Abnormal reflex venous function in patients with neuromediated syncope

Pharmacologic prevention of recurrent vasovagal syncope: A systematic review and network meta-analysis of randomized controlled trials

Vasovagal syncope (VVS) is a transient loss of consciousness that currently imposes a high burden on health care systems with limited evidence of the comparative efficacy of available pharmacologic interventions. This study aims to compare all pharmacologic therapies suggested in randomized controlled trials (RCTs) through systematic review and network meta-analysis. A systematic search in PubMed, Embase, Web of Science, and Cochrane Library was conducted to identify RCTs evaluating pharmacologic therapies for patients with VVS. The primary outcome was spontaneous VVS recurrence. The secondary outcome was a positive head-up tilt test (HUTT) after receiving intervention, regarded as a lower level of evidence. Pooled risk ratio (RR) with 95% confidence interval (CI) was calculated using random-effect network meta-analysis. Pairwise meta-analysis for comparison with placebo was also performed when applicable. The surface under the cumulative ranking curve analysis was conducted to rank the treatments for each outcome. Twenty-eight studies with 1744 patients allocated to different medications or placebo were included. Network meta-analysis of the reduction in the primary outcome showed efficacy for midodrine (RR 0.55; 95% CI 0.35-0.85) and fluoxetine (especially in patients with concomitant anxiety) (RR 0.36; 95% CI 0.16-0.84). In addition, midodrine and atomoxetine were superior to other treatment options, considering positive HUTT (RR 0.37; 95% CI 0.23-0.59; and RR 0.49; 95% CI 0.28-0.86, respectively). Overall, midodrine was the only agent shown to reduce spontaneous syncopal events. Fluoxetine also seems to be beneficial but should be studied further in RCTs. Our network meta-analysis did not find evidence of the efficacy of any other medication.

Counter pressure maneuvers for syncope prevention: A semi-systematic review and meta-analysis

Physical counter pressure maneuvers (CPM) are movements that are recommended to delay or prevent syncope (fainting) by recruiting the skeletal muscle pump to augment cardiovascular control. However, these recommendations are largely based on theoretical benefit, with limited data evaluating the efficacy of CPM to prevent syncope in the real-world setting. We conducted a semi-systematic literature review and meta-analysis to assess CPM efficacy, identify literature gaps, and highlight future research needs. Articles were identified through a literature search (PubMed, April 2022) of peer-reviewed publications evaluating the use of counter pressure or other lower body maneuvers to prevent syncope. Two team members independently screened records for inclusion and extracted data. From 476 unique records identified by the search, 45 met inclusion criteria. Articles considered various syncopal conditions (vasovagal = 12, orthostatic hypotension = 8, postural orthostatic tachycardia syndrome = 1, familial dysautonomia = 2, spinal cord injury = 1, blood donation = 10, healthy controls = 11). Maneuvers assessed included hand gripping, leg fidgeting, stepping, tiptoeing, marching, calf raises, postural sway, tensing (upper, lower, whole body), leg crossing, squatting, “crash” position, and bending foreword. CPM were assessed in laboratory-based studies (N = 28), the community setting (N = 4), both laboratory and community settings (N = 3), and during blood donation (N = 10). CPM improved standing systolic blood pressure (+ 14.8 ± 0.6 mmHg, p < 0.001) and heart rate (+ 1.4 ± 0.5 bpm, p = 0.006), however, responses of total peripheral resistance, stroke volume, or cerebral blood flow were not widely documented. Most patients experienced symptom improvement following CPM use (laboratory: 60 ± 4%, community: 72 ± 9%). The most prominent barrier to employing CPM in daily living was the inability to recognize an impending faint. Patterns of postural sway may also recruit the skeletal muscle pump to enhance cardiovascular control, and its potential as a discrete, proactive CPM needs further evaluation. Physical CPM were successful in improving syncopal symptoms and producing cardiovascular responses that may bolster against syncope; however, practical limitations may restrict applicability for use in daily living.

Midodrine for the Prevention of Vasovagal Syncope : A Randomized Clinical Trial

BACKGROUND

Recurrent vasovagal syncope is common, responds poorly to treatment, and causes physical trauma and poor quality of life. Midodrine prevents hypotension and syncope during tilt tests in patients with vasovagal syncope.

OBJECTIVE

To determine whether midodrine can prevent vasovagal syncope in usual clinical conditions.

DESIGN

Randomized, double-blind, placebo-controlled clinical trial. (ClinicalTrials.gov: NCT01456481).

SETTING

25 university hospitals in Canada, the United States, Mexico, and the United Kingdom.

PATIENTS

Patients with recurrent vasovagal syncope and no serious comorbid conditions.

INTERVENTION

Patients were randomly assigned 1:1 to placebo or midodrine and followed for 12 months.

MEASUREMENTS

The primary outcome measure was the proportion of patients with at least 1 syncope episode during follow-up.

RESULTS

The study included 133 patients who had had a median of 6 syncope episodes in the prior year (median age, 32 years; 73% female). Compared with patients receiving placebo, fewer patients receiving midodrine had at least 1 syncope episode (28 of 66 [42%] vs. 41 of 67 [61%]). The relative risk was 0.69 (95% CI, 0.49 to 0.97; P = 0.035). The absolute risk reduction was 19 percentage points (CI, 2 to 36 percentage points), and the number needed to treat to prevent 1 patient from having syncope was 5.3 (CI, 2.8 to 47.6). The time to first syncope was longer with midodrine (hazard ratio, 0.59 [CI, 0.37 to 0.96]; P = 0.035; log-rank P = 0.031). Adverse effects were similar in both groups.

LIMITATION

Small study size, young and healthy patients, relatively short observation period, and high proportion of patients from 1 center.

CONCLUSION

Midodrine can reduce the recurrence of syncope in healthy, younger patients with a high syncope burden.

PRIMARY FUNDING SOURCE

The Canadian Institutes of Health Research.

Pathophysiology of neurally mediated syncope: Role of cardiac output and total peripheral resistance

Syncope is a common clinical condition occurring even in otherwise healthy people without underlying cardiovascular disease. Neurally mediated syncope is by far the most common cause of syncope in individuals without any structural heart disease. Based on traditional wisdom, loss of sympathetic tone with relaxation of vascular smooth muscle is the key mechanism underlying the pathophysiology of syncope, especially in patients without an acute decrease in heart rate. However, this concept has recently been challenged. Some microneurographic studies indicate that sympathetic withdrawal may not always be a prerequisite even for the development of classic "vasodepressor" forms of syncope. Conversely, a decrease in cardiac output appears to be a determinant factor for syncope in most circumstances. This article reviews the relative contribution of cardiac output versus sympathetic vasoconstriction in neurally mediated syncope in otherwise healthy individuals. It is suggested that a moderate to severe fall in cardiac output with or without vasodilatation may contribute to syncope.

What is brain fog? An evaluation of the symptom in postural tachycardia syndrome

PURPOSE

Adolescents with postural tachycardia syndrome (POTS) often experience ill-defined cognitive impairment referred to by patients as "brain fog." The objective of this study was to evaluate the symptom of brain fog as a means of gaining further insight into its etiology and potential palliative interventions.

METHODS

Eligible subjects who reported having been diagnosed with POTS were recruited from social media web sites. Subjects were asked to complete a 38-item questionnaire designed for this study, and the Wood mental fatigue inventory (WMFI).

RESULTS

Responses were received from 138 subjects with POTS (88 % female), ranging in age from 14 to 29 years; 132 subjects reported brain fog. WMFI scores correlated with brain fog frequency and severity (P < 0.001). The top ranked descriptors of brain fog were "forgetful," "cloudy," and "difficulty focusing, thinking and communicating." The most frequently reported brain fog triggers were fatigue (91 %), lack of sleep (90 %), prolonged standing (87 %), dehydration (86 %), and feeling faint (85 %). Although aggravated by upright posture, brain fog was reported to persist after assuming a recumbent posture. The most frequently reported interventions for the treatment of brain fog were intravenous saline (77 %), stimulant medications (67 %), salt tablets (54 %), intra-muscular vitamin B-12 injections (48 %), and midodrine (45 %).

CONCLUSIONS

Descriptors for "brain fog" are most consistent with it being a cognitive complaint. Factors other than upright posture may play a role in the persistence of this symptom. Subjects reported a number of therapeutic interventions for brain fog not typically used in the treatment of POTS that may warrant further investigation.

Rationale for the prevention of syncope trial IV: assessment of midodrine

BACKGROUND

Vasovagal syncope is a common problem associated with a poor quality of life, which improves when the frequency of syncope is reduced. Effective pharmacological therapies for vasovagal syncope have been elusive. Midodrine is a pro-drug whose primary metabolite is an alpha-1 adrenoreceptor agonist. A few studies have suggested that it may be beneficial in syncope, but all have had significant methodological limitations. A placebo-controlled clinical trial of midodrine for the prevention of vasovagal syncope is needed.

STRUCTURE OF STUDY

The prevention of syncope trial IV (POST 4) is a multicenter, international, randomized, placebo-controlled study of midodrine in the prevention of vasovagal syncope. The primary end point is the time to first recurrence of syncope. Patients will be randomized 1:1 to receive midodrine 10-30 mg/day or matching placebo, and followed for 1 year. Secondary end points include syncope frequency, presyncope, and quality of life. Primary analysis will be performed with an intention-to-treat approach, with a secondary on-treatment analysis.

POWER CALCULATIONS

A total sample size of 112, split equally between the two groups, achieves 85 % power to detect a 50 % relative risk reduction when the event rates are 55 and 27.5 % in the placebo and midodrine arms. Allowing for 20 % dropout, we propose to enroll 140 patients.

REGISTRATION

POST 4 is registered with http://www.clinicaltrials.gov (NCT01456481).

IMPLICATIONS

This study will be the first adequately powered trial to determine whether midodrine is effective in preventing vasovagal syncope. If it is effective, then midodrine may become the first-line pharmacological therapy for this condition.

Orthostatic intolerance and the headache patient

Orthostatic intolerance (OI) refers to a group of clinical conditions, including postural orthostatic tachycardia syndrome (POTS) and neurally mediated hypotension (NMH), in which symptoms worsen with upright posture and are ameliorated by recumbence. The main symptoms of chronic orthostatic intolerance syndromes include light-headedness, syncope or near syncope, blurring of vision, headaches, problems with short-term memory and concentration, fatigue, intolerance of low impact exercise, palpitations, chest pain, diaphoresis, tremulousness, dyspnea or air hunger, nausea, and vomiting. This review discusses what is known about the pathophysiology of this disorder, potential treatments, and understanding its role in the patient with chronic headache pain.

Steep fall in cardiac output is main determinant of hypotension during drug-free and nitroglycerine-induced orthostatic vasovagal syncope

BACKGROUND

How much of the hypotension occurring during postural syncope is cardiac output-mediated and how much can be ascribed to a fall in systemic vascular resistance are unknown. The contribution of both determinants may be influenced by the use of vasoactive drugs.

OBJECTIVE

The purpose of this study was to assess the determinants of hypotension during drug-free and nitroglycerine (NTG)-induced vasovagal presyncope in routine tilt table testing.

METHODS

In this retrospective study, a total of 56 patients (37 female; age 36 +/- 19 years) with suspected vasovagal syncope and a positive tilt test at two clinical centers were selected. In 29 patients, presyncope was provoked by 0.4 mg sublingual NTG, administered in the 60 degrees head-up tilt position. In the other 27 patients, presyncope was provoked by passive tilt alone. Finger arterial pressure was monitored continuously, and left ventricular stroke volume was computed from pressure pulsations.

RESULTS

After NTG administration, heart rate rose, and peak heart rate was similar in all patients. Use of NTG did not affect circulatory patterns precipitating a vasovagal response. On average in all patients, marked hypotension was mediated by an approximately 50% fall in cardiac output, whereas systemic vascular resistance was well maintained until presyncope.

CONCLUSION

Hypotension during routine tilt testing is cardiac output-mediated, and the mechanism appears independent of the use of 0.4 mg sublingual NTG. The study data challenge the conventional idea of systemic vasodilation as the overriding cause of hypotension during postural syncope.

Impact of age on the vasovagal response provoked by sublingual nitroglycerine in routine tilt testing

NTG (nitroglycerine) is used in routine tilt testing to elicit a vasovagal response. In the present study we hypothesized that with increasing age NTG triggers a more gradual BP (blood pressure) decline due to a diminished baroreflex-buffering capacity. The purpose of the present study was to examine the effect of NTG on baroreflex control of BP in patients with distinct age-related vasovagal collapse patterns. The study groups consisted of 29 patients (16-71 years old, 17 females) with clinically suspected VVS (vasovagal syncope) and a positive tilt test. Mean FAP (finger arterial pressure) was monitored continuously (Finapres). Left ventricular SV (stroke volume), CO (cardiac output) and SVR (systemic vascular resistance) were computed from the pressure pulsations (Modelflow). BRS (baroreflex sensitivity) was estimated in the time domain. In the first 3 min after NTG administration, BP was well-maintained in all patients. This implied an adequate arterial resistance response to compensate for steeper reductions in SV and CO with increasing age. HR (heart rate) increased and the BRS decreased after NTG administration. The rate of mean FAP fall leading to presyncope was inversely related to age (r=0.51, P=0.005). Accordingly, patients with a mean FAP fall >1.44 mmHg/s (median) were generally younger compared with patients with a slower mean FAP-fall (30+/-10 years compared with 51+/-17 years; P=0.001). The main determinant of the rate of BP fall on approach of presyncope was the rate of fall in HR (r=0.75, P<0.001). It was concluded that, in older patients, sublingual NTG provokes a more gradual BP decline compared with younger patients. This gradual decline cannot be ascribed to failure of the baroreflex-buffering capacity with increasing age. Age-related differences in the laboratory presentation of a vasovagal episode depend on the magnitude of the underlying bradycardic response.

The venous system is the main determinant of hypotension in patients with vasovagal syncope

AIMS

In patients with vasovagal syncope (VVS), a neural reflex appears the main determinant of hypotension leading to loss of consciousness; whether hypotension is mainly due to involvement of the arterial system or the venous system remains a debated issue. The aim of the present study was to assess which of these two systems is responsible for the fall in blood pressure (BP) in patients with VVS; to this end, a haemodynamic study was carried out not only before and during loss of consciousness but also during the recovery phase.

METHODS AND RESULTS

Beat-to-beat recordings of heart rate (HR), BP (volume-clamp method) and stroke volume (SV) (modelflow method), cardiac output (CO), and total peripheral resistance (TPR) were made at rest, during unmedicated tilt testing (TT) and recovery from loss of consciousness in 18 patients with a history of syncope (age 45+/-23 years) and positive response to TT. Blood pressure showed a significant fall during prodromal symptoms and a further fall at the beginning of loss of consciousness, together with a fall in SV, CO, and HR, and a slight, but significant, increase in TPR. At the beginning of recovery, BP showed a significant increase and a further increase 5 min later, together with an increase in SV, CO, and HR without significant changes in TPR.

CONCLUSION

These results suggest that in VVS the fall in BP is mainly caused by reduced venous return to the heart. The arterial system does not appear to be the main determinant of the fall of BP; however, the system appears unable to make the appropriate compensatory changes.

Responses to Tilt test in young and elderly patients with syncope of unknown origin

The aim of the present study was to assess the cardiovascular autonomic function and responses to tilt test in young and elderly patients with syncope of unknown origin. We evaluated two groups of patients with previous unexplained syncope: 192 older subjects (112 males, 80 females, mean age 67.2 +/- 6.8 years) and 188 young subjects (102 males, 86 females, mean age 25 +/- 9 years). All patients underwent ambulatory electrocardiogram (ECG) monitoring, to evaluate time domain indices of heart rate variability (HRV), and head-up tilt test in the morning after an overnight fast. The responses of positive tilt test were classified using the VAsovagal Syncope International Study (VASIS) criteria: mixed (VASIS-1), cardioinhibition (VASIS-2A), severe cardioinhibition/asystole (VASIS-2B), pure vasodepression (VASIS-3). All the time-domain HRV indexes were lower in the older than in young subjects. The rate of positive responses was not different in the two groups. In elderly group the positive head-up tilt test responses showed: a pure vasodepressive response (VASIS 3) in 126 patients (65%), a mixed (VASIS-1) response in 25 patients (13%), a cardioinhibitory (VASIS-2A) response in 13 patients (7%). Only 28 (14.6%) of elderly group patients had negative head-up tilt test response. In contrast, in young group the positive head-up tilt test responses showed: 114 patients (61.2%) a mixed (VASIS-1) vasovagal response, 40 patients (22.3%) a cardioinhibitory (VASIS-2A) response, four (2.1%) patients a severe cardioinhibitory (VASIS-2B) and four (2.1%) patients a pure vasodepression (VASIS-3) response, respectively. The tilt test was negative in response in 26 young patients (12.2%). Our results confirm that the head-up tilt test may be useful in assessing unexplained syncope, since it is seen to be positive in 85% of elderly patients and 86% in young patients. In our subjects, vasodepressive response was the most frequent cause of syncope in older subjects, while vasovagal response is the commonest cause of syncope of young patients. This different behaviour in the elderly may be is explained with physiological aging, which is associated with a reduction of sympathetic-parasympathetic control on the cardiac rhythm, demonstrated by reduction in all the time domain HRV indices.

Dysfunctional baroreflex regulation of sympathetic nerve activity in patients with vasovagal syncope

BACKGROUND

The interplay of resting muscle sympathetic nerve activity (MSA) and the baroreceptor reflex in patients with vasovagal syncope remains elusive. Hence, the aim of the present study was to investigate MSA, baroreceptor sensitivity, heart rate, and blood pressure under resting conditions and during orthostatic stress in patients with a history of vasovagal syncope.

METHODS AND RESULTS

MSA was measured using microneurography at rest and during lower body negative pressure (LBNP) to mimic orthostatic stress in patients with a history of vasovagal syncope (n=10) and in age-matched healthy controls (n=8). Heart rate and blood pressure were simultaneously recorded. Cardiac baroreceptor sensitivity was calculated with the spectral technique (alpha coefficient). Resting MSA in the patients with syncope was significantly increased as compared with controls (42.4+/-2.3 versus 26.5+/-3.6 bursts/min, P=0.001), whereas activation of MSA during orthostatic stress in the patient group was significantly blunted (5.1+/-1.6 versus 15.2+/-2.1 bursts/min at LBNP -50 mm Hg, P=0.002). In the patients with syncope, cardiac baroreceptor sensitivity was significantly reduced under supine resting conditions (8.5+/-0.7 versus 13.0+/-1.1 ms/mm Hg, P=0.001), as well as under orthostatic stress (7.3+/-0.7 versus 13.4+/-1.5 ms/mm Hg, P=0.003).

CONCLUSIONS

This study shows that in patients with vasovagal syncope, resting MSA is increased and baroreflex regulation during orthostatic stress is blunted, thus leading to impaired MSA adaptation. These results provide new insights into mechanisms of vasovagal syncope and suggest that pharmacological modulation of baroreceptor sensitivity may represent a promising treatment of neuromediated syncope.

Pooling in chronic orthostatic intolerance: arterial vasoconstrictive but not venous compliance defects

BACKGROUND

Orthostatic intolerance is characterized by postural tachycardia syndrome (POTS) with exaggerated tachycardia, orthostatic symptoms, and "pooling" (which comprises acrocyanosis and dependent edema when upright). My colleagues and I tested the hypothesis that pooling results from increased venous compliance in POTS patients.

METHODS AND RESULTS

Fifteen patients aged 13 to 19 years were compared with 11 healthy, age-matched controls. The POTS group was divided into patients with high venous pressure (P(v)>20 mm Hg) and normal P(v) on the basis of resting supine P(v) obtained in previous work. Subjects were studied using strain gauge plethysmography to measure blood flow, P(v), and the venous compliance volume-pressure relation while supine and during incremental head-up tilt testing at -10 degrees, 0 degrees, 20 degrees, and 35 degrees. Volume-pressure relations of controls and POTS patients with normal P(v) and high P(v) were not different and were unchanged by orthostasis. Supine leg peripheral resistance was greater than control resistance in patients with high P(v) (54+/-9 versus 30+/-6 mm Hg x mL(-1) x 100 mL(-1) x min(-1)) and less than control resistance in patients with normal P(v) (17+/-2 mm Hg x mL(-1) x 100 mL(-1) x min(-1)). On upright tilt, resistance decreased in high P(v) to approximate resistance in normal P(v). Resistance in controls increased throughout tilt. Leg P(v) increased in patients with normal P(v) and in controls but remained unchanged in the high P(v) group.

CONCLUSIONS

The findings suggest that pooling in POTS is due to blunted arterial vasoconstriction, which produces passive redistribution of blood within peripheral venous capacitance beds. Venous compliance in POTS is similar to that in control subjects.

Decrease in cardiac output and muscle sympathetic activity during vasovagal syncope

The importance of cardiac output (CO) to blood pressure level during vasovagal syncope is unknown. We measured thermodilution CO, mean blood pressure (MBP), and leg muscle mean sympathetic nerve activity (MSNA) each minute during 60 degrees head-up tilt in 26 patients with recurrent syncope. Eight patients tolerated tilt (TT) for 45 min (mean age 60 +/- 5 yr) and 15 patients developed syncope during tilt (TS) (mean age 58 +/- 4 yr, mean tilt time 15.4 +/- 2 min). In TT patients, CO decreased during the first minute of tilt (from 3.2 +/- 0.2 to 2.5 +/- 0.3 l x min(-1) x m(-2), P = 0.001) and thereafter remained stable between 2.5 +/- 0.3 (P = 0.001) and 2.4 +/- 0.2 l x min(-1) x m(-2) (P = 0.004) at 5 and 45 min, respectively. In TS patients, CO decreased during the first minute (from 3.3 +/- 0.2 to 2.7 +/- 0.1 l x min(-1) x m(-2), P = 0.02) and was stable until 7 min before syncope, falling to 2.0 +/- 0.2 at syncope (P = 0.001). Regression slopes for CO versus time during tilt were -0.01 min(-1) in TT versus -0.1 l x min(-1) x m(-2) x min(-1) in TS (P = 0.001). However, MBP was more closely correlated to total peripheral resistance (R = 0.56, P = 0.001) and MSNA (R = 0.58, P = 0.001) than CO (R = 0.32, P = 0.001). In vasovagal reactions, a progressive decline in CO may contribute to hypotension some minutes before syncope occurs.

Orthostasis fails to produce active limb venoconstriction in adolescents

Orthostasis is characterized by translocation of blood from the upper body and thorax into dependent venous structures. Although active splanchnic venoconstriction is known to occur, active limb venoconstriction remains controversial. Based on prior work, we initially hypothesized that active venoconstriction does occur in the extremities during orthostasis in response to baroreflex activation. We investigated this hypothesis in the arms and legs of 11 healthy volunteers, aged 13-19 yr, using venous occlusion strain gauge plethysmography to obtain the forearm and calf blood flows and to compute the capacitance vessel volume-pressure compliance relation. Subjects were studied supine and at -10, +20, and +35 degrees to load the baroreflexes. With +20 degrees of tilt, blood flow decreased and limb arterial resistance increased significantly (P < 0.05) compared with supine. With +35 degrees of tilt, blood flow decreased, limb arterial resistance increased, and heart rate increased, indicating parasympathetic withdrawal and sympathetic activation with arterial vasoconstriction. The volume-pressure relation was unchanged by orthostatic maneuvers. The results suggest that active venoconstriction in the limbs is not important to mild orthostatic response.

Endothelium-derived nitric oxide contributes to the regulation of venous tone in humans

BACKGROUND

Although nitric oxide (NO) is known to play an important part in the regulation of arterial tone, little is known about its role in veins. The aim of this study was to investigate the role of basal and stimulated NO activity in the regulation of tone of the human venous capacitance bed.

METHODS AND RESULTS

We measured venous tone using radionuclide forearm venous plethysmography in 24 healthy subjects with no cardiovascular risk factors. In 13 subjects, basal NO activity was assessed by measuring the effects on venous tone of an intra-arterial infusion of the NO synthase inhibitor N-monomethyl-L-arginine (L-NMMA). In the remaining 11 subjects, stimulated NO activity was evaluated by measuring the effects of an intra-arterial infusion of incremental doses of carbachol, followed in a subgroup by coinfusion with L-NMMA. Infusion of carbachol caused dose-dependent venodilation, with a maximal reduction in forearm venous tone of 40.1+/-12.5% (P<0.0001). Carbachol-induced venodilation was inhibited by L-NMMA (48.9+/-6.2% reversal of maximal venodilation, P<0.01). Infusion of L-NMMA alone caused venoconstriction (9.1+/-6.4% increase in venous tone, P=0.002).

CONCLUSIONS

Human forearm capacitance veins exhibit both stimulated and basal NO activity, which indicates that NO contributes not only to the regulation of venous tone but also to resting venous tone in healthy human subjects.

Inadequate sympathovagal balance in response to orthostatism in patients with unexplained syncope and a positive head up tilt test

AIM

To analyse the immediate response of heart rate variability (HRV) in response to orthostatic stress in unexplained syncope.

SUBJECTS

69 subjects, mean (SD) age 42 (18) years, undergoing 60 degrees head up tilt to evaluate unexplained syncope.

METHODS

Based on 256 second ECG samples obtained during supine and upright phases, spectral analyses of low (LF) and high frequency (HF) bands were calculated, as well as the LF/HF power ratio, reflecting the sympathovagal balance. All variables were measured just before tilt during the last five minutes of the supine position, during the first five minutes of head up tilt, and just before the end of passive tilt.

RESULTS

Symptoms occurred in 42 subjects (vasovagal syncope in 37; psychogenic syncope in five). Resting haemodynamics and HRV indices were similar in subjects with and without syncope. Immediately after assuming the upright posture, adaptation to orthostatism differed between the two groups in that the LF/HF power ratio decreased by 11% from supine (from 2.7 (1.5) to 2.4 (1.2)) in the positive test group, while it increased by 11.5% (from 2.8 (1.5) to 3.1 (1.7)) in the negative test group (p = 0.02). This was because subjects with a positive test did not have the same increment in LF power with tilting as those with a negative test (11% v 28%, p = 0.04), while HF power did not alter. A decreased LF/HF power ratio persisted throughout head up tilt and was the only variable found to discriminate between subjects with positive and negative test results (p = 0.005, multivariate analysis). During the first five minutes of tilt, a decreased LF/HF power ratio occurred in 33 of 37 subjects in the positive group and three of 27 in the negative group. Thus a decreased LF/HF ratio had 89% sensitivity, 89% specificity, a 92% positive predictive value, and an 86% negative predictive value.

CONCLUSIONS

Through the LF/HF power ratio, spectral analysis of HRV was highly correlated with head up tilt results. Subjects developing syncope late during continued head up tilt have a decrease in LF/HF ratio immediately after assuming the upright posture, implying that although symptoms have not developed the vasovagal reaction may already have begun. This emphasises the major role of the autonomic nervous system in the genesis of vasovagal (neurally mediated) syncope.

[Possible contribution of alpha-adrenergic abnormalities to cerebral ischemia in the patients with sinus bradycardia. Analysis by pharmacologic autonomic nervous test]

BACKGROUND

Syncope of patients with bradyarrhythmia is perceived as severe sign of low cardiac output caused by bradycardia and as a major criteria for pacemaker implantation (PMI). However, it has been reported that PMI can not always prevent syncope; it has been suggested that not bradycardia but an abnormality of the autonomic nervous system plays a part in syncope.

PURPOSE

To investigate the relation between autonomic nervous dysfunction and syncope in cases of sinus bradycardia (SB).

SUBJECTS

Thirty-nine patients with SB were divided into two groups according to the presence (group S, n = 16, 46.9 +/- 20.0 years) or absence (group N, n = 23, 40.4 +/- 17.6 years) of syncope or presyncope.

METHODS

Corrected sinus node recovery time (CSNRT) was measured by electrophysiologic study. Pharmacologic autonomic nervous tests were performed as follows in a quiet room. Increased HR by application of 0.04 mg/kg atropine (para-tone), and by 0.004 microgram/kg/min isoproterenol divided by 0.004 (beta-sens) were evaluated, beta-tone was obtained by subtracting HR after application of propranolol (0.2 mg/kg) from that of atropine. Basal beta-sympathetic activity was evaluated by beta-sec that was obtained by beta-tone/beta-sens. Increased SBP by application of 0.4 microgram/kg/min phenylephrine divided by 0.4 (alpha-sens) was evaluated. alpha-tone was obtained by subtracting minimum SBP after 0.2 mg/kg phentolamine from SBP after application of propranolol. Basal alpha-sympathetic activity was evaluated by alpha-sec, that was obtained by alpha-tone/alpha-sens.

RESULT

There were no significant differences in basal clinical characteristics (age, sex, cardiac function) between the groups. The parameters of the functions of parasympathetic and beta-sympathetic receptors (para-tone, beta-sens, beta-tone, beta-sec) showed no significant differences between the groups, alpha-sens was attenuated (P < 0.01) and alpha-sec was augmented (P < 0.0001) significantly in group S.

CONCLUSION

It was suggested that syncope or presyncope in SB patients could be attributed to failure of vasoconstriction mediated by alpha-sympathetic receptor but to severity of sinus node dysfunction.

Orthostatic intolerance in the chronic fatigue syndrome

This study aims to investigate the prevalence and pathophysiology of orthostatic intolerance (OI) and its potential contribution to symptoms of a group of unselected patients with chronic fatigue syndrome (CFS). Seventy five patients (65 women, 10 men) with CFS were evaluated. During an initial visit, a clinical suspicion as to the likelihood of observing laboratory evidence of OI was assigned. Laboratory investigation consisted of beat-to-beat recordings of heart rate, blood pressure (Finapres), and stroke volume (impedance cardiograph) while supine and during 80 degrees head-up tilt (HUT), during rhythmic deep breathing (6 breaths/min) and during the Valsalva maneuver. The responses of 48 age-matched healthy controls who had no history of OI were used to define the range of normal responses to these three maneuvers. Forty percent of patients with CFS had OI during head-up tilt. Sixteen exhibited neurally-mediated syncope alone, seven tachycardia (> 35 bpm averaged over the whole of the head-up tilt) and six a mixture of tachycardia and syncope. Eight of 48 controls exhibited neurally-mediated syncope. The responses to the Valsalva maneuver and to deep breathing were similar in controls and patients. On average, the duration of disease and patient age were significantly less and the onset of symptoms was more often subacute in patients with OI than in those without OI. We conclude that there exists a clinically identifiable subgroup of patients with CFS and OI that differs from control subjects and from those with CFS without OI for whom treatment specifically aimed at improving orthostatic tolerance may be indicated.

Neurally mediated hypotension and chronic fatigue syndrome

A substantial body of clinical evidence now supports an association between various forms of hypotension and both idiopathic chronic fatigue and the chronic fatigue syndrome (CFS). Patients with CFS have a high prevalence of neurally mediated hypotension, and open treatment of this autonomic dysfunction has been associated with improvements in CFS symptoms. Randomized trials are now in progress to evaluate the efficacy of treatments directed at neurally mediated hypotension in those with CFS patients, and the results of these trials should help guide more basic inquiries into the mechanisms of orthostatic intolerance in affected individuals.

Bedside autonomic function testing in patients with vasovagal syncope

The factors that determine the individual susceptibility to vasovagal syncope (VVS) are largely unknown, including the role of the autonomic nervous system. We therefore studied common vagal and sympathetic reflexes in 12 patients with VVS (mean age 37 (18-75) years, 6 men). The Valsalva maneuver and deep breathing were performed to assess vagal responsiveness and mental arithmetic stress; the cold-pressor test and isometric handgrip were performed to assess sympathetic responsiveness. Standing up was performed to assess the combined responsiveness. With the exception of a subnormal response to deep breathing in one patient, all vagal tests were normal. In contrast, the response to mental stress, the cold-pressor test, and isometric handgrip was subnormal in 4 patients, 6 patients, and 4 patients, respectively. The response to standing up was normal in all patients. It is concluded that many patients with VVS are characterized by normal vagal responsiveness, but sympathetic hyporesponsiveness. Sympathetic hyporesponsiveness might explain the inadequate vasoconstruction, which plays an important, early role in VVS.

Pathophysiologic basis for vasodepressor syncope

The current knowledge regarding the pathophysiologic basis of the vasodepressor response was reviewed. The balance of evidence indicates that the mechanoreceptor hypothesis seems unlikely to be the sole afferent alteration that leads to the vasodepressor response. Alternative afferent mechanisms should include neurohumoral mediated sympathoinhibition triggered by opioid mechanisms as well as impaired endothelial and NO responses to orthostatic stress in susceptible individuals. It is possible that impaired cardiovagal and sympathetic outflow control of arterial baroreceptors is enhanced by the aforementioned mechanisms. The role of central sympathoinhibition and vagal excitation triggered directly from pathways within the temporal lobe or triggered by alterations in regional cerebral blood flow should be considered as potential alternative mechanisms. Efferent autonomic outflow during vasodepressor syncope include sympathetic neural outflow withdrawal in addition to activation of parasympathetic outflow to the heart and abdominal viscera. Further human research is needed to understand the underlying mechanisms that result in the described neural and vascular responses.

Neural monitoring of vasovagal syncope

Head-up tilt testing has become a valuable and widely accepted diagnostic tool for evaluation of patients with vasovagal syncope. This test has afforded clinical researchers the opportunity to focus on the hemodynamic, humoral, and neural changes that accompany syncope. We review the animal and clinical studies that provide insight into the possible pathophysiological mechanisms involved in vasovagal syncope. Hemodynamic measurements in patients with vasovagal syncope suggest that a relative decrease in ventricular size and increase in cardiac contractility may be seen in many patients with vasovagal syncope. Patients with vasovagal syncope have also demonstrated numerous "exaggerated" neurohumoral responses to syncope. Differential changes in plasma levels of epinephrine, renin, endothelin, vasopressin, cortisol, prolactin, beta endorphins, and substance P have been reported by some investigators either prior to or during a syncopal episode in patients with vasovagal syncope. The precise pathophysiological significance of these measurements is unknown at the present time. Measurements of autonomic tone may be accomplished indirectly with analysis of heart rate variability or baroreflex slope, or directly by sympathetic neural recordings of the peroneal nerve. We have demonstrated decreased baroreflex slopes in patients with vasovagal syncope. Using microneurography, we and others have demonstrated decreased sympathetic nerve activity occurring 11 +/- 3 seconds prior to syncope during head-up tilt table testing. A variety of other abnormal reflexes, including blunted forearm blood flow responses during exercise, have been demonstrated by others. These observations suggest that pacing instituted after the event may not be as helpful as the use of a hemodynamic sensor that will result in the initiation of pacing prior to sympathetic withdrawal or modify the decrease in sympathetic tone that occurs prior to syncope.