BASELINE SYMPATHETIC ACTIVITY IS ELEVATED IN PATIENTS WITH ATRIAL FIBRILLATION

Atrial fibrillation, or AF, is an abnormal rhythm of the heart that is common in many people, particularly in individuals over 60 years of age. The association of AF with other cardiovascular diseases and the potential relation to sudden cardiac death (or cardiac arrest) is not known. This study investigated the baseline activity of the sympathetic nervous system. The sympathetic nervous system is the primary branch of the nervous system that mediates the response to a stress. The findings of this study strongly suggest that sympathetic nervous system activity is increased in AF which may also increase incidence of progression of other cardiovascular disease states including potentially fatal ventricular dysrhythmias. Purpose (a): Most cardiovascular diseases are associated with high sympathetic nerve activity (SNA), and elevations in SNA are known to increase the progression of many forms of cardiovascular disease. Atrial fibrillation (AF) is a serious cardiac dysrhythmia that afflicts a substantial percentage of the population and is known to result in increased risk of blood clotting. This results in a higher risk for pulmonary emboli and stroke. These complications tend to be associated with a generalized fatigue and reduced exercise capacity due to decreased cardiac output. However, the effect of atrial fibrillation on SNA is unknown, thus the purpose of this study was to determine the effect of AF on baseline SNA independent of other factors. Methods (b): Two studies were performed in patients with AF. First, eight patients with drug-refractory AF were studied before and after completion of an AV nodal ablation to normalize the ventricular rate. Microneurographic recordings of SNA were obtained continuously during and after completion of the procedure. Upon effective AV nodal ablation, the patient was immediately paced with a temporary pacemaker inserted in the ventricular apex via vascular access. Ventricular pacing was conducted at the same rate as the patients ventricular rate during their episodes of AF. Baseline SNA was determined prior to ablation and during ventricular pacing. A comparison of SNA was made between the AF state and during the artificially paced ventricular rate, which as stated above was conducted at the mean of their ventricular rate during AF. Second, nine patients with paroxysmal AF were studied during AF and after cardioversion of AF during sinus rhythm. Microneurographic recordings of SNA were obtained continuously during AF and during sinus rhythm. Statistical comparisons were performed for each study with a paired Student's T test to determine the difference in SNA between conditions with AF and without AF. Results (c): For study 1, mean ventricular rates during AF were 93 + 4 bpm (range= 82-128 bpm) and the post-ablation pacing rates were the same and were maintained at a constant regularity with no inter-beat difference. Baseline SNA decreased significantly from 2836 + 332 units to 2291 + 298 units (p < 0.03). For study 2, mean ventricular rates during AF were 98 + 3 bpm (range= 78-140 bpm) and the post-cardioversion sinus rhythm rates were 77 + 2 bpm (range= 62-91 bpm). The baseline SNA decreased significantly from 3755 + 401 units to 2329 + 339 units (p < 0.01). In each case, the measurement of SNA was based on 100 heartbeats, and thus was independent of the rate. Conclusions (d): These data support the hypothesis that baseline SNA is elevated in atrial fibrillation and appears to be independent of rate. These findings support the hypothesis that AF may also increase the risk of progression of other cardiovascular disease states including potentially fatal ventricular dysrhythmias.