Neurally mediated syncope: a review of cardiac and arterial receptors

Evaluation of the effect of radiofrequency catheter ablation on autonomic function in patients with atrioventricular nodal reentrant tachycardia by head-up tilt table test

Background:

One of the recommended treatments for atrioventricular nodal reentrant tachycardia (AVNRT), is radiofrequency catheter ablation (RFCA). However, RFCA may affect the autonomic system. This study aims to evaluate the effect of RFCA on autonomic system in patients with PSVT by head-up tilt table (HUTT) test.

Materials and Methods:

In a before–after study, 22 patients with PSVT were enrolled. Data were collected with a data collection form that included two parts. Electrocardiogram (ECG), echocardiogram, 24-h Holter monitoring, HUTT test, heart rate variability (HRV) indexes, and symptoms of all patients were recorded 24 h before and 1 month after the ablation. Wilcoxon, McNemar, Mann–Whitney U, and Chi-square tests were used to analyze the data.

Results:

Of the total 22 patients, 31.8% were male and 68.2% were female. There were significant differences in heart palpitation (P < 0.0001) and non-specific symptoms (P = 0.031) and no significant difference in head-up tilt test results and HRV indices before and after RFCA. The results showed that there were no significant differences in specific and non-specific symptoms in patients with AVNRT with positive and negative HUTT before and after RFCA.

Conclusions:

The observed difference in heart palpitation and non-specific symptoms emphasized the role of AVNRT in causing these symptoms. Autonomic dysfunction is more probably an accompanying condition of AVNRT than causing symptoms. We could not find any significance in the results of HUTT after RFCA. HUTT cannot determine or predict the symptoms after RFCA.

Severe autonomic failure in moderate to severe aortic stenosis: prevalence and association with hemodynamics and biomarkers

BACKGROUND

Severe autonomic failure (SAF) refers to combined abnormalities in reflex and tonic autonomic function. SAF indicates increased risk of death in post-infarction and heart failure patients, but has not been studied in aortic stenosis (AS). Here, we investigated SAF in patients with AS and tested its correlation with hemodynamic and biochemical markers.

METHODS

We prospectively enrolled 174 patients with moderate to severe AS in sinus rhythm (age 76 ± 9 years; mean aortic valve area 0.9 ± 0.3 cm(2)). Heart rate turbulence (as marker of autonomic reflex activity) and deceleration capacity (as marker of autonomic tonic activity) were calculated from 24-h Holter recordings. According to the previously published technology, SAF was considered present if both factors were abnormal.

RESULTS

44 (25.3%) of the 174 patients had signs of SAF. Patients with SAF had lower left ventricular ejection fraction (LVEF: 48.1 vs. 54.8%; p = 0.002), lower mean aortic gradients (28 vs. 34 mmHg, p = 0.019), higher systolic pulmonary artery pressures (46.8 vs. 40.9 mmHg, p = 0.028), higher levels of brain natriuretic peptide (905 vs. 407 ng/l; p = 0.003) and higher levels of high sensitive troponin I (0.65 vs. 0.24 μg/l; p = 0.013). Impaired LVEF (≤50%) was the only independent factor associated with SAF, but only explained autonomic abnormalities in less than half of the patients.

CONCLUSIONS

In patients with moderate to severe AS prevalence of SAF is high. SAF correlates with hemodynamic and biochemical markers indicating increased risk. Future studies should evaluate the prognostic value of SAF in patients with AS.

Tilt table testing with video EEG monitoring in the evaluation of patients with unexplained loss of consciousness

It is well established that convulsive movements often accompany syncopal events yet many patients with these clinical features are misdiagnosed with seizures and often referred to epilepsy centers because they are refractory to treatment with anticonvulsant medications. Tilt table testing is the gold standard for diagnosing vasodepressor syncope, but it can fail to provide clinical details that help distinguish convulsive syncope from epileptic seizures and psychogenic events. This study evaluates the diagnostic utility of the addition of video and EEG monitoring during tilt table testing for patients with refractory episodes of unexplained loss of consciousness. Retrospective analysis was performed of 40 consecutive patients who were referred to the Emory Epilepsy Center and underwent tilt table testing with concomitant video-EEG between March 1, 2007 and December 1, 2008. EEG was recorded throughout the study in addition to video recording and single channel EKG. Events were classified as vasodepressor syncope, presyncope, or psychogenic. Tilt combined with video EEG was diagnostic in 26/40 (65%) of patients. Vasodepressor syncope was seen in 17/40 (42.5%), 9 of which had associated involuntary movements. Three patients experienced psychogenic non-epileptic events. Antiepileptic drugs (AEDs) were being prescribed for 17 patients, 7 of which were discontinued as a result of the testing. The majority of patients (38/40) had undergone prior neurological and cardiac evaluation with routine EEG, neuroimaging and/or Holter monitoring. Patients with convulsive syncope are often misdiagnosed and treated with AEDs despite prior neurodiagnostic and cardiac evaluation. Tilt table testing with video-EEG is useful in patients with refractory episodes of unexplained loss of consciousness and can avoid expensive non-diagnostic evaluations as well as ongoing treatment with unnecessary AEDs.

Theoretical and electrophysiological evidence for axial loading about aortic baroreceptor nerve terminals in rats

Arterial baroreceptors are essential for neurocirculatory control, providing rapid hemodynamic feedback to the central nervous system. The pressure-dependent discharge of carotid and aortic baroreceptor afferents has been extensively studied. A common assumption has been that circumferential deformation of the arterial wall is the predominant mechanical force affecting baroreceptor discharge. However, in vivo the arterial tree is under significant longitudinal tension, leading to the hypothesis that axially directed forces may contribute to baroreceptor function. To test this hypothesis, we utilized a combination of finite element modeling methods and an in vitro rat aortic arch preparation. Model formulation utilized traditional analytic constructs available in the literature followed by refinement of model material parameters through direct comparison of computationally and experimentally generated pressure-diameter curves. The numerical simulations strongly indicated a functional role for axial loading within the region of the baroreceptive nerve terminal. This prediction was confirmed through single-fiber recording of baroreceptor nerve discharge under conditions with and without longitudinal tension in the vessel preparation. The recordings (n = 5) demonstrated that longitudinal tension significantly (P < 0.02) lowered both the pressure threshold (P(th), mmHg) for baroreceptor discharge and sensitivity (S(th), Hz/mmHg). The effect was nearly instantaneous and sustained; i.e., under longitudinal tension average P(th) was 84 +/- 3 mmHg and S(th) was 0.71 +/- 0.15 Hz/mmHg, which immediately increased to a P(th) of 94 +/- 4 mmHg and a S(th) of 1.20 +/- 0.32 Hz/mmHg with loss of axial tension. Possible explanations of how an abrupt change in axial loading could result in a synchronized increase in afferent drive of the baroreceptor reflex, and the potentiating effect this could have on neurogenically mediated orthostatic intolerance are discussed.

Perioperative vasovagal syncope with focus on obstetric anesthesia

Vasovagal syncope refers to a reflex cardiovascular depression that gives rise to loss of consciousness with bradycardia and profound vasodilatation. This response commonly occurs during regional anesthesia, hemorrhage or supine inferior vena cava compression in pregnancy. The changes in circulatory response from the normal maintenance of arterial pressure to parasympathetic activation and sympathetic inhibition may cause severe hypotension. This change is triggered by reduced cardiac venous return as well as episodes of emotional stress, excitement or pain. Occasionally, these vasovagal responses may be unpredictable and may dramatically proceed to asystole with circulatory collapse, and may even result in death. In these circumstances, hypotension may be more severe than that caused by bradycardia alone, because of unappreciated vasodilatation. Regional anesthesia, decreased venous return, hemorrhage and abnormal fetal presentation cumulatively increase the risk of vasovagal syncope in cesarean section patients. When a vasovagal response occurs, ephedrine is the drug of first choice because of its combined action on the heart and peripheral blood vessels. Epinephrine must be used early in established cardiac arrest, especially after high regional anesthesia.

Reversal of Inappropriate Peripheral Vascular Responses in Hypertrophic Cardiomyopathy

OBJECTIVES

We assessed the frequency of abnormal forearm vasodilator responses during lower body negative pressure (LBNP) in 21 non-obstructive hypertrophic cardiomyopathy (HCM) patients (31 +/- 8 [20 to 43] years) with abnormal blood pressure response (ABPR) to exercise and the effects of three drugs used to treat vasovagal syncope (propranolol, clonidine, and paroxetine) in a double-blind crossover study.

BACKGROUND

Some HCM patients have an ABPR to exercise, which may be due to paradoxical peripheral vasodilatation. A similar proportion has paradoxical forearm vasodilatation during central volume unloading using LBNP. These abnormal reflexes may be caused by left ventricular mechanoreceptor activation. Similar mechanisms may also contribute to some cases of vasovagal syncope.

METHODS

Blood pressure changes were assessed during exercise, and forearm vascular responses and baroreceptor sensitivity were assessed during LBNP using plethysmography.

RESULTS

Nine (43%) patients (group A) had paradoxical vasodilator responses (forearm vascular resistance [FVR] fell by 7.5 +/- 4.6 U), and 12 (57%) patients (group B) had normal vasoconstrictor responses during LBNP (FVR increased by 7.7 +/- 4.9 U). Paroxetine augmented systolic blood pressure (SBP) during exercise in group A (21 +/- 6 mm Hg vs. 14 +/- 11 mm Hg at baseline, p = 0.02); no effect was detected in group B. Paroxetine reversed paradoxical vascular responses during LBNP in seven (78%) patients from group A. Propranolol and clonidine had no significant effect on SBP during exercise but reversed paradoxical vascular responses in some patients from group A (n = 5 and n = 3).

CONCLUSIONS

Paradoxical vasodilatation during LBNP occurs in 40% of patients with ABPR during exercise and is reversed by propranolol, clonidine, and paroxetine. Paroxetine also improved SBP response to exercise.

Dysfunctional baroreflex regulation of sympathetic nerve activity in patients with vasovagal syncope

BACKGROUND

The interplay of resting muscle sympathetic nerve activity (MSA) and the baroreceptor reflex in patients with vasovagal syncope remains elusive. Hence, the aim of the present study was to investigate MSA, baroreceptor sensitivity, heart rate, and blood pressure under resting conditions and during orthostatic stress in patients with a history of vasovagal syncope.

METHODS AND RESULTS

MSA was measured using microneurography at rest and during lower body negative pressure (LBNP) to mimic orthostatic stress in patients with a history of vasovagal syncope (n=10) and in age-matched healthy controls (n=8). Heart rate and blood pressure were simultaneously recorded. Cardiac baroreceptor sensitivity was calculated with the spectral technique (alpha coefficient). Resting MSA in the patients with syncope was significantly increased as compared with controls (42.4+/-2.3 versus 26.5+/-3.6 bursts/min, P=0.001), whereas activation of MSA during orthostatic stress in the patient group was significantly blunted (5.1+/-1.6 versus 15.2+/-2.1 bursts/min at LBNP -50 mm Hg, P=0.002). In the patients with syncope, cardiac baroreceptor sensitivity was significantly reduced under supine resting conditions (8.5+/-0.7 versus 13.0+/-1.1 ms/mm Hg, P=0.001), as well as under orthostatic stress (7.3+/-0.7 versus 13.4+/-1.5 ms/mm Hg, P=0.003).

CONCLUSIONS

This study shows that in patients with vasovagal syncope, resting MSA is increased and baroreflex regulation during orthostatic stress is blunted, thus leading to impaired MSA adaptation. These results provide new insights into mechanisms of vasovagal syncope and suggest that pharmacological modulation of baroreceptor sensitivity may represent a promising treatment of neuromediated syncope.

Cardiovascular collapse caused by carbon dioxide insufflation during one-lung anaesthesia for thoracoscopic dorsal sympathectomy

Carbon dioxide insufflation into the pleural space during one-lung anaesthesia for thoracoscopic surgery is used in some centres to improve surgical access, even though this practice has been associated with well-described cardiovascular compromise. The present report is of a 35-year-old woman undergoing thoracoscopic left dorsal sympathectomy for hyperhidrosis. During one-lung anaesthesia the insufflation of carbon dioxide into the non-ventilated hemithorax for approximately 60 seconds, using a pressure-limited gas inflow, was accompanied by profound bradycardia and hypotension that resolved promptly with the release of the gas. Possible mechanisms for the cardiovascular collapse are discussed, and the role of carbon dioxide insufflation as a means of expediting lung collapse for procedures performed using single-lung ventilation is questioned.

Perioperative bradycardia and asystole: relationship to vasovagal syncope and the Bezold-Jarisch reflex

Reflex cardiovascular depression with vasodilation and bradycardia has been variously termed vasovagal syncope, the Bezold-Jarisch reflex and neurocardiogenic syncope. The circulatory response changes from the normal maintenance of arterial pressure, to parasympathetic activation and sympathetic inhibition, causing hypotension. This change is triggered by reduced cardiac venous return as well as through affective mechanisms such as pain or fear. It is probably mediated in part via afferent nerves from the heart, but also by various non-cardiac baroreceptors which may become paradoxically active. This response may occur during regional anaesthesia, haemorrhage or supine inferior vena cava compression in pregnancy; these factors are additive when combined. In these circumstances hypotension may be more severe than that caused by bradycardia alone, because of unappreciated vasodilation. Treatment includes the restoration of venous return and correction of absolute blood volume deficits. Ephedrine is the most logical choice of single drug to correct the changes because of its combined action on the heart and peripheral blood vessels. Epinephrine must be used early in established cardiac arrest, especially after high regional anaesthesia.