Sustained atrial fibrillation induced by carbachol, methacholine and bethanechol

Interrelationships Between the Autonomic Nervous System and Atrial Fibrillation

Mechanisms responsible for atrial fibrillation are not completely understood but the autonomic nervous system is a potentially potent modulator of the initiation, maintenance, termination and ventricular rate determination of atrial fibrillation. Complex interactions exist between the parasympathetic and sympathetic nervous systems on the central, ganglionic, peripheral, tissue, cellular and subcellular levels that could be responsible for alterations in conduction and refractoriness properties of the heart as well as the presence and type of triggered activity, all of which could contribute to atrial fibrillation. These dynamic inter-relationships may also be altered dependent upon other neurohumoral modulators and cardiac mechanical effects from ventricular dysfunction and congestive heart failure. The clinical implications regarding the effects of the autonomic nervous system in atrial fibrillation are widespread. The effects of modulating ganglionic input into the atria may alter the presence or absence of atrial fibrillation as has been highlighted from ablation investigations. This article reviews what is known regarding the inter-relationships between the autonomic nervous system and atrial fibrillation and provides state of the art information at all levels of autonomic interactions.

Direction-dependent conduction abnormalities in a canine model of atrial fibrillation due to chronic atrial dilatation

Chronic rapid atrial pacing (RAP) leads to changes that perpetuate atrial fibrillation (AF). Chronic atrial dilatation due to mitral regurgitation (MR) also increases AF inducibility, but it is not clear whether the underlying mechanism is similar. Therefore, we have investigated atrial electrophysiology in a canine MR model (mitral valve avulsion, 1 mo) using high-resolution optical mapping and compared it with control dogs and with the canine RAP model (6-8 wk of atrial pacing at 600 beats/min, atrioventricular block, and ventricular pacing at 100 beats/min). At followup, optical action potentials were recorded using a 16 x 16 photodiode array from 2 x 2-cm left atrial (LA) and right atrial (RA) areas in perfused preparations, with pacing electrodes around the field of view to study direction dependency of conduction. Action potential duration at 80% repolarization (APD(80)) was not different between control and MR but was reduced in RAP atria. Conduction velocities during normal pacing were not different between groups. However, the MR LA showed increased conduction heterogeneity during pacing at short cycle lengths and during premature extrastimuli, which frequently caused pronounced regional conduction slowing. Conduction in the MR LA during extrastimulation also displayed a marked dependence on propagation direction. These phenomena were not observed in the MR RA and in control and RAP atria. Thus both models form distinctly different AF substrates; in RAP dogs, the decrease in APD(80) may stabilize reentry. In MR dogs, regional LA conduction slowing and increased directional dependency, allowing unidirectional conduction block and preferential paths of conduction, may account for increased AF inducibility.

Cardiac arrhythmia monitoring during bronchial provocation test with methacholine

STUDY OBJECTIVES

During a bronchial provocation test (BPT), the performance of maximal inspiratory-expiratory maneuvers, causing abrupt and marked shifts in intrathoracic pressure, may increase the risk of cardiac arrhythmias. Moreover, the inhalation of methacholine (MCh), a cholinergic agonist agent, could favor the development of unwelcome cardiovascular events, namely, cardiac arrhythmias.

SUBJECTS AND METHODS

We studied the number and severity of cardiac arrhythmias by ECG-Holter monitoring before, during, and after BPTs with MCh challenge in a group of 46 consecutive nonselected subjects (28 men and 18 women) with clinical indications for BPT, without preexisting cardiovascular diseases, and not receiving arrhythmogenic drugs. The subjects performed a routine pulmonary function test (PFT), followed by BPT, during ECG-Holter monitoring. Determination of the serum potassium concentration, a baseline arterial blood gas analysis, and monitoring of oxyhemoglobin saturation also were performed.

RESULTS

We found no significant increase in the number of supraventricular and ventricular arrhythmias during the performance of PFTs and of BPTs with MCh in the subjects, either with or without bronchial hyperresponsiveness (BHR). However, during the performance of BPTs, we observed a significant reduction in mean heart rate.

CONCLUSIONS

Our results indicate that the performance of PFTs and BPTs with MCh does not increase the cardiac arrhythmogenic risk in subjects without cardiovascular diseases, as well as in those with BHR, suggesting that these tests are safe to perform in most subjects.

Induction of atrial fibrillation and flutter in dogs using methacholine

Systemic infusion of methacholine has been used to facilitate induction of atrial fibrillation. However, the dose-response relationship, reproducibility and effect of anesthetic agents on induction are not well understood. The use of methacholine to facilitate electrical induction of sustained (>10 minutes duration) atrial fibrillation or flutter was examined. In 25 dogs induction of atrial arrhythmias was attempted using a series of ten 50 Hz trains of 10 seconds duration delivered via an endocardial catheter in the baseline anaesthetized state and subsequently in the presence of graded doses of intravenous methacholine (maximum 5 microg/kg/min). Studies were repeated in 13 dogs to assess reproducibility. Twelve dogs (48%) had inducible sustained atrial flutter or fibrillation lasting greater than 10 minutes in the baseline state. During infusion of methacholine the remaining 13 (52%) dogs also had inducible sustained atrial flutter or fibrillation (mean infusion rate 1.6 +/- 1.9 microg/kg/min). Induction of sustained atrial flutter or fibrillation was reproducible in all but one dog. The type of anesthetic did not significantly affect inducibility. Induction of prolonged atrial fibrillation or flutter is possible in the baseline anaesthetized state in approximately half of dogs using high frequency programmed electrical stimulation. The yield of inducible sustained atrial fibrillation or flutter with programmed stimulation during intravenous infusion of methacholine was increased to 100%. Induction of sustained atrial fibrillation or flutter was highly reproducible.

Effects of PACAP-38 on the SA nodal pacemaker activity in autonomically decentralized hearts of anesthetized dogs

Pituitary adenylate cyclase-activating polypeptide (PACAP) receptors exist, but the physiologic role of PACAP is unclear in the heart in situ. We investigated effects of PACAP-38 on sinus rate and on the negative chronotropic response to acetylcholine (ACh) or stimulation of the intracardiac parasympathetic nerve fibers to the sinoatrial nodal region in the automatically decentralized heart of the open chest, anesthetized dog. PACAP-38 (0.1-1 nmol) injected directly into the sinus node artery caused transient positive followed by negative chronotropic responses. Both pretreatment with atropine and tetrodotoxin inhibited the negative chronotropic responses to PACAP-38. However, hexamethonium did not block the negative responses to PACAP-38. After treatment with PACAP-38 (0.1-1 nmol), ACh induced atrial fibrillation significantly (p < 0.01). On the other hand, the negative chronotropic responses to intracardiac parasympathetic stimulation were not changed. These results suggest that (a) PACAP-38 induces negative chronotropic responses and liberates ACh from intracardiac postganglionic parasympathetic nerves, and that (b) PACAP-38 reduces ACh-induced atrial fibrillation threshold in the dog heart in situ.