Influence of locomotor muscle group III/IV afferents on cardiovascular and ventilatory responses in human heart failure during submaximal exercise

Abstract

The purpose of this study is to determine the influence of locomotor muscle group III/IV afferent inhibition on central and peripheral hemodynamics at multiple levels of submaximal cycling exercise in patients with heart failure with reduced ejection fraction (HFrEF). Eleven patients with HFrEF and nine healthy matched controls were recruited. The participants performed a multiple stage [i.e., 30 W, 50%peak workload (WL), and a workload eliciting a respiratory exchange ratio (RER) of ∼1.0] exercise test with lumbar intrathecal fentanyl (FENT) or placebo (PLA). Cardiac output ($\dot{\text{Q}}$tot) was measured via open-circuit acetylene wash-in technique and stroke volume was calculated. Leg blood flow ($\dot{\text{Q}}$l) was measured via constant infusion thermodilution and leg vascular conductance (LVC) was calculated. Radial artery and femoral venous blood gases were measured. For HFrEF, stroke volume was higher at the 30 W (FENT: 110 ± 21 vs. PLA: 100 ± 18 mL), 50%peak WL (FENT: 113 ± 22 vs. PLA: 103 ± 23 mL), and RER = 1.0 (FENT: 119 ± 28 vs. PLA: 110 ± 26 mL) stages, whereas heart rate and systemic vascular resistance were lower with fentanyl than with placebo (all, P < 0.05). $\dot{\text{Q}}$tot in HFrEF and $\dot{\text{Q}}$tot, stroke volume, and heart rate in controls were not different between fentanyl and placebo (all, P > 0.19). During submaximal exercise, controls and patients with HFrEF exhibited increased leg vascular conductance (LVC) with fentanyl compared with placebo (all, P < 0.04), whereas no differences were present in $\dot{\text{Q}}$l or O₂ delivery with fentanyl (all, P > 0.20). Taken together, these findings provide support for locomotor muscle group III/IV afferents playing a role in integrative control mechanisms during submaximal cycling exercise in patients with HFrEF and older controls.

NEW & NOTEWORTHY Patients with HFrEF exhibit severe exercise intolerance. One of the primary peripheral mechanisms contributing to exercise intolerance in patients with HFrEF is locomotor muscle group III/IV afferent feedback. However, it is unknown whether these afferents impact the central and peripheral responses during submaximal cycling exercise. Herein, we demonstrate that inhibition of locomotor muscle group III/IV afferent feedback elicited increases in stroke volume during submaximal exercise in HFrEF, but not in healthy controls.

INTRODUCTION

Autonomic adjustments to exercise are mediated by central command and the exercise pressor reflex, while modulated by the arterial baroreceptors (1, 2). These adjustments are prevalent in the cardiovascular response to exercise, which include increases in blood pressure and heart rate. The afferent arm of the exercise pressor reflex comprises group III (predominantly mechanically sensitive) and group IV (predominantly metabolically sensitive) afferents located surrounding muscle fibers as well as within interstitial tissue of muscle, and adventitia of venules, arterioles, and lymphatics within skeletal muscle (3–8). In healthy young adults, this reflex afferent feedback is obligatory for normal cardiovascular function during exercise. Specifically, previous studies have demonstrated that inhibition of µ-opioid receptor sensitive group III/IV muscle afferent feedback via lumbar intrathecal fentanyl results in attenuated increases in cardiac output ($\dot{\text{Q}}$tot), leg blood flow ($\dot{\text{Q}}$l), leg vascular conductance (LVC), and blood pressure during exercise (9–11). Recent evidence indicates that aging influences the cardiovascular responses with locomotor muscle afferent inhibition during exercise. Specifically, older adults have minimal or no changes in $\dot{\text{Q}}$tot, stroke volume, $\dot{\text{Q}}$l_, and O₂ delivery, but increases in LVC with fentanyl compared with placebo during small muscle mass submaximal exercise and peak cycling exercise (12–14). It is unclear, however, how locomotor muscle group III/IV afferents impact the cardiovascular responses during submaximal cycling exercise in older adults.

Patients with heart failure with reduced ejection fraction (HFrEF) exhibit exercise limitation with locomotor muscle afferent feedback-mediated cardiovascular impairments as a proposed contributory mechanism (15). In support of this, we recently found that locomotor muscle afferent inhibition via intrathecal fentanyl elicited greater peak oxygen uptake (V̇o_2peak), $\dot{\text{Q}}$tot, stroke volume, $\dot{\text{Q}}$l, and LVC at maximal exercise in patients with HFrEF (14). However, it is unclear if these findings translate to submaximal cycling exercise as exercise intensity can modulate the degree of the metaboreflex-mediated response in blood pressure, $\dot{\text{Q}}$tot, and systemic vascular resistance in healthy adults (16). Furthermore, the influence of locomotor muscle afferent feedback on peripheral and in particular, central hemodynamics during submaximal exercise in patients with HFrEF remains unresolved. For example, studies using a canine model have found that stimulation of the locomotor muscle metaboreflex during submaximal exercise constrains stroke volume in HFrEF, but not in healthy canines (17–20). In contrast to these findings, locomotor afferent inhibition during single-leg knee-extensor submaximal exercise resulted in attenuated increases in $\dot{\text{Q}}$tot and stroke volume, but improved $\dot{\text{Q}}$l and LVC in patients with HFrEF (12). Therefore, the purpose of this study was to determine the influence of locomotor muscle group III/IV afferent feedback on central and peripheral hemodynamics during cycling exercise at multiple submaximal intensities [i.e., 30 W, 50%peak workload (WL), and a workload resulting in a respiratory exchange ratio (RER) of ∼1.0] in older control participants and patients with HFrEF. This multiple submaximal intensity cycling protocol allowed for comparisons of the cardiovascular responses with and without locomotor muscle afferent inhibition at absolute and relative exercise intensities as well as a workload to match for metabolic load (i.e., RER = 1.0). We hypothesized that locomotor muscle afferent inhibition in 1) older adult participants would result in improvements in LVC and 2) patients with HFrEF would result in improvements in stroke volume and LVC during submaximal cycling exercise.

METHODS

Participants

Eleven patients with HFrEF and nine healthy matched control participants were recruited for this study and provided written informed consent. The participant demographics, peak exercise responses, and CO₂ chemosensitivity for all the patients with HFrEF and eight of the controls presented herein were previously reported (one control participant did not have V̇o_2peak data with placebo and fentanyl and, thus was not included previously) (14). The patients with HFrEF were recruited from the Mayo Clinic Heart Failure Service and the Cardiovascular Health Clinic. Inclusion criteria for patients with HFrEF included 1) diagnosis of ischemic or dilated cardiomyopathy with symptom duration >1 year, 2) stable HF symptoms (>3 mo), 3) left ventricular ejection fraction ≤40%, 4) body mass index <35 kg/m², 5) nonsmokers with a smoking history of <15 pack-years, and 6) no diagnosis of coexisting pulmonary disease. Patients with HFrEF performed all testing, while remaining on standard pharmacological therapy to reduce potential for any decompensation and increase generalizability to free-living patients in the community. Control participants were matched for sex and age to the patients with HFrEF and free of cardiovascular, pulmonary, and muscular diseases. All aspects of this study were approved by the Mayo Clinic Institutional Review Board and conformed to the Declaration of Helsinki.

Experimental Design

For this randomized, single-blind placebo-controlled, crossover design study, participants performed all protocols and measurements during three study visits as previously described (14). The first study visit consisted of familiarization with all experimental measurements and protocols. Then, the participants performed an incremental exercise test to volitional fatigue to calculate the 50% V̇o_2peak workload for study visits 2 and 3. The second and third study visits consisted of lower lumbar intrathecal injection of fentanyl or placebo (in randomized order with at least 7 days separating these visits) followed by a single exercise cycling test comprised of multiple submaximal stages in order of increasing intensity (i.e., 30 W, 50%peak WL, and RER = 1.0) for 5 min. On study visits 2 and 3, cardiovascular and ventilatory variables and blood gases were measured at rest as well as during each submaximal stage. The focus of the present study was to investigate the influence of locomotor muscle group III/IV afferent inhibition on cardiovascular responses during submaximal exercise and, as such, this data was reported herein. The central and peripheral hemodynamic responses with fentanyl at the RER = 1.0 stage were previously compared with placebo responses at V̇o_2peak in a subset of patients with HFrEF (n = 6) (14).

Ventilation, Gas Exchange, and Cardiac Output

On all study visits, ventilatory and gas exchange variables were collected (MedGraphics CPX/D, St. Paul, MN) and averaged over 30 s (14, 21–23). Heart rate was measured continuously via electrocardiogram. Open-circuit acetylene wash-in technique was used to measure $\dot{\text{Q}}$tot. Specifically, the participants breathed into a nonrebreathing three-way pneumatic switching valve connected to a pneumotachometer (Hans Rudolph, Kansas City, MO) and gas mass spectrometry (Perkin Elmer MGA-1100, Wesley, MA) integrated with custom analysis software (24–26). The pneumatic switching valve allowed for rapid switching from room air to the gas mixture (0.65% C₂H₂, 21% O₂, 9% He, and balanced N₂). Stroke volume was calculated as the quotient between $\dot{\text{Q}}$tot and heart rate. Systemic vascular resistance (SVR) was calculated by dividing mean arterial pressure (MAP) by $\dot{\text{Q}}$tot.

Intra-Arterial Blood Pressure, Blood Sampling, and $\dot{\text{Q}}$l

On the second and third visits, a radial catheter (FA-04020; Arrow International Inc., Reading, PA) was placed for blood sampling and arterial pressure measurement. The arterial pressure recordings were exported to a digital oscilloscope from the pressure transducer (PX-MI099; Edwards Lifesciences, Irvine, CA) for offline analysis.

$\dot{\text{Q}}$l was measured using the constant infusion thermodilution technique (14, 25, 27). Two 18-gauge, 4.0-French, high-flow catheters (Royal Flush Plus Angiographic Catheter, Cook Medical Inc. Bloomington, IN) were inserted into the left femoral vein immediately distal to the inguinal ligament and advanced ∼8 and 20 cm toward the heart. A thin Teflon-coated thermocouple (IT-18 Physi-temp Instruments, Clifton, NJ) was inserted through the more proximal catheter and then this catheter was removed leaving the thermistor and infusion catheter secured. Iced saline (∼3–5°C) was infused for 15–20 s until femoral vein blood temperature was reduced and stabilized at the lower temperature. Saline infusion rate was adjusted with a roller pump controller to achieve a ∼1°C decrease in femoral vein temperature. The volume of infusate was determined by measuring the slope of the change in weight of the saline reservoir over time with a displacement transducer (FT10C, Grass Instruments, Quincy, MA). All data signals were exported to a digital oscilloscope for offline analysis. During each stage, duplicate measures of constant infusion thermodilution were performed and $\dot{\text{Q}}$l was calculated using the thermal-balance principle and doubled to provide two-leg blood flow values (14, 25, 27). LVC was calculated as the quotient of $\dot{\text{Q}}$l and MAP. Blood gases were measured in duplicate, averaged, and used to calculate leg arterial and venous O₂ content [$\text{C}{\text{a}}_{{\text{O}}_2}$ = (1.34 × Hb × $\text{S}{\text{a}}_{{\text{O}}_2}$) + ($\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$ × 0.0031) and $\text{C}{\text{v}}_{{\text{O}}_2}$ = (1.34 × Hb × $\text{S}{\text{v}}_{{\text{O}}_2}$) + (${\text{Pv}}_{{\text{O}}_2}$ × 0.0031)]. Leg V̇o₂ was calculated as $\dot{\text{Q}}$l multiplied by leg $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$. Leg O₂ delivery was calculated as $\dot{\text{Q}}$l multiplied by $\text{C}{\text{a}}_{{\text{O}}_2}$. $\dot{\text{Q}}$l data were not valid in a small subset of the HFrEF and control (n = 2 and n = 3, respectively) participants and, as a result, $\dot{\text{Q}}$l and variables calculated with $\dot{\text{Q}}$l (i.e., leg V̇o₂, LVC, and leg O₂ delivery) are reported in n = 9 patients with HFrEF and n = 6 control.

Lower Lumbar Intrathecal Fentanyl Injection

On the second and third visits, participants were seated in an upright flexed position and the skin and subcutaneous tissues were cleaned and anesthetized at the L3-L4 vertebral interspace with 2–4 mL of 1% lidocaine under aseptic technique (14, 21, 22). During the fentanyl study visit, a 22-g Whitaker needle was advanced to the subarachnoid space, with placement confirmed by visualization of free-flowing cerebrospinal fluid. A small amount of free-flowing cerebrospinal fluid was aspirated and 1 mL of fentanyl (0.05 mg/mL) was injected. The participants remained in the seated position to minimize the cephalad migration of fentanyl. The placebo study visit was identical to the fentanyl study visit except the advancement of the needle to the subarachnoid space was simulated after subcutaneous local anesthesia.

CO₂ Rebreathing Testing

Central chemosensitivity was assessed 2–3 times via CO₂ rebreathing testing following the exercise testing on study visits 2 and 3. The participants breathed on a mouthpiece connected to a pneumatic switching valve and 6-L rebreathing bag (5% CO₂ and 95% O₂). Following breathing room air for 2 min, participants were switched to the rebreathing bag for 4 min. The slope of ventilation versus end-tidal partial pressure of CO₂ was used as an index of central CO₂ chemosensitivity (14, 21, 22).

Statistical Analyses

Values are reported as means ± standard deviation (SD). Statistical analyses were performed using SigmaStat 2.0 (Jandel Scientific, San Rafael, CA). Normality and equal variance were assessed using the Shapiro–Wilk and Levene tests, respectively and nonparametric tests were used when appropriate. Participant characteristics and the workload during the RER = 1.0 stage with fentanyl compared with placebo were compared using unpaired t tests. Cardiovascular, ventilatory, and metabolic variables as well as blood gases were compared within blockade (placebo vs. fentanyl) and across (30 W vs. 50%peak WL vs. RER = 1.0) exercise stages using a two-way repeated analysis of variance (ANOVA) and were performed separately for controls and patients with HFrEF. Tukey’s post hoc test was performed when appropriate. Statistical significance was set at P < 0.05.

RESULTS

Participant Characteristics

These data are a subset of data from a previously published study (14). Hemodynamics with placebo and fentanyl at rest and V̇o_2peak are presented in that manuscript. Participant characteristics are shown in Table 1. Age, height, weight, and body mass index were not different between controls and HFrEF.

Table 1.

Participant characteristics

	Control	HFrEF	P Value
n	9	11
Age, yr	57 ± 10	61 ± 9	0.29
Sex, men/women	5/4	9/2	0.23
Height, cm	172 ± 10	176 ± 9	0.35
Weight, kg	79 ± 14	92 ± 15	0.06
Body mass index, kg/m2	27 ± 4	30 ± 5	0.10
V̇o2peak, L/min‡	2.3 ± 0.6	1.8 ± 0.5	0.06
V̇o2peak, mL/kg/min‡	29.1 ± 7.8	19.5 ± 6.3	<0.01
Peak Watts, W‡	160 ± 45	96 ± 34	<0.01
LV ejection fraction, %		34 ± 9
NYHA class: I/II		3/8
ACE I or ARBs		10 (91)
β-Blocker		11 (100)
Aspirin		6 (55)
Diuretics		9 (82)

Means ± SD. ACE, angiotensin-converting enzyme; ARB, angiotensin-receptor blockers; HFrEF, heart failure with reduced ejection fraction; LV, left ventricular; NYHA, New York Heart Association.

^‡Data from study visit 1.

Resting Responses with Fentanyl

As previously discussed (14), resting V̇o₂, carbon dioxide production (V̇co₂), RER, ventilation (V̇e), MAP as well as systolic and diastolic blood pressures were not different with fentanyl compared with placebo for controls or HFrEF (all, P > 0.05). Furthermore, there were no differences in resting $\dot{\text{Q}}$_TOT, stroke volume, heart rate, or SVR with fentanyl compared with placebo for controls or for patients with HFrEF (all, P > 0.05).

Submaximal Exercise Responses with Fentanyl

For controls, there were no differences in V̇o₂ and V̇co₂ between fentanyl and placebo during exercise (Table 2), whereas RER was higher with fentanyl than placebo at the 30 W and 50%peak WL stages (both, P = 0.01–0.047). The workload during the RER = 1.0 stage was not different between conditions for controls (P = 0.40). Using the V̇o_2peak with fentanyl and placebo from our recent study (14), the relative intensity during the RER = 1.0 stage was lower for controls with fentanyl compared with placebo (FENT: 73 ± 8 vs. PLA: 84 ± 10% V̇o_2peak, P = 0.02). Diastolic blood pressure was lower at each exercise stage for controls with fentanyl compared with placebo, whereas systolic blood pressure was lower at the RER = 1.0 stage (all, P = <0.01–0.05). For controls, significant differences existed between fentanyl and placebo for MAP (blockade, P < 0.01 (${\text{η}}_{\text{partial}}^{\text{2}}$: 0.62); blockade × exercise stage, P = 0.24), but not for SVR (blockade, P = 0.27; blockade × exercise stage, P = 0.88) (Fig. 1). MAP was lower at each exercise stage with fentanyl compared with placebo (30 W: FENT 115 ± 21 vs. PLA 123 ± 21 mmHg; 50%peak WL: FENT 117 ± 21 vs. PLA 129 ± 21 mmHg; RER = 1.0: FENT 123 ± 23 vs. PLA 135 ± 21 mmHg) (all, P = <0.01–0.01). With fentanyl compared with placebo, the SVR responses were 13.5 ± 3.2 versus 14.0 ± 3.3 mmHg/L/min for 30 W (FENT vs. PLA), 11.2 ± 3.5 versus 11.9 ± 4.1 mmHg/L/min for 50%peak WL, and 10.0 ± 3.0 versus 10.5 ± 3.5 mmHg/L/min for RER = 1.0 for controls. For HFrEF, there were no differences in V̇o₂, V̇co₂, RER, or systolic blood pressure between fentanyl and placebo during exercise (Table 3). The workload during the RER = 1.0 stage with fentanyl was not different compared with placebo for HFrEF (P = 0.31). Using the V̇o_2peak with fentanyl and placebo from our recent study (14), the relative intensity during the RER = 1.0 stage was not different between conditions for patients with HFrEF (FENT: 76 ± 13 vs. PLA: 82 ± %V̇o_2peak, P = 0.15). For HFrEF, diastolic blood pressure was lower at each exercise stage with fentanyl compared with placebo (all, P < 0.01). For HFrEF, differences existed between fentanyl and placebo for MAP and SVR (blockade, P = <0.01–0.02 (${\text{η}}_{\text{partial}}^{\text{2}}$: 0.32–0.48); blockade × exercise stage, P = 0.05–0.54) (Fig. 1). For HFrEF, MAP (30 W: FENT 94 ± 13 vs. PLA 99 ± 13 mmHg; 50%peak WL: FENT 93 ± 14 vs. PLA 99 ± 14 mmHg; RER = 1.0: FENT 93 ± 13 vs. PLA 103 ± 13 mmHg) and SVR (50%peak WL: FENT 10.0 ± 2.8 vs. PLA 10.7 ± 2.8 mmHg/L/min; RER = 1.0: FENT 8.6 ± 2.5 vs. PLA 9.6 ± 3.0 mmHg/L/min) were lower at each exercise stage with fentanyl compared with placebo (all, P = <0.01–0.04), except for the 30 W stage for SVR (30 W: FENT 11.2 ± 1.8 vs. PLA 11.8 ± 1.9 mmHg/L/min, P = 0.11).

Table 2.

Submaximal exercise data with placebo and fentanyl for control

	30 W		50% Peak Workload		RER = 1.0		ANOVA
	Placebo	Fentanyl	Placebo	Fentanyl	Placebo	Fentanyl	Blockade	Interaction
Workload, W	30 ± 0	30 ± 0	72 ± 39	72 ± 39	113 ± 59	107 ± 47
V̇o2, mL/kg/min	11.6 ± 1.8	11.7 ± 1.7	17.7 ± 6.4	17.6 ± 6.0	23.5 ± 9.9	21.8 ± 6.4	0.50	0.38
V̇o2, L/min	0.9 ± 0.1	0.9 ± 0.1	1.4 ± 0.4	1.4 ± 0.4	1.8 ± 0.7	1.7 ± 0.5	0.56	0.46
V̇co2, L/min	0.8 ± 0.1	0.8 ± 0.1	1.2 ± 0.4	1.3 ± 0.5	1.8 ± 0.8	1.7 ± 0.6	0.93	0.43
RER	0.84 ± 0.04	0.88 ± 0.06*	0.91 ± 0.04	0.96 ± 0.06*	0.99 ± 0.05	1.01 ± 0.06	0.01	0.52
SBP, mmHg	210 ± 44	199 ± 46	224 ± 48	210 ± 49	248 ± 47	225 ± 53*	0.04	0.31
DBP, mmHg	80 ± 10	74 ± 10*	81 ± 11	71 ± 9*	78 ± 9	72 ± 9*	<0.01	0.02
$\text{C}{\text{a}}_{{\text{O}}_2}$, mL/dL	18.6 ± 2.0	17.8 ± 2.5*	18.6 ± 2.0	17.8 ± 2.4*	18.7 ± 2.2	17.7 ± 2.5*	0.02	0.81
Arterial Hb, g/dL	14.2 ± 1.6	14.0 ± 1.8	14.2 ± 1.6	13.9 ± 1.7	14.3 ± 1.6	13.9 ± 1.7	0.21	0.67
$\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$, mmHg	95 ± 7	85 ± 13*	97 ± 9	86 ± 13*	97 ± 12	85 ± 13*	0.01	0.67
$\text{S}{\text{a}}_{{\text{O}}_2}$, %	96 ± 1	94 ± 5	96 ± 1	94 ± 5	96 ± 1	94 ± 4	0.18	0.96
Arterial lactate, mmol/L	1.1 ± 0.6	1.2 ± 0.6	1.8 ± 0.6	2.2 ± 0.8	3.2 ± 1.1	4.0 ± 1.2	0.06	0.11
pHa	7.40 ± 0.01	7.38 ± 0.04	7.40 ± 0.01	7.37 ± 0.05	7.39 ± 0.03	7.34 ± 0.05*	0.03	0.01
$\text{C}{\text{v}}_{{\text{O}}_2}$, mL/dL	6.7 ± 1.8	6.3 ± 1.5	5.9 ± 2.0	5.1 ± 1.6	4.9 ± 1.5	4.2 ± 1.4	0.07	0.24
Venous Hb, g/dL	14.0 ± 1.4	13.7 ± 1.3	14.2 ± 1.6	13.9 ± 1.7	14.1 ± 1.3	14.0 ± 1.8	0.36	0.47
${\text{Pv}}_{{\text{O}}_2}$, mmHg	23 ± 3	24 ± 3	22 ± 3	22 ± 3	21 ± 2	21 ± 2	0.45	0.08
$\text{S}{\text{v}}_{{\text{O}}_2}$, %	35 ± 8	34 ± 8	31 ± 9	27 ± 8	26 ± 7	22 ± 7	0.10	0.11
Venous lactate, mmol/L	1.5 ± 1.0	1.4 ± 0.7	2.1 ± 0.7	2.5 ± 0.9	3.9 ± 1.0	4.5 ± 1.2	0.29	0.12
RPE (Borg 6–20)	9.3 ± 2.3	8.4 ± 1.7	11.2 ± 1.5	10.3 ± 1.7	14.7 ± 2.3	11.9 ± 2.1*	0.02	0.11
Dyspnea (Borg 1–10)	1.7 ± 1.2	1.2 ± 0.9	2.7 ± 0.9	2.2 ± 1.1	4.7 ± 2.2	3.3 ± 1.3	0.11	0.38

Means ± SD. $\text{C}{\text{a}}_{{\text{O}}_2}$, arterial oxygen content; $\text{C}{\text{v}}_{{\text{O}}_2}$, venous oxygen content; DBP, diastolic blood pressure; Hb, hemoglobin; MAP, mean arterial pressure; Pa_O2, arterial oxygen pressure; ${\text{Pv}}_{{\text{O}}_2}$, venous oxygen pressure; RER, respiratory exchange ratio; RPE, rating of perceived exertion; $\text{S}{\text{a}}_{{\text{O}}_2}$, arterial oxygen saturation; SBP, systolic blood pressure; $\text{S}{\text{v}}_{{\text{O}}_2}$, venous oxygen saturation; V̇o₂, oxygen uptake; V̇co₂, carbon dioxide production.

^*Significantly different than placebo.

Figure 1.

Mean arterial pressure (MAP) and systemic vascular resistance (SVR) during submaximal exercise. MAP (A and B) and SVR (C and D) during submaximal exercise with fentanyl (FENT) and placebo (PLA) in control (n = 9, 5M/4W) and heart failure with reduced ejection fraction (HFrEF) (n = 11, 9M/2W). MAP was lower for control and HFrEF at each submaximal exercise stage with fentanyl compared with placebo (all, P < 0.04). For HFrEF, SVR was lower at each submaximal exercise stage with fentanyl compared with placebo (all, P < 0.04), except for the 30 W stage (P = 0.11). WL, workload. *Significantly different than PLA. Data for control and HFrEF were analyzed separately using two-way repeated ANOVA with Tukey’s post hoc tests.

Table 3.

Submaximal exercise data with placebo and fentanyl for HFrEF

	30 W		50% Peak Workload		RER = 1.0		ANOVA
	Placebo	Fentanyl	Placebo	Fentanyl	Placebo	Fentanyl	Blockade	Interaction
Workload, W	30 ± 0	30 ± 0	49 ± 23	49 ± 23	75 ± 29	80 ± 29
V̇o2, mL/kg/min	10.7 ± 1.6	10.3 ± 1.3	12.9 ± 2.6	12.9 ± 2.5	15.6 ± 2.8	15.9 ± 3.1	0.92	0.31
V̇o2, L/min	1.0 ± 0.1	0.9 ± 0.1	1.2 ± 0.3	1.2 ± 0.3	1.4 ± 0.4	1.5 ± 0.4	0.96	0.48
V̇co2, L/min	0.9 ± 0.2	0.8 ± 0.1	1.1 ± 0.3	1.1 ± 0.3	1.4 ± 0.4	1.5 ± 0.4	0.93	0.47
RER	0.88 ± 0.06	0.87 ± 0.05	0.91 ± 0.04	0.91 ± 0.07	0.98 ± 0.05	0.99 ± 0.06	0.85	0.38
SBP, mmHg	163 ± 27	165 ± 33	165 ± 29	164 ± 33	174 ± 27	168 ± 31	0.71	0.23
DBP, mmHg	68 ± 10	59 ± 7*	67 ± 10	58 ± 9*	67 ± 10	56 ± 8*	<0.01	0.31
$\text{C}{\text{a}}_{{\text{O}}_2}$, mL/dL	18.8 ± 2.1	18.4 ± 2.3	18.7 ± 2.1	18.4 ± 2.3	18.7 ± 2.1	18.4 ± 2.3	0.28	0.92
Arterial Hb, g/dL	14.5 ± 1.6	14.4 ± 1.7	14.5 ± 1.6	14.4 ± 1.7	14.5 ± 1.6	14.5 ± 1.6	0.67	0.10
$\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$, mmHg	89 ± 8	84 ± 11*	88 ± 11	82 ± 13*	88 ± 11	83 ± 13*	0.02	0.73
$\text{S}{\text{a}}_{{\text{O}}_2}$, %	95 ± 1	94 ± 2	95 ± 2	94 ± 3	95 ± 2	94 ± 3	0.09	0.56
Arterial lactate, mmol/L	1.7 ± 1.4	1.7 ± 1.4	1.9 ± 1.2	1.9 ± 1.3	2.6 ± 1.3	2.6 ± 1.0	0.57	0.43
pHa	7.40 ± 0.05	7.38 ± 0.05*	7.40 ± 0.04	7.38 ± 0.04*	7.39 ± 0.05	7.36 ± 0.04*	<0.01	0.10
$\text{C}{\text{v}}_{{\text{O}}_2}$, mL/dL	4.7 ± 1.2	4.4 ± 1.0	4.3 ± 1.2	4.1 ± 1.4	3.9 ± 1.3	3.3 ± 1.2	0.19	0.07
Venous Hb, g/dL	14.6 ± 1.6	14.6 ± 1.6	14.6 ± 1.5	14.6 ± 1.7	14.6 ± 1.6	14.4 ± 1.6	0.61	0.42
${\text{Pv}}_{{\text{O}}_2}$, mmHg	20 ± 2	19 ± 1	20 ± 1	19 ± 2	19 ± 1	19 ± 1	0.60	0.97
$\text{S}{\text{v}}_{{\text{O}}_2}$, %	24 ± 6	22 ± 4	22 ± 6	21 ± 7	20 ± 7	17 ± 6	0.25	0.14
Venous lactate, mmol/L	1.9 ± 1.6	1.8 ± 1.5	2.1 ± 1.4	2.2 ± 1.3	3.0 ± 1.5	3.1 ± 1.0*	0.33	0.03
RPE (Borg 6–20)	11.4 ± 3.1	10.5 ± 2.8	13.1 ± 2.0	11.7 ± 2.3*	15.6 ± 3.2	13.6 ± 2.6*	<0.01	0.31
Dyspnea (Borg 1–10)	2.9 ± 2.0	2.2 ± 1.9	3.3 ± 1.8	2.8 ± 1.1	5.3 ± 2.6	4.0 ± 2.0*	<0.01	0.40

Means ± SD. $\text{C}{\text{a}}_{{\text{O}}_2}$, arterial oxygen content; $\text{C}{\text{v}}_{{\text{O}}_2}$, venous oxygen content; DBP, diastolic blood pressure; Hb, hemoglobin; MAP, mean arterial pressure; $\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$, arterial oxygen pressure; ${\text{Pv}}_{{\text{O}}_2}$, venous oxygen pressure; RER, respiratory exchange ratio; RPE, rating of perceived exertion; ${\text{Sa}}_{{\text{O}}_2}$, arterial oxygen saturation; SBP, systolic blood pressure; ${\text{Sv}}_{{\text{O}}_2}$, venous oxygen saturation; V̇co₂, carbon dioxide production; V̇o₂, oxygen uptake.

^*Significantly different than placebo.

Central Hemodynamic Responses with Fentanyl

For controls, $\dot{\text{Q}}$tot, stroke volume, and heart rate were not significantly different between placebo and fentanyl during submaximal exercise (blockade, P = 0.20–0.79; blockade × exercise stage, P = 0.74–0.89) (Fig. 2). With fentanyl compared with placebo, the stroke volume responses were 105 ± 33 versus 105 ± 26 mL for 30 W (FENT vs. PLA), 113 ± 33 versus 115 ± 39 mL for 50%peak WL, and 115 ± 37 versus 116 ± 34 mL for RER = 1.0 for controls. With fentanyl compared with placebo, the heart rate responses were 87 ± 13 versus 88 ± 14 beats/min for 30 W (FENT vs. PLA), 100 ± 12 versus 105 ± 15 beats/min for 50%peak WL, and 116 ± 16 versus 122 ± 21 beats/min for RER = 1.0 for controls. For HFrEF, stroke volume and heart rate were different with fentanyl compared with placebo [blockade, P < 0.01 (${\text{η}}_{\text{partial}}^{\text{2}}$: 0.40–52); blockade × exercise stage, P = 0.92–0.98), whereas no differences were present for $\dot{\text{Q}}$tot (blockade, P = 0.87; blockade × exercise stage, P = 0.83) (Fig. 2). For HFrEF, stroke volume was higher (30 W: FENT 110 ± 21 vs. PLA 100 ± 18 mL; 50%peak WL: FENT 113 ± 22 vs. PLA 103 ± 23 mL; RER = 1.0: FENT 119 ± 28 vs. PLA 110 ± 26 mL) and heart rate was lower (30 W: FENT 79 ± 16 vs. PLA 87 ± 15 beats/min; 50%peak WL: FENT 87 ± 16 vs. PLA 95 ± 16 beats/min; RER = 1.0: FENT 98 ± 20 vs. PLA 105 ± 23 beats/min) with fentanyl compared with placebo (all, P = <0.01–0.02). In patients with HFrEF, there was a significant relationship between the changes in systolic blood pressure and stroke volume with fentanyl compared with placebo at 50%peak WL (r = −0.65, P = 0.03), but not at the 30 W or RER = 1.0 stages (P = 0.58–0.83).

Figure 2.

Cardiac output ($\dot{\mathrm{Q}}$tot), stroke volume, and heart rate during submaximal exercise. $\dot{\mathrm{Q}}$tot (A and B), stroke volume (C and D), and heart rate (E and F) during submaximal exercise with fentanyl (FENT) and placebo (PLA) in control (n = 9, 5M/4W) and heart failure with reduced ejection fraction (HFrEF) (n = 11, 9M/2W). No differences were present in $\dot{\mathrm{Q}}$tot with fentanyl compared with placebo for control and HFrEF (P > 0.19). Patients with HFrEF had a higher stroke volume and lower heart rate at each exercise stage with fentanyl compared with placebo (all, P < 0.02). WL, workload. *Significantly different than PLA. Data for control and HFrEF were analyzed separately using two-way repeated ANOVA with Tukey’s post hoc tests.

Peripheral Hemodynamic Responses with Fentanyl

For controls, significant differences between placebo and fentanyl existed for LVC [blockade, P = 0.03 (${\text{η}}_{\text{partial}}^{\text{2}}$: 0.44); blockade × exercise stage, P = 0.28], but not $\dot{\text{Q}}$l (blockade, P = 0.21; blockade × exercise stage, P = 0.90) (Fig. 3). With fentanyl compared with placebo, LVC was higher at the 50%peak WL (FENT: 0.060 ± 0.029 vs. PLA: 0.050 ± 0.026 L/min/mmHg) and RER = 1.0 stages (FENT: 0.080 ± 0.034 vs. PLA: 0.067 ± 0.032 L/min/mmHg) for controls (both, P = 0.01–0.04), but not at 30 W (FENT: 0.041 ± 0.017 vs. PLA: 0.036 ± 0.014 L/min/mmHg, P = 0.22). For controls, there were no significant differences in leg V̇o₂, O₂ delivery, or $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$ between placebo and fentanyl (blockade, P = 0.33–0.67; blockade × exercise stage, P = 0.31–0.99) (Fig. 4). For HFrEF, significant differences between placebo and fentanyl existed for LVC [blockade, P = 0.03 (${\text{η}}_{\text{partial}}^{\text{2}}$: 0.17); blockade × exercise stage, P = 0.41], but not $\dot{\text{Q}}$l (blockade, P = 0.97; blockade × exercise stage, P = 0.49) (Fig. 3). For HFrEF, LVC was higher at the RER = 1.0 stage with fentanyl compared with placebo (FENT: 0.079 ± 0.020 vs. PLA: 0.066 ± 0.019 L/min/mmHg, P = 0.02), but not at 30 W (FENT: 0.052 ± 0.019 vs. PLA: 0.048 ± 0.014 L/min/mmHg) or 50%peak WL (FENT: 0.063 ± 0.026 vs. 0.058 ± 0.021 L/min/mmHg) (both, P = 0.35–0.47). No significant differences were present for leg V̇o₂, O₂ delivery, or $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$ for HFrEF between placebo and fentanyl (blockade, P = 0.64–0.95; blockade × exercise stage, P = 0.16–0.57) (Fig. 4).

Figure 3.

Leg blood flow ($\dot{\mathrm{Q}}$l) and leg vascular conductance (LVC) during submaximal exercise. $\dot{\mathrm{Q}}$l (A and B) and LVC (C and D) during submaximal exercise with fentanyl (FENT) and placebo (PLA) in control (n = 6, 4M/2W) and heart failure with reduced ejection fraction (HFrEF) (n = 9, 7M/2W). With fentanyl compared with placebo, no differences were present in $\dot{\mathrm{Q}}$l for control and HFrEF (P > 0.20), while LVC was higher during the 50%peak workload (WL) and respiratory exchange ratio (RER) = 1.0 stages for control and the RER = 1.0 stage for HFrEF (all, P < 0.04). *Significantly different than PLA. Data for control and HFrEF were analyzed separately using two-way repeated ANOVA with Tukey’s post hoc tests.

Figure 4.

Leg O₂ uptake (V̇o₂; A and B), O₂ delivery (C and D), and $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$ (E and F) during submaximal exercise with fentanyl (FENT) and placebo (PLA) in control (n = 6, 4M/2W) and heart failure with reduced ejection fraction (HFrEF) (n = 9, 7M/2W) (except leg $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$, which included all control (n = 9, 5M/4W) and HFrEF (n = 11, 9M/2W) participants. There were no differences in leg V̇o₂, O₂ delivery or $\text{C}{\text{a}}_{{\text{O}}_2}$-$\text{C}{\text{v}}_{{\text{O}}_2}$ with fentanyl compared to placebo for control and HFrEF (all, P > 0.16). WL, workload. *Significantly different than PLA. Data for control and HFrEF were analyzed separately using two-way repeated ANOVA with Tukey’s post hoc tests.

Arterial and Femoral Venous Blood Gases with Fentanyl

For controls, $\text{C}{\text{a}}_{{\text{O}}_2}$ and $\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$ were lower with fentanyl compared with placebo at all exercise stages (all, P = <0.01–0.03), while pHa was lower at the RER = 1.0 stage (P < 0.01) (Table 2). For HFrEF, $\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$ and pHa were lower with fentanyl compared with placebo at each exercise stage (P = <0.01–0.05) (Table 3). Furthermore, venous lactate was higher with fentanyl than placebo at RER = 1.0 stage for HFrEF (P = 0.03).

Ventilatory Responses with Fentanyl

For controls, V̇e/V̇co₂ and $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ were significantly different between placebo and fentanyl during submaximal exercise (blockade, P = <0.01–0.01; blockade × exercise stage, P = 0.03–0.10) (Fig. 5). V̇e/V̇co₂ was lower at the 50%peak WL and RER = 1.0 stages for controls with fentanyl compared with placebo during exercise (both, P = <0.01–0.04). From placebo to fentanyl, $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ was higher at the RER = 1.0 stage for controls (P < 0.01). For HFrEF, V̇e/V̇co₂ and $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ were significantly different between placebo and fentanyl during submaximal exercise (blockade, P = 0.02–0.04; blockade × exercise stage, P = 0.23–0.69). V̇e/V̇co₂ was lower and $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ was higher at each exercise stage with fentanyl compared with placebo for HFrEF (all, P = 0.01–0.05). At the 50%peak WL stage, there was a negative relationship between V̇e/V̇co₂ with placebo and the % change in V̇e/V̇co₂ with fentanyl in controls (r = −0.70, P = 0.04), HFrEF (r = −0.73, P = 0.01), and with the whole cohort (r = −0.70, P < 0.01), whereas no other significant relationships were present (P = 0.09–0.78).

Figure 5.

V̇e/V̇co₂ (A and B) and $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ (C and D) during submaximal exercise with fentanyl (FENT) and placebo (PLA) in control (n = 9, 5M/4W) and heart failure with reduced ejection fraction (HFrEF) (n = 11, 9M/2W). V̇e/V̇co₂ was lower at 50%peak workload (WL) and respiratory exchange ratio (RER) = 1.0 stages for controls and at each submaximal exercise stage for HFrEF (all, P ≤ 0.05). $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ was higher at the RER = 1.0 stage for controls and each submaximal exercise stage for HFrEF (all, P < 0.04). *Significantly different than PLA. Data for control and HFrEF were analyzed separately using two-way repeated ANOVA with Tukey’s post hoc tests.

Exertional Symptoms with Fentanyl

For controls, significant differences between placebo and fentanyl existed for RPE (Table 2). RPE was lower at the RER = 1.0 stage for controls with fentanyl compared with placebo (P < 0.01). For HFrEF, significant differences between placebo and fentanyl existed for RPE and dyspnea (Table 3). RPE was lower at the 50%peak WL and RER = 1.0 stages and dyspnea was lower at the RER = 1.0 stage with fentanyl compared with placebo for HFrEF (P = <0.01–0.03).

Central Chemosensitivity

There were no changes in the V̇e/$\text{P}\text{E}{\text{T}}_{\text{C}{\text{O}}_2}$ slope between placebo and fentanyl in controls (PLA: 2.8 ± 0.7 vs. FENT: 2.8 ± 0.7 L/min/mmHg, P = 0.99) or HFrEF (PLA: 2.1 ± 0.9 vs. FENT: 2.1 ± 0.7 L/min/mmHg, P = 0.91).

DISCUSSION

Major Findings

The major novel findings of the present study are threefold. First, locomotor muscle afferent inhibition resulted in significant increases in stroke volume and decreases in heart rate resulting in similar $\dot{\text{Q}}$tot compared with placebo in patients with HFrEF during submaximal cycling exercise. Second, HFrEF participants exhibited greater LVC during submaximal cycling exercise with locomotor muscle afferent inhibition, while $\dot{\text{Q}}$l and leg O₂ delivery were not altered. Third, the older controls exhibited increased LVC, but no changes in $\dot{\text{Q}}$l and leg O₂ delivery with locomotor muscle afferent inhibition during submaximal cycling exercise. Taken together, these findings provide support for locomotor muscle group III/IV afferents playing a role in integrative control mechanisms during submaximal cycling exercise in patients with HFrEF and older controls.

Group III/IV Afferents and Central Hemodynamics during Exercise in HFrEF

In the present study, we found that locomotor muscle afferent inhibition elicited a higher stroke volume (∼10%) and lower heart rate during submaximal exercise in patients with HFrEF culminating in similar $\dot{\text{Q}}$tot (between conditions). To date, there has been conflicting findings regarding the impact of locomotor muscle afferent feedback on cardiac responses during exercise. For example, Amann et al. (12) found that locomotor muscle afferent inhibition resulted in attenuated increases in heart rate, stroke volume, and $\dot{\text{Q}}$tot during submaximal single-leg knee-extensor exercise in patients with HFrEF. In contrast, inhibition of these afferents in patients with HFrEF resulted in a higher $\dot{\text{Q}}$tot via increased stroke volume (as heart rate was not different between placebo and fentanyl) during maximal cycling exercise (14). Furthermore, previous studies using a HFrEF-induced canine model have shown that locomotor muscle metaboreflex activation during submaximal exercise (via partial reductions in $\dot{\text{Q}}$l) decreased $\dot{\text{Q}}$tot via reductions in stroke volume compared with free-flowing condition potentially due to impairments in contractility (17–20). What are potential mechanisms responsible for the stroke volume and heart rate responses with locomotor muscle afferent feedback in HFrEF presented herein? Left ventricular afterload may have decreased via attenuated sympathetically mediated vasoconstriction with locomotor muscle afferent inhibition contributing to the higher stroke volume during cycling exercise (as suggested by the reduced SVR with FENT). Alternatively, locomotor muscle afferent inhibition may have attenuated the heart response and led to a greater stroke volume response. In addition, previous studies have found that unloading the respiratory muscles during submaximal exercise elicited increases in stroke volume in patients with HFrEF (but not controls) (25, 28, 29). In the present study, the attenuated ventilatory response with locomotor muscle afferent inhibition (see Group III/IV Afferents and Ventilatory Response during Exercise in HFrEF section) during submaximal exercise may have partially “unloaded” the respiratory muscles also contributing to the improved stroke volume response during exercise in HFrEF. Finally, diastolic dysfunction may have also contributed to the attenuated stroke volume response with locomotor muscle afferent feedback in patients with HFrEF (30, 31). Taken together, these findings indicate locomotor muscle afferent feedback impacts the stroke volume and heart rate response during submaximal exercise in HFrEF, but not $\dot{\text{Q}}$tot.

Locomotor muscle afferent inhibition did not alter the cardiac response during submaximal cycling exercise in the healthy older adults. These findings are consistent with previous studies reporting minimal or no differences in the cardiac response during small muscle mass submaximal exercise or peak cycling exercise with fentanyl compared with placebo in older adults (12, 13). It is important to note that locomotor muscle afferent inhibition results in attenuated increases in $\dot{\text{Q}}$tot, stroke volume, and heart rate during submaximal single-leg knee-extensor exercise in young adults suggesting these afferents significantly contribute to the cardiac response in younger adults (11, 13, 14). Therefore, the impact of locomotor muscle group III/IV afferent feedback on the cardiac response during exercise appears to be age-specific (13, 32).

Group III/IV Afferents and Peripheral Hemodynamics during Exercise in HFrEF

In the present study, we found locomotor muscle afferent inhibition enhanced LVC, but not $\dot{\text{Q}}$l during submaximal cycling exercise at the RER = 1.0 stage (∼76%–82% of V̇o_2peak) for patients with HFrEF. A previous study incorporating the submaximal single-leg knee-extensor exercise model in HFrEF found that afferent inhibition resulted in greater LVC, $\dot{\text{Q}}$l, and O₂ delivery (in the face of lower $\dot{\text{Q}}$tot) (12). The underlying explanation(s) for the differential peripheral responses with afferent inhibition with respect to $\dot{\text{Q}}$l, and O₂ delivery between the present and this previous study is unclear but may include the different methodologies used to assess peripheral hemodynamics, the unilateral and/or small muscle mass nature of this prior experimental model, which likely limits the competition between vascular beds, and/or different HF severity and etiology between studies. In the present study, we also found that locomotor muscle afferent inhibition elicited increases in LVC, but not in $\dot{\text{Q}}$l and O₂ delivery at the 50%peak WL and RER = 1.0 exercise stages in the controls. Consistent with our findings in healthy control adults, previous studies using the same single-leg knee extensor exercise model found that afferent inhibition resulted in increased LVC during submaximal exercise, but not $\dot{\text{Q}}$l, and O₂ delivery (12, 13). In both the controls and HFrEF patients, the decreased perfusion pressure was compensated for by increased LVC during exercise with fentanyl (compared with placebo) culminating in similar $\dot{\text{Q}}$l and O₂ delivery. These findings are in contrast to younger adults in which afferent inhibition results in decreases in $\dot{\text{Q}}$l due to decreases in perfusion pressure and LVC (11, 13).

Group III/IV Afferents and Ventilatory Response during Exercise in HFrEF

In the present study, the ventilatory response was attenuated with locomotor muscle afferent inhibition across a range of submaximal exercise intensities in patients with HFrEF. Specifically, patients with HFrEF exhibited attenuated V̇e/V̇co₂ responses and subsequently elevated $\text{P}{\text{a}}_{\text{C}\text{O}}{}_{{}_2}$ with inhibition of these locomotor afferents during submaximal cycling exercise. This is consistent with previous work from our laboratory showing that inhibition of these locomotor muscle afferents reduced the ventilatory response (i.e., V̇e/V̇co₂) in HFrEF during constant workload exercise at 60% peak workload (21). The controls exhibited an attenuated ventilatory response at the 50%peak WL and the RER = 1.0 stages with fentanyl compared with placebo. These findings are consistent with some, but not all studies investigating the influence of locomotor muscle afferent inhibition on ventilatory control during exercise in older adults (13, 14, 21, 22). In addition, patients with HFrEF and controls with the greatest ventilatory response during submaximal exercise at the RER = 1.0 stage had the largest attenuation in their ventilatory response with locomotor muscle afferent inhibition. These findings suggest that these afferents significantly contribute to the augmentation of the ventilatory response during exercise in older controls and patients with HFrEF, which have implications for cardiac output distribution during exercise (25, 29, 33–35). Taken together, these findings suggest that locomotor muscle group III/IV afferent feedback in patients with HFrEF and older adults significantly contribute to the ventilatory response across the spectrum of submaximal intensities.

Methodological Considerations

The experimental design assessing simultaneous central and peripheral mechanisms (via gold standard methodology) across multiple submaximal exercise stages in human HF are major strengths of the present study. However, there are several methodological considerations that may have influenced our results. We acknowledge the relatively small sample size and homogenous nature of our groups. Future studies incorporating a larger, more heterogeneous HF group may be necessary to confirm our findings. Furthermore, it is important to note that the 50%peak WL intensity was determined from study visit 1. As locomotor muscle afferent inhibition via lumbar intrathecal fentanyl resulted in a greater V̇o_2peak in patients with HFrEF (14), the 50%peak WL was at a lower (albeit not a significantly different) relative intensity with fentanyl than placebo for patients with HFrEF. In addition, future studies including additional cardiac measures are prudent to better understand the mechanisms by which locomotor muscle afferent feedback impacts stroke volume during exercise in patients with HFrEF. Finally, locomotor muscle activation was not assessed in the present study. Previous studies have reported that fentanyl does not impact locomotor muscle activation via electromyography during submaximal exercise at matched workloads in healthy young adults (9, 10); however, future studies are necessary to translate these findings to patients with HFrEF.

Conclusion

In patients with HFrEF, our findings indicate that locomotor muscle group III/IV muscle afferents do not impact $\dot{\text{Q}}$tot (but modulate stroke volume and/or heart rate). Furthermore, inhibition of these afferents resulted in augmented LVC (but not $\dot{\text{Q}}$l) during submaximal exercise in patients with HFrEF and older controls. Future studies focused on interventions, such as exercise training (36, 37), to ameliorate the abnormal locomotor muscle afferent-mediated responses are necessary to improve the cardiovascular response to submaximal exercise in patients with HFrEF.

GRANTS

This work was supported by the National Institutes of Health under Grants HL126638 (to T. P. Olson), HL128526 (to B. A. Borlaug), HL139854 (to M. J. Joyner), T32AR56950 (to M. L. Keller-Ross), T32HL007111 (to J. R. Smith), K12 HD065987 (to J. R. Smith), and American Heart Association Grant 18POST3990251 (to J. R. Smith). This publication was also made possible through the support of the Mary Kathryn and Michael B. Panitch Career Development Award in Hypertension Research Honoring Gary Schwartz, M.D. (to J. R. Smith).

DISCLOSURES

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

AUTHOR CONTRIBUTIONS

J.R.S., M.J.J., T.B.C., B.A.B., M.L.K-R., E.H.V.T., and T.P.O. conceived and designed research; J.R.S., M.J.J., T.B.C., B.A.B., M.L.K-R., E.H.V.T,, and T.P.O. performed experiments; J.R.S. analyzed data; J.R.S. interpreted results of experiments; J.R.S. prepared figures; J.R.S. drafted manuscript; J.R.S., M.J.J., T.B.C., B.A.B., M.L.K-R., E.H.V.T., and T.P.O. edited and revised manuscript; J.R.S., M.J.J., T.B.C., B.A.B., M.L.K-R., E.H.V.T., and T.P.O. approved final version of manuscript.