Mechanistic Insights and Knowledge Gaps in the Effects of Radiation Therapy on Cardiac Arrhythmias.

Stereotactic body radiation therapy (SBRT) is an innovative modality for the treatment of refractory ventricular arrhythmias (VAs). Phase 1/2 clinical trials have demonstrated the remarkable efficacy of SBRT at reducing VA burden (by >85%) in patients with good short-term safety. SBRT as an option for VA treatment delivered in an ambulatory nonsedated patient in a single fraction during an outpatient session of 15 to 30 minutes, without added risks of anesthetic or surgery, is clinically relevant. However, the underlying mechanism remains unclear. Currently, the clinical dosing of SBRT has been derived from preclinical studies aimed at inducing transmural fibrosis in the atria. The propitious clinical effects of SBRT appear earlier than the time course for fibrosis. This review addresses the plausible mechanisms by which radiation alters the electrophysiological properties of myocytes and myocardial conduction to impart an antiarrhythmic effect, elucidate clinical observations, and point the direction for further research in this promising area.

Radiation therapy for ventricular arrhythmias.

Ventricular arrhythmias (VA) can be life-threatening arrhythmias that result in significant morbidity and mortality. Catheter ablation (CA) is an invasive treatment modality that can be effective in the treatment of VA where medications fail. Recurrence occurs commonly following CA due to an inability to deliver lesions of adequate depth to cauterise the electrical circuits that drive VA or reach areas of scar responsible for VA. Stereotactic body radiotherapy is a non-invasive treatment modality that allows volumetric delivery of energy to treat circuits that cannot be reached by CA. It overcomes the weaknesses of CA and has been successfully utilised in small clinical trials to treat refractory VA. This article summarises the current evidence for this novel treatment modality and the steps that will be required to bring it to the forefront of VA treatment.

Evaluation of contemporary methods to determine vascular impedance.

The past decade has seen considerable growth in therapeutics and device technologies to treat patients with hypertension and other cardiovascular disease states. Uncoupling ventriculo-arterial interactions in these patients, however, is often complex and not adequately accounted for by arterial pressure or vascular resistance measurement alone. In reality, the global vascular load presented to the left ventricle (LV) includes both steady-state and pulsatile components. Whereas steady-state load is best represented by the vascular resistance, pulsatile load, which incorporates wave reflections and arterial stiffness, may oscillate during various phases of the cardiac cycle and is best determined by the vascular impedance (Z). In recent years, measurement of Z has become more readily accessible through an array of simultaneous applanation tonometry, echocardiography and cardiac magnetic resonance (CMR) techniques. In the following review, we evaluate existing and newer methods to assess Z so as to better understand the pulsatile characteristics of the human circulation in hypertension and other cardiovascular disease states.