Normal and abnormal circadian profiles of heart autonomic balance, evaluated by time-related common indicator of heart rate variability

OBJECTIVE

Heart rate variability (HRV) is an universally accepted method for assessing the heart autonomic balance (HAB). At the same time HRV is a highly specific method, but with a low sensitivity to the cardiovascular diseases (CVD). We found that HRV indices may be applied to obtain differentiated characteristics of the HAB for individual CVD.

METHODS

We assessed the changes taking place in the circadian characteristic of the autonomic balance, which has a definite profile with and without the principal CVD. We proposed adequate time-related autonomic balance indicator (TRABI) for evaluating the changes in the circadian nature of HAB by comparing the values of the HRV indices during rest and upon vegetative nervous system stimulation during two intervals in the 24-hour period in which there is a physiologically determined difference in the balance.

RESULTS

The normal circadian characteristic of HAB is distinguished by a slight prevalence of the circadian nature of the parasympathetic component. The mean value of the assessments of the HRV indices using TRABI in healthy individuals, according to data from the study during rest and with the handgrip test is 0.100, according to data from the study during rest and with the Valsalva maneuver is 0.141. The mean values of the scores of the indices for HRV using TRABI in the comparative studies during rest and with the two stimulation tests are accordingly: with hypertension- 0.132 and 0.047; with morning myocardial infarction incidents- 0.177 and 0.107; with non-morning myocardial infarction incidents- 0.082 and 0.053. The patients with unstable angina in the population studied have a good long-term prognosis and the mean values in this case in the two comparative studies are 0.180 and 0.211.

CONCLUSIONS

Cardiovascular diseases have a characteristic abnormal circadian nature of the autonomic balance. In our opinion, the proposed time-related autonomic balance indicator broadens, the opportunities for using HRV for HAB assessment. The method for evaluating the circadian changes in HAB through the HRV indices during rest and with stimulation during different time intervals is convenient and accessible for clinical use.

Transthoracic impedance study with large self-adhesive electrodes in two conventional positions for defibrillation

External defibrillation requires the application of high voltage electrical impulses via large external electrodes, placed on selected locations on the thorax surface. The position of the electrodes is one of the major determinants of the transthoracic impedance (TTI) which influences the intracardiac current flow during electric shock and defibrillation success. The variety of factors which influence TTI measurements raised our interest to investigate the range of TTI values and the temporal TTI variance during long-term application of defibrillation self-adhesive electrodes in two conventional positions on the patient's chest--position 1 (sub-clavicular/sub-axillar position) and position 2 (antero-posterior position). The prospective study included 86 randomly selected volunteers (39 male and 49 female, 67 patients with normal skin, 13 patients with dry skin and 6 patients with greasy skin, 16 patients with chest pilosity and 70 patients without chest pilosity). The TTI was measured according to the interelectrode voltage drop obtained by passage of a low-amplitude high-frequency current (32 kHz) between the two self-adhesive electrodes (active area about 92 cm2). For each patient, the TTI values were measured within 10 s, 1 min and 5 min after sticking the electrodes to the skin surface, independently for the two tested electrode positions. We found that the expected TTI range is between 58 Omega and 152 Omega for position 1 and between 55 Omega and 149 Omega for position 2. Although the two TTI ranges are comparable, we measured significantly higher TTI mean of about (107.2 +/- 22.3) Omega for position 1 compared to (96.6 +/- 19.2) Omega for position 2 (p = 0.001). This fact suggested that the antero-posterior position of the electrodes is favourable for defibrillation. Within the investigated time interval of 5 min, we observed a significant TTI reduction with about 6.9% (7.4 Omega/107.2 Omega) for position 1 and about 5.3% (5.1 Omega/96.6 Omega) for position 2. We suppose that the long-term application of self-adhesive electrodes would lead to improvement of the physical conditions for conduction of the defibrillation current and to diminution of energy loss in the electrode-skin contact impedance. We found that gender is important when position 1 is used because women have significantly higher TTI (111 +/- 20.3) Omega compared to the TTI of men (102.6 +/- 24) Omega (p = 0.0442). Although we found some specifics of the electrode-skin contact layer, we can conclude that because of the insignificant differences in TTI, the operator of the defibrillator paddles does not need to take into consideration the skin type and pilosity of the patients. Analysis of the correlations between TTI and the individual patient characteristics (chest size, weight, height, age) showed that these patient characteristics are unreliable factors for prediction of the TTI values and optimal defibrillation pulse parameters and energy.

Dataset of manually measured QT intervals in the electrocardiogram

Background

The QT interval and the QT dispersion are currently a subject of considerable interest. Cardiac repolarization delay is known to favor the development of arrhythmias. The QT dispersion, defined as the difference between the longest and the shortest QT intervals or as the standard deviation of the QT duration in the 12-lead ECG is assumed to be reliable predictor of cardiovascular mortality.

The seventh annual PhysioNet/Computers in Cardiology Challenge, 2006 addresses a question of high clinical interest: Can the QT interval be measured by fully automated methods with accuracy acceptable for clinical evaluations?

Method

The PTB Diagnostic ECG Database was given to 4 cardiologists and 1 biomedical engineer for manual marking of QRS onsets and T-wave ends in 458 recordings. Each recording consisted of one selected beat in lead II, chosen visually to have minimum baseline shift, noise, and artifact.

In cases where no T wave could be observed or its amplitude was very small, the referees were instructed to mark a 'group-T-wave end' taking into consideration leads with better manifested T wave.

A modified Delphi approach was used, which included up to three rounds of measurements to obtain results closer to the median.

Results

A total amount of 2*5*548 Q-onsets and T-wave ends were manually marked during round 1. To obtain closer to the median results, 8.58 % of Q-onsets and 3.21 % of the T-wave ends had to be reviewed during round 2, and 1.50 % Q-onsets and 1.17 % T-wave ends in round 3.

The mean and standard deviation of the differences between the values of the referees and the median after round 3 were 2.43 ± 0.96 ms for the Q-onset, and 7.43 ± 3.44 ms for the T-wave end.

Conclusion

A fully accessible, on the Internet, dataset of manually measured Q-onsets and T-wave ends was created and presented in additional file: 1 (Table 4) with this article. Thus, an available standard can be used for the development of automated methods for the detection of Q-onsets, T-wave ends and for QT interval measurements.

Time-related heart autonomic balance characteristics in healthy subjects

Changes of the heart autonomic balance between morning (8-9 h) and afternoon (14-15 h) measurements were studied in 22 healthy subjects. The selection of these two daytime periods was substantiated by the established higher risk of cardiovascular incidents in the morning and the relative balance of the vegetative nervous system in the afternoon hours. The changes were analysed by RR-variability indices from ECG recordings in the resting state and with vegetative nervous system stimulation by the handgrip test and Valsalva manoeuvre. It was shown that there were no significant differences between the morning and afternoon values of the respective indices, between morning and afternoon handgrip tests and between morning and afternoon Valsalva manoeuvres. However, there were significant differences in comparison of the index values between resting state and handgrip test and resting state and Valsalva manoeuvre, both from morning and afternoon measurements. Moreover, the significantly differing indices were clustered in different groupings, when comparing resting state recordings with morning and afternoon stimulation tests. For this reason, the authors introduced an indicator for time-related autonomic balance changes. The indicator evaluates the power of each RR-variability index to respond to changes in the autonomic control, in comparisons between resting state and stimulation data in the morning and afternoon measurements. These evaluations showed low power of the frequency-domain indices to respond to time-related autonomic balance changes in stimulation, with respect to the resting state. The time-domain indices have considerably higher power to react to relative morning and afternoon changes in the two vegetative nervous system components. Based on the estimations of the RR-variability indices obtained by the introduced indicator, a profile was constructed to represent time-related heart autonomic balance changes in healthy subjects. It was established that in spite of the relative stability of the vegetative nervous system in healthy subjects, hypersympatheticotonia and relatively lower parasympathetic tone were present in the risky morning hours.

Diagnostic value of the RR-variability indicators for mild hypertension

Experimental and clinical studies have shown that an imbalance of the cardiovascular system autonomous regulation can lead to complications. In this study we have aimed to investigate whether the vegetative control of patients with mild hypertension is disturbed and to evaluate the power of the RR-variability indicators to detect the disturbances. RR variability has been studied in two groups of individuals: normotensive and mildly hypertensive. The indicators of the heart rate variability (HRV) from the 'Cardiovit CS-100 ECG Module' electrocardiograph have been used. The HRV is known to exhibit circadian variations, therefore RR intervals were recorded in two separate sessions: morning (8-9 am) and afternoon (2-3 pm). Each session included three tests: in a resting state, with a handgrip test and with the Valsalva manoeuvre. The multivariate statistical method was applied separately to the morning and afternoon data. A number of classification algorithms were synthesized, yielding correct recognition of normotensive and mildly hypertensive subjects of 91.9% and 94.2% for the morning and afternoon sessions, respectively. We have thoroughly discussed the substantiating reasons for the high statistical significance and reliability of the discrimination obtained. The high sensitivity and specificity of the RR-variability indicators included in the classification models suggest correlative changes of the nervous system vegetative balance in mildly hypertensive patients. The results of the study show the possibility for statistically significant diagnosis of mild hypertension, for prediction of ventricular arrhythmia, left ventricular hypertrophy and ischaemic heart disease.