Time and frequency domain analyses of heart rate variability during orthostatic stress in patients with neurally mediated syncope

Vasovagal syncope: An overview of pathophysiological mechanisms

Syncope is a short-term transient loss of consciousness, characterized by rapid onset and complete spontaneous recovery. According to the 2018 European Society of Cardiology guidelines, three different types of syncope have been identified. However, all forms of syncope share a common final pathophysiological event, global cerebral hypoperfusion, which results from the inability of the circulatory system to maintain blood pressure at the level required to efficiently supply blood to the brain. The vasovagal syncope (VVS) is the most common form of syncope. Although, VVS is generally harmless, its frequent occurrence can negatively affect quality of life and increase the risk of adverse events. The pathophysiological mechanisms underlying VVS remain obscure. The multifaceted nature of VVS presents a veritable challenge to understanding this condition and developing preventative strategies. Thus, the aim of this review was to discuss the factors contributing to the pathogenesis of VVS and provide guidance for future research.

Simultaneous beat-to-beat heart rate and systolic blood pressure variability in patients with and without neurally mediated syncope

Introduction: Autonomic changes play an essential role in the genesis of neurally mediated syncope (NMS). The aim of this study was to compare the changes of the autonomic nervous system (ANS) by measuring spectral indices of beat-to-beat systolic blood pressure and heart rate variability (SBPV and HRV) in ranges of low frequency (LF), high frequency (HF), and the LF/HF ratio during head-up tilt test (HUTT) in patients with and without a syncope response.

Methods: In this case-control study of 46 patients with a suspected history of unexplained syncope, data were recorded separately during the typical three phases of HUTT. Patients who developed syncope were designated as the case group and the rest as the control group.

Results: Thirty one patients experienced syncope during HUTT. Resting HRV and SBPV indices were significantly lower in cases than controls. After tilting in the syncope group, both HF and LF powers of SBPV showed a significant and gradual decrease. LF/HF in HRV increased in both groups similarly during the test but in SBPV, mainly driven by oscilations in its LF power, it increased significantly more during the first two phases of the test in syncope patients only to paradoxically decrease during active tilt (P< 0.001).

Conclusion: Our findings show an abnormal autonomic function in patients with syncope, both at rest and tilting. Fluctuations of spectral indices of beat-to-beat SBPV, a potential noval index of pure sympathetic activity, show an exaggerated response during tilt and its withdrawal before syncope.

Efficient syncope prediction from resting state clinical data using wavelet bispectrum and multilayer perceptron neural network

Neurally mediated syncope (NMS) is the most common type of syncope, and head up tilt test (HUTT) is, so far, the most appropriate tool to identify NMS. In this work, an effort to predict the NMS before performing the HUTT is attempted. To achieve this, the heart rate variability (HRV) at rest and during the first minutes of tilting position during HUTT was analyzed using both time and frequency domains. Various features from HRV regularity and complexity, along with wavelet higher-order spectrum (WHOS) analysis in low-frequency (LF) and high-frequency (HF) bands were examined. The experimental results from 26 patients with history of NMS have shown that at rest, a time domain entropy measure and WHOS-based features in LF band exhibit significant differences between positive and negative HUTT as well as among 10 healthy subjects and NMS patients. The best performance of multilayer perceptron neural network (MPNN) was achieved by using an input vector consisted of WHOS-based HRV features in the LF zone and systolic blood pressure from the resting period, yielding an accuracy of 89.7%, assessed by 5-fold cross-validation. The promising results presented here pave the way for an early prediction of the HUTT outcome from resting state, contributing to the identification of patients at higher risk NMS. The HRV analysis along with systolic blood pressure at rest predict NMS using a multilayer perceptron neural network.

Association of Postural Instability with Autonomic Dysfunction in Early Parkinson’s Disease

Background: There have been several pathologic data that support an association between postural instability (PI) and autonomic dysfunction in Parkinson’s disease (PD). The purpose of this study was to investigate the correlation of PI and autonomic deficits in early PD. Methods: We collected 17 patients with a diagnosis of early PD. PI was assessed by computerized dynamic posturography (CDP). Standardized autonomic function test (AFT) and time and frequency domain spectral analysis of heart rate variability (HRV) were performed. CDP data obtained from the 21 patients were compared to that from age- and sex-matched healthy controls. We collected HRV data from 18 other age- and sex-matched controls. All patients were evaluated in the “OFF” state. We used Mann–Whitney U-test to compare parameters of CDP between the early PD and control groups. Spearman correlation was used for correlation analysis between parameters of CDP and autonomic function test in PD patients. Results: Most patients (76.5%) showed mild or moderate autonomic dysfunction in the standardized AFT. In CDP, sensory ratios of equilibrium score (e.g., visual and vestibular) and composite scores were significantly lower in PD patients than in controls. In HRV, the low-frequency/high-frequency ratio during the tilt and the gap of low- frequency/high-frequency ratio from supine to tilt were significantly different in both groups. The parameters of time and frequency domains of HRV reflecting parasympathetic function were correlated with equilibrium scores for somatosensory organization test in CDP. Discussion: PI was associated with parasympathetic autonomic dysfunction in early PD. This result was in accordance with a previous assumption that PI in PD is related to parasympathetic cholinergic neuron loss in the brainstem.

Cardiac Autonomic Response to Active Standing in Adults With Vasovagal Syncope

PURPOSE

Little is known about the autonomic response to active standing in vasovagal syncope, and most works have focused on children or adolescents. The aim of this work was to study the changes in cardiac autonomic modulation in adult patients with vasovagal syncope through heart rate variability analysis with linear and short-term complexity (alpha-1) indexes during supine position and active standing, in patients with positive or negative head-up tilt test (HUTT).

METHODS

Twenty-five patients with vasovagal syncope were included. Heart rate variability linear and short-term complexity (alpha-1) indexes were recorded during an active standing test (15 minutes in each position) and compared among patients grouped by HUTT outcome and between positions.

RESULTS

During supine position, positive HUTT (+HUTT) patients had longer mean RR (1016 [850-1051] milliseconds), higher pNN50 (17.7 [9.2-26.2]), lower sympathovagal balance (1.3 [0.5-1.7]), and alpha-1 (0.9 [0.8-1.0]) than negative HUTT (-HUTT) patients (871 [776-969] milliseconds, 8.8 [2.1-14.5], 2.9 [1.3-3.9], and 1.2 [1.0-1.1], respectively). During active standing, heart rate and alpha-1 increased in both groups; in +HUTT patients, pNN50 decreased, whereas sympathovagal balance increased. The magnitude of change between positions of sympathovagal balance and alpha-1 was 6.1 and 4.8 times larger in +HUTT than -HUTT patients, respectively.

CONCLUSIONS

The underlying cardiac autonomic mechanism in vasovagal syncope may involve different autonomic patterns in subjects with a history of recurrent syncope and +HUTT or -HUTT.

Predicting the outcome of head-up tilt test using heart rate variability and baroreflex sensitivity parameters in patients with vasovagal syncope

PURPOSE

The aim of the study was to investigate whether a statistical model could be used for an early prediction of the head-up tilt test (HUTT) outcome from heart rate variability (HRV) and baroreflex sensitivity (BRS) data obtained during early stages of the HUTT.

METHODS

A modified Italian protocol was used for HUTT in 105 patients with a previous history of vasovagal syncope. Beat-to-beat heart rate and blood pressure were continuously recorded. Fast Fourier transformation was used for spectral analysis of HRV and a sequence technique for measuring the BRS.

RESULTS

Linear statistical models based on HRV and BRS data from the first 15 min of HUTT were no more accurate than always naively predicted majority class that a syncope will occur (average model out-of-sample accuracy 56.2 ± 5.1 % vs. majority class relative frequency 54.2 %). Even when HRV and BRS data from the first 30 min were used in the model, we did not obtain any predictions of meaningful practical value (75.0 ± 5.1 % accuracy vs. 72.2 % majority class).

CONCLUSIONS

While there are discernible and meaningful differences between HUTT-P and HUTT-N subjects, they are not sufficient to discriminate between the two groups and predict a syncope early in the HUTT. The results might improve with a larger set of subjects; however, we can conclude that it is not likely that syncope predictions of practical value can be obtained from aggregate HRV spectral analysis and BRS values.

Real-Time Prediction of Neurally Mediated Syncope

Neurally mediated syncope (NMS) patients suffer from sudden loss of consciousness, which is associated with a high rate of falls and hospitalization. NMS negatively impacts a subject's quality of life and is a growing cost issue in our aging society, as its incidence increases with age. In this paper, we present a solution for prediction of NMS, which is based on the analysis of the electrocardiogram (ECG) and photoplethysmogram (PPG) alone. Several parameters extracted from ECG and PPG, associated with reflectory mechanisms underlying NMS in previous publications, were combined in a single algorithm to detect impending syncope. The proposed algorithm was evaluated in a population of 43 subjects. The feature selection, distance metric selection, and optimal threshold were performed in a subset of 30 patients, while the remaining data from 13 patients were used to test the final solution. Additionally, a leave-one-out cross-validation scheme was also used to evaluate the performance of the proposed algorithm yielding the following results: sensitivity (SE)--95.2%; specificity (SP)--95.4%; positive predictive value (PPV)--90.9%; false-positive rate per hour (FPRh)-0.14 h(-1), and prediction time (aPTime)--116.4 s.

One swallow does not a summer make: safety and renal denervation

Renal denervation is increasingly being adopted as a treatment option in patients with resistant hypertension. The long-term safety of this procedure is unknown. Though the procedure interrupts the sympathetic nerves at the renal level, it also has effects on other organ beds, notably the heart and vasculature. These effects have been purported to be clinically beneficial and thus formed a rationale for examining the role of renal denervation in other conditions, including heart failure, arrhythmia, obstructive sleep apnoea and the metabolic syndrome. There is a theoretical concern that attenuating the renal sympathetic nerves might cause orthostatic hypotension or syncope. From the limited data available from hypertension trials, the procedure has not been associated with excessive episodes of syncope and this is supported by mechanistic tilt table data in asymptomatic patients. Ultimately, the safety of this technique will only be established once we have larger phase III/IV studies.

Orthostatic tolerance is difficult to predict in recurrent syncope patients

OBJECTIVES

We tested the hypothesis that detailed anthropometric and hemodynamic measurements predict orthostatic tolerance in neurally mediated syncope patients. In addition, we tested whether orthostatic tolerance is related to syncope frequency in real life.

BACKGROUND

Earlier studies in patients with neurally mediated syncope suggested that orthostatic heart rate and blood pressure responses predict the tilt table responses with high sensitivity and specificity.

METHODS

We analyzed data from 157 consecutive patients (n = 100 exploratory cohort, n = 57 confirmatory cohort) with recurrent syncope in whom orthostatic tolerance was quantified as the time to (pre)syncope during head-up tilt testing combined with lower body negative pressure. We measured heart rate, brachial blood pressure, cardiac stroke volume, heart rate and blood pressure variability, and spontaneous baroreflex sensitivity supine and early during head-up tilt.

RESULTS

The orthostatic heart rate increase showed the strongest correlation with orthostatic tolerance. The best multivariate model including age, supine diastolic blood pressure, supine blood pressure variability, as well as tilt-induced changes in diastolic blood pressure and heart rate explained no more that 40% of the variability in orthostatic tolerance. The model failed to predict orthostatic tolerance in the confirmatory cohort. Frequency or number of free-living syncopal episodes were only weakly related to orthostatic tolerance.

CONCLUSIONS

In patients with neurally mediated syncope, orthostatic tolerance in the clinical laboratory is difficult to predict with a wide range of anthropometric and cardiovascular measurements and correlates poorly with syncope occurrence in real life.

The autonomic nervous system dysregulation in response to orthostatic stress in children with neurocardiogenic syncope

Neurocardiogenic syncope is a common disorder, which is considered as a benign condition. However, sudden loss of conscience and muscle tone causes anxiety among the family members due to its similarity to sudden death. Autonomic nervous system dysregulation is thought to be responsible in the aetiology. Heart rate variability is used for assessment of autonomic nervous system.We evaluated 24 children between 6 and 18 years (mean plus or minus standard deviation is equal to 12.5 plus or minus 3.28, with neurocardiogenic syncope and 10 healthy controls, mean plus or minus standard deviation is equal to 12.48 plus or minus 3.27) by using 24 hour Holter monitorisation and head-up tilt test. Heart rate variability analysis was performed using the Holter recordings obtained both during head-up tilt test and throughout the day.Our results revealed that, there is no significant difference between the study and the control groups in terms of the mean heart rate and all indices of the heart rate variability (p > 0.05). However, during the first 5 minutes of the head-up tilt test, standard deviation of all RR intervals and root mean square of successive differences were significantly lower in the syncope group compared with the control group, 42.17 plus or minus 12.56 versus 60.10 plus or minus 33.10 and 21.26 plus or minus 8.87 versus 36.80 plus or minus 31.03; p-values 0.02 and 0.03, respectively.In conclusion; autonomic functions in children with neurocardiogenic syncope are similar to healthy children. However, sympathetic hyperactivation occurs during the early phase of orthostatic stress in children with neurocardiogenic syncope comparing to healthy controls. Parasympathetic innervation is not sufficient in compensation of this sympathetic hyperactivation. Management strategy in neurocardiogenic syncope should be based on these pathophysiologic mechanisms.

Vulnerability to simple faints is predicted by regional differences in brain anatomy

Neurocardiogenic syncope (NCS, simple fainting) is a common and typically benign familial condition, which rarely may result in traumatic injury or hypoxic convulsions. NCS is associated with emotional triggers, anxiety states and stress. However, the etiology of NCS, as a psychophysiological process, is poorly understood. We therefore investigated the relationship between NCS and brain anatomy. We studied a non-clinical sample of eighteen individuals with histories characteristic of NCS, and nineteen matched controls who had never fainted. We recorded fainting frequency, resting heart rate variability measures and anxiety levels. Structural T1-weighted magnetic resonance images (MRI) were acquired at 1.5 T. Associations between brain morphometry (regional gray and white matter volumes) and NCS, resting physiology and anxiety were tested using voxel-based morphometry (VBM). Compared to controls, NCS participants had lower regional brain volume within medulla and midbrain (a priori regions of interest). Moreover, across NCS individuals, lower gray matter volume in contiguous regions of left caudate nucleus predicted enhanced parasympathetic cardiac tone, fainting frequency and anxiety levels. Our findings provide preliminary evidence for a hierarchical anatomical basis to NCS. First, differences in the volume of brainstem centers supporting cardiovascular homeostasis may relate to constitutional predisposition to NCS. Second, differences in the structural organization of the caudate nucleus in NCS individuals may relate to fainting frequency via interactions between emotional state and parasympathetic control of the heart. These observations highlight the application of VBM to the identification of neurovisceral mechanisms relevant to psychosomatic medicine and the neuroscience of emotion.

Autonomic responses to orthostatic stress in head-up tilt testing: relationship to test-induced prolonged asystole

BACKGROUND

Prolonged asystole is sometimes an extreme manifestation of neurally mediated syncope.

HYPOTHESIS

To investigate the mechanism of head-up tilt testing-induced prolonged (life-threatening) cardiac asystole, we measured temporal changes in frequency domain heart rate variability indices in 25 patients with syncope of undetermined etiology.

METHODS

Head-up tilt testing (80 degrees) was performed in 25 patients for up to 40 min or until asystole or syncope occurred. Three patients (Group 1; 37 +/- 13 years, 1 man 2 women) had an episode of prolonged cardiac asystole (> or = 10 s) during testing, necessitating cardiopulmonary resuscitation. Syncope, but no asystole, was induced in 10 patients (Group 2; 48 +/- 31 years, 6 men, 4 women), and 12 patients (Group 3; 55 +/- 20 years, 5 men, 7 women) failed to show asystole or syncope during testing. Power spectra of low (0.04-0.15 Hz) and high (0.15-0.40 Hz) frequency, and total (0.01-1.00 Hz) frequency spectra were measured in consecutive 2 min segments throughout the test.

RESULTS

Maximally changed values in heart rate, systolic blood pressure, and heart rate variability indices during testing were compared among the three groups (maximally changed values did not include the values during tilt-induced symptoms). High frequency spectra in Groups 2 and 3, but not in Group 1, decreased during the test. High frequency spectra, low frequency spectra, and total spectra in Group 1 were significantly higher than those in Groups 2 and 3 during testing. In Group 1 patients, findings at test-induced asystole were consistent with exaggerated sympathetic and concurrent persistent parasympathetic activity.

CONCLUSION

Unusual autonomic responses to orthostatic stress can cause prolonged asystole, and this autonomic nerve dysregulation may relate to asystolic episodes associated with cardiovascular collapse.

Precursors of syncope in linear and non-linear parameters of heart rate variability during pediatric head-up tilt test

The aim of this study was to identify precursors of neurocardiogenic syncope (NCS) in ECG parameters of children and adolescents who undergo head-up tilt testing (HUTT). Established linear and non-linear parameters of heart rate variability (HRV) were calculated minute-by-minute in 44 young patients with an NCS history (age 7-20 years, 22 female, 22 male, positive HUTT) and 34 age-matched healthy controls (age 7-20 years, 17 female, 17 male, negative HUTT) during 60 degrees HUTT. The influence of age and gender on the response to tilting was studied using the Wilcoxon rank sum test. We also assessed the predictive power of individual features with respect to HUTT outcome within three temporal windows (0-2 min after tilt; 2-5 min after tilt, and 5-2 min before tilt-back) by means of receiver operating characteristics analysis. Our results indicate that prediction of HUTT outcome should be restricted to subjects in or after puberty. In this group, descriptors based on co-occurrence or joint-recurrence analysis in recurrence plots showed promising predictive power (up to 86% sensitivity and 90% specificity, area under the curve 0.91). Time-domain parameters (standard deviation of differences for successive RR intervals) reached 75% sensitivity and 80% specificity. We conclude that HRV analysis within the first 5 min after tilting provides significant information on HUTT outcome and may be useful in conjunction with more sophisticated classification strategies for shortening HUTT duration in subjects after puberty.

Predictor of a negative tilt-table test result based on early heart rate variation in patients with a history of syncope

Lack of early sympathetic overreactivity measured by early increase in heart rate has been shown to predict a negative tilt-table (TT) test result. The goal of this study was to evaluate this observation in patients with a history of syncope. The author retrospectively identified 27 patients with a history of syncope undergoing TT testing. The difference between maximal heart rate during the first 10 minutes of the tilting and baseline heart rate was used for early heart rate change. There was a significant association between the probability of a negative TT test result and failure to increase early heart rate over 19 beats per minute. The sensitivity of a negative TT test result was 93.3%, with a specificity of 66.7% (odds ratio, 0.1; confidence interval, 0.14-0.69; P=.001). This study confirms previous findings that failure to increase the heart rate early during a TT test can predict low probability of syncope.

The pattern of activation of the sympathetic nervous system during tilt-induced syncope

A 49-year-old patient with a history of situational syncope and minimal electrocardiographic signs of accessory pathway is described. The evidence for pre-excitation was present only during the sympathetic activation caused by exercise testing and isoprenaline infusion. This phenomenon served as an indicator of significant adrenergic drive to the heart after the tilt-induced syncope. The meaning of the observed electrocardiographic changes in the course of neurocardiogenic reaction and its contribution to the understanding of the sympatho-vagal balance during vasovagal syncope is discussed. The lack of preexcitation signs during syncope and its appearance several seconds after the syncope-related sinus pause indicates sympathetic withdrawal before and shortly after the asystole. The possible pathophysiological mechanisms are discussed.

Neurocardiogenic mechanisms of unexplained syncope in idiopathic dilated cardiomyopathy

Syncope in patients with advanced heart failure is a sign of poor prognosis. The cause of syncope in patients with dilated cardiomyopathy (DC) is not fully recognized and may remain elusive even after standardized evaluation. The purpose of the present study was to examine the implication of neurally mediated mechanisms in the pathophysiology of syncopal episodes in patients with DC. Twenty-six patients (21 men, 5 women; mean age 59 +/- 2 years, range 38 to 79) with DC and left ventricular ejection fractions <or=40% were included in the study. Thirteen patients with unexplained syncope or presyncope and a control group of 13 patients without unexplained syncope underwent head-up tilt tests with clomipramine challenge. The 2 groups were matched with regard to age, gender, and left ventricular ejection fractions, and there were no major differences in terms of medication. Heart rate variability analysis and plethysmography of forearm flow were performed during the tilt tests. Blood samples were also drawn for catecholamine measurements. In the group with histories of unexplained syncope, the head-up tilt test results were positive in 11 patients (84.6%). Sympathetic and parasympathetic heart rate indexes were markedly stimulated, while catecholamine concentrations and blood flow changes indicated sympathetic withdrawal during tilting. In the control group, the head-up tilt test results were negative in 12 patients (92.3%). In conclusion, neurally mediated mechanisms seem to be implicated in the pathophysiology of syncope in patients with DC and should therefore be considered in the differential diagnosis of syncopal episodes of unexplained origin.

Is vasovagal syncope a disease?

Vavovagal syncope (VVS) is not generally associated with cardiovascular, neurological or other diseases, and, therefore, represents an isolated manifestation. Isolated VVS cannot be regarded as a disease for several reasons: spontaneous syncope occurs in about half of individuals during their lives, and the unidentified neural pathways involved in the vasovagal response are probably present in all healthy humans, with individual differences in susceptibility; VVS is induced during tilt testing in several subjects with no history of syncope; during haemorrhagic shock, the vasovagal reaction can be observed in subjects with no history of syncope; about 20% of astronauts, who are selected on the basis of their great resistance to orthostatic stress, experience syncope or presyncope on landing after a short-duration space flight; to date, no genetic basis of VVS has been demonstrated; subjects with VVS are generally normotensive and, importantly, have normal blood pressure regulation apart from the episodes of syncope; hormonal disorders or a generalized state of autonomic involvement, although frequently investigated, have never been clearly demonstrated. Isolated VVS should be distinguished from those forms that start in old age and which are often associated with cardiovascular or neurological disorders, and other dysautonomic disturbances such as carotid sinus hypersensitivity, post-prandial hypotension, and symptoms of autonomic dysfunction. In these subjects, VVS appears as an expression of a pathological process, i.e. a disease, mainly related to a generalized involvement of the autonomic nervous system, which is not yet well-defined from a nosological point of view.

Spectral and time-domain analyses of heart-rate variability during head-upright tilt-table testing in children with neurally mediated syncope

Neurocardiac syncope (NS) is a common cause of syncope in children. The mechanism, though related to abnormalities in autonomic function, has not been fully elucidated, particularly in pediatric patients. This study assessed the heart-rate variability (HRV) response to head-upright tilt-table test (HUT) in children with NS and normal volunteers. Spectral and time-domain analysis of HRV was used to assess changes in autonomic function in 27 children (9 male, mean age 12.3 +/- 1.6 years) with a history of at least one episode of syncope and positive passive HUT and 27 age-matched normal volunteers with negative passive HUT before and during postural tilt and to attempt to relate such changes to specific types of hemodynamic response to tilt. Frequency-domain measurements of the high-(HF) and low-(LF) frequency bands and the ratio LF/HF were derived from Holter recordings and computed by fast Fourier analysis for 5-min intervals. Time-domain measurements of the SDNN, SDNNI, SDANN, RMSSD, and triangular index were derived from 24-h Holter recordings. There were no significant differences between clinical characteristics, time-domain, and basal frequency domain parameters of the groups. Mean values of LF and LF/HF ratio was increased and HF was decreased significantly in response to tilt in both patient and control groups. Mean values of LF and LF/HF ratio were higher and HF was lower compared to controls immediately after tilt. LF and LF/HF ratio showed a statistically significant decrease and a significant increase in HF during syncope in patients. The three subgroups of patients had similar patterns of changes in autonomic activity. The results of this study show that although the basal autonomic function was similar to that of the control group, patients with NS have a different pattern of response to the HUT. In our study, patients with NS demonstrated an exaggerated response to the HUT. This exaggerated response may be the factor that activates the pathological reflexes of NS. The pathological mechanism leading to NS appears to be independent of the specific type of hemodynamic response to HUT.

Orthostatic decrease in cardiac chaos during the head-up tilt test in patients with vasovagal syncope

BACKGROUND

Autonomic dysfunction contributes to orthostatic intolerance in vasovagal syncope (VVS), but as it has not been identified by spectral analysis of heart rate variability (HRV) in previous studies, the present hypothesis was that nonlinear analysis of HRV would identify the orthostatic intolerance in VVS.

METHODS AND RESULTS

Twenty-six patients with VVS and 14 matched controls were subjected to 80-degree head-up tilt test (positive: 13 patients; negative: 13 patients and 14 controls). There were no differences in the orthostatic changes in the indices of spectral analyses of HRV among the 3 groups. The Lyapunov exponent (LE) was calculated from 200 consecutive RR-intervals to investigate chaotic behavior, and cardiac chaos was defined as the incidence of the presence of a positive finite LE. Orthostatic decreases in cardiac chaos were observed in the VVS patients (both the positive and negative groups), although there was no orthostatic decrease in the control group (ANOVA: p = 0.008). The receiver-operator characteristic curve indicated that cardiac chaos during the tilt identified VVS regardless of the results of the tilt (p < 0.001, sensitivity: 85.7%, specificity: 96.2%).

CONCLUSIONS

The decrease in cardiac chaos during the tilt test was specific to patients with VVS, even if their response to the test was negative.

Causal Cross-Spectral Analysis of Heart Rate and Blood Pressure Variability for describing the Impairment of the Cardiovascular Control in Neurally Mediated Syncope

A causal approach to the calculation of coherence and transfer function between systolic pressure (SP) and RR interval variability was applied in eight patients and eight control subjects during prolonged tilt test for investigating the impairment of cardiovascular control related to neurally mediated syncope. The causal analysis showed a depressed baroreflex regulation in resting patients, with reduced gain and increased latency from SP to RR, and a drop of the baroreflex coupling immediately before syncope. These findings, which were not elicited by traditional cross-spectral analysis, strongly suggest the use of the causal approach for the study of syncope mechanisms.

Autonomic Nervous System Changes in Vasovagal Syncope:

Spectral analysis of heart rate variability was used to compare the changes in autonomic function during tilting in young and older patients with vasovagal syncope. Twenty-four young (age 28 +/- 8 years) and 31 older (56 +/- 5 years) patients with unexplained syncope and a positive tilt test and 25 controls (age 48 +/- 12 years) were included in the study. Frequency-domain measurements of the low (LF) (0.06-0.15 Hz) and high (HF) (0.15-0.40 Hz) frequency bands and the ratio of LF to HF were computed from Holter recordings for 4-minute intervals before and immediately after tilting and just before the end in all groups. Syncopal patients showed a different pattern of response to tilting from controls in all spectral indexes. Young and older patients showed the same pattern of changes in all measurements, even though certain differences were observed. The LF after tilting reduced more in the older (-20 +/- 7% vs -14 +/- 5%, P < 0.001), while HF reduced more in young patients (-17 +/- 8% vs -8 +/- 3%, P < 0.001). Young patients showed mainly a cardioinhibitory type (71%) of response whereas a vasodepressor type response predominated (68%) in the older patients. The autonomic nervous system appears to play an important role in the pathophysiological mechanism of vasovagal syncope. This role is similar in young and in older patients and this should be taken into account in the therapeutic approach to the condition. Specific differences between age groups may be related to the type of vasovagal syncope.

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.

Heart rate and blood pressure variability in subjects with vasovagal syncope

Autonomic nervous system control in subjects with vasovagal syncope is controversial. In the present study, we used short-term spectral analysis to evaluate autonomic control in subjects with recurrent vasovagal syncope. We assessed the ability of spectral indices of HR (heart rate) variability to predict tilt-test responses. A series of 47 outpatients with recurrent vasovagal syncope and with positive responses to head-up tilt testing underwent a further study of RR variability during controlled breathing at rest and during tilt testing. During controlled breathing, RR interval variability of total power (TPRR; P<0.001), low-frequency power (LFRR; P<0.05), high-frequency power (HFRR; P<0.001) and HF expressed in normalized units (HFnuRR; P<0.001) were all higher, and LF expressed in normalized units (LFnuRR) and LF/HF ratio were lower in subjects with vasovagal syncope than in controls (P<0.001). To assess the ability of spectral components of RR variability to predict tilt-test responses, we prospectively studied 109 subjects with recurrent vasovagal syncope. The two normalized measures, HFnuRR and LFnuRR, determined during controlled breathing alone predicted a positive tilt-test response (sensitivity, 76%; specificity, 99%; positive predictive value, 96%; and negative predictive value, 90%). During tilting, subjects with vasovagal syncope had lower SBP (systolic blood pressure; P<0.05), LF component of peak SBP variability (LFSBP) and LFnuRR than controls, and higher TPRR, HFRR, HFnuRR and α HF (P<0.001). These spectral data indicate that vagal sinus modulation is increased at rest in subjects with vasovagal syncope. Spectral analysis of RR variability during controlled breathing, a procedure that predicts tilt-test responses, could be a useful guide in choosing the method of tilt testing.

Vestibular function interferes in cardiovascular reflexes [corrected]

BACKGROUND

Experimental work indicates that the vestibular system participates in autonomic reflexes during body movement and postural changes. However, there are no studies of cardiovascular reflexes during vertigo due to human acute vestibular lesions.

METHODS

We assessed the response to active change of posture and hand immersion in cold water in seven patients with unilateral peripheral vestibular failure (vestibular neuritis) and seven age/sex-matched healthy subjects in acute phase (72 h from vertigo onset) and at 2 weeks of follow-up.

RESULTS

During acute phase, patients showed decreased blood pressure response during cold hand test (p < 0.05). Upright stance induced deficient decrease of the respiratory component of heart rate variability (p < 0.05) with lack of increase in low frequency/high frequency (LF/HF) ratio. At 2 weeks of follow-up, these abnormalities improved.

CONCLUSIONS

Results suggest that acute vestibular lesions can interfere with cardiovascular autonomic responses in humans. This may reflect disruption of normal vestibulo-autonomic reflexes.

Insulin sensitivity in young women with vasovagal syncope

BACKGROUND

Insulin, in addition to its known metabolic effects, has sympatho-excitatory and vasodilatory actions on muscular blood vessels. The goal of this study was to evaluate insulin sensitivity in young women with vasovagal syncope and positive tilt test results (HUT+) and to compare it with that in patients with negative tilt test results (HUT-) and in control subjects without a history of syncope.

METHODS

Different indices of insulin sensitivity were obtained by an oral glucose tolerance test (OGTT) in 13 young women with syncope and HUT+ (age 26.8 +/- 9.1 years, body mass index 20.4 +/- 2.1), 8 patients with HUT- (age 26 +/- 5.6 years, body mass index 21.9 +/- 2.4), and 13 control subjects without syncope and HUT- (age 28.9 +/- 8.8 years, body mass index 23.1 +/- 1.7). The following parameters were assessed: fasting glucose and insulin levels (G(0), I(0)); G(0)/I(0) ratio; G(0) x I(0); areas under the curve for glucose and insulin; homeostatic model assessment (HOMA); quantitative insulin sensitivity check index (QUICKI); and composite whole-body insulin sensitivity index (ISI).

RESULTS

G(0) and I(0) values were significantly lower in patients with HUT+ than in control subjects (G(0) 4.9 vs 81.9, P <.05, I(0) 4.7 vs 9.1, P <.005). All the fasting values-based indices (ie, HOMA 0.9 vs 1.9, P <.005) and the ISI (12.8 vs 7.1, P =.01) differed significantly in both groups. None of the parameters showed significant differences between patients with HUT- and control subjects. Sixty-one percent of patients with HUT+ had a vasovagal reaction during OGTT.

CONCLUSIONS

Young women with vasovagal syncope and HUT+ have a greater insulin sensitivity. They have a propensity to reproduce symptoms during the OGTT. This hypersensitivity could be one of the predisposing factors for vasovagal episodes.

Orthostatic hypotension, 2001

Upright posture requires rapid and effective circulatory and neurologic compensations to maintain blood pressure and consciousness. Although it has been recognized over the past 100 years or more that the act of standing may cause hypotension in patients with autonomic dysfunction, only recently several of the pathophysiologic mechanisms resulting in orthostatic intolerance have been discovered. In patients with orthostatic hypotension, failure of reflex vasoconstriction causes pooling of blood in the legs during standing. Not everyone with a postural blood pressure drop requires treatment, nor does everyone with posturally induced symptoms have orthostatic hypotension. This review will discuss current knowledge of a broad, heterogeneous group of disturbances in the autonomic nervous system, each of which is manifested by hypotension, orthostatic intolerance, and often syncope.

Orthostatic intolerance: different abnormalities in the neural sympathetic response to a gravitational stimulus

In this paper we shall focus on the different abnormalities in the neural sympathetic response to a gravitational stimulus, characterising syndromes with symptoms of orthostatic intolerance. In Vaso vagal Syncope, an increase or a reduction of cardiac and vascular sympathetic modulation have been described in occasional and habitual fainters, respectively. Pure Autonomic Failure (PAF) is characterized by a global cardiovascular denervation. Accordingly, the spectral markers of cardiac and vascular sympathetic modulation are absent or reduced. However, a concomitant vagal diminished activity is present. In Chronic Orthostatic Intolerance (COI), the most common form of dysautonomia in young female, an abnormal regional distribution of sympathetic discharge has been hypothesized during standing. Indeed, an overall increased sympathetic activity is present in recumbent position; during tilt a blunted vascular sympathetic discharge, with a concomitant exaggerated cardiac sympathetic modulation, is evident. Baroreflex Failure is a syndrome that may result from neck surgery or irradiation due to different forms of regional cancer. It is characterized by a volatility of blood pressure and heart rate, without habitual orthostatic hypotension. In the present paper, we describe a case of Baroreflex Failure with marked orthostatic hypotension in spite of a huge muscle sympathetic nerve activity (MSNA) and high levels of plasma cathecolamines. The most relevant finding was the absence of any coordinate rythmicity in blood pressure, heart rate and MSNA, both at rest and during tilt, particularly in the frequency band likely to be related with sympathetic modulation, i.e. at 0.1 Hz. We hypothesize that the absence of 0.1 Hz spontaneous fluctuations might play a role in sustaining orthostatic hypotension.

Elevated parasympathetic nerve tone in isoproterenol-induced neurally mediated syncope during head-up tilt testing

To clarify the autonomic nerve mechanisms whereby isoproterenol induces vasovagal reaction, 32 patients with recurrent syncopal episodes of unknown origin underwent head-up tilt testing. Syncope was induced in 11 patients by a control tilt (control/+ group). The remaining 21 patients (control/- group) underwent the procedure during isoproterenol infusion. Syncope was induced in 13 (isoproterenol/+ group), but not in the remaining 8 patients (isoproterenol/- group). High-frequency component (HF) and low-frequency component (LF)/HF, which are indices of heart rate variability, were assessed for 2-min intervals at baseline (T1) during the initial phase (T2) and at the end (T3) of tilting in each test. The ratios of HF and LF/HF at T2/T1 and T3/T2 were calculated. The HF value at T3 in the control/+ group was greater than that in the control/- group (1.47+/-0.91 vs 0.75+/-0.34; p<0.05). The HF value at T3 in the isoproterenol/+ group was significantly greater than those in the isoproterenol/- group (1.19+/-1.04 vs 0.43+/-0.23; p<0.05). The HF value at T2 in the isoproterenol/+ tended to be greater than in the isoproterenol/- group (1.30+/-0.85 vs 0.66+/-0.53; p=0.07). The ratio of HF for T2/T1 in isoproterenol/+ was greater than in the other groups (2.04+/-1.26 in isoproterenol/+ vs 0.58+/-0.39 in control/-, 0.47+/-0.18 in control/+ and 0.71+/-0.45 in isoproterenol/-; p=0.0001), whereas that of HF for T3/T2 in the control/+ group was greater than in the other groups (1.48+/-0.62 in control/+ vs 0.97+/-0.33 in control/-; p<0.05, 0.79+/-0.35 in isoproterenol/- and 0.88+/-0.37 in isoproterenol/+; p<0.01). No difference was observed in either LF/HF or the ratio of LF/HF among the groups. The early and persistent increase in parasympathetic nerve tone provoked by isoproterenol plays an important role in enhancing the vasovagal reaction during head-up tilt testing.

Distinct hemodynamic profiles in patients with vasovagal syncope: a heterogeneous population

OBJECTIVE

The objective was to investigate mechanisms of vasovagal syncope by identifying laboratory techniques that characterize cardiovascular profiles in patients with vasovagal syncope.

BACKGROUND

The triggering mechanisms of vasovagal syncope are complex. The patient population is likely heterogeneous. We hypothesized that distinct hemodynamic profiles are definable with provocative maneuvers.

METHODS

Three groups of subjects were matched for age and gender: 16 patients with a history of syncope and an inducible vasovagal response during passive tilt table testing (70 degrees, 45 min, group I), 16 with a history of syncope, negative passive tilt table testing but positive isoproterenol tilt table testing (0.05 microg/kg per min, 70 degrees, 10 min, group II), and 16 control subjects. Beat-to-beat hemodynamic functions were determined noninvasively by photo-plethysmography and impedance cardiography.

RESULTS

At baseline, hemodynamic functions were not different among the three groups (supine). In response to tilt before any symptoms developed, total peripheral resistance decreased 9% +/- 14% in group I from baseline supine to tilt position but increased 27% +/- 18% in group II and 28% +/- 17% in controls (p < 0.001). Responses to isoproterenol were not significantly different between group II and controls in supine position. In response to tilt during isoproterenol infusion before any symptoms developed, total peripheral resistance decreased 24% +/- 20% in group II and increased 20% +/- 48% in controls (p = 0.002).

CONCLUSIONS

Group I patients may have impaired ability to increase vascular resistance during orthostatic stress. The inability to overcome isoproterenol-induced vasodilatation during tilt is important in triggering a vasovagal response in group II patients. These data suggest that the population with vasovagal response is heterogeneous. Distinct hemodynamic profiles in response to various provocative maneuvers are definable with noninvasive, continuous monitoring techniques.

24-hour heart rate variability in patients with vasovagal syncope

Since alterations in the autonomic nervous system are thought to play a major role in the pathogenesis of vasovagal syncope, we characterized the chronic autonomic profile of 44 patients with syncope and 20 healthy subjects by means of heart rate variability using 24-hour Holter recordings (time- and frequency-domain indexes), and evaluated whether the different types of responses to tilting (vasodepressive versus cardioinhibitory) could be associated with different cardiac autonomic patterns. Twenty-three patients exhibited a positive response to tilting, which was vasodepressive in 11 patients and cardioinhibitory in 12 patients. All vasodepressive patients had a standard deviation of the averages of NN (SDANN) intervals in all 5-minute segments lower than 100 ms. Patients with vasodepressive syncope also had significantly lower values of RMSSD (the 24-hour square root of the mean of the sum of the squares of differences between adjacent normal RR intervals) than those with cardioinhibitory response, and lacked the day-night rhythm of the low frequency/high frequency ratio. However, only SDANN values correctly identified patients with vasodepressive response to tilting. We conclude that (1) the population of patients with vasovagal syncope is heterogeneous, (2) patients with vasodepressive syncope have a peculiar chronic autonomic profile as assessed by 24-hour heart rate variability analysis, and (3) the evaluation of the autonomic profile in 24-hour Holter recordings could be of value in the diagnosis of patients with syncope.

Frequency-domain heart rate variability in 24-hour Holter recordings: role of spectral method to assess circadian patterns and pharmacological autonomic modulation

Different spectral methodologies for heart rate variability were recently shown to provide the same qualitative results in the context of passive tilt test. However, the impact of the method and the use of normalized power units in long-term ECG monitoring is still debated. Autoregressive and Fast Fourier transform (FFT) spectral approaches were applied to assess circadian modulation and the effect of beta-blocker administration in mild hypertensive patients who underwent continuous ambulatory ECG recording (n = 44, 51 +/- 12 years, 30 men). Spectral analysis was applied to 5-minute sequences and spectral parameters representative of each circadian period (24 hour, day, night) were calculated. In baseline recordings, FFT spectral method provided a smaller estimate of total and very low frequency powers. On the contrary, low- and high-frequency components were systematically larger with FFT. Circadian variations were in favor of an increased overall nocturnal variability but of a reduced low frequency normalized power with both spectral methods. Chronic oral administration of beta-blocker induced an increase of all spectral components except for an unchanged low-frequency normalized power, independently from the spectral approach. In spite of quantitative differences, the qualitative assessment of circadian patterns and beta-blockade effect by autoregressive- and FFT-based spectral analyses is equivalent. The low-frequency component of heart rate variability cannot be considered a reliable direct marker of sympathetic activity in long-term ambulatory ECG recording.

Neurally mediated syncope induced by lung cancer--a case report

The authors present hemodynamic and autonomic features of recurrent and episodic neurally mediated syncope in a man with lung cancer involving afferent vagus. He revealed extreme hypotension with bradycardia occurring during sitting or standing. A head-up tilt test also induced syncope. However, syncope attacks no longer occurred 2 weeks after admission. Alternatively, the paralyses of the left recurrent laryngeal nerve and the left phrenic nerve developed. It is suggested that the lung cancer involved upper rootlets of the left vagus and caused transient hypersensitivity of baroreceptor function that resulted in neurally mediated syncope.

Frequency-domain analysis of heart rate variability during positive and negative head-up tilt test: importance of age

The study of autonomic behavior during a head-up tilt test (HUT) has been deemed important to understand the loss of consciousness mechanism. Though HRV in patients with HUT(+) and HUT(-) has been compared, few trials emphasized the importance of age. HRV in frequency domain was analyzed based on 5-minute samples in the supine position, and between 5 and 10 minutes during early tilt test (R1) in 102 patients with one or more episodes of syncope (mean age 44.3 +/- 20.8, range 15-85 years, 55 women). Two subgroups were selected afterwards: (1) young patients between 15 and 35 years of age (41 patients) and (2) elderly patients aged 60 or more (36 patients). The following parameters were taken into account: the sum of low (LF) and high frequency (HF) (LF and HF in absolute values and in normalized units), the LF/HF ratio (L/H ratio), and the percentage of change between baseline and R1 values. The HRV behavior in young and elderly patients with positive and negative HUT was established. We then analyzed the correlation between HRV and age and HUT outcome. A multiple regression analysis encompassing age, HUT outcome, gender, and number of syncope episodes was performed. In young patients, the LF and HF areas and the L/H ratio changed significantly between baseline and R1. The L/H ratio increases from baseline to R1. Conversely, these differences were not significant in the elderly. No differences between HUT(+) and HUT(-) within the same age group were observed. Age related significantly to practically all HRV parameters analyzed, whereas the tilt test outcome correlates poorly with HF normalized units and LF normalized units during R1, and the L/H ratio changes between baseline and R1. By means of a multivariate analysis, only age shows a significant correlation with the HRV values. Despite an all age triggering of vasovagal syncope during HUT, the young and elderly patients' autonomic behavior differs. The young considerably increase their sympathovagal balance during HUT, whereas the elderly have a mitigated autonomic response. No significant differences were observed during the first minutes of the test between those with a HUT(+) and those with a HUT(-) within the same age group. Age, and not the HUT response, is the major determinant of the autonomic behavior during early HUT.

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.

Differences in heart rate variability between cardioinhibitory and vasodepressor responses to head-up tilt table testing

BACKGROUND

Patients with syncope show different responses to head-up tilt (HUT) test, which may be due to different pathophysiological mechanisms.

METHODS

HUT (70 degrees) was performed in 24 patients who experienced recurrent syncope. Nine patients had a cardioinhibitory (CI) response, 7 patients had a vasodepressor (VD) response, and 8 patients had a mixed (MX) response. Heart rate variability was analyzed at 60-sec periods during HUT.

RESULTS

Total spectrum (TS) was greater at rest and 1 min after syncope in the CI and MX groups as compared to the VD group. Low frequency spectrum (LF) was significantly greater during rest and the first minute after syncope in the CI groups as compared with the VD group. After the rest period, the CI and MX groups showed more elevated high frequency spectrum (HF) values than the VD group (p < 0.01). One minute after syncope, the HF increased in the CI and MX groups but not in the VD group (p < 0.01). The VD group showed higher LF/HF ratio from the beginning of rest (3.9 +/- 4.1) as compared to the CI and MX groups (p < 0.01). This difference was most significant 2 min before syncope occurred. The CI and MX groups showed greater pNN50 and rMSSD as compared to the VD group.

CONCLUSIONS

Our results suggest that vagal tone is higher in subjects showing cardioinhibitory and mixed responses to HUT. In contrast, patients with a vasodepressor response showed predominantly sympathetic activity. These findings suggest that there are different pathophysiological mechanisms underlying syncope.

Relationship between blood pressure and heart-rate variability during graded head-up tilt

To investigate the relationship between change in blood pressure and autonomic nerve activity, two types of head-up tilt experiments were performed. One was a 30 degrees, 45 degrees, 60 degrees, and 90 degrees graded tilt-up, in which tilt angles were changed at 6-min intervals and 5 min were spent at each angle. The other was a 10-min lasting head-up tilt at 60 degrees. Electrocardiogram (ECG) and blood pressure (Finapres) of 18 healthy non-smoking subjects (9 men, 9 women) were recorded during the experiments. Heart-rate variability was examined by general spectral analysis (GSA). The high-frequency/total-area ratio (HF/TO) showed a decrease as the tilt angle increased. Compared with the values at the 0 degrees position, these changes were statistically significant (P < 0.05). The low-frequency/HF ratio (LF/HF) showed a significant (P < 0.05) difference between 0 degrees and 90 degrees, and between 30 degrees and 90 degrees. Some of the subjects could not maintain their blood pressure during either of the head-up tilt experiments, and they showed only a slight change in HF/TO and LF/HF. This result confirmed that immediate responses to head-up tilt reflect autonomic nerve activity. Hence, changes in the frequency components were found to be an index of autonomic nerve activity, and they explained the individual differences observed in the ability to control blood pressure during a transition to upright posture.

Cardiac autonomic patterns preceding occasional vasovagal reactions in healthy humans

BACKGROUND

The wide range of clinical presentation of orthostatic vasovagal syncope suggests different underlying changes in the cardiac autonomic modulation.

METHODS AND RESULTS

To evaluate the beat-by-beat modifications in the neural control of heart period preceding a syncopal event, we studied RR interval variability in 22 healthy subjects who experienced fainting for the first time during a 90 degrees head-up tilt and in 22 control subjects by means of time-variant power spectral analysis. Sympathetic and vagal modulations to the sinoatrial node were assessed by the normalized power of the low-frequency (LF, approximately 0.1-Hz) and high-frequency (HF, approximately 0.25-Hz) oscillatory components of RR variability. When the patients were supine, no differences were observed in the hemodynamic and spectral parameters of the 2 groups. During the tilt procedure, RR, LFNU, and HFNU (NU=normalized units) values were relatively stable in control subjects. During early tilt (T1), subjects with syncope had reduced RR intervals compared with control subjects. In 13 subjects with syncope, RR decreased while LFNU and LF/HF increased in the last minute of tilt before syncope (T2). Conversely, in the remaining 9 fainters, LFNU and LF/HF decreased from T1 to T2 and HFNU increased slightly.

CONCLUSIONS

Two different patterns may be recognized in the cardiac autonomic changes preceding an occasional vasovagal event, namely, one characterized by a progressive increase of the marker of cardiac sympathetic modulation up to the onset of syncope, the other by a sympathetic inhibition with an impending vagal predominance. The recognition of different pathophysiological mechanisms in fainters may have important therapeutic implications.

Neurally mediated hypotension and autonomic dysfunction measured by heart rate variability during head-up tilt testing in children with chronic fatigue syndrome

Recent investigations suggest a role for neurally mediated hypotension (NMH) in the symptomatology of chronic fatigue syndrome (CFS) in adults. Our previous observations in children with NMH and syncope (S) unrelated to CFS indicate that the modulation of sympathetic and parasympathetic tone measured by indices of heart rate variability (HRV) is abnormal in children who faint during head-up tilt (HUT). In order to determine the effects of autonomic tone on HUT in children with CFS we performed measurements of HRV during HUT in 16 patients aged 11-19 with CFS. Data were compared to 26 patients evaluated for syncope and with 13 normal control subjects. After 30 minutes supine, patients were tilted to 80 degrees for 40 minutes or until syncope occurred. Time domain indices included RR interval, SDNN, RMSSD, and pNN50. An autoregressive model was used to calculate power spectra. LFP (.04-.15 Hz), HFP (.15-.40Hz), and TP (.01-.40Hz). Data were obtained supine (baseline) and after HUT. Thirteen CFS patients fainted (CFS+, 5/13 pure vasodepressor syncope) and three patients did not (CFS-). Sixteen syncope patients fainted (S+, all mixed vasodepressor-cardioinhibitory) and 10 did not (S-). Four control patients fainted (Control+, all mixed vasodepressor-cardioinhibitory) and nine did not (Control-). Baseline indices of HRV were not different between Control+ and S+, and between Control- and S-, but were depressed in S+ compared to S-. HRV indices were strikingly decreased in CFS patients compared to all other groups. With tilt, SDNN, RMSSD, and pNN50 and spectral indices decreased in all groups, remaining much depressed in CFS compared to S or control subjects. With HUT, sympathovagal indices (LFP/HFP, nLFP, and nHFP) were relatively unchanged in CFS, which contrasts with the increase in nLFP with HUT in all other groups. With syncope RMSSD, SDNN, LFP, TP, and HFP increased in S+ (and Control+), suggesting enhanced vagal heart rate regulation. These increases were not observed in CFS+ patients. CFS is associated with NMH during HUT in children. All indices of HRV are markedly depressed in CFS patients, even when compared with already low HRV in S+ or Control+ patients. Sympathovagal balance does not shift toward enhanced sympathetic modulation of heart rate with HUT and there is blunting in the overall HRV response with syncope during HUT. Taken together these data may indicate autonomic impairment in patients with CFS.

Heart rate variability in passive tilt test: comparative evaluation of autoregressive and FFT spectral analyses

The dynamic response of the autonomic nervous system during tilting is assessed by changes in the low (LF) and high frequency (HF) components of the RR series power spectral density (PSD). Although results of many studies are consistent, some doubts related to different methodologies remain. Specifically, the respective relevance of autoregressive (AR) and fast Fourier transform (FFT) methods is often questioned. Beat-to-beat RR series were recorded during 90 degrees passive tilt in 18 healthy subjects (29 +/- 5 years, eight females). FFT-based (50% overlap, Hanning window) and AR-based (Levinson-Durbin algorithm) PSDs were calculated on the same RR intervals. Powers in very low frequency (VLF: < 0.04 Hz), LF (0.04-0.15 Hz), and HF (0.15-0.40 Hz) bands were calculated either by spectrum integration (FFT and ARIN), by considering the highest AR component in each band (ARHP), or by summation of all AR components (ARAP). LF and HF raw powers (ms2) were normalized by total power (%P) and by total power after removal of the VLF component (nu). AR and FFT total powers were not different, regardless of body position. In supine condition, when compared to ARHP and ARAP, FFT underestimated VLF and overestimated LF, whereas in tilt position FFT overestimated HF and underestimated LF. However, supine/tilt trends were consistent in all methods showing a clear reduction of HF and a less marked increase of LF. Both normalization procedures provided a significant LF increase and further magnified the HF decrease. Results obtained with ARIN were remarkably close to those obtained with FFT. In conclusion, significant differences between AR and FFT spectral analyses do exist, particularly in supine position. Nevertheless, dynamic trends provided by the two approaches are consistent. Normalization is necessary to evidence the LF increase during tilt.

Reproducibility of tilt table testing in patients with vasovagal syncope and its relation to variations in autonomic nervous system activity

To assess the variability of head-up tilt table testing, 35 patients with vasovagal syncope, shown by a positive tilt table test, underwent a second test 1 week later. Also, on the day before each test, spectral and time-domain indexes of heart rate variability were derived from Holter recordings to examine the stability of autonomous nervous system activity in these patients. Fifteen healthy volunteers served as a control group and also underwent two tilt table tests with prior Holter monitoring. Twenty-one (60%) of the 35 patients had a second positive test. None of the patients in the control group experienced syncope during either of the tests. The heart rate variability measures in the control group varied slightly from 1 day to the other, in contrast to the syncopal patients, where only low frequency spectral power and the mean of all 5-minute standard deviations of RR intervals showed comparable behavior. The indexes which reflect parasympathetic activity exhibited significant fluctuations in the syncopal patients. These fluctuations were due entirely to the patients who did not reproduce the outcome of the tilt table test, where high parasympathetic tone was associated with the positive test and normal parasympathetic tone with the negative test. In contrast, the patients with two positive tests had high parasympathetic tone during both test periods, with low individual variability. In conclusion, patients with vasovagal syncope show variations in vagal autonomic tone and appear to be more prone to syncope when their parasympathetic tone is elevated.

Spectral analysis of heart rate variability during tilt-table testing in patients with vasovagal syncope

Spectral analysis of heart rate variability was used to assess changes in autonomic function in 44 patients with vasovagal syndrome and 20 normal controls before and during postural tilt and to attempt to relate such changes to specific types of haemodynamic response to tilt. Frequency domain measurements of the high (HF) and low (LF) frequency bands and the ratio LF/HF were derived from Holter recordings, computed by Fast Fourier Analysis for 4 min intervals immediately before tilt testing, immediately after tilting and just before the end of the test. In the syncopal patients the mean values of LF and HF decreased significantly in response to tilting, while the LF/HF ratio remained constant. All parameters showed a statistically significant increase just before the onset of syncope. In the control group there was an increase in the LF and LF/HF ratio and a decrease in the HF immediately after tilting. The three subgroups of patients had similar patterns of changes in autonomic activity. The results of this study show that syncopal patients have a different pattern of response to the tilting test. The pathological mechanism leading to vasovagal syncope appears to be independent of the specific type of haemodynamic response to tilt testing.

Review article: heart rate and blood pressure control in vasovagal syncope

Vasovagal syncope is characterized by transient failure of usually reliable physiologic mechanisms responsible for maintaining both systemic arterial pressure and cerebral blood flow. Two circulatory phenomena are almost universally present: systemic arterial vasodilation and bradycardia. A third phenomenon, cerebrovascular constriction, has also been described but its contribution to the faint is less well established. The neural reflex pathways responsible for triggering the circulatory changes in the vasovagal faint are incompletely understood, but have recently been the subject of renewed interest. In part, this interest probably stems from the frequency with which vasovagal symptoms are now recognized to be the cause of fainting spells. Additionally, however, there is an increasingly recognized need to develop treatment strategies for those affected patients in whom recurrent vasovagal symptoms are particularly troublesome. It is the goal of this discussion to focus on those aspects of circulatory control, and in particular on potential interactions among certain neural and humoral systems, which may contribute to the inappropriate physiologic responses associated with the vasovagal faint.