Detection of different recumbent body positions from the electrocardiogram.

Changes in body position alter the relative angle between ECG electrodes and the mean electric axis of the heart. These changes influence the time interval during which the projection of the electric dipole, on any ECG lead, is positive (R-wave). In this study, measurements of R-wave duration (RWD) were used to identify changes in body position, and two of its uncorrelated features were used to classify each heartbeat into four basic groups relating to four body positions (supine, prone, left-side, right-side). Data were acquired from healthy volunteers during controlled condition experiments that included well-defined sequences of body positions and simultaneous recordings of ECG leads I, II and III. Results showed over 90% correct identifications of body position changes when using any of the three leads. Lead II had the best performance for the classification of body position and correctly classified 80% of heartbeats. Classification did not improve for a combination of two leads. The technique can be used to reveal additional important clinical information and can be easily implemented, in a variety of applications where ECG is recorded, such as sleep studies, Holter recordings and ischaemia detection.

Estimation of autonomic response based on individually determined time axis.

The analysis of the time-dependence of autonomic response requires: 1. A reliable procedure for the quantification of autonomic activity under nonsteady conditions, such as an algorithm for time-frequency decomposition (ex. SDA. Wigner-Ville, or others). 2. The choice of an adequate time scale for focusing on the data: (a) the regular, universal time scale, independent of the unsteady physiological conditions, or (b) a time axis defined by specific events related to an applied perturbation, as the indicators of specific experimental or physiological conditions, so that each individual is considered according to his own intrinsic time scale. The alignment of the various subjects according to their intrinsic time scale, reflecting their individual response mechanisms, may help to disclose a common pattern of autonomic function. Using an absolute time scale to align and average results for different subjects may obscure the underlying mechanisms. Several examples of autonomic challenges are presented, in which the use of an individual time scale contributes to unveil a typical response pattern: tilt test in vasovagal syncope, the autonomic effect of active standing on hypertension, and the autonomic response to acute hypoxia.

Autonomic cardiovascular control in children with obstructive sleep apnea.

Autonomic cardiorespiratory control changes with sleep-wake states and is influenced by sleep-related breathing disorders. Power spectrum (PS) analysis of instantaneous fluctuations in heart rate (HR) is used to investigate the role of the autonomic nervous system (ANS) in cardiorespiratory control. The two spectral regions of interest are the low frequency component (LF) and high frequency component (HF). The aim of the present study was to investigate the autonomic cardiorespiratory control in children with obstructive sleep apnea (OSA) syndrome. We studied 10 children with OSA versus 10 normal children. All subjects underwent whole night polysomnography. Spectral analysis of the HR and breathing signals was performed for 256 second long, artifact-free epochs in each sleep-wake state. The LF power was higher in the OSA group compared with control subjects for all states, reflecting enhanced sympathetic activity in OSA subjects. The results indicated sympathetic predominance during REM sleep in all subjects and parasympathetic predominance in slow wave sleep only in controls. The autonomic balance (LF/HF) was significantly higher in OSA patients than in control subjects, at all stages during night sleep, and while awake before sleep onset. An index of overall autonomic balance (ABI) was computed for each subject and correlated well with the measured respiratory disturbance index (RDI).

Home monitoring for infants at high risk for the sudden infant death syndrome.

The rate of sudden infant death syndrome (SIDS) In Israel is relatively low (0.5-0.9:1,000). Home cardiorespiratory monitoring (HM) is an accepted practice in infants at high risk for SIDS. We report our experience with 261 infants who were referred to our SIDS prevention program. They included: 52 preterm infants with apneas and bradycardias, 83 SIDS siblings (3 twins), 22 infants of drug-addicted mothers, and 104 infants after an idiopathic apparent life-threatening event (ALTE). HM was performed in 40 of 52 preterms, 38 of 83 SIDS siblings, all 22 infants of addicted mothers and 67 of 104 post-ALTE. All received 24 h/day medical and technical backup as well as emotional support, and were closely followed until 15 months of age. None of the 261 infants died. Five infants experienced six ALTEs that required resuscitative measures; another 28 infants had monitor alarms judged as real by the caregivers. The average duration of HM was 3.2 months (range 1-7). In 8 of 167 cases the parents stopped HM earlier than recommended, and in 34 of 167 cases (20%), parents continued HM beyond the time when discontinuation was recommended by the medical personnel. Among the caregivers, 85% found HM to be reassuring and stated that it helped them to conduct a normal life. We suggest that in our population, HM may have a favorable effect on family life. With close backup and support, most families will benefit from HM and will gain reassurance that will enable them to conduct normal life.

Estimation of fast vagal response by time-dependent analysis of heart rate variability in normal subjects.

In this study, the selective windowed discrete Fourier transform algorithm (SDA) for time-frequency analysis was applied on non-stationary heart rate signals, recorded during vagal perturbations. These perturbations were achieved in healthy subjects (aged 6-42 years) by inducing the oculocardiac reflex and the diving response. The results showed that the SDA can detect and quantify the expected, although brief, increase in vagal tone, by displaying a marked transient increase in the respiratory peak of the time-dependent spectrum. It allowed us to demonstrate an age-dependent reduction of the vagal response, obtained from the respiratory peak of the heart rate time-dependent spectrum. The SDA approach is thus an adequate tool for the evaluation of an instantaneous change in vagal activity, as well as steady-state vagal activity, including the detection of a malfunction or an exaggerated response of the parasympathetic tone. Since changes in heart rate control are expressed by a transient response, they would not have been detected by a standard, steady-state spectral analysis of heart rate variability, which requires the consideration of long and steady trace lengths and would therefore cause smearing of such fast changes. Time-dependent, or transient changes in autonomic control can thus be detected and quantified by SDA time-frequency analysis.

Fluctuations in autonomic nervous activity during sleep displayed by power spectrum analysis of heart rate variability.

OBJECTIVE: The use of an efficient noninvasive method to investigate the autonomic nervous system and cardiovascular control during sleep. BACKGROUND: Beat-to-beat heart rate variability displays two main components: a low-frequency (LF) one representing sympathetic and parasympathetic influence and a high-frequency (HF) component of parasympathetic origin. Sympathovagal balance can be defined as LF/HF ratio. METHODS/DESIGN: We reviewed normal, standardly staged all-night polysomnograms from 10 healthy children aged 6 to 17 years. Recorded 256-second traces of heart rate and respiration were sampled. Power spectra of instantaneous heart rate and respiration were computed using a fast Fourier transform method. RESULTS: The study revealed a decrease in LF during sleep, with minimal values during non-REM slow-wave sleep and elevated levels similar to those of wakefulness during REM. HF increased with sleep onset, reaching maximal values during slow-wave sleep, and behaved as a mirror image of LF. LF/HF ratio displayed changes similar to those in LF. CONCLUSION: The sympathetic predominance that characterizes wakefulness decreases during non-REM sleep, is minimal in slow-wave sleep, and surges toward mean awake levels during REM sleep. The autonomic balance is shifted toward parasympathetic predominance during slow-wave sleep. This noninvasive method used to outline autonomic activity achieves results that are in complete agreement with those obtained with direct invasive tools.

Spectral analysis of heart rate in vasovagal syncope: the autonomic nervous system in vasovagal syncope.

Spectral analysis of heart rate fluctuations was used to investigate the role of the autonomic nervous system in the pathogenesis of vasovagal syncope. Nine adolescents with a history of at least three episodes of vasovagal syncope and nine age-matched healthy controls were studied. All subjects were tested in supine position and at a 60 degrees inclination for 60 min or less if syncope developed. Blood pressure and heart rate were measured, while the ECG and respiration traces were recorded on magnetic tape for later spectral analysis. Baseline heart rate was lower in control subjects than in patients, increased with tilt in both groups, and remained lower in the control subjects throughout the experiment. Baseline systolic and diastolic blood pressure was similar in both groups. Diastolic blood pressure initially increased with tilt in all subjects and decreased significantly thereafter in patients. Pulse pressure was lower in patients throughout the experiment. The heart rate power spectra displayed a higher baseline level of low frequency fluctuations in the control group. The high frequency fluctuations component was similar in all subjects. The results of the test, regarding haemodynamic parameters and autonomic control of the heart rate, as expressed by low and high frequency fluctuations, are consistent with a reduced sympathetic reserve in the individuals with previous episodes of syncope.