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Published in final edited form as: Hypertension. 2021 Nov 1;78(6):1750–1759. doi: 10.1161/HYPERTENSIONAHA.121.17875

Locomotor Muscle Microvascular Dysfunction in Heart Failure with Preserved Ejection Fraction

Michael A Francisco 1,2,*, Joshua F Lee 1,2,*, Zachary Barrett-O’Keefe 2,3, H Jonathan Groot 2,3, Stephen M Ratchford 2,4, Kanokwan Bunsawat 1, Jeremy K Alpenglow 3, John J Ryan 1, Jose N Nativi 1, Russell S Richardson 1,2,3, D Walter Wray 1,2,3
PMCID: PMC8585708  NIHMSID: NIHMS1746643  PMID: 34719934

Abstract

While there is emerging evidence of peripheral microvascular dysfunction in patients with heart failure with preserved ejection fraction that may be related to systemic inflammation and redox imbalance, disease-related changes in locomotor muscle microvascular responsiveness has not been determined. This study combined passive leg movement and biomarker assessments of inflammation and oxidative damage to determine the magnitude and mechanisms of lower limb microvascular function in patients with heart failure with preserved ejection fraction (71±1yrs; n=44) compared to healthy, similarly-aged controls (68±2yrs; n=39). Leg blood flow, heart rate, mean arterial pressure, and stroke volume were assessed, and plasma biomarkers of inflammation and oxidative damage were also determined. A significantly attenuated passive leg movement-induced peak change in leg blood flow (263±25 vs. 371±31 mL/min, heart failure with preserved ejection fraction vs. control) and leg vascular conductance (2.99±0.32 vs. 3.88±0.34 mL/min/mmHg, heart failure with preserved ejection fraction vs. control) was observed in patients compared to controls. Similarly, the total hyperemic response to passive leg movement, expressed as leg blood flowAUC and leg vascular conductanceAUC, was ~40% less in patients with heart failure with preserved ejection fraction vs. control. Significantly greater C-reactive protein, interleukin 6, and malondialdehyde were observed in patients with heart failure with preserved ejection fraction, but were not correlated with passive leg movement responses. These data provide new evidence of a decline in lower limb microvascular function within a milieu of vascular inflammation that may contribute to locomotor muscle dysfunction in patients with heart failure with preserved ejection fraction.

Keywords: HFpEF, hyperemia, vascular function, nitric oxide, inflammation

Journal Subject Codes: Heart Failure, Peripheral Vascular Disease, Endothelium, Hemodynamics, Biomarkers

INTRODUCTION

It is predicted that heart failure (HF) and other forms of cardiovascular disease will exceed 40% of the U.S. population by 2030 [1], making HF a burgeoning public health crisis. Just over half of HF patients have a preserved left ventricular ejection fraction (HFpEF) [2,3], and the prevalence of this form of HF relative to HF with reduced ejection fraction (HFrEF) is increasing 1% annually [4]. While HFpEF and HFrEF share a similar clinical presentation that is characterized by exertional dyspnea and exercise intolerance [2,3,5–8], the numerous proven therapies for patients with HFrEF [9] have been unsuccessful in the treatment of patients with HFpEF [10]. This lack of success with standard therapies highlights the truly unique nature of this clinical syndrome and demonstrates the need for a more comprehensive understanding of HFpEF pathophysiology in this phenotypically distinct patient group.

One of the most prominent unifying theories concerning the pathophysiology of HFpEF is the role of non-cardiac comorbidities (i.e. obesity, hypertension, diabetes mellitus, chronic obstructive pulmonary disease, and chronic kidney disease) that accompany this clinical syndrome and result in systemic inflammation [11]. Indeed, circulating inflammatory biomarkers appear to be distinctly elevated in patients with HFpEF [12], and this inflammation has been associated with myocardial remodeling and dysfunction [11] through inflammatory activation of the coronary microvascular endothelium, oxidative stress, and impaired endothelial-cardiomyocyte nitric oxide (NO) signaling [13]. How this systemic inflammation impacts peripheral vascular function in patients with HFpEF remains unclear.

To date, a small number of studies have sought to evaluate peripheral vascular function using traditional, non-invasive testing (i.e. arterial flow-mediated dilation; FMD) in patients with HFpEF, and findings have been equivocal [14–18]. Our group recently documented a distinct pattern of upper limb peripheral vascular dysfunction in HFpEF patients specific to the microvasculature [19], which parallels a growing body of literature demonstrating microvascular dysfunction in this patient group [13,16,20,21]. However, the exact vasodilatory pathways involved in HFpEF-specific peripheral microvascular dysfunction remain somewhat unclear [22,23].

The peripheral hemodynamic response to passive leg movement (PLM) fills this void by providing an index of predominantly NO-mediated (~80%) microvascular function in the lower extremities [24]. Unlike FMD and RH measurements in the upper extremity, the PLM test involves continuous movement of the knee joint through a 90-degree range of motion, eliciting a transient increase in limb blood flow which is somewhat correlated with FMD testing [25]. Thus, the assessment of lower limb microvascular function using the PLM methodology represents a unique approach for assessment of NO bioavailability in the HFpEF patient population. The PLM test has been used extensively to assess and differentiate lower limb microvascular responses in healthy individuals [24,26,27], the elderly [28,29], and various disease states [30–32], making it a, potentially, clinically relevant non-invasive test of lower limb peripheral vascular function in patients with HFpEF.

Therefore, the overall goal of this investigation was to combine biomarkers of inflammation and oxidative stress with a novel assessment of primarily NO-dependent vasodilation to more fully characterize disease-specific adaptations in the lower extremity microvasculature of patients with HFpEF. We hypothesized that biomarkers of inflammation and oxidative stress would be elevated, and that PLM-induced hyperemia would be attenuated, in patients with HFpEF compared to healthy, similarly aged controls.

METHODS

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Ethical Approval

All experimental procedures were approved by the University of Utah and Salt Lake City Veterans Affairs Medical Center Institutional Review Boards. All subjects provided oral and written informed consent prior to participation in the study, as set forth by the Declaration of Helsinki.

Subjects

A total of 83 subjects (n = 44 HFpEF patients, n = 39 healthy, similarly aged controls) participated in the current study. Subject characteristics for all individuals and disease specific characteristics of the patient group are documented in Table 1 and 2, respectively. Details regarding inclusion/exclusion criteria are provided in the online data supplement.

Table 1.

Subject characteristics

Subject Characteristics Controls HFpEF P value
Subjects, N (males/females) 39 (20/19) 43 (18/25) ---
Age, yrs 68 ± 2 71 ± 1 0.19
Body mass, kg 77 ± 3 98 ± 4* <0.001
Height, cm 170 ± 2 169 ± 2 0.82
Body mass index, kg/m2 26 ± 1 34 ± 1* <0.001
Body surface area, 1.91 ± 0.04 2.00 ± 0.05* <0.001
Resting Heart rate, beats/min 59 ± 1 65 ± 2* 0.003
Mean arterial blood pressure, mmHg 100 ± 3 92 ± 3* 0.04
Glucose, mg/dL 84 ± 3 97 ± 3* <0.001
Cholesterol, mg/dL 207 ± 8 170 ± 9* <0.001
High-density lipoprotein, mg/dL 55 ± 3 47 ± 3* 0.015
Low-density lipoprotein, mg/dL 132 ± 5 96 ± 8* <0.001
Triglycerides, mg/dL 126 ± 15 173 ± 17* 0.04
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Data are mean ± SEM.

*

Significantly different vs. Controls

Table 2.

HFpEF-specific characteristics.

Clinical Characteristics HFpEF
Echocardiography
Ejection fraction (%) 61 ± 2
LV IVSd (cm) 1.17 ± 0.04
LV PWd (cm) 1.22 ± 0.11
LV ID diastole (cm) 4.52 ± 0.12
LD ID systole (cm) 4.58 ± 0.07
Peak E wave (cm/s) 98 ± 5
Peak A wave (cm/s) 92 ± 7
E/A ratio 1.36 ± 0.17
E’ lateral wall (cm/s) 8 ± 1
E/E’ ratio 22 ± 8
Disease-specific Biomarkers
BNP (pg/ml) 150 ± 26
NT-pro BNP (pg/ml) 620 ± 165
ST2 (ng/ml) 45 ± 6
Galectin-3 (ng/ml) 15 ± 1
Cystatin C (ng/ml) 17 ± 1
Comorbidities
COPD (%) 7
Diabetes (%) 33
CAD (%) 29
Hypertension (%) 71
Medications
Beta receptor blocker (%) 45
ACEi or ARB (%) 60
Loop diuretics (%) 81
Diuretic (%) 69
Statin (%) 67
Calcium channel blockers (%) 19
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LV IVSD, left ventricle interventricular septum; LV PWd, left ventricle posterior wall diameter; LV ID, left ventricle internal diameter; COPD, chronic obstructive pulmonary disease; CAD, coronary artery disease; angiotensin converting enzyme inhibitor; angiotensin receptor blocker; BNP, B-type natriuretic peptide; NT-proBNP, NT-pro B-type natriuretic peptide; ST2, suppression of tumorigenicity 2. Data are mean ± SEM or %.

Passive Leg Movement Protocol

The PLM procedure has been described in detail elsewhere [33]. After 20 minutes of supine rest and prior to leg movement, baseline measures were assessed for 1 minute. PLM was then performed by a trained investigator who articulated the subject’s knee joint through a 90° range of motion (full leg extension (180°) to flexion (90°)) at 1Hz for 1 minute. PLM has been previously shown to have good reliability as a non-invasive index of microvascular function [34]

Measurements

Common femoral arterial blood velocity and vessel diameter were assessed using ultrasound Doppler (GE a Logiq 7). Heart rate (HR) was monitored continuously from a standard three-lead ECG, and arterial blood pressure was measured using finger photoplethysmography (Finometer). Stroke volume (SV) was estimated using the Modelflow method [35], which, in combination with HR, was used to estimate cardiac output (CO). Plasma biomarkers of inflammation and oxidative stress were determined using standard quantitative methods. Additional methodological details are provided in the online data supplement.

Statistical Approach

Statistical analysis was performed using commercially available software (SigmaStat 3.10, Systat Software, Point Richmond, CA). Physical characteristics, resting cardiovascular measures, PLM-induced peak and total blood flow (AUC) responses and plasma biomarkers were compared between groups using Student’s unpaired t-tests to identify any group differences. A two-way ANOVA was used to compare differences between groups across time during PLM. When a significant main effect was observed a Holm-Sidak post hoc analysis was performed. The Significance was set at P < 0.05 and data are presented as group mean ± SEM. Exact P-values are given unless otherwise noted.

RESULTS

Characteristics of HFpEF Patients and healthy controls.

Patient and control characteristics are presented in Table 1. Despite being well-matched for age and height, patients with HFpEF had higher BMI, BSA, resting HR, and lower resting MAP (All P < 0.05; Table 1). Additionally, patients with HFpEF also had higher resting blood glucose levels despite having lower overall cholesterol, HDL, and LDL levels (Table 1). Disease-specific characteristics and pharmacological intervention information for patients with HFpEF patients are presented in Table 2. All 43 of the patients with HFpEF were currently taking one or more blood pressure lowering and/or cholesterol altering medications (Table 2).

Peripheral Responses to Passive Leg Movement.

Resting LBF and vascular conductance were not different between the two cohorts (both P > 0.05). At the onset of PLM, LBF and leg vascular conductance increased immediately above baseline in both groups (Figure 1A and 2A). Peak LBF during PLM was ~10% lower in patients with HFpEF (593 ± 34 mL/min) compared to controls (651 ± 35 mL/min), yet the difference did not reach statistical significance (P = 0.24). Similarly, peak conductance was not different between HFpEF patients (6.89 ± 0.54 mL/min/mmHg) and controls (6.77 ± 0.41 mL/min/mmHg) (P = 0.89). However, in HFpEF patients compared to controls the peak change in LBF (263 ± 25 mL/min and 2.99 ± 0.32 mL/min/mmHg, respectively, P = 0.008, Figure 1B) and leg vascular conductance (371 ± 31 mL/min and 3.88 ± 0.34 mL/min/mmHg, respectively, P = 0.049, Figure 2B) was significantly blunted. Notably, these differences in LBF and vascular conductance occurred in the absence of any differences in the MAP response, or peak change in MAP, during PLM between groups.

FIGURE 1:

FIGURE 1:

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Panel A: Changes in leg blood flow during 1 minute of passive leg movement in controls and patients with HFpEF. Area under the curve (AUC) was calculated as the summed second-by-second values during passive movement. Panel B: Peak change in leg blood flow achieved during 1 minute of passive leg movement in controls and patients with HFpEF. *Significantly different than control, P < 0.05. HFpEF n = 43; Control n = 39. Data are mean ± SEM

FIGURE 2:

FIGURE 2:

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Panel A: Changes in leg vascular conductance during 1 minute of passive leg movement in controls and patients with HFpEF. Area under the curve (AUC) was calculated as the summed second-by-second values during passive movement. Panel B: Peak change in leg vascular conductance achieved during 1 minute of passive leg movement in controls and patients with HFpEF. *Significantly different than control, P < 0.05. HFpEF n = 40; Control n = 35. Data are mean ± SEM

Although both groups exhibited the typical transient increases in LBF and conductance in response to PLM, LBF and leg vascular conductance not only increased to a greater extent in controls compared to patients with HFpEF, but remained elevated relative to the patients with HFpEF throughout PLM (LBF P = 0.012; vascular conductance P = 0.008; Figure 1A and 2A, respectively) Furthermore, when PLM-induced changes in LBF and vascular conductance were assessed as AUC, the differences between groups were even greater (Figure 1A and 2A), with patients with HFpEF having a ~ 40% lower LBFAUC and vascular conductanceAUC (P = 0.009 and P = 0.024, respectively).

Central Hemodynamic Responses to Passive Leg Movement.

Immediately prior to PLM, resting HR (P = 0.03), SV (P < 0.01), and CO (P < 0.001) were all higher in patients with HFpEF (65 ± 2 bpm, 107 ± 5 ml, 6.8 ± 0.3 ml/min, respectively) compared to controls (59 ± 1 bpm, 87 ± 5 ml, 4.9 ± 0.3 ml/min, respectively). Which may be due to the higher BMIs observed in HFpEF patients and the added demand for perfusion of excess adipose tissue (Table 1). There were no differences in peak change in HR (9 ± 2 vs. 7 ± 1 bpm, HFpEF vs. Control, P = 0.28) or peak change in CO (0.8 ± 0.1 vs. 1.0 ± 0.2 ml/min, HFpEF vs. Control, P = 0.37) during PLM. However, peak change in stroke volume was higher in patients with HFpEF compared to controls (11 ± 1 ml vs. 9 ± 1ml, HFpEF vs. Control, P = 0.01).

Inflammation, Antioxidant Capacity and Oxidative Stress.

Despite no group differences for the pro-inflammatory cytokine TNFα (2.08 ± 0.25 vs. 1.85 ± 0.15 pg ml−1, HFpEF vs. Control; Figure 3A), patients with HFpEF had significantly greater plasma IL-6 concentrations (5.09 ± 0.87 vs. 2.7 ± 0.37 pg ml−1, HFpEF vs. Control; Figure 3C). Additionally, systemic inflammation, as assessed by CRP was greater in patients with HFpEF compared to controls (4637 ± 601 and 1020 ± 113 ng ml−1, respectively; Figure 3A). Protein oxidation, assessed by plasma protein carbonyl, was not different between groups (0.13 ± 0.01 vs. 0.13 ± 0.02 nmol mg−1, HFpEF vs. control; Figure 4A). Conversely, lipid peroxidation, as assessed by plasma malondialdehyde, was elevated in patients with HFpEF compared to controls (4.06 ± 0.57 vs. 1.30 ± 0.14 μM, HFpEF vs. control; Figure 4B). No notable correlations between any biomarkers of inflammation and oxidative stress and PLM responses were observed.

FIGURE 3:

FIGURE 3:

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Serum biomarkers of inflammation, as assessed by C-reactive protein (panel A), Tumor Nercrosis Factor-α (panel B), and Interleukin-6 (panel C) in controls and patients with HFpEF. * Significantly different than control, P < 0.05. HFpEF n = 27; Control n = 35. Data are mean ± SEM

FIGURE 4:

FIGURE 4:

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Serum biomarkers of oxidative damage, as assessed by protein carbonyl (panel A, HFpEF n = 26; Control n = 33) and malondialdehyde (panel B, HFpEF n = 27; Control n = 35) in controls and patients with HFpEF. * Significantly different than control, P < 0.05. Data are mean ± SEM

DISCUSSION

This study sought to characterize lower limb microvascular function in patients with HFpEF compared to healthy, similarly aged controls and to characterize the relationship between plasma biomarkers of inflammation, oxidative damage, and the blood flow response to PLM. We observed a significantly diminished PLM-induced hyperemic response, whether quantified as the peak change in LBF or LBFAUC, in patients with HFpEF compared to healthy, aged matched controls. In light of previous studies that have identified NO as the predominant factor governing the increase in LBF during PLM, these findings build upon an emerging line of evidence for a disease-related decline in NO bioavailability in this patient population. Patients with HFpEF also exhibited elevated biomarkers for both inflammation and oxidative damage, though a significant relationship was not observed between these biomarkers and PLM responses. Taken together, these findings provide new evidence for a decline in NO-mediated lower limb microvascular function within a milieu of systemic inflammation and redox imbalance that likely contribute to locomotor muscle microvascular function in patients with HFpEF.

Peripheral Vascular Response to PLM in HFpEF.

In contrast to HFrEF, where peripheral vascular dysfunction has been well documented, few studies have sought to evaluate peripheral vascular function using traditional testing (i.e. FMD) in patients with HFpEF, and the findings have been equivocal [14–18]. Despite this, several investigations have recently demonstrated impaired microvascular function, assessed by peripheral arterial tonometry (PAT), in patients with HFpEF [16,20,21]. Our group has previously documented attenuated upper limb vascular function in patients with HFpEF at the level of the microcirculation, assessed by RH, despite an apparent preservation of conduit artery FMD when corrected for shear stimulus [19]. These findings are in agreement with a growing body of literature supporting the importance of microvascular dysfunction and NO bioavailability to HFpEF pathophysiology [13,16,20,21]. However, recent studies have questioned the NO-dependent nature of both FMD [36–38] and RH [22] tests. Based in part on this lack of specificity for the NO pathway, our group has developed the PLM paradigm as an additional assessment of NO-dependent microvascular function in the leg.

PLM is unique in that this test directly interrogates the leg, which plays a major role in human locomotion and exercise capacity, and thus this mode of assessment is relevant to the physical challenges faced by patients with HFpEF. We have previously documented that the PLM hyperemic response is largely NO-mediated [24], and this has been corroborated in subsequent studies [39,40]. Therefore, the current study sought to utilize this novel vascular assessment to better characterize microvascular dysfunction and NO bioavailability in patients with HFpEF.

Our current finding is that PLM-induced hyperemia, as quantified by LBF and leg vascular conductanceAUC, was ~40% attenuated in patients with HFpEF, despite a preserved MAP response (Figure 1A and 2A). Additionally, we observed a ~ 25% lower peak change in LBF and vascular conductance (Figure 1B and 2B) during PLM in patients with HFpEF compared to controls. Together, these responses to PLM testing indicate a clear decrement in microvascular function in patients with HFpEF and suggest that disease-related maladaptations in the peripheral vasculature of the lower limbs may contribute to the severe exercise intolerance in this patient population [41]. This finding of impaired microvascular responsiveness in an ambulatory muscle group is particularly relevant in the context of previous studies from our group [42,43] and others [44,45] that have collectively identified a decline in exercising muscle blood flow in patients with HFpEF. Although it is important to acknowledge that exercising limb blood flow is governed by a complex combination of regulatory processes [46], it is tempting to speculate that the disease-related decline in microvascular reactivity observed in the present study is an important contributor to the overall decrement in muscle blood flow and exercise tolerance in this patient group [47]. Thus, data from the present study extends this previous work identifying microvascular dysfunction in patients with HFpEF [42–45], and provides new evidence of this phenomenon in the microvasculature of the locomotor muscle of this population.

The observed attenuation in the hyperemic response to PLM in patients with HFpEF also offers important new insight in terms of NO bioavailability in their peripheral microvasculature. Previous work from our group [24,39] and others [40] has indicated that the hyperemic response to PLM in young, healthy individuals is ~80% NO-dependent and, as such, provides a reliable non-invasive biomarker of NO bioavailability. While this may be indicative of attenuated NO bioavailability in patients with HFpEF, it is also important to note that healthy aging has, likewise, been documented to limit the NO-mediated component of the PLM-induced hyperemic response [48]. Despite this, NO still plays a small role in PLM-mediated vasodilation in healthy aged humans that might become further diminished in highly sedentary or diseased populations. Indeed, the magnitude of attenuation in the overall vasodilatory response (vascular conductance AUC) in patients with HFpEF compared to controls (Figure 2A) was similar to that previously observed in the elderly following NO synthase inhibition [48]. This indicates, albeit indirectly, that the attenuated PLM-induced hyperemic response in patients with HFpEF may be due to attenuated bioavailable NO that is distinct from that lost as a consequence of aging per se. With this in mind, our current findings highlight the NO signaling pathway as an important target for therapies to improve symptoms, and potentially outcomes, for patients with HFpEF.

Inflammation and Oxidative Stress.

In HFrEF, systemic inflammation and oxidative stress have been implicated as major contributors to the reduction in NO bioavailability [49,50] and the development of peripheral vascular dysfunction [31,51–54]. Oxidative stress can negatively impact NO bioavailability by uncoupling endothelial NOS [55] and increasing NO degradation from existing NO pools [56], which, in turn, is further exacerbated by the production of superoxide. Studies aimed at attenuating oxidative stress through antioxidant supplementation have been successful in improving endothelial function in both HFrEF and HFpEF [54,57] and have highlighted the deleterious effects of a redox imbalance [58] on endothelial function.

In the pathogenesis of patients with HFpEF, current convention holds that aging as well non-cardiac comorbidities [11] (i.e. obesity, hypertension, diabetes mellitus, chronic obstructive pulmonary disease, and chronic kidney disease) initiate chronic systemic inflammation, as evidenced from elevated plasma biomarkers of inflammation [12] in patients with HFpEF. This systemic inflammation may then affect myocardial remodeling and dysfunction through the retardation of the coronary microvasculature and subsequent redox imbalance. Indeed, myocardial biopsies from patients with HFpEF have revealed a clear association with microvascular endothelial inflammatory activation, oxidative stress, and attenuated NO-dependent signaling from endothelial cells to cardiomyocytes [13]. Despite this evidence for cardiac involvement, the impact of inflammation and associated oxidative stress on peripheral vascular function is not well characterized in patients with HFpEF. Thus, the present study sought to explore potential mechanisms responsible for the observed decrement in PLM-induced hyperemia in patients with HFpEF by assessing plasma biomarkers of systemic inflammation and oxidative damage.

This study documented that CRP concentrations were elevated in patients with HFpEF compared to controls (Figure 3A), which is consistent with previous findings in this patient group [57,59]. Additionally, in patients with HFpEF the pro-inflammatory cytokine IL-6 concentration (Figure 3B) was significantly elevated, while TNFα concentrations (Figure 3C) tended to be higher compared to controls, which are also consistent with previous findings in these patients [57]. An elevation in these biomarkers is particularly compelling, given that aging itself is associated with increased expression of CRP, IL-6, and TNFα [60], and antihypertensive pharmacological interventions used to treat HFpEF patients are recognized to attenuate circulating pro-inflammatory mediators [61]. Despite the robust inflammation exhibited by the patients with HFpEF, compared to similarly aged control subjects, correlation analysis failed to reveal any significant association with the PLM-induced LBF responses. Regardless, these findings highlight a pro-inflammatory vascular milieu that may result in decreased NO bioavailability and/or oxidative damage. Indeed, the concentration of malondialdehyde, an indicator of lipid peroxidation, was elevated in patients with HFpEF (Figure 4B), while there was no difference in protein oxidation (assessed by plasma protein carbonyl) between the two groups (Figure 4A). The measures of oxidative damage in this study likely represent some degree of ongoing redox imbalance in patients with HFpEF. Inflammation and the resultant oxidative stress from either an increased production of reactive oxygen species or attenuated antioxidant capacity have been documented to negatively impact NO-bioavailability and endothelial function in the vasculature [54,57,58,62], suggesting that this axis of inflammation, oxidative stress, and NO may represent a potential target for therapeutic interventions. Indeed, we have recently identified the efficacy of interventions targeting oxidative stress to improve vascular outcomes in a variety of patient groups [62–64], including HFpEF [57]. However, whether interventional studies aimed at attenuating inflammation and oxidative stress could improve lower limb microvascular function in patients with HFpEF has yet to be examined, and therefore represents an area that is certainly worthy of further study.

Experimental Considerations

Given that the current study is the first to utilize PLM testing in HFpEF to examine lower limb microvascular function, the a priori decision was made to enroll healthy older controls based on the desire to establish that any observed microvascular dysfunction in patients with HFpEF was not a function of aging per se [48]. However, we acknowledge that matching patients and controls in this manner precludes consideration of how comorbidities common to the HFpEF phenotype may differentially affect microvascular function. By design, no prescribed medications were withheld on study days, which enabled the opportunity to study these patients in a “real‐world” setting. While we cannot exclude the potential for commonly prescribed HFpEF medications to influence the observed vascular responses, the persistence of marked microvascular dysfunction in the face of pharmacotherapeutics known to improve vascular outcomes [65,66] supports the relevance of our findings in further characterizing HFpEF pathophysiology. We also acknowledge that the lack of a statistically significant correlation between blood flow responses to PLM and biomarkers of inflammation and oxidative stress limits some aspects of data interpretation, which is likely due to a relatively small sample size. Finally, we recognize that systemic inflammation commonly associated with the HFpEF phenotype may impact multiple organ systems, including skeletal muscle, which may in turn influence vascular function. Inflammatory activation of locomotor muscle protein degradation and suppression of protein synthesis remains an understudied aspect of HFpEF pathophysiology that may be a root cause of muscle dysfunction and exercise intolerance in this patient population [67]. As such, therapies aimed at modifying catabolic processes and post translational modifications downstream of inflammation and oxidative stress may also improve exercise tolerance in patients with HFpEF.

Perspectives

The present study has identified a marked attenuation in PLM-induced hyperemia in patients with HFpEF compared to healthy similarly aged controls, a finding that contributes to the growing body of literature highlighting the importance of peripheral microvascular dysfunction to HFpEF pathophysiology. Additionally, the current study extends prior findings of microvascular function in the forearm [19] to the lower limb, which provides new insight into functional limitations related to activities of daily living in these patients. The observation that biomarkers of inflammation and oxidative damage are elevated in patients with HFpEF, combined with evidence that PLM-induced hyperemia is largely NO mediated [24], highlights the potential of the inflammation-oxidative stress-NO axis as a potential target for therapies that may favorably affect microvascular health and, ultimately, improve clinical status in this patient group. Indeed, the NO pathway has been targeted in several recent clinical trials in HFpEF, primarily using NO precursors that utilize the non-enzymatic nitrate-nitrite-NO pathway [68]. Unfortunately, this approach of supplementing the “alternative pool” of bioactive NO does not address the underlying mechanisms contributing to the disease-related dysfunction in endogenous, enzymatic NO signaling in patients with HFpEF. Thus, it is conceivable that pharmacologic interventions aimed at modulating specific steps in the NO-GC-CGMP signaling cascade, including administration of NO substrate (i.e. L-Citrulline), eNOS cofactors (i.e. tetrahydrobiopterin, BH4) or sGC stimulators, will prove more efficacious in patients with HFpEF.

Supplementary Material

Online Supplement

NOVELTY and SIGNIFICANCE.

What is New?

This study has demonstrated, for the first time, a clear decrement in lower limb microvascular dysfunction in the presence of elevated markers of inflammation and oxidative stress in patients with heart failure with preserved ejection fraction (HFpEF).

What is Relevant?

The passive leg movement test is a non-invasive assessment of microvascular function that is predominantly nitric oxide (NO)-mediated. Therefore, the observed microvascular dysfunction in HFpEF patients may be due to decreased bioavailable NO that is distinct from that lost as a consequence of aging per se.

Summary.

This study provides new evidence for a decline in NO-mediated lower limb microvascular function within a milieu of systemic inflammation and redox imbalance that likely contribute to locomotor muscle microvascular function in patients with HFpEF.

Acknowledgements

The authors thank all subjects for their cheerful participation in the study.

Funding Sources

This study was funded in part by the National Institutes of Health (R01 HL118313, D.W.W.; R56 AG057584, R.S.R; T32 HL139451, K.B.) and the U.S. Department of Veterans Affairs (RX001311 and CX002152, D.W.W.; E6910-R, E1697-R, E1433-P, E9275-L, and E1572-P R.S.R).

Footnotes

Disclosures

No conflicts of interest, financial or otherwise, are declared by the authors.

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