The Function Of Canine Third Fat Pad And The Relationship Between Autonomic Nerve System And Atrial Fibrillation

IntroductionPulmonary veins (PVs) are very important in the initiation and maintenance of AF. Most AF is initiated by the focal ectopic beats in the myocardial sleeves of PVs. However, change of autonomic tone especially the vagal tone may play a role in the generation of ectopic PV activity.Autonomic ganglionated plexis (GPs) also play an important role in the initiation and maintenance of atrial fibrillation. These intrinsic cardiac GPs mainly distribute diffusely in the three major cardiac fat pads (FP):the right pulmonary vein fat pad (RPV FP), the inferior vena cava–left atrial fat pad (IVC-LA FP), and the third FP (SVC-Ao FP). Studies have demonstrated the SVC-AO FP receives the most vagal fibers that innervate the atrium. It appears to be the "head station" of the atrial vagal inputs. However, the function of, the neuronal cell types present in this FP, and the specific efferent pathway of the autonomic nervous system to the atrium remain unclear.Therefore, the goals of the present study were four-fold. First, we performed the premature stimulation (S1S2) and the burst stimulation (S1S1) at distal left superior pulmonary vein(LSPV) and left inferior pulmonary vein (LIPV) to investigate if SVC-Ao FP ablation can reduce the inducibility of AF originated in PVs. Second, we established a Langendorff-perfused canine heart model to investigate the function of the SVC-Ao FP. Third, we determined the neuronal cell types present in the SVC-Ao FP by fluorescent staining. Fourth, we performed BDA tracing to study the specific efferent autonomic pathways in the SVC-Ao FP.Part One: AF initiation from distal pulmonary veins by electric stimulationBackground: premature stimulus and short-period tachyrhythmia can avtive the focal ectopic sites in PVs and initiate AF. However, whether electric stimulation can well simulate these ectopic beats and induce AF succesfuly is still not clear. Aim: To measure the inducing rate of AF from distal PVs by electric stimulation. Methods: 10 dogs were anesthetized with Sumianxin. The left superior pulmonary vein (LSPV) and the left inferior pulmonary vein (LIPV) were exposed. Programmed stimulation (S1S2) and burst stimulation (S1S1) protocol were performed at distal LSPV and LIPV respectively to induce AF. Results: the AF inducing rate by S1S2 and S1S1 (2 min) was 20%, 50% respectively; while the total inducing rate was 35%.Conclusions—Programmed stimulation and burst stimulation protocol can well simulate focal ectopic beats in PVs. AF can be initiated by the two protocols even in normal condition, but the inducing rate was low. Part Two: Epicardial ablation of vagal inputs in canine third fat pad for prevention of atrial fibrillation originated in left pulmonary veinBackground: Catheter ablation of the third fat pad (SVC-Ao FP) could prevent vagally mediated atrial fibrillation (AF). But the effect of the SVC-Ao FP ablation on AF originated in pulmonary veins is unknown. Aim: To investigate whether SVC-Ao FP ablation could reduce AF initiated from pulmonary veins.Methods: In 40 dogs, the atrial effective refractory period (AERP), left superior and inferior pulmonary vein effective refractory period (LSPVERP and LIPVERP), and the dispersion of AERP (dAERP) were mesured at baseline, during SVC-Ao FP stimulation and after ablation. Programmed stimulation (S1S2) and burst stimulation (S1S1) protocol were performed at distal LSPV and LIPV respectively to measure AF inducibility before and after ablation. Results: SVC-Ao FP stimulation shortened AERP(130±10 ms vs.116±16 ms, P<0.01,n=40), LPVERP(136±12 ms vs.122±14 ms,P<0.01, n=20), LIPVERP(138±10 ms vs. 120±13 ms, P<0.01), and increased dAERP(23±6 ms vs. 31±6 ms, P<0.01). After ablation, AERP LSPVERP and LIPVERP prolonged (130±10 ms vs 138±7ms, P<0.05), (136±12 ms vs. 146±18 ms,P<0.05) and (138±10 ms vs. 149±14 ms, P<0.05), dAERP decreased (23±6 ms vs 19±5 ms, P<0.05), and the incidence of AF from LSPV or LIPV reduced (40% vs75%/90%) and (30% vs65%/80%). S1S2 could not induce AF after ablation; S1S1 could still provoke AF but mean cycle length drives (CLD) decreased significantly(LSPV: 145±20 ms vs 160±29 ms /194±15 ms,P<0.05) and (LIPV:135±21 ms vs 164±33 ms /193±16 ms, P<0.05).Conclusions—Ablation of SVC-Ao FP changes the electrophysiological properties of both the atriums and the Left pulmonary vein (LPV), and decreases the inducibility of AF originated in LPV.Part Three: The function, neuronal cell type, and efferent autonomic pathways of the canine third fat padBackgrounds: the third fat pad (SVC-Ao FP) receives most vagal nerve fibers that innervate the atrium. Modulation of this FP provokes atrial vagal responses. However, the function and structure of SVC-Ao FP, as well as the relationship between the FP and atrial fibrillation remain unknown. Aim: This study investigated the function, neuronal cell types, and nerve fiber projections of the SVC-Ao FP. Methods: Langendorff-perfused canine heart models were established in 20 dogs, and the atrial effective refractory period (AERP), dispersion of AERP (dAERP), pulmonary vein effective refractory period (PVERP), spontaneous cycle length (SCL), AV nodal conduction time, and window of vulnerability (WOV) were measured at baseline during SVC-Ao FP stimulation and after excision. Nissl's staining and fluorescent dual label staining (TH and ChAT) were performed to determine the neuronal cell types in the SVC-Ao FP of five dogs. BDA, TH, and ChAT triple immunofluorescence staining were performed for another five dogs to study the efferent autonomic pathway of the SVC-Ao FP. Results: SVC-Ao FP simulation significantly shortened the baseline AERP (129±11 ms vs. 115±17 ms,P<0.05), LSPVERP (122±14 ms vs. 136±12 ms,P<0.05) and LPVERP (120±16 ms vs. 133±14 ms, P<0.05), increased the dAERP (21±6 ms vs. 29±7 ms,P<0.05), and significantly widened the WOV (40±12ms vs. 23±10 ms, P<0.01). However, the above indices revealed an adverse change after excision (P<0.05). Adrenergic and cholinergic neurons were both present in the SVC-Ao FP, and the majority of the neurons were of the cholinergic subtype. Autonomic nerve fibers projected directly to the LA (left atrium) and LPV (left pulmonary vein) from the SVC-Ao FP, but the fibers did not go directly to the RA (right atrium) or the right pulmonary vein (RPV). Conclusion: Both adrenergic and cholinergic neurons exist in the SVC-Ao FP. This FP innervates LA and LPV directly and serves as the lower nerve center that is the sole modulator of autonomic atrial activity.