Physiological blunting during pregnancy extends to induced relaxation.

Pregnancy is associated with dramatic physiological changes in virtually every major organ system. Cardiovascular adjustments alone include increased blood volume, heart rate, and cardiac output, decreased systemic vascular resistance, and decreased arterial pressure (Fu & Levine, 2009; Monga, 1999). Indicators of autonomic balance derived from measures of heart rate variability in both frequency and time domains change over the course of pregnancy. For example, a lower sympathetic to parasympathetic ratio, which reflects greater vagal input, has been reported in early pregnancy followed by reduced vagal activity coupled with enhanced sympathetic activation later in pregnancy (DiPietro, Costigan, & Gurewitsch, 2005; Ekholm & Erkkola, 1996; Kuo, Chen, Yang, Lo, & Tsai, 2000; Voss et al., 2000). Pregnancy is also accompanied by fundamental changes to other physiological systems, including the hypothalamic-pituitary-adrenal axis (HPA). In pregnancy, the well-known negative feedback loop between glucocorticoids and hypothalamic control of corticotrophin-releasing hormone (CRH) is modified by the positive feedback loop between glucocorticoids and placental CRH (Robinson, Emanuel, Frim, & Majzoub, 1988). This unique alteration to normal physiology is accompanied by a substantial increase in cortisol as gestation advances (Mastorakos & Ilias, 2003). A number of studies have reported that, in addition to the dynamic changes in the physiological milieu, pregnancy is associated with blunting of responsiveness to physical and psychological challenges. A fairly extensive body of research has relied on well known cardiovascular reflex maneuvers, such as attempted forced exhalation against a closed airway (i.e., Valsalva maneuver), changes in posture, and isometric exercise (e.g., hand grip). This literature has documented diminished heart rate, heart rate variability, and catecholamine responses to these challenges during pregnancy (Barron, Mujais, Zinaman, Bravo, & Lindheimer, 1986; Ekholm, Piha, Antila, & Erkkola, 1993; Matthews & Rodin, 1992). Pregnancy is further associated with a blunted renin response to thermal stress (Vaha-Eskeli, Erkkola, Schenin, & Seppanen, 1992) coupled with attenuated pain perception (Saisto, Kaaja, Ylikorkala, & Halmesmaki, 2001) and failure to mount a cortisol response (Kammerer, Adams, vonCastelberg, & Glover, 2002) to cold pressor tests. In contrast to the extensive literature on physical challenges, few studies evaluate the effects of pregnancy on modulating responses to cognitive or psychological challenges using direct comparisons to non-pregnant samples. In one such study, pregnant women displayed reduced diastolic responsivity to mental arithmetic and mirror image tracing tasks, but not differential heart rate responsiveness, at 21 to 23 weeks gestation (Matthews & Rodin, 1992). In contrast, pregnant women showed significantly blunted heart rate and electrodermal (i.e., sympathetic) reactivity and recovery responses to administration of the Stroop Color Word test at both 24 and 36 weeks gestation (DiPietro et al., 2005). A more recent study did not find significant differences in the responsiveness of non-pregnant and pregnant participants (at either 13 to 18 weeks or 26 to 31 weeks gestation) to the Trier Social Stress Test (TSST) in either heart rate or variability (Klinkenberg et al., 2009). Analysis of neuroendocrine responses of participants in the same study revealed attenuated salivary α-amylase responses, a primarily sympathetic derivative, to the TSST in pregnant women at both gestational periods. There was no difference in cortisol reactivity between pregnant and non-pregnant participants, although there was diminished (i.e., prolonged) cortisol recovery to the experimentally induced surge in pregnant women at the earlier gestational age (Nierop, Klinkenberg, Nater, Zimmermann, & Ehlert, 2006). In summary, there is compelling evidence that pregnancy is associated with blunted responsivity to autonomic maneuvers that manipulate the cardiovascular system, and to a lesser extent, pain perception related to thermal challenges. Parallel data are generally supportive but less extensive regarding cognitive or psychological laboratory challenges. Evidence for attenuation of responsivity to laboratory challenges as pregnancy progresses in studies that include only pregnant women (e.g., DiPietro et al., 2005; Entringer et al., 2010; Glynn, Schetter, Wadhwa, & Sandman, 2004; Nierop et al., 2006) also provides support for this concept. Together, this research is typically interpreted as supporting the position that the physiological milieu of pregnancy confers stress-buffering effects on the developing fetus, and as such, is protective against potentially deleterious effects of maternal stress (deWeerth & Buitelaar, 2005; Glynn, 2010a). Similar hypotheses have been offered to reconcile observed reductions in fearful behavior in other species (e.g., Vierin & Bouissou, 2001). However, it is equally possible that these observations reveal an overall dampening of maternal physiological lability to all experiences, both positive and negative, thereby promoting a condition of measured physiological homeostasis for the fetus. To the extent that pregnancy is characterized by a dampening of autonomic responsivity in general, one might expect attenuation of autonomic responses to laboratory-based cognitive or psychological challenges designed to increase physiological arousal (i.e., Stroop Color Word test or the TSST), and to manipulations designed to decrease arousal (i.e., progressive relaxation) since both result in changes to the intrauterine milieu. If, however, physiological blunting is observed in response to manipulations that heighten arousal due to their potential downstream effects on the fetus such as decreased blood flow, it is possible that pregnant women might show an enhanced response to conditions that facilitate the types of maternal physiological alterations that may be of benefit to the fetus. Relaxation techniques may fall into this category. The relaxation response is generally regarded as the antithesis of the stress response (Jacobs, 2001). Relaxation techniques including guided imagery, meditation, and yoga have been found to reduce psychological distress while decreasing sympathetic and increasing parasympathetic activity in non-pregnant populations (Tang et al., 2009; Vempati & Telles, 2002). Several manipulations designed to induce relaxation in laboratory settings have been shown to be effective at garnering expected cardiovascular system changes in pregnant women including reductions in heart rate and/or blood pressure (Teixeira, Martin, Prendiville, & Glover, 2005; Urech et al., 2010), alterations to both low and high frequency bands within heart rate variability (Satyapriya, Nagendra, Nagarathna, & Padmalatha, 2009), and reducing resistance in the umbilical artery (DiPietro, Costigan, Nelson, Gurewitsch, & Laudenslager, 2008). Reductions in cortisol, ACTH, norepinerphine, and noradrenaline have also been observed, although it is more difficult to ascribe these to the relaxation manipulation itself as opposed to simple rest (DiPietro et al., 2008; Teixeira et al., 2005; Urech et al., 2010). To our knowledge, no study has compared the relaxation response in pregnant and non-pregnant women. Doing so has practical implications for both the design of relaxation interventions for pregnant women as well as an explicit evaluation of the stress buffering hypothesis. This study assessed responsivity to a guided imagery relaxation protocol in healthy nulliparous women and non-pregnant women of similar age. The measures selected for study include heart rate, the most commonly-used cardiovascular indicator in prior studies. As heart rate is governed by multiple autonomic influences and non-neural factors, it is fairly non-specific; for this reason, we also included respiratory sinus arrhythmia (RSA), a well-known indicator of parasympathetic activation that corresponds primarily to vagal input to the heart (Berntson, Cacioppo, & Quigley, 1993). In addition, electrodermal activity data were collected, measured as changes in skin conductance (SC) mediated by eccrine glands which are uniquely innervated by sympathetic processes (Venables, 1991). Intervals between breaths were quantified to provide both peak to trough fluctuations for RSA quantification and to ascertain maternal compliance to the relaxation task demands. We anticipated that both pregnant and non-pregnant women would display a relaxation response consistent with enhanced parasympathetic (i.e., RSA) and reduced sympathetic activity (i.e., SC) but that the relaxation response of pregnant women would be attenuated.