Assessment of Quadratic Nonlinear Cardiorespiratory Couplings During Tilt-Table Test by Means of Real Wavelet Biphase.

OBJECTIVE: In this paper, a method for assessment of quadratic phase coupling (QPC) between respiration and heart rate variability (HRV) is presented. METHODS: First, a method for QPC detection is proposed named real wavelet biphase (RWB). Then, a method for QPC quantification is proposed based on the normalized wavelet biamplitude (NWB). A simulation study has been conducted to test the reliability of RWB to identify QPC, even in the presence of constant delays between interacting oscillations, and to discriminate it from quadratic phase uncoupling. Significant QPC was assessed based on surrogate data analysis. Then, quadratic cardiorespiratory couplings were studied during a tilt-table test protocol of 17 young healthy subjects. RESULTS: Simulation study showed that RWB is able to detect even weak QPC with delays in the range of [Formula: see text] s, which are usual in the autonomic nervous system (ANS) control of heart rate. Results from the database revealed a significant reduction ([Formula: see text]) of NWB between respiration and both low and high frequencies of HRV in head-up tilt position compared to early supine. CONCLUSION: The proposed technique detects and quantifies robustly QPC and is able to track the coupling between respiration and various HRV components during ANS changes. SIGNIFICANCE: The proposed method can help to assess alternations of nonlinear cardiorespiratory interactions related to ANS dysfunction and physiological regulation of HRV in cardiovascular diseases.

The Value of Tilt Testing and Autonomic Nervous System Assessment.

Tilt testing for the investigation of syncope was introduced in 1986. Since then, it has propagated worldwide as a routine test to determine the cause of transient loss of consciousness when that event has not been diagnosed by history, including that of a witness, physical examination, supine and erect blood pressures, and 12-lead electrocardiogram. Tilt testing allows reproduction of syncope with monitoring of physiologic parameters including electrocardiogram, beat-to-beat blood pressure, electroencephalogram, and middle cerebral artery blood-flow velocity. As a result, much has been learned about syncope.

[Assessment of vasovagal origin of syncope based on history features].

Study aim was to elaborate questionnaire for diagnostics of vasovagal syncope (VVS) based on data of anamnesis. We examined 182 patients (mean age 37.1+/-14.3 years, 78 men, 104 women). Initial examination included anamnesis, physical examination, electrocardiography at rest, measurement of blood pressure in orthostasis. Each patient was asked 82 questions describing duration of symptoms, characteristics of episodes of loss of consciousness, symptoms of prodromal period before loss on consciousness and in the period of recovery. As a standard method of VVS diagnostics of we used tilt test (TT) according to Westminster or Italian protocols. Using methods of statistical analysis we created questionnaire for diagnostics of VVS. Tilt-positive group comprised 108 patients (age 35.9+/-14.6 years, 45% men), tilt-negative group comprised 74 patients (age 36.1+/-14.3 years, 39% men). Of 82 testing questions 8 had probability ratio (PR) >1 and were significant predictors (<0.05) of positive TT. Seven questions had <1 and were significant predictors (<0.05) of negative TT. These questions were included into logistical regression analysis. The final variant of the questionnaire comprises 15 vasovagal origin questions. Total score necessary for diagnosis of VVS is more or equal 1. Sensitivity of questionnaire for prediction of positive result of TT was 95%, specificity - 57%. This allows to using it as a screening test for selection of further method of investigation in patients with episodes of loss of consciousness.

Assessment of the dynamic interactions between heart rate and arterial pressure by the cross time-frequency analysis.

In this study, a framework for the characterization of the dynamic interactions between RR variability (RRV) and systolic arterial pressure variability (SAPV) is proposed. The methodology accounts for the intrinsic non-stationarity of the cardiovascular system and includes the assessment of both the strength and the prevalent direction of local coupling. The smoothed pseudo-Wigner-Ville distribution (SPWVD) is used to estimate the time-frequency (TF) power, coherence, and phase-difference spectra with fine TF resolution. The interactions between the signals are quantified by time-varying indices, including the local coupling, phase differences, time delay, and baroreflex sensitivity (BRS). Every index is extracted from a specific TF region, localized by combining information from the different spectra. In 14 healthy subjects, a head-up tilt provoked an abrupt decrease in the cardiovascular coupling; a rapid change in the phase difference (from 0.37 ± 0.23 to -0.27 ± 0.22 rad) and time delay (from 0.26 ± 0.14 to -0.16 ± 0.16 s) in the high-frequency band; and a decrease in the BRS (from 23.72 ± 7.66 to 6.92 ± 2.51 ms mmHg(-1)). In the low-frequency range, during a head-up tilt, restoration of the baseline level of cardiovascular coupling took about 2 min and SAPV preceded RRV by about 0.85 s during the whole test. The analysis of the Eurobavar data set, which includes subjects with intact as well as impaired baroreflex, showed that the presented methodology represents an improved TF generalization of traditional time-invariant methodologies and can reveal dysfunctions in subjects with baroreflex impairment. Additionally, the results also suggest the use of non-stationary signal-processing techniques to analyze signals recorded under conditions that are usually supposed to be stationary.

Frequency domain assessment of the coupling strength between ventricular repolarization duration and heart period during graded head-up tilt.

We test the hypothesis that the degree of correlation between ventricular repolarization duration (VRD) and heart period (HP) carries information on cardiac autonomic regulation. The degree of correlation was assessed in the frequency domain using squared coherence function during an experimental protocol known to gradually induce a shift of sympathovagal balance toward sympathetic predominance (ie, graded head-up tilt). We observed a progressive decrease of squared coherence with tilt table inclination, thus confirming the working hypothesis. The VRD-HP uncoupling occurs in the high-frequency band, centered on the respiratory rate, thus suggesting that vagal withdrawal is responsible for the VRD-HP uncoupling.

Dynamic assessment of spontaneous baroreflex sensitivity by means of time-frequency analysis using either RR or pulse interval variability.

In this study we propose a method to continuously assess the changes of spontaneous baroreflex sensitivity (BRS). Systolic arterial pressure and RR intervals are analyzed by time-frequency analysis to estimate their instantaneous powers as well as the time-course of their spectral coherence. The BRS estimated in classical frequency bands is compared to the BRS estimated in dynamic frequency bands centered on respiratory frequency. The possibility of obtaining reliable estimations of the BRS using the pulse interval from the pressure signal as a surrogate of the RR is considered. Results on a tilt table test database suggest that is possible to obtain reliable BRS estimates just from the analysis of the pressure signal, without the need of ECG recordings.

What is the optimal duration of tilt testing for the assessment of patients with suspected postural tachycardia syndrome?

AIMS: The aim of this study is to define the optimal duration of tilt testing for the assessment of patients with suspected postural tachycardia syndrome (POTS). METHODS AND RESULTS: This was a case-control study. Cases were identified retrospectively from a database of patients referred with orthostatic intolerance (OI). All met the diagnostic criteria for POTS. Controls were enrolled prospectively. All subjects underwent tilting to 70 degrees for 40 min if tolerated. Continuous monitoring was provided by a Finometer. Analysis of responses to tilting was performed on 28 cases and 28 controls. The mean age in the case group was 23.6 and in the control group was 26.2. The majority was female in both groups (cases = 4F:3M, controls = 2F:1M). All cases met the criteria for POTS within 7 min of orthostasis. No controls demonstrated a sustained tachycardia. The prevalence of vasovagal syncope (VVS) was 36% in cases vs. 7% in controls (P = 0.02) and 25% in the remaining patients (n = 233) on the OI database (P = 0.259). CONCLUSION: A 10 min tilt will diagnose POTS in the majority of patients. It will not, however, be sufficient to identify the overlap that exists between POTS and VVS. The optimal duration of tilt testing in patients suspected of POTS is 40 min.

Tilt table exercise echocardiography assessment in the diagnosis of coronary artery disease.

OBJECTIVES: To assess the diagnostic performance of per-exercise echocardiography using a recent tilt table to investigate for coronary artery disease with a significance threshold for stenosis > or =70%. METHODS: 104 consecutive patients (93 men) referred for elective coronary angiography underwent tilt table exercise echocardiography in which second harmonic imaging was used systematically. Images were recorded and interpreted by the same operator. RESULTS: The investigation was contributory in 81 patients (target HR at least 85% of the age-predicted maximal HR or criterion for discontinuation). The most common reason for a non-contributory investigation (negative investigation but submaximal or uninterpretable investigation) was submaximal exercise (91%). The prevalence of significant coronary artery disease in the cohort of 81 contributory investigations was 38%. Sensitivity and specificity to detect significant lesions was 90% and 94% respectively. The positive and negative predictive values were 90% and 94% respectively. The three false negatives involved distal lesions. The three false positives occurred in patients with regional wall motion abnormalities in the context of various heart diseases (severe mitral incompetence, diabetic and hypertensive cardiomyopathy, left bundle branch block). CONCLUSION: Using a high performance echocardiography machine with second harmonic imaging per-exercise tilt table echocardiography offers excellent diagnostic performance to detect significant coronary artery disease under everyday clinical practice conditions.

Probabilistic assessment of Autonomic Nervous System fluctuations during tilt table tests.

A number of reports have advocated the use of Heart Rate Variability (HRV) as a non invasive method of monitoring the Autonomic Nervous System (ANS). In the anesthesia and critical care monitoring settings, the development of an instrument able to provide real-time information about the ANS state at different stages of any procedure would provide improved safety for patients undergoing diagnostic or therapeutic interventions. However, real-time analysis of HRV can be particularly challenging since larger effective lengths of observation provide better spectral resolution. Our study explores a probabilistic approach that analyzes changes in HRV parameters obtained from an autoregressive (AR) model technique using Burg's methods to evaluate very short observation windows while preserving appropriate frequency resolution. These HRV parameters are continuosly compared to a baseline state, and a probability trend is updated during provocative maneuvers. Preliminary results show that trends from classical parameters such as RMSSD and LFn are consistent and reliable instruments capable of providing significant information about ANS fluctuations in a timely fashion.

Assessment of orthostatic fluid shifts with strain gauge plethysmography.

We evaluated the use of strain gauge plethysmography (SGP) for the assessment of orthostatic fluid shifts during head up tilt (HUT). Subjects wore a parachute harness fixed to the tilt table to avoid muscle tension in the lower limbs during HUT. 22 Healthy subjects (9 women) were tilted for 5 minutes. Calf volume changes as measured by SGP, surface EMG, heart rate and blood pressure were measured continuously. Ten subjects underwent a second tilt test during which circulation in one leg was occluded with a pressure cuff at 250 mmHg. During HUT with occlusion, calf volume increased in the non-occluded leg by 1.9+/-0.3% (mean +/- SEM) and 0.2+/-0.2% in the occluded leg (p<0.001). During HUT without occlusion a significant correlation (r = 0.9) was found between measurements of the left and right leg with a mean difference of 0.03+/-0.1%. HUT did not cause significant changes of surface EMG. An unexpected gender effect was found: calf volume increased significantly more in men than in women. Men were significantly taller, but the hemodynamic response to HUT did not differ between both sexes. The gender effect on orthostatic increases of calf volume remained significant after adjustment for heart-to-calf distance. SGP during HUT with a parachute harness is a new, promising method to assess orthostatic fluid shifts. The gender differences in orthostatic pooling in the calf may be explained by a higher calf compliance in men together with a greater hydrostatic pressure due to a greater height in men.

Assessment of cardiac autonomic modulation during graded head-up tilt by symbolic analysis of heart rate variability.

Two symbolic indexes, the percentage of sequences characterized by three heart periods with no significant variations (0V%) and that with two significant unlike variations (2UV%), have been found to reflect changes in sympathetic and vagal modulations, respectively. We tested the hypothesis that symbolic indexes may track the gradual shift of the cardiac autonomic modulation during an incremental head-up tilt test. Symbolic analysis was carried out over heart period variability series (250 cardiac beats) derived from ECG recordings during a graded head-up tilt test (0, 15, 30, 45, 60, 75, and 90 degrees ) in 17 healthy subjects. The percentage of subjects showing a significant linear correlation (Spearman rank-order correlation) with tilt angles was utilized to evaluate the performance of symbolic analysis. Spectral analysis was carried out for comparison over the same series. 0V% progressively increased with tilt angles, whereas 2UV% gradually decreased. The decline of 2UV% was greater than the increase of 0V% at low tilt angles. Linear correlation with tilt angles was exhibited in a greater percentage of subjects for 0V% and 2UV% than for any spectral index. Our findings suggest that symbolic analysis performed better than spectral analysis and, thus, is a suitable methodology for assessment of the subtle changes of cardiac autonomic modulation induced by a graded head-up tilt test. Moreover, symbolic analysis indicates that the changes of cardiac sympathetic and vagal modulations observed during this protocol were reciprocal but characterized by different absolute magnitudes.

[Assessment of arterial compliance and elasticity during graded head-up tilt in healthy people]. / Ocena podatnosci i eltancji tetniczej czasie stopniowanej pionizacji zdrowych.

UNLABELLED: Head-up tilting causes many changes in cardiovascular system (CVS), and stimulates sympathetic nervous system, which leads to increase of total peripheral resistance (TPR) and acceleration of heart rate (HR). The hemodynamic changes influence pulse wave shape and mechanical properties of the arterial system. Arterial compliance (C) and elastance (Ea) are parameters used to describe mechanical properties of the arterial system. The study aim was to evaluate arterial stiffness by evaluation of arterial elastance and compliance during graded head-up tilting and assessment of the relationship between the change of the tilt angle and those parameters in healthy young subjects. MATERIAL AND METHODS: The study was conducted in 32 healthy volunteers (21-30 years old, 17 female). After 20 minutes of supine rest, the head-up tilting was performed with the use of a tilt-table with electrical engine. The tilt-table was stopped for 1 minute after reaching the angle of 15 degrees and its multiples (i.e. 30, 45, 60, 75 and 90 degrees). Hemodynamic parameters were measured by means of chest bioelectrical impedance method (BioZcom monitor, CardioDynamics, USA). The association of the sine of the tilt angle with Ea and C was calculated with the use of Pearson correlation. RESULTS: A strong positive correlation between the sine of the tilt angle and Ea was noted (r = 0. 5084; p < 0.0001). A negative correlation between the change of the tilt angle during head-up tilting and C was found (r = -0.4608; p < 0.0001). CONCLUSION: During graded head-up tilting, arterial stiffness increases in young healthy people.

Assessment of autonomic activity during daily life of patients with head-up tilt-induced prolonged asystole.

Autonomic activity is important in the pathophysiology of neurally mediated syncope (NMS) patients diagnosed by the head-up tilt (HUT) test, and prolonged systole induced by HUT is associated with a malignant vasovagal syndrome. This study investigated whether or not daily autonomic activity evaluated by heart rate variability (HRV) assessed through 24-hour ambulatory ECG monitoring was augumented in HUT-induced asystole (AS; > 30 s) and whether or not HRV differed between NMS patients with and without AS. Ambulatory ECG monitoring was performed in 176 patients with suspected NMS and in 62 age-matched healthy control subjects (C group, age: 36 +/- 13 yr), with HRV time and frequency domain indices measured within 48 hours of HUT. Six patients displayed prolonged AS (PAS group, age: 33 +/- 12 yr), and 109 patients had positive HUT (P group, age: 29 +/- 17 yr), while 61 patients had negative HUT (N group, age: 40 +/- 18 yr). Several HRV indices in the P group were higher than those in the N and C groups. However, HRV indices did not differ between the P group and the PAS group. It is concluded that impairment of autonomic activity specifically related to orthostatic stress could be important in the pathophysiology of HUT-induced prolonged asystole, while impairment of autonomic activity in daily life is not as important in NMS patients with HUT-induced prolonged asystole.

Lumbosacral position sense during pelvic tilting in men and women without low back pain: test development and reliability assessment.

STUDY DESIGN: A single group test-retest design to evaluate the reproducibility of lumbosacral position sense measurements. OBJECTIVES: To develop a measure of position sense in the lumbosacral area and to determine test-retest reliability. BACKGROUND: Proprioception, muscle control, and coordination training could be the key issues in resolving neuromuscular dysfunction in patients with low back pain, but there are no standard ways to assess these parameters. METHODS AND MEASURES: A piezoresistive accelerometer attached to the skin over the sacrum was used to research the repositioning accuracy of active pelvic tilting, between days, of 14 young nonimpaired subjects (20 to 26 years of age) in standing. RESULTS: The mean absolute error for repositioning accuracy (the difference between criterion and matching positions) was 1.81 degrees (+/- 0.85). The intraclass correlation coefficient between measurements obtained on days 1 and 2 was moderate (R = 0.51). The average standard error of measurement associated with the intraclass correlation coefficient was 0.5 degree (95% confidence interval = +/- 0.99 degree). CONCLUSIONS: These findings suggest that the proposed test is sensitive with moderate test-retest reliability to examine lumbosacral position sense in healthy subjects. Further adjustments in the testing protocol are needed to improve the test-retest reliability.