Causal relationships between heart period and systolic arterial pressure during graded head-up tilt.

In physiological conditions, heart period (HP) affects systolic arterial pressure (SAP) through diastolic runoff and Starling's law, but, the reverse relation also holds as a result of the continuous action of baroreflex control. The prevailing mechanism sets the dominant temporal direction in the HP-SAP interactions (i.e., causality). We exploited cross-conditional entropy to assess HP-SAP causality. A traditional approach based on phases was applied for comparison. The ability of the approach to detect the lack of causal link from SAP to HP was assessed on 8 short-term (STHT) and 11 long-term heart transplant (LTHT) recipients (i.e., less than and more than 2 yr after transplantation, respectively). In addition, spontaneous HP and SAP variabilities were extracted from 17 healthy humans (ages 21-36 yr, median age 29 yr; 9 females) at rest and during graded head-up tilt. The tilt table inclinations ranged from 15 to 75° and were changed in steps of 15°. All subjects underwent recordings at every step in random order. The approach detected the lack of causal relation from SAP to HP in STHT recipients and the gradual restoration of the causal link from SAP to HP with time after transplantation in the LTHT recipients. The head-up tilt protocol induced the progressive shift from the prevalent causal direction from HP to SAP to the reverse causality (i.e., from SAP to HP) with tilt table inclination in healthy subjects. Transformation of phases into time shifts and comparison with baroreflex latency supported this conclusion. The proposed approach is highly efficient because it does not require the knowledge of baroreflex latency. The dependence of causality on tilt table inclination suggests that "spontaneous" baroreflex sensitivity estimated using noncausal methods (e.g., spectral and cross-spectral approaches) is more reliable at the highest tilt table inclinations.

Testing the involvement of baroreflex during general anesthesia through Granger causality approach.

Baroreflex sensitivity (BRS) is commonly assessed from spontaneous fluctuations of heart period (HP) and systolic arterial pressure (SAP) during general anesthesia. Unfortunately, general anesthesia depresses autonomic function and, consequently, spontaneous SAP variations could not be capable to drive HP changes, thus preventing the use of spontaneous variability to infer BRS. We applied two Granger causality approaches (F-test and Wald test) during two anesthesiological strategies (i.e. sevoflurane plus remifentanil or propofol plus remifentanil). We found a significant Granger-causality from SAP to HP independently of the anesthesiological strategy; thus suggesting that techniques estimating BRS from spontaneous variability can be utilized during general anesthesia.

Non-stationarities significantly distort short-term spectral, symbolic and entropy heart rate variability indices.

The autonomic regulation is non-invasively estimated from heart rate variability (HRV). Many methods utilized to assess autonomic regulation require stationarity of HRV recordings. However, non-stationarities are frequently present even during well-controlled experiments, thus potentially biasing HRV indices. The aim of our study is to quantify the potential bias of spectral, symbolic and entropy HRV indices due to non-stationarities. We analyzed HRV series recorded in healthy subjects during uncontrolled daily life activities typical of 24 h Holter recordings and during predetermined levels of robotic-assisted treadmill-based physical exercise. A stationarity test checking the stability of the mean and variance over short HRV series (about 300 cardiac beats) was utilized to distinguish stationary periods from non-stationary ones. Spectral, symbolic and entropy indices evaluated solely over stationary periods were contrasted with those derived from all the HRV segments. When indices were calculated solely over stationary series, we found that (i) during both uncontrolled daily life activities and controlled physical exercise, the entropy-based complexity indices were significantly larger; (ii) during uncontrolled daily life activities, the spectral and symbolic indices linked to sympathetic modulation were significantly smaller and those associated with vagal modulation were significantly larger; (iii) while during uncontrolled daily life activities, the variance of spectral, symbolic and entropy rate indices was significantly larger, during controlled physical exercise, it was smaller. The study suggests that non-stationarities increase the likelihood to overestimate the contribution of sympathetic control and affect the power of statistical tests utilized to discriminate conditions and/or groups.

Robotic treadmill training improves cardiovascular function in spinal cord injury patients.

BACKGROUND: Body weight supported treadmill training (BWSTT) assisted with a robotic driven gait orthosis (DGO) is an emerging tool in rehabilitating patients with lost sensorimotor function. Few information about the effects of BWSTT on cardiovascular system are available. The purpose of this study was to determine the effects of BWSTT on: 1) left ventricular (LV) systo-diastolic function; 2) coronary flow reserve (CFR); 3) endothelial function in patients with lost sensorimotor function due to neurologic lesions. METHODS: Fourteen adults (males 10, age 50.6±17.1years) with motor incomplete spinal cord injuries (SCI) due to trauma or spondylotic diseases underwent standard echocardiographic examination, non invasive assessment of CFR by dipyridamole stress echo and determination of plasma asymmetric dimethylarginine (ADMA) levels at baseline and after 6weeks of BWSTT. RESULTS: At post training evaluation we observed lower LV end-diastolic (P=0.0164) and end-systolic volumes (P=0.0029) with increased ejection fraction (EF) (P=0.0266). We also observed a LV interventricular septum (IVS) (P=0.00469) increase. At the same time, we detected an improvement of LV diastolic function as witnessed by the reduction of isovolumic relaxation time (IVRT) (P=0.0404) and deceleration time (DT) (P=0.0405) with an increased E/A ratio (P=0.0040). Improved CFR (P=0.020) and reduced plasma ADMA levels (P=0.0005) have been observed too, in association with a reduction of the inflammatory status (C-reactive protein (CRP) (P=0.0022) and erythrocyte sedimentation rate (ESR) (P=0.0005)). CONCLUSION: For the first time, this study demonstrated that 6weeks of BWSTT improved not only the sensorimotor function but also systo-diastolic LV function, CFR and endothelial dysfunction associated with a reduction of the inflammatory status in patients with incomplete SCI.

Psychophysiological signals associated with affective states.

We present a preliminary quantitative study aimed at developing an optimal standard protocol for automatic classification of specific affective states as related to human- computer interactions. This goal is mainly achieved by comparing standard psychological test-reports to quantitative measures derived from simultaneous non-invasive acquisition of psychophysiological signals of interest, namely respiration, galvanic skin response, blood volume pulse, electrocardiogram and electroencephalogram. Forty-three healthy students were exposed to computer-mediated stimuli, while wearable non-invasive sensors were applied in order to collect the physiological data. The stimuli were designed to elicit three distinct affective states: relaxation, engagement and stress. In this work we report how our quantitative analysis has helped in redefining important aspects of the protocol, and we show preliminary findings related to the specific psychophysiological patterns correlating with the three target affective states. Results further suggest that some of the quantitative measures might be useful in characterizing specific affective states.

RR-SAP causality in heart transplant recipients.

An information domain approach to the assessment of causality was applied to the beat-to-beat variability of heart period and systolic arterial pressure to test the open loop condition along baroreflex in heart transplant recipients. The closed loop between heart period and systolic arterial pressure was detected as open at the level of the baroreflex if systolic arterial pressure is more easily predictable from heart period than vice versa according to a conditional entropy approach. We found that in short-term heart transplant (STHT) recipients (less than 2 years after transplantation) the closed loop between heart period and systolic arterial pressure was open at the level of baroreflex. Baroreflex appeared to be involved in the heart period regulation in long-term heart transplant (LTHT) recipients (more than 2 years after transplantation). The significant linear correlation of causality index on the number of months after transplantation suggests that baroreflex control recovers after transplantation.

Effects of robot-driven gait orthosis treadmill training on the autonomic response in rehabilitation-responsive stroke and cervical spondylotic myelopathy patients.

Body weight supported treadmill training (BWSTT) assisted with a robotic-driven gait orthosis is utilized in rehabilitation of individuals with lost motor skills. A typical rehabilitation session included: sitting, standing, suspension, robotic-assisted walking at 1.5 and 2.5km/h, respectively with 50% body weight support and recovery. While the effects of robotic-assisted BWSTT on motor performances were deeply studied, the influences on the cardiovascular control are still unknown. The aim of the study was to evaluate in stroke (ST) and cervical spondylotic myelopathy (CSM) patients: (1) the autonomic response during a traditional robotic-assisted BWSTT session of motor rehabilitation; (2) the effects of 30 daily sessions of BWSTT on cardiovascular regulation. The autonomic response was assessed through symbolic analysis of short-term heart rate variability in 11 pathologic subjects (5 ST and 6 CSM patients) whose motor skills were improved as a result of the rehabilitation therapy. Results showed variable individual responses to the rehabilitation session in ST patients at the beginning of the therapy. At the end of the rehabilitation process, the responses of ST patients were less variable and more similar to those previously observed in healthy subjects. CSM patients exhibited an exaggerated vagal response to the fastest walking phase during the first rehabilitative session. This abnormal response was limited after the last rehabilitative session. We conclude that robotic-assisted BWSTT is helpful in restoring cardiovascular control in rehabilitation-responsive ST patients and limiting vagal responses in rehabilitation-responsive CSM patients.

Open loop linear parametric modeling of the QT variability.

Open loop linear parametric modeling approach was applied to describe the variability of the ventricular depolarization and repolarization duration (i.e. the QT interval from the ECG). Several model structures were compared. The model maximizing the goodness of fit describes the QT interval as a linear combination of its own past values plus two exogenous influences (i.e. heart period interval and respiration) and a colored noise. When this model was applied to a protocol imposing a progressive increase of the sympathetic activity and modulation (i.e. the graded head-up tilt), the goodness of fit gradually decreased, thus suggesting a progressive uncoupling between QT duration and heart period that cannot be the result of influences unrelated to heart period changes.

Evaluation of the autonomic response in healthy subjects during treadmill training with assistance of a robot-driven gait orthosis.

Body weight supported treadmill training assisted with a robotic driven gait orthosis is an emerging clinical tool helpful to restore gait in individuals with loss of motor skills. However, the autonomic response during this rehabilitation protocol is not known. The aim of the study was to evaluate the autonomic response during a routine protocol of motor rehabilitation through spectral and symbolic analyses of short-term heart rate variability in a group of 20 healthy subjects (11 men, mean age 25+/-3.8 years). The protocol included the following phases: (1) sitting position; (2) standing position; (3) suspension during subject instrumentation; (4 and 5) robotic-assisted treadmill locomotion at 1.5km/h and 2.5km/h respectively with partial body weight support; (6) standing recovery after exercise. Results showed a significant tachycardia associated with the reduction in variance during the suspended phase of the protocol compared to the sitting position. Spectral analysis did not demonstrate any significant autonomic response during the entire protocol, while symbolic analysis detected an increase in sympathetic modulation during body suspension and an increase of vagal modulation during walking. These results could be used to improve understanding of the cardiovascular effects of rehabilitation in subjects undergoing robotic driven gait orthosis treadmill training.