Reflexly evoked coactivation of cardiac vagal and sympathetic motor outflows: observations and functional implications

Resting Heart Rate Affects Heart Response to Cold-Water Face Immersion Associated with Apnea

The regular cardiac response to immersion of the face in cold water is reduction in heart rate (HR). The highly individualized and unpredictable course of the cardiodepressive response prompted us to investigate the relationship between the cardiac response to face immersion and the resting HR. The research was conducted with 65 healthy volunteers (37 women and 28 men) with an average age of 21.13 years (20-27 years) and a BMI of 21.49 kg/m² (16.60-28.98). The face-immersion test consisted of stopping breathing after maximum inhaling and voluntarily immersing the face in cold water (8-10 °C) for as long as possible. Measurements included determination of minimum, average, and maximum HR at rest and minimum and maximum HR during the cold-water face-immersion test. The results indicate a strong relationship between the cardiodepressive reaction of the immersion of the face and the minimum HR before the test, as well as a relationship between the maximum HR during the test and the maximum HR at rest. The results also indicate a strong influence of neurogenic HR regulation on the described relationships. The parameters of the basal HR can, therefore, be used as prognostic indicators of the course of the cardiac response of the immersion test.

The Influence of Trigeminocardiac Reflex on Postoperative Cardiac Adverse Events in Patients Undergoing Cerebellopontine Angle Tumor Resections: A Case-Control Study

OBJECTIVE

To assess the potential impact of the trigeminocardiac reflex (TCR) on postoperative adverse cardiac events and to identify predictors of the TCR in cerebellopontine angle surgery.

METHODS

Patients undergoing elective cerebellopontine angle surgery from October 1, 2015, to September 30, 2020, were recruited consecutively for this retrospective case-control study. The TCR was evaluated by reviewing electronic anesthesia records and defined as a drop in heart rate was >20%. Controls were identified from the same retrospective cohort and matched by age, sex, and similar (±5 days) surgery date in the ratio of 1:2.

RESULTS

Of 2446 patients, 68 (2.78%) experienced TCR episodes. A total of 97 TCR episodes occurred among the 68 patients. In 2 TCR episodes, severe cardiac complications developed after surgery-myocardial injury in one case and cardiac arrest in the other case. The prevalence of adverse cardiovascular events was higher in the TCR group (60.3% vs. 36.0%, P = 0.001) than in the control group. The independent risk factor for the TCR in the multivariate condition logistic regression was tumor compression of the brainstem (odds ratio = 2.36, 95% confidence interval 1.40-3.95; P = 0.001).

CONCLUSIONS

Intraoperative TCR episodes seemed to be associated with postoperative adverse cardiac events in patients undergoing cerebellopontine angle surgery. Moreover, tumor compression of the brainstem might be a risk factor for TCR episodes.

Autonomic Cardiovascular Control in Health and Disease

Autonomic neural control of the cardiovascular system is formed of complex and dynamic processes able to adjust rapidly to mitigate perturbations in hemodynamics and maintain homeostasis. Alterations in autonomic control feature in the development or progression of a multitude of diseases with wide-ranging physiological implications given the neural system's responsibility for controlling inotropy, chronotropy, lusitropy, and dromotropy. Imbalances in sympathetic and parasympathetic neural control are also implicated in the development of arrhythmia in several cardiovascular conditions sparking interest in autonomic modulation as a form of treatment. A number of measures of autonomic function have shown prognostic significance in health and in pathological states and have undergone varying degrees of refinement, yet adoption into clinical practice remains extremely limited. The focus of this contemporary narrative review is to summarize the anatomy, physiology, and pathophysiology of the cardiovascular autonomic nervous system and describe the merits and shortfalls of testing modalities available. © 2023 American Physiological Society. Compr Physiol 13:4493-4511, 2023.

Heart Rate Variability at Rest Predicts Heart Response to Simulated Diving

A characteristic feature of the cardiac response to diving is the uncertainty in predicting individual course. The aim of the study was to determine whether resting regulatory heart rate determinants assessed before diving may be predictors of cardiac response in a simulated diving test. The research was conducted with 65 healthy volunteers (37 women and 28 men) with an average age of 21.13 years (20-27 years) and a BMI of 21.49 kg/m² (16.60-28.98). The simulated diving test consisted of stopping breathing after maximum inhaling and voluntarily immersing the face in water (8-10 °C) for as long as possible. The measurements included heart rate variability (HRV) analysis before diving and determination of the course of the cardiac response to diving-minimum and maximum heart rate (HR). The results indicate that minimum HR during diving (MIN_div) is dependent on the short-term HRV measures, which proves the strong influence of the parasympathetic system on the MIN_div. The lack of dependence of MIN_div on short-term HRV in women may be associated with differences in neurogenic HR regulation in women and men. In conclusion, cardiac response to simulated diving is strictly dependent on the autonomic regulation of the heart rhythm under resting conditions. The course of the cardiac response to diving and its relationship with resting HRV appears to be gender dependent.

The Bezold-Jarisch Reflex in a Patient With Stanford Type A Acute Aortic Dissection

BACKGROUND

The Bezold-Jarisch reflex is a depressor reflex of the heart due to the preferential distribution of vagal nerves in the inferior wall of the left ventricle. We report a case of Stanford type A acute aortic dissection, in which coronary spastic angina caused the Bezold-Jarisch reflex in the acute phase.

CASE REPORT

A 53-year-old man presented with left chest pain and cold sweating. An electrocardiogram was normal and the high-sensitivity cardiac troponin T level was negative. A diagnosis of Stanford type A acute aortic dissection was made based on computed tomography (CT); there was no evidence of ischemic heart disease on coronary CT angiogram obtained simultaneously. While waiting for emergency surgical repair, chest pain worsened, followed by bradycardia and hypotension, along with ST-segment elevations in the inferior leads, all of which were resolved by conservative treatment. During surgery, no evidence to suggest an extension of the dissection to the ostium of the right coronary artery was observed. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: The present case highlights the importance of recognizing the Bezold-Jarisch reflex because this depressor reflex may require different management than other conditions.

Differentiation of skin incision and laparoscopic trocar insertion via quantifying transient bradycardia measured by electrocardiogram

Most surgical procedures involve structures deeper than the skin. However, the difference in surgical noxious stimulation between skin incision and laparoscopic trocar insertion is unknown. By analyzing instantaneous heart rate (IHR) calculated from the electrocardiogram, in particular the transient bradycardia in response to surgical stimuli, this study investigates surgical noxious stimuli arising from skin incision and laparoscopic trocar insertion, and their difference. Thirty-five patients undergoing laparoscopic cholecystectomy were enrolled in this prospective observational study. Sequential surgical steps including umbilical skin incision (11 mm), umbilical trocar insertion (11 mm), xiphoid skin incision (5 mm), xiphoid trocar insertion (5 mm), subcostal skin incision (3 mm), and subcostal trocar insertion (3 mm) were investigated. IHR was derived from electrocardiography and calculated by the modern time-varying power spectrum. Similar to the classical heart rate variability analysis, the time-varying low frequency power (tvLF), time-varying high frequency power (tvHF), and tvLF-to-tvHF ratio (tvLHR) were calculated. Prediction probability (P_K) analysis and global pointwise F-test were used to compare the statistical performance between indices and the heart rate readings from the patient monitor. Analysis of IHR showed that surgical stimulus elicits a transient bradycardia, followed by the increase of heart rate. Transient bradycardia is more significant in trocar insertion than skin incision (p < 0.001 for tvHF). The IHR change quantifies differential responses to different surgical intensity. Serial P_K analysis demonstrates de-sensitization in skin incision, but not in laparoscopic trocar insertion. Quantitative indices present the transient bradycardia introduced by noxious stimulation. The results indicate different effects between skin incision and trocar insertion.

Nociceptive pulmonary-cardiac reflexes are altered in the spontaneously hypertensive rat

KEY POINTS

We investigated the cardiovascular and respiratory responses of the normotensive Wistar-Kyoto (WKY) rat and the spontaneously hypertensive (SH) rat to inhalation and intravenous injection of the noxious stimuli allyl isothiocyanate (AITC). AITC inhalation evoked atropine-sensitive bradycardia in conscious WKY rats, and evoked atropine-sensitive bradycardia and atenolol-sensitive tachycardia with premature ventricular contractions (PVCs) in conscious SH rats. Intravenous injection of AITC evoked bradycardia but no tachycardia/PVCs in conscious SHs, while inhalation and injection of AITC caused similar bradypnoea in conscious SH and WKY rats. Anaesthesia (inhaled isoflurane) inhibited the cardiac reflexes evoked by inhaled AITC but not injected AITC. Data indicate the presence of a de novo nociceptive pulmonary-cardiac reflex triggering sympathoexcitation in SH rats, and this reflex is dependent on vagal afferents but is not due to steady state blood pressure or due to remodelling of vagal efferent function.

ABSTRACT

Inhalation of noxious irritants/pollutants activates airway nociceptive afferents resulting in reflex bradycardia in healthy animals. Nevertheless, noxious pollutants evoke sympathoexcitation (tachycardia, hypertension) in cardiovascular disease patients. We hypothesize that cardiovascular disease alters nociceptive pulmonary-cardiac reflexes. Here, we studied reflex responses to irritants in normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive (SH) rats. Inhaled allyl isothiocyanate (AITC) evoked atropine-sensitive bradycardia with atrial-ventricular (AV) block in conscious WKY rats, thus indicating a parasympathetic reflex. Conversely, inhaled AITC in conscious SH rats evoked complex brady-tachycardia with both AV block and premature ventricular contractions (PVCs). Atropine abolished the bradycardia and AV block, but the atropine-insensitive tachycardia and PVCs were abolished by the β₁ -adrenoceptor antagonist atenolol. The aberrant AITC-evoked reflex in SH rats was not reduced by acute blood pressure reduction by captopril. Surprisingly, intravenous AITC only evoked bradycardia in conscious SH and WKY rats. Furthermore, anaesthesia reduced the cardiac reflexes evoked by inhaled but not injected AITC. Nevertheless, anaesthesia had little effect on AITC-evoked respiratory reflexes. Such data suggest distinct differences in nociceptive reflex pathways dependent on cardiovascular disease, administration route and downstream effector. AITC-evoked tachycardia in decerebrate SH rats was abolished by vagotomy. Finally, there was no difference in the cardiac responses of WKY and SH rats to vagal efferent electrical stimulation. Our data suggest that AITC inhalation in SH rats evokes de novo adrenergic reflexes following vagal afferent activation. This aberrant reflex is independent of steady state hypertension and is not evoked by intravenous AITC. We conclude that pre-existing hypertension aberrantly shifts nociceptive pulmonary-cardiac reflexes towards sympathoexcitation.

Biphasic changes in spontaneous cardiovagal baroreflex sensitivity during passive hyperthermia

Successful adaptation to passive hyperthermia requires continual adjustment of circulation, which is mediated mainly by the autonomic nervous system. The goal of this study was to explore the alterations in spontaneous cardiovagal baroreflex sensitivity (BRS) during exposure to a hot environment. To continuously follow changes in core body temperature (Tc), haemodynamics, and BRS, male Wistar-Kyoto rats were implanted with telemetric transmitters. BRS at an ambient temperature of 23 °C was not steady but oscillated with a maximum power in the range of 0.02–0.2 Hz. Exposure to hot air immediately shifted the distribution of BRS to higher values, although Tc remained unchanged (37.2 (0.3) °C), and the average BRS changed from 1.3 (0.3) to 3 (1.4) ms.mmHg⁻¹, p < 0.0001. The degree of initial cardiovagal baroreflex sensitization explained 57% of the variability in the time to the onset of arterial pressure decline (p = 0.0114). With an increasing Tc (>38.8 (0.6) °C), BRS non-linearly declined, but haemodynamic parameters remained stable even above a Tc of 42 °C when the cardiovagal baroreflex was virtually non-operative. Abrupt full desensitization of the cardiovagal baroreflex with a muscarinic blocker did not induce arterial pressure decline. Our data indicate that a progressive decrease in BRS during passive hyperthermia does not induce haemodynamic instability. The positive association between initial cardiovagal baroreflex sensitization and the time to the onset of arterial pressure decline may reflect the potential protective role of parasympathetic activation during exposure to a hot environment.

Is Adenosine Action Common Ground for NREM Sleep, Torpor, and Other Hypometabolic States?

This review compares two states that lower energy expenditure: non-rapid eye movement (NREM) sleep and torpor. Knowledge on mechanisms common to these states, and particularly on the role of adenosine in NREM sleep, may ultimately open the possibility of inducing a synthetic torpor-like state in humans for medical applications and long-term space travel. To achieve this goal, it will be important, in perspective, to extend the study to other hypometabolic states, which, unlike torpor, can also be experienced by humans.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

Cycling before and after Exhaustion Differently Affects Cardiac Autonomic Control during Heart Rate Matched Exercise

During cycling before (PRE) and after exhaustion (POST) different modes of autonomic cardiac control might occur due to different interoceptive input and altered influences from higher brain centers. We hypothesized that heart rate variability (HRV) is significantly affected by an interaction of the experimental period (PRE vs. POST) and exercise intensity (HIGH vs. LOW; HIGH = HR > HR at the lactate threshold (HR_LT), LOW = HR ≤ HR_LT) despite identical average HR.

Methods: Fifty healthy volunteers completed an incremental cycling test until exhaustion. Workload started with 30 W at a constant pedaling rate (60 revolutions · min⁻¹) and was gradually increased by 30 W · 5 min⁻¹. Five adjacent 60 s inter-beat (R-R) interval segments from the immediate recovery period (POST 1–5 at 30 W and 60 rpm) were each matched with their HR-corresponding 60 s-segments during the cycle test (PRE 1–5). An analysis of covariance was carried out with one repeated-measures factor (PRE vs. POST exhaustion), one between-subject factor (HIGH vs. LOW intensity) and respiration rate as covariate to test for significant effects (p < 0.050) on the natural log-transformed root mean square of successive differences between adjacent R-R intervals (lnRMSSD_60s).

Results: LnRMSSD_60s was significantly affected by the interaction of experimental period × intensity [F_{(1, 242)} = 30.233, p < 0.001, η_p² = 0.111]. LnRMSSD_60s was higher during PRE compared to POST at LOW intensity (1.6 ± 0.6 vs. 1.4 ± 0.6 ms; p < 0.001). In contrast, at HIGH intensity lnRMSSD_60s was lower during PRE compared to POST (1.0 ± 0.4 vs. 1.2 ± 0.4 ms; p < 0.001).

Conclusion: Identical net HR during cycling can result from distinct autonomic modulation patterns. Results suggest a pronounced sympathetic-parasympathetic coactivation immediately after the cessation of peak workload compared to HR-matched cycling before exhaustion at HIGH intensity. On the opposite, at LOW intensity cycling, a stronger coactivational cardiac autonomic modulation pattern occurs during PRE-exhaustion if compared to POST-exhaustion cycling. The different autonomic modes during these phases might be the result of different afferent and/or central inputs to the cardiovascular control centers in the brainstem.

Heart-Rate Variability-More than Heart Beats?

Heart-rate variability (HRV) is frequently introduced as mirroring imbalances within the autonomous nerve system. Many investigations are based on the paradigm that increased sympathetic tone is associated with decreased parasympathetic tone and vice versa. But HRV is probably more than an indicator for probable disturbances in the autonomous system. Some perturbations trigger not reciprocal, but parallel changes of vagal and sympathetic nerve activity. HRV has also been considered as a surrogate parameter of the complex interaction between brain and cardiovascular system. Systems biology is an inter-disciplinary field of study focusing on complex interactions within biological systems like the cardiovascular system, with the help of computational models and time series analysis, beyond others. Time series are considered surrogates of the particular system, reflecting robustness or fragility. Increased variability is usually seen as associated with a good health condition, whereas lowered variability might signify pathological changes. This might explain why lower HRV parameters were related to decreased life expectancy in several studies. Newer integrating theories have been proposed. According to them, HRV reflects as much the state of the heart as the state of the brain. The polyvagal theory suggests that the physiological state dictates the range of behavior and psychological experience. Stressful events perpetuate the rhythms of autonomic states, and subsequently, behaviors. Reduced variability will according to this theory not only be a surrogate but represent a fundamental homeostasis mechanism in a pathological state. The neurovisceral integration model proposes that cardiac vagal tone, described in HRV beyond others as HF-index, can mirror the functional balance of the neural networks implicated in emotion-cognition interactions. Both recent models represent a more holistic approach to understanding the significance of HRV.

Heart rate regulation in diving sea lions: the vagus nerve rules

Recent publications have emphasized the potential generation of morbid cardiac arrhythmias secondary to autonomic conflict in diving marine mammals. Such conflict, as typified by cardiovascular responses to cold water immersion in humans, has been proposed to result from exercise-related activation of cardiac sympathetic fibers to increase heart rate, combined with depth-related changes in parasympathetic tone to decrease heart rate. After reviewing the marine mammal literature and evaluating heart rate profiles of diving California sea lions (Zalophus californianus), we present an alternative interpretation of heart rate regulation that de-emphasizes the concept of autonomic conflict and the risk of morbid arrhythmias in marine mammals. We hypothesize that: (1) both the sympathetic cardiac accelerator fibers and the peripheral sympathetic vasomotor fibers are activated during dives even without exercise, and their activities are elevated at the lowest heart rates in a dive when vasoconstriction is maximal, (2) in diving animals, parasympathetic cardiac tone via the vagus nerve dominates over sympathetic cardiac tone during all phases of the dive, thus producing the bradycardia, (3) adjustment in vagal activity, which may be affected by many inputs, including exercise, is the primary regulator of heart rate and heart rate fluctuations during diving, and (4) heart beat fluctuations (benign arrhythmias) are common in marine mammals. Consistent with the literature and with these hypotheses, we believe that the generation of morbid arrhythmias because of exercise or stress during dives is unlikely in marine mammals.

Muscular contraction mode differently affects autonomic control during heart rate matched exercise

The precise contributions of afferent feedback to cardiovascular and respiratory responses to exercise are still unclear. The aim of this crossover study was to assess whether and how autonomic cardiovascular and respiratory control differed in response to dynamic (DYN) and isometric contractions (ISO) at a similar, low heart rate (HR) level. Therefore, 22 healthy males (26.7 ± 3.6 yrs) performed two kinds of voluntary exercises at similar HR: ISO and DYN of the right quadriceps femoris muscle. Although HR was eqivalent (82 ± 8 bpm for DYN and ISO, respectively), rating of exertion, blood pressures, and rate pressure product were higher, whereas breathing frequency, minute ventilation, oxygen uptake and carbon dioxide output were significantly lower during ISO. Tidal volume, end-tidal partial pressures of O2 and CO2, respiratory exchange ratio and capillary blood lactate concentration were comparable between both contraction modes. Heart rate variability (HRV) indicators, SDNN, HF-Power and LF-Power, representing both vagal and sympathetic influences, were significantly higher during ISO. Sample entropy, a non-linear measure of HRV was also significantly affected by contraction mode. It can be concluded that, despite the same net effect on HR, the quality of cardiovascular control during low intensity exercise is significantly different between DYN and ISO. HRV analysis indicated a sympatho-vagal coactivation during ISO. Whether mechanoreceptor feedback alone, a change in central command, or the interaction of both mechanisms is the main contributor of the distinct autonomic responses to the different exercise modes remains to be elucidated.

The preparatory set: a novel approach to understanding stress, trauma, and the bodymind therapies

Basic to all motile life is a differential approach/avoid response to perceived features of environment. The stages of response are initial reflexive noticing and orienting to the stimulus, preparation, and execution of response. Preparation involves a coordination of many aspects of the organism: muscle tone, posture, breathing, autonomic functions, motivational/emotional state, attentional orientation, and expectations. The organism organizes itself in relation to the challenge. We propose to call this the "preparatory set" (PS). We suggest that the concept of the PS can offer a more nuanced and flexible perspective on the stress response than do current theories. We also hypothesize that the mechanisms of body-mind therapeutic and educational systems (BTES) can be understood through the PS framework. We suggest that the BTES, including meditative movement, meditation, somatic education, and the body-oriented psychotherapies, are approaches that use interventions on the PS to remedy stress and trauma. We discuss how the PS can be adaptive or maladaptive, how BTES interventions may restore adaptive PS, and how these concepts offer a broader and more flexible view of the phenomena of stress and trauma. We offer supportive evidence for our hypotheses, and suggest directions for future research. We believe that the PS framework will point to ways of improving the management of stress and trauma, and that it will suggest directions of research into the mechanisms of action of BTES.

Somatic experiencing: using interoception and proprioception as core elements of trauma therapy

Here we present a theory of human trauma and chronic stress, based on the practice of Somatic Experiencing(®) (SE), a form of trauma therapy that emphasizes guiding the client's attention to interoceptive, kinesthetic, and proprioceptive experience. SE™ claims that this style of inner attention, in addition to the use of kinesthetic and interoceptive imagery, can lead to the resolution of symptoms resulting from chronic and traumatic stress. This is accomplished through the completion of thwarted, biologically based, self-protective and defensive responses, and the discharge and regulation of excess autonomic arousal. We present this theory through a composite case study of SE treatment; based on this example, we offer a possible neurophysiological rationale for the mechanisms involved, including a theory of trauma and chronic stress as a functional dysregulation of the complex dynamical system formed by the subcortical autonomic, limbic, motor and arousal systems, which we term the core response network (CRN). We demonstrate how the methods of SE help restore functionality to the CRN, and we emphasize the importance of taking into account the instinctive, bodily based protective reactions when dealing with stress and trauma, as well as the effectiveness of using attention to interoceptive, proprioceptive and kinesthetic sensation as a therapeutic tool. Finally, we point out that SE and similar somatic approaches offer a supplement to cognitive and exposure therapies, and that mechanisms similar to those discussed in the paper may also be involved in the benefits of meditation and other somatic practices.

The mammalian diving response: an enigmatic reflex to preserve life?

The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

Is there consistency and specificity of autonomic changes during emotional episodes? Guidance from the Conceptual Act Theory and psychophysiology

The consistency and specificity of autonomic nervous system (ANS) responses during emotional episodes remains a topic of debate with relevance for emotional concordance. We present a recent model of how mental states are constructed, the Conceptual Act Theory (CAT), and then review findings from existing meta-analyses and a qualitative review along with studies using pattern classification of multivariate ANS patterns to determine if there is across-study evidence for consistency and specificity of ANS responses during emotional episodes. We conclude that there is thus far minimal evidence for ANS response consistency and specificity across studies. We then review the current understanding of the functional and anatomical features of ANS including its efferent and afferent connections with the central nervous system, which suggests the need to reformulate how we conceptualize ANS response consistency and specificity. We conclude by showing how this reformulation is consistent with the CAT, and how the model suggests when we would and would not expect to see consistency and specificity in ANS responses, and concordance more generally, during emotional episodes.

Sympathetic nerve activity and simulated diving in healthy humans

The goal of our study was to develop a simple and practical method for simulating diving in humans using facial cold exposure and apnea stimuli to measure neural and circulatory responses during the stimulated diving reflex. We hypothesized that responses to simultaneous facial cold exposure and apnea (simulated diving) would be synergistic, exceeding the sum of responses to individual stimuli. We studied 56 volunteers (24 female and 32 male), average age of 39 years. All subjects were healthy, free of cardiovascular and other diseases, and on no medications. Although muscle sympathetic nerve activity (MSNA), blood pressure, and vascular resistance increased markedly during both early and late phases of simulated diving, significant reductions in heart rate were observed only during the late phase. Total MSNA during simulated diving was greater than combined MSNA responses to the individual stimuli. We found that simulated diving is a powerful stimulus to sympathetic nerve traffic with significant bradycardia evident in the late phase of diving and eliciting synergistic sympathetic and parasympathetic responses. Our data provide insight into autonomic triggers that could help explain catastrophic cardiovascular events that may occur during asphyxia or swimming, such as in patients with obstructive sleep apnea or congenital long QT syndrome.

Influence of hypoxia and hypercapnia on sleep state-dependent heart rate variability behavior in newborn lambs

STUDY OBJECTIVES

Although hypercapnia and/or hypoxia are frequently present during chronic lung disease of infancy and have also been implicated in sudden infant death syndrome (SIDS), their effect on cardiac autonomic regulation remains unclear. The authors' goal is to test that hypercapnia and hypoxia alter sleep-wake cycle-dependent heart rate variability (HRV) in the neonatal period.

DESIGN

Experimental study measuring HRV during sleep states in lambs randomly exposed to hypercapnia, hypoxia, or air.

SETTING

University center for perinatal research in ovines (Sherbrooke, Canada). INSERM-university research unit for signal processing (Rennes, France).

PARTICIPANTS

Six nonsedated, full-term lambs.

INTERVENTIONS

Each lamb underwent polysomnographic recordings while in a chamber flowed with either air or 21% O(2) + 5% CO(2) (hypercapnia) or 10% O(2) + 0% CO(2) (hypoxia) on day 3, 4, and 5 of postnatal age.

MEASUREMENTS AND RESULTS

Hypercapnia increased the time spent in wakefulness and hypoxia the time spent in quiet sleep (QS). The state of alertness was the major determinant of HRV characterized with linear or nonlinear methods. Compared with QS, active sleep (AS) was associated with an overall increase in HRV magnitude and short-term self-similarity and a decrease in entropy of cardiac cycle length in air. This AS-related HRV pattern persisted in hypercapnia and was even more pronounced in hypoxia.

CONCLUSION

Enhancement of AS-related sympathovagal coactivation in hypoxia, together with increased heart rate regularity, may be evidence that AS + hypoxia represent a particularly vulnerable state in early life. This should be kept in mind when deciding the optimal arterial oxygenation target in newborns and when investigating the potential involvement of hypoxia in SIDS pathogenesis.

Synchronized activation of sympathetic vasomotor, cardiac, and respiratory outputs by neurons in the midbrain colliculi

The superior and inferior colliculi are believed to generate immediate and highly coordinated defensive behavioral responses to threatening visual and auditory stimuli. Activation of neurons in the superior and inferior colliculi have been shown to evoke increases in cardiovascular and respiratory activity, which may be components of more generalized stereotyped behavioral responses. In this study, we examined the possibility that there are “command neurons” within the colliculi that can simultaneously drive sympathetic and respiratory outputs. In anesthetized rats, microinjections of bicuculline (a GABAA receptor antagonist) into sites within a circumscribed region in the deep layers of the superior colliculus and in the central and external nuclei of the inferior colliculus evoked a response characterized by intense and highly synchronized bursts of renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA). Each burst of RSNA had a duration of ∼300–400 ms and occurred slightly later (peak to peak latency of 41 ± 8 ms) than the corresponding burst of PNA. The bursts of RSNA and PNA were also accompanied by transient increases in arterial pressure and, in most cases, heart rate. Synchronized bursts of RSNA and PNA were also evoked after neuromuscular blockade, artificial ventilation, and vagotomy and so were not dependent on afferent feedback from the lungs. We propose that the synchronized sympathetic-respiratory responses are driven by a common population of neurons, which may normally be activated by an acute threatening stimulus.

Evaluating the physiological significance of respiratory sinus arrhythmia: looking beyond ventilation-perfusion efficiency

We conducted a theoretical study of the physiological significance of respiratory sinus arrhythmia (RSA), a phenomenon used as an index of cardiac vagal tone and wellbeing, whereby the heart rate (HR) increases during inspiration and decreases during expiration. We first tested the hypothesis that RSA improves gas exchange efficiency but found that although gas exchange efficiency improved with slow and deep breathing and with increased mean heart rate, this was unrelated to RSA. We then formulated and tested a new hypothesis: that RSA minimizes the work done by the heart while maintaining physiological levels of arterial carbon dioxide. We tested the new hypothesis using two methods. First, the HR for which the work is minimized was calculated using techniques from optimal control theory. This calculation was done on simplified models that we derived from a previously published model of gas exchange in mammals. We found that the calculated HR was remarkably similar to RSA and that this became more profound under slow and deep breathing. Second, the HR was prescribed and the work done by the heart was calculated by conducting a series of numerical experiments on the previously published gas exchange model. We found that cardiac work was minimized for RSA-like HR functions, most profoundly under slow and deep breathing. These findings provide novel insights into potential reasons for and benefits of RSA under physiological conditions.

REM sleep-like episodes of motoneuronal depression and respiratory rate increase are triggered by pontine carbachol microinjections in in situ perfused rat brainstem preparation

Hypoglossal nerve activity (HNA) controls the position and movements of the tongue. In persons with compromised upper airway anatomy, sleep-related hypotonia of the tongue and other pharyngeal muscles causes increased upper airway resistance, or total upper airway obstructions, thus disrupting both sleep and breathing. Hypoglossal nerve activity reaches its nadir, and obstructive episodes are longest and most severe, during rapid eye movement stage of sleep (REMS). Microinjections of a cholinergic agonist, carbachol, into the pons have been used in vivo to investigate the mechanisms of respiratory control during REMS. Here, we recorded inspiratory-modulated phrenic nerve activity and HNA and microinjected carbachol (25-50 nl, 10 mm) into the pons in an in situ perfused working heart-brainstem rat preparation (WHBP), an ex vivo model previously validated for studies of the chemical and reflex control of breathing. Carbachol microinjections were made into 40 sites in 33 juvenile rat preparations and, at 24 sites, they triggered depression of HNA with increased respiratory rate and little change of phrenic nerve activity, a pattern akin to that during natural REMS in vivo. The REMS-like episodes started 151 ± 73 s (SD) following microinjections, lasted 20.3 ± 4.5 min, were elicited most effectively from the dorsal part of the rostral nucleus pontis oralis, and were prevented by perfusion of the preparation with atropine. The WHBP offers a novel model with which to investigate cellular and neurochemical mechanisms of REMS-related upper airway hypotonia in situ without anaesthesia and with full control over the cellular environment.

Impact of the human circadian system, exercise, and their interaction on cardiovascular function

The risk of adverse cardiovascular events peaks in the morning (≈9:00 AM) with a secondary peak in the evening (≈8:00 PM) and a trough at night. This pattern is generally believed to be caused by the day/night distribution of behavioral triggers, but it is unknown whether the endogenous circadian system contributes to these daily fluctuations. Thus, we tested the hypotheses that the circadian system modulates autonomic, hemodynamic, and hemostatic risk markers at rest, and that behavioral stressors have different effects when they occur at different internal circadian phases. Twelve healthy adults were each studied in a 240-h forced desynchrony protocol in dim light while standardized rest and exercise periods were uniformly distributed across the circadian cycle. At rest, there were large circadian variations in plasma cortisol (peak-to-trough ≈85% of mean, peaking at a circadian phase corresponding to ≈9:00 AM) and in circulating catecholamines (epinephrine, ≈70%; norepinephrine, ≈35%, peaking during the biological day). At ≈8:00 PM, there was a circadian peak in blood pressure and a trough in cardiac vagal modulation. Sympathetic variables were consistently lowest and vagal markers highest during the biological night. We detected no simple circadian effect on hemostasis, although platelet aggregability had two peaks: at ≈noon and ≈11:00 PM. There was circadian modulation of the cardiovascular reactivity to exercise, with greatest vagal withdrawal at ≈9:00 AM and peaks in catecholamine reactivity at ≈9:00 AM and ≈9:00 PM. Thus, the circadian system modulates numerous cardiovascular risk markers at rest as well as their reactivity to exercise, with resultant profiles that could potentially contribute to the day/night pattern of adverse cardiovascular events.

Cardiac response to startle stimuli in larval zebrafish: sympathetic and parasympathetic components

Central regulation of cardiac output via the sympathetic and parasympathetic branches of the autonomic nervous system allows the organism to respond to environmental changes. Sudden onset stimuli, startle stimuli, are useful probes to study central regulatory responses to the environment. In mammals, startle stimuli induce a transient bradycardia that habituates with repeated stimulation. Repeated presentation of the stimulus results in tachycardia. In this study, we investigate the behavioral regulation of heart rate in response to sudden stimuli in the zebrafish. Larval zebrafish show a stereotyped heart rate response to mild electrical shock. Naïve fish show a significant increase in interbeat interval that resolves in the 2 s following stimulation. This transient bradycardia decreases on repeated exposure to the stimulus. Following repeated stimulation, the fish become tachycardic within 1 min of stimulation. Both the transient bradycardia and following tachycardia responses are blocked with administration of the ganglionic blocker hexamethonium, demonstrating that these responses are mediated centrally. The transient bradycardia is blocked by the muscarinic antagonist atropine, suggesting that this response is mediated by the parasympathetic system, while the following tachycardia is specifically blocked by the beta-adrenergic antagonist propranolol, suggesting that this response is mediated by the sympathetic nervous system. Together, these results demonstrate that at the larval stage, zebrafish actively regulate cardiac output to changes in their environment using both the parasympathetic and sympathetic branches of the autonomic nervous system, a behavioral response that is markedly similar to that observed in mammals to similar sudden onset stimuli.

The cardiovascular and endocrine responses to voluntary and forced diving in trained and untrained rats

The mammalian diving response, consisting of apnea, bradycardia, and increased total peripheral resistance, can be modified by conscious awareness, fear, and anticipation. We wondered whether swim and dive training in rats would 1) affect the magnitude of the cardiovascular responses during voluntary and forced diving, and 2) whether this training would reduce or eliminate any stress due to diving. Results indicate Sprague-Dawley rats have a substantial diving response. Immediately upon submersion, heart rate (HR) decreased by 78%, from 453 +/- 12 to 101 +/- 8 beats per minute (bpm), and mean arterial pressure (MAP) decreased 25%, from 143 +/- 1 to 107 +/- 5 mmHg. Approximately 4.5 s after submergence, MAP had increased to a maximum 174 +/- 3 mmHg. Blood corticosterone levels indicate trained rats find diving no more stressful than being held by a human, while untrained rats find swimming and diving very stressful. Forced diving is stressful to both trained and untrained rats. The magnitude of bradycardia was similar during both voluntary and forced diving, while the increase in MAP was greater during forced diving. The diving response of laboratory rats, therefore, appears to be dissimilar from that of other animals, as most birds and mammals show intensification of diving bradycardia during forced diving compared with voluntary diving. Rats may exhibit an accentuated antagonism between the parasympathetic and sympathetic branches of the autonomic nervous system, such that in the autonomic control of HR, parasympathetic activity overpowers sympathetic activity. Additionally, laboratory rats may lack the ability to modify the degree of parasympathetic outflow to the heart during an intense cardiorespiratory response (i.e., the diving response).

Autonomic activity and somatic symptoms in response to success vs. failure on a cognitive task: a comparison of chronic abdominal pain patients and well children

OBJECTIVES

To compare autonomic nervous system (ANS) activity and somatic symptoms in chronic abdominal pain (CAP) patients and well children during (a) resting baseline, (b) training in a cognitive task, and (c) random assignment to success vs. failure on the task.

METHODS

The ECG was continuously recorded with a dual lead system (Biopac) in 45 CAP patients and 22 well children, ages 9-16 years (mean age=12.3). Heart rate variability (HRV) was analyzed during the 5-min resting baseline, training, and success/failure on the task. Performance expectations were assessed before the task. Gastrointestinal (GI) and non-GI somatic symptoms were assessed before and after the task.

RESULTS

Compared to well children, CAP patients reported lower expectations for their task performance and higher GI symptoms (P's<.05). During success, CAP patients exhibited significant increases in both sympathetic (P<.05) and parasympathetic (P<.05) activity, whereas well children exhibited no change in ANS activity. During failure, CAP patients exhibited significant increases in somatic symptoms (<.05) but no change in ANS activity.

CONCLUSIONS

The lower performance expectations of CAP patients compared to well children may have influenced their experience of success and contributed to differences in their autonomic activity.

Effects of head and body cooling on hemodynamics during immersed prone exercise at 1 ATA

Immersion pulmonary edema (IPE) is a condition with sudden onset in divers and swimmers suspected to be due to pulmonary arterial or venous hypertension induced by exercise in cold water, although it does occur even with adequate thermal protection. We tested the hypothesis that cold head immersion could facilitate IPE via a reflex rise in pulmonary vascular pressure due solely to cooling of the head. Ten volunteers were instrumented with ECG and radial and pulmonary artery catheters and studied at 1 atm absolute (ATA) during dry and immersed rest and exercise in thermoneutral (29-31 degrees C) and cold (18-20 degrees C) water. A head tent varied the temperature of the water surrounding the head independently of the trunk and limbs. Heart rate, Fick cardiac output (CO), mean arterial pressure (MAP), mean pulmonary artery pressure (MPAP), pulmonary artery wedge pressure (PAWP), and central venous pressure (CVP) were measured. MPAP, PAWP, and CO were significantly higher in cold pool water (P < or = 0.004). Resting MPAP and PAWP values (means +/- SD) were 20 +/- 2.9/13 +/- 3.9 (cold body/cold head), 21 +/- 3.1/14 +/- 5.2 (cold/warm), 14 +/- 1.5/10 +/- 2.2 (warm/warm), and 15 +/- 1.6/10 +/- 2.6 mmHg (warm/cold). Exercise values were higher; cold body immersion augmented the rise in MPAP during exercise. MAP increased during immersion, especially in cold water (P < 0.0001). Except for a transient additive effect on MAP and MPAP during rapid head cooling, cold water on the head had no effect on vascular pressures. The results support a hemodynamic cause for IPE mediated in part by cooling of the trunk and extremities. This does not support the use of increased head insulation to prevent IPE.

Sympathetic and cardiovascular activity during cataplexy in narcolepsy

Autonomic nervous system activity changes have been described during cataplexy as playing a role in triggering it. To confirm these previous findings, we investigated the time course of sympathetic and cardiovascular activities during cataplexy. We made for the first time microneurographic recordings of 10 cataplectic episodes in three patients with hypocretin-deficient narcolepsy. During microneurography, muscle sympathetic nerve activity (MSNA) was recorded simultaneously with heart rate (HR), respiratory movements, arterial finger blood pressure (BP), electroencephalography, electro-oculogram and superficial electromyogram. Results showed no significant autonomic changes before the onset of the cataplectic episodes. Cataplexy was associated with a significant increase in MSNA and BP compared with baseline, whereas HR was markedly decreased. An irregular breathing pattern mainly characterized by apnea typically occurred during the attacks. In conclusion, our findings did not show significant changes in autonomic activity prior to cataplexy onset, ruling out a triggering role of the autonomic system. However, cataplexy was associated with co-activation of sympathetic and parasympathetic autonomic systems, a pattern reminiscent of that reported during the vigilance reaction in animals.

Nonuniformity in the von Bezold-Jarisch reflex

The von Bezold-Jarisch reflex (BJR) is a vagally mediated chemoreflex from the heart and lungs, causing hypopnea, bradycardia, and inhibition of sympathetic vasomotor tone. However, cardiac sympathetic nerve activity (CSNA) has not been systematically compared with vasomotor activity during the BJR. In 11 urethane-anesthetized (1-1.5 g/kg iv), artificially ventilated rats, we measured CSNA simultaneously with lumbar sympathetic activity (LSNA) while the BJR was evoked by right atrial bolus injections of phenylbiguanide (0.5, 1.0, 1.5, and 2 microg). Nerve and heartbeat responses were analyzed by calculating normalized cumulative sums. LSNA and heartbeats were always reduced by the BJR. An excitatory "rebound" component often followed the inhibition of LSNA but never outweighed it. For CSNA, however, excitation usually (in 7 of 11 rats) outweighed any initial inhibition, such that the net response to phenylbiguanide was excitatory. The differences in net response between LSNA, CSNA, and heartbeats were all significant (P < 0.01). A second experimental series on seven rats showed that methyl atropine (1 mg/kg iv) abolished the bradycardia of the BJR, whereas subsequent bilateral vagotomy substantially reduced LSNA and CSNA responses, both excitatory and inhibitory. These findings show that, during the BJR, 1) CSNA is often excited, 2) there may be coactivation of sympathetic and parasympathetic drives to the heart, 3) divergent responses may be evoked simultaneously in cardiac vagal, cardiac sympathetic, and vasomotor nervous pathways, and 4) those divergent responses are mediated primarily by the vagi.