Viktor Avbelj

Two Proximal Skin Electrodes — A Respiration Rate Body Sensor

We propose a new body sensor for extracting the respiration rate based on the amplitude changes in the body surface potential differences between two proximal body electrodes. The sensor could be designed as a plaster-like reusable unit that can be easily fixed onto the surface of the body. It could be equipped either with a sufficiently large memory for storing the measured data or with a low-power radio system that can transmit the measured data to a gateway for further processing. We explore the influence of the sensors position on the quality of the extracted results using multi-channel ECG measurements and considering all the pairs of two neighboring electrodes as potential respiration-rate sensors. The analysis of the clinical measurements, which also include reference thermistor-based respiration signals, shows that the proposed approach is a viable option for monitoring the respiration frequency and for a rough classification of breathing types. The obtained results were evaluated on a wireless prototype of a respiration body sensor. We indicate the best positions for the respiration body sensor and prove that a single sensor for body surface potential difference on proximal skin electrodes can be used for combined measurements of respiratory and cardiac activities.

Wireless network of bipolar body electrodes

A wireless bipolar body electrode has been developed, taking into account the limitations posed by signal-to-noise ratio, dimensions, wireless technology, limited power consumption and requirements for post-processing. A small set of wireless bipolar electrodes, placed at optimal locations on the body surface, are connected into a network that is able to acquire real-time data. Measured signals, sampled with local frequencies of different electrodes, are synchronized to a global clock. Thus, complex post-processing that requires signals aligned in time is enabled. As an interesting example, a standard 12-channel ECG synthesis from a small network of three wireless bipolar electrodes is shown.

Beat-to-beat repolarisation variability in body surface electrocardiograms.

The repolarisation variability in body surface electrocardiograms has been evaluated by beat-to-beat QT interval variability. Interpolated R-peak time and template T-wave matching algorithms were used to determine the characteristic time points of the R-wave and T-wave, respectively. The T-wave time can be determined accurately and robustly by searching for the best match between a template T-wave and measured T-waves. The authors studied 5 min multichnnel ECG recordings (35 channels) measured in 20 healthy subjects. A QT variability of 2.24±0.79 ms was obtained (1.15±0.30 ms, if linear detrend was used), which is significantly lower than that reported in several other studies. To explore this discrepancy, the sensitivity of the template matching algorithm to periodic and random noise on the ECG was estimated by a simulation study. The results showed that the repolarisation variability depended on selection of the appropriate lead, the signal-to-noise ratio and the effectiveness of baseline correction. Lead II of a standard 12-lead ECG is a reasonable choice for QT variability analysis; however, precordial leads V3-V6 could be better with regard to the amplitude of the T-wave. Poor signal-to-noise ratios can lead to unrealistic values for repolarisation variability.

Assessment of Cardiac Autonomic Regulation and Ventricular Repolarization after Off-Pump Coronary Artery Bypass Grafting

BACKGROUND Altered autonomic regulation precipitates cardiac arrhythmias and increases the risk of sudden cardiac death. This risk is further increased by changes in ventricular repolarization. Autonomic regulation is deranged in patients after myocardial on-pump revascularization. We aimed to clarify how off-pump coronary artery bypass grafting (CABG) affects postoperative cardiac autonomic regulation and ventricular repolarization within 4 weeks after CABG. METHODS Forty-two patients (mean age, 61.9 +/- 9.3 years; mean EURO score 2.6 +/- 1.9) were electively admitted for off-pump CABG. The electrocardiographic and respiratory waveform recordings were performed in the afternoon in the supine position for 10 minutes. Autonomic modulation was assessed using heart rate variability analysis. Power spectra were computed from 5-minute stable RR intervals using Fourier Transform analysis. Total power of spectra was defined in the range of 0.01 to 0.40 Hz, high-frequency power within 0.15 to 0.40 Hz, and low-frequency power within 0.04 to 0.15 Hz. Normalized power was defined as a ratio of power in each band/total power. The high- and low-frequency power as well as their normalized values indicated cardiac vagal and sympathetic modulation, respectively. Ventricular repolarization was assessed using QT interval, QT interval variability, and QT-RR interdependence analysis. QT intervals were determined from the beginning of the 5-minute segments. QT interval variability was evaluated by a T-wave template-matching algorithm. Pearson correlation between length of RR and QT interval was applied to study QT-RR characteristics. The results were tested for significance using the Fisher exact test, nonpaired t test, and analysis of variance; a P <.05 was considered significant. RESULTS The frequency of arrhythmic events and heart rate increased from the fourth to the seventh postoperative day and returned to preoperative levels 4 weeks after CABG. Heart rate variability measures indicating autonomic modulation remained depressed even 4 weeks after the procedure. QT variability index increased from -1.2 +/- 0.5 to -0.8 +/- 0.4 on the fourth day after the operation (P <.05) and returned to -1.0 +/- 0.5 4 weeks after CABG (P = not significant). QT-RR correlation decreased from 0.41 to 0.23 (P <.05) and remained significantly impaired as long as 4 weeks after CABG. CONCLUSIONS Observed faster heart rates until 1 week after off-pump CABG imply excessive adrenergic activation, which is comparable to on-pump CABG procedure rates. The results indicate profound autonomic derangement and loss of rate-dependent regulation after off-pump CABG even 4 weeks after operation. Restituted repolarization as assessed by QT variability index 4 weeks postoperatively corresponded with decreased frequency of rhythm disturbances 4 weeks after CABG. The loss of coupling between QT and RR intervals shows increased electrical instability postoperatively, which may serve as an additional promoter for postoperative arrhythmias, especially at higher heart rates.

Multichannel ECG measurement system

Multichannel ECG is an important instrumentation for experimental and clinical electrophysiology. Both the software and the front-end design issues are described for a 128 channel system which can record the signals up to 100 seconds. The basic analysis and presentation of the acquired ECG signals are also given with examples of body surface potential maps. Thanks to the frequency response of the front-end down to 0.05 Hz, the system can show high resolution isointegral maps of ST segment shift. The system is modular and can be modified for different number of channels, sample-rate, bandwidth and quantization resolution.

Effects of Beating- versus Arrested-Heart Revascularization on Cardiac Autonomic Regulation and Arrhythmias

BACKGROUND Altered autonomic regulation after cardiac operations precipitates cardiac arrhythmias, affects repolarization, and increases the risk of sudden cardiac death. We sought to clarify how the 2 different techniques of coronary artery bypass grafting (CABG), namely conventional CABG using cardiopulmonary bypass (on-pump) and beating-heart CABG without cardiopulmonary bypass (off-pump), affect cardiac autonomic regulation and arrhythmic disturbances postoperatively. METHODS We included 57 consecutive patients, 28 in the on-pump group and 29 in the off-pump group. The electro-cardiographic recordings were performed on the preoperative day and the fourth, seventh, and twenty-eighth day after operation. Fifteen-minute digital recordings were taken; one channel was used to record electrocardiogram and the other breathing. Detailed analyses of arrhythmia, heart rate, and heart rate variability indices were performed on respective days to assess sympathetic and parasympathetic modulation of the heart and relate it to detected arrhythmic disturbances. RESULTS Total power, low-frequency power, which indicates baroreceptor-mediated sympathetic modulation, and high-frequency power, indicating parasympathetic vagal modulation, declined significantly in both groups after CABG (P < .001); however, 7 days after CABG, total and high-frequency power were better preserved in the off-pump group. Mean RR interval was longer in the off-pump group at 7 (P= .006) and 28 days (P= .008) after surgery. The total incidence of arrhythmic events was higher in the on-pump group on the seventh day (P = .017, adjusted odds ratio = 8.6, 95% confidence interval 1.4-80.3). CONCLUSIONS The results show profound impairment of cardiac autonomic regulation after CABG, showing better preserved cardiac autonomic modulation 7 days after beating-heart revascularization. Evidence suggests that slower restoration of heart rate and increased incidence of arrhythmic events after CABG using cardiopulmonary bypass are the result not only of impaired cardiac autonomic regulation but also of the involvement of additional factors of nonautonomic origin.

Beat-to-beat QT interval variability before and after cardiac surgery.

Non-uniform recovery of excitability may be essential in triggering malignant ventricular tachycardia after cardiac surgery. Thirty-five channels ECG was recorded for 6 min in 27 patients before and after heart surgery and in 20 control subjects. Off-line analysis was performed. RR interval duration, RR SD, QT SD and power spectra of RR variability were computed from 256 s stable RR and QT interval series. When compared to controls, patients had decreased RR SD and increased QT SD before surgery (p<0.002 and p<0.0005, respectively); RR SD further decreased and QT SD increased after the surgery (p<0.0001 and p<0.0002, respectively). Increase of QT variability and decrease of RR variability after cardiac surgery may reflect disrupted electrophysiological stability of the myocardium and thus electrophysiological substrate for triggering malignant arrhythmia.

The Effect of 4-week Rehabilitation on Heart Rate Variability and QTc Interval in Patients with Chronic Obstructive Pulmonary Disease

Abstract Chronic obstructive pulmonary disease negatively affects the autonomic nervous system and increases risks of arrhythmias and sudden cardiac death. Electrocardiogram (ECG) recordings were used to compare parameters of heart rate variability and QTc interval in patients with COPD and healthy individuals. The effects of a 4-week program of rehabilitation in patients with COPD were also evaluated by comparing pre- and post-rehabilitation ECGs with age- and sex-matched control COPD patients not participating in the program. Heart rate, average NN, SDNN, RMSSD, pNN50, TP, LF, HF, LF/HF, and QTc were analyzed. Rehabilitation effects were evaluated using the St. Georges respiratory questionnaire (SGRQ), the 6-min walk test (6MWT), and the incremental shuttle walking test (ISWT). In comparison with the healthy individuals, the patients with COPD had higher heart rate (p < 0.05) and reduced average NN, SDNN, RMSSD, pNN50, HF, LF, and TP (all p < 0.05) but similar QTc interval (p = 0.185). During rehabilitation, SDNN and TP (p < 0.05 for both) increased, as did the results for 6MWT, ISWT, and SGRQ (all p < 0.05). No significant change of QTc interval was observed within or between the two groups of patients with COPD. Change in SDNN correlated with a clinically relevant difference in SGRQ (r = 0.538, p = 0.021). It is concluded that patients with COPD demonstrate reduced parameters of heart rate variability and that these can be improved in a rehabilitation program, thus improving health-related quality of life.

Breathing rates and heart rate spectrograms regarding body position in normal subjects

The right lateral body position has been proposed as an effective vagal enhancer. However, the possibility of breathing affecting heart rate power spectra in different body positions has not been assessed. The level of vagal modulation in various body positions in normal subjects was estimated by calculating heart rate power spectra. The results suggest that the levels of vagal modulation do not necessarily reflect a change due to assuming different body position, but might be the consequence of changed breathing patterns. Before adopting the right lateral body position as vagal enhancing, the contribution of varying breathing pattern should be eliminated.

Multifractality in heartbeat dynamics in patients undergoing beating-heart myocardial revascularization

BACKGROUND The multifractal approach of HRV analysis offers new insight into the mechanisms of autonomic modulation of the diseased hearts and has a potential to depict subtle changes in cardiac autonomic nervous control not revealed by conventional linear and non-linear analyses in various conditions like heart failure or stable angina pectoris. The aim of this study was to employ the multifractality approach in cardiac surgery patients and evaluate the multifractality before and after beating-heart myocardial revascularization (off-pump CABG). METHODS Twenty-four hour Holter recordings were performed pre- and postoperatively in 60 patients undergoing off-pump CABG. Selected conventional time- and frequency-domain linear HRV indices were calculated from the 24h and 5 min ECG segments, and preselected multifractal parameters τ(q=2), τ(q=3), h_top and Δh were determined for daytime (12:00-18:00) and nighttime (00:00-06:00) periods of the ECG recordings using Ivanovs method. Mean differences over time were tested using paired-samples t-test and exact Wilcoxon matched-pairs test. The results are reported as mean ± SD and median with interquartile range. A p value of <0.05 was considered statistically significant. RESULTS All selected conventional linear HRV parameters decreased significantly after off pump CABG (p from <0.001-0.015). Preoperatively, multifractal parameter τ(q=2) was -0.60 ± 0.12 and -0.54 ± 0.12, τ(q=3) -0.52 ± 0.18 and -0.49 ± 0.17, h_top 0.20 ± 0.07 and 0.15 ± 0.07 and Δh 0.31 ± 0.14 and 0.17 ± 0.14 for daytime and nighttime periods, respectively. Postoperatively, τ(q=2) and τ(q=3) were significantly higher for daytime (-0.49 ± 0.15, p<0.001 and -0.43 ± 0.23, p=0.015), whereas h_top and Δh were significantly higher for both daytime and nighttime (0.25 ± 0.07, p<0.001 and 0.19 ± 0.06, p=0.002 for h_top and 0.41 ± 0.20, p=0.003 and 0.31 ± 0.19, p < 0.001 for Δh, respectively). All pre- and postoperative parameters, except τ(q=2) and τ(q=3) preoperatively, were significantly lower for nighttime as compared to daytime periods. CONCLUSIONS A significant breakdown of multifractal complexity and anti-correlation behavior with a significant sympathetic overdrive and a concomitant parasympathetic withdrawal occurs after off-pump CABG. The circadian pattern of multifractality regains its day-night variation in the first week after the surgical procedure.