Echocardiographic demonstration of decreased left ventricular dimensions and vigorous myocardial contraction during syncope induced by head-up tilt

Upright postures and isoproterenol infusion for provocation of neurocardiogenic syncope: a comparison of standing and head-up tilting

Head-up tilt testing has proved to be useful in provocation of neurocardiogenic syncope. The purpose of this study was to examine whether simply assuming an upright posture by standing can be an alternative to the head-up tilt testing for diagnosis of neurocardiogenic syncope. Eighty-four patients with recurrent unexplained syncope and 22 normal volunteers were recruited into the study. Forty-seven patients with syncope and all normal volunteers received the standing test. Thirty-seven of the patients with syncope received head-up tilt testing (90 degrees). All subjects lay down for 5 minutes and then assumed an upright posture until syncope or presyncope occurred or until a maximum of 10 minutes was reached in each stage of the test. The tests included four stages: baseline and infusion of 1, 2, or 3 micrograms/min isoproterenol in each of the successive stages. Five subjects could not tolerate the procedure, and further testing was terminated. Overall, the standing test was positive in 83% of the patients with syncope, and its specificity was 74%. The head-up tilt testing was positive in 75% of the patients with syncope. The duration of assuming an upright posture before occurrence of syncope or presyncope was significantly longer in the syncope-tilting group in the third stage (p < 0.01) and the fourth stage (p < 0.05) compared with the syncope-standing group. However, the curves of the time course for cumulative positive rates were not significantly different (p = 0.0739) in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac pacing during neurocardiogenic (vasovagal) syncope

Head-up tilt testing is increasingly being used as a diagnostic modality in patients with unexplained syncope who are thought to have neurocardiogenic (vasovagal) mechanisms of syncope. Although large-scale placebo-controlled trials are still awaited, pharmacologic therapy is usually effective in preventing syncope or presyncope in this patient population. However, the role of permanent pacemaker therapy remains controversial. Because hypotension is usually associated with paradoxical bradycardia and occasionally asystole, it has been argued that permanent pacemaker therapy may be useful in preventing syncope and, thus, injury, in the so-called "malignant vasovagal cardioinhibitory response" in which the onset of syncope is thought to be abrupt. The onset of hypotension, however, usually precedes bradycardia during neurocardiogenic syncope, and pacing may thus not prevent syncope or presyncope in these patients. The role of cardiac pacing in patients with neurocardiogenic syncope is reviewed.

Plasma catecholamines and cyclic AMP response during head-up tilt test in patients with neurocardiogenic (vasodepressor) syncope

To examine hemodynamic, plasma catecholamines, and cyclic AMP changes during tilt in patients with neurocardiogenic (vasodepressor) syncope, six patients underwent 80 degrees head-up tilt test for 10 minutes with isoproterenol infusion (1-3 micrograms/min). Venous blood was sampled in the supine position, at 3 minutes of tilt, and at the onset of vasodepressor reaction during tilt. All patients had previous tilt studies in which vasodepressor syncope had been induced reproducibly (mean 3.3 episodes in each patient). Syncope was induced at 6.1 +/- 0.4 minutes of tilt with an infusion of isoproterenol (mean 1.7 +/- 0.3 micrograms/min). Although arterial pressure and heart rate did not change significantly between in the supine position and at 3 minutes of tilt, plasma norepinephrine increased significantly at 3 minutes of tilt (0.44 +/- 0.10 ng/mL; P < 0.05) and at the onset of vasodepressor reaction (0.49 +/- 0.12 ng/mL; P < 0.01) compared to the supine position with isoproterenol (0.34 +/- 0.10 ng/mL). Also, cyclic AMP (cAMP) increased significantly at 3 minutes of tilt (25.3 +/- 2.0 pmol/mL; P < 0.005) and at the onset of vasodepressor reaction (29.6 +/- 1.7 pmol/mL; P < 0.005) compared to the supine position with isoproterenol (20.4 +/- 1.9 pmol/mL). After administration of selective beta 1-blocker, metoprolol (40 mg/day), plasma norepinephrine, and cAMP during tilt did not change significantly compared to baseline tilt. However, metoprolol prevented the syncope in 3 of 6 patients. After administration of beta 1-, beta 2-blocker, propranolol (30 mg/day), cAMP at 3 minutes of tilt decreased significantly compared to the baseline tilt (16.9 +/- 1.4 pmol/mL vs 25.3 +/- 2.0 pmol/mL; P < 0.05) and propranolol prevented the syncope in all six patients. We concluded that the increase of cAMP may play an important role for the induction of vasodepressor reaction in patients with neurocardiogenic (vasodepressor) syncope. The concentration of cAMP showed more sensitive response to vasodepressor reaction than that of norepinephrine.

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.

Pathophysiological aspects of neurocardiogenic syncope: current concepts and new perspectives

Neurocardiogenic syncope is both a common and complex clinical disorder. Although recent research has clarified some of the pathophysiological mechanisms involved, much still remains either unknown or incompletely understood. Further investigation into this condition will not only enhance our knowledge of this and other related disorders, but will shed greater light on the influences of the brain and autonomic system on heart rate and blood pressure regulation and aid in our understanding of the complex interrelationships of neurocardiology.

Failure to decrease parasympathetic tone during upright tilt predicts a positive tilt-table test

The most frequently proposed mechanism for vasodepressor syncope is based on cardiac mechanoreceptor activation by augmented sympathetic tone. Because of the central role of the autonomic nervous system in this response, we hypothesized that the responses of the sympathetic and parasympathetic nervous systems (as assessed by analysis of heart rate variability) to orthostatic stress would differentiate patients with a positive from those with a negative tilt-table response. We therefore evaluated 28 patients undergoing tilt-table testing for presumed vasodepressor syncope. Based on 5-minute electrocardiographic samples obtained during the supine and upright phases (without isoproterenol infusion), we computed the mean RR interval, reflecting integrated cardiac sympathetic and parasympathetic tone, as well as the root-mean-square of successive differences of the RR intervals (RMSSD), a measure of high-frequency heart rate variability that is correlated with parasympathetic tone. Eleven patients had a negative and 17 a positive tilt response. There were no differences between the groups at baseline. In response to upright tilt, the mean RR decreased by a similar magnitude in both groups. In contrast, RMSSD decreased by 36% (p = 0.05) in response to upright tilt in patients with a negative response, but did not change significantly in patients with a positive tilt response. Absence of a decrease in RMSSD in response to orthostatic stress had 100% specificity and 41% sensitivity for predicting a positive test result. Thus, failure of withdrawal of parasympathetic tone (as assessed by RMSSD) during upright tilt predicts a positive tilt response.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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)

Effects of beta-adrenergic blockade on vasodepressor reaction in patients with vasodepressor syncope

The mechanism of vasodepressor reaction induced by head-up tilt test in patients with vasodepressor syncope is not clearly understood. We hypothesized that an abnormal alteration of beta-adrenergic transmission could be involved in these patients. We measured plasma catecholamine concentration during tilt and density of beta-adrenoceptors and tested the effects of beta-adrenergic blockers in the prevention of the vasodepressor reaction in patients with vasodepressor syncope. Ten patients had reproducibly induced vasodepressor syncope (mean 3.1 +/- 0.3 episodes in each patient) with head-up tilt (80 degrees) for 10 minutes with isoproterenol infusion (1 to 3 micrograms/min). Syncope occurred at 6.3 +/- 1.7 minutes during tilt with isoproterenol infusion. The plasma norepinephrine concentration before administration of beta-blocker was significantly elevated during tilt compared to the supine position (0.347 +/- 0.079 ng/ml in supine position with isoproterenol vs 0.468 +/- 0.082 ng/ml at 3 minutes of tilt, p < 0.001, and vs 0.503 +/- 0.106 ng/ml at the onset of vasodepressor reaction, p < 0.005; n = 8). Plasma norepinephrine after administration of the selective beta 1-adrenergic blocker metoprolol (40 mg/day) was similarly elevated (0.282 +/- 0.071 ng/ml in supine position with isoproterenol vs 0.390 +/- 0.078 ng/ml at 3 minutes of tilt, p < 0.05, n = 6; and vs 0.547 +/- 0.152 ng/ml at the onset of vasodepressor reaction, p < 0.001, n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic and volumetric response of the normal left ventricle to upright tilt testing

The purpose of this study were to determine the specificity of the head-up tilt test in normal subjects when a graded isoproterenol infusion is used, and to evaluate the role of dynamic ventricular volume change during head-up tilt as a mechanism of syncope. We prospectively studied 12 normal volunteers, each of whom underwent an upright tilt test for 10 minutes at 80 degrees with and without an infusion of isoproterenol. A subgroup of five subjects had a third tilt test during administration of a combination of esmolol and isoproterenol. Blood pressure, heart rate, and left ventricular volumes and flow (obtained with Doppler echocardiography) were recorded in the following sequence: while supine, during upright tilt, while supine with isoproterenol, and during upright tilt with isoproterenol. During the initial head-up tilt, one subject had syncope. An additional eight subjects had presyncope or syncope during head-up tilt with isoproterenol. The remaining three subjects were asymptomatic. In subjects with syncope or near-syncope ("responders"), heart rate increased with isoproterenol but decreased markedly, to 76 +/- 5 beats/min, by the end of the protocol. Systolic blood pressure rose slightly above baseline during isoproterenol but fell from 118 +/- 4 to 85 +/- 5 mm Hg during head-up tilt with isoproterenol. The three asymptomatic subjects had only one significant change, an increase in heart rate with isoproterenol. In the five responders undergoing three tilt tests, left ventricular volume decreased significantly at end diastole (94 +/- 25 vs 58 +/- 22 ml) and end systole (34 +/- 13 vs 18 +/- 6 ml) when supine baseline is compared with initial upright tilt.(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic control of cardiovascular function: clinical evaluation in health and disease

The high- and low-pressure baroreceptor reflexes are integral to the control of blood pressure by the autonomic nervous system. Tests of the integrity of these baroreflexes make it possible to identify the site of autonomic dysfunction in patients with orthostatic hypotension. Clinical characteristics and typical results of autonomic testing in patients with autonomic failure, with carotid sinus hypersensitivity, and with hyperadrenergic autonomic dysfunction are described in this review.

Vasodepressor reaction induced by inferior vena cava occlusion and isoproterenol in the rat. Role of beta 1- and beta 2-adrenergic receptors

BACKGROUND

Testing for the susceptibility of vasodepressor reaction in humans involves the combination of restriction of venous return by passive upright tilting and the administration of isoproterenol. We developed an experimental rat model in which vasodepressor reactions are induced when the inferior vena cava is occluded during an infusion of isoproterenol. The reactions are characterized by the development of paradoxical bradycardia during the period of inferior vena cava occlusion.

METHODS AND RESULTS

Inferior vena cava occlusion was performed for 60 seconds, and the maximal changes in RR interval were measured during seven states as follows: (1) when inferior vena cava occlusion was performed under control conditions in 40 rats, the rate accelerated in all 40 rats (delta RR, -15.6 +/- 1.9 milliseconds in 25 rats, P < .001; delta RR, -13.3 +/- 1.7 milliseconds in 10 rats, P < .001); (2) when inferior vena cava occlusion was performed in 25 rats during an infusion of isoproterenol, a vasodepressor reaction was observed in all rats as the heart rate slowed (delta RR, +92.7 +/- 8.3 milliseconds, P < .001); (3) when inferior vena cava occlusion was performed in 10 rats during an infusion of dobutamine, a selective beta 1-agonist, a vasodepressor reaction was observed in all rats as the heart rate slowed (delta RR, +63.3 +/- 10.6 milliseconds, P < .001); (4) when inferior vena cava occlusion was performed in 5 rats during an infusion of salbutamol, a selective beta 2-agonist, vasodepressor reaction was not observed as the heart rate accelerated in all rats (delta RR, -11.4 +/- 2.8 milliseconds, P < .002); (5) the vasodepressor reaction induced by either dobutamine or isoproterenol was inhibited by atenolol, a selective beta 1-adrenergic receptor antagonist; (6) the vasodepressor reaction induced by isoproterenol was inhibited by propranolol (lipophilic) and sotalol (nonlipophilic) beta-blockers and there was a dose-dependent attenuation by propranolol of the maximal RR interval slowing during inferior vena cava occlusion; and (7) butoxamine, a selective beta 2-adrenergic receptor antagonist, attenuated but did not block the vasodepressor reaction observed during an infusion of isoproterenol.

CONCLUSIONS

Reduced cardiac volume combined with beta 1-adrenergic stimulation can stimulate a vasodepressor reaction in rats. beta 2-Adrenergic receptors play little or no role in the reaction. The vasodepressor reaction can be blocked by selective or nonselective beta 1-adrenergic antagonists independent of the drug's ability to penetrate the central nervous system. The application of these findings to humans remains to be elucidated.

Short AV interval VDD pacing does not prevent tilt induced vasovagal syncope in patients with cardioinhibitory vasovagal syndrome

Eleven subjects (mean age 50 years, range 33-71 years), who had previously received permanent dual chamber pacemakers for cardioinhibitory vasovagal syncope, underwent paired Westminster protocol tilt tests, one with short AV delay VDD pacing and one without pacing, to test the hypothesis that continuous ventricular pacing would prevent the cardiac initiation of vasovagal syncope. Nine (82%) of the paced tilts produced positive vasovagal outcomes compared with seven (64%) of the unpaced tilts. No important differences in the heart rate or blood pressure behavior during tilt or the time to positive vasovagal outcomes were observed between the paired tilts. There was more accelerated syncope/presyncope once symptoms had developed during the paced tilts of subjects in whom both study tilts were positive, although this did not reach statistical significance (P = 0.054). This study shows that atrial synchronous ventricular pacing does not prevent the initiation, or progression, of tilt induced vasovagal syncope in predisposed subjects.

Assessment of effects of autonomic stimulation and blockade on the signal-averaged electrocardiogram

BACKGROUND

Signal-averaged ECG is a noninvasive test designed to detect "late potentials." The effects of alterations in autonomic tone on the signal-averaged ECG have not been evaluated systematically.

METHODS AND RESULTS

The effects of autonomic stimulation and blockade on the signal-averaged ECG were evaluated in 14 healthy subjects (8 men and 6 women; age, 28.5 +/- 4.8 years) on 2 separate days. The signal-averaged ECG was recorded at baseline and after physiological and pharmacologic beta-adrenergic stimulation (tilt, exercise, and epinephrine and isoproterenol infusions), sequential and combined beta-adrenergic and parasympathetic blockade, and alpha-adrenergic stimulation before and after parasympathetic blockade. Analysis was performed with a bidirectional filter (40-Hz high-pass). Significant changes in the signal-averaged QRS duration from baseline (105.1 +/- 12.0 milliseconds) were noted with tilt (96.8 +/- 8.8 milliseconds), tilt after double blockade (97.5 +/- 9.0 milliseconds), epinephrine (110.5 +/- 11.8 milliseconds), and isoproterenol (99.6 +/- 12.6 milliseconds). Changes in the root-mean-square voltage of the terminal 40 milliseconds and the low-amplitude (< 40 microV) signal duration paralleled the changes in the QRS duration.

CONCLUSIONS

The signal-averaged ECG does not measure only "fixed" parameters but rather is altered under a variety of physiological and pharmacologic conditions. Upright tilt leads to shortening of the QRS duration before and after autonomic blockade; thus, the decrease in QRS duration with tilt may be related to factors other than changes in autonomic tone. These findings have implications for interpretation of the results of signal-averaged ECG.

Circulatory and catecholamine changes during head-up tilt testing in neurocardiogenic (vasovagal) syncope

Changes in heart rate, arterial pressure, norepinephrine and epinephrine levels were compared in 19 consecutive patients (10 men and 9 women, mean age 46 +/- 16 years) with neurocardiogenic syncope and 11 age- and sex-matched control subjects (5 men and 6 women, mean age 49 +/- 15 years) during head-up tilt testing. Norepinephrine and epinephrine levels were measured at the baseline supine position, in the initial upright position, every 90 seconds during the 70 degrees upright tilt, at the time of termination due to hypotension and syncope (or at 15 minutes in control subjects), and at 40 seconds and 1 minute and 40 seconds in the supine position after terminating the head-up tilt test. Baseline norepinephrine, epinephrine and heart rate were slightly higher in patients. Despite a significant decrease in mean arterial pressure during head-up tilt testing in patients (51 +/- 20 mm Hg; p < 0.001), norepinephrine levels in patients and control subjects at the time of terminating the head-up tilt test were comparable (459 +/- 204 vs 473 +/- 172 pg/ml). A fivefold increase in epinephrine levels (73 +/- 53 to 345 +/- 260 pg/ml; p < 0.01) were seen in patients, whereas control subjects had insignificant change (38 +/- 16 to 65 +/- 44 pg/ml). It is concluded that diminished neuronal sympathetic activity and enhanced adrenomedullary activity is demonstrated during head-up tilt testing in patients with neurocardiogenic syncope.

Nitroglycerin infusion during upright tilt: a new test for the diagnosis of vasovagal syncope

The aim of our present study was to assess the value of nitroglycerin infusion during upright posture as a new provocative test for diagnosis of vasovagal syncope. To this purpose 40 patients with unexplained syncope (17 men and 23 women, mean age 47 years) and 25 asymptomatic control subjects with negative baseline head-up tilt underwent two other tilting tests, one during nitroglycerin infusion and one during isoproterenol infusion. The protocol of the nitroglycerin test consisted of a maximum of five successive stages of 5 minutes in the supine position plus 10 minutes 80-degree upright tilt at progressively increasing infusion rates (increments of 0.86 microgram/kg/hr every stage). During the nitroglycerin test a positive response (syncope in association with sudden hypotension and bradycardia) occurred in 21 (53%) patients with unexplained syncope, an exaggerated response (minor symptoms in association with slowly increasing hypotension alone) occurred in 10 (25%), a negative response in 9 (22%), and drug intolerance in 0. During the isoproterenol test these percentages were 25%, 25%, 32%, and 18%, respectively. Only 2 (8%) control subjects had a positive response to nitroglycerin test and 2 (8%) to isoproterenol test. Thus the nitroglycerin test seems to be a useful alternative tool for diagnosis of vasovagal syncope; it is equally specific but more sensitive and feasible than the isoproterenol test.

Comparison of cardiac pacing with drug therapy in the treatment of neurocardiogenic (vasovagal) syncope with bradycardia or asystole

BACKGROUND

The efficacy of permanent cardiac pacing in patients with neurocardiogenic (or vasovagal) syncope associated with bradycardia or asystole is not clear. We compared the efficacy of cardiac pacing with that of oral drug therapy in the prevention of hypotension and syncope during head-up tilt testing.

METHODS

Among 70 patients with a history of syncope in whom hypotension and syncope could be provoked during head-up tilt testing, 22 had bradycardia (a heart rate < 60 beats per minute, with a decline in the rate by at least 20 beats per minute) or asystole along with hypotension during testing. There were 9 men and 13 women, with a mean (+/- SD) age of 41 +/- 17 years. Head-up tilt testing was repeated during atrioventricular sequential pacing (in 20 patients with sinus rhythm) or ventricular pacing (in 2 patients with atrial fibrillation). Regardless of the results obtained during artificial pacing, all the patients subsequently had upright-tilt testing repeated during therapy with oral metoprolol, theophylline, or disopyramide.

RESULTS

During the initial tilt test, 6 patients had asystole and 16 had bradycardia along with hypotension. Despite artificial pacing, the mean arterial pressure during head-up tilt testing still fell significantly, from 97 +/- 19 to 57 +/- 19 mm Hg (P < 0.001); 5 patients had syncope, and 15 had presyncope. By contrast, 19 patients who later received only medical therapy (metoprolol in 10, theophylline in 3, and disopyramide in 6), 2 patients who received both metoprolol and atrioventricular sequential pacing, and 1 patient who received only atrioventricular sequential pacing had negative head-up tilt tests. After a median follow-up of 16 months, 18 of the 19 patients who were treated with drugs alone (94 percent) remained free of recurrent syncope or presyncope, whereas the patient treated only with permanent dual-chamber pacemaker had recurrent syncope.

CONCLUSIONS

In patients with neurocardiogenic syncope associated with bradycardia or asystole, drug therapy is often effective in preventing syncope, whereas artificial pacing is not.

Usefulness of metoprolol for unexplained syncope and positive response to tilt testing in young persons

The efficacy of intravenous metoprolol in preventing symptoms during a repeat tilt test was compared with the outcome of chronic oral treatment in 21 patients (14 female, 7 male), age 8 to 20 years (mean 13 +/- 3) with unexplained syncope (> or = 1 episode) and a positive response to tilt testing. A positive response was defined as the development of either syncope or presyncope. During the initial tilt test, a positive response occurred during baseline (14 patients) or isoproterenol (0.03 to 0.1 microgram/kg/min) infusion (7 patients) with a cardioinhibitory (1 patient), vasodepressor (5 patients) or mixed (15 patients) pattern. Metoprolol (0.1 to 0.2 mg/kg) was administered intravenously. During the repeat tilt test, response was negative in 18 patients, including 11 of 14 patients with a positive response in the baseline and 7 of 7 patients with a positive response during isoproterenol infusion. Metoprolol (0.8 to 2.8 mg/kg/day) was administered orally to 15 patients for an average of 10 months. Symptoms were absent (7 patients) or improved (2 patients); metoprolol was discontinued because of adverse effects (3 patients) or recurrence of symptoms (3 patients). In 7 of 12 patients with a negative response and 2 of 3 patients with a positive response after intravenous metoprolol, oral administration of metoprolol prevented or improved symptoms without adverse effects. Many young patients (60%) with recurrent syncope obtained symptomatic improvement from chronic oral metoprolol treatment without adverse effects; repeat tilt testing after intravenous metoprolol did not appear to offer any additional information than would have been obtained from a trial of chronic oral treatment.

Impaired immediate vasoconstrictor responses in patients with recurrent neurally mediated syncope

Immediate responses to head-up tilt were determined in 78 consecutive patients with unexplained syncope undergoing 45-minute tilt tests at 60 degrees. Thirty-four patients developed neurally mediated syncope (mean time to syncope 18 minutes), 40 tolerated the full duration of tilt, and 4 were unable to complete the study but did not develop syncope. Blood pressure, heart rate, forearm blood flow and forearm vascular resistance were measured at baseline and after 2 and 5 minutes of tilt. Syncopal and nonsyncopal patients were well-matched for age and baseline hemodynamic parameters. There was no difference between the groups in heart rate or blood pressure at 2 minutes, but there was a small but significant difference in percent reduction in mean arterial pressure at 5 minutes. After 2 and 5 minutes of tilt, mean forearm blood flow was 2.4 and 2.6 ml/min/100 ml, respectively, in syncopal patients compared with 1.6 (p < 0.05) and 1.7 ml/min/100 ml (p < 0.01), respectively, in patients who tolerated 45 minutes of tilt. In syncopal patients, forearm vascular resistance was 51.0 and 44.0 at 2 and 5 minutes, respectively, whereas in nonsyncopal patients, it was 82.4 (p < 0.02) and 73.1 (p < 0.001), respectively. These differences remained consistent when only data for patients developing syncope after > 15 minutes were included in the analysis. Patients with neurally mediated syncope have clearly demonstrable abnormalities in vascular control immediately after assumption of the upright posture. The results shed new light on the pathophysiology of neurally mediated syncope.

Use of intravenous esmolol to predict efficacy of oral beta-adrenergic blocker therapy in patients with neurocardiogenic syncope

The usefulness of esmolol in predicting the efficacy of treatment with an oral beta-adrenergic blocking agent was evaluated in 27 consecutive patients with neurocardiogenic syncope. Seventeen patients had a positive head-up tilt test response at baseline and 10 patients required intravenous isoproterenol for provocation of hypotension. All patients were then given a continuous esmolol infusion (500 micrograms/kg per min loading dose for 3 min followed by 300 micrograms/kg per min maintenance dose) and rechallenged with a head-up tilt test at baseline or with isoproterenol. Of the 17 patients with a positive baseline tilt test response, 11 continued to have a positive response to esmolol challenge. Sixteen patients (including all 10 patients with a positive tilt test response with isoproterenol) exhibited a negative response to upright tilt during esmolol infusion. Irrespective of their response to esmolol infusion, all patients had a follow-up tilt test with oral metoprolol after an interval of greater than or equal to 5 half-lives of the drug. All 16 patients (100%) with a negative tilt test response during esmolol infusion had a negative tilt test response with oral metoprolol. Of the 11 patients with a positive tilt test response during esmolol infusion, 10 (90%) continued to have a positive response with oral metoprolol. It is concluded that in the electrophysiology laboratory, esmolol can accurately predict the outcome of a head-up tilt response to oral metoprolol. This information may be helpful in formulating a therapeutic strategy at the initial head-up tilt test in patients with neurocardiogenic syncope.