A placebo-controlled trial of intravenous and oral disopyramide for prevention of neurally mediated syncope induced by head-up tilt

Vasodepressor syncope. Diagnosis and management

Vasodepressor syncope is a common medical problem that can be diagnosed through an accurate history and upright tilt testing. In most cases, patients experience a striking decrease in syncopal episodes following a tilt test, and long-term therapy is not necessary. In the rare patient who experiences no prodrome and continues to experience injury-causing syncope, empiric therapy with drugs or dual-chamber pacing has to be considered despite the lack of controlled trials establishing the efficacy of such therapies.

Neurally mediated syncope and syncope due to autonomic failure: differences and similarities

Syncope is a transient loss of consciousness and postural tone caused by a global reduction of blood flow to the brain. Abnormalities in autonomic cardiovascular control can impair blood supply to the brain and produce syncope in two different disorders: autonomic failure and neurally mediated syncope. In autonomic failure, sympathetic efferent activity is chronically impaired so that vasoconstriction is deficient, upon standing blood pressure always falls (i.e., orthostatic hypotension), and syncope or presyncope occurs. Conversely, in neurally mediated syncope, the failure of sympathetic efferent vasoconstrictor traffic (and hypotension) occurs episodically and in response to a trigger. Between syncopal episodes, patients with neurally mediated syncope have normal blood pressure and orthostatic tolerance. This article reviews the characteristics of autonomic failure and describes in more detail the pathophysiology, diagnosis, and treatment of neurally mediated syncope.

Syncope. A neurologist's viewpoint

Syncope is caused by a global reversible reduction of blood flow to the brain. Three hemodynamic abnormalities can cause syncope: (1) a fall in systemic blood pressure because of ineffective control of peripheral vascular resistance, (2) an acute decrease in cardiac output, and (3) an acute increase in cerebrovascular resistance. Complicating the differential diagnosis of syncope are other causes of loss of consciousness, such as seizures, metabolic disorders, and psychiatric disorders, which may simulate syncope.

Neural circulatory control in vasovagal syncope

The orthostatic volume displacement associated with the upright position necessitates effective neural cardiovascular modulation. Neural control of cardiac chronotropy and inotropy, and vasomotor tone aims at maintaining venous return, thus opposing gravitational pooling of blood in the lower part of the body. The present concept of the vasovagal response or "common faint" implicates the development of inappropriate cardiac slowing due to sudden augmentation of efferent vagal activity, and arteriolar dilatation by sudden reduction or cessation of sympathetic activity. The venous pooling associated with lasting orthostatic stress results in development of central hypovolemia. At a certain point during the ongoing reflex adaptation to the hypovolemia in progress, a depressor reflex is set in train. The depressor reflex input along this second "peripheral" afferent pathway is postulated to originate from various sites in the cardiovascular system but remains uncertain. The common faint in humans is of both vaso- and vagal origin; the pure vagal response is less common than its vasodepressor variant. There is strong evidence for an early loss of vasomotor tone in the majority of fainting subjects. Blocking the vagus nerve or cardiac pacing is not of much help in preventing vasovagal syncope; though atropine or pacing may prevent bradycardia in vasovagal fainting, they have never been proven to prevent hypotension. Baroreflex modulation of autonomic outflow remains present during the presyncopal stages until it becomes offset by an opposing depressor reflex with relative bradycardia and relaxation of arterial resistance vessels. The nature of the vasodilatation associated with the vasovagal response has still not been settled.

Clinical experience with Thera DR rate-drop response pacing algorithm in carotid sinus syndrome and vasovagal syncope. The International Rate-Drop Investigators Group

This study examined the effectiveness of cardiac pacing using the Thera DR rate-drop response algorithm for prevention of recurrent symptoms in patients with carotid sinus syndrome (CSS) or vasovagal syncope. The algorithm comprises both diagnostic and treatment elements. The diagnostic element consists of a programmable "window" used to identify heart rate changes compatible with an evolving neurally mediated syncopal episode. The treatment arm consists of pacing at a selectable rate and for a programmable duration. Forty-three patients (mean age 53 +/- 20.4 years) with CSS alone (n = 8), CSS in conjunction with vasovagal syncope (n = 4), or vasovagal syncope alone (n = 31) were included. Thirty-nine had recurrent syncope, while the remaining four reported multiple presyncopal events. Prior to pacing, 40 +/- 152 syncopal episodes (range from 1 to approximately 1,000 syncopal events) over the preceding 56 +/- 84.5 months. Postpacing follow-up duration was 204 +/- 172 days. Three patients have been lost to follow-up and in one patient the algorithm was disabled. Among the remaining 39 individuals, 31 (80%) indicated absence or diminished frequency of symptoms, or less severe symptoms. Twenty-three patients (23/29, or 59%) were asymptomatic with respect to syncope or presyncope. Sixteen patients had symptom recurrences. Of these, seven experienced syncope (7/39, or 18%) and 9 (29%) had presyncope: the majority of patients with recurrences (6/7 syncope and 7/9 presyncope) were individuals with a history of vasovagal syncope. Consequently, although symptoms were observed during postpacing follow-up, they appeared to be of reduced frequency and severity. Thus, our findings suggest that a transient period of high rate pacing triggered by the Thera DR rate-drop response algorithm was beneficial in a large proportion of highly symptomatic patients with CSS or vasovagal syncope.

Head-up tilt test. A nonspecific method of evaluating patients with bifascicular block

BACKGROUND

Patients with bifascicular block have an increased risk of syncopal attacks, but the underlying mechanism often remains unclear despite an extensive diagnostic workup. The head-up tilt test has been established as an important diagnostic tool in the unmasking of vasovagal syncope in patients with unexplained syncope. Its role in the evaluation of patients with bifascicular block has not been studied.

METHODS AND RESULTS

A head-up tilt test, using a 60 degrees angle of tilt for 45 minutes without pharmacological provocation, was performed in 25 patients with bifascicular block and syncope that remained unexplained after an extensive invasive and noninvasive electrophysiological investigation. As a control group, 25 subjects with bifascicular block without syncope, matched for age, sex, left ventricular function, and underlying heart disease, were included. A positive head-up tilt test was found in 7 (28%) of the syncope patients and in 8 (32%) of the control subjects (P = NS). Six patients, of whom 3 had a positive tilt test, had recurrent syncopal attacks during 32 months of follow-up. None of the control subjects had syncope during follow-up.

CONCLUSIONS

This study gives rise to serious concern regarding the specificity of the head-up tilt test in patients with bifascicular block. A head-up tilt test should therefore be interpreted with caution, and its role as a diagnostic tool in this patient category remains to be established.

Response to beta blockers in patients with neurocardiogenic syncope: how to predict beneficial effects

No definitive data are available about the possibility of predicting improvement in patients with neurocardiogenic syncope treated with beta blockers. Among 112 patients with syncope and a positive head-up tilt test (HUT), independent predictors for prevention of symptoms with beta blockers were determined using the Cox proportional hazards model. Each patient underwent HUT at 70 degrees for 20 minutes both in the drug-free state and during isoproterenol infusion given to increase the heart rate by at least 25%. Fifty-nine patients had a positive HUT during isoproterenol infusion and 53 in the drug-free state. All patients were then given esmolol infusion at 500 micrograms/kg per minute for 3 minutes followed by 300 micrograms/kg per minute maintenance dose. HUT was then repeated as previously described with or without isoproterenol, depending upon the initial positive response. Regardless of the response during esmolol, all patients were treated with metoprolol 50 to 100 mg twice daily. At follow-up, 36 patients experienced symptom relapse. Four of them had negative HUT on esmolol, whereas the remaining 32 did not respond to the acute infusion of esmolol. Only four patients with positive HUT on esmolol had a favorable response to metoprolol. Patients responding to metoprolol were older (55 +/- 12 years vs 42 +/- 15 years, P < 0.05). Response to metoprolol was predicted by a negative test on esmolol (P < 0.0001) and a positive HUT on isoproterenol (P < 0.001). Age older than 42 years was also associated with a higher likelihood of metoprolol success (P < 0.02).

CONCLUSION

Acute challenge with esmolol infusion appears to be an accurate predictor of response to chronic beta blockers, together with age and a positive HUT during low-dose isoproterenol infusion.

Effect of beta blockers on the time to first syncope recurrence in patients after a positive isoproterenol tilt table test

Isoproterenol-headup tilt table testing provides a diagnosis of neuromediated syncope in many patients who faint. The involvement of beta-adrenoceptor stimulation in the provocation of syncope suggests that beta blockers might chronically prevent syncope. To assess this, a cohort of 153 syncope patients (age 39 +/- 20 years) underwent baseline assessment of demographic variables, symptomatic burden, and hemodynamic and clinical responses to tilt testing. Fifty-two patients then received beta blockers, and 101 did not receive drug therapy. The primary outcome was the time to the first recurrent syncopal spell. Actuarial survival analysis was used. Syncope recurred in 17 of 52 patients who received beta blockers and in 28 of 101 patients who were untreated. The actuarial probability of remaining free of syncope was similar in both groups. For example, the probability of remaining free of syncope 12 months following the tilt test was 0.72 in both populations. Thus, treatment with beta blockers may not have a significant effect in preventing syncope recurrence following a positive tilt test.

Syncope and the autonomic nervous system

The autonomic nervous system plays a central role in the maintenance of hemodynamic stability. Dysfunction of this complex regulatory system can lead to the development of loss of consciousness. This article summarizes our current understanding of the role of the autonomic nervous system in maintaining a stable blood pressure and heart rate under normal and abnormal physiologic conditions. The role of baroreceptors, mechanoreceptors, chemoreceptors, vascular reactivity, and the interaction of these sensor systems with the central nervous system as a whole are reviewed. Current concepts related to the mechanisms of unexplained syncope and the "state-of-the-art" diagnostic and treatment options are also discussed.

Recurrence of neurocardiogenic syncope without pharmacologic interventions

In 54 patients with positive tilt and who refused medical therapy, we observed a significant decrease in the frequency of syncopal spells after diagnosis and counseling. However, symptoms were reported at follow-up by 64.8% of the patients and were predicted by the frequency and total number of syncope episodes before upright tilt.

Can serial tilt testing be used to evaluate therapy in neurally mediated syncope?

The role of serial head-up tilt testing for the evaluation of therapeutic efficacy of neurally mediated syncope is reviewed. The evidence available suggests that guiding therapy based on serial head-up tilt response may not be appropriate, and large placebo-controlled trials should be conducted to address this issue.

Isoproterenol infusion provokes vasovagal response without upright tilt in a patient exhibiting syncopal episodes

We report a case of a patient with vasovagal syncope, in whom isoproterenol infusion provoked vasovagal response without upright tilting. We subjected the patient, who had had two previous syncopal and several presyncopal episodes, to upright tilting with isoproterenol infusion. Before a control tilt was performed for 10 min (80 degrees), the patient was placed in the supine position for 5 min. The control tilt did not provoke a vasovagal response. With isoproterenol being infused at a dose of 1 mu g/min, the sequence of positioning in the supine position for 5 min and upright tilting for 10 min was repeated. This dose of isoproterenol infusion did not provoke any vasovagal response in the patient, either in the supine or in the upright position. When the dose of isoproterenol infusion was then increased to 2 mu g/min, the heart rate increased to 121/min, but then suddenly dropped to 74/min; systemic arterial pressure simultaneously fell from 148/80 to 108/80 mmHg. The patient complained of palpitation and anxiety, and showed profound cold sweating. The drop in the heart rate and the fall in blood pressure occurred when the patient was in the supine position, indicating that, unlike upright tilting with isoproterenol infusion, venous return was not decreased at the beginning of vasovagal response in this setting. This observation suggests that isoproterenol infusion, even without upright tilting, may provoke the vasovagal response in some patients.

Efficacy of pindolol for treatment of vasovagal syncope

The purpose of this study was to evaluate the efficacy, safety, and tolerance of pindolol as initial therapy for vasovagal syncope. Head-up tilt table testing (HUT) was performed on 192 patients for syncope or near-syncope of unknown cause. Forty-four (23%) patients had a positive HUT for vasovagal syncope, and 28 (64%) received oral pindolol as initial therapy. Three patients were lost to follow-up; of the remaining 25 patients (mean age 60 +/- 22 years), 15 were women, 14 had syncope, and 11 had near-syncope. At 14 +/- 6 months' follow-up, 16 (64%) patients were without recurrence or side effects from pindolol. Of the 9 patients who stopped taking pindolol, 3 were switched to another regimen for recurrent symptoms, 2 stopped because of side effects, and 4 did not comply with the regimen. In conclusion, pindolol appears to be safe and effective as initial treatment for vasovagal syncope.

Observations on midodrine in a case of vasodepressor neurogenic syncope

A 43-year-old man presented with recurrent syncope and dizziness after he had a dual chamber pacemaker fitted for presumed sino-atrial disease. Head-up tilt produced vasodepressor neurocardiogenic syncope, despite appropriate heart rate support during pacing, and reproduced symptoms. Symptoms were not improved by disopyramide. A double-blind cross-over trial of midodrine, an alpha-receptor agonist, was effective in reducing symptoms: it abolished syncope and reduced frequency and severity of dizziness, coupled with improved haemodynamic responses to head-up tilt.

Evolution of patients with clinical neurocardiogenic (vasovagal) syncope not subjected to specific treatment

The purpose of this prospective study was to analyze the natural history of patients with clinical neurocardiogenic syncope. The results obtained with some therapeutic strategies in the prevention of neurocardiogenic syncope are encouraging. However, the benefit they represent when compared with the natural history of syncope has not been clearly established. Fifty-six consecutive patients with either (1) one or more episodes of syncope and a positive head-up tilt test or (2) typical history of vasovagal syncope despite a negative tilt test were monitored during a mean period of 16.1 +/- 10.2 months. Patients were discharged without any specific medication. There were no deaths during the follow-up period. Recurrences were found in 5 (8.9%) patients. The risk of recurrence was 7% after 1 year and 15% after 21 months. The prognosis of patients with clinical neurocardiogenic syncope is excellent as far as survival is concerned. Recurrences of episodes without specific treatment were uncommon after diagnosis had taken place. This finding strengthens the need to evaluate the effect of drugs with placebo control groups and to select the highest-risk group for treatment.

Left ventricular dimensions and autonomic balance during head-up tilt differ between patients with isoproterenol-dependent and isoproterenol-independent neurally mediated syncope

OBJECTIVES

This study sought to elucidate differences in mechanisms of neurally mediated syncope between patients with syncope induced by head-up tilt alone and those requiring isoproterenol infusion to induce syncope during head-up tilt.

BACKGROUND

Some patients with neurally mediated syncope require isoproterenol to induce syncope during head-up tilt (isoproterenol dependent), and others do not (isoproterenol independent). Differences in mechanisms between these two groups have not been well elucidated.

METHODS

A 60 degrees head-up tilt test was performed in 13 patients with isoproterenol-independent syncope (Group I, mean [+/- SD] age 28 +/- 12 years), 14 patients with isoproterenol-dependent syncope (Group II, mean age 34 +/- 14 years) and 20 control subjects without syncope (Group III, mean age 32 +/- 12 years). Left ventricular size and contractility were determined by echocardiography, and sympathovagal balance was determined with power spectral analysis of heart rate variability using a maximal entropy method.

RESULTS

Group I patients had smaller left ventricular dimensions than Group II and III during baseline tilt. During head-up tilt with isoproterenol infusion (0.01 to 0.04 microgram/kg body weight per min), left ventricular dimensions decreased to the same extent in Groups II and III, but fractional shortening was greater in Group II than in Group III at the end of the tilt. The ratio of low (0.05 to 0.15 Hz) to high frequency (0.15 to 1.0 Hz) component became greater in Group I than in Groups II and III during the last period of baseline tilt. However, the ratio was greater in Group II than in Group III during the last period of the tilt with isoproterenol.

CONCLUSIONS

Patients with isoproterenol-independent syncope had an exaggerated decrease in left ventricular size and sympathetic predominance preceding syncope during head-up tilt. In contrast, in patients with isoproterenol-dependent syncope, similar changes in autonomic nervous balance were evident only during isoproterenol infusion in addition to head-up tilt.

Diagnostic accuracy of a low-dose isoproterenol head-up tilt protocol

The rate of positive head-up tilt (HUT), specificity, and same-day reproducibility of an HUT at 60 degrees combined with a low-dose isoproterenol infusion was assessed in the following patients: 120 consecutive patients with recurrent unexplained syncope, 30 healthy patients in a control group, and 30 patients with documented syncope not related to a vasodepressor reaction. HUT was positive in 61% (73 of 120) of patients with unexplained syncope. The false-positive rate in both the control and documented syncope groups was 6.6%. The mean isoproterenol dose infused was 1.4 +/- 0.5 microgram/min, 1.3 +/- 0.4 micrograms/min, 1.3 +/- 0.5 microgram/min, respectively (p = NS). HUT was positive during the drug free stage in 30 (25%) of 120 patients, and isoproterenol infusion was necessary in the remaining 43 (36%) patients. Immediate reproducibility was assessed in 75 patients, and HUT response was reproduced in 37 (82%) of 45 patients with a baseline positive HUT and in 28 (93%) of 30 patients with a baseline negative response. Overall, "sensitivity," specificity, and reproducibility were 61%, 93%, and 86%, respectively. Clinical variables that increased the probability of a positive outcome were age < or = 50 years and two or more syncopal episodes in the preceding 6 months in the absence of structural heart disease. These data support the use of an HUT protocol with low-dose isoproterenol infusion for the assessment of patients with recurrent syncope.

Efficacy of different treatment strategies for neurocardiogenic syncope

OBJECTIVES

The purpose of this study was to evaluate the efficacy of different therapeutic approaches for patients with a history of syncope and positive head-up tilt testing.

BACKGROUND

Head-up tilt testing has gained broad acceptance as a reliable diagnostic method for the assessment of patients with recurrent unexplained syncope. However, once the diagnosis is established, there is no consensus on the most appropriate treatment. In this respect, efficacy of drug therapy in preventing recurrence of symptoms in such patients is not entirely clear, and controversies exist regarding the need to confirm the effects of pharmacological interventions.

METHODS

Clinical follow-up was obtained in 303 patients with a history of syncope and positive head-up tilt testing. After the diagnostic head-up tilt, patients were assigned to different therapeutic approaches according to their preference or logistic impediments. Of 303 patients, 44 received empiric therapy, 210 were treated with medications proven effective during repeated head-up tilt testing, and 49 refused or discontinued medical therapy. The three groups were similar with regard to age, sex, and clinical presentation. The mean follow-up was 2.8 +/- 1.8 years. Among the patients treated according to head-up tilt guided therapy, 130 were on beta blockers, 35 on theophylline, 10 on ephedrine, 31 on disopyramide, and 4 on miscellaneous regimens. Empiric treatment consisted of beta blockers in 37 of 44 patients and other drugs in the remaining patients.

RESULTS

During the follow-up, recurrence of symptoms was experienced in 12 (6%) of the 210 patients receiving the head-up tilt guided therapy, 16 (36%) of 44 in the empiric therapy group, and 33 (67%) of 49 in the no therapy group. Recurrence of symptoms in patients on empiric or no therapy was significantly more frequent as compared to the head-up tilt guided therapy group (P < 0.01).

CONCLUSIONS

In patients with unexplained syncope and positive upright tilt testing, therapeutic strategies identified on the basis of response during head-up tilt have a more positive impact on the recurrence of symptoms during follow-up.

Limitations of head-up tilt test for evaluating the efficacy of therapeutic interventions in patients with vasovagal syncope: results of a controlled study of etilefrine versus placebo

OBJECTIVES

This study assessed the efficacy of oral etilefrine (10 mg three times a day) in preventing a positive response to head-up tilt testing.

BACKGROUND

Previous reports have suggested that oral etilefrine can be effective either in preventing a positive response to head-up tilt testing or in reducing syncopal recurrences in patients with vasovagal syncope. Up to now most studies assessing drug therapy in these patients have been uncontrolled.

METHODS

This was a randomized double-blind crossover study of etilefrine versus placebo in 30 consecutive patients with syncope and a baseline positive head-up tilt test. After the first test, patients had no treatment for 3 days and were randomized to receive etilefrine or placebo for 4 additional days. They underwent tilt testing under treatment and again after 3 days of washout; they then received the alternative treatment for 4 days, and a third test was performed.

RESULTS

Head-up tilt test results were negative in 13 (43%) patients with etilefrine and 15 (50%) with placebo (p = NS). Therefore, the statistical power of the study was only 10%. The rate of positive responses decreased with repeated testing irrespective of the assigned treatment: A positive response was obtained during the second head-up tilt test in 20 patients (10 with placebo, 10 with etilefrine) but in only 12 during the third (7 with etilefrine, 5 with placebo) (p < 0.05).

CONCLUSIONS

Oral etilefrine (10 mg three times a day) was not superior to placebo in preventing a positive response to head-up tilt testing. Despite a low statistical power, the high rate of negative response with placebo (50%) suggests that controlled trials are needed to assess the real efficacy of any treatment in patients with vasovagal syncope.

Time and frequency domain analyses of heart rate variability during orthostatic stress in patients with neurally mediated syncope

The role of autonomic balance during upright tilt in patients with neurally mediated syncope is unclear. To assess the characteristics of autonomic tone during orthostatic stress, 15 patients (mean age 32 years) with recurrent episodes of syncope (> or = 2) and a positive response to a 30-minute 60 degrees upright tilt were compared with the following control groups: (1) 15 patients (mean age 33.5 years) with > or = 2 episodes of recurrent syncope and a negative tilt response, and (2) 15 age- and sex-matched healthy volunteers (mean age 34 years) with no previous history of presyncope or syncope. Time domain measurements assessed were mean RR interval, standard deviation of normal RR intervals, and percentage of normal consecutive RR intervals differing by > 50 ms. Frequency domain measurements of the low-frequency (LF) and high-frequency (HF) bands were obtained, and the LF/HF ratio was also calculated. All variables were calculated in the supine position and during the first 5 minutes of upright tilt. No significant difference was observed in the time and frequency domain variables in the supine position between control groups with a negative head-up tilt response and the group with a positive response. The percentage of normal consecutive RR intervals differing by > 50 ms during the first 5 minutes of head-up tilt was significantly higher in the group with positive tilt tests than in the controls (25 +/- 12% vs 7 +/- 4%, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential therapeutic responses of patients with isoproterenol-dependent and isoproterenol-independent vasodepressor syncope

Orthostatic stress during tilt table testing (TTT) is used to examine patients who may have vasodepressor syncope. This response is thought to be mediated by activation of left ventricular mechanoreceptors. Isoproterenol, by increasing the rate of discharge of these mechanoreceptors, has been proposed to increase the sensitivity of TTT without decreasing its specificity. This mechanism is not, however, totally consistent with recent observations of vasodepressor responses after cardiac transplantation in patients with denervated hearts. These reports and data showing that not all sympathomimetic agents increase the sensitivity of TTT suggest that more than one mechanism may be responsible for a positive TTT result. Therefore we hypothesized that patients with positive TTT results tests not requiring isoproterenol (iso-independent) would have a different clinical and therapeutic response than patients who required isoproterenol (iso-dependent). One hundred sixty-one consecutive patients who underwent TTT for the evaluation of unexplained syncope were included in the study. TTT was performed without and during isoproterenol infusion. A positive TTT result was defined as syncope or presyncope with a sudden decrease in systolic blood pressure and reproduction of the patient's clinical symptoms. Patients with a positive TTT result underwent a second test after 1 to 2 weeks of therapy with an oral beta-blocking agent; if the result remained positive, TTT was performed again with other agents until a satisfactory therapeutic response was obtained. Sixty-six (41%) of 161 patients had a positive result; 18 (27%) were iso-independent, and 48 (73%) were iso-dependent. There were no significant differences in age, gender, or presence of underlying heart disease between these two groups.(ABSTRACT TRUNCATED AT 250 WORDS)