Differential effects of digitalis on chemoreflex responses in humans

A Methodological Perspective on the Function and Assessment of Peripheral Chemoreceptors in Heart Failure: A Review of Data from Clinical Trials

Augmented peripheral chemoreceptor sensitivity (PChS) is a common feature of many sympathetically mediated diseases, among others, and it is an important mechanism of the pathophysiology of heart failure (HF). It is related not only to the greater severity of symptoms, especially to dyspnea and lower exercise tolerance but also to a greater prevalence of complications and poor prognosis. The causes, mechanisms, and impact of the enhanced activity of peripheral chemoreceptors (PChR) in the HF population are subject to intense research. Several methodologies have been established and utilized to assess the PChR function. Each of them presents certain advantages and limitations. Furthermore, numerous factors could influence and modulate the response from PChR in studied subjects. Nevertheless, even with the impressive number of studies conducted in this field, there are still some gaps in knowledge that require further research. We performed a review of all clinical trials in HF human patients, in which the function of PChR was evaluated. This review provides an extensive synthesis of studies evaluating PChR function in the HF human population, including methods used, factors potentially influencing the results, and predictors of increased PChS.

The Role of Pharmacological Treatment in the Chemoreflex Modulation

From a physiological point of view, peripheral chemoreceptors (PCh) are the main sensors of hypoxia in mammals and are responsible for adaptation to hypoxic conditions. Their stimulation causes hyperventilation—to increase oxygen uptake and increases sympathetic output in order to counteract hypoxia-induced vasodilatation and redistribute the oxygenated blood to critical organs. While this reaction promotes survival in acute settings it may be devastating when long-lasting. The permanent overfunctionality of PCh is one of the etiologic factors and is responsible for the progression of sympathetically-mediated diseases. Thus, the deactivation of PCh has been proposed as a treatment method for these disorders. We review here physiological background and current knowledge regarding the influence of widely prescribed medications on PCh acute and tonic activities.

The Effect of Carotid Chemoreceptor Inhibition on Exercise Tolerance in Chronic Heart Failure

Purpose

Chronic heart failure (CHF) is characterized by heightened sympathetic nervous activity, carotid chemoreceptor (CC) sensitivity, marked exercise intolerance and an exaggerated ventilatory response to exercise. The purpose of this study was to determine the effect of CC inhibition on exercise cardiovascular and ventilatory function, and exercise tolerance in health and CHF.

Methods

Twelve clinically stable, optimally treated patients with CHF (mean ejection fraction: 43 ± 2.5%) and 12 age- and sex-matched healthy controls were recruited. Participants completed two time-to-symptom-limitation (TLIM) constant load cycling exercise tests at 75% peak power output with either intravenous saline or low-dose dopamine (2 μg⋅kg^–1⋅min^–1; order randomized). Ventilation was measured using expired gas data and operating lung volume data were determined during exercise by inspiratory capacity maneuvers. Cardiac output was estimated using impedance cardiography, and vascular conductance was calculated as cardiac output/mean arterial pressure.

Results

There was no change in TLIM in either group with dopamine (CHF: saline 13.1 ± 2.4 vs. dopamine 13.5 ± 1.6 min, p = 0.78; Control: saline 10.3 ± 1.2 vs. dopamine 11.5 ± 1.3 min, p = 0.16). In CHF patients, dopamine increased cardiac output (p = 0.03), vascular conductance (p = 0.01) and oxygen delivery (p = 0.04) at TLIM, while ventilatory parameters were unaffected (p = 0.76). In controls, dopamine improved vascular conductance at TLIM (p = 0.03), but no other effects were observed.

Conclusion

Our findings suggest that the CC contributes to cardiovascular regulation during full-body exercise in patients with CHF, however, CC inhibition does not improve exercise tolerance.

Effects of the recombinant form of the natural human B-type natriuretic peptide and levosimendan on pulmonary hyperventilation and chemosensivity in heart failure

BACKGROUND

The origin of dyspnea in chronic heart failure (HF) is multifactorial, and excessive ventilation is thought to play a role in inducing this symptom. Chemosensivity is augmented in HF, correlates with increased pulmonary ventilation (VE), and is an adverse prognostic marker. Despite increased blood levels of natriuretic peptides in clinical conditions associated with dyspnea, their effect on pulmonary VE and chemoreceptor activity remains unexplored.

METHODS

We tested in a prospective, placebo-controlled, three-way cross-over, double-blind randomized study the effects of the recombinant form of the natural human B-type natriuretic peptide (R-BNP) in comparison with placebo and levosimendan on chemoreflex sensitivity at rest, as well as their effects on pulmonary VE, systemic blood pressure, heart rate and sympathetic serum activity both at rest and during exercise.

RESULTS

Eleven stable chronic HF patients were randomized to sessions of 6-min treadmill-walking tests during placebo, or levosimendan or R-BNP intravenous infusion in the following conditions: room air, hypoxia, and hypercapnia. R-BNP administration determined higher pulmonary ventilatory response at rest and during exercise (P < 0.001) consequent to a boost of respiratory rate (P < 0.001) under room air and hypoxia conditions. Norepinephrine blood levels increased from rest to exercise in all conditions without differences among placebo, levosimendan, and R-BNP effects. BNP blood levels remained unchanged.

CONCLUSIONS

The novelty of the present findings is that R-BNP infusion in HF patients can boost pulmonary ventilatory response at rest and during exercise.

Effects of enoximone on peripheral and central chemoreflex responses in humans

cAMP plays an important role in peripheral chemoreflex function in animals. We tested the hypothesis that the phosphodiesterase inhibitor and inotropic medication enoximone increases peripheral chemoreflex function in humans. In a single-blind, randomized, placebo-controlled crossover study of 15 men, we measured ventilatory, muscle sympathetic nerve activity, and hemodynamic responses to 5 min of isocapnic hypoxia, 5 min of hyperoxic hypercapnia, and 3 min of isometric handgrip exercise, separated by 1 wk, with enoximone and placebo administration. Enoximone increased cardiac output by 120 ± 3.7% from baseline ( P < 0.001); it also increased the ventilatory response to acute hypoxia [13.6 ± 1 vs. 11.2 ± 0.7 l/min at 5 min of hypoxia, P = 0.03 vs. placebo (by ANOVA)]. Despite a larger minute ventilation and a smaller decrease in O2 desaturation (83 ± 1 vs. 79 ± 2%, P = 0.003), the muscle sympathetic nerve response to hypoxia was similar between enoximone and placebo (123 ± 6 and 117 ± 6%, respectively, P = 0.28). In multivariate regression analyses, enoximone enhanced the ventilatory ( P < 0.001) and sympathetic responses to isocapnic hypoxia. Hyperoxic hypercapnia and isometric handgrip responses were not different between enoximone and placebo ( P = 0.13). Enoximone increases modestly the chemoreflex responses to isocapnic hypoxia. Moreover, this effect is specific for the peripheral chemoreflex, inasmuch as central chemoreflex and isometric handgrip responses were not altered by enoximone.