The pulmonary vascular response to combined activation of the muscle metaboreflex and mechanoreflex

Cardiovascular responses to combined mechanoreflex and metaboreflex activation in healthy adults: effects of sex and low- versus high-hormone phases in females

Females generally have smaller blood pressure (BP) responses to isolated muscle mechanoreflex and metaboreflex activation compared with males, which may explain sex differences in BP responses to voluntary exercise. The mechanoreflex may be sensitized during exercise, but whether mechanoreflex-metaboreflex interactions differ by sex or variations in sex hormones remains unknown. Thirty-one young healthy subjects (females, n = 16) performed unilateral passive cycling (mechanoreflex), active cycling (40% peak Watts), postexercise circulatory occlusion (PECO; metaboreflex), and passive cycling combined with PECO (combined mechanoreflex and metaboreflex activation). Beat-to-beat BP, heart rate, inactive leg vascular conductance, and active leg muscle oxygenation were measured. Ten females underwent exploratory testing during low- and high-hormone phases of their self-reported menstrual cycle or oral contraceptive use. Systolic BP and heart rate responses did not differ between sexes during active cycling [Δ30 ± 9 vs. 29 ± 11 mmHg (males vs. females), P = 0.9; Δ33 ± 8 vs. 35 ± 6 beats/min, P = 0.4] or passive cycling with PECO (Δ26 ± 11 vs. 21 ± 10 mmHg, P = 0.3; Δ14 ± 7 vs. 18 ± 15 beats/min, P = 0.3). Passive cycling with PECO revealed additive, not synergistic, effects for systolic BP [males: Δ23 ± 14 vs. 26 ± 11 mmHg (sum of isolated passive cycling and PECO vs. combined activation); females: Δ26 ± 11 vs. 21 ± 12 mmHg, interaction P = 0.05]. Results were consistent in subset analyses with sex differences in active cycling BP (P > 0.1) and exploratory analyses of hormone phase (P > 0.4). Despite a lack of statistical equivalence, no differences in cardiovascular responses were found during combined mechanoreflex-metaboreflex activation between sexes or hormone levels. These results provide preliminary data regarding the involvement of muscle mechanoreflex-metaboreflex interactions in mediating sex differences in voluntary exercise BP responses.NEW & NOTEWORTHY The muscle mechanoreflex may be sensitized by metabolites during exercise. We show that cardiovascular responses to combined mechanoreflex (passive cycling) and metaboreflex (postexercise circulatory occlusion) activation are primarily additive and do not differ between males and females, or across variations in sex hormones in females. Our findings provide new insight into the contributions of muscle mechanoreflex-metaboreflex interactions as a cause for prior reports that females have smaller blood pressure responses to voluntary exercise.

Differential contributions of cardiac, coronary and pulmonary artery vagal mechanoreceptors to reflex control of the circulation

Distinct populations of stretch-sensitive mechanoreceptors attached to myelinated vagal afferents are found in the heart and adjoining coronary and pulmonary circulations. Receptors at atrio-venous junctions appear to be involved in control of intravascular volume. These atrial receptors influence sympathetic control of the heart and kidney, but contribute little to reflex control of systemic vascular resistance. Baroreceptors at the origins of the coronary circulation elicit reflex vasodilatation, like feedback control from systemic arterial baroreceptors, as well as having characteristics that could contribute to regulation of mean pressure. In contrast, feedback from baroreceptors in the pulmonary artery and bifurcation is excitatory and elicits a pressor response. Elevation of pulmonary arterial pressure resets the vasomotor limb of the systemic arterial baroreflex, which could be relevant for control of sympathetic vasoconstrictor outflow during exercise and other states associated with elevated pulmonary arterial pressure. Ventricular receptors, situated mainly in the inferior posterior wall of the left ventricle, and attached to unmyelinated vagal afferents, are relatively inactive under basal conditions. However, a change to the biochemical environment of cardiac tissue surrounding these receptors elicits a depressor response. Some ventricular receptors respond, modestly, to mechanical distortion. Probably, ventricular receptors contribute little to tonic feedback control; however, reflex bradycardia and hypotension in response to chemical activation may decrease the work of the heart during myocardial ischaemia. Overall, greater awareness of heterogeneous reflex effects originating from cardiac, coronary and pulmonary artery mechanoreceptors is required for a better understanding of integrated neural control of circulatory function and arterial blood pressure.

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

Passive bilateral leg cycling with concomitant regional circulatory occlusion for testing mechanoreflex-metaboreflex interactions in humans

Purpose

The exercise pressor reflex (EPR) plays a fundamental role in physiological reactions to exercise in humans and in the pathophysiology of cardiovascular disorders. There is no “gold standard” method for EPR assessment; therefore, we propose a new protocol for testing interactions between the muscle mechanoreflex and metaboreflex (major components of EPR).

Methods

Thirty-four healthy subjects (mean age [± standard deviation] 24 ± 4 years, 22 men) were enrolled in the study. During the study, the hemodynamic and ventilatory parameters of these subjects were continuously monitored using our proposed assessment method. This assessment method consists of an initial 5-min rest period (baseline) followed by 5 min of passive cycling (PC) on an automated cycle ergometer (mechanoreceptor stimulation), after which tourniquet cuffs located bilaterally on the upper thighs are inflated for 3 min to evoke venous and arterial regional circulatory occlusion (CO) during PC (metaboreceptor stimulation). Deflation of the tourniquet cuffs is followed by a second 5 min of PC and finally by a 5-min recovery time. The control test comprises a 5-min rest period, followed by 3 min of CO only and a final 5-min recovery.

Results

Mean arterial pressure (MAP) and minute ventilation (MV) increased significantly during PC (MAP: from 90 ± 9.3 to 95 ± 9.7 mmHg; MV: from 11.5 ± 2.5 to 13.5 ± 2.9 L/min; both p < 0.05) and again when CO was applied (MAP: from 95 ± 9.7 to 101 ± 11.0 mmHg; MV: from 13.5 ± 2.9 to 14.8 ± 3.8 L/min; both p < 0.05). In the control test there was a slight increase in MAP during CO (from 92 ± 10.5 to 94 ± 10.0 mmHg; p < 0.05) and no changes in the ventilatory parameters.

Conclusion

Bilateral leg passive cycling with concomitant circulatory occlusion is a new, simple and effective method for testing interactions between the mechanoreflex and metaboreflex in humans.

Electronic supplementary material

The online version of this article (10.1007/s10286-020-00717-x) contains supplementary material, which is available to authorized users.

Faster oxygen uptake, heart rate, and ventilatory kinetics in stepping compared with cycle ergometry in patients with COPD during moderate-intensity exercise

Step tests are a stressful and feasible cost-effective modality to evaluate aerobic performance. However, the eccentric in addition to concentric muscle contractions of the legs on stepping emerge as a potential speeding factor for cardioventilatory and metabolic adjustments towards a steady-state, since eccentric contractions would prompt an earlier and stronger mechanoreceptor activation, as well as higher heart rate/cardiac output adjustments to the same metabolic demand. Moreover, shorter tests are ideal for exercise-limited subjects. Nine subjects with chronic obstructive pulmonary disease were invited to participate in comprehensive lung function tests and constant work tests performed on different days at a 90% gas exchange threshold for 6 min, in single-step tests or cycle ergometry. After careful monoexponential regression modelling, statistically relevant faster phase II time constants for oxygen uptake (45 ± 18 s vs 53 ± 17 s, p = 0.017) and minute ventilation (61 ± 13 s vs 74 ± 17 s, p = 0.027) were observed in the 6-min step tests compared with cycle ergometry, respectively. Despite an absence of heart rate time constant difference (43 ± 20 s vs 69 ± 46 s, p = 0.167), there was a significantly faster rate constant toward a steady state for heart rate (p = 0.02). In addition, 4-min compared with 6-min analysis presented similar results (p > 0.05), providing an appropriate steady-state. We conclude that step tests might elicit faster time constants compared with cycle ergometry, at the same average metabolic level, and 4-min analysis has similar mean errors compared with 6-min analysis within an acceptable range. New studies, comprising mechanisms and detailed physiological backgrounds, are necessary.

Pulmonary artery wave propagation and reservoir function in conscious man: impact of pulmonary vascular disease, respiration and dynamic stress tests

KEY POINTS

Wave travel plays an important role in cardiovascular physiology. However, many aspects of pulmonary arterial wave behaviour remain unclear. Wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery in subjects with and without pulmonary hypertension during spontaneous respiration and dynamic stress tests. Arterial wave energy decreased during expiration and Valsalva manoeuvre due to decreased ventricular preload. Wave energy also decreased during handgrip exercise due to increased heart rate. In pulmonary hypertension patients, the asymptotic pressure at which the microvascular flow ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by waves increased. The reservoir and excess pressures decreased during Valsalva manoeuvre but remained unchanged during handgrip exercise. This study provides insights into the influence of pulmonary vascular disease, spontaneous respiration and dynamic stress tests on pulmonary artery wave propagation and reservoir function.

ABSTRACT

Detailed haemodynamic analysis may provide novel insights into the pulmonary circulation. Therefore, wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery to characterize changes in wave propagation and reservoir function during spontaneous respiration and dynamic stress tests. Right heart catheterization was performed using a pressure and Doppler flow sensor tipped guidewire to obtain simultaneous pressure and flow velocity measurements in the pulmonary artery in control subjects and patients with pulmonary arterial hypertension (PAH) at rest. In controls, recordings were also obtained during Valsalva manoeuvre and handgrip exercise. The asymptotic pressure at which the flow through the microcirculation ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by arterial waves increased in PAH patients compared to controls. The systolic and diastolic rate constants also increased, while the diastolic time constant decreased. The forward compression wave energy decreased by ∼8% in controls and ∼6% in PAH patients during expiration compared to inspiration, while the wave speed remained unchanged throughout the respiratory cycle. Wave energy decreased during Valsalva manoeuvre (by ∼45%) and handgrip exercise (by ∼27%) with unaffected wave speed. Moreover, the reservoir and excess pressures decreased during Valsalva manoeuvre but remained unaltered during handgrip exercise. In conclusion, reservoir-excess pressure analysis applied to the pulmonary artery revealed distinctive differences between controls and PAH patients. Variations in the ventricular preload and afterload influence pulmonary arterial wave propagation as demonstrated by changes in wave energy during spontaneous respiration and dynamic stress tests.

Influence of the Metaboreflex on Pulmonary Vascular Capacitance in Heart Failure

PURPOSE

An impaired metaboreflex is associated with abnormal ventilatory and peripheral vascular function in heart failure (HF), whereas its influence on cardiac function or pulmonary vascular pressure remains unclear. We aimed to assess whether metabolite-sensitive neural feedback (metaboreflex) from locomotor muscles via postexercise regional circulatory occlusion (RCO) attenuates pulmonary vascular capacitance (GXCAP) and/or circulatory power (CircP) in patients with HF.

METHODS

Eleven patients with HF (NYHA class, I/II; ages, 51 ± 15 yr; ejection fraction, 32% ± 9%) and 11 age- and gender-matched controls (ages, 43 ± 9 yr) completed three cycling sessions (4 min, 60% peak oxygen uptake (V˙O2)). Session 1 was a control trial including normal recovery (NR). Session 2 or 3 included bilateral upper thigh pressure tourniquets inflated suprasystolic at end of exercise (RCO) for 2-min recovery with or without inspired CO2 (RCO + CO2) (randomized). Mean arterial pressure, HR, and V˙O2 were continuously measured. Estimates of central hemodynamics; CircP = (V˙O2 × mean arterial pressure)/weight; oxygen pulse index (O2pulseI = (V˙O2/HR)/body surface area); and GXCAP = O2pulseI × end-tidal partial pressure CO2 were calculated.

RESULTS

At rest and end of exercise, CircP and GXCAP were lower in HF versus those in controls (P < 0.05), with no differences between transients (P > 0.05). At 2-min recovery, GXCAP was lower during RCO versus that during NR in both groups (72 ± 23 vs 98 ± 20 and 73 ± 34 vs 114 ± 35 mL·beat·mm Hg·m, respectively; P < 0.05), whereas CircP did not differ between transients (P > 0.05). Differences (% and Δ) between baseline and 2-min recovery among transients suggest that metaboreflex attenuates GXCAP in HF. Differences (% and Δ) between baseline and 2-min recovery among transients suggest that metaboreflex may attenuate CircP in controls.

CONCLUSIONS

The present observations suggest that locomotor muscle metaboreflex activation may influence CircP in controls but not in HF. However, metaboreflex activation may evoke decreases in GXCAP (increased pulmonary vascular pressures) in HF and controls.

Intrathecal fentanyl blockade of afferent neural feedback from skeletal muscle during exercise in heart failure patients: Influence on circulatory power and pulmonary vascular capacitance

BACKGROUND

Secondary pulmonary hypertension is common in heart failure (HF) patients. We hypothesized that inhibition of feedback from locomotor muscle group III/IV neurons contributes to reduced pulmonary vascular pressures independent of changes in cardiac function during exercise in HF.

METHODS

9 HF patients (ages, 60 ± 2; EF, 26.7 ± 1.9%; New York Heart Association classes, I-III) and 9 age/gender matched controls (ages, 63 ± 2) completed five-minutes of constant-load cycling (65% Workloadpeak) with intrathecal fentanyl or placebo on randomized separate days. Mean arterial pressure (MAP), heart rate (HR), end-tidal partial pressure of CO2 (PETCO2), and oxygen consumption (VO2) were measured at rest and exercise. Non-invasive surrogates for cardiac power (circulatory power, CircP=VO2 × MAP), stroke volume (oxygen pulse, O2pulse=VO2/HR), and pulmonary arterial pressure (GXCAP=O2pulse × PETCO2) were calculated.

RESULTS

At rest and end-exercise, differences between fentanyl versus placebo were not significant for CircP in HF or controls. Differences between fentanyl versus placebo for GXCAP were not significant at rest in HF or controls. At end-exercise, GXCAP was significantly higher with fentanyl versus placebo in HF (691 ± 59 versus 549 ± 38 mL/beat × mmHg), but not controls (536 ± 59 versus 474 ± 43 mL/beat × mmHg). Slopes (rest to end-exercise) for GXCAP were significantly higher with fentanyl versus placebo in HF (95.1 ± 9.8 versus 71.6 ± 6.0 mL/beat × mmHg), but not controls (74.3 ± 9.5 versus 60.8 ± 6.5 mL/beat × mmHg). CircP slopes did not differ between fentanyl versus placebo in HF or controls (p>0.05).

CONCLUSION

We conclude that feedback from locomotor muscle group III/IV neurons may evoke increases in pulmonary vascular pressures independent of changes in cardiac function during exercise in HF.