Michael A. Navakatikyan

A real-time algorithm for the quantification of blood pressure waveforms

A real-time algorithm for quantification of biological oscillatory signals, such as arterial blood pressure (BP) is proposed which does not require user intervention and works on waveforms complicated by rapid changes in the mean level, frequency, or by the presence of arrhythmia. The algorithm is based on the continous independent assessment of the refractory period (RP). In the first stage, a sample of the signal is band-pass filtered. During the next stage: 1) the local maxima in the filtered signal are identified and their pulse amplitudes (PA) measured on the side opposite to the possible notch position and 2) those maxima whose PA exceeds some threshold are selected and an array of RP values is formed as a fraction of the moving estimate of the interval between successive selected peaks. Finally, the original signal is analyzed by means of two moving averages (MAs) with short and long averaging time intervals. The true peaks are determined as the maxima between intersections of MAs if the peak-to-peak or the intersection-to-intersection intervals since the previous peak and the previous intersection exceed the R.P. The algorithm proved to be superior against three commercially available heartbeat detectors yielding an error rate of 0.09%.

Resonance in the renal vasculature evoked by activation of the sympathetic nerves

We examined the ability of different frequencies in sympathetic nerve activity (SNA) to induce oscillations in renal blood flow (RBF). In anesthetized rabbits the renal nerves were stimulated using modulated sine patterns (base frequency 5 Hz, 5-ms duration pulses) that varied in amplitude between 0 and 10 V at a frequency between 0.04 and 1.0 Hz. The strengths of the induced oscillations in RBF were calculated using spectral analysis. Although faster rhythms in simulated SNA >0.6 Hz contributed to the level of vascular tone, 95% of the power in the frequency response curve was below this frequency, indicating a low-pass filtering/integrating characteristic of the vasculature. Frequencies <0.6 Hz were associated with increasing ability to induce oscillations in RBF. The ability of an SNA rhythm at 0.6 Hz to induce a rhythm in RBF was 21 times less than that at 0.25 Hz. At 0.16 Hz there was a distinct peak in the frequency response curve, indicating the vasculature was more sensitive in this frequency band to sympathetic stimulation. Blockade of endogenous nitric oxide by N G-nitro-l-arginine methyl ester (l-NAME; 20 mg/kg) did not alter resting RBF levels nor was the low-pass filtering/integrating characteristic of the vasculature to nerve stimulation changed (i.e., the curve was not shifted left or right); however, there was a selective increase in the sensitivity to stimulation at 0.16 Hz, i.e., larger oscillations in RBF were evoked. These results indicate an ability of SNA to induce resonant oscillations in the renal vasculature and that there may be active and passive modulators of these responses. Naturally occurring oscillations in SNA <0.6 Hz are likely to contribute to the dynamic control of RBF, ensuring it responds rapidly and with high gain to the stimuli of daily life, while filtering out the faster oscillations ensures stable glomerular filtration.We examined the ability of different frequencies in sympathetic nerve activity (SNA) to induce oscillations in renal blood flow (RBF). In anesthetized rabbits the renal nerves were stimulated using modulated sine patterns (base frequency 5 Hz, 5-ms duration pulses) that varied in amplitude between 0 and 10 V at a frequency between 0.04 and 1.0 Hz. The strengths of the induced oscillations in RBF were calculated using spectral analysis. Although faster rhythms in simulated SNA >0.6 Hz contributed to the level of vascular tone, 95% of the power in the frequency response curve was below this frequency, indicating a low-pass filtering/integrating characteristic of the vasculature. Frequencies <0.6 Hz were associated with increasing ability to induce oscillations in RBF. The ability of an SNA rhythm at 0.6 Hz to induce a rhythm in RBF was 21 times less than that at 0.25 Hz. At 0.16 Hz there was a distinct peak in the frequency response curve, indicating the vasculature was more sensitive in this frequency band to sympathetic stimulation. Blockade of endogenous nitric oxide by NG-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg) did not alter resting RBF levels nor was the low-pass filtering/integrating characteristic of the vasculature to nerve stimulation changed (i.e., the curve was not shifted left or right); however, there was a selective increase in the sensitivity to stimulation at 0.16 Hz, i.e., larger oscillations in RBF were evoked. These results indicate an ability of SNA to induce resonant oscillations in the renal vasculature and that there may be active and passive modulators of these responses. Naturally occurring oscillations in SNA <0.6 Hz are likely to contribute to the dynamic control of RBF, ensuring it responds rapidly and with high gain to the stimuli of daily life, while filtering out the faster oscillations ensures stable glomerular filtration.

Differential regulation of the oscillations in sympathetic nerve activity and renal blood flow following volume expansion

Renal sympathetic nerve activity (RSNA) and renal blood flow (RBF) both show oscillations at various frequencies but the functional significance and regulation of these oscillations is not well understood. To establish whether the strength of these oscillations is under differential control we measured the frequency spectrum of RSNA and RBF following volume expansion in conscious rabbits. Seven days prior to experiment animals underwent surgery to implant an electrode for recording renal nerve activity and a flow probe for recording RBF. Volume expansion (Haemaccel, 1.5 ml min(-1) kg(-1) for 15 min) resulted in a 25 +/- 5% decrease in mean RSNA, paralleled by an increase in RBF to 60 +/- 12 ml min(-1) from resting levels of 51 +/- 11 ml min(-1). Renal denervated rabbits did not show an increase in RBF with volume expansion. Arterial baroreflexes were unaltered by volume expansion. Spectral analysis of the different frequencies in RSNA showed oscillations in RSNA between 0.2 and 0.4 Hz were selectively decreased following volume expansion (14 +/- 3 to 6 +/- 1% of total power in RSNA at < 3 Hz). A corresponding decrease in the strength of oscillations in RBF at this frequency was also seen (20 +/- 6 to 8 +/- 2%). In contrast, the strength of respiratory (0.8-2.0 Hz) and cardiac (3-6 Hz) related rhythms did not change with volume expansion. These results show that selective changes in the different frequency components of RSNA can occur. We suggest that input from cardiopulmonary receptors and/or other vascular beds, and/or altered vascular resistance after volume expansion can reduce the strength of the 0.3 Hz oscillation independent of changes in arterial baroreflex control of RSNA.

Peak-to-peak amplitude in neonatal brain monitoring of premature infants.

OBJECTIVE To assess the strength of association between alternative measures of electroencephalographic (EEG) signal peak-to-peak amplitude (ppA) and postmenstrual age (PMA) among a cohort of extremely premature infants. METHODS 177 Two-channel EEG recordings 3-6h long were collected from 26 infants born before 29weeks of gestation. The raw EEG was converted into four different continuous measures of ppA: amplitude-integrated EEG (aEEG), range-EEG (rEEG), Gotman and Gloors half-wave decomposition (HWD), and root of mean squares (RMS). For each ppA-measure EEG indices including mean, median, and 5% margins; indices of spread (width, standard deviation, coefficient of variation), and asymmetry were calculated for each 1min epoch. The medians of each index for the entire recording were tested for association with PMA using linear mixed models. RESULTS The log-transformed values of aEEG and rEEG indices of spread were highly associated with PMA (fixed effects R(β)(2)=0.84-0.89). CONCLUSIONS Indices of spread by aEEG or rEEG can be used as indicators of neonatal brain maturation. However, rEEG produces the absolute values that most closely approximate the raw EEG amplitudes. SIGNIFICANCE The indices of spread and rEEG as a measure of ppA provide a basis for improvements in neonatal EEG monitoring.

The sympathetic nervous system's role in regulating blood pressure variability

This article focuses on how sympathetic nerve activity (SNA) contributes to the variability seen in blood pressure. Specifically, it examines the following questions: why do oscillations occur at certain frequencies, why do only certain frequencies of oscillations in SNA induce oscillations in the vasculature, and what may be the functional purpose of these oscillations.

THE DYNAMICS OF THE LAW OF EFFECT: A COMPARISON OF MODELS

Dynamical models based on three steady-state equations for the law of effect were constructed under the assumption that behavior changes in proportion to the difference between current behavior and the equilibrium implied by current reinforcer rates. A comparison of dynamical models showed that a model based on Navakatikyans (2007) two-component functions law-of-effect equations performed better than models based on Herrnsteins (1970) and Davison and Hunters (1976) equations. Navakatikyans model successfully described the behavioral dynamics in schedules with negative-slope feedback functions, concurrent variable-ratio schedules, Vaughans (1981) melioration experiment, and experiments that arranged equal, and constant-ratio unequal, local reinforcer rates.

Automatic measurement of interburst interval in premature neonates using range EEG

OBJECTIVE To explore the direct measure of EEG amplitude (range EEG, rEEG) for detection of interburst intervals (IBIs) and bursts in neonates. METHODS Previously described 177 two-channel EEG recordings 3-6h long from 26 preterm infants (median gestational age of 26 weeks) at 23-38 weeks post-menstrual age (PMA) without major abnormalities were used to test four definitions of IBI detection algorithms with various settings of the parameters. RESULTS As the basis for all four algorithms we developed the aggregation of rEEG signal over the channels by taking its maximum, and method of EEG trace selection at different phases of sleep-wake cycle (with different degree of discontinuity). The two less restrictive algorithms - with one and two amplitude thresholds - turned to be the most promising definitions. There were enough IBI detections for analysis, with no substantial difference in mean and maximum values of intervals. The longest IBI were measured at the location of greater discontinuity. Values of bursts and IBI indices as well as association with PMA were close to the published normative values derived manually. CONCLUSIONS rEEG as a direct measure of EEG amplitude can be used for detection of bursts and IBI. SIGNIFICANCE The automatic measurement of IBI based on rEEG provides a basis for improvements in neonatal brain monitoring.