Cardiac vagal control in depression: A critical analysis

Abstract

Rapidly developing research has found abnormal cardiac vagal control (CVC) in several physical and mental health conditions. CVC findings in depression are mixed, and the degree to which CVC is compromised in depression is unclear. A meta-analysis of 13 rigorous cross-sectional studies reveals that a diagnosis of depression exerts a small-to-medium effect size on CVC, and explains only about 2% of the overall variance in CVC. More robust data may emerge from alternative approaches to the depression–CVC relationship, such as the use of CVC to predict the course of the disorder. Despite the vigor of recent work on CVC and depression, overall findings are suggestive rather than conclusive. Methodological desiderata and priorities for future research are discussed, including the need to clarify the etiological significance of CVC.

Introduction

There has been a recent surge of research on biological predictors of health. One active area of work has been on cardiac vagal control (CVC), as indexed by the beat-to-beat variability in the timing of heart beats. A substantial scientific literature on CVC has accumulated. In fact, if one types CVC-relevant keywords such as “heart rate variability” and “respiratory sinus arrhythmia” into the PsycINFO database, these phrases return 538 and 201 hits, respectively. Indeed, the study of CVC has become sufficiently large and differentiated as to constitute a vigorous subfield, as seen in the convening of this special issue of Biological Psychology.

CVC is associated with an impressive array of physical and mental health outcomes. Included among the psychological correlates of high CVC are social competence (Eisenberg et al., 1995) and resiliency in the face of stressors (e.g. Fabes and Eisenberg, 1997). In turn, low CVC has been associated with negative mental health outcomes including anxiety (e.g. Thayer et al., 1996), disorders of impulse control (Beauchaine, 2001), and childhood behavior problems (e.g. Pine et al., 1998). Moreover, CVC exhibits relations with physical health. Low CVC is associated with a number of health problems; for instance, it is a prognostic indicator of risk for cardiac disease even after adjustment for known cardiovascular risk factors, and low CVC has been shown to confer increased relative risk of cardiovascular disease mortality (Dekker et al., 1997, Dekker et al., 2000), and increased risk of coronary insufficiency, myocardial infarction, and death from both coronary heart disease and congestive heart failure (Tsuji et al., 1996).

The explosion of CVC correlates, while highlighting the relevance of CVC for psychobiological functioning, also suggests the need to take stock of existing findings (Beauchaine, 2001). For example, how should low resting levels of CVC be interpreted, when it is observed in illnesses as diverse as dsypepsia (Haug et al., 1994) and posttraumatic stress disorder (Cohen et al., 1997)? With the mushrooming number of findings, comparative analyses and close analyses of CVC in the context of individual disorders are sorely needed. This paper joins this effort by conducting a systematic analysis of the relation between CVC and one crippling psychiatric condition, major depressive disorder (MDD).

There are compelling reasons to investigate CVC in major depressive disorder. MDD not only involves the kinds of self-regulatory deficits that are often coincident with abnormal CVC, it also imposes a heavy burden of morbidity and mortality on society. In fact, MDD afflicts nearly one-fifth of the population over the lifetime (Kessler, 2002), and it is a leading cause of psychiatric hospitalizations, accounting for over 20% of economic costs for all mental illness (Greenberg et al., 1993). The high prevalence and pervasive impact of depression underscore the need to elucidate factors, such as CVC, that may underlie depressed persons deficits in self-regulation and be implicated in the generation or maintenance of depressive episodes.

The literature on depression and CVC – though rapidly growing – has yet to receive a comprehensive review. Accordingly, this article will consider the depression–CVC relationship in several sections. The first briefly defines CVC and its theorized significance for general psychobiological adaptation and for major depression. The second considers the cross-sectional literature on levels of CVC in depression, including (a) the mixed reports of effects for CVC and depression, (b) the role of confounding factors that may contribute to this mixed pattern, and (c) methodological strategies to minimize these confounds. To estimate effect sizes of depression more accurately, the third section presents two meta-analyses of cross-sectional studies of depression and levels of CVC. Relatively modest effect sizes for depression are obtained; three possible explanations of these data are discussed. Alternative approaches to CVC and depression are presented in the fourth section, with indications that these approaches – such as the use of CVC to predict the course of depression – may yield a more robust and meaningful harvest. The paper concludes by highlighting several priorities for future research, including the need to clarify causal relations between cardiac vagal control and depression.

Mammalian heart rate exhibits considerable beat-to-beat variation. Much of this variation reflects the parasympathetic control of heart rate by brain stem regions that project to the heart via the vagus nerve. Vagal nerve activity thus leads to regular oscillations in heart rate. Cardiac vagal control, essentially, reflects the degree to which there is tonic vagal influence on the heart, and more specifically reflects the extent of variability in heart rate that is gated by the respiratory cycle. That is, during inspiration, there is decreased vagal activity and heart rate speeds up, whereas, during expiration, vagal activity is reinstated and heart rate slows down. This coupling of heart rate variability to the respiratory cycle is called respiratory sinus arrhythmia (RSA), estimates of which index the vagal control of the heart (e.g. Berntson et al., 1997).

These regular heart rate oscillations are typically assessed via an electrocardiogram (ECG) taken from a peripheral site (i.e. the chest wall). As discussed in more detail elsewhere in this special issue, several techniques exist to isolate the faster vagal respiratory-coupled influences on heart rate variability (0.15–0.50 Hz) from slower sources of heart rate variability (<0.04 Hz) that reflect a blend of sympathetic and parasympathetic influences and are believed to be influenced by temperature, vasomotor, hormonal, and metabolic regulation (see Taylor et al., 1998). This article will focus upon the most commonly accepted techniques for quantifying CVC, which include frequency (e.g. high frequency power of heart period) and time-domain measures (e.g. root mean square of successive R–R differences). Although distinctions between time and frequency-domain measures should not be ignored, these measures tend to be strongly intercorrelated and in most contexts will be valid indicators of CVC.

CVC is of interest in part for the non-invasive window it affords onto activity in the parasympathetic nervous system, which is believed to perform broad homeostatic, growth, and restorative functions. Porges, 1995, Porges, 1997 has offered an influential account of the functional and adaptive significance of CVC. Briefly, Polyvagal Theory links developments in the evolution of the mammalian autonomic nervous system to affective experience, emotional expression, facial gestures, vocal communication, and contingent social behavior. The poly in polyvagal refers to evidence of two vagal systems, functionally indicated by two branches of the vagus nerve that show first differentiation in mammals. The theory has focused on the evolutionarily newer branch of the vagus, in particular, an efferent pathway that emerges from a brain stem area called the nucleus ambiguous. This branch of the vagus terminates on a number of visceral organs (heart, soft palate, pharynx, larynx, esophagus, bronchi, facial muscles) that are notable for their role in emotion and communication. In other words, this vagal efferent projection, in addition to generating CVC, is theorized to play a critical role in motor pathways involved in vocalizations, facial expressions, and the communication of internal states to others, in other words, a large proportion of the unique social and survival behaviors observed in mammals.

An important aspect of this vagal pathway is its responsiveness to environmental demand. When a mammal is at rest, the vagal pathway works to “brake” energy expenditure (e.g. inhibiting the sympathetic influences to the heart, dampening the HPA axis). Accordingly, mammalian CVC is highest during unchallenged situations (e.g. non-REM sleep). In the context of the heart, the role of the vagal pathway in energy conservation is clear: the vagal input to the sino-atrial node of the heart slows heart rate well below its autonomous rhythm. Importantly, as environmental conditions become more taxing, this vagal brake can be actively and rapidly withdrawn to meet several metabolic demands, including increased attention and information processing, exercise, or coping with threats to life or limb (Suess et al., 1994, George et al., 1989). In sum, the application and withdrawal of the vagal “brake” is thought to play an important role in enabling the greater flexibility of mammalian behavioral routines (relative to other animals like reptiles).

In tying the vagal pathway to the evolution of attention, motor behavior, emotion, and communication in mammals, Polyvagal Theory has obvious implications for human adaptation. More specifically, high resting levels of CVC and a high capacity to appropriately withdraw CVC in the face of environmental demand are thought to facilitate physical and psychological functioning. In turn, low levels of resting CVC and a low capacity to appropriately withdraw CVC are expected to predict poor outcomes, reflecting weak regulatory control over attentional and emotional systems as well as behavioral inflexibility. In sum, Polyvagal Theory outlines the ontogenic and phylogenic significance of the vagal pathway and its role in self-regulation and self-regulatory problems. Because this theory operates at a relatively high level of abstraction, additional work is required to apply the theory to depression, or to other categories in psychopathology research (i.e. DSM-IV disorders).

Polyvagal Theory posits that CVC subserves social engagement and flexible adjustment to environmental demands. If so, one would expect CVC to be compromised in depression. First, depression is clearly associated with reduced social engagement (Rottenberg and Gotlib, 2004). Depressed persons not only withdraw from social activities, they also report experiencing social impairments that range from rejection, marital difficulties, and social isolation, to poor relationship quality with parents, spouses, and friends (reviewed in Segrin and Abramson, 1994). Second, depression is also clearly associated with inflexible responding to environmental demands (Rottenberg, 2005). For example, during naturalistic social interactions depressed persons often appear unresponsive, displaying fewer facial expressions and evidencing reduced gaze behavior (Ellgring, 1989). Moreover, experimental findings converge on the idea that depressed persons respond inflexibly to changing environments. For example, in paradigms that expose participants to both positive and negative emotional stimuli, depressed persons exhibit a constricted range of response in their self-report of emotion (e.g. Rottenberg et al., 2002a, Rottenberg et al., 2005), facial expressions (e.g. Greden et al., 1986) and startle magnitude (e.g. Allen et al., 1999). Interestingly, the phenomenology of depression itself often involves perceptions that the environment is unchanging: patients often describe their world as being flat, dull, and empty, and remark that “everything is the same” (Healy, 1993). In sum, the kinds of inflexibility that are theoretically identified with compromised CVC appear evident in depressed persons.

Theory being clear, we are left with the empirical question: Do depressed persons actually exhibit compromised CVC? I now take up this empirical literature, first focusing on a body of work that compares levels of CVC between depressed and non-depressed persons.