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The structure of 9-year time series data for Sea Surface Temperature (SST), Chlorophyll a (Chl-a) and Total Suspended Solids (TSS), derived from the Visible Infrared Imaging Radiometer Suite (VIIRS), was examined in this study. Authors found that there exists strong seasonality among the three variables with spatial heterogeneity along the Korean South Coast (KSC). In specific, SST was in phase with Chl-a, but out of phase with TSS by six months. A strong inversed spectral power with six-month phase-lag was found between Chl-a and TSS. This could be attributed to different dynamics and environmental settings. For example, Chl-a concentration seemed to have strong positive correlation with SST indicating typical seasonality of marine biogeochemical processes such as primary production; while a strong negative correlation between TSS and SST might have been influenced by changes in physical oceanographic processes, such as stratification and monsoonal wind-driven vertical mixing. In addition, the strong east-west heterogeneity of Chl-a suggests that the marine coastal environments are predominantly governed by distinct local hydrological conditions and human activities associated with land cover and land use, while the east-west spatial pattern revealed in TSS timeseries was associated with the gradient of tidal forcings and topographical changes keeping tidally induced resuspension low eastward.

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We propose higher-order detrending moving-average cross-correlation analysis (DMCA) to assess the long-range cross-correlations in cardiorespiratory and cardiovascular interactions. Although the original (zeroth-order) DMCA employs a simple moving-average detrending filter to remove non-stationary trends embedded in the observed time series, our approach incorporates a Savitzky-Golay filter as a higher-order detrending method. Because the non-stationary trends can adversely affect the long-range correlation assessment, the higher-order detrending serves to improve accuracy. To achieve a more reliable characterization of the long-range cross-correlations, we demonstrate the importance of the following steps: correcting the time scale, confirming the consistency of different order DMCAs, and estimating the time lag between time series. We applied this methodological framework to cardiorespiratory and cardiovascular time series analysis. In the cardiorespiratory interaction, respiratory and heart rate variability (HRV) showed long-range auto-correlations; however, no factor was shared between them. In the cardiovascular interaction, beat-to-beat systolic blood pressure and HRV showed long-range auto-correlations and shared a common long-range, cross-correlated factor. This article is part of the theme issue 'Advanced computation in cardiovascular physiology: new challenges and opportunities'.

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Understanding the mechanisms of oxygen supply regulation, which involves the respiratory and cardiovascular systems, during human adaptation to intense physical activity, accompanied by hypoxemia, is important for the management of a training process. The objectives of this study were to investigate the cardiorespiratory coherence (CRC) changes in the low-frequency band in response to hypoxic exposure and to verify a dependence of these changes upon sports qualification level in athletes. Twenty male runners aged 17-25 years were exposed to acute normobaric hypoxia (10% O2) for 10 min. Respiration, gas exchange, and heart rate were measured at baseline, during hypoxia, and after the exposure. To evaluate cardiorespiratory coupling, squared coherence was calculated based on 5-s averaged time series of heart and respiratory rhythms. Based on sports qualification level achieved over 4 years after the experimental testing, athletes were retrospectively divided into two groups, one high level (HLG, n = 10) and the other middle level (MLG, n = 10). No differences in anthropometric traits were observed between the groups. In the pooled group, acute hypoxia significantly increased CRC at frequencies 0.030-0.045 Hz and 0.075 Hz. In response to hypoxia, oxygen consumption decreased in HLG, and carbon dioxide production and ventilation increased in MLG. At 0.070-0.080 Hz frequencies in hypoxia, the CRC in HLG was higher than in MLG. Thus, highly qualified athletes enhance intersystem integration in response to hypoxia. This finding can be a physiological sign for the prognosis of qualification level in runners.

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AIM: To study cerebral autoregulation (CA) in region of the stenotic carotid artery. MATERIAL AND METHODS: The study involved 35 patients with critical stenosis of the carotid arteries, including 24 patients were asymptomatic and 11 patients with symptomatic course. Blood flow velocity in middle cerebral arteries was monitored using Multi Dop X (DWL, Germany) with simultaneous noninvasive systemic blood pressure registration (CNAP, Austria). CA was assessed by calculating the phase shift (PS) between spontaneous oscillations of blood flow velocity and blood pressure within the range of systemic Mayer waves (80-120 mHz). RESULTS: In 18 patients, the CA indicators were in the normal range (PS 1.2±0.3 rad). Seventeen patients, including asymptomatic as well as symptomatic types, had impaired CA (PS 0.2±0.2 rad and 0.3±0.2 rad, respectively). Reconstructive surgery, irrespective of clinical manifestations, led to the significant increase in PS (p<0.001) in the early postoperative period (0.9±0.5 and 0.9±0.3 rad, respectively). CONCLUSION: A significant variability in the cerebrovascular reserve capacity in symptomatic and asymptomatic types of carotid artery stenosis was found. CA can be used in determining the indications for surgical treatment and evaluation of its effectiveness in patients with stenosis of carotid arteries.

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Chronic obstructive pulmonary disease (COPD) is a chronic airway disease with increased airway resistance. This study investigated the common characteristics of electrocardiographic (ECG) and nostril airflow signals in COPD patients using cross-spectral analysis. Heart rate variability (HRV) measures and cross-spectral (cs) measures of ECG and nostril airflow were compared in COPD patients and normal subjects, and correlated with their clinical characteristics. We found that cross-spectral analysis can lead to a significant increase in normalized high-frequency power (nHFPcs) and a significant decrease in normalized very low-frequency power (nVLFPcs), normalized low-frequency power (nLFPcs), and low-/high-frequency power ratio (LHRcs) in both normal subjects and COPD patients, as compared with their corresponding HRV measures. Further analysis showed that the percentage increase in nHFP (%nHFP) and the percentage decrease in LHR (%LHR) due to cross-spectral analysis in COPD patients were significantly smaller than those of normal subjects. All cross-spectral measures of ECG and nostril airflow in COPD patients did not significantly correlate with their pulmonary function characteristics. However, the nHFPcs correlated significantly and negatively with body mass index (BMI) in both normal subjects and COPD patients, and the %nHFP correlated significantly and negatively with BMI in COPD patients. We conclude that cross-spectral analysis of ECG and nostril airflow signals could lead to reduced enhancement in the high-frequency component in the cross spectrum of COPD patients. The magnitude of reduced enhancement in the high-frequency component in the cross-spectrum was related to the BMI of the patients. Cross-spectral analysis of ECG and nostril airflow might be used to assess the cardiovascular-related functions of COPD patients.

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BACKGROUND AND PURPOSE: To assess dynamic cerebral autoregulation (CA) in patients with acute ischaemic stroke of undetermined aetiology, within 72h of stroke onset. MATERIALS AND METHODS: In 6 patients with ischaemic stroke of undetermined aetiology (aged 66±9 years, National Institutes of Health Stroke Scale [NIHSS] score on admission: 4.0, range: 4-11), selected based on screening of 118 consecutive ischaemic stroke patients and in 14 volunteers (aged 62±10 years), we continuously monitored RR intervals (RRI), mean arterial pressure (MAP) by means of photoplethysmography, mean cerebral blood flow velocity (CBFV) using transcranial Doppler ultrasonography, end-tidal CO2 (ETCO2) and respiration during 2-min deep breathing paced at 6min(-1) (0.1Hz). To assess CA, we evaluated the impact of breathing-induced MAP oscillations on fluctuations of CBFV in the hemispheres with stroke, the non-involved hemispheres and randomly selected hemispheres of controls by applying cross-spectral analysis and calculating coherence, transfer function gain (CBFV-MAP gain) and phase shift angle between the two oscillating signals. RESULTS: Phase shift angle between MAP and CBFV oscillations showed values >0 and was significantly reduced in the hemispheres without stroke as compared to controls (0.39±0.95 vs. -1.59±0.33rad, p=0.015), whereas in the hemispheres with stroke, phase shift angle did not differ significantly from that observed in the control hemispheres. Clinical status of stroke patients significantly improved at discharge from the hospital (NIHSS: 2.0, range: 1-8, p=0.028). CONCLUSIONS: During the first days of ischaemic stroke of undetermined aetiology, dynamic cerebral autoregulation is compromised in the non-affected hemisphere, but not in the hemisphere with ischaemic lesion.

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Sustained water immersion is thought to modulate orthostatic tolerance to an extent dependent on the duration and repetition over consecutive days of the diving sessions. We tested this hypothesis investigating in ten healthy subjects the potential changes in the cardiovascular response to head-up tilt induced by single and multiple resting air dives. Parametric cross-spectral analysis of spontaneous RR interval and systolic arterial pressure variability was performed in three experimental sessions: before diving (BD), after single 6-hour dive (ASD), and after multiple 6-hour dives (AMD, 5 consecutive days with 18-hour surface interval). From this analysis, baroreflex sensitivity (BRS) was computed as spectral power ratio (αBRS), non-causal transfer function gain (tfBRS) and causal transfer function gain (γBRS) evaluated at low frequency (0.04-0.14Hz) in the supine position (su) as well as in the standing upright position in the early tilt (et) and late tilt (lt) epochs. We found that, while αBRS decreased significantly in et and lt compared to su during all sessions, tfBRS and γBRS decreased during ASD and AMD but not during BD; moreover γBRS evidenced a progressive decrease from BD to ASD and to AMD in both et and lt epochs. These results indicate the necessity of following a causal approach for the estimation of BRS in the frequency domain, and suggest a progressive impairment of the baroreflex response to postural stress after single and multiple dives, which may reflect symptoms of increasing orthostatic intolerance.

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BACKGROUND: Total spinal anesthesia (TSA) after injections of local anesthetics into the intrathecal space during epidural anesthesia is not rare. TSA anesthetizes cranial nerves and peripheral nerves, causes specific circulatory disturbances related to autonomic imbalance. Spectral analysis of heart rate variability (HRV) and blood pressure variability (BPV) provide a dynamic assessment of sympathetic and parasympathetic tone. Cross-spectral analysis has been used to emphasize this dynamic baroreflex control of HR as a frequency-dependent phenomenon and allows an assessment of baroreflex function. To examine the effects of TSA on the autonomic nervous system, we used spectral and cross-spectral analytic METHODS. METHODS: We investigated 14 rats before and after TSA. Power spectral densities of blood pressure (BP) and heart rate (HR) were estimated by fast Fourier transform. To evaluate the effect of TSA on baroreflex function, the cross spectral gain, phase, and coherence between beat-to-beat BP and HR signals were calculated by using transfer function analysis. RESULTS: With the onset of TSA, BP and HR significantly decreased. TSA significantly decreased the low frequency (LF) and high frequency (HF) components of HRV and BPV. Baroreflex sensitivity (BRS) obtained from transfer function gain between these variables was significantly decreased. CONCLUSIONS: TSA reduces the LF and HF components of HRV and BPV. These suggest that TSA in rats decreases both parasympathetic and sympathetic drive. Moreover, the decrease in BRS suggests impairment of cardiac baroreflex buffering function during TSA.

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