseases. In addition, we investigated the extent to which VO2 kinetics at the onset of the 6MWT were associated with exercise capacity, morbidity and mortality.

METHODS

VO2 kinetics of 204 patients and 16 healthy controls were obtained using mobile telemetric cardiopulmonary monitoring during a 6MWT. A new mean response time (MRT) index (wMRT) was developed to quantify VO2 kinetics by correcting MRT for work rate. The differences in wMRT between disease categories as well as the association between wMRT and patients' exercise capacity and outcome - time to hospitalization/death- were tested.

RESULTS

The assessment of a robust wMRT was feasible in 86% (244/284) patients. wMRT was increased in patients compared to healthy controls (p <0.001). wMRT was largest in patients with pulmonary arterial hypertension (PAH). There were significant associations between wMRT and exercise capacity in all patients. High wMRT was found to be associated with a high rate of death and re-hospitalization in patients with CHF (p = 0.024). In patients with pulmonary diseases and pulmonary hypertension wMRT was not associated with outcome (p = 0.952).

CONCLUSIONS

Submaximal exercise performance of patients is reduced. O2 kinetics at the onset of exercise are associated with exercise capacity in all patients. wMRT was found to be an important prognostic factor in patients with congestive heart failure (CHF), but not with pulmonary diseases.

Dietary nitrate accelerates postexercise muscle metabolic recovery and O2 delivery in hypoxia

We tested the hypothesis that the time constants (τ) of postexercise T2* MRI signal intensity (an index of O2 delivery) and muscle [PCr] (an index of metabolic perturbation, measured by (31)P-MRS) in hypoxia would be accelerated after dietary nitrate (NO3 (-)) supplementation. In a double-blind crossover design, eight moderately trained subjects underwent 5 days of NO3 (-) (beetroot juice, BR; 8.2 mmol/day NO3 (-)) and placebo (PL; 0.003 mmol/day NO3 (-)) supplementation in four conditions: normoxic PL (N-PL), hypoxic PL (H-PL; 13% O2), normoxic NO3 (-) (N-BR), and hypoxic NO3 (-) (H-BR). The single-leg knee-extension protocol consisted of 10 min of steady-state exercise and 24 s of high-intensity exercise. The [PCr] recovery τ was greater in H-PL (30 ± 4 s) than H-BR (22 ± 4 s), N-PL (24 ± 4 s) and N-BR (22 ± 4 s) (P < 0.05) and the maximal rate of mitochondrial ATP resynthesis (Qmax) was lower in the H-PL (1.12 ± 0.16 mM/s) compared with H-BR (1.35 ± 0.26 mM/s), N-PL (1.47 ± 0.28 mM/s), and N-BR (1.40 ± 0.21 mM/s) (P < 0.05). The τ of postexercise T2* signal intensity was greater in H-PL (47 ± 14 s) than H-BR (32 ± 10 s), N-PL (38 ± 9 s), and N-BR (27 ± 6 s) (P < 0.05). The postexercise [PCr] and T2* recovery τ were correlated in hypoxia (r = 0.60; P < 0.05), but not in normoxia (r = 0.28; P > 0.05). These findings suggest that the NO3 (-)-NO2 (-)-NO pathway is a significant modulator of muscle energetics and O2 delivery during hypoxic exercise and subsequent recovery.

Hemodynamic responses to small muscle mass exercise in heart failure patients with reduced ejection fraction

To better understand the mechanisms responsible for exercise intolerance in heart failure with reduced ejection fraction (HFrEF), the present study sought to evaluate the hemodynamic responses to small muscle mass exercise in this cohort. In 25 HFrEF patients (64 ± 2 yr) and 17 healthy, age-matched control subjects (64 ± 2 yr), mean arterial pressure (MAP), cardiac output (CO), and limb blood flow were examined during graded static-intermittent handgrip (HG) and dynamic single-leg knee-extensor (KE) exercise. During HG exercise, MAP increased similarly between groups. CO increased significantly (+1.3 ± 0.3 l/min) in the control group, but it remained unchanged across workloads in HFrEF patients. At 15% maximum voluntary contraction (MVC), forearm blood flow was similar between groups, while HFrEF patients exhibited an attenuated increase at the two highest intensities compared with controls, with the greatest difference at the highest workload (352 ± 22 vs. 492 ± 48 ml/min, HFrEF vs. control, 45% MVC). During KE exercise, MAP and CO increased similarly across work rates between groups. However, HFrEF patients exhibited a diminished leg hyperemic response across all work rates, with the most substantial decrement at the highest intensity (1,842 ± 64 vs. 2,675 ± 81 ml/min; HFrEF vs. control, 15 W). Together, these findings indicate a marked attenuation in exercising limb perfusion attributable to impairments in peripheral vasodilatory capacity during both arm and leg exercise in patients with HFrEF, which likely plays a role in limiting exercise capacity in this patient population.

Causes of exercise intolerance in heart failure with preserved ejection fraction: searching for consensus

Exercise intolerance is one of the cardinal symptoms of heart failure with preserved ejection fraction (HFpEF). We review its mechanistic basis using evidence from exercise studies. One barrier to a consensus understanding of the pathophysiology is heterogeneity of the patient population. Therefore, we pay special attention to varying study definitions of the disease and their possible impact on the causal factors that are implicated. We then discuss the role of exercise testing and its potential to subtype HFpEF in to more homogeneous mechanism-based subclasses.

The Alberta Heart Failure Etiology and Analysis Research Team (HEART) study

Background

Nationally, symptomatic heart failure affects 1.5-2% of Canadians, incurs $3 billion in hospital costs annually and the global burden is expected to double in the next 1–2 decades. The current one-year mortality rate after diagnosis of heart failure remains high at >25%. Consequently, new therapeutic strategies need to be developed for this debilitating condition.

Methods/Design

The objective of the Alberta HEART program (http://albertaheartresearch.ca) is to develop novel diagnostic, therapeutic and prognostic approaches to patients with heart failure with preserved ejection fraction. We hypothesize that novel imaging techniques and biomarkers will aid in describing heart failure with preserved ejection fraction. Furthermore, the development of new diagnostic criteria will allow us to: 1) better define risk factors associated with heart failure with preserved ejection fraction; 2) elucidate clinical, cellular and molecular mechanisms involved with the development and progression of heart failure with preserved ejection fraction; 3) design and test new therapeutic strategies for patients with heart failure with preserved ejection fraction. Additionally, Alberta HEART provides training and education for enhancing translational medicine, knowledge translation and clinical practice in heart failure. This is a prospective observational cohort study of patients with, or at risk for, heart failure. Patients will have sequential testing including quality of life and clinical outcomes over 12 months. After that time, study participants will be passively followed via linkage to external administrative databases. Clinical outcomes of interest include death, hospitalization, emergency department visits, physician resource use and/or heart transplant. Patients will be followed for a total of 5 years.

Discussion

Alberta HEART has the primary objective to define new diagnostic criteria for patients with heart failure with preserved ejection fraction. New criteria will allow for targeted therapies, diagnostic tests and further understanding of the patients, both at-risk for and with heart failure.

Trial registration

ClinicalTrials.gov NCT02052804.

New insights into the pathophysiology of cardiogenic shock: the role of the microcirculation

PURPOSE OF REVIEW

The ultimate goal of therapy for cardiogenic shock is to restore microcirculatory function and thereby restore the oxygen supply to sustain cellular function. Therapeutic measures mainly focus on improving pressure-derived macrocirculatory parameters. However, it is increasingly clear that to achieve significant progress in treatment, microcirculatory physiopathological mechanisms must be considered.

RECENT FINDINGS

Microcirculatory function deteriorated during cardiogenic shock and improved after treatment. Postcardiogenic shock microcirculatory disturbances, both myocardial and peripheral, were a prognostic factor for the long-term outcome. Hypothermia, whether pharmacologically or physically induced, improved postresuscitation myocardial and cerebral function, an effect associated with improved postresuscitation microcirculation. The impact of cardiogenic shock on cerebral and myocardial microcirculation could be evaluated with MRI. In severe heart failure, pharmacological interventions improved microcirculation. An assessment of the microcirculation was often performed using handheld video microscopy for direct observation of the sublingual microcirculation, which proved to be useful for evaluating the effects of interventions during cardiogenic shock. A large multicenter study on critically ill patients is now being conducted using this technique.

SUMMARY

Cardiogenic shock induces microcirculatory disorders that can be monitored and influenced in various manners, both pharmacologically and physically. In addition to global hemodynamic optimization, interventions must also ameliorate the microcirculation.

Group III/IV muscle afferents impair limb blood in patients with chronic heart failure

OBJECTIVE

To better understand the hemodynamic and autonomic reflex abnormalities in heart-failure patients (HF), we investigated the influence of group III/IV muscle afferents on their cardiovascular response to rhythmic exercise.

METHODS

Nine HF-patients (NYHA class-II, mean left ventricular ejection-fraction: 27 ± 3%) performed single leg knee-extensor exercise (25/50/80% peak-workload) under control conditions and with lumbar intrathecal fentanyl impairing μ-opioid receptor-sensitive muscle afferents.

RESULTS

Cardiac-output (Q) and femoral blood-flow (QL) were determined, and arterial/venous blood samples collected at each workload. Exercise-induced fatigue was estimated via pre/post-exercise changes in quadriceps strength. There were no hemodynamic differences between conditions at rest. During exercise, Q was 8-13% lower with Fentanyl-blockade, secondary to significant reductions in stroke volume and heart rate. Lower norepinephrine spillover during exercise with Fentanyl revealed an attenuated sympathetic outflow that likely contributed to the 25% increase in leg vascular conductance (p<0.05). Despite a concomitant 4% reduction in blood pressure, QL was 10-14% higher and end-exercise fatigue attenuated by 30% with Fentanyl-blockade (p<0.05).

CONCLUSION/PRACTICE/IMPLICATIONS

Although group III/IV muscle afferents play a critical role for central hemodynamics in HF-patients, it also appears that these sensory neurons cause excessive sympatho-excitation impairing QL which likely contributes to the exercise intolerance in this population.

Skeletal muscle abnormalities and exercise intolerance in older patients with heart failure and preserved ejection fraction

Heart failure (HF) with preserved ejection fraction (HFPEF) is the most common form of HF in older persons. The primary chronic symptom in HFPEF is severe exercise intolerance, and its pathophysiology is poorly understood. To determine whether skeletal muscle abnormalities contribute to their severely reduced peak exercise O2 consumption (Vo2), we examined 22 older HFPEF patients (70 ± 7 yr) compared with 43 age-matched healthy control (HC) subjects using needle biopsy of the vastus lateralis muscle and cardiopulmonary exercise testing to assess muscle fiber type distribution and capillarity and peak Vo2. In HFPEF versus HC patients, peak Vo2 (14.7 ± 2.1 vs. 22.9 ± 6.6 ml·kg(-1)·min(-1), P < 0.001) and 6-min walk distance (454 ± 72 vs. 573 ± 71 m, P < 0.001) were reduced. In HFPEF versus HC patients, the percentage of type I fibers (39.0 ± 11.4% vs. 53.7 ± 12.4%, P < 0.001), type I-to-type II fiber ratio (0.72 ± 0.39 vs. 1.36 ± 0.85, P = 0.001), and capillary-to-fiber ratio (1.35 ± 0.32 vs. 2.53 ± 1.37, P = 0.006) were reduced, whereas the percentage of type II fibers was greater (61 ± 11.4% vs. 46.3 ± 12.4%, P < 0.001). In univariate analyses, the percentage of type I fibers (r = 0.39, P = 0.003), type I-to-type II fiber ratio (r = 0.33, P = 0.02), and capillary-to-fiber ratio (r = 0.59, P < 0.0001) were positively related to peak Vo2. In multivariate analyses, type I fibers and the capillary-to-fiber ratio remained significantly related to peak Vo2. We conclude that older HFPEF patients have significant abnormalities in skeletal muscle, characterized by a shift in muscle fiber type distribution with reduced type I oxidative muscle fibers and a reduced capillary-to-fiber ratio, and these may contribute to their severe exercise intolerance. This suggests potential new therapeutic targets in this difficult to treat disorder.

The "second wind" in McArdle's disease patients during a second bout of constant work rate submaximal exercise

Patients with McArdle's disease (McA) typically show the "second-wind" phenomenon, a sudden decrease in heart rate (HR) and an improved exercise tolerance occurring after a few minutes of exercise. In the present study, we investigated whether in McA a first bout of exercise determines a second wind during a second bout, separated by the first by a few minutes of recovery. Eight McA (44 ± 4 yr) and a control group of six mitochondrial myopathy patients (51 ± 6 yr) performed two repetitions (CWR1 and CWR2) of 6-min constant work rate exercise (∼50% of peak work rate) separated by 6-min (SHORT) or 18-min (LONG) recovery. Pulmonary O2 uptake (Vo2), HR, cardiac output, rates of perceived exertion, vastus lateralis oxygenation {changes in deoxygenated Hb and myoglobin Mb concentrations, Δ[deoxy(Hb+Mb)], by near-infrared spectroscopy} were determined. In McA, Vo2 (0.86 ± 0.2 vs. 0.95 ± 0.1 l/min), HR (113 ± 10 vs. 150 ± 13 beats/min), cardiac output (11.6 ± 0.6 vs. 15.0 ± 0.8 l/min), and rates of perceived exertion (11 ± 2 vs. 14 ± 3) were lower, whereas Δ[deoxy(Hb+Mb)] was higher (14.7 ± 2.3 vs. -0.1 ± 4.6%) in CWR2-SHORT vs. CWR1; the "overshoot" of Δ[deoxy(Hb+Mb)] and the "slow component" of Vo2 kinetics disappeared in CWR2-SHORT. No differences (vs. CWR1) were observed in McA during CWR2-LONG, or in mitochondrial myopathy patients during both CWR2-SHORT and -LONG. A second-wind phenomenon was observed in McA during the second of two consecutive 6-min constant-work rate submaximal exercises. The second wind was associated with changes of physiological variables, suggesting an enhanced skeletal muscle oxidative metabolism. The second wind was not described after a longer (18-min) recovery period.

Therapeutic effects of serelaxin in acute heart failure

Over the past few decades, research on the peptide hormone, relaxin, has significantly improved our understanding of its biological actions under physiological and diseased conditions. This has facilitated the conducting of clinical trials to explore the use of serelaxin (human recombinant relaxin). Acute heart failure (AHF) is a very difficult to treat clinical entity, with limited success so far in developing new drugs to combat it. A recent phase-III RELAX-AHF trial using serelaxin therapy given during hospitalization revealed acute (ameliorated dyspnea) and chronic (improved 180-day survival) effects. Although these findings support a substantial improvement by serelaxin therapy over currently available therapies for AHF, they also raise key questions and stimulate new hypotheses. To facilitate the development of serelaxin as a new drug for heart disease, joint efforts of clinicians, research scientists and pharmacological industries are necessary to study these questions and hypotheses. In this review, after providing a brief summary of clinical findings and the pathophysiology of AHF, we present a working hypothesis of the mechanisms responsible for the observed efficacy of serelaxin in AHF patients. The existing clinical and preclinical data supporting our hypotheses are summarized and discussed. The development of serelaxin as a drug provides an excellent example of the bilateral nature of translational research. 

Effects of interleukin-1 blockade with anakinra on aerobic exercise capacity in patients with heart failure and preserved ejection fraction (from the D-HART pilot study)

Heart failure with preserved ejection fraction (HFpEF) is a clinical syndrome of exercise intolerance due to impaired myocardial relaxation and/or increased stiffness. Patients with HFpEF often show signs of chronic systemic inflammation, and experimental studies have shown that interleukin-1 (IL-1), a key proinflammatory cytokine, impairs myocardial relaxation. The aim of the present study was to determine the effects of IL-1 blockade with anakinra on aerobic exercise capacity in patients with HFpEF and plasma C-reactive protein (CRP) >2 mg/L (reflecting increased IL-1 activity). A total of 12 patients were enrolled in a double-blind, randomized, placebo-controlled, crossover trial and assigned 1:1 to receive 1 of the 2 treatments (anakinra 100 mg or placebo) for 14 days and an additional 14 days of the alternate treatment (placebo or anakinra). The cardiopulmonary exercise test was performed at baseline, after the first 14 days, and after the second 14 days of treatment. The placebo-corrected interval change in peak oxygen consumption was chosen as the primary end point. All 12 patients enrolled in the present study and receiving treatment completed both phases and experienced no major adverse events. Anakinra led to a statistically significant improvement in peak oxygen consumption (+1.2 ml/kg/min, p = 0.009) and a significant reduction in plasma CRP levels (-74%, p = 0.006). The reduction in CRP levels correlated with the improvement in peak oxygen consumption (R = -0.60, p = 0.002). Three patients (25%) had mild and self-limiting injection site reactions. In conclusion, IL-1 blockade with anakinra for 14 days significantly reduced the systemic inflammatory response and improved the aerobic exercise capacity of patients with HFpEF and elevated plasma CRP levels.

Skeletal muscle microvascular oxygenation dynamics in heart failure: exercise training and nitric oxide-mediated function

Chronic heart failure (CHF) impairs nitric oxide (NO)-mediated regulation of skeletal muscle O2 delivery-utilization matching such that microvascular oxygenation falls faster (i.e., speeds PO2mv kinetics) during increases in metabolic demand. Conversely, exercise training improves (slows) muscle PO2mv kinetics following contractions onset in healthy young individuals via NO-dependent mechanisms. We tested the hypothesis that exercise training would improve contracting muscle microvascular oxygenation in CHF rats partly via improved NO-mediated function. CHF rats (left ventricular end-diastolic pressure = 17 ± 2 mmHg) were assigned to sedentary (n = 11) or progressive treadmill exercise training (n = 11; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min; -14% grade downhill running) groups. PO2mv was measured via phosphorescence quenching in the spinotrapezius muscle at rest and during 1-Hz twitch contractions under control (Krebs-Henseleit solution), sodium nitroprusside (SNP; NO donor; 300 μM), and N(G)-nitro-l-arginine methyl ester (L-NAME, nonspecific NO synthase blockade; 1.5 mM) superfusion conditions. Exercise-trained CHF rats had greater peak oxygen uptake and spinotrapezius muscle citrate synthase activity than their sedentary counterparts (p < 0.05 for both). The overall speed of the PO2mv fall during contractions (mean response time; MRT) was slowed markedly in trained compared with sedentary CHF rats (sedentary: 20.8 ± 1.4, trained: 32.3 ± 3.0 s; p < 0.05), and the effect was not abolished by L-NAME (sedentary: 16.8 ± 1.5, trained: 31.0 ± 3.4 s; p > 0.05). Relative to control, SNP increased MRT in both groups such that trained CHF rats had slower kinetics (sedentary: 43.0 ± 6.8, trained: 55.5 ± 7.8 s; p < 0.05). Improved NO-mediated function is not obligatory for training-induced improvements in skeletal muscle microvascular oxygenation (slowed PO2mv kinetics) following contractions onset in rats with CHF.

Skeletal muscle capillary function: contemporary observations and novel hypotheses

The capillary bed constitutes a vast surface that facilitates exchange of O2, substrates and metabolites between blood and organs. In contracting skeletal muscle, capillary blood flow and O2 diffusing capacity, as well as O2 flux, may increase two orders of magnitude above resting values. Chronic diseases, such as heart failure and diabetes, and also sepsis impair these processes, leading to compromised energetic, metabolic and, ultimately, contractile function. Among researchers seeking to understand blood-myocyte exchange in health and the basis for dysfunction in disease, there is a fundamental disconnect between microcirculation specialists and many physiologists and physiologist clinicians. While the former observe capillaries and capillary function directly (muscle intravital microscopy), the latter generally use indirect methodologies (e.g. post-mortem tissue analysis, 1-methyl xanthine, contrast-enhanced ultrasound, permeability-surface area product) and interpret their findings based upon August Krogh's observations made nearly a century ago. 'Kroghian' theory holds that only a small fraction of capillaries support red blood cell (RBC) flux in resting muscle, leaving the vast majority to be 'recruited' (i.e. to initiate RBC flux) during contractions, which would constitute the basis for increasing surface area for capillary exchange and reducing capillary-mitochondrial diffusion distances. Experimental techniques each have their strengths and weaknesses, and often the correct or complete answer to a problem emerges from integration across multiple technologies. Today, Krogh's entrenched 'capillary recruitment' hypothesis is challenged by direct observations of capillaries in contracting muscle, which is something that he and his colleagues could not do. Moreover, in the peer-reviewed scientific literature, application of a range of contemporary physiological technologies, including intravital microscopy of contracting muscle, magnetic resonance, near-infrared spectroscopy and phosphorescence quenching, combined with elegant in situ and in vivo models, suggest that the role of the capillary bed, at least in contracting muscle, is subserved without the necessity for de novo capillary recruitment of previously non-flowing capillaries. When viewed within the context of the capillary recruitment hypothesis, this evidence casts serious doubt on the interpretation of those data that are based upon Kroghian theory and indirect methodologies. Thus, today a wealth of evidence calls for a radical revision of blood-muscle exchange theory to one in which most capillaries support RBC flux at rest and, during contractions, capillary surface area is 'recruited' along the length of previously flowing capillaries. This occurs, in part, by elevating capillary haematocrit and extending the length of the capillary available for blood-myocyte exchange (i.e. longitudinal recruitment). Our understanding of blood-myocyte O2 and substrate/metabolite exchange in health and the mechanistic basis for dysfunction in disease demands no less.

The effects of dietary fish oil on exercising skeletal muscle vascular and metabolic control in chronic heart failure rats

Impaired vasomotor control in chronic heart failure (CHF) is due partly to decrements in nitric oxide synthase (NOS) mediated vasodilation. Exercising muscle blood flow (BF) is augmented with polyunsaturated fatty acid (PUFA) supplementation via fish oil (FO) in healthy rats. We hypothesized that FO would augment exercising muscle BF in CHF rats via increased NO-bioavailability. Myocardial infarction (coronary artery ligation) induced CHF in Sprague-Dawley rats which were subsequently randomized to dietary FO (20% docosahexaenoic acid, 30% eicosapentaenoic acid, n = 15) or safflower oil (SO, 5%, n = 10) for 6-8 weeks. Mean arterial pressure (MAP), blood [lactate], and hindlimb muscles BF (radiolabeled microspheres) were determined at rest, during treadmill exercise (20 m·min(-1), 5% incline) and exercise + N(G)-nitro-l-arginine-methyl-ester (l-NAME) (a nonspecific NOS inhibitor). FO did not change left ventricular end-diastolic pressure (SO: 14 ± 2; FO: 11 ± 1 mm Hg, p > 0.05). During exercise, MAP (SO: 128 ± 3; FO: 132 ± 3 mm Hg) and blood [lactate] (SO: 3.8 ± 0.4; FO: 4.6 ± 0.5 mmol·L(-1)) were not different (p > 0.05). Exercising hindlimb muscle BF was lower in FO than SO (SO: 120 ± 11; FO: 93 ± 4 mL·min(-1)·100 g(-1), p < 0.05) but was not differentially affected by l-NAME. Specifically, 17 of 28 individual muscle BF's were lower (p < 0.05) in FO demonstrating that PUFA supplementation with FO in CHF rats does not augment muscle BF during exercise but may lower metabolic cost.

A validated model of oxygen uptake and circulatory dynamic interactions at exercise onset in humans

At the onset of muscular exercise, the kinetics of pulmonary O2 uptake (Vo2P) reflect the integrated dynamic responses of the ventilatory, circulatory, and neuromuscular systems for O2 transport and utilization. Muscle O2 uptake (Vo2m) kinetics, however, are dissociated from Vo2P kinetics by intervening O2 capacitances and the dynamics of the circulation and ventilation. We developed a multicompartment computational model (MCM) to investigate these dynamic interactions and optimized and validated the MCM using previously published, simultaneously measured Vo2m, alveolar O2 uptake (Vo2A), and muscle blood flow (Qm) in healthy young men during cycle ergometry. The model was used to show that 1) the kinetics of Vo2A during exercise transients are very sensitive to preexercise blood flow distribution and the absolute value of Qm, 2) a low preexercise Qm exaggerates the magnitude of the transient fall in venous O2 concentration for any given Vo2m kinetics, necessitating a tighter coupling of Qm/Vo2m (or a reduction in the available work rate range) during the exercise transient to avoid limits to O2 extraction, and 3) information regarding exercise-related alterations in O2 uptake and blood flow in nonexercising tissues and their effects on mixed venous O2 concentration is required to accurately predict Vo2A kinetics from knowledge of Vo2m and Qm dynamics. Importantly, these data clearly demonstrate that Vo2A kinetics are nonexponential, nonlinear distortions of Vo2m kinetics that can be explained in a MCM by interactions among circulatory and cellular respiratory control processes before and during exercise.

The TNF-α/sphingosine-1-phosphate signaling axis drives myogenic responsiveness in heart failure

Heart failure (HF) is hallmarked by an increase in total peripheral resistance (TPR) that compensates for the drop in cardiac output. While initially allowing for the maintenance of mean arterial pressure at acceptable levels, the long-term upregulation of TPR is prone to compromise cardiac performance and tissue perfusion, and to ultimately accelerate disease progression. Augmented vasoconstriction of terminal arteries, the site of TPR regulation, is cooperatively driven by mechanisms such as: (i) endothelial dysfunction, (ii) increased sympathetic activity and (iii) enhanced pressure-induced myogenic responsiveness. Herein, we review emerging evidence that the increase in myogenic responsiveness is central to the long-term elevation of TPR in HF. On a molecular level, this augmented intrinsic response is governed by an activation of the tumor necrosis factor-α (TNF-α)/sphingosine-1-phosphate signaling axis in microvascular smooth muscle cells. The beneficial effect of TNF-α scavenging strategies on tissue perfusion in HF mouse models adds to the gaining momentum to revisit the use of anti-TNF-α treatment modalities in discrete HF patient populations.

Effects of nitrate supplementation via beetroot juice on contracting rat skeletal muscle microvascular oxygen pressure dynamics

NO3(-) supplementation via beetroot juice (BR) augments exercising skeletal muscle blood flow subsequent to its reduction to NO2(-) then NO. We tested the hypothesis that enhanced vascular control following BR would elevate the skeletal muscle O2 delivery/O2 utilization ratio (microvascular PO2, PmvO2) and raise the PmvO2 during the rest-contractions transition. Rats were administered BR (~0.8 mmol/kg/day, n=10) or water (control, n=10) for 5 days. PmvO2 was measured during 180 s of electrically induced (1 Hz) twitch spinotrapezius muscle contractions. There were no changes in resting or contracting steady-state PmvO2. However, BR slowed the PmvO2 fall following contractions onset such that time to reach 63% of the initial PmvO2 fall increased (MRT1; control: 16.8±1.9, BR: 24.4±2.7 s, p<0.05) and there was a slower relative rate of PmvO2 fall (Δ1PmvO2/τ1; control: 1.9±0.3, BR: 1.2±0.2 mmHg/s, p<0.05). Despite no significant changes in contracting steady state PmvO2, BR supplementation elevated the O2 driving pressure during the crucial rest-contractions transients thereby providing a potential mechanism by which BR supplementation may improve metabolic control.

Skeletal muscle oxidative function in vivo and ex vivo in athletes with marked hypertrophy from resistance training

Oxidative function during exercise was evaluated in 11 young athletes with marked skeletal muscle hypertrophy induced by long-term resistance training (RTA; body mass 102.6 ± 7.3 kg, mean ± SD) and 11 controls (CTRL; body mass 77.8 ± 6.0 kg). Pulmonary O2 uptake (Vo2) and vastus lateralis muscle fractional O2 extraction (by near-infrared spectroscopy) were determined during an incremental cycle ergometer (CE) and one-leg knee-extension (KE) exercise. Mitochondrial respiration was evaluated ex vivo by high-resolution respirometry in permeabilized vastus lateralis fibers obtained by biopsy. Quadriceps femoris muscle cross-sectional area, volume (determined by magnetic resonance imaging), and strength were greater in RTA vs. CTRL (by ∼40%, ∼33%, and ∼20%, respectively). Vo2peak during CE was higher in RTA vs. CTRL (4.05 ± 0.64 vs. 3.56 ± 0.30 l/min); no difference between groups was observed during KE. The O2 cost of CE exercise was not different between groups. When divided per muscle mass (for CE) or quadriceps muscle mass (for KE), Vo2 peak was lower (by 15-20%) in RTA vs. CTRL. Vastus lateralis fractional O2 extraction was lower in RTA vs. CTRL at all work rates, during both CE and KE. RTA had higher ADP-stimulated mitochondrial respiration (56.7 ± 23.7 pmol O2·s(-1)·mg(-1) ww) vs. CTRL (35.7 ± 10.2 pmol O2·s(-1)·mg(-1) ww) and a tighter coupling of oxidative phosphorylation. In RTA, the greater muscle mass and maximal force and the enhanced mitochondrial respiration seem to compensate for the hypertrophy-induced impaired peripheral O2 diffusion. The net results are an enhanced whole body oxidative function at peak exercise and unchanged efficiency and O2 cost at submaximal exercise, despite a much greater body mass.