Separation Between Virtual Sources Modifies the Response of Cardiac Tissue to Field Stimulation

INTRODUCTION: While it is now understood that the tissue geometry and the electric field distribution are important in generating virtual electrodes, the effects of interaction between a collection of electrodes have not been examined. To develop a basis for understanding such interactions, we have studied a single pair of oppositely polarized virtual sources. Although such oppositely polarized pairs of virtual electrodes can be generated by a variety of field distributions and tissue geometries, we examine one simple system that incorporates the salient features of source interaction. METHODS AND RESULTS: Our model system is a homogeneous tissue strip stimulated by a uniform extracellular field. To clarify virtual source interaction, we show that field stimulated tissue can be equivalently polarized by a set of intracellular current sources with magnitude and distribution defined by the generalized activating function. In our model system, an intracellular current source is produced at one edge of the tissue and an intracellular current sink at the other. Therefore, the tissue length acts to modulate the overlap, or interaction, between the polarizations arising from each source. To quantify the effects of source interaction, the chronaxie and rheobase values of the strength-duration relation were determined for source separations varying between 1.0 cm and 100 microm (active membrane dynamics were modeled with the Luo-Rudy phase I formulation). At all separations >3.0 mm, the chronaxie was constant at 3.09 msec and the rheobase was 0.38 V/cm. Under 0.2 mm, the chronaxie decreased to 0.55 msec while the rheobase increased linearly with the inverse of source separation. The dependence of these parameters on separation primarily reflects passive electrotonic interactions between the two virtual electrodes. However, the exact values are strongly dependent upon active tissue properties-largely the inward rectifier potassium channel and activation of the sodium current. CONCLUSION: Tissue excitation in response to field stimulation is strongly modulated by the proximity of, and therefore the interaction between, oppositely polarized virtual electrode sources.     

Life Satisfaction and Subsequent Physical,Behavioral, and Psychosocial Health in Older Adults

Policy Points

• Several intergovernmental organizations (Organisation for Economic Co‐operation and Development, World Health Organization, United Nations) are urging countries to use well‐being indicators (e.g., life satisfaction) in addition to traditional economic indicators when making important policy decisions.
• As the number of governments implementing this new approach grows, so does the need to continue evaluating the health and well‐being outcomes we might observe from policies aimed at improving life satisfaction.
• The results of this study suggest that life satisfaction is a valuable target for policies aiming to enhance several indicators of psychosocial well‐being, health behaviors, and physical health outcomes.

ContextSeveral intergovernmental organizations (Organisation for Economic Co‐operation and Development, World Health Organization, United Nations) are urging countries to use well‐being indicators (e.g., life satisfaction) in addition to traditional economic indicators when making important policy decisions. As the number of governments implementing this new approach grows, so does the need to continue evaluating the health and well‐being outcomes we might observe from policies aimed at improving life satisfaction.MethodsWe evaluated whether positive change in life satisfaction (between t₀;2006/2008 and t₁;2010/2012) was associated with better outcomes on 35 indicators of physical, behavioral, and psychosocial health and well‐being (in t₂;2014/2016). Data were from 12,998 participants in the University of Michigan''s Health and Retirement Study—a prospective and nationally representative cohort of US adults over age 50.FindingsParticipants with the highest (versus lowest) life satisfaction had better subsequent outcomes on some physical health indicators (lower risk of pain, physical functioning limitations, and mortality; lower number of chronic conditions; and higher self‐rated health) and health behaviors (lower risk of sleep problems and more frequent physical activity), and nearly all psychosocial indicators (higher positive affect, optimism, purpose in life, mastery, health mastery, financial mastery, and likelihood of living with spouse/partner; and lower depression, depressive symptoms, hopelessness, negative affect, perceived constraints, and loneliness) over the 4‐year follow‐up period. However, life satisfaction was not subsequently associated with many specific health conditions (i.e., diabetes, hypertension, stroke, cancer, heart disease, lung disease, arthritis, overweight/obesity, or cognitive impairment), other health behaviors (i.e., binge drinking or smoking), or frequency of contact with children, family, or friends.ConclusionsThese results suggest that life satisfaction is a valuable target for policies aiming to enhance several indicators of psychosocial well‐being, health behaviors, and physical health outcomes.     

Survival of Transvenous ICD Leads in Young Patients

Background : In adults, transvenous implantable cardioverter defibrillator (ICD) lead failure rates are significant, and their occurrence increases with time from implant. There are limited data in children. The goal of this study was to assess lead survival in young patients undergoing ICD implantation at a single center. Methods : Records of patients under 21 years old with transvenous ICD leads implanted at our center from June 1997 to August 2007 were retrospectively reviewed. Age, weight, height, diagnosis, lead and generator model, venous access technique, generator position, pacing thresholds, lead impedance, and R wave size were recorded. “Lead failure” was defined as any lead problem requiring surgical intervention to restore proper function to the ICD system. Results : Seventy‐one transvenous leads were included (70 patients). Average age at implant was 14.8 years (range 5.7–19.5). All the devices were implanted by a single operator (HMS). Venous access was obtained via cephalic cutdown in 66/71. Mean follow‐up time was 2.8 years (range 0.2–7.8 years, median 2.3 years). There were no infections requiring explantation. There were four lead failures. Three were lead fractures, occurring 12, 13, and 19 months after implant. The fourth lead failed when an arrhythmia was not appropriately detected, and a second dedicated rate‐sensing lead was thus implanted. Univariate analysis did not identify any variable to be a significant predictor of lead failure. Kaplan–Meier survival analysis demonstrated 5‐year lead survival at 89.6%. Conclusions : ICD lead survival in children, when performed by an experienced operator, is similar to that found in adults. (PACE 2010; 33:186–191)     

Fetal growth from mid‐ to late pregnancy is associated with infant development: the Generation R Study

Aim The aim of this study was to investigate within a population‐based cohort of 4384 infants (2182 males, 2202 females) whether fetal growth from early pregnancy onwards is related to infant development and whether this potential relationship is independent of postnatal growth. Method Ultrasound measurements were performed in early, mid‐, and late pregnancy. Estimated fetal weight was calculated using head and abdominal circumference and femur length. Infant development was measured with the Minnesota Infant Development Inventory at 12 months (SD 1.1mo, range 10–17mo). Information on postnatal head size and body weight at 7 months was obtained from medical records. Results After adjusting for potential confounders and for postnatal growth, faster fetal weight gain from mid‐ to late pregnancy predicted a reduced risk of delayed social development (odds ratio [OR] 0.82; 95% confidence interval [CI] 0.71–0.95, p=0.008), self‐help abilities (OR 0.84; 95% CI 0.73–0.98, p=0.023), and overall infant development (OR 0.65; 95% CI 0.49–0.87, p=0.003). Similar findings were observed for fetal head growth from mid‐ to late pregnancy. Interpretation Faster fetal growth predicts a lower risk of delayed infant development independent of postnatal growth. These results suggest that reduced fetal growth between mid‐ and late pregnancy may determine subsequent developmental outcomes.     

Coil Occlusion of the Arterial Duct

In 1976 Poretmann introduced a catheter-mediated method of closure of the arterial duct. The first device to achieve widespread use was the Rashkind double umbrella. Closure rates were of the order of 80% with one device. The method was expensive and occlusion with Gianturco coils was shown to be as effective in small ducts. More recently, controlled release coils have proved to be as effective and have considerable popularity because they appear to carry a lower risk of embolization from the duct, and closure rates with one or more coils implanted at one procedure are in excess of 95%. Coil occlusion is safe and effective. However, large ducts still need other types of devices or must be closed surgically.     

Initial Experience with Symbios Model 7008 Pacemaker

Le Symbios 7008 est capable de traiter les bradycardies et les tachycardies. Nous rapportons ici notre expérience initiale chez 21 patients chez qui ce boitier a été implanté. Tous présentaient des tachyarythmies supraventriculaires qui ont été arretées par stimulation auriculaire rapide (19 pts) ou séquentielle. Plus de la moitié des patients présentaient aussi des bradyarythmies nécessitant une stimulation et ont done ainsi bénéficié des deux fonctions du boitier. En l'état actuel des choses, l'expérience est favorable.