Preliminary report: modulation of parasympathetic nervous system tone influences oesophageal pain hypersensitivity.

OBJECTIVES: Autonomic nervous system dysfunction has been implicated in visceral hypersensitivity. However, the specific contribution of the parasympathetic nervous system (PNS) is unclear. We aimed to determine whether physiological and pharmacological manipulation of parasympathetic tone influences the development of hypersensitivity in a validated model of acid-induced oesophageal pain. DESIGN: Prior to, and following, a 30-min distal oesophageal infusion of 0.15 M hydrochloric acid, pain thresholds to electrical stimulation were determined in the proximal non-acid exposed oesophagus in healthy subjects. Validated sympathetic (skin conductance response) and parasympathetic (cardiac vagal tone) parameters were measured at baseline and continuously thereafter. In study 1, 55 subjects were randomised in a pragmatic blinded crossover design to receive deep breathing or un-paced breathing during acid infusion. In study 2, 32 subjects were randomised in a blinded, crossover design to receive intravenous atropine or placebo (saline) with deep breathing during acid infusion. RESULTS: Study 1: Deep breathing increased cardiac vagal tone (2.1±2.3 vs -0.3±2.3, p=0.0006) with concomitant withdrawal of skin conductance response (-0.6±4.9 vs 3±4.8, p=0.03) in comparison with un-paced breathing. Deep breathing prevented the development of acid-induced oesophageal hypersensitivity in comparison with sham breathing (p=0.0001). Study 2: Atropine, in comparison with placebo, blocked the attenuating effect of deep breathing on the development of acid-induced oesophageal hypersensitivity (p=0.046). CONCLUSIONS: The development of oesophageal hyperalgesia is prevented by physiologically increasing parasympathetic tone. This effect is pharmacologically blocked with atropine, providing evidence that the PNS influences the development of oesophageal pain hypersensitivity.

Normal values and reproducibility of the real-time index of vagal tone in healthy humans: a multi-center study.

BACKGROUND: The parasympathetic nervous system has been implicated in the pathogenesis of a number of gastrointestinal disorders including irritable bowel syndrome. Within the field, cardiometric parameters of parasympathetic/vagal tone are most commonly derived from time, or frequency, domain analysis of heart rate variability (HRV), yet it has limited temporal resolution. Cardiac vagal tone (CVT) is a non-invasive beat-to-beat measure of brainstem efferent vagal activity that overcomes many of the temporal limitations of HRV parameters. However, its normal values and reproducibility in healthy subjects are not fully described. The aim of this study was to address these knowledge gaps. METHODS: 200 healthy subjects (106 males, median age 28 years, range 18-59 years) were evaluated across three study centers. After attachment of CVT recording equipment, 20 min of data (resting/no stimulation) was acquired. 30 subjects, selected at random, were restudied after 1 year. RESULTS: The mean CVT was 9.5±4.16 linear vagal scale (LVS). Thus, the normal range (mean±2 standard deviations) for CVT based on this data was 1.9-17.8 LVS. CVT correlated negatively with heart rate (r=-0.6, P=0.001). CVT reproducibility over 1 year, as indexed by an intra-class correlational coefficient of 0.81 (95% confidence interval 0.64-0.91), was good. CONCLUSIONS: In healthy subjects, the normal range for CVT should be considered to be 1.9-17.8 LVS and is reproducible over 1 year. Future research utilizing CVT should refer to these values although further study is warranted in patient groups.