Peripheral muscle ergoreceptors and ventilatory response during exercise recovery in heart failure

Mechanism of augmented exercise hyperpnea in chronic heart failure and dead space loading

Patients with chronic heart failure (CHF) suffer increased alveolar VD/VT (dead-space-to-tidal-volume ratio), yet they demonstrate augmented pulmonary ventilation such that arterial [Formula: see text] ( [Formula: see text] ) remains remarkably normal from rest to moderate exercise. This paradoxical effect suggests that the control law governing exercise hyperpnea is not merely determined by metabolic CO2 production ( [Formula: see text] ) per se but is responsive to an apparent (real-feel) metabolic CO2 load ( [Formula: see text] ) that also incorporates the adverse effect of physiological VD/VT on pulmonary CO2 elimination. By contrast, healthy individuals subjected to dead space loading also experience augmented ventilation at rest and during exercise as with increased alveolar VD/VT in CHF, but the resultant response is hypercapnic instead of eucapnic, as with CO2 breathing. The ventilatory effects of dead space loading are therefore similar to those of increased alveolar VD/VT and CO2 breathing combined. These observations are consistent with the hypothesis that the increased series VD/VT in dead space loading adds to [Formula: see text] as with increased alveolar VD/VT in CHF, but this is through rebreathing of CO2 in dead space gas thus creating a virtual (illusory) airway CO2 load within each inspiration, as opposed to a true airway CO2 load during CO2 breathing that clogs the mechanism for CO2 elimination through pulmonary ventilation. Thus, the chemosensing mechanism at the respiratory controller may be responsive to putative drive signals mediated by within-breath [Formula: see text] oscillations independent of breath-to-breath fluctuations of the mean [Formula: see text] level. Skeletal muscle afferents feedback, while important for early-phase exercise cardioventilatory dynamics, appears inconsequential for late-phase exercise hyperpnea.

Cardiac resynchronization therapy reduces metaboreflex contribution to the ventilatory response in heart failure population

Background. Metaboreflex overactivation has been proprosed to explain exaggerated hyperventilation in heart failure population. We investigated the metaboreflex activation after cardiac resynchronization therapy (CRT). Methods. 10 heart failure patients (mean left ventricular ejection fraction (LVEF) 27 ± 4%) schedulded for CRT implantation were prospectively studied. At baseline and after 6 month follow up two maximal cardiopulmonary exercise tests with and without regional circulatory occlusion (RCO) during recovery were performed. RCO was achieved by inflation of bilateral upper thigh tourniquets 30 mmHg above peak systolic blood pressure during 3 minutes after peak exercise. Metaboreflex contribution to the ventilatory response was assessed as the difference in ventilatory data at the third minute during recovery between the two tests (Δ). Results. Patients had enhanced VE/VCO(2) slope (40 ± 9) and an evident metaboreflex contribution to the high ventilatory response (ΔVE: 3 ± 4 L/min; P = 0.05, ΔRR: 4.5 ± 4/min; P = 0.003 and ΔVE/VCO(2): 5.5 ± 4; P = 0.007). 6 months after CRT implantation, NYHA class, LVEF, peak VO(2) and VE/VCO(2) were significantly improved (1.4 ± 0.5; P < 0.001, 42 ± 7%; P < 0.001, 16.5 ± 3 mL/kg/min; P = 0.003; 33 ± 10; P = 0.01). Metaboreflex contribution to VE, RR, and VE/VCO(2) was reduced compared with baseline (P = 0.08, P = 0.01 and P = 0.4 resp.). Conclusion. 6 months after CRT metaboreflex contribution to the ventilatory response is reduced.

[The ventilatory response in chronic heart failure population]

Symptom-limited exercise test with peak oxygen consumption measurement possesses a strong prognostic value in chronic heart failure. This parameter allowing notably the selection of patients for heart transplant. Nevertheless, sub maximal effort and beta blocker therapy tend to limit its prognostic value. The ventilatory response evaluated by the minute ventilation - carbon dioxide production (VE/VCO2) linear regression slope during effort is generally considered to be a significant predictor of mortality and hospitalizations in HF population. An enhanced ventilatory response is correlated with a poorer prognostic. In addition, this parameter is not influenced by the intensity of the effort neither by the betablocker therapy. But, physiological determinants are not clear yet. Aim of our study is to confirm the important place of the (VE/VCO2) slope in HF patient's evaluation.

Influence of locomotor muscle metaboreceptor stimulation on the ventilatory response to exercise in heart failure

BACKGROUND

Whether locomotor muscle afferent neural activity contributes to exercise hyperpnea and symptoms of dyspnea in heart failure (HF) is controversial. We examined the influence of metaboreceptor stimulation on ventilation with and without maintaining end-exercise end-tidal CO(2) levels.

METHODS AND RESULTS

Eleven patients with HF aged 51+/-5 years (ejection fraction, 32+/-3%; New York Heart Association class, 1.6+/-0.2) and 11 age- and gender-matched healthy control participants aged 43+/-3 years were studied. Participants underwent 3 steady-state cycling sessions at 60% of peak oxygen consumption for 4 minutes. The first exercise session was a baseline control trial. Bilateral thigh tourniquets were inflated to suprasystolic pressure at end exercise for 2 minutes during 2 of the trials (regional circulatory occlusion) with the addition of inspired CO(2) to maintain end-exercise partial pressure of end-tidal CO(2) during 1 trial (regional circulatory occlusion+CO(2)). Minute ventilation was measured continuously throughout each trial. At 2 minutes postexercise during the baseline control trial in patients with HF, minute ventilation was 54% of end exercise, whereas the control group averaged 41% (P=0.11). During regional circulatory occlusion in patients with HF, minute ventilation was 60% of end exercise; however, the control group averaged 35% (P<0.001). During regional circulatory occlusion+CO(2), the minute ventilation of patients with HF averaged 67% of end exercise, whereas the control group averaged 44% (P<0.001).

CONCLUSIONS

These data suggest that increased afferent neural activity from the large locomotor muscles associated with metabolites generated during exercise contribute to the augmented ventilatory response to exercise in patients with HF.

Contribution of skeletal muscle 'ergoreceptors' in the human leg to respiratory control in chronic heart failure

The role of skeletal muscle ergoreceptors (afferents sensitive to muscle contraction, differentiated into metaboreceptors, sensitive to metabolic changes, and mechanoreceptors, sensitive to mechanical changes) in the genesis of the increased ventilatory drive in chronic heart failure is controversial. We have aimed to clarify the contribution of muscle metaboreceptors in the leg to ventilation and to compare this with the contribution of mechanoreceptors. Eighteen heart failure patients and 12 controls were studied. Metaboreceptor and mechanoreceptor responses were measured in the leg by bicycle exercise with and without regional circulatory occlusion during recovery, and by active and equivalent passive limb movement, respectively.Patients, in comparison with controls, had a lower peak VO2 (Oxygen uptake) (18.1+/-1.6 vs. 24.5+/-2.5 ml min(-1) kg(-1), P< 0.05), and an evident metaboreceptor contribution to the ventilatory response (3.5+/-1.6 vs. -4.0+/-1.3 l min(-1), P<0.001). Passive limb movement increased ventilation in both patients and controls (+3.7+/-0.4 and +2.9+/-0.5 l min(-1) from baseline, P<0.003), but this was associated with an increase in VO2 (+0.1+/-0.01 and +0.1+/-0.02 l min(-1) from baseline, P<0.001). The ratio of the increase in ventilation to the increase in VO2 during passive movement was not significantly higher than that during active exercise for either patients or controls, suggesting a limited contribution from the mechanoreceptors. In chronic heart failure the presence of a muscle metaboreceptor reflex is also demonstrated in the leg, while mechanoreceptors exhibited a non-significant contribution in both patients and controls. The hypothesis of a peripheral origin of symptoms of exertional intolerance in this syndrome is confirmed as being mainly due to metabolic stimulation of the muscle metaboreceptors.