Optimal method to achieve consistently low defibrillation energy requirements

Toward a More Efficient Implementation of Antifibrillation Pacing

We devise a methodology to determine an optimal pattern of inputs to synchronize firing patterns of cardiac cells which only requires the ability to measure action potential durations in individual cells. In numerical bidomain simulations, the resulting synchronizing inputs are shown to terminate spiral waves with a higher probability than comparable inputs that do not synchronize the cells as strongly. These results suggest that designing stimuli which promote synchronization in cardiac tissue could improve the success rate of defibrillation, and point towards novel strategies for optimizing antifibrillation pacing.

The dilemma of ICD implant testing

Ventricular fibrillation (VF) has been induced at implantable cardioverter defibrillator (ICD) implant to ensure reliable sensing, detection, and defibrillation. Despite its risks, the value was self-evident for early ICDs: failure of defibrillation was common, recipients had a high risk of ventricular tachycardia (VT) or VF, and the only therapy for rapid VT or VF was a shock. Today, failure of defibrillation is rare, the risk of VT/VF is lower in some recipients, antitachycardia pacing is applied for fast VT, and vulnerability testing permits assessment of defibrillation efficacy without inducing VF in most patients. This review reappraises ICD implant testing. At implant, defibrillation success is influenced by both predictable and unpredictable factors, including those related to the patient, ICD system, drugs, and complications. For left pectoral implants of high-output ICDs, the probability of passing a 10 J safety margin is approximately 95%, the probability that a maximum output shock will defibrillate is approximately 99%, and the incidence of system revision based on testing is < or = 5%. Bayes' Theorem predicts that implant testing identifies < or = 50% of patients at high risk for unsuccessful defibrillation. Most patients who fail implant criteria have false negative tests and may undergo unnecessary revision of their ICD systems. The first-shock success rate for spontaneous VT/VF ranges from 83% to 93%, lower than that for induced VF. Thus, shocks for spontaneous VT/VF fail for reasons that are not evaluated at implant. Whether system revision based on implant testing improves this success rate is unknown. The risks of implant testing include those related to VF and those related to shocks alone. The former may be due to circulatory arrest alone or the combination of circulatory arrest and shocks. Vulnerability testing reduces risks related to VF, but not those related to shocks. Mortality from implant testing probably is 0.1-0.2%. Overall, VF should be induced to assess sensing in approximately 5% of ICD recipients. Defibrillation or vulnerability testing is indicated in 20-40% of recipients who can be identified as having a higher-than-usual probability of an inadequate defibrillation safety margin based on patient-specific factors. However, implant testing is too risky in approximately 5% of recipients and may not be worth the risks in 10-30%. In 25-50% of ICD recipients, testing cannot be identified as either critical or contraindicated.

The effects of electrode position on the excitability of rat atria during postnatal development

Cardiac excitability is determined by the direction of the electric field, which is defined by the positioning of electrodes. However, important morphological and physiological modifications that happen during the postnatal development of the heart may affect the cardiac threshold. In this work we have evaluated the effect of electrode positioning on the excitability threshold of isolated Wistar rat atria (left auricles) during postnatal development. This was performed by determining the parameters of strength-duration curves for stimuli (rheobase, chronaxie and normalized minimum pulse energy) of atria from rats at ages (days) 5, 15, 30, 60, 90 and 120. These parameters were determined using electric field stimulation in four different orientations (apex-base, base-apex, left-right and right-left). Atrial rheobase decreased by 1.5- to 4-fold with animal age and was altered by electric field orientation in a diversified way, whereas atrial chronaxie increased only with animal age. The minimum pulse energy decreased two- to nine-fold with ageing. This was mainly due to rheobase dependence with electric field direction. We showed that the appropriate cardiac stimulation depends on the effects of three combined factors (pulse parameters, electrode position and animal age) on the atrial tissue excitability.

The pharmacodynamics of parenteral glycoprotein IIb/IIIa inhibitors

Glycoprotein (GP) IIb/IIIa antagonists are a unique class of antiplatelet agents introduced for the management of patients undergoing percutaneous coronary intervention (PCI) and those presenting with unstable angina or non-ST segment elevation (NSTE) myocardial infarction (MI), collectively recognized as acute coronary syndromes (ACS). Eptifibatide, abciximab, and tirofiban HCl are three GPIIb/IIIa antagonists approved for use by the Food and Drug Administration. Of the three agents, eptifibatide is approved for use in both PCI and NSTE ACS patient populations, whereas abciximab is indicated for patients undergoing PCI, and tirofiban is approved for patients with NSTE ACS. Dose selection for the initial trials using the three parenteral antagonists was based on in vitro and ex vivo pharmacodynamic assays conducted under different blood collection and platelet function assay conditions. Recent comparative pharmacodynamics studies, which used newly defined and standardized assay conditions, indicate that the platelet aggregation inhibition achieved with these dosing regimens is variable. Therefore, the differences in clinical efficacy as evidenced in the more recent clinical studies (e.g., Enhanced Suppression of the Platelet Receptor GPIIb/IIIa using Integrilin Therapy [ESPRIT], Global Use of Strategies to Open Occluded Coronary Arteries IV Acute Coronary Syndromes [GUSTO-IV ACS], and Do Tirofiban HCl and ReoPro Give Similar Efficacy Outcomes Trial [TARGET]) may be related to the variable antiplatelet effects of the approved dose regimens.