Steven M. Markowitz

Differential Effects of Adenosine on Focal and Macroreentrant Atrial Tachycardia

Focal and Macroreentrant Atrial Tachycardia. Introduction: The effects of adenosine on atrial tachycardia (AT) remain controversial, and the mechanistic implications of adenosine termination have not been fully established. The purpose of this study was to elucidate the differential effects of adenosine on focal and macroreentrant AT and describe the characteristics of adenosine‐sensitive AT.

Relationship of Reverse Anatomical Remodeling and Ventricular Arrhythmias After Cardiac Resynchronization

Ventricular Arrhythmias and Reverse Remodeling. Introduction: Cardiac resynchronization (CRT) affects reverse anatomical remodeling in patients with heart failure. CRT has also been associated with fewer ventricular arrhythmias and reduced sudden death in some clinical trials, but the predictors and mechanism of the antiarrhythmic actions of CRT have not been well defined. The purpose of this study is to investigate the relationship of reverse anatomical remodeling to ventricular arrhythmias in CRT patients.

Adenosine-sensitive ventricular tachycardia: a conceptual approach.

Adenosine‐Sensitive VT. Idiopathic ventricular tachycardia (VT) is a term that refers to tachycardia that arises from ventricles devoid of apparent structural abnormalities. This form of VT is now recognized to be related to several distinct entities and includes a reentrant form typically located in the region of the left posterior fascicle, an automatic form that may originate from either ventricle, and a form that originates from the right ventricular outflow tract. This last type can account for up to 80% of cases of idiopathic VT and with few exceptions can be further subdivided into repetitive monomorphic VT and paroxysmal stress‐induced VT, Evidence has accumulated suggesting that both forms of VT are related to cAMP‐mediated triggered activity. The experimental underpinnings of this conclusion as well as the clinical characteristics of this form of idiopathic VT are elucidated in this review.

Mechanism-Specific Effects of Adenosine on Ventricular Tachycardia

There is no universally accepted method by which to diagnose clinical ventricular tachycardia (VT) due to cAMP‐mediated triggered activity. Based on cellular and clinical data, adenosine termination of VT is thought to be consistent with a diagnosis of triggered activity. However, a major gap in evidence mitigates the validity of this proposal, namely, defining the specificity of adenosine response in well‐delineated reentrant VT circuits. To this end, we systematically studied the effects of adenosine in a model of canine reentrant VT and in human reentrant VT, confirmed by 3‐dimensional, pace‐ and substrate mapping.

Recovery of Atrioventricular Conduction After Pacemaker Placement Following Cardiac Valvular Surgery

Atrioventricular block (AVB) occurs commonly after valve surgery, and permanent pacemaker (PPM) implantation is often required. However, the rate and time course of spontaneous recovery of AV conduction in these patients is not known. The goal of this study was to define the rate and risk factors for late high‐grade AVB in patients who have PPM implantation for this indication.

ICD Implantation and Arrhythmia‐Free Survival in Patients with Depressed LV Function Following Surgery for Valvular Heart Disease

Background: Although prophylactic implantable cardioverter‐defibrillator (ICD) implantation is beneficial in patients with severe ischemic cardiomyopathy, it is unclear whether patients with cardiomyopathy due to valvular heart disease have a similar benefit.

To test or not to test during defibrillator implantation? A reassessment of the conventional wisdom.

From the inception of implantable cardioverter defibrillators (ICDs), defibrillation testing has been conducted at the time of implantation. Initially, defibrillation testing was performed through implanted epicardial patches and an external defibrillator to test the efficacy of this arrangement in terminating ventricular fibrillation (VF). Testing in this manner assured the appropriateness of implanting a generator only after verifying successful defibrillation with the chosen lead configuration. The concept emerged of an adequate “safety margin” for defibrillation as an implant criterion, traditionally accepted as a defibrillation threshold (DFT) at least 10 J below the maximum energy of the device.1 With the introduction of endocardial leads, defibrillation testing remained an important aspect of implantation, as ≈15% patients were found to have high DFTs with monophasic shocks and required revision for new high-voltage coils or epicardial patches.2 The practice of defibrillation testing continues as standard clinical practice, although several groups have recently questioned the role for routine testing.3-5 These arguments center around the probabilistic nature of defibrillation and the high efficacy of current generation ICDs, but no large scale prospective trials have approached this issue directly. In this issue of the Journal, the study by Gula et al. add to the growing body of literature challenging universal defibrillation testing.6 In addressing this question, the authors performed a decision analysis to compare the strategies of routine testing versus no testing at the time of implantation, and they quantify the long-term survival benefit, if any, obtained by the routine testing strategy. They find that routine defibrillation testing confers no significant survival advantage, with nearly identical 5-year survival rates associated with each strategy (59.7% vs. 59.4%) calculated with their baseline probabilities. These results hold over a wide range of assumptions. Even if inadequate DFTs are common (up to 20% of the ICD population) or if defibrillation efficacy of spontaneous VT/VF is low in patients with high DFTs, they still find no significant survival advantage to ICD testing. In a highly unlikely “worse case scenario”—if high DFTs occur in 20% of patients and defibrillation efficacy is 0% in such patients—there is a small but statistically significant difference in 5-year survival favoring ICD testing, with a survival advantage of only 3.2%. Similarly, survival

Eligibility of Pacemaker Patients for Subcutaneous Implantable Cardioverter Defibrillators

The subcutaneous implantable cardioverter defibrillator (ICD) has emerged as a viable therapeutic option for patients who are deemed high risk for sudden cardiac death. Previous studies have shown that 7–15% of patients are not candidates for the S‐ICD based on their intrinsic QRS/T‐wave morphology. Presently, it is not known if the S‐ICD can be considered as supplementary therapy in patients who are ventricularly paced. We sought to determine the proportion of ventricularly paced patients who would qualify for an S‐ICD.

Defibrillator implantation in the elderly: patients are older, but are physicians wiser?

Health care reform in the U.S. has focused public attention on the cost of services and expected outcomes of our medical interventions. With finite resources, society is forced to confront difficult questions of which services to offer to which patients, and pressure will increase to base coverage decisions on objective evidence of benefit. A therapy that will likely attract scrutiny is implantable cardioverter defibrillator (ICD) implantation for primary prevention of sudden death. Randomized clinical trials have clearly shown absolute survival benefits for ICD implantation in the postinfarction and heart failure populations,1,2 and long-term follow-up has shown accrual of the survival benefit over time. In an 8year follow-up of the Multicenter Automatic Defibrillator Implantation Trial (MADIT-II) cohort, mortality among ICD recipients decreased by 37%, and the number needed to treat to save one life decreased from 17 (in the initial 2-year study) to 6 by 8 years of follow-up.3 A natural question to arise from these studies is whether some populations do not benefit from ICD implantation. As rates of nonsudden and noncardiac deaths increase, the survival benefit from ICDs will be reduced. Hence, efforts have intensified to identify comorbidities that impact survival in ICD recipients and incorporate these in risk scores that may have clinical utility. Several studies have shown that patients with a large number of comorbidities have reduced ICD survival benefit. For example, an analysis of the MADIT-II data set showed that a risk score of five clinical variables (one of which is age >70 years) can identify patients likely or not to have a survival advantage. No significant risk reduction was seen in those at very low risk of sudden death as well as those with multiple comorbidities who are likely to succumb from a competing cause of death.4 A risk prediction model of mortality in heart failure was recently applied to the Sudden Cardiac Death In Heart Failure Trial (SCD-HeFT) population and showed that patients at the highest risk of mortality derive little if any survival benefit after prophylactic ICD implantation.5 In real world practice, new therapies such as ICD implantation are often applied to patients with clinical characteristics that differ from those included in ran-

T‐Wave Alternans and ST Depression Assessment Identifies Low Risk Individuals with Ischemic Cardiomyopathy in the Absence of Left Ventricular Hypertrophy

Although ECG left ventricular hypertrophy (LVH) by Cornell product (CP) predicts increased mortality in patients with ischemic cardiomyopathy (ICM), those without CP LVH remain at relatively high risk. We examined whether T‐wave alternans (TWA) testing and ST depression can improve risk stratification in these patients.