α-Adrenergic effects on low-frequency oscillations in blood pressure and R-R intervals during sympathetic activation.

The present study was designed to address the contribution of α-adrenergic modulation to the genesis of low-frequency (LF; 0.04-0.15 Hz) oscillations in R-R interval (RRi), blood pressure (BP) and muscle sympathetic nerve activity (MSNA) during different sympathetic stimuli. Blood pressure and RRi were measured continuously in 12 healthy subjects during 5 min periods each of lower body negative pressure (LBNP; -40 mmHg), static handgrip exercise (HG; 20% of maximal force) and postexercise forearm circulatory occlusion (PECO) with and without α-adrenergic blockade by phentolamine. Muscle sympathetic nerve activity was recorded in five subjects during LBNP and in six subjects during HG and PECO. Low-frequency powers and median frequencies of BP, RRi and MSNA were calculated from power spectra. Low-frequency power during LBNP was lower with phentolamine versus without for both BP and RRi oscillations (1.6 ± 0.6 versus 1.2 ± 0.7 ln mmHg(2), P = 0.049; and 6.9 ± 0.8 versus 5.4 ± 0.9 ln ms(2), P = 0.001, respectively). In contrast, the LBNP with phentolamine increased the power of high-frequency oscillations (0.15-0.4 Hz) in BP and MSNA (P < 0.01 for both), which was not observed during saline infusion. Phentolamine also blunted the increases in the LBNP-induced increase in frequency of LF oscillations in BP and RRi. Phentolamine decreased the LF power of RRi during HG (P = 0.015) but induced no other changes in LF powers or frequencies during HG. Phentolamine resulted in decreased frequency of LF oscillations in RRi (P = 0.004) during PECO, and a similar tendency was observed in BP and MSNA. The power of LF oscillation in MSNA did not change during any intervention. We conclude that α-adrenergic modulation contributes to LF oscillations in BP and RRi during baroreceptor unloading (LBNP) but not during static exercise. Also, α-adrenergic modulation partly explains the shift to a higher frequency of LF oscillations during baroreceptor unloading and muscle metaboreflex activation.

Reducing the effect of respiration in baroreflex sensitivity estimation with adaptive filtering.

Cardiac baroreflex is described by baroreflex sensitivity (BRS) from blood pressure and heart rate interval (RRi) fluctuations. However, respiration affects both blood pressure and RRi via mechanisms that are not necessarily of baroreflex origin. To separate the effects of baroreflex and respiration, metronome-guided breathing in a high frequency band (HF, 0.25-0.4 Hz) and a low frequency spectral band (LF, 0.04-0.15 Hz) have therefore been commonly used for BRS estimation. The controlled breathing may, however, change the natural functioning of the autonomic system and interfere BRS estimates. To enable usage of spontaneous breathing, we propose an adaptive LMS-based filter for removing the respiration effect from the BRS estimates. ECG, continuous blood pressure and respiration were measured during 5 min spontaneous and 5 min controlled breathing at 0.25 Hz in healthy males (n = 24, 33+/-7 years). BRS was calculated with spectral methods from the LF band with and without filtering. In those subjects whose spontaneous breathing rate was <0.15 Hz, the BRS(LF) values were overestimated, whereas the adaptive filtering reduced the bias significantly. As a conclusion, the adaptive filter reduces the distorting effect of respiration on BRS values, which enables more accurate estimation of BRS and the usage of spontaneous breathing as a measurement protocol.

ECG-derived respiration methods: adapted ICA and PCA.

Respiration is an important signal in early diagnostics, prediction, and treatment of several diseases. Moreover, a growing trend toward ambulatory measurements outside laboratory environments encourages developing indirect measurement methods such as ECG derived respiration (EDR). Recently, decomposition techniques like principal component analysis (PCA), and its nonlinear version, kernel PCA (KPCA), have been used to derive a surrogate respiration signal from single-channel ECG. In this paper, we propose an adapted independent component analysis (AICA) algorithm to obtain EDR signal, and extend the normal linear PCA technique based on the best principal component (PC) selection (APCA, adapted PCA) to improve its performance further. We also demonstrate that the usage of smoothing spline resampling and bandpass-filtering improve the performance of all EDR methods. Compared with other recent EDR methods using correlation coefficient and magnitude squared coherence, the proposed AICA and APCA yield a statistically significant improvement with correlations 0.84, 0.82, 0.76 and coherences 0.90, 0.91, 0.85 between reference respiration and AICA, APCA and KPCA, respectively.

Impact and management of physiological calibration in spectral analysis of blood pressure variability.

Physiological calibration (Physiocal) improves the quality of continuous blood pressure (BP) signal from finger. However, the effects of Physiocal on spectral characteristics of systolic BP (SBP) variability are not well-known. We tested the hypothesis that the use of Physiocal may alter the results on SBP variability when compared with BP recording without Physiocal. Continuous BP was recorded simultaneously from fingers of both arms during 10-min standing by two Nexfin devices, one with (ON) and the other without (OFF) Physiocal (n = 19). Missing SBP values in ON signal were linearly interpolated over Physiocal sequences (ONinter). The OFF signal was analyzed without any corrections (OFFreference) and after linear interpolation of corresponding sequences when Physiocal appeared in the ON signal (OFFinter). Mean low frequency power of SBP oscillations (LFSBP, 0.04-0.15 Hz) did not differ between the OFFreference, OFFinter, and ONinter. However, LFSBP deviated more from OFFreference when analyzed from ONinter compared with the analysis from OFFinter [median (interquartile range): 14.7 (4.6-38.6) vs. 0.9 (0.5-1.8) %, p < 0.05]. In conclusion, the use of Physiocal had a significant effect on the spectral SBP variability that overwhelms the impact of linear interpolation of short data sequences. Therefore, caution is needed when comparing SBP variability between BP datasets acquired with and without Physiocal.

Impact of type 2 diabetes on cardiac autonomic responses to sympathetic stimuli in patients with coronary artery disease.

Type 2 diabetes (T2D) has shown limited impact on cardiac autonomic function in patients with cardiac disease at rest. The effect of T2D on autonomic responses to sympathetic stimuli, such as passive tilt and static exercise, is not well known in patients with coronary artery disease (CAD). Heart rate, arterial pressure, and their variability along with baroreflex sensitivity (BRS) were analyzed at supine rest and during passive head-up tilt (TILT) and static handgrip exercise (HG) in CAD patients with (T2D+, n=68, 61±6 years, 14 women) and without T2D (T2D-, n=68, 62±6 years, 17 women). The effect of T2D at rest and in responses to TILT and HG was examined. In T2D+, the normalized low-frequency (0.04-0.15 Hz) power of R-R intervals was higher at rest (44±17 vs. 38±17 nu, p=0.015) and its response to TILT and HG was lower than that in T2D- (8±21 vs. 2±17 nu, p=0.041 and 3±18 vs. -4±15 nu, p=0.019, respectively). Vagally mediated heart rate variability indices and BRS were not different between T2D+ and T2D-. We concluded that T2D has a specific impact on low-frequency oscillation of R-R interval among patients with angiographically documented CAD. This may indicate increased basal sympathetic modulation of sinoatrial node and lower sympathetic responsiveness to sympathetic activation by baroreceptor unloading and exercise pressor response. Limited effects of T2D on vagally mediated heart rate variability and baroreflex were observed in the patients with CAD.

Multimodal emotion recognition by combining physiological signals and facial expressions: a preliminary study.

Lately, multimodal approaches for automatic emotion recognition have gained significant scientific interest. In this paper, emotion recognition by combining physiological signals and facial expressions was studied. Heart rate variability parameters, respiration frequency, and facial expressions were used to classify person's emotions while watching pictures with emotional content. Three classes were used for both valence and arousal. The preliminary results show that, over the proposed channels, detecting arousal seem to be easier compared to valence. While the classification performance of 54.5% was attained with arousal, only 38.0% of the samples were classified correctly in terms of valence. In future, additional modalities as well as feature selection will be utilized to improve the results.

Cardiovascular autonomic nervous system function and aerobic capacity in type 1 diabetes.

Impaired cardiovascular autonomic nervous system (ANS) function has been reported in type 1 diabetes (T1D) patients. ANS function, evaluated by heart rate variability (HRV), systolic blood pressure variability (SBPV), and baroreflex sensitivity (BRS), has been linked to aerobic capacity (VO(2peak)) in healthy subjects, but this relationship is unknown in T1D. We examined cardiovascular ANS function at rest and during function tests, and its relations to VO(2peak) in T1D individuals. Ten T1D patients (34 ± 7 years) and 11 healthy control (CON; 31 ± 6 years) age and leisure-time physical activity-matched men were studied. ANS function was recorded at rest and during active standing and handgrip. Determination of VO(2peak) was obtained with a graded cycle ergometer test. During ANS recordings SBPV, BRS, and resting HRV did not differ between groups, but alpha1 responses to maneuvers in detrended fluctuation analyses were smaller in T1D (active standing; 32%, handgrip; 20%, medians) than in CON (active standing; 71%, handgrip; 54%, p < 0.05). VO(2peak) was lower in T1D (36 ± 4 ml kg(-1) min(-1)) than in CON (45 ± 9 ml kg(-1) min(-1), p < 0.05). Resting HRV measures, RMSSD, HF, and SD1 correlated with VO(2peak) in CON (p < 0.05) and when analyzing groups together. These results suggest that T1D had lower VO(2peak), weaker HRV response to maneuvers, but not impaired cardiovascular ANS function at rest compared with CON. Resting parasympathetic cardiac activity correlated with VO(2peak) in CON but not in T1D. Detrended fluctuation analysis could be a sensitive detector of changes in cardiac ANS function in T1D.

HRV and EEG based indicators of stress in children with Asperger syndrome in audio-visual stimulus test.

Asperger syndrome (AS) is a neurobiological condition which is characterized by poor skills in social communication, and restricted and repetitive patterns of behavior and interests. We studied whether stress-related indices of heart rate variability (HRV) and electroencephalography (EEG) are different in children with AS than normal controls. We analyzed retrospectively the data of the test where audiovisual stimuli were used. We hypothesized that this test is a stressful situation for individuals with AS and they would have a greater reaction than control subjects. EEG and one-channel electrocardiography (ECG) were collected for children with diagnosis of AS (N = 20) and their age-matched controls (N = 21). HRV indices, frontal EEG asymmetry index and brain load index were calculated. HRV based indices revealed increased sympathetic activity during the test in children with AS. EEG based indices increased more in children with AS during the test compared to baseline. Thus, the children with AS seems to have a greater reaction to stressful situation.

Sympatho-vagal interaction in the recovery phase of exercise.

Reciprocal autonomic regulation occurs during incremental exercise. We hypothesized that sympatho-vagal interplay may become altered after exercise because of the differences in recovery patterns of autonomic arms. The cardiac vagal activity was assessed by measurement of beat-to-beat R-R interval oscillations using a Poincaré plot method (SD1), and muscle sympathetic nervous activity (MSNA) was measured from peroneus nerve by a microneurography technique during and after exercise in 16 healthy subjects. Autonomic regulation was compared between the rest and after exercise (3·5 ± 1·0 min after exercise) at equal heart rates (HR). SD1 was at the equal level at the recovery phase (40 ± 21 ms) compared to the resting condition (38 ± 16 ms, P = ns) at comparable HR (57 ± 10 for both). MSNA was higher at the recovery phase (40 ± 19 burst per 100 heartbeats) than at rest (25 ± 13 burst per 100 heartbeats, P<0·0001). The difference of MSNA activity between rest and late recovery phase had a strong positive correlation with the difference in SD1 (r = 0·78, P<0·001) at equal HRs. Subjects who have a higher sympathetic activity in the recovery phase of exercise have a more augmented cardiac vagal activity resulting in an accentuated sympatho-vagal outflow. The altered autonomic interaction observed here may partly explain the clustering of various cardiovascular events to the recovery phase of exercise.

Frequency of slow oscillations in arterial pressure and R-R intervals during muscle metaboreflex activation.

Altered frequency of slow (0.04-0.15Hz) arterial pressure and R-R interval oscillations has been observed in various diseases but the mechanisms for this frequency shift are unclear. The median (Med) frequencies of slow R-R interval and blood pressure (BP) oscillations were recorded in 11 healthy subjects with paced breathing (0.25Hz) during muscle metaboreflex and baroreflex mediated sympathetic stimuli: 1) handgrip exercise (HG) followed by post-exercise circulatory occlusion (PECO), 2) handgrip exercise during ischemia by circulatory occlusion (IHG) and 3) passive head-up tilt (TILT). Med(BP) shifted to the higher frequency during HG, PECO and IHG (from 0.070+/-0.009Hz to 0.088+/-0.013, 0.085+/-0.015 and 0.099+/-0.013Hz, respectively, p<0.01 for all) but not during TILT (0.078+/-0.012Hz). Similarly, Med(R-R) shifted to the higher frequency during HG, PECO and IHG (from 0.072+/-0.009Hz to 0.085+/-0.014, 0.085+/-0.016 and 0.095+/-0.015Hz, respectively, p<0.01 for all) but not during TILT (0.075+/-0.012Hz). Med(BP) and Med(R-R) were higher during IHG compared to HG and lower during TILT compared to both HG and IHG (p<0.01 for all). We conclude that the sympathetic stimulus induced by muscle metaboreflex is an important mechanism increasing the frequency of slow oscillations in arterial pressure and R-R intervals. The present results give new insight to understand the physiology underlying the frequency of slow arterial pressure and R-R interval oscillations.

Low-frequency oscillations in R-R interval and blood pressure across the continuum of cardiovascular risk.

The purpose of this study was to assess the power and the frequency of low-frequency (LF; 0.04-<0.15 Hz) oscillations in systolic blood pressure (SBP) and R-R interval (RRi) across the continuum of risk of cardiovascular disease, including age. A potential confound in such determinations is low spontaneous breathing frequency in some individuals. We measured beat-to-beat SBP, RRi and respiration in healthy YOUNG (33±3 years) and OLDER subjects (62±5 years) and older patients with hypertension (HT, 61±5 years), coronary artery disease without (CAD, 62±5 years) and with type 2 diabetes (CAD+DM, 62±4 years, n=28 for all groups) during spontaneous breathing at supine rest. Power (Power(LF)) and median frequency (Med(LF)) of LF oscillations were calculated by power spectral analysis after removing respiratory effects by least-mean-square adaptive filtering. OLDER had higher Power(LF-SBP) (5.5±3.0 vs. 3.4±2.5 mmHg(2), p=0.002) and lower Power(LF-RRi) than YOUNG (339±460 vs. 575±422 ms(2), p=0.001) whereas neither variable differed between OLDER and patient groups. Med(LF-SBP) (0.072±0.009 vs. 0.080±0.011 Hz, p=0.005) and Med(LF-RRi) (0.072±0.010 vs. 0.079±0.013 Hz, p=0.027) were lower in OLDER compared with YOUNG. Compared with OLDER, Med(LF-RRi) was lower in CAD (0.065±0.006 Hz, p=0.015) and CAD +DM (0.066±0.008 Hz, p=0.012); whereas CAD+DM had also lower Med(LF-SBP) (0.065±0.006 Hz, p=0.012). No differences were observed between OLDER and HT and between CAD and CAD+DM in these variables. We concluded that age is major determinant of the power of LF oscillations in SBP and RRi at rest, whereas the median frequency of these oscillations is altered also by coronary artery disease.

Time-frequency representation of cardiovascular signals during handgrip exercise.

Altering cardiovascular oscillation is present during various interventions, e.g. in autonomic tests. Traditional spectral analysis suffers from nonstationary data and poor time resolution when studying the dynamics and frequency of cardiovascular signals. Smoothed pseudo Wigner-Ville representation method was applied when analyzing handgrip (HG) data of healthy men (n = 11). Prevalent low frequency (PLF) and LF/HF powers were estimated from the time-frequency representation (TFR). According to the experimental results, PLF increases during HG with both heart rate interval (RRi) and systolic blood pressure (SBP) data. In addition, TFR revealed the increasing LF power during the HG with both RRi and SBP, while HF power increases with SBP and decreases with RRi. This is interpreted as an altered sympatho-vagal balance during the hand grip protocol.

Muscle sympathetic nerve activity at rest compared to exercise tolerance.

Cardiovascular autonomic function is associated with physical performance and exercise training adaptation. The association between physical performance and sympathetic regulation is not well known. We hypothesized that sympathetic nervous system activity is associated with physical performance among male runners. The study population included 26 healthy male club runners [age 33 +/- 5 years, body mass index (BMI) 24 +/- 1 kg/m(2), VO(2max) 58 +/- 5 ml kg(-1) min(-1); mean +/- SD]. Muscle sympathetic nerve activity (MSNA) was assessed from the peroneal nerve by the microneurography technique during 5 min of supine rest. Physical performance was assessed by time to exhaustion during treadmill running. The mean resting MSNA was 20 +/- 6 bursts min(-1) (range 6-34). The mean time to exhaustion was 1,005 +/- 136 s (range 720-1260). When the study group was divided into tertiles according to their running performance (866 +/- 69, 994 +/- 30 and 1154 +/- 71 s in time to exhaustion, P < 0.0001 between the groups), MSNA was lower (P = 0.032) in the group with the best running performance (16 +/- 5 bursts min(-1)) compared to those with the worst running performance (23 +/- 7 bursts min(-1)). In conclusion, baseline sympathetic activity, measured by a microneurography at rest, may be associated with the maximal running performance of healthy subjects.

Novel spectral indexes of heart rate variability as predictors of sudden and non-sudden cardiac death after an acute myocardial infarction.

BACKGROUND: Various indexes of 24-hour heart rate variability (HRV) have been able to predict all-cause mortality after an acute myocardial infarction (AMI), but their value in predicting specific modes of cardiac death has been limited. AIM: The aim of this study was to assess the role of two novel spectral indexes of HRV as predictors of either sudden (SCD) or non-sudden cardiac death after an AMI. Method. We used two novel methods of spectral analysis of HRV: 1) the high-frequency (HF) spectral component, V(index), calculated as an average HF power from the most linear portion of HF power versus the R-R interval regression curve, and 2) the prevalent low-frequency oscillation of heart rate (PLF). V(index), conventional HRV measures, and PLF were analyzed from 24-hour Holter recordings of 590 patients with a recent AMI. RESULTS: During the mean follow-up of 39+/-14 months, SCD occurred in 3% (n = 17) and non-sudden cardiac death in 5% (n = 28) of the patients. In univariate analysis, V(index) was the most potent predictor of SCD (RR: 6.0, 95% CI: 1.7-20.7, P<0.01), also remaining the most powerful predictor of SCD after adjustment for clinical variables and ejection fraction (RR: 4.2, 95% CI: 1.2-15.2, P<0.05). PLF was a potent predictor of non-sudden cardiac death (RR: 13.9, 95% CI: 5.9-32.5, P<0.001), but it did not predict SCD. CONCLUSIONS: Novel spectral HRV analysis methods, V(index) and PLF, provide significant information of the risk of the specific mode of death after an AMI.