Chagas disease: Impaired vagal modulation has been demonstrated, enhanced parasympathetic activity remains to be proved

Muscarinic antibodies and heart rate responses to dynamic exercise and to the Valsalva maneuver in chronic chagasic patients

We have studied the cardiac chronotropic responses to the Valsalva maneuver and to dynamic exercise of twenty chronic chagasic patients with normal left ventricular function and no segmental wall abnormalities by two-dimensional echocardiogram. The absolute increase in heart rate of the patients (Δ = 21.5 ± 10 bpm, M±SD) during the maneuver was significantly diminished when compared to controls (Δ = 31.30 ± 70, M±SD, p = 0.03). The minimum heart rate (58.24 ± 8.90 vs. 62.80 ± 10, p = 0.68) and the absolute decrease in heart rate at the end of the maneuver (Δ = 38.30 ± 13 vs. Δ = 31.47 ± 17, p = 0.10) were not different from controls. The initial heart rate acceleration during dynamic exercise (Δ = 12 ± 7.55 vs. Δ = 19 ± 7.27, M±SD, p = 0.01) was also diminished, but the heart rate recovery during the first ten seconds was more prominent in the sero-positive patients (Median: 14, Interquartile range: (9.75-17.50 vs. 5(0-8.75, p = 0.001). The serum levels of muscarinic cardiac auto-antibodies were significantly higher in the chagasic patients (Median: 34.58, Interquartile Range: 17-46.5, Optical Density) than in controls (Median: 0, Interquartile Range: 0-22.25, p = 0.001) and correlated significantly and directly (r = 0.68, p = 0.002) with early heart rate recovery during dynamic exercise. The results of this investigation indirectly suggest that, the cardiac muscarinic auto-antibodies may have positive agonist effects on parasympathetic heart rate control of chagasic patients.

Cardiac autonomic control mechanisms in the pathogenesis of chagas' heart disease

Primary abnormalities of the autonomic nervous system had been postulated as the pathogenic mechanisms of myocardial damage, in patients with Chagas disease. However, recent investigations indicate that these abnormalities are secondary and amenable to treatment with beta-adrenergic blockers. Moreover, muscarinic cardiac autoantibodies appear to enhance parasympathetic activity on the sinus node. Therefore, the purpose of this paper is to analyze how knowledge on Chagas' disease evolved from being initially considered as a primary cardioneuromyopathy to the current status of a congestive cardiomyopathy of parasitic origin.

Enhanced parasympathetic activity in Chagas disease still stands in need of proof

Cardiac vagal modulation impairment is a typical feature of Chagas disease (ChD) and is correlated to antagonistic anti-M2 autoantibodies activity. In this letter, we analyze the hypothesis that, in ChD, anti-muscarinic antibody activity could simultaneously increase parasympathetic tonus (decreasing heart rate) and decrease vagal modulation over cardiac sinus node (reducing heart rate variability - HRV). Although attractive, this hypothesis is not supported by empirical data, since reduction of vagal-specific HRV indexes has been demonstrated in the absence of bradycardia. Moreover, anti-muscarinic antibody levels correlate with HRV indexes, but not with heart rate. In the absence of reduced heart rate and, consequently, of tonic vagal enhancement, it is impossible to explain the reduction of vagal-mediated HRV by continuous and strong muscarinic activity of autoantibodies in ChD.

Comments on the letter by Ribeiro et al.: impairment of parasympathetic-mediated autonomic modulation of the heart in Chagas' cardiomyopathy: parasympathetic modulation vs. tonus

Chagas' disease is complex immune-mediated disease originated after Trypanosome cruzi transmission, and a major cause of heart failure in Latin American continent. Auto-antibodies directed to type 2 muscarinic parasympathetic (M2) receptors seem to play key roles on the pathogenesis of heart disease, in particular in the impairment of the cardiac autonomic modulation. When talking about M2 agonistic effects, one should first argue about the differences between the terms 'modulation' and 'tonus' before describing possible autonomic influences on the heart.