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Concept: Bradycardia

28

Rationale: Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is caused by mutations in cardiac ryanodine receptor (RyR2) or calsequestrin (Casq2) genes. Sinoatrial node dysfunction associated with CPVT may increase the risk for ventricular arrhythmia. Objective: To test the hypothesis that CPVT is suppressed by supraventricular overdrive stimulation. Methods and Results: Using CPVT mouse models (Casq2-/- and RyR2(R4496C)+/- mice), the effect of increasing sinus heart rate was tested by pretreatment with atropine and by atrial overdrive pacing. Increasing intrinsic sinus rate with atropine before catecholamine challenge suppressed ventricular tachycardia (VT) in 86% of Casq2-/- mice (6/7) and significantly reduced the ventricular arrhythmia (VA) score (atropine: 0.6±0.2 vs. vehicle: 1.7±0.3, p<0.05). Atrial overdrive pacing completely prevented VA in 16/19 (84%) Casq2-/- and in 7/8 (88%) RyR2(R4496C)+/- mice and significantly reduced ventricular premature beats in both CPVT models (p<0.05). Rapid pacing also prevented spontaneous calcium waves and triggered beats in isolated CPVT myocytes. In humans, heart-rate dependence of CPVT was evaluated by screening a CPVT patient registry for antiarrhythmic drug-naïve individuals that reached >85% of their maximum predicted heart rate during exercise testing. All 18 CPVT patients who fulfilled the inclusion criteria exhibited VA before reaching 87% of maximum heart rate. In six CPVT patients (33%), VA were paradoxically suppressed as sinus heart rates increased further with continued exercise. Conclusions: Accelerated supraventricular rates suppress VAs in two CPVT mouse models and in a subset of CPVT patients. Hypothetically, atrial overdrive pacing may be a therapy for preventing exercise-induced VT in treatment-refractory CPVT patients.

Concepts: Cardiology, Heart, Ventricular tachycardia, Supraventricular tachycardia, Heart rate, Sinoatrial node, Tachycardia, Bradycardia

25

Temporary pacemakers are used in a variety of critical care settings. These life-saving devices are reviewed in 2 major categories in this review: first, the insertion and management of epicardial pacemakers after and during cardiac surgery; and second, the insertion of transvenous temporary pacemakers for the emergent treatment of bradyarrhythmias. Temporary epicardial pacemakers are used routinely in patients recovering from cardiac surgery. Borrowing from advances in cardiac resynchronization therapy there are many theoretical and untested benefits to pacing the postoperative cardiac surgery patient. Temporary transvenous pacing is traditionally an emergency procedure to stabilize patients suffering from hemodynamically unstable bradyarrhythmia. We review the traditional and expanding use of transvenous pacemakers inside and outside the operating room.

Concepts: Patient, Hospital, Physician, Review, Tradition, Artificial pacemaker, Transcutaneous pacing, Bradycardia

18

Active flight requires the ability to efficiently fuel bursts of costly locomotion while maximizing energy conservation during non-flying times. We took a multi-faceted approach to estimate how fruit-eating bats (Uroderma bilobatum) manage a high-energy lifestyle fueled primarily by fig juice. Miniaturized heart rate telemetry shows that they use a novel, cyclic, bradycardic state that reduces daily energetic expenditure by 10% and counteracts heart rates as high as 900 bpm during flight. Uroderma bilobatum support flight with some of the fastest metabolic incorporation rates and dynamic circulating cortisol in vertebrates. These bats will exchange fat reserves within 24 hr, meaning that they must survive on the food of the day and are at daily risk of starvation. Energetic flexibly in U. bilobatum highlights the fundamental role of ecological pressures on integrative energetic networks and the still poorly understood energetic strategies of animals in the tropics.

Concepts: Biodiversity, Metabolism, Energy, Muscle, Heart rate, Conservation of energy, Bradycardia, Energy conservation

7

Endurance athletes exhibit sinus bradycardia, that is a slow resting heart rate, associated with a higher incidence of sinus node (pacemaker) disease and electronic pacemaker implantation. Here we show that training-induced bradycardia is not a consequence of changes in the activity of the autonomic nervous system but is caused by intrinsic electrophysiological changes in the sinus node. We demonstrate that training-induced bradycardia persists after blockade of the autonomous nervous system in vivo in mice and in vitro in the denervated sinus node. We also show that a widespread remodelling of pacemaker ion channels, notably a downregulation of HCN4 and the corresponding ionic current, If. Block of If abolishes the difference in heart rate between trained and sedentary animals in vivo and in vitro. We further observe training-induced downregulation of Tbx3 and upregulation of NRSF and miR-1 (transcriptional regulators) that explains the downregulation of HCN4. Our findings provide a molecular explanation for the potentially pathological heart rate adaptation to exercise training.

Concepts: Brain, Cardiology, Cardiac electrophysiology, Autonomic nervous system, Parasympathetic nervous system, Cardiac pacemaker, Electrical conduction system of the heart, Bradycardia

4

Blunted tachycardia during hypotension is a characteristic feature of patients with autonomic failure, but the range has not been defined. This study reports the range of orthostatic heart rate (HR) changes in patients with autonomic failure caused by neurodegenerative synucleinopathies.

Concepts: Cardiology, Orthostatic hypotension, Bradycardia

4

Permanent cardiac pacing is the only effective treatment for symptomatic bradycardia, but complications associated with conventional transvenous pacing systems are commonly related to the pacing lead and pocket. We describe the early performance of a novel self-contained miniaturized pacemaker.

Concepts: Cardiology, Cardiac electrophysiology, Heart rate, Cardiac pacemaker, Artificial pacemaker, Transcutaneous pacing, Bradycardia

3

Rationale: Downregulation of the pacemaking ion channel, HCN4, and the corresponding ionic current, If, underlies exercise training-induced sinus bradycardia in rodents. If this occurs in humans, it could explain the increased incidence of bradyarrhythmias in veteran athletes and it will be important to understand the underlying processes. Objective: To test the role of HCN4 in the training-induced bradycardia in human athletes and investigate the role of micro-RNAs (miRs) in the repression of HCN4. Methods and Results: As in rodents, the intrinsic heart rate was significantly lower in human athletes than non-athletes and in all subjects the rate-lowering effect of the HCN selective blocker, ivabradine, was significantly correlated with the intrinsic heart rate, consistent with HCN repression in athletes. Next generation sequencing and qPCR showed remodelling of miRs in the sinus node of swim-trained mice. Computational predictions highlighted a prominent role for miR-423-5p. Interaction between miR-423-5p and HCN4 was confirmed by a dose-dependent reduction in HCN4 3'-UTR luciferase reporter activity on co-transfection with precursor miR-423-5p (abolished by mutation of predicted recognition elements). Knockdown of miR-423-5p with antimiR-423-5p reversed training-induced bradycardia via rescue of HCN4 and If Further experiments showed that, in the sinus node of swim-trained mice, upregulation of miR-423-5p (intronic miR) and its host gene, NSRP1, is driven by an upregulation of the transcription factor Nkx2.5. Conclusions: HCN remodelling likely occurs in human athletes as well as rodent models. miR-423-5p contributes to training-induced bradycardia by targeting HCN4. This work presents the first evidence of miR control of HCN4 and heart rate. miR-423-5p could be a therapeutic target for pathological sinus node dysfunction in veteran athletes.

Concepts: Cardiology, Heart, Action potential, Ion channels, Rodent, Cardiac pacemaker, Bradycardia

3

Abnormalities in the specialized cardiac conduction system may result in slow heart rate or mechanical dyssynchrony. Here we apply optogenetics, widely used to modulate neuronal excitability, for cardiac pacing and resynchronization. We used adeno-associated virus (AAV) 9 to express the Channelrhodopsin-2 (ChR2) transgene at one or more ventricular sites in rats. This allowed optogenetic pacing of the hearts at different beating frequencies with blue-light illumination both in vivo and in isolated perfused hearts. Optical mapping confirmed that the source of the new pacemaker activity was the site of ChR2 transgene delivery. Notably, diffuse illumination of hearts where the ChR2 transgene was delivered to several ventricular sites resulted in electrical synchronization and significant shortening of ventricular activation times. These findings highlight the unique potential of optogenetics for cardiac pacing and resynchronization therapies.

Concepts: Cardiology, Heart, Cardiac pacemaker, Artificial pacemaker, Channelrhodopsin, Optogenetics, Electrical conduction system of the heart, Bradycardia

2

Pacemakers (PM) are used for managing sick sinus syndrome (SSS). This study evaluates predictors and trends of PM implantation for SSS.

Concepts: Cardiac electrophysiology, Artificial pacemaker, Sick sinus syndrome, Bradycardia

2

BACKGROUND: Atropine has is currently recommended to facilitate haemodynamic stability during critical care intubation. Our objective was to determine whether atropine use at induction influences ICU mortality. METHODOLOGYPRINCIPAL FINDINGS: A 2-year prospective, observational study of all first non-planned intubations, September 2007-9 in PICU and Intensive Care Transport team of Hôpital Robert Debré, Paris, 4 other PICUs and 5 NICUs in the Paris Region, France. Follow-up was from intubation to ICU discharge. A propensity score was used to adjust for patient specific characteristics influencing atropine prescription. 264/333 (79%) intubations were included. The unadjusted ICU mortality was 7.2% (9/124) for those who received atropine compared to 15.7% (22/140) for those who did not (OR 0.42, 95%CI 0.19-0.95, p = 0.04). One child died during intubation (1/264, 0.4%). Two age sub-groups of neonates (≤28 days) and older children (>28 days, <8 years) were examined. This difference in mortality arose from the higher mortality in children aged over one month when atropine was not used (propensity score adjusted OR 0.22, 95%CI 0.06-0.85, p = 0.028). No effect was seen in neonates (propensity score adjusted OR 1.3, 95%CI 0.31-5.1 p = 0.74). Using the propensity score, atropine maintained the mean heart rate 45.9 bpm above that observed when no atropine was used in neonates (95%CI 34.3-57.5, p<0.001) and 43.5 bpm for older children (95%CI 25.5-61.5 bpm, p<0.001). CONCLUSIONSSIGNIFICANCE: Atropine use during induction was associated with a reduction in ICU mortality in children over one month. This effect is independent of atropine's capacity to attenuate bradycardia during intubation which occurred similarly in neonates and older children. This result needs to be confirmed in a study using randomised methodology.

Concepts: Scientific method, Observational study, Observation, Heart rate, Propensity score, Atropine, Paris, Bradycardia