Vasovagal syncope and Darwinian fitness

Vagus nerve stimulation primes platelets and reduces bleeding in hemophilia A male mice

Deficiency of coagulation factor VIII in hemophilia A disrupts clotting and prolongs bleeding. While the current mainstay of therapy is infusion of factor VIII concentrates, inhibitor antibodies often render these ineffective. Because preclinical evidence shows electrical vagus nerve stimulation accelerates clotting to reduce hemorrhage without precipitating systemic thrombosis, we reasoned it might reduce bleeding in hemophilia A. Using two different male murine hemorrhage and thrombosis models, we show vagus nerve stimulation bypasses the factor VIII deficiency of hemophilia A to decrease bleeding and accelerate clotting. Vagus nerve stimulation targets acetylcholine-producing T lymphocytes in spleen and α7 nicotinic acetylcholine receptors (α7nAChR) on platelets to increase calcium uptake and enhance alpha granule release. Splenectomy or genetic deletion of T cells or α7nAChR abolishes vagal control of platelet activation, thrombus formation, and bleeding in male mice. Vagus nerve stimulation warrants clinical study as a therapy for coagulation disorders and surgical or traumatic bleeding.

An ethologically based view into human fear

The quality of the defensive response to a threat depends on the elements that trigger the fear response. The current classification system of phobias does not account for this. Here, we analyze the fear-eliciting elements and discern the different types of fears that originate from them. We propose Pain, Disgust, Vasovagal response, Visual-vestibular and postural interactions, Movement and Speed, Distance and Size, Low and mid-level visual features, Smell, and Territory and social status. We subdivide phobias according to the fear-eliciting elements most frequently triggered by them and their impact on behavior. We discuss the implications of a clinical conceptualization of phobias in humans by reconsidering the current nosology. This conceptualization will facilitate finding etiological factors in defensive behavior expression, fine-tuning exposure techniques, and challenging preconceived notions of preparedness. This approach to phobias leads to surprising discoveries and shows how specific responses bear little relation to the interpretation we might later give to them. Dividing fears into their potentially fear-eliciting elements can also help in applying the research principles formulated by the Research Domain Criteria initiative.

"Complex" Vasovagal Syncope: A Zebra Among Horses

Background: Vasovagal syncope (VVS) occurs due to cerebral hypoperfusion from a fall in blood pressure, with accompanying bradycardia in most cases. Seizure and/or asystole may accompany VVS, though their prediction within the VVS cohort remains elusive.

Objective: To further characterize VVS and to find predictive features of “complex” VVS (defined as VVS associated with seizures and/or asystole).

Methods: We reviewed medical records of all patients who were referred for orthostatic intolerance and had a definite VVS during the head-up tilt table testing (HUTT). The following variables were recorded: cardiovascular indices during HUTT, autonomic testing results, and semiology of asystole and/or seizure when present. Simple frequency and correlation analysis were performed using the ANOVA.

Results: A total of 78 independent VVS were recorded in 60 patients of which 24% were not preceded by presyncope. Vasodepressor (45%) and mixed (38%) VVS were the most prevalent types. Eighteen (23%) were complex VVS; five had an associated seizure (SySz), nine were accompanied by asystole (SyAs), and four had both (SySzAs). Males were significantly more likely to have complex VVS. Mean asystole duration was somewhat longer in the SyAsSz group. The severity of bradycardia significantly correlated with complex VVS and was a predictor of SySz. Autonomic abnormalities were frequent but did not distinguish the two VVS subgroups. Seizures had multiple distinguishing features from those typically associated with epileptic seizures.

Conclusions: The underlying pathophysiologic mechanisms of complex VVS remain unclear, but the severity of cerebral hypoperfusion due to bradycardia likely plays a key role in seizure generation.

Novel functional advanced echocardiography for the assessment of myocardial mechanics in children with neurocardiogenic syncope - a blinded prospective speckle tracking head-up tilt-table challenge study

Background

Data on left ventricular (LV) function in patients with neurocardiogenic syncope (NS) is conflicting in adults and lacking in children. The aim of this study was to analyze LV myocardial performance in children with NS at rest and during head-up tilt-table (HUTT) testing.

Methods

This is the first study to combine HUTT and speckle-tracking echocardiography (STE) in children with NS. 43 consecutive normotensive pediatric patients with NS (mean age 13.9 ± 2.6 years, 51% female) and 41 sex- and age-matched healthy controls were included in the study. The study groups consisted of 21 patients with a positive HUTT reaction (HUTT+) and 22 with a negative HUTT reaction (HUTT-). STE was used to analyze peak systolic LV myocardial strain and strain rate.

Results

Conventional echocardiographic parameters were similar in all analyzed groups. When compared to healthy controls, children with NS had depressed levels of circumferential strain rate (p = 0.032) and significantly depressed longitudinal strain rate (p <  0.001) at rest. Interestingly, during HUTT testing LV global strain and strain rate were similar in both groups. LV strain rate was lowest in HUTT+ followed by HUTT- and control subjects both at rest and during HUTT.

Conclusions

Resting LV longitudinal strain rate is attenuated in children with NS, especially in those with a positive HUTT response. This is further evidence that NS patients feature altered cardiac mechanics rendering them prone to vasovagal perturbations that can ultimately result in collapse.

Trial registration

Witten/Herdecke University ethics committee clinical study number: UWH-73-2014.

Typical vasovagal syncope as a "defense mechanism" for the heart by contrasting sympathetic overactivity

Many observations suggest that typical (emotional or orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. Some authors have hypothesized this type of syncope as a "defense mechanism" for the organism and a few theories have been postulated. Under the human violent conflicts theory, the VVS evolved during the Paleolithic era only in the human lineage. In this evolutionary period, a predominant cause of death was wounding by a sharp object. This theory could explain the occurrence of emotional VVS, but not of the orthostatic one. The clot production theory suggests that the vasovagal reflex is a defense mechanism against hemorrhage in mammals. This theory could explain orthostatic VVS, but not emotional VVS. The brain self-preservation theory is mainly based on the observation that during tilt testing a decrease in cerebral blood flow often precedes the drop in blood pressure and heart rate. The faint causes the body to take on a gravitationally neutral position, and thereby provides a better chance of restoring brain blood supply. However, a decrease in cerebral blood flow has not been demonstrated during negative emotions, which trigger emotional VVS. Under the heart defense theory, the vasovagal reflex seems to be a protective mechanism against sympathetic overactivity and the heart is the most vulnerable organ during this condition. This appears to be the only unifying theory able to explain the occurrence of the vasovagal reflex and its associated selective advantage, during both orthostatic and emotional stress.

Postsynaptic α1-Adrenergic Vasoconstriction Is Impaired in Young Patients With Vasovagal Syncope and Is Corrected by Nitric Oxide Synthase Inhibition

BACKGROUND

Syncope is a sudden transient loss of consciousness and postural tone with spontaneous recovery; the most common form is vasovagal syncope (VVS). During VVS, gravitational pooling excessively reduces central blood volume and cardiac output. In VVS, as in hemorrhage, impaired adrenergic vasoconstriction and venoconstriction result in hypotension. We hypothesized that impaired adrenergic responsiveness because of excess nitric oxide can be reversed by reducing nitric oxide.

METHODS AND RESULTS

We recorded cardiopulmonary dynamics in supine syncope patients and healthy volunteers (aged 15-27 years) challenged with a dose-response using the α1-agonist phenylephrine (PE), with and without the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine, monoacetate salt (L-NMMA). Systolic and diastolic pressures among control and VVS were the same, although they increased after L-NMMA and saline+PE (volume and pressor control for L-NMMA). Heart rate was significantly reduced by L-NMMA (P<0.05) for control and VVS compared with baseline, but there was no significant difference in heart rate between L-NMMA and saline+PE. Cardiac output and splanchnic blood flow were reduced by L-NMMA for control and VVS (P<0.05) compared with baseline, while total peripheral resistance increased (P<0.05). PE dose-response for splanchnic flow and resistance were blunted for VVS compared with control after saline+PE, but enhanced after L-NMMA (P<0.001). Postsynaptic α1-adrenergic vasoconstrictive impairment was greatest in the splanchnic vasculature, and splanchnic blood flow was unaffected by PE. Forearm and calf α1-adrenergic vasoconstriction were unimpaired in VVS and unaffected by L-NMMA.

CONCLUSIONS

Impaired postsynaptic α1-adrenergic vasoconstriction in young adults with VVS can be corrected by nitric oxide synthase inhibition, demonstrated with our use of L-NMMA.

Nitric oxide synthase inhibition restores orthostatic tolerance in young vasovagal syncope patients

OBJECTIVE

Syncope is sudden transient loss of consciousness and postural tone with spontaneous recovery; the most common form is vasovagal syncope (VVS). We previously demonstrated impaired post-synaptic adrenergic responsiveness in young VVS patients was reversed by blocking nitric oxide synthase (NOS). We hypothesised that nitric oxide may account for reduced orthostatic tolerance in young recurrent VVS patients.

METHODS

We recorded haemodynamics in supine VVS and healthy volunteers (aged 15-27 years), challenged with graded lower body negative pressure (LBNP) (-15, -30, -45 mm Hg each for 5 min, then -60 mm Hg for a maximum of 50 min) with and without NOS inhibitor N^G-monomethyl-L-arginine acetate (L-NMMA). Saline plus phenylephrine (Saline+PE) was used as volume and pressor control for L-NMMA.

RESULTS

Controls endured 25.9±4.0 min of LBNP during Saline+PE compared with 11.6±1.4 min for fainters (p<0.001). After L-NMMA, control subjects endured 24.8±3.2 min compared with 22.6±1.6 min for fainters. Mean arterial pressure decreased more in VVS patients during LBNP with Saline+PE (p<0.001) which was reversed by L-NMMA; cardiac output decreased similarly in controls and VVS patients and was unaffected by L-NMMA. Total peripheral resistance increased for controls but decreased for VVS during Saline+PE (p<0.001) but was similar following L-NMMA. Splanchnic vascular resistance increased during LBNP in controls, but decreased in VVS patients following Saline+PE which L-NMMA restored.

CONCLUSIONS

We conclude that arterial vasoconstriction is impaired in young VVS patients, which is corrected by NOS inhibition. The data suggest that both pre- and post-synaptic arterial vasoconstriction may be affected by nitric oxide.

Understanding of anesthesia - Why consciousness is essential for life and not based on genes

Anesthesia and consciousness represent 2 mysteries not only for biology but also for physics and philosophy. Although anesthesia was introduced to medicine more than 160 y ago, our understanding of how it works still remains a mystery. The most prevalent view is that the human brain and its neurons are necessary to impose the effects of anesthetics. However, the fact is that all life can be anesthesized. Numerous theories have been generated trying to explain the major impact of anesthetics on our human-specific consciousness; switching it off so rapidly, but no single theory resolves this enduring mystery. The speed of anesthetic actions precludes any direct involvement of genes. Lipid bilayers, cellular membranes, and critical proteins emerge as the most probable primary targets of anesthetics. Recent findings suggest, rather surprisingly, that physical forces underlie both the anesthetic actions on living organisms as well as on consciousness in general.

Disgust stimuli reduce heart rate but do not contribute to vasovagal symptoms

BACKGROUND AND OBJECTIVES

The vasovagal response demonstrates a unique form of stress response, common in medical settings yet provoked by a variety of blood-injury-injection stimuli. This study aimed to better understand the psychophysiological mechanisms of the vasovagal response..

METHODS

16 undergraduates with and 42 without a self-reported history of fainting watched five 3-5 min videos with different emotional content. One documentary clip (Neutral condition) described a campus environmental project while another (Blood/Injury) depicted portions of an open heart surgery. Three additional clips were also used, including Medical, Threat, and Contamination stimuli. Vasovagal symptoms and physiological variables were assessed during each video.

RESULTS

As predicted, while the disgust-related stimuli (Blood/Injury, Medical, Contamination) were associated with generally lower heart rate, the Blood/Injury video produced the highest symptoms and the only significant difference between previous fainters and non-fainters. The physiological measures also revealed that participants with a fainting history experienced higher stroke volume and lower systolic blood pressure throughout, as well as several main effects of video.

LIMITATIONS

An additional decrease in systolic blood pressure and respiration produced by watching the Blood/Injury video may have been sufficient to trigger symptoms in some, though results also suggest that systemic variables do not entirely explain susceptibility to symptoms. More careful evaluation of regional blood flow may be required.

CONCLUSIONS

Participants who had previously experienced strong vasovagal responses displayed what appeared to be an anticipatory response to the Blood/Injury video. Finally, disgust stimuli may reduce heart rate but do not appear to contribute to vasovagal symptoms.

Vasovagal Syncope: Hypothesis Focusing on Its Being a Clinical Feature Unique to Humans

Humans live primarily in the upright position; as a result, there is a constant struggle between gravity and needed supply of blood flow to the brain. In certain circumstances brain blood supply may become temporarily insufficient, resulting in syncope. Among the numerous causes of syncope in humans, vasovagal syncope (VVS) is by far the most common. However, despite intensive research, many aspects of the pathophysiology of VVS remain unknown; among these, one of the least well understood is the basis for why VVS is restricted, among vertebrates, to Homo sapiens. In this manuscript we review proposals that have been offered in an attempt to address the issue of the origin of VVS and, although highly speculative, we suggest a new hypothesis (the "brain theory") to try to address the question of why humans, to the exclusion of other species, remain susceptible to VVS. This theory suggests that VVS evolved to offer protection to the brain's functional integrity under certain conditions of severe threat. Although seemingly a disadvantageous evolutionary adaptation, the faint causes the body to take on a gravitationally neutral position, and thereby provides a better chance of restoring brain blood supply and preserving long-term brain function.

The vasovagal response during confrontation with blood-injury-injection stimuli: the role of perceived control

The vasovagal response (VVR) is a common medical problem, complicating and deterring people from various procedures. It is an unusual stress response given the widespread decreases in physiological activity. Nevertheless, VVR involves processes similar to those observed during episodes of strong emotions and pain. We hypothesized that heightened perceived control would reduce symptoms of VVR. Eighty-two young adults were randomly assigned to perceived control or no perceived control conditions during exposure to a stimulus video of a mitral valve surgery, known to trigger VVR in non-medical personnel. Perceived control was manipulated by allowing some participants to specify a break time, though all received equivalent breaks. Outcomes included subjective symptoms of VVR, anxiety, blood pressure, heart rate, and other measures derived from impedance cardiography. Compared to participants with perceived control, participants with no perceived control reported significantly more vasovagal symptoms and anxiety, and experienced lower stroke volume, cardiac output, and diastolic blood pressure. Participants who were more fearful of blood were more likely to benefit from perceived control in several measures. Perceived control appears to reduce vasovagal symptoms. Results are discussed in terms of cognition and emotion in VVR.

Sepsis, venous return, and teleology

An understanding of heart-circulation interaction is crucial to our ability to guide our patients through an episode of septic shock. Our knowledge has advanced greatly in the last one hundred years. There are, however, certain empirical phenomena that may lead us to question the wisdom of our prevailing treatment algorithm. Three extreme but iatrogenically possible haemodynamic states exist. Firstly, inappropriately low venous return; secondly, overzealous arteriolar constriction; and finally, misguided inotropy and chronotropy. Following an unsuccessful fluid challenge, it would be logical to first set the venous tone, then set the cardiac rate and contractility, and finally set the peripheral vascular resistance. It is hypothesized that a combination of dihydroergotamine, milrinone and esmolol should be superior to a combination of noradrenaline and dobutamine for surviving sepsis.

Vasovagal Syncope As A Manifestation Of An Evolutionary Selected Trait

Some observations suggest that typical (emotional or orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. We conducted an extensive bibliographic research on the vasovagal reactions in animals, including humans, in order to investigate the possible factors that may explain the origin and evolution of VVS. We found two processes which appear relevant for the investigation of VVS evolution: fear/threat bradycardia (alarm bradycardia) in animals, mainly during tonic immobility and vasovagal reflex during hemorrhagic shock (thoracic hypovolemia) both in animals and humans. The available data suggest that VVS in humans, alarm bradycardia in animals and the vasovagal reflex during hemorrhagic shock share the same physiological mechanisms and that is indicative of a common evolutionary root. However, during the vasovagal reflex loss of consciousness occurs in humans, but it is absent (or extremely rare) in animals. That can be explained as a by-product due to the erect position and the large brain evolved in our species. If the vasovagal reflex persisted for millions of years along the vertebrates evolutionary history, we can reasonably assume that it has a function and it is not harmful. It could be neutral or beneficial, but the available data suggest it is beneficial; likely, it evolved as an advantageous response to stressful and possibly dangerous heart conditions. Emotional or orthostatic vasovagal reflex is preceded by enhanced sympathetic activity, which is harmful and possibly dangerous. The transient inhibition of the sympathetic system, together with activation of the vagal tone , characterizes VVS. The consequent slowing of the heart rate induced by the vasovagal reflex may constitute a beneficial break of the cardiac pump, thereby reducing myocardial oxygen consumption. We suggest that typical VVS should be regarded as a selected response, which probably evolved in the ancient past as a defense mechanism of the organism within some ancestral group(s) of vertebrates.

Common syndromes of orthostatic intolerance

The autonomic nervous system, adequate blood volume, and intact skeletal and respiratory muscle pumps are essential components for rapid cardiovascular adjustments to upright posture (orthostasis). Patients lacking sufficient blood volume or having defective sympathetic adrenergic vasoconstriction develop orthostatic hypotension (OH), prohibiting effective upright activities. OH is one form of orthostatic intolerance (OI) defined by signs, such as hypotension, and symptoms, such as lightheadedness, that occur when upright and are relieved by recumbence. Mild OI is commonly experienced during intercurrent illnesses and when standing up rapidly. The latter is denoted "initial OH" and represents a normal cardiovascular adjustment to the blood volume shifts during standing. Some people experience episodic acute OI, such as postural vasovagal syncope (fainting), or chronic OI, such as postural tachycardia syndrome, which can significantly reduce quality of life. The lifetime incidence of ≥1 fainting episodes is ∼40%. For the most part, these episodes are benign and self-limited, although frequent syncope episodes can be debilitating, and injury may occur from sudden falls. In this article, mechanisms for OI having components of adrenergic hypofunction, adrenergic hyperfunction, hyperpnea, and regional blood volume redistribution are discussed. Therapeutic strategies to cope with OI are proposed.

Does disgust increase parasympathetic activation in individuals with a history of fainting? A psychophysiological analysis of disgust stimuli with and without blood-injection-injury association

People with blood-injection-injury fear can faint when being confronted with blood, injections or injuries. Page (1994) holds that people with blood-injury phobia faint, because they are disgust sensitive and disgust facilitates fainting by eliciting parasympathetic activity. We tested the following two hypotheses: (1) Disgusting pictures elicit more disgust in blood-injection-injury-anxious people with a history of fainting than they do in controls. (2) Disgust causes parasympathetic activation. Subjects were 24 participants with high blood-injection-injury fear and a history of fainting in anxiety relevant situations and 24 subjects with average blood-injection-injury fear and no fainting history. We analyzed self-reported feelings of disgust, anxiety and faintness and reactions in heart rate, skin conductance, blood pressure and respiratory sinus arrhythmia during the confrontation with disgusting pictures with and without blood content. We did not find any evidence that the blood-injection-injury anxious subjects were more disgust sensitive than the control subjects and we also did not find any evidence that disgust elicits parasympathetic activation.

The effects of blood-draw and injection stimuli on the vasovagal response

Vasovagal reactions (VVR) are common, complicating and deterring people from various medical procedures. A recent perspective (R. R. Diehl, ) suggests that VVR developed from the adaptive process of hemorrhagic fainting, perhaps as a means of preparing for anticipated blood loss. The primary goal of this study was to compare vasovagal symptoms during intravenous-injection and blood-draw videos. Sixty-two young adults watched the videos. Vasovagal symptoms were assessed with self-report, blood pressure, and heart rate variability. As predicted, participants reported more vasovagal symptoms and anxiety following the blood-draw video. Sympathetic nervous system activity (low- to high-frequency ratio) decreased during both videos but significantly more during the blood-draw video, although this could be reversed by the Applied Tension technique. Results are discussed in terms of the relevance of specific stimuli and emotions in VVR.

The relation between disgust-sensitivity, blood-injection-injury fears and vasovagal symptoms in blood donors: disgust sensitivity cannot explain fainting or blood donation-related symptoms

BACKGROUND AND OBJECTIVES

Page's (1994) prominent theory for the explanation of fainting in blood-injection-injury situations holds that disgust sensitivity contributes to syncopal reactions. We investigated if blood donation-related vasovagal symptoms (1) or fainting related to blood donations (2) are associated with disgust sensitivity.

METHODS

In an online sample of 361 blood donors, we assessed blood-injection-injury fears, disgust sensitivity, history of blood donation related fainting and retrospective self-ratings of vasovagal symptoms. For the assessment of blood-injection-injury fears we used the BII-Q which has excellent psychometric properties and does not confound disgust and anxiety sensitivity. Vasovagal symptoms were measured by the Blood Donation Reactions Inventory (BDRI) which captures mild and strong vasovagal symptoms and has been used in previous studies with blood donors.

RESULTS

Disgust sensitivity did not significantly contribute to the explanation of self-reported vasovagal symptoms in a regression model with gender, blood-injection-injury fear and disgust sensitivity as predictors. We did not find any significant group differences in disgust sensitivity for blood donors with or without a fainting history (statistical power = 0.95) and a Bayesian model selection procedure showed that it is more likely that both groups are equally disgust sensitive than it is that the fainters are more disgust sensitive.

LIMITATIONS

Further research is required to confirm the findings in prospective studies.

CONCLUSION

Our results indicate that disgust sensitivity is not relevant for the development of vasovagal syncopes.

Relation between perceived blood loss and vasovagal symptoms in blood donors

INTRODUCTION/RESULTS: This study examined whether a belief of significant blood loss may be associated with vasovagal symptoms, irrespective of actual blood loss. Individual differences in vasovagal symptoms among blood donors who had an equivalent amount of blood withdrawn were significantly associated with their rating of perceived blood loss.

CONCLUSION

The anticipation or belief of blood loss, and perhaps more remotely associated ideas, may trigger processes similar to those induced by actual hemorrhage though further research is required to address other possibilities such as the inflation of ratings by vasovagal symptoms.

Fear-related predictors of vasovagal symptoms during blood donation: it's in the blood

A recent theory proposes that emotional fainting developed from an earlier adaptive characteristic, fainting in response to hemorrhage. Despite potential loss of consciousness, a dramatic decrease in blood pressure improves chances of survival in animals with severe wounds by reducing blood loss and facilitating clotting. Humans may have developed the characteristic of emotional fainting as a response to anticipated blood loss. This idea suggests that people with stronger fears of blood should be especially susceptible to fainting and milder vasovagal symptoms such as dizziness and lightheadedness. Two samples of young adult blood donors (N = 276 and 663) who completed the Medical Fears Survey (MFS) were studied. Items from the MFS related to fears of blood, needles, and mutilation were used to predict self-reported dizziness and nurse-initiated treatment for vasovagal reactions. In both samples, fears of experiencing or seeing blood loss were more closely associated with both subjective and objective measures of vasovagal reactions, despite the fact that other fears (e.g., fears related to needles) were more common overall. Better understanding of the mechanisms of vasovagal reactions has both theoretical and clinical implications, such as improving means of coping with invasive medical procedures.

Estradiol potentiates hypothalamic vasopressin and oxytocin neuron activation and hormonal secretion induced by hypovolemic shock

Estrogen receptors are located in important brain areas that integrate cardiovascular and hydroelectrolytic responses, including the subfornical organ (SFO) and supraoptic (SON) and paraventricular (PVN) nuclei. The aim of this study was to evaluate the influence of estradiol on cardiovascular and neuroendocrine changes induced by hemorrhagic shock in ovariectomized rats. Female Wistar rats (220-280 g) were ovariectomized and treated for 7 days with vehicle or estradiol cypionate (EC, 10 or 40 μg/kg, sc). On the 8th day, animals were subjected to hemorrhage (1.5 ml/100 g for 1 min). Hemorrhage induced acute hypotension and bradycardia in the ovariectomized-oil group, but EC treatment inhibited these responses. We observed increases in plasma angiotensin II concentrations and decreases in plasma atrial natriuretic peptide levels after hemorrhage; EC treatment produced no effects on these responses. There were also increases in plasma vasopressin (AVP), oxytocin (OT), and prolactin levels after the induction of hemorrhage in all groups, and these responses were potentiated by EC administration. SFO neurons and parvocellular and magnocellular AVP and OT neurons in the PVN and SON were activated by hemorrhagic shock. EC treatment enhanced the activation of SFO neurons and AVP and OT magnocellular neurons in the PVN and SON and AVP neurons in the medial parvocellular region of the PVN. These results suggest that estradiol modulates the cardiovascular responses induced by hemorrhage, and this effect is likely mediated by an enhancement of AVP and OT neuron activity in the SON and PVN.

Early activation of the coagulation system during lower body negative pressure

We considered that a moderate reduction of the central blood volume (CBV) may activate the coagulation system. Lower body negative pressure (LBNP) is a non-invasive means of reducing CBV and, thereby, simulates haemorrhage. We tested the hypothesis that coagulation markers would increase following moderate hypovolemia by exposing 10 healthy male volunteers to 10 min of 30 mmHg LBNP. Thoracic electrical impedance increased during LBNP (by 2.6 +/- 0.7 Omega, mean +/- SD; P < 0.001), signifying a reduced CBV. Heart rate was unchanged during LBNP, while mean arterial pressure decreased (84 +/- 5 to 80 +/- 6 mmHg; P < 0.001) along with stroke volume (114 +/- 22 to 96 +/- 19 ml min(-1); P < 0.001) and cardiac output (6.4 +/- 2.0 to 5.5 +/- 1.7 l min(-1); P < 0.01). Plasma thrombin-antithrombin III complexes increased (TAT, 5 +/- 6 to 19 +/- 20 microg l(-1); P < 0.05), indicating that LBNP activated the thrombin generating part of the coagulation system, while plasma D-dimer was unchanged, signifying that the increased thrombin generation did not cause further intravascular clot formation. The plasma pancreatic polypeptide level decreased (13 +/- 11 to 6 +/- 8 pmol l(-1); P < 0.05), reflecting reduced vagal activity. In conclusion, thrombin generation was activated by a modest decrease in CBV by LBNP in healthy humans independent of the vagal activity.

The influence of vasovagal response on the coagulation system

BACKGROUND

Vasovagal response (VVR) is provoked by a reduced venous blood return to the heart as a reaction to orthostatic stress and to haemorrhage. Recently, two cases were reported showing elevated plasma concentration of von-Willebrand-factor (VWF) and factor VIII (FVIII) after VVR due to venapuncture. Although the effect of epinephrine as trigger for VWF liberation is known, a connection between VVR and activation of the coagulation system has not been studied systematically.

METHODS

We examined 21 subjects with lower body negative pressure. We measured the plasma concentration of von-Willebrand-factor-antigen (VWF:Ag), the activity of von-Willebrand-factor-Ristocetin-Cofactor (VWF:RiCo) and FVIII at several stress-levels and consecutively split up the different VWF-multimers.

RESULTS

In 16 of 21 subjects VVR could be induced. These subjects showed a significant increase of VWF:Ag concentration in plasma and an increase of FVIII and VWF:RiCo activity. The five individuals who experienced all stress-levels without VVR did not show any changes in their clotting factor levels.

CONCLUSION

VVR leads to measurable changes in the coagulation system. This might be a further diagnostic tool in treating patients with syncope.

The origin of vasovagal syncope: to protect the heart or to escape predation?

Major lines of evidence suggest that classical (emotional and orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. It is, therefore, reasonable to investigate the possible factors that may explain its origin and evolution. We reviewed the data available in the literature on the vasovagal reaction in humans and animals in order to identify possible similarities that might provide insight into the evolution of VVS. We found two processes which appear relevant to the investigation of VVS evolution: fear and threat bradycardia in animals, and the vasovagal reflex during hemorrhagic shock in humans and animals. We suggest that VVS in humans involves physiological mechanisms similar to those found in other vertebrates, and that this may indicate a common evolutionary root. The available data seem to suggest that VVS evolved as an advantageous response to inescapable predators or to stressful and possibly dangerous heart conditions. The inhibition of the sympathetic system, together with activation of the vagal system, characterizes VVS. The consequent slowing of the heart rate induced by VVS may constitute a beneficial break of the cardiac pump, thereby reducing myocardial oxygen consumption. We suggest that classical VVS did not evolve recently in the modern human lineage; rather, it should be regarded as a selected response, which probably evolved in the ancient past as a "defense mechanism" of the organism within some ancestral group(s) of vertebrates.