Accessory atrioventricular myocardial pathways in mouse heart development: substrate for supraventricular tachycardias.

Atrioventricular reentry tachycardia (AVRT) requiring an accessory atrioventricular pathway (AP) is the most common type of arrhythmia in the perinatal period. The etiology of these arrhythmias is not fully understood as well as their capability to dissipate spontaneously in the first year of life. Temporary presence of APs during annulus fibrosus development might cause this specific type of arrhythmias. To study the presence of APs, electrophysiological recordings of ventricular activation patterns and immunohistochemical analyses with antibodies specifically against atrial myosin light chain 2 (MLC-2a), Periostin, Nkx2.5, and Connexin-43 were performed in embryonic mouse hearts ranging from 11.5 to 18.5 days post-conception (dpc). The electrophysiological recordings revealed the presence of functional APs in early (13.5-15.5 dpc) and late (16.5-18.5 dpc) postseptated stages of mouse heart development. These APs stained positive for MLC-2a and Nkx2.5 and negative for Periostin and Connexin-43. Longitudinal analyses showed that APs gradually decreased in number (p = 0.003) and size (p = 0.035) at subsequent developmental stages (13.5-18.5 dpc). Expression of periostin was observed in the developing annulus fibrosus, adjacent to APs and other locations where formation of fibrous tissue is essential. We conclude that functional APs are present during normal mouse heart development. These APs can serve as transient substrate for AVRTs in the perinatal period of development.

Electrical activation of sinus venosus myocardium and expression patterns of RhoA and Isl-1 in the chick embryo.

UNLABELLED: Electrical Activity and RhoA in the Embryo. INTRODUCTION: Myocardium at the venous pole (sinus venosus) of the heart has gained clinical interest as arrhythmias can be initiated from this area. During development, sinus venosus myocardium is incorporated to the primary heart tube and expresses different markers than primary myocardium. We aimed to elucidate the development of sinus venosus myocardium, including the sinoatrial node (SAN), by studying expression patterns of RhoA in relation to other markers, and by studying electrical activation patterns of the developing sinus venosus myocardium. METHODS AND RESULTS: Expression of RhoA, myocardial markers cTnI and Nkx2.5, transcription factors Isl-1 and Tbx18, and cation channel HCN4 were examined in sequential stages in chick embryos. Electrical activation patterns were studied using microelectrodes and optical mapping. Embryonic sinus venosus myocardium is cTnI and HCN4 positive, Nkx2.5 negative, complemented by distinct patterns of Isl-1 and Tbx18. During development, initial myocardium-wide expression of RhoA becomes restricted to right-sided sinus venosus myocardium, comprising the SAN. Electrophysiological measurements revealed initial capacity of both atria to show electrical activity that in time shifts to a right-sided dominance, coinciding with persistence of RhoA, Tbx18, and HCN4 and absence of Nkx2.5 expression in the definitive SAN. CONCLUSION: Results show an initially bilateral electrical potential of sinus venosus myocardium evolving into a right-sided activation pattern during development, and suggest a role for RhoA in conduction system development. We hypothesize an initial sinus venosus-wide capacity to generate pacemaker signals, becoming confined to the definitive SAN. Lack of differentiation toward a chamber phenotype would explain ectopic pacemaker foci.

Accessory atrioventricular myocardial connections in the developing human heart: relevance for perinatal supraventricular tachycardias.

BACKGROUND: Fetal and neonatal atrioventricular (AV) reentrant tachycardias can be life-threatening but resolve in most cases during the first year of life. The transient presence of accessory AV myocardial connections during annulus fibrosus development may explain this phenomenon. METHODS AND RESULTS: A total of 45 human embryonic, fetal, and neonatal sectioned hearts (4 to 36 weeks of development) were studied immunohistochemically. Accessory myocardial AV connections were quantified and categorized according to their specific location, and 3D reconstructions were made. Between 4 and 6 weeks of development, the atrial and ventricular myocardium was continuous at the primitive AV canal. At 6 to 10 weeks, numerous accessory myocardial AV connections were identified in the left (45%), right (35%), and septal (20%) regions of the AV junction. Most right-sided accessory connections comprised distinct myocardial strands, whereas left-sided connections consisted of larger myocardial continuities. At 10 to 20 weeks, all accessory AV connections comprised discrete myocardial strands and gradually decreased in number. The majority of accessory connections were located in the right AV junction (67%), predominantly in the lateral aspect (45%). Seventeen percent of the accessory connections were observed in the left AV junction, and 16% were observed in the septal region. 3D reconstructions of the developing AV nodal area at these stages demonstrated multiple AV node-related accessory connections. From 20 weeks until birth, and in neonatal hearts, no further accessory myocardial AV connections were observed. CONCLUSIONS: Isolation of the AV junction is a gradual and ongoing process, and right lateral accessory myocardial AV connections in particular are commonly found at later stages of normal human cardiac development. These transitory accessory connections may act as substrate for AV reentrant tachycardias in fetuses or neonates.

Epicardium-derived cells in development of annulus fibrosis and persistence of accessory pathways.

BACKGROUND: The developmental mechanisms underlying the persistence of myocardial accessory atrioventricular pathways (APs) that bypass the annulus fibrosis are mainly unknown. In the present study, we investigated the role of epicardium-derived cells (EPDCs) in annulus fibrosis formation and the occurrence of APs. METHODS AND RESULTS: EPDC migration was mechanically inhibited by in ovo microsurgery in quail embryos. In ovo ECGs were recorded in wild-type (n=12) and EPDC-inhibited (n=12) hearts at Hamburger-Hamilton (HH) stages 38 to 42. Subsequently, in these EPDC-inhibited hearts (n=12) and in additional wild-type hearts (n=45; HH 38-42), ex ovo extracellular electrograms were recorded. Electrophysiological data were correlated with differentiation markers for cardiomyocytes (MLC2a) and fibroblasts (periostin). In ovo ECGs showed significantly shorter PR intervals in EPDC-inhibited hearts (45+/-10 ms) than in wild-type hearts (55+/-8 ms, 95% CI 50 to 60 ms, P=0.030), whereas the QRS durations were significantly longer in EPDC-inhibited hearts (29+/-14 versus 19+/-2 ms, 95% CI 18 to 21 ms, P=0.011). Furthermore, ex ovo extracellular electrograms (HH 38-42) displayed base-first ventricular activation in 44% (20/45) of wild-type hearts, whereas in all EPDC-inhibited hearts (100%, 12/12), the ventricular base was activated first (P<0.001). Small periostin- and MLC2a-positive APs were found mainly in the posteroseptal region of both wild-type and EPDC-inhibited hearts. Interestingly, in all (n=10) EPDC-inhibited hearts, additional large periostin-negative and MLC2a-positive APs were found in the right and left lateral free wall coursing through marked isolation defects in the annulus fibrosis until the last stages of embryonic development. CONCLUSIONS: EPDCs play an important role in annulus fibrosis formation. EPDC outgrowth inhibition may result in marked defects in the fibrous annulus with persistence of large APs, which results in ventricular preexcitation on ECG. These APs may provide a substrate for postnatally persistent reentrant arrhythmias.

Periostin expression by epicardium-derived cells is involved in the development of the atrioventricular valves and fibrous heart skeleton.

The epicardium is embryologically formed by outgrowth of proepicardial cells over the naked heart tube. Epicardium-derived cells (EPDCs) migrate into the myocardium, contributing to myocardial architecture, valve development, and the coronary vasculature. Defective EPDC formation causes valve malformations, myocardial thinning, and coronary defects. In the atrioventricular (AV) valves and the fibrous heart skeleton isolating atrial from ventricular myocardium, EPDCs colocalize with periostin, a matrix molecule involved in remodeling. We investigated whether proepicardial outgrowth inhibition affected periostin expression and how this related to development of the AV valves and fibrous heart skeleton. Periostin expression by epicardium and EPDCs was confirmed in vitro in primary cultures of human and quail EPDCs. Disturbing EPDC formation in quail embryos reduced periostin expression in the endocardial cushions and AV junction. Disturbed fibrous tissue development resulted in AV myocardial connections reflected by preexcitation electrocardiographic patterns. We conclude that EPDCs are local producers of periostin. Disturbance of EPDC formation results in decreased cardiac periostin levels and hampers the development of fibrous tissue in AV junction and the developing AV valves. The resulting cardiac anomalies might link to Wolff-Parkinson White syndrome with persistent AV myocardial connections.

Persistence of functional atrioventricular accessory pathways in postseptated embryonic avian hearts: implications for morphogenesis and functional maturation of the cardiac conduction system.

BACKGROUND: During heart development, the ventricular activation sequence changes from a base-to-apex to an apex-to-base pattern. We investigated the possibility of impulse propagation through remnants of atrioventricular (AV) connections in quail hearts. METHODS AND RESULTS: In 86 hearts (group A, HH30-34, n=15; group B, HH35-44, n=65; group C, 5 to 6 months, n=6) electrodes were positioned at the left atrium, right ventricular base, left ventricular (LV) base, and LV apex. In group A, LV base activation preceded LV apex activation in the majority of cases (60%; 9 of 15), whereas hearts in group B primarily demonstrated an LV apex-to-base activation pattern (72%; 47 of 65). Interestingly, in group B, the right ventricular base (17%; 11 of 65) or LV base (8%; 5 of 65) exhibited premature activation in 25% (16 of 65) of cases, whereas in 26% (17 of 65), the right ventricular base or LV base was activated simultaneously with the LV apex. Morphological analysis confirmed functional data by showing persistent muscular AV connections in embryonic hearts. Interestingly, all myocardial AV connections stained positive for periostin, a nonmyocardial marker. Longitudinal analysis (HH35-44) demonstrated a decrease in both the number of hearts that exhibited premature base activation (P=0.015) and the number (P=0.004) and width (P=0.179) of accessory AV pathways with developmental stage in a similar time course. In the adult quail hearts, accessory myocardial AV pathways were functionally and morphologically absent. CONCLUSIONS: Thus, impulse propagation through persistent accessory AV connections remains possible at near-hatching stages (HH44) of development, which may provide a substrate for AV reentrant arrhythmias in perinatal life. Periostin positivity and absence of AV pathways in the adult heart suggest that these connections eventually lose their myocardial phenotype, which implicates ongoing AV ring isolation perinatally and postnatally.

Low-energy radiofrequency catheter ablation as therapy for supraventricular tachycardia in a premature neonate.

UNLABELLED: A premature neonate with hydrops was born at 32 weeks of gestation after successful direct fetal amiodarone therapy via cordocentesis for incessant supraventricular tachycardia. After birth the tachycardia could not be controlled despite high doses of amiodarone and flecainide and the patient developed severe respiratory and circulatory failure. After 3 weeks, weighing 2 kg, he underwent successful and uncomplicated catheter ablation of a left free-wall accessory pathway using low-energy radiofrequency. CONCLUSION: radiofrequency catheter ablation is rarely used in neonates, but when used with caution may provide the optimal treatment.