The diagnostic value of ATP testing in patients with unexplained syncope

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.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

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.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Syncope in adults: terminology, classification, and diagnostic strategy

Syncope is a relatively common clinical syndrome that is a subset of a broader range of conditions that cause transient loss of consciousness (TLOC). Other TLOC conditions include seizures, concussions, and intoxications. However, despite frequent confusion in the medical literature, syncope is and should be clearly distinguished from those other causes of TLOC by virtue of its pathophysiology; specifically, syncope is the result of self-terminating inadequacy of global cerebral nutrient perfusion, while the other forms of TLOC have different etiologies. Unfortunately, the diagnostic evaluation of syncope remains for the most part poorly managed and inefficient. Careful history-taking, physical examination, and judicious use of tests can not only increase the diagnostic yield of the evaluation, but also increase patient safety and reduced overall healthcare costs.

New approaches to the treatment and prevention of neurally mediated reflex (neurocardiogenic) syncope

Neurally mediated reflex syncope (sometimes referred to as neurocardiogenic syncope), encompasses a group of disorders of which the best known and most frequently occurring forms are the vasovagal (or common) faint, and carotid sinus syndrome. Postmicturition syncope, defecation syncope, cough syncope, and other situational reflex faints are also included among these conditions. With the exception of carotid sinus syndrome in which cardiac pacing is effective, treatment of most neurally mediated reflex faints is shifting from reliance on various drugs to greater emphasis on education and nonpharmacologic therapy. Initial management should include counseling of patients regarding recognition of early warning symptoms, and avoidance of precipitating factors. Volume expansion with salt tablets or electrolyte-containing beverages and patient education on how to perform isometric arm contractions and/or leg crossing in order to abort impending syncope are also important. Thereafter, tilt-training has demonstrated benefit in several clinical studies. When symptoms remain despite the above-noted interventions, pharmacologic therapy with midodrine or a nonselective b-blocker can be considered. In the case of most neurally mediated reflex faints, permanent cardiac pacing should be reserved only for those older patients with significant bradycardia or asystole at time of syncope when all other interventions have failed.