An insight into the autonomic and haemodynamic mechanisms underlying reflex syncope in children and adolescents: a multiparametric analysis.

Around 15% of children and adolescents experience at least one episode of syncope until adulthood. Excluding cardiac disease, the majority of syncopes are of reflex origin and benign in nature. In this situation, a tilt test is conducted to reproduce symptoms and to evaluate cardiovascular adaptations to orthostatism, but its mechanisms are not yet well defined. Here, we investigated haemodynamics and autonomic activity during tilt in young patients. Patients (n=113) with unexplained syncope were enrolled. Tilt followed a standard protocol without provocative agents. A positive response (fainters) was defined as a sudden development of syncope or presyncope associated with hypotension, bradycardia, or both. Haemodynamic parameters, autonomic activity, and baroreflex sensibility were evaluated. Data were analysed on baseline; immediately after tilting; on tilt adaptation; before fainting or before tilt-down for non-fainters; and on tilt-down. A total of 45 patients experienced syncope after a mean time of 18 minutes. During tilting up, fainters showed lower blood pressure and peripheral resistance values, which decreased progressively with time together with baroreflex sensibility. Sympathetic tone increased massively along time till syncope. No changes in cardiac output and heart rate were observed. Results show a strong effort of the autonomic nervous system to adapt to orthostatic stress through different magnitudes of sympathetic output, which was maximal before syncope without apparent modifications of parasympathetic tone. These changes suggest an imbalance between both branches of the autonomic nervous system, not enabling a time-progressive adaptation and leading the subject to faint.

[Tilt training increases vasoconstrictor reserve in patients with neurocardiogenic syncope]. / O treino de ortostatismo (tilt training) aumenta a reserva vasoconstritora em doentes com síncope reflexa neurocardiogénica.

UNLABELLED: Neurocardiogenic syncope (NCS) is a common clinical entity resulting from an excessive reflex autonomic response, particularly during orthostatism. Treatment options are controversial and of limited effectiveness. Tilt training (TT) is a promising option to treat these patients. However, its mechanism of action and clinical impact remain unclear. OBJECTIVE: To characterize hemodynamic and autonomic responses during a TT program in patients with NCS refractory to conventional measures. METHODS: We studied 28 patients (50% male, mean age 41±14 years) without structural heart disease, with NCS documented by tilt testing. The TT program included 9 tilt sessions (3 times a week, 30 min) (60° - 6 sessions, 70° - 3 sessions), under ECG and blood pressure monitoring combined with home orthostatic self-training and 10° head-up during sleep. Systolic volume, cardiac output, total peripheral resistance, baroreflex sensitivity and heart-rate variability were computed. Patients were reassessed at 1 month and every 6 months for a maximum of 36 months (24±12 months). RESULTS: Over the course of the TT program there was a significant increase in total peripheral resistance (1485±225 vs. 1591±187 dyn·s·cm(-5), p<0.05), with a decrease in standard deviation (206±60 vs. 150±42, p<0.05). During follow-up, syncope recurred in five patients (19%), with a significant reduction in the number of episodes (4.0±3.2/patient in the 12 months before TT vs. 1.4±0.8/patient post-TT, p<0.05). CONCLUSION: In refractory NCS, TT may be an effective therapeutic option, with long-term benefits. These results appear to be due to an increase in vasoconstrictor reserve combined with a reduction in its variance.

[Acute electrophysiological modulation of the atria and pulmonary veins: effects of sympathetic and parasympathetic interaction on atrial fibrillation inducibility]. / Modulação eletrofisiológica das auriculas e veias pulmonares: interação da estimulação simpática e parassimpática na inducibilidade de fibrilhação aguda auricular num modelo experimental in vivo.

UNLABELLED: Atrial fibrillation (AF) is a complex disease with multiple mechanisms, involving the interaction between the autonomic nervous system (ANS), electrophysiological properties of the atria and pulmonary veins (PVs), and vulnerability for AF. AIM: We assessed the effects of acute vagal (vagus_stim) and sympathetic stimulation (symp_stim) on atrial conduction, atrial and PV refractoriness and inducibility of AF in an in vivo rabbit model with preserved autonomic innervation. METHODS: An open-epicardial approach was used in 17 anesthetized and artificially ventilated New Zealand white rabbits. The ECG was recorded with bipolar subcutaneous electrodes placed in the four limbs. Electrograms were obtained with four monopolar electrodes placed epicardially along the atria, and a circular electrode adapted to the proximal PVs. The cervical vagus nerve and thoracic sympathetic trunk were stimulated with bipolar electrodes. Epicardial activation was recorded in sinus rhythm, and effective refractory periods (ERPs) and conduction times from the high-lateral right atrium (RA) to the high-lateral left atrium (LA) and PVs were quantified at baseline and during vagus_stim, symp_stim, or combined vagal and sympathetic stimulation (dual_stim). Burst pacing (50 Hz, 10 s), alone or combined with vagus_stim, symp_stim or dual_stim, was performed in the right (RAA) and left atrial appendage (LAA) and PVs to test for AF inducibility. RESULTS: At baseline, ERPs were higher in the LAA and there was a delay in the conduction time from RA to PV, compared to the mean activation time from RA to LA. During vagus_stim or dual_stim, ERP decreased significantly at all sites, and baseline interatrial activation times changed from 20 +/- 4 ms to 30 +/- 10 ms and 31 +/- 11 ms, respectively (p < 0.05). Symp_stim resulted in a significant decrease in ERPs only in the LAA, and a reduction of the interatrial interval to 16 +/- 11 ms (p < 0.05 vs baseline). AF inducibility ranged from 35% to 53% with baseline 50 Hz pacing, 65% to 76% during vagus_stim or symp_stim, and 75% to 100% with dual_stim (p < 0.05). AF duration increased significantly during ANS stimulation. In two-thirds of the animals with longer inducible AF, the arrhythmia ceased immediately after cessation of vagus_stim. CONCLUSIONS: In the fully innervated rabbit heart in vivo, acute ANS stimulation shortens atrial and PV refractoriness, and significantly changes atrial conduction times, promoting AF induction and prolonging the arrhythmia. This underscores the importance of acute variations in ANS tone and its interactions in the pathophysiology of AF.

Interplay of upper and lower motor neuron degeneration in amyotrophic lateral sclerosis.

OBJECTIVE: We studied motor unit recruitment to test a new method to identify motor unit firing rate (FR) variability. METHODS: We studied 68 ALS patients, with and without upper neuron signs (UMN) in lower limbs, 24 patients with primary lateral sclerosis (PLS), 13 patients with spinal cord lesion and 39 normal subjects. All recordings were made from tibialis anterior muscles of normal strength. Subjects performed a very slight contraction in order to activate 2 motor units in each recording. 5-7 motor unit pairs were recorded in each subject. Mean consecutive differences (MCD) were calculated for each pair of potentials. The mean MCD for each muscle was estimated as the mean from the total number of pairs recorded. Ap value<0.01 was accepted as significant. RESULTS: MCD of FR frequency was less in the subjects with spinal cord lesion and PLS. In addition, the FR frequency of the 1st motor unit in a pair of units was markedly reduced in PLS, and in subjects with spinal cord lesions. CONCLUSION: These results support a lower threshold and reduced FR fluctuation in spinal motor neurons of spastic patients. SIGNIFICANCE: This method can be developed for detection of UMN lesions.

Lead toxicity promotes autonomic dysfunction with increased chemoreceptor sensitivity.

Mortality and morbidity by toxic metals is an important issue of occupational health. Lead is an ubiquitous heavy metal in our environment despite having no physiological role in biological systems. Being an homeostatic controller is expected that the autonomic nervous system would show a degree of impairment in lead toxicity. In fact, sympathoexcitation associated to high blood pressure and tachypnea has been described together with baroreflex dysfunction. However, the mechanisms underlying the autonomic dysfunction and the interplay between baro- and chemoreflex are not yet fully clarified. The angiotensinogenic PVN-NTS axis (paraventricular nucleus of the hypothalamus - nucleus tractus solitarius axis) is a particularly important neuronal pathway that could be responsible for the autonomic dysfunction and the cardiorespiratory impairment in lead toxicity. Within the current work, we addressed in vivo, baro- and chemoreceptor reflex behaviour, before and after central angiotensin inhibition, in order to better understand the cardiorespiratory autonomic mechanisms underlying the toxic effects of long-term lead exposure. For that, arterial pressure, heart rate, respiratory rate, sympathetic and parasympathetic activity and baro- and chemoreceptor reflex profiles of anaesthetized young adult rats exposed to lead, from foetal period to adulthood, were evaluated. Results showed increased chemosensitivity together with baroreceptor reflex impairment, sympathetic over-excitation, hypertension and tachypnea. Chemosensitivity and sympathetic overexcitation were reversed towards normality values by NTS treatment with A-779, an angiotensin (1-7) antagonist. No parasympathetic changes were observed before and after A-799 treatment. In conclusion, angiotensin (1-7) at NTS level is involved in the autonomic dysfunction observed in lead toxicity. The increased sensitivity of chemoreceptor reflex expresses the clear impairment of autonomic outflow to the cardiovascular and respiratory systems induced by putative persistent, long duration, alert reaction evoked by the long term exposure to lead toxic effects. The present study brings new insights on the central mechanisms implicated in the autonomic dysfunction induced by lead exposure which are relevant for the development of additional therapeutic options to tackle lead toxicity symptoms.

Reversing gene expression in cardiovascular target organs following chronic depression of the paraventricular nucleus of hypothalamus and rostral ventrolateral medulla in spontaneous hypertensive rats.

BACKGROUND: Chronic overexpression of an inwardly rectifying potassium channel (hKir2.1) in the paraventricular nucleus of the hypothalamus (PVN) and in the rostral ventrolateral medulla (RVLM) to suppress neuronal excitability, resulted in a long term decrease of blood pressure and sympathetic output in spontaneously hypertensive rats (SHR). OBJECTIVE: Evaluate gene expression in end-organs of SHR after a chronic overexpression of hKir2.1 channels in either the PVN or RVLM. METHODS: mRNA levels of 16 genes known to be involved with blood pressure regulation were evaluated using RT-PCR in tissues from the heart, common carotid artery and kidney of SHR submitted to chronic depression of PVN and RVLM excitability using a lentiviral vector (LVhKir2.1). RESULTS: In SHR hearts in which either the PVN or RVLM were injected with LVhKir2.1, there was a downregulation of angiotensin II receptor 1b (AT1), ATPase, Ca(2+)-transporter, troponin T2 and tropomyosin2 (only in RVLM) relative to the sham group. In the kidney of SHR with LVhKir2.1 injections in PVN and RVLM, angiotensinogen, angiotensin II receptor2 (AT2) and endothelin1 were all upregulated compared to sham. In the carotid artery, endothelin2, endothelin receptor A and B were up-regulated following LVhKir2.1 in to either the PVN or RVLM relative to sham. CONCLUSION: Chronic overexpression of hKir2.1 channels in PVN and RVLM, promoted a BP decrease with up-regulation of angiotensinogen and AT2 genes expression in the kidney and down-regulation of AT1 in the heart of SHR. Thus, we demonstrate the potential efficacy of central manipulation to protect against end-organ damage in essential hypertension.

The arterial baroreflex effectiveness index in risk stratification of chronic heart failure patients who are candidates for cardiac resynchronization therapy.

INTRODUCTION: Baroreflex function is an independent marker of prognosis in heart failure (HF). However, little is known about its relation to response to cardiac resynchronization therapy (CRT). The aim of this study is to assess arterial baroreflex function in HF patients who are candidates for CRT. METHODS: The study population consisted of 25 patients with indication for CRT, aged 65±10 years, NYHA functional class ≥III in 52%, QRS width 159±15 ms, left ventricular ejection fraction (LVEF) 29±5%, left ventricular end-systolic volume (LVESV) 150±48 ml, B-type natriuretic peptide (BNP) 357±270 pg/ml, and peak oxygen consumption (peak VO2) 18.4±5.0 ml/kg/min. An orthostatic tilt test was performed to assess the baroreflex effectiveness index (BEI) by the sequence method. This group was compared with 15 age-matched healthy individuals. RESULTS: HF patients showed a significantly depressed BEI during tilt (31±12% vs. 49±18%, p=0.001). A lower BEI was associated with higher BNP (p=0.038), lower peak VO2 (p=0.048), and higher LVESV (p=0.031). By applying a cut-off value of 25% for BEI, two clusters of patients were identified: lower risk cluster (BEI >25%) QRS 153 ms, LVESV 129 ml, BNP 146 pg/ml, peak VO2 19.0 ml/kg/min; and higher risk cluster (IEB ≤25%) QRS 167 ms, LVESV 189 ml, BNP 590 pg/ml, peak VO2 16.2 ml/kg/min. CONCLUSIONS: Candidates for CRT show depressed arterial baroreflex function. Lower BEI was observed in high-risk HF patients. Baroreflex function correlated closely with other clinical HF parameters. Therefore, BEI may improve risk stratification in HF patients undergoing CRT.

Autonomic modulation in a patient with syncope and paroxysmal atrial-fibrillation.

We report a case of a patient with recurrent syncope and paroxysmal atrial fibrillation whose clinical status greatly improved after a period of orthostatic training. The potential efficacy of this non-pharmacological measure in modulating the autonomic tone is discussed below.