The terms “programmed stimulation” and “entrainment,” in the context of this article, refer to specific methods of pacing the heart. Typically, these techniques are used to gather information about the cardiac conduction system, which can then be used to guide the treatment of heart rhythm disorders.  As an example of its utility, these techniques demonstrated that most ventricular tachycardia in the setting of ischemic heart disease is reentrant rather than automatic.  These observations have led to the development of anti-tachycardia pacing as a painless alternative to high-energy implantable cardioverter-defibrillator (ICD) shocks and have been instrumental in guiding the ablative therapy of ventricular and supraventricular arrhythmias. 
Programmed stimulation, which is a means of entrainment, is most commonly used during invasive electrophysiologic studies, although it may also be accomplished to some degree through an existing pacemaker or implanted defibrillator. Most of this article focuses on the principles involved, which can be applied in either situation.
This article also discusses some of the historical basis for the techniques and the general electrophysiologic principles involved in programmed stimulation of the heart, although a comprehensive review of the subject is beyond the scope of this article. While these techniques are typically used to guide ablation, the discussion of ablation per se is also beyond the scope of this article.
The most common indication for programmed stimulation is the evaluation of tachycardias, especially when ablation is planned. Other indications for programmed stimulation include the evaluation of syncope in the setting of structural heart disease, stratification of the risk of sudden death in patients with a history of remote myocardial infarction (MI), assessment of the success of ventricular tachycardia ablation, and evaluation of patients with remote MI who have symptoms suggestive of ventricular arrhythmia.  In years past, it was common to perform serial programmed stimulation to assess the efficacy of antiarrhythmic drug therapy; in the current era of routine ICD use, this is less frequently performed.
When these techniques are used for risk stratification with regard to ventricular arrhythmias, one must recognize that the substrate plays a critical role in determining the value of programmed stimulation. While a large body of evidence supports the use of programmed stimulation in ischemic cardiomyopathy, its use in the evaluation of nonischemic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, and Brugada syndrome is less clear. [5, 6, 7, 8]
While electrocardiography (ECG) and ambulatory monitoring remain the mainstay of diagnosis in suspected bradyarrhythmias, invasive electrophysiologic testing continues to have a role, especially in evaluating sinus and atrioventricular (AV) nodal function when noninvasive means have been unrevealing.
Programmed stimulation has a few absolute contraindications. As a rule, patients with unstable angina should be excluded from any aggressive pacing protocols. Patients with decompensated heart failure should be medically optimized prior to an electrophysiology study or other forms of programmed stimulation except when the heart failure is a result of the arrhythmia being investigated. In some instances, programmed stimulation is performed in the evaluation of hemodynamically unstable rhythms; in these cases, hemodynamic support with intraaortic balloon pumps and/or intravenous pressors may be indicated.
When the procedure is performed transvenously, rather than through an existing pacemaker or ICD; bacteremia, deep venous thrombosis at the planned access site, or the presence of an untreated bleeding diathesis are considered absolute contraindications. As with any procedure, the inability to give informed consent is also an absolute contraindication.
The procedural room should be equipped with emergency equipment, including a crash cart and two external defibrillators. The support staff should have appropriate training and experience.
Careful positioning of diagnostic catheters is always prudent. In particular, the right ventricular catheter should be directed toward the right ventricular septum rather than the true apex, as this is the thinnest part of the ventricle. Patients on chronic steroid therapy are at particular increased risk of cardiac perforation, and extra care should be taken when positioning catheters in this cohort.
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- Periprocedural Care