Vagal Tone: Myths and Realities

Abstract

The possibility, heard during a conference session while I was a graduate student, of measuring vagal tone from heart rate changes during respiration, hooked me into heart rate variability research. Vagal tone seemed to be a sort of Holy Grail. Anything that promised to increase it would be life sustaining, and that claim would be testable in an objective way. Loss of vagal tone could serve as an early warning signal and catastrophe could be prevented. The truth has proved to be far more complex, but it is my impression, as a reviewer of numerous manuscripts in the field that the essence of this myth persists. Where then does a more nuanced view of this field take us?The first issue is the confusion between vagal tone and what is really measurable from heart rate variability (HRV) that is vagal modulation of heart rate. Indeed, I am embarrassed to admit that one of the first review articles I wrote in this field (in 1993) was entitled "Heart rate variability: A measure of cardiac autonomic tone." 1 Malik and Camm clarified this distinction in the same year in their article "Components of heart rate variability-What they really mean and what we really measure." 2 Understanding the difference between vagal tone, as directly measured by vagus nerve activity and vagal modulation as measured by HRV is critical. Although direct measurement of peripheral (although, importantly not cardiac) sympathetic nerve traffic is now feasible, direct measurement of vagal activity is too invasive to be practical. Thus, we are limited to two measurements. Basal vagal tone can be estimated by giving atropine and measuring heart rate in the absence of all parasympathetic activity. This, however, does not provide any information about vagal tone under dynamic or stress conditions. Alternatively, vagal modulation of heart rate can be estimated, under controlled laboratory conditions or in an ambulatory setting, using appropriate manipulations of the beat-to-beat changes in heart rate and, as demonstrated in the current article by Kuo et al., 3 under certain circumstances, reflect vagus nerve traffic. However, the relationship between vagal activity and HRV does not appear to be linear, and a lack of vagally modulated HRV could indicate either a nonresponsive or a saturated system. In human studies, for example, Goldberger et al., 4 using phenylephrine and nitroprusside to stimulate and reduce vagally modulated baroreflex activity in subjects under beta-blockade, found that the relationship between HRV and parasympathetic effect was far from linear. They observed an ascending limb where HRV increased with increasing parasympathetic effect, followed by a plateau level. After that, HRV decreased with increasing parasympathetic effect. Moreover, this relationship was not uniform across subjects. A useful analogy might be to imagine that we can measure the height of the wave (modulation) but not the depth of the ocean (tone) using HRV.A second problem is equally important in clinical research. The various HRV indices that reflect vagal modula...

Diurnal variation and dispersion in QT interval in cirrhosis: Relation to haemodynamic changes

“…By contrast, in patients with abnormal sympathetic nervous activity, the length of the QT C may be related to an index of enhanced sympathetic nervous activity, like the circulating level of noradrenalin [5,11]. Heart rate variability primarily reflects the tonic vagal activity [50]. Several previous studies have confirmed the presence of autonomic neuropathy in patients with cirrhosis [7,12,13], and the present study confirms the reduced heart rate variability as evidenced by the substantial reduction of all values of indices of heart variability from the present study (Table 6) with a relation to central and arterial hypovolaemia (Fig.…”

Evaluating Critical Reflection for Postgraduate Students in Computing

“…The relationship between critical reflection skills and rationality required in the rest of the research process is something that requires further examination. Hunt (1997) and Stein (2000) are in agreement that we venture in murky grounds when we explore the domain between critical reflection and rationality. Some authors (Hunt, 1997;Imel, 1998;Stein, 2000) also believe that the journey to becoming critical reflective is one that ventures onto paths that neither educator or student may ever have been before.…”

“…I think that clear boundaries and environment setting are more important than 'teaching'. (Stein, 2000) also believes 'how to teach different levels of critical reflection 'is still in need of additional research'. Stein also suggests that the effectiveness of commonly employed methods to teach critical reflection like journal-keeping is unclear.…”

“…Common tools utilized for teaching critical reflection include a reflective journal (Goatley, 1999;Hunt, 1997;Imel, 1998;Schon, 1987;Stein, 2000), the use of dialogue in small groups (Boys, 1999;Brookfield, 1987), practice-based assignments (Boud, Keogh, & Walker, 1995;Evans, 1997) and learning as narrative process (Umphrey, 2004). It is only the skills acquired during the process of keeping a reflective journal that are considered in this paper.…”

Using A Reflective Court Report To Integrate And Assess Reflective Practice In Law

“…Further, Mezirow construed "critical reflection" as "an analytical discussion but also a 'contextual awareness'; that is, an appreciation of historical, social and political perspectives and underpinning assumptions" (2004,185). Stein (2000) merged contextual reflection with "cultural forces" such as "race, gender, ethnicity, institutional policies, and personal expertise" (p. 3).…”