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

Contribution of Peripheral Chemoreceptors to Exercise Intolerance in Heart Failure

Peripheral chemoreceptors (PChRs), because of their strategic localization at the bifurcation of the common carotid artery and along the aortic arch, play an important protective role against hypoxia. Stimulation of PChRs evokes hyperventilation and hypertension to maintain adequate oxygenation of critical organs. A relationship between increased sensitivity of PChRs (hyperreflexia) and exercise intolerance (ExIn) in patients with heart failure (HF) has been previously reported. Moreover, some studies employing an acute blockade of PChRs (e.g., using oxygen or opioids) demonstrated improvement in exercise capacity, suggesting that hypertonicity is also involved in the development of ExIn in HF. Nonetheless, the precise mechanisms linking dysfunctional PChRs to ExIn remain unclear. From the clinical perspective, there are two main factors limiting exercise capacity in HF patients: subjective perception of dyspnoea and muscle fatigue. Both have many determinants that might be influenced by abnormal signalling from PChRs, including: exertional hyperventilation, oscillatory ventilation, ergoreceptor oversensitivity, and augmented sympathetic tone. The latter results in reduced muscle perfusion and altered muscle structure. In this review, we intend to present the milieu of abnormalities tied to malfunctioning PChRs and discuss their role in the complex relationships leading, ultimately, to ExIn.

Cardiac resynchronization therapy restores muscular metaboreflex control

INTRODUCTION

The muscular metaboreflex, whose activation regulates blood flow during isometric and aerobic exercise, is blunted in patients with heart failure (HF), and cardiac resynchronization therapy (CRT) may restore this regulatory reflex.

OBJECTIVE

To evaluate metaboreflex responses after CRT.

METHODS

Thirteen HF patients and 12 age-matched healthy control subjects underwent the following evaluations (pre- and post-CRT implantation in the patient group): (a) heart rate, blood pressure, and forearm blood flow measurements; (b) muscle sympathetic nerve activity (MSNA) evaluation; and (c) peak oxygen consumption (VO_2peak ). Examinations were performed at rest, during moderate isometric exercise (IE), and during forearm ischemia (metaboreflex activation). The primary outcome was the increment in MSNA during limb ischemia compared to the rest moment (ΔMSNA rest to metaboreflex activation).

RESULTS

After CRT, rest MSNA decreased in the HF participants: 50.4 ± 9.2 bursts/min pre-CRT vs 34.0 ± 14.4 bursts/min post-CRT, P = .001, accompanied by an improvement in systolic blood pressure and in rate-pressure product. MSNA during limb ischemia decreased: 56.6 ± 11.5 bursts/min pre-CRT vs 43.6 ± 12.7 bursts/min post-CRT, P = .001, and the ΔMSNA rest to metaboreflex activation increased: 0% (interquartile range [IQR)], -7 to 9) vs 13% (IQR, 5-30), P = .03. An augmentation of mean blood pressure during limb ischemia post-CRT was noticed: 94 mmHg (IQR, 81-104) vs 110 mmHg (IQR, 100-117), P = .04. CRT improved VO_2peak , and this improvement was correlated with diminution in ΔMSNA pre- to post-CRT at rest moment (r_s = -0.74, P = .006).

CONCLUSION

CRT provides metaboreflex sensitization and MSNA enhancement. The restoration of sympathetic responsiveness correlates with the improvement in functional capacity.

Clinical usefulness of response profiles to rapidly incremental cardiopulmonary exercise testing

The advent of microprocessed "metabolic carts" and rapidly incremental protocols greatly expanded the clinical applications of cardiopulmonary exercise testing (CPET). The response normalcy to CPET is more commonly appreciated at discrete time points, for example, at the estimated lactate threshold and at peak exercise. Analysis of the response profiles of cardiopulmonary responses at submaximal exercise and recovery, however, might show abnormal physiologic functioning which would not be otherwise unraveled. Although this approach has long been advocated as a key element of the investigational strategy, it remains largely neglected in practice. The purpose of this paper, therefore, is to highlight the usefulness of selected submaximal metabolic, ventilatory, and cardiovascular variables in different clinical scenarios and patient populations. Special care is taken to physiologically justify their use to answer pertinent clinical questions and to the technical aspects that should be observed to improve responses' reproducibility and reliability. The most recent evidence in favor of (and against) these variables for diagnosis, impairment evaluation, and prognosis in systemic diseases is also critically discussed.