Adenosine-induced atrioventricular block in patients with unexplained syncope: the diagnostic value of ATP testing

Adenosine, Adenosine Receptors and Neurohumoral Syncope: From Molecular Basis to Personalized Treatment

Adenosine is a ubiquitous nucleoside that is implicated in the occurrence of clinical manifestations of neuro-humoral syncope (NHS). NHS is characterized by a drop in blood pressure due to vasodepression together with cardio inhibition. These manifestations are often preceded by prodromes such as headaches, abdominal pain, feeling of discomfort or sweating. There is evidence that adenosine is implicated in NHS. Adenosine acts via four subtypes of receptors, named A1 (A1R), A2A (A2AR), A2B (A2BR) and A3 (A3R) receptors, with all subtypes belonging to G protein membrane receptors. The main effects of adenosine on the cardiovascular system occurs via the modulation of potassium ion channels (IK Ado, K ATP), voltage-gate calcium channels and via cAMP production inhibition (A1R and A3R) or, conversely, through the increased production of cAMP (A2A/BR) in target cells. However, it turns out that adenosine, via the activation of A1R, leads to bradycardia, sinus arrest or atrioventricular block, while the activation of A2AR leads to vasodilation; these same manifestations are found during episodes of syncope. The use of adenosine receptor antagonists, such as theophylline or caffeine, should be useful in the treatment of some forms of NHS. The aim of this review was to summarize the main data regarding the link between the adenosinergic system and NHS and the possible consequences on NHS treatment by means of adenosine receptor antagonists.

Is the Adenosine Test Obsolete in the Clinical Assessment of Syncope of Unknown Origin?

Syncope is a common clinical condition affecting 50% of the general population; however, its exact pathophysiology and underlying mechanisms remain elusive. The adenosine test (ADT) has been proposed as a complementary diagnostic test in the work-up of syncope of unknown origin aiming to further elucidate the underlying pathogenetic mechanism of spontaneous syncope. Although ADT has not been endorsed by the recent European Society of Cardiology guidelines on syncope management, the use of a quick, safe and non-invasive test which can contribute to an accurate diagnosis and rationalised therapy, may deserve further consideration. This review summarises the evidence on the role of ADT in the investigation and management of syncope of unknown origin and highlights future perspectives in this area. The authors also analyse the current challenges and research targets on adenosine plasma levels and its receptors due to the involvement of the adenosine pathway in the ADT response.

Adenosine and neurohumoral syncope

Either central or peripheral baroreceptor reflex abnormalities and/or alterations in neurohumoral mechanisms play a pivotal role in the genesis of neurally mediated syncope. Thus, improving our knowledge of the biochemical mechanisms underlying specific forms of neurally mediated syncope (more properly termed 'neurohumoral syncope') might allow the development of new therapies that are effective in this specific subgroup. A low-adenosine phenotype of neurohumoral syncope has recently been identified. Patients who suffer syncope without prodromes and have a normal heart display a purinergic profile which is the opposite of that observed in vasovagal syncope patients and is characterized by very lowadenosine plasma level values, low expression of A2A receptors and the predominance of the TC variant in the single nucleotide c.1364 C>T polymorphism of the A2A receptor gene. The typical mechanism of syncope is an idiopathic paroxysmal atrioventricular block or sinus bradycardia, most often followed by sinus arrest. Since patients with low plasma adenosine levels are highly susceptible to endogenous adenosine, chronic treatment of these patients with theophylline, a non-selective adenosine receptor antagonist, is expected to prevent syncopal recurrences. This hypothesis is supported by results from series of cases and from two controlled studies.

Adenosine and the Cardiovascular System: The Good and the Bad

Adenosine is a nucleoside that impacts the cardiovascular system via the activation of its membrane receptors, named A₁R, A_2AR, A_2BR and A₃R. Adenosine is released during hypoxia, ischemia, beta-adrenergic stimulation or inflammation and impacts heart rhythm and produces strong vasodilation in the systemic, coronary or pulmonary vascular system. This review summarizes the main role of adenosine on the cardiovascular system in several diseases and conditions. Adenosine release participates directly in the pathophysiology of atrial fibrillation and neurohumoral syncope. Adenosine has a key role in the adaptive response in pulmonary hypertension and heart failure, with the most relevant effects being slowing of heart rhythm, coronary vasodilation and decreasing blood pressure. In other conditions, such as altitude or apnea-induced hypoxia, obstructive sleep apnea, or systemic hypertension, the adenosinergic system activation appears in a context of an adaptive response. Due to its short half-life, adenosine allows very rapid adaptation of the cardiovascular system. Finally, the effects of adenosine on the cardiovascular system are sometimes beneficial and other times harmful. Future research should aim to develop modulating agents of adenosine receptors to slow down or conversely amplify the adenosinergic response according to the occurrence of different pathologic conditions.

Adenosine hypersensitivity and atrioventricular block

Adenosine is a ubiquitous substance that is released under several physiological and pathological conditions and has cardiovascular effects including cardioinhibition and vasodilation. It has been shown to be an important modulator implicated in several forms of syncope. In patients with chronic low plasma levels of adenosine, a transient release of endogenous adenosine can be sufficient to block conduction in the atrioventricular node and induce prolonged asystole; conversely, when plasma adenosine levels are chronically high, adenosine release is responsible for vasodepression. Distinct purinergic profiles in patients presenting with syncope have recently been correlated with the clinical presentation: "low-adenosine patients," prone to asystole, may present with idiopathic atrioventricular block, carotid sinus syndrome, or syncope with no or very brief prodromes and normal heart; "high-adenosine patients," prone to vasodilation, experience vasovagal syncope. This pathophysiological classification may have therapeutic implications.

Syncope and paroxysmal atrioventricular block

Current literature reveals three types of paroxysmal atrioventricular block (AVB) that can cause syncope: Intrinsic paroxysmal atrioventricular block is due to an intrinsic disease of the AV conduction system; this type of "cardiac syncope" is also called Stokes-Adams attack; Extrinsic vagal paroxysmal atrioventricular block is linked to the effect of the parasympathetic nervous system on cardiac conduction and is one of the mechanisms involved in "reflex syncope." Extrinsic idiopathic paroxysmal atrioventricular block is associated with low levels of endogenous adenosine and is supposed to be one of the mechanisms involved in "low-adenosine syncope." These three types of paroxysmal AVB present different clinical and electrocardiographic features. Additionally, the efficacy of cardiac pacing and theophylline therapy to prevent syncopal recurrences is also different for these three types of AVB.

Adenosine 5'-Triphosphate Test in the Management of Patients With Syncope

The response to adenosine 5'-triphosphate (ATP) identifies patients with syncope who might benefit from pacemaker therapy (ATP test). Two measures have been used to determine the outcome of the ATP test, which have lead to contrasting conclusions regarding its utility: (1) the duration of cardiac pause (CP) mainly due to AV block and (2) the longest RR interval (RRmax). We tested the hypothesis that the discrepancy regarding the utility of the ATP test is mainly because of the different way the 2 measures determine the outcome of the test. Post hoc analysis was applied to data obtained from patients with syncope (n = 33) with a positive and negative ATP test based on the CP duration and RRmax, respectively, subjected to pacemaker therapy. In 19 and 14 patients, the pacemaker was programmed to function as AAI pacing at 30 ppm (control) and as DDD pacing at 70 ppm, respectively. During the follow-up period of 17.0 ± 8.6 months, syncope recurred in only 1 of the 14 patients with DDD pacing; in contrast, 10 of 19 patients with AAI30 pacing experienced syncope within the first 5.3 ± 5.2 months of follow-up (P < 0.009; recurrence rate). The ATP test, the outcome of which is determined by the CP measure, is a useful diagnostic test for the identification of patients with bradycardic syncope who may benefit from pacemaker therapy; the identification of such patients would be missed when the RRmax measure is used to determine the outcome of the test. The efficacy of DDD pacing suggests that atrioventricular nodal conduction block is the primary cause of syncope in patients with a positive ATP test based on the CP measure.

Electrophysiological Testing for the Investigation of Bradycardias

In this article we review the role of electrophysiological testing in patients presenting with bradycardia due to sinus node or atrioventricular node disease. In sinus bradycardia the role of electrophysiology studies is not established. In AV conduction disturbances, an electrophysiology study may be necessary both for the establishment of atrioventricular block as the main cause of symptoms, and for identification of the anatomic site of block that may dictate the potential need of permanent pacing.

Idiopathic Paroxysmal Atrio-Ventricular Block. What is The Mechanism?

Idiopathic paroxysmal atrioventricular (AV) block poses a true diagnostic challenge. What is clear about this entity is the confusion about its definition and consequently about its etiology. According to certain sources, the diagnosis of this block requires the lack of a structural cardiac pathology that justifies the observed manifestations and an absence of electrocardiographic disorders prior to an episode. The clinical presentation of idiopathic paroxysmal AV block does not differ from that of another cardiogenic syncope or of a vasovagal syncope with a significant cardioinhibitory component. With respect to the mechanism that explains this block, it has been postulated that patients with low basal adenosine levels exhibit hyperaffinity of the A2 receptors of the AV node. Variations in plasma adenosine levels may favor episodes of paroxysmal AV block. The diagnosis of this block is complex and can require years to determine. Routine electrophysiological examination of these patients is not cost effective due to the low sensitivity and specificity of this approach. Numerous groups have supported the use of an implantable loop recorder to substantiate AV block paroxysms and assess their clinical correlations. Permanent stimulation devices are utilized to reduce syncopal recurrence.

Syncope and Idiopathic (Paroxysmal) AV Block

Syncope due to idiopathic AV block is characterized by: 1) ECG documentation (usually by means of prolonged ECG monitoring) of paroxysmal complete AV block with one or multiple consecutive pauses, without P-P cycle lengthening or PR interval prolongation, not triggered by atrial or ventricular premature beats nor by rate variations; 2) long history of recurrent syncope without prodromes; 3) absence of cardiac and ECG abnormalities; 4) absence of progression to persistent forms of AV block; 5) efficacy of cardiac pacing therapy. The patients affected by idiopathic AV block have low baseline adenosine plasma level values and show an increased susceptibility to exogenous adenosine. The APL value of the patients with idiopathic AV block is much lower than patients affected by vasovagal syncope who have high adenosine values.

The use of adenosine and adenosine triphosphate testing in the diagnosis, risk stratification and management of patients with syncope: current evidence and future perspectives

Syncope is a significant source of cardiovascular-related morbidity yet the etiology is frequently obscure and the identification of patients at highest risk is challenging. Adenosine (AD) and adenosine triphosphate (ATP) administrations have been suggested as potentially useful non-invasive tools in the diagnostic workup of patients with neurally-mediated or bradycardia-related syncope. It has been postulated that both compounds by modulating the autonomic innervation in the heart and exerting negative chronotropic and dromotropic effects in the conduction system, may unmask the mechanism of syncope. However, the clinical implications derived from the efficacy of both tests in the investigation of syncope remain unclear mainly due to inconclusive and occasionally contradictory results of published studies. This review article summarizes recent and past information in the use of ATP and AD in the investigation of syncope with emphasis on clinical trials. We present the current level of evidence for the use of these agents in clinical practice, identify areas where further research is warranted and highlight the future perspectives of these agents as complements to an accurate risk-stratification of patients with syncope.

Syncope associated with documented paroxysmal atrioventricular block reproduced by adenosine 5' triphosphate injection

AIMS

We sought to investigate in patients with syncope the relationship between documented paroxysmal atrioventricular block (AVB) of unknown mechanism and AVB induced by adenosine triphosphate (ATP) injection.

METHODS AND RESULTS

We selected patients >45 years free of structural heart disease with syncope related to paroxysmal AVB documented by Holter or in-hospital monitoring, but without any trigger suggestive of vasovagal origin and with normal baseline electrocardiogram. Adenosine triphosphate test was performed according to the usual protocol. Nine patients (all females; mean age 66 ± 14.6 years; range: 48-81 years) matching the abovementioned criteria particularly documented spontaneous complete AVB with long ventricular pauses. Their mean QRS duration was 86.6 ± 14.1 ms and the mean PR interval was 161 ± 21.3 ms. In all patients, ATP induced a long ventricular pause related to AVB (mean duration 13.2 s; range from 7 to 56 s). After a mean follow-up duration of 42 ± 36 months, electrocardiogram (ECG) remained unchanged without progression to permanent AVB or appearance of intraventricular conduction disturbances.

CONCLUSION

Some patients, predominantly older females, with 'normal' heart and ECG, have syncope associated with spontaneous AVB of unknown origin reproduced during the ATP test. They do not develop permanent AVB during follow-up. This unusual behaviour could be interpreted as an abnormal susceptibility to ATP and these patients could be considered to have 'ATP-sensitive AVB'. In this subgroup of syncope patients ATP test is useful.

ATPace™: injectable adenosine 5'-triphosphate : Diagnostic and therapeutic indications

ATPace™, a novel injectable formulation of adenosine 5'-triphosphate (ATP), is developed by Cordex Pharma, Inc. (Cordex) as a diagnostic and therapeutic drug for the management of cardiac bradyarrhythmias. Extracellular ATP exerts multiple effects in various cell types by activating cell-surface receptors known as P2 receptors. In the heart, ATP suppresses the automaticity of cardiac pacemakers and atrioventricular (AV) nodal conduction via adenosine, the product of its degradation by ecto-enzymes, as well as by triggering a cardio-cardiac vagal reflex. ATP, given as a rapid intravenous bolus injection, has been used since the late 1940s as a highly effective and safe therapeutic agent for the acute termination of reentrant paroxysmal supraventricular tachycardia (PSVT) involving the AV node. In addition, preliminary studies have shown that ATP can also be used as a diagnostic agent for the identification of several cardiac disorders including sinus node dysfunction (sick sinus syndrome), dual AV nodal pathways, long QT syndrome, and bradycardic syncope. The US Food and Drug Administration has approved Cordex formulation for ATP as an Investigational New Drug and two pathways for its marketing approval; one therapeutic, i.e., acute termination of paroxysmal PSVT, and the other diagnostic, i.e., the identification of patients with bradycardic syncope who can benefit from pacemaker therapy. The scientific rationale for the development of ATPace™ is discussed.

Syncope due to idiopathic paroxysmal atrioventricular block: long-term follow-up of a distinct form of atrioventricular block

OBJECTIVES

We present data on patients with syncope due to paroxysmal atrioventricular (AV) block unexplainable in terms of currently known mechanisms.

BACKGROUND

Paroxysmal AV block is known to be due to intrinsic AV conduction disease or to heightened vagal tone.

METHODS

We evaluated 18 patients presenting with unexplained syncope who had: 1) normal baseline standard electrocardiogram (ECG); 2) absence of structural heart disease; and 3) documentation, by means of prolonged ECG monitoring at the time of syncopal relapse, of paroxysmal third-degree AV block with abrupt onset and absence of other rhythm disturbances before or during the block.

RESULTS

The study group consisted of 9 men and 9 women, mean age 55 ± 19 years, who had recurrent unexplained syncope for 8 ± 7 years and were subsequently followed up for as long as 14 years (4 ± 4 years on average). The patients had no structural heart disease, standard ECG was normal, and electrophysiological study was negative. In all patients, prolonged ECG monitoring documented paroxysmal complete AV block with 1 or multiple consecutive pauses (mean longest pause: 9 ± 7 s at the time of syncope); AV block occurred without P-P cycle lengthening or PR interval prolongation. During the observation time, no patient had permanent AV block; on permanent cardiac pacing, no patient had further syncopal recurrences.

CONCLUSIONS

Common clinical and electrophysiological features define a distinct form of syncope due to idiopathic paroxysmal AV block characterized by a long history of recurrent syncope, absence of progression to persistent forms of AV block, and efficacy of cardiac pacing therapy.

The intravenous adenosine test: a new test for the identification of bradycardia pacing indications? A pilot study in subjects with bradycardia pacing indications, vasovagal syncope and controls

BACKGROUND

Intravenous adenosine has recently been used in the diagnosis of unexplained syncope, but there is no consensus as to the meaning of a 'positive' test. The objective is to determine the sensitivity and specificity of intravenous adenosine testing in the diagnosis of bradycardia-pacing indications [sinus node dysfunction(SND), atrio-ventricular block (AVB) and cardio-inhibitory carotid sinus syndrome (CSS)].

DESIGN

Pilot cohort study.

METHODS

Patients-(i) Bradycardia-pacing group: Consecutive patients referred for pacing for SND, AVB and CSS; (ii) Consecutive head-up tilt (HUT)-positive VVS patients. Controls-(i) Simple controls (S-Con: normal examination/ECG) and (ii) Electrophysiology controls (EP-Con: consecutive subjects referred for accessory pathway ablation). Pacing referrals and EP-Con had electrophysiology studies to confirm referral diagnosis and exclude others. All subjects had bolus injection of 20 mg intravenous adenosine during continuous ECG and blood pressure monitoring (positive test: >or=6 s asystole, >or=10 s high-degree AVB post-injection). Sensitivity, specificity, safety and tolerability of the test were measured.

RESULTS

Of 264 potential participants (4 SND, 8 AVB, 7 CSS, 10 VVS, 10 EP-Con and 11 S-Con) 50 were studied. All (100%) of the bradycardia-pacing group were adenosine test-positive, as were 6 (60%) VVS. None (0%) and 3 (27%) of the EP- and S-Con groups were positive. Adenosine testing was 100% sensitive and 86% specific for bradycardia-pacing indications, and 100% specific using the diagnostically 'clean' EP-Con results. There were no significant adverse or side effects.

CONCLUSION

Adenosine testing reliably identified patients with definitive bradycardia-pacing indications in whom alternative diagnoses were excluded. Further work is needed to evaluate the role of this test in the diagnosis of unexplained syncope.

The value of adenosine test in the diagnosis of sick sinus syndrome: susceptibility of sinus and atrioventricular node to adenosine in patients with sick sinus syndrome and unexplained syncope

AIMS

Patients (pts) with sick sinus syndrome (SSS) and unexplained syncope show increased susceptibility of sinus and atrioventricular node (AVN) to intravenous adenosine, respectively. Our aim is to assess the diagnostic value of adenosine test in pts with SSS, as well as to evaluate the response of AVN to adenosine either in pts with unexplained syncope or in pts with syncope and known SSS.

METHODS AND RESULTS

The effect of adenosine administration on the sinus and AVN was studied in a population consisted of 19 pts with clinical SSS (group SSS), 7 pts with syncope of unknown origin (group SUO), and 12 control subjects (group C). We calculated the maximum corrected sinus node recovery time (CSNRT), after overdrive pacing of the atrium at cycle lengths of 600, 500, and 400 ms and compared this value with the longest sinus pause, following adenosine administration corrected to the basic cycle length (ADSNRT). The longest R-R interval during atrioventricular block in response to adenosine injection (ADAVB) was also measured. Adenosine was given in a bolus dose of 0.15 mgr/kg through a femoral or large antecubital vein. There was a significant difference in the mean values of CSNRT among the three groups: group SSS (651 +/- 228 ms) > group SUO (284 +/- 100 ms) = group C (291 +/- 117 ms), F(2.35) = 19.078, P = 0.000. A significant difference was also found with ADSNRT: group SSS (5437 +/- 6863 ms) > group SUO (122 +/- 120 ms) = group C (801 +/- 1897 ms), F(2.35) = 4.513, P = 0.018. Using 525 ms as a cutoff value indicating sinus node dysfunction, CSNRT had a sensitivity of 74% and specificity of 100% for diagnosis of SSS while ADSNRT had 94% and 84%, respectively. Higher values of ADAVB in pts with SSS (10659 +/- 5872) and SUO (10026 +/- 7092) in comparison with controls (3615 +/- 5002) were measured, F(2.35) = 5.697, P = 0.007. No difference in the degree of ADAVB was found between the pts with SUO (10026 +/- 7092 ms) and syncope in the presence of SSS (12058 +/- 6787 ms), F(1.15) = 0.356, P = 0.56.

CONCLUSION

Adenosine test appears to be at least comparable with CSNRT in making the diagnosis of SSS and may be considered as an alternative non-invasive test for confirmation of suspected SSS. No difference in the susceptibility of AVN to adenosine between the pts with syncope in the presence of SSS and those with unexplained syncope was found, suggesting that adenosine test cannot be used to diagnose atrioventricular block as the cause of syncope.

Evaluation of syncope

Syncope is not an uncommon condition. The evaluation can be as frustrating as the treatment. The role of a detailed history and physical examination cannot be underestimated and both remain a cornerstone in the evaluation of syncope. A systematic approach may not only lead to correct diagnosis in most cases, but also ultimately will be cost effective. This review emphasizes the systematic approach for the evaluation of this challenging problem.

Adenosine test in the diagnosis of unexplained syncope: marker of conducting tissue disease or neurally mediated syncope?

Adenosine test (supine administration of a 20 mg intravenous bolus with electrocardiographic and blood pressure monitoring) has been endorsed by the European Society of Cardiology guidelines on syncope management as an 'experimental' test in the diagnosis of unexplained syncope. The test is quick and cheap, but there is no consensus as to what condition, if any, the adenosine test is exposing, with conducting tissue disease and neurally mediated syncope proposed by various authors. In this article, we review the possible mechanisms underlying a positive adenosine test, its safety, and a comprehensive examination of the literature supporting each of the putative causal diagnoses.

Epidemiology, clinical features, and follow-up of patients with syncope and a positive adenosine triphosphate test result

OBJECTIVES

We sought to evaluate epidemiology, clinical features, and outcomes of patients with syncope and an abnormal response to adenosine triphosphate (ATP).

BACKGROUND

Syncope remains of unknown origin in almost 30% of the patients. Injection of ATP induces in some of these patients, but not in control patients, a ventricular pause > or =6 s.

METHODS

Patients with syncope of unknown origin had an intravenous injection of 20 mg of ATP. All patients had a tilt test.

RESULTS

Among 214 patients, 19 (8.9%) had a positive ATP test result. The proportion of positive test results was higher (p < 0.002) in women (14.3%) than in men (2.2%). Ten patients (4.7%) had positive ATP and tilt test results. These patients (exclusively women) were older (p < 0.05) at the time of their fist syncope than the 67 patients with a negative ATP test result but a positive tilt test result. There was a trend for these two test results to be correlated (p = 0.07). Side effects were of short duration and benign. The mean duration of pauses was longer in women (p = 0.009). During a mean period of 31 +/- 14 months, recurrences of syncope were reported in 25% of patients.

CONCLUSIONS

The ATP test is a safe test with an "abnormal" result in <10% of patients with syncope of unknown origin. The profile of these patients is characteristic.

An implantable loop recorder study of highly symptomatic vasovagal patients: the heart rhythm observed during a spontaneous syncope is identical to the recurrent syncope but not correlated with the head-up tilt test or adenosine triphosphate test

OBJECTIVES

The aim of this study was to analyze the heart rhythm during spontaneous vasovagal syncope (VVS) in highly symptomatic patients with implantable loop recorders (ILR) and to correlate this rhythm with the heart rhythm observed during head-up tilt test (HUT).

BACKGROUND

Heart rhythm obtained during provocative condition is often used to guide therapy in VVS. To date there is no conclusive evidence that the heart rhythm observed during a positive HUT can predict heart rhythm during VVS or that the heart rhythm observed during a spontaneous syncope will be identical to the recurrent syncope.

METHODS

Twenty-five consecutive VVS patients (age 60.2 +/- 17.1 years; 14 women,) presenting with frequent syncopes (6.9 +/- 4.6 episodes/year) and a positive HUT (cardioinhibitory in 8 patients) were implanted with an ILR. Seven of them also had a positive adenosine triphosphate (ATP) test.

RESULTS

Follow-up was 17.0 +/- 3.6 months. Thirty VVS were observed in 12 patients. Nine episodes showed bradycardia of <40 beats/min or asystole; progressive sinus bradycardia preceding sinus arrest was the most frequent electrocardiographic finding. Twenty-one syncopes occurred without severe bradycardia. The heart rhythm observed during the first syncope was identical to the recurrence. No correlation was found between slow heart rate at the ILR interrogation and a cardioinhibitory HUT response (p = 1.0) or a positive ATP test (p = 1.0).

CONCLUSIONS

In highly symptomatic patients with VVS, the heart rhythm observed during spontaneous syncope does not correlate with the HUT. The heart rhythm during the first spontaneous syncope is identical to the recurrent syncope.

Relation of clinical presentation of syncope to the age of patients

The aim of the study was to evaluate the effect of patient age on the clinical presentation of syncope and to establish the diagnostic value of clinical history in older patients. A total of 485 consecutive patients with unexplained syncope referred to secondary and tertiary hospitals were divided into 2 predefined age groups: 224 patients <65 years and 261 patients > or =65 years. The diagnostic criteria for the cause of syncope were developed before the beginning of the study. The clinical features of syncope were analyzed using a standard 46-item form. A cardiac cause of syncope was established in 27 younger (12%) and 89 older (34%) patients. A neurally mediated cause was established in 154 younger (68%) and 142 older (54%) patients. In patients > or =65 years, the clinical features of cardiac and neurally mediated syncope were very similar. The diagnosis of the cause of syncope was possible on the basis of the history alone in 26% younger and 5% older patients (p <0.0001). Myoclonic movements, effort syncope, and supine position during loss of consciousness were the most specific (97%, 99%, and 99%, respectively) diagnostic criteria for a cardiac cause of syncope in older patients, but, all together, they accounted for only 14% of patients. In conclusion, compared with younger patients, the medical history has a limited value in the diagnosis of the cause of syncope in older patients. The specificity of some features is high but these can be observed in only a minority of patients.

[The adenosine-5'-triphosphate (ATP) test: a diagnostic tool in the management of syncope of unknown origin. Basic and clinical aspects]

The prevalence of syncope increases in elderly population. An inappropriate reflex reaction of the autonomous nervous system in specific circumstances is responsible for symptoms in more than 50% of cases. These neurocardiogenic or vasovagal syncopes are due to a vasoplegia or to a cardio-inhibitory reflex or to the association of both mechanisms. Reproducing the symptoms may contribute to identify the responsible mechanism of syncope; this objective is partly reached by the head-up tilt test--a provocative test--which reproduces symptoms in about half of the cases, mainly by provoking a severe vasodilation with fall of blood pressure. On the opposite, the ATP test (20 mg i.v. bolus)--a descriptive test--provokes a strong cardio-inhibitory reflex independent from external factors like body position but it must be undertaken in a calm environment for preventing any anticipative sympathetic reaction. The test is considered positive if ATP produces a cardiac pause longer than 10 seconds and related symptoms are not taken into account for assessing the final result. ATP test positivity increases with age and with the presence of cardiac diseases. Permanent dual chamber pacing at 70 bpm has been shown to reduce significantly the syncope recurrences in patients with positive ATP test and to have no influence on patients with a negative test. The use of ATP test should be extended to all syncope screening. Its positivity identifies a group of patients in whom the mechanism of the syncope is susceptible to be improved by a permanent dual chamber pacing.

Significance of adenosine-induced atrioventricular block in patients with unexplained syncope

OBJECTIVES

The purpose of this study was to test whether a prolonged (>/=6 seconds) period of AV block in response to adenosine triphosphate (ATP) identifies additional patients at risk for bradycardia who may benefit from pacemaker implantation.

BACKGROUND

Bradycardia is a common etiology for syncope in patients without underlying structural heart disease. Conventional testing using electrophysiologic and tilt table studies often fail to identify patients prone to episodes of symptomatic bradycardia.

METHODS

Adenosine testing was performed in 92 consecutive patients (64 women, age 55 +/- 21 years) with syncope of uncertain origin referred for tilt table testing. The adenosine test measured the maximal R-R interval after bolus administration of intravenous adenosine 150 mug/kg to upright patients. A positive adenosine AV block response was defined as a maximal R-R interval > or =6 seconds.

RESULTS

A total of 21 patients (23%) had a positive response. During mean follow-up of 14.3 +/- 5.9 months, 14 patients (16%) had recurrent syncope. Among patients with a positive adenosine response, 3 patients (14%) had recurrent syncope. In comparison, 11 of 69 patients (16%) without adenosine-induced AV block had recurrent syncope (P = 1.00).

CONCLUSIONS

Prolonged adenosine-induced AV block in patients with unexplained syncope failed to predict recurrent syncopal episodes. These data do not support therapeutic interventions (e.g., pacemaker implantation) based on a positive adenosine AV block response alone.

Paroxysmal atrioventricular block: clinical experience with 20 patients

BACKGROUND

Paroxysmal atrioventricular (AV) block is an ill-defined entity, previously described in sporadic cases in association with vasovagal reaction, coronary angiography and distal conduction disease.

METHODS

We describe 20 patients (10 women) aged 26 to 80 years with symptomatic paroxysmal AV block.

RESULTS

Eight patients had ischemic heart disease-three with dilated cardiomyopathy, and two with co-existing carotid sinus hypersensitivity. Eight were taking chronic AV blockers. In five patients, the paroxysmal AV block occurred during a vagal reaction, in one during migranotic headaches, in one following aortic valve replacement and in one while recovering from acute myocardial infarction. The events lasted between 2.2 and 36 s. In 10 patients, the QRS configuration on the electrocardiogram was wide. Immediate treatment consisted of intravenous atropine and fluid supplements in two patients, discontinuation of the AV blocking agents in four, and the insertion of a temporary pacemaker in eight. Seventeen patients required a permanent pacemaker.

CONCLUSIONS

Paroxysmal AV block is an underestimated clinical entity related to vagal reaction, AV blocking drugs and distal conduction disease. Most of our patients eventually required implantation of a permanent pacemaker.

Single-stage adenosine tilt testing in patients with unexplained syncope

INTRODUCTION

We previously have shown that a 3-minute single-stage adenosine tilt test has a diagnostic yield comparable to a two-stage protocol consisting of a 30-minute drug-free tilt followed by a 15-minute isoproterenol tilt. In this study, we sought to further define the clinical utility of adenosine tilt testing in patients with unexplained syncope by prospectively evaluating test specificity and determining predictors of a positive test response.

METHODS AND RESULTS

The specificity of single-stage adenosine tilt testing was determined using 30 control subjects. To determine the diagnostic yield of this protocol, adenosine tilts were performed in 129 patients with unexplained syncope. The adenosine tilt test protocol had high specificity (100%) but a low overall diagnostic yield (18%). However, the yield was affected significantly by age. In patients </=40 years of age, the tilt test was positive in 15 (41%) of 37 patients, which was significantly greater than the yield in patients between the ages of 41 and 64 years (6/41 patients [15%], P = 0.012) and those >/=65 years of age (2/41 patients [5%], P < 0.0001).

CONCLUSION

These data support single-stage adenosine tilt testing in patients </=40 years of age because the diagnostic yield of the test is maximal in this group and the test can be completed in </=3 minutes. Conversely, the diagnostic yield of adenosine tilt testing in patients >40 years of age is low, suggesting that the clinical utility of this protocol is limited in these patients.

Pharmacological treatment of reflex syncope

Patient education, identification of possible triggers of syncope and reassurance are a central feature of the management of patients with reflex syncope. Patients should be advised as to the importance of adequate hydration and taught physical countermaneuvers to enhance cardiac venous return. These maneuvers are sufficient for most patients, however, for a small number of patients who continue to have recurrent syncopal events, pharmacological intervention may be considered. Volume expansion can be enhanced with salt and fludrocortisone. Agents from diverse pharmacological classes have been used to attenuate the reflex response, enhance vasoconstriction and attenuate vagal outflow. Alpha adrenoreceptor agonists, anticholinergic agents, theophylline, beta adrenoreceptor antagonists, serotonin reuptake inhibitors and disopyramide are the most widely studied. None of these agents has shown a consistent therapeutic benefit in clinical trials.

Paroxysmal atrioventricular block in young patients

We describe 17 patients (8 girls, and 9 boys), aged 9.6 +/- 5.7 years, with paroxysmal atrioventricular block (PAVB), a condition rarely described in children. Holter monitoring documented the PAVB in 15 patients, and tilt test was performed in 4 patients (positive in 1). The electrocardiograph (ECG) was normal in 7 patients. Two patients had acquired and 11 patients had congenital heart disease (CHD). Syncope or presyncope were present in 7 patients. A normal ECG was significantly more frequent in symptomatic patients. Pauses were significantly longer in girls and in children <5 years. PAVB was recorded only during nocturnal hours in 6 patients and throughout the day in the others. The sinus rate decreased during PAVB in 6 patients and increased in 4 (generally younger girls with symptoms). Permanent pacemakers were implanted in 13 patients, including 7 asymptomatic patients with CHD and severe bradycardia. During follow-up (3.7 +/- 2.5 years), 1 patient developed complete AVB. Although PAVB was still present in 91% of paced patients, symptoms did not recur because pacing prevented the pauses. In conclusion, PAVB is a rare arrhythmia. Autonomic nervous system dysfunction seems to play an etiological role and permanent pacing was an effective treatment.

Selective use of diagnostic tests inpatients with syncope of unknown cause

OBJECTIVE

We sought to prospectively assess the diagnostic yielding of a protocol in which electrophysiologic studies (EPS), tilt-table tests (TTTs), and loop recorder implantation are selectively used.

BACKGROUND

The optimal strategy in the diagnosis of patients with syncope of unknown cause has not been defined.

METHODS

A total of 184 consecutive patients with syncope of unknown cause were classified into two groups. Group A consisted of 72 patients fulfilling any of the following criteria: 1) presence of structural heart disease or family history of sudden death; 2) abnormal electrocardiogram; 3) significant non-symptomatic arrhythmia on Holter monitoring; and 4) paroxysmal palpitations immediately before or after syncope. These patients initially underwent an EPS and, if this study was negative, TTT. In the remaining 112 patients (group B), TTT was performed.

RESULTS

The EPS was positive in 32 patients (44%) in group A. The TTT was positive in 80 patients (71%) in group B. An additional patient had carotid sinus hypersensitivity. In patients of group A with a negative EPS, the TTT was positive in 23 (57%). A loop recorder was implanted in 15 patients from group A with negative conventional testing, and diagnostic activation was obtained in seven patients. Overall, a positive diagnosis was achieved in 143 patients (78%).

CONCLUSIONS

In patients with syncope of unknown cause, selective use of EPS or TTT leads to a positive diagnosis in >70% of the cases. An implantable loop recorder can be useful in non-diagnosed cases.

Mechanism of syncope in patients with positive adenosine triphosphate tests

OBJECTIVES

We prospectively evaluated the mechanism of syncope in patients with positive adenosine triphosphate (ATP) tests (defined as the induction of atrioventricular [AV] block with a ventricular pause >/=6 s after an intravenous bolus of 20 mg ATP).

BACKGROUND

Patients with unexplained syncope tend to have more positive ATP tests results than those without syncope.

METHODS

An implantable loop recorder (ILR) was inserted in 36 ATP-positive patients (69 +/- 10 years; 22 women; median of 6 syncopal episodes); 15 of them also had a positive response to tilt testing.

RESULTS

During the follow-up of 18 +/- 9 months, 18 patients (50%) had syncopal recurrence and 16 (44%) had an electrocardiographically documented episode: AV block (n = 3: paroxysmal in 2 and permanent in 1), AV block followed by sinus arrest (n = 1), sinus arrest (n = 5), sinus bradycardia <40 beats/min (n = 2), normal sinus rhythm (n = 2), sinus tachycardia (n = 1), rapid atrial fibrillation (n = 1), and ectopic atrial tachycardia (n = 1). Bradycardia was documented in a total of 11 cases (69%), and a long ventricular pause (4 to 29 s) was present in eight cases (50%). All three patients with ILR-documented AV block had previously had a negative tilt test, whereas seven of eight with ILR-documented sinus bradycardia or sinus arrest had previously had a positive tilt test.

CONCLUSIONS

In patients with adenosine-sensitive syncope, the mechanism of syncope is heterogeneous, although bradycardia is the most frequent finding. Adenosine triphosphate-induced AV block predicts AV block as the mechanism of spontaneous syncope in only a few tilt-negative patients.

Diagnosis and treatment of neurally mediated syncope

BACKGROUND

Syncope is caused by a severe but reversible reduction in blood flow to the brain stem neurons responsible for supporting consciousness (reticular activating system). Neurally mediated syncope, also referred to as vasovagal or reflex syncope, is the most frequent cause of loss of consciousness in apparently normal subjects.

REVIEW SUMMARY

Neurally mediated syncope is believed to be a reflex response with afferent, central, and efferent pathways. Characteristic autonomic changes in neurally mediated syncope are an increase in parasympathetic efferent activity causing bradycardia and a reduction in sympathetic vasoconstrictor outflow causing vasodilatation. Premonitory symptoms, such as nausea, diaphoresis, abdominal discomfort, and blurred vision, are caused by autonomic activation and are distinguishing features of neurally mediated syncope. Neurally mediated syncope frequently has a characteristic trigger, although this may not be apparent. Testing orthostatic tolerance during passive head-up tilt is the best available diagnostic procedure to evaluate patients with syncope in whom a cardiac cause has been excluded. In many cases, once the diagnosis of neurally mediated syncope is confirmed, it may suffice to reassure the patient and teach him to avoid known triggers and to recognize and act upon early warning symptoms. Because subjects with neurally mediated syncope may potentially be sodium depleted, increasing salt intake can be beneficial in improving their orthostatic intolerance.

CONCLUSIONS

Neurally mediated syncope is the most common form of syncope in healthy adults. The best diagnostic tools are the clinical history and passive head-up tilt. The best treatment strategies are the avoidance of triggering factors as well as intravascular volume expansion.

Alterations in systemic and local colonic hemodynamic variables associated with intravenous infusion of ATP-MgCl2 in healthy anesthetized horses

OBJECTIVE

To characterize alterations in systemic and local colonic hemodynamic variables associated with IV infusion of ATP-MgCl2 in healthy anesthetized horses.

ANIMALS

12 adult horses.

PROCEDURE

Six horses were given ATP-MgCl2, IV, beginning at a rate of 0.1 mg of ATP/kg of body weight/min with incremental increases until a rate of 1.0 mg/kg/min was achieved. The remaining 6 horses were given an equivalent volume of saline (0.9% NaCl) solution over the same time period. Colonic and systemic hemodynamic variables and colonic plasma nitric oxide (NO) concentrations were determined before, during, and after infusion.

RESULTS

Infusion of ATP-MgCl2 caused a rate-dependent decrease in systemic and colonic vascular resistance, principally via its vasodilatory effects. A rate of 0.3 mg of ATP/kg/min caused a significant decrease in systemic and colonic arterial pressure and colonic vascular resistance without a significant corresponding decrease in colonic arterial blood flow. Consistent alterations in NO concentrations of plasma obtained from colonic vasculature were not detected, despite profound vasodilatation of the colonic arterial vasculature.

CONCLUSIONS AND CLINICAL RELEVANCE

Results revealed that IV infusion of ATP-MgCl2 may be beneficial in maintaining colonic perfusion in horses with ischemia of the gastrointestinal tract, provided a sufficient pressure gradient exists to maintain blood flow.

The adenosine triphosphate test: a bedside diagnostic tool for identifying the mechanism of supraventricular tachycardia in patients with palpitations

OBJECTIVES

This study assesses the value of the "ATP test" (injection of adenosine triphosphate [ATP] during sinus rhythm) for identifying patients with palpitations of unclear etiology who actually have atrioventricular (AV) nodal re-entry tachycardia (AVNRT) or AV re-entry tachycardia (AVRT).

BACKGROUND

Because AVNRT and AVRT can be cured with radiofrequency ablation, documentation of spontaneous AVNRT or AVRT usually prompts referral for electrophysiologic (EP) evaluation. However, these paroxysmal arrhythmias may elude clinical diagnosis. We recently showed that administration of ATP during sinus rhythm often reveals dual AV node physiology or a concealed accessory pathway (AP) in patients with documented AVNRT or AVRT. Thus, we postulated that the ATP test could identify patients with palpitations who actually have AVNRT or AVRT and would therefore benefit from EP evaluation.

METHODS

One hundred forty-six patients (54 with "palpitations without documented arrhythmias" and 92 with "documentation of arrhythmias of unclear mechanism") underwent a noninvasive ATP test. ATP was injected during sinus rhythm using 10 mg increments. The ATP test was considered positive when prospectively defined signs of dual AV node physiology or concealed AP were disclosed in the electrocardiogram. These findings were correlated with the results of EP evaluation.

RESULTS

A positive ATP test predicted induction of AVNRT or AVRT with a positive predictive value of 93% (sensitivity 71%) but a negative predictive value of 37% (specificity 76%).

CONCLUSIONS

A bedside ATP test identifies patients with palpitations who are likely to have AVNRT or AVRT (and who are therefore likely to benefit from EP evaluation) with a high positive predictive value.

Diagnostic value of history in patients with syncope with or without heart disease

OBJECTIVES

We sought to establish what historical findings are predictive of the cause of syncope.

BACKGROUND

The clinical features of the various types of syncope have not been systematically investigated.

METHODS

Three hundred forty-one patients with syncope were prospectively evaluated. Each patient was interviewed using a standard questionnaire. A cause of syncope was assigned using standardized diagnostic criteria.

RESULTS

A cardiac cause of syncope was established in 23% of the patients, a neurally mediated cause in 58% and a neurologic or psychiatric cause in 1%, and in the remaining 18%, the cause of syncope remained unexplained. In a preliminary analysis including age, gender and the presence of suspected or certain heart disease after the initial evaluation, only heart disease was an independent predictor of a cardiac cause of syncope (odds ratio 16, p = 0.00001), with a sensitivity of 95% and a specificity of 45%. In contrast, the absence of heart disease allowed us to exclude a cardiac cause of syncope in 97% of the patients. In patients with certain or suspected heart disease, the most specific predictors of a cardiac cause were syncope in the supine position or during effort, blurred vision and convulsive syncope. Significant and specific predictors of a neurally mediated cause were time between the first and last syncopal episode >4 years, abdominal discomfort before the loss of consciousness and nausea and diaphoresis during the recovery phase. In the patients without heart disease, palpitation was the only significant predictor of a cardiac cause.

CONCLUSIONS

The presence of suspected or certain heart disease after the initial evaluation is a strong predictor of a cardiac cause of syncope. A few historical findings are useful to predict cardiac and neurally mediated syncope in patients with and without heart disease.

Diagnostic evaluation and management of patients with syncope

This article focuses on the evaluation of patients with syncope, a symptom not a disease. Syncope is a transient loss of consciousness associated with loss of postural tone with spontaneous recovery. The authors discuss the utility of an indications for different diagnostic tests, the indications for hospital admission, and the management of patients with certain known causes of syncope, including vasovagal and arrhythmic.

Spontaneous paroxysmal atrioventricular block in patients with positive tilt tests and negative electrophysiologic studies

A subgroup of patients with neurocardiogenic syncope and negative electrophysiologic studies and adenosine tests (in 5 of 6 cases), who developed symptomatic paroxysmal atrioventricular block in the natural, ambulatory state, had positive tilt tests without advanced block. Lack of concordance between electrocardiographic changes may have reflected differential effects of the autonomic nervous system in the sinus and atrioventricular nodes, occurring in diverse circumstances and less likely because of the protocol used for tilt testing.

Clinical features of adenosine sensitive syncope and tilt induced vasovagal syncope

AIM

To evaluate the possible relation between adenosine sensitive syncope and tilt induced vasovagal syncope.

METHODS

An ATP test and a head up tilt test were performed in 175 consecutive patients with syncope of uncertain origin. The ATP test consisted of the rapid intravenous injection of 20 mg of ATP; a positive response was defined as the induction of a ventricular pause (maximum RR interval) >/= 6000 ms. The head up tilt test was performed at 60 degrees for 45 minutes; if negative, 0.4 mg oral glyceryl trinitrate spray was given and the test continued for a further 20 minutes; a positive response was defined as induction of syncope in the presence of bradycardia, hypotension, or both.

RESULTS

Of the 121 patients with a positive response, 77 (64%) had a positive head up tilt alone, 18 (15%) had a positive ATP test alone, and in 26 (21%) both ATP and head up tilt were positive. Compared with the patients with isolated positive head up tilt, those with isolated positive ATP were older (mean (SD) age, 68 (10) v 45 (20) years), had a lower median number of syncopal episodes (2 v 3), a shorter median duration of syncopal episodes (4 v 36 months), a lower prevalence of situational, vasovagal, or triggering factors (11% v 64%), a lower prevalence of warning symptoms (44% v 71%), and a higher prevalence of systemic hypertension (22% v 5%) and ECG abnormalities (28% v 9%). The patients with a positive response to both tests had intermediate features. Of the 44 positive responses to the ATP test, atrioventricular block was the cause of the ventricular pause in 43; of the 29 positive cardioinhibitory responses to head up tilt, sinus arrest was present in 23 cases and atrioventricular block in six.

CONCLUSIONS

ATP and head up tilt tests identify different populations of patients affected by syncope; these have different general clinical features, different histories of syncopal episodes, and different mechanism sites of action. Therefore, adenosine sensitive syncope and tilt induced vasovagal syncope are two distinct clinical entities.