Dose-dependent effect of dobutamine on chemoreflex activity in healthy volunteers

The effect of hypoxemia on muscle sympathetic nerve activity and cardiovascular function: a systematic review and meta-analysis

We conducted a systematic review and meta-analysis to determine the effect of acute poikilocapnic, high-altitude, and acute isocapnia hypoxemia on muscle sympathetic nerve activity (MSNA) and cardiovascular function. A comprehensive search across electronic databases was performed until June 2021. All observational designs were included: population (healthy individuals); exposures (MSNA during hypoxemia); comparators (hypoxemia severity and duration); outcomes (MSNA; heart rate, HR; and mean arterial pressure, MAP). Sixty-one studies were included in the meta-analysis. MSNA burst frequency increased by a greater extent during high-altitude hypoxemia [P < 0.001; mean difference (MD), +22.5 bursts/min; confidence interval (CI) = -19.20 to 25.84] compared with acute poikilocapnic hypoxemia (P < 0.001; MD, +5.63 bursts/min; CI = -4.09 to 7.17) and isocapnic hypoxemia (P < 0.001; MD, +4.72 bursts/min; CI = -3.37 to 6.07). MSNA burst amplitude was only elevated during acute isocapnic hypoxemia (P = 0.03; standard MD, +0.46 au; CI = -0.03 to 0.90), and MSNA burst incidence was only elevated during high-altitude hypoxemia [P < 0.001; MD, 33.05 bursts/100 heartbeats; CI = -28.59 to 37.51]. Meta-regression analysis indicated a strong relationship between MSNA burst frequency and hypoxemia severity for acute isocapnic studies (P < 0.001) but not acute poikilocapnia (P = 0.098). HR increased by the same extent across each type of hypoxemia [P < 0.001; MD +13.81 heartbeats/min; 95% CI = 12.59-15.03]. MAP increased during high-altitude hypoxemia (P < 0.001; MD, +5.06 mmHg; CI = 3.14-6.99), and acute isocapnic hypoxemia (P < 0.001; MD, +1.91 mmHg; CI = 0.84-2.97), but not during acute poikilocapnic hypoxemia (P = 0.95). Both hypoxemia type and severity influenced sympathetic nerve and cardiovascular function. These data are important for the better understanding of healthy human adaptation to hypoxemia.

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 hemodynamic response to constant dobutamine infusion: the effect of ADRB1 389 polymorphism and sex

OBJECTIVES

Prolonged activation of the β-1 adrenergic receptor (ADRB1) is associated with receptor desensitization. This process has been suggested to have important pathophysiological and clinical implications in conditions such as congestive heart failure. The contribution of genetic factors to this process is a subject of ongoing research. We have previously shown that the ADRB1 389 polymorphism affects the response to incremental dose infusion of the ADRB agonist dobutamine. The aim of the current study was to determine whether the ADRB1 389 polymorphism affects the hemodynamic response to constant dose infusion of dobutamine in healthy patients.

PATIENTS AND METHODS

Healthy patients were recruited according to their ADRB1 49 and 389 genotypes [15 Arg389Arg, 10 Gly389Arg, and 10 Gly389Gly patients (all Ser49Ser), 21 men and 14 women]. Following a standardized protocol of dose increase, 6 mcg/kg/min dobutamine was infused over 2 h. Heart rate (HR), blood pressure (BP), and active plasma renin (PR) were measured. Standardized exercise (1 min) was performed at three time points during infusion.

RESULTS

In all patients, resting systolic BP was significantly decreased during infusion [144.4±11.5 vs. 140.3±12.2 mmHg (mean±SD), P=0.007]. There was no change in HR, and PR following 120 min of dobutamine infusion. ADRB1 389 genotypes were not associated with HR, systolic BP, and PR changes during dobutamine infusion (all P>0.05, repeated measures analysis of variance). Sex was associated with response to dobutamine. Among women, but not in men, resting HR significantly increased, and diastolic blood pressure (DBP) significantly decreased during dobutamine infusion [HR: 76.0±7.3 to 86.3±17.5 beats per minute (P=0.023), and DBP 78.5±8.49 mmHg to 72.36±6.16 (P=0.041) (repeated measures analysis of variance)].

CONCLUSION

In healthy patients, the ADRB1 389 genotype was not associated with hemodynamic changes during constant dobutamine infusion. In women, but not in men, HR significantly increased and DBP decreased during 2 h of infusion.

Direct evidence of sympathetic hyperactivity in patients with vasospastic angina

BACKGROUND

The autonomic nervous system is reported to be involved in the pathogenesis of vasospastic angina (VSA). Studies based on heart rate variability analysis have shown conflicting results with both a reduction and an enhancement of sympathetic nervous system (SNS) activity in patients with Prinzmetal's variant angina, but direct assessment has never been performed. The aim of our study was to evaluate the SNS activity using microneurography in patients with VSA.

METHODS AND RESULTS

The SNS was evaluated by measuring the muscle sympathetic nerve activity (MSNA) with microneurography in 15 patients with VSA confirmed by positive ergonovine provocation test and 15 controls subjects negative for the provocation test. Over the baseline period, SNS activity was higher in patients with VSA compared with control patients (56.8 ± 5 vs. 49.3 ± 6.3 burst/min, p < 0.001, respectively). During mental stress, SNS activity increased significantly only in patients with VSA, which still presented a higher SNS activity than control patients (66.1 ± 7.2 vs. 53.6 ± 8.7 burst/min; p < 0.001, respectively). Furthermore only VSA patients showed significant hemodynamic modifications with an increase in mean arterial blood pressure (96.2 ± 13.4 vs. 86.6 ± 9.6 mmHg in VSA patients and control subjects, respectively; p < 0.05).

CONCLUSION

Our results provide the first direct evidence of lasting increased sympathetic activity that is worsened by mental stress in patients with VSA. These results suggest that SNS participate to the pathogenesis of VSA by enhancing coronary vascular tone.

Hyperventilation in normoxia following myocardial infarction in rats: a shift in the set point of the hypoxic ventilatory response

AIM

The peripheral chemoreflex is augmented in heart failure, and it may contribute to sympathoexcitation. This study aimed to investigate both the chemoreflex and the cardiac sympathetic nerve activity in the acute-stage post-myocardial infarction.

METHODS

Myocardial infarction was induced in male adult Sprague-Dawley rats by permanent ligation of the left anterior descending coronary artery. Within-animal repeated measure assessment of normoxic and hypoxic ventilation patterns was determined with whole-body plethysmography and compared to sham-operated controls. Cardiac function, morphology and cardiac sympathetic nerve activity were determined 14 days later.

RESULTS

Infarction induced increases in normoxic ventilation through increases in tidal volume within 3 days. At the same time points, the hypoxic ventilatory response to short durations (10 min) of hypoxia (8, 10 and 12% inspired O2 ) was blunted. At the end of the experiment (D14), increases in nerve activity, specifically through increased firing rate, and significant cardiac dysfunction (ejection fraction 43%) were observed in myocardial infarction (MI) group.

CONCLUSIONS

An augmentation of normoxic ventilation caused by myocardial infarction occurs before the amplification of the hypoxic ventilatory response. It occurs much earlier following myocardial injury than previously demonstrated and may have a role in initiating cardiac sympathoexcitation. The difference in the augmentation of hypoxic response between early and late stages post-myocardial infarction suggest that the initial change in the chemoreflex is an alteration to the operating point of chemoreflex.

Pharmacology of levosimendan: inotropic, vasodilatory and cardioprotective effects

WHAT IS KNOWN AND OBJECTIVE

Positive inotropic agents are frequently used in acute decompensated heart failure (ADHF) due to left ventricular systolic dysfunction. These agents are known to improve cardiac performance and peripheral perfusion in the short-term treatment. However, several preclinical and clinical studies emphasized detrimental effects of these drugs on myocardial oxygen demand and on sympathetic tone entailing arrhythmogenesis. Levosimendan is an inotropic agent with an original mechanism of action. This review focuses on major data available for levosimendan.

METHODS

A literature search was conducted in the PubMed database by including studies published in English using combinations of the following key words, levosimendan, inotropic drugs and acute heart failure. Furthermore, bibliographies of selected references were also evaluated for relevant articles. The collection for this review was limited to the most recently available human and animal data.

RESULTS AND DISCUSSION

Levosimendan's vasodilatory and cardioprotective effects are mediated by calcium sensitization of contractile proteins and opening of adenosine triphosphate (ATP)-dependent K+ channels in vascular smooth muscle cells and on mitochondrial ATP-sensitive potassium [mito.K(ATP)] channels. This inotropic agent has mild PDE inhibitory action. Unlike other inotropic agents, levosimendan improves cardiac performance without activating the sympathetic nervous system. Moreover, there are evidences that levosimendan has additional anti-inflammatory and anti-apoptotic properties that prevent cardiac toxicity and contributes to positive hemodynamic response of the drug. Four randomized trials evaluated the effects of levosimendan on mortality in patients with acute decompensated chronic heart failure; nevertheless, a clear benefit has not been demonstrated so far. Although levosimendan is indicated for the treatment of ADHF (class of recommendation IIa, level of evidence B), it is has not been approved in all countries.

WHAT IS NEW AND CONCLUSION

This review summarizes the characteristics and the current knowledge of the literature on levosimendan and its active metabolite OR-1896.

The impact of acute and chronic catecholamines on respiratory responses to hypoxic stress in the rat

Chronic catecholamine production is associated with desensitisation and down-regulation of adrenergic receptors and occurs in conditions, such as heart failure and myocardial infarction. The effects of further acute adrenergic stimulation, which may occur during exercise, and their subsequent effects on chemosensitivity and ventilation are unclear. Chronic isoprenaline (ISO) increased ventilation by 50 % (P < 0.05) yet the sensitivity to graded hypoxia was preserved. Acute noradrenaline (NA) in control animals led to a doubling of ventilation in hyperoxia (P < 0.001), and this difference was preserved in graded hypoxia (P < 0.001). Yet, combination of NA + ISO did not increase ventilation beyond ISO at baseline or in hypoxia. ISO, NA, and NA + ISO all induced a metabolic acidosis (P < 0.05) with enhanced ventilation in partial compensation. Carotid sinus nerve (CSN) section led to a partial loss of catecholamine-induced augmentation in ventilation (P < 0.05), yet direct recording from CSN in vitro suggests catecholamine is inhibitory for CSN discharge. These observations suggest that chronic catecholamine exposure may result in decreased exercise performance as a direct consequence of the hyperpnea to compensate for an increased metabolic rate coupled with acidosis and leading to increased central chemosensitivity. A limited contribution from peripheral chemoreceptors was noted but was not a consequence of catecholamine stimulation of the carotid body.

Excessive sympathetic activation in heart failure with chronic renal failure: role of chemoreflex activation

OBJECTIVE

Sympathetic activation contributes both to the initiation and progression of heart failure. The role of chronic renal failure (CRF) in determining sympathetic overactivity in chronic heart failure (CHF) patients is unknown. We tested the hypothesis that in CHF patients, CRF could lead to increase sympathetic activity through tonic activation of excitatory chemoreceptor afferents.

METHODS

We conducted a double-blind, randomized, vehicle-controlled study to examine the effect of chemoreflex deactivation on muscle sympathetic nerve activity in CHF patients with or without CRF. We compared effect of breathing 100% oxygen for 15 min in 15 stable CHF patients with CRF and 15 control CHF patients matched for age, sex, blood pressure and BMI.

RESULTS

The baseline muscle sympathetic nerve activity was significantly elevated in CHF patients with CRF as compared with simple CHF patients (61 +/- 3 versus 42 +/- 4 bursts/min; P < 0.01). Administration of 100% oxygen led to a significant decrease in muscle sympathetic nerve activity in CHF patients with CRF (from 61 +/- 3 to 55 +/- 4 bursts/min; P < 0.05). By contrast, neither 100% oxygen nor room air changed muscle sympathetic nerve activity or hemodynamics in patients with solely CHF.

CONCLUSION

Tonic activation of excitatory chemoreflex afferents contributes to increased efferent sympathetic activity to muscle circulation and to blood pressure control in CHF patients with CRF. These findings may have important implications for understanding how CRF contributes to the progression of CHF and increases morbidity and mortality in CHF patients.

Facial cooling and peripheral chemoreflex mechanisms in humans

AIM

Reductions in arterial oxygen partial pressure activate the peripheral chemoreceptors which increase ventilation, and, after cessation of breathing, reduce heart rate. We tested the hypothesis that facial cooling facilitates these peripheral chemoreflex mechanisms.

METHODS

Chemoreflex control was assessed by the ventilatory response to hypoxia (10% O2 in N2) and the bradycardic response to voluntary end-expiratory apnoeas of maximal duration in 12 young, healthy subjects. We recorded minute ventilation, haemoglobin O2 saturation, RR interval (the time between two R waves of the QRS complex) and the standard deviation of the RR interval (SDNN), a marker of cardiac vagal activity throughout the study. Measurements were performed with the subject's face exposed to air flow at 23 and 4 degrees C.

RESULTS

Cold air decreased facial temperature by 11 degrees C (P < 0.0001) but did not affect minute ventilation during normoxia. However, facial cooling increased the ventilatory response to hypoxia (P < 0.05). The RR interval increased by 31 +/- 8% of the mean RR preceding the apnoea during the hypoxic apnoeas in the presence of cold air, compared to 17 +/- 5% of the mean RR preceding the apnoea in the absence of facial cooling (P < 0.05). This increase occurred despite identical apnoea durations and reductions in oxygen saturation. Finally, facial cooling increased SDNN during normoxia and hypoxia, as well as during the apnoeas performed in hypoxic conditions (all P < 0.05).

CONCLUSION

The larger ventilatory response to hypoxia suggests that facial cooling facilitates peripheral chemoreflex mechanisms in normal humans. Moreover, simultaneous diving reflex and peripheral chemoreflex activation enhances cardiac vagal activation, and favours further bradycardia upon cessation of breathing.

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.