A number of studies have addressed the role of the ANS in modulating visceral perception in FGD. Chen and Orr demonstrated enhanced sympathetic dominance to esophageal acid infusion in patients with gastroesophageal reflux disease (GERD), which appeared to be secondary to decreased vagal tone in these subjects (121). During acid infusion, there was a significant decrease in LF band power (a measure of sympathetic tone) in the control group, which was unchanged in the patient group, whereas the HF band power (a measure of vagal tone) was lower during all the infusion periods in the GERD group. The findings suggest the autonomic effects of acid infusion are different between healthy subjects and GERD patients. Indeed, the between-group comparisons did reveal a significant group difference during acid infusion, with GERD patients demonstrating a significantly larger LF/HF ratio compared with controls (p < 0.05). The healthy controls who had heartburn with acid infusion did not have a different LF/HF ratio from the controls who had no symptoms with acid infusion. These data seem to suggest that alterations in autonomic balance may play a role in modulating visceral sensation. The observed decrease in HF band power in the patient group (corresponding with a reduction in vagal tone) was proposed to be the cause of the increase in the LF/HF ratio; however, the ratio could also result from an increase in the LF band power (sympathetic component). This latter possibility was not borne out in the data, and therefore conflicts with the findings of Iovino et al. where increased sympathetic activity (albeit experimental) corresponded with heightened visceral sensitivity (61).
Increased sympathetic activity has been demonstrated in patients with IBS. Heitkemper et al. studied urinary catecholamine (NE and epinephrine) and cortisol levels in women diagnosed with IBS against women who reported similar symptoms but did not seek health care services and asymptomatic control women (122). Women with IBS had significantly higher urinary levels of all of these neuroendocrine indicators of arousal suggesting heightened sympathetic nervous system activation. Whether greater symptom distress in the IBS women resulted in increased sympathetic activation and health care seeking or the higher sympathetic activation increased pain perception leading to health care seeking is unclear. These investigators later demonstrated significantly lower parasympathetic tone and higher ANS balance in constipation-predominant compared to diarrhea-predominant subgroups of IBS but only when symptom severity scores were high. No difference was seen between IBS and control women, and between subgroups with IBS on autonomic function tests in the absence of severe symptoms, highlighting the importance of assessing symptom severity in these patients (123).
Diminished variability in heart rate and skin conductance has been demonstrated in anxiety, and these are likely to be due to the interaction of both the sympathetic and parasym-pathetic nervous systems. Piccirillo et al. demonstrated that healthy adults with higher anxiety scores have lower LF and HF power values, and demonstrate a higher LF/HF ratio compared to those that report lower anxiety scores on questionnaires (124). The significantly higher LF power suggested cardiac sympathetic hyperactivity. Other work has suggested that the variations in power spectral components in anxiety are associated with reduced vagal modulation of cardiac control (125). The mechanisms whereby anxiety can modulate the ANS, HPA axis, and eventually pain perception are incompletely understood, but this area of research is likely to gain interest due to their prevalence in FGD and the advancement of monitoring technologies.
Although much of the literature assessing the role of the ANS in various disorders involves power spectral analysis of HRV, the delineation into LF and HF band powers and the inference that the ratio of these two components can provide an idea of sympathovagal balance. This is controversial. The LF variability is a product of both sympathetic and parasym-pathetic influences on the heart, and as a result any change in LF power cannot be accurately taken as index of alterations in sympathetic cardiac control (126). The notion of sympathovagal balance has been questioned as its autonomic constructs are not always reciprocally controlled and can vary independently or demonstrate coactivation or coinhibition, particularly in the setting of stress and fear (127). HRV holds considerable promise for providing insights into the modulatory role of the ANS in health and disease, and for clarifying the relationship between psychological processes and observed physiological responses, but careful quantification and interpretation of data seems paramount.
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Gastroesophageal reflux disease is the medical term for what we know as acid reflux. Acid reflux occurs when the stomach releases its liquid back into the esophagus, causing inflammation and damage to the esophageal lining. The regurgitated acid most often consists of a few compoundsbr acid, bile, and pepsin.