Michael L. Daley

Design of optimal digital filter using a parallel genetic algorithm

This paper describes the use of a parallel genetic algorithm to design a direct form of a finite word length (FWL), finite impulse response (FIR) low pass digital filter. The results of the proposed design technique are compared to two other proposed methods, an integer programming technique and roundoff of the optimal floating point set of coefficients. Using the Chebyshev metric as the comparison criterion, design of the low pass filter by the parallel genetic algorithm technique proved to be superior to the other methodologies. >

Detection of loss of cerebral vascular tone by correlation of arterial and intracranial pressure signals

With the use of a laboratory model, arterial and intracranial pressure signals were obtained under conditions of intact regulation of cerebral blood flow and massive dilation. During elevated intracranial pressure and intact regulation, positive pressure inhalation appears to briefly occlude venous flow into the cranial sinuses during inspiration. As a result, the intracranial pressure and arterial pressure signals are not similar. In contrast, when maximal dilation causes failure of regulation of cerebral blood flow, the intracranial pressure signal is approximately proportional to the arterial pressure signal. Comparison of the cross-correlation function derived from the intracranial and arterial pressure signals to the autocorrelation function of the arterial signal reveals that the two correlation functions are: (1) different during intact regulation and (2) nearly identical during dilation induced failure of regulation of cerebral blood flow.<<ETX>>

Assessment of Cerebrovascular Autoregulation. Changes of Highest Modal Frequency of Cerebrovascular Pressure Transmission With Cerebral Perfusion Pressure

Background and Purpose— Development of a method to continuously assess cerebrovascular autoregulation of patients with traumatic brain injury would facilitate therapeutic intervention and thus reduce secondary complications. Methods— Changes in arterial blood pressure (ABP), intracranial pressure (ICP), cerebral blood flow velocity (CBFV), and pial arteriolar diameter (PAD) induced by acute pressor challenge (norepinephrine; 1 μg/[kg · min]) were evaluated in both uninjured and fluid percussion injured piglets equipped with cranial windows. The linear correlation coefficient and corresponding slope of the regression line of the relationship between highest modal frequency (HMF) of cerebrovascular pressure transmission of ABP to ICP and cerebral perfusion pressure (CPP) were determined for each challenge. Results— For all uninjured piglets, pressor challenge resulted in an inverse relationship between HMF and CPP characterized by significant negative correlation values and negative corresponding regression line slopes with respective group mean values (±SD) of −0.50 (±0.14) and −0.6 (±0.44) Hz/mm Hg, respectively. Consistent with functional autoregulation of the uninjured preparations, pressor challenge resulted in a decrease of PAD, and CBFV remained relatively constant. For all injured piglets, pressor challenge resulted in direct relationship between HMF and CPP, characterized by positive correlation values and corresponding regression line slopes with group mean values of 0.48 (±0.21) and 1.13 (±2.08) Hz/mm Hg, respectively. Consistent with impaired autoregulation, PAD and CBFV increased during pressor challenge after brain injury. Conclusions— Evaluation of changes of the HMF of cerebrovascular pressure transmission with respect to CPP changes permits continuous monitoring of cerebral autoregulation.

Heme oxygenase inhibition reduces neuronal activation evoked by bicuculline in newborn pigs

Carbon monoxide (CO) is a product of heme degradation by heme oxygenase (HO) that is highly expressed in the brain. The present study addresses the hypothesis that CO can be involved in brain neuronal function. The effects of the HO inhibitor, tin protoporphyrin (SnPP), on brain electrical activity and pial arteriolar diameter were examined using quantitative electroencephalography (EEG) and cranial window techniques in the bicuculline model of sustained generalized seizures in newborn pigs. SnPP (3 mg/kg i.v.) inhibits brain HO as indicated by blocking cerebral vasodilation to heme and decreasing CO concentration in cortical periarachnoid cerebrospinal fluid. The quantitative spectral analysis of digitalized scalp EEG recordings was performed to determine the EEG amplitude and spectral power within a 1-15-Hz frequency range. SnPP did not affect basal brain EEG parameters. Bicuculline (3 mg/kg i.v.) immediately (in <1 min) evoked bursts of brain electrical activity characterized by four- to seven-fold increases in EEG amplitude and power in all analyzed frequency bands that occurred simultaneously with cerebral vasodilation. Increased EEG activity and cerebral vasodilation were sustained for a 2h period. SnPP inhibited cerebral vasodilation but did not affect the EEG amplitude evoked by bicuculline. However, 20-40% reductions of the power in 7.5 Hz (theta), 10 and 12.5 Hz (alpha), and a 15-Hz (beta) bands, the major evoked EEG spectral components, were observed for the duration of seizures in SnPP-treated animals. These findings suggest that endogenous CO can have proconvulsant action and affect neuronal activation during seizures.

Cerebrovasodilatory Contribution of Endogenous Carbon Monoxide During Seizures in Newborn Pigs

Carbon monoxide (CO) and the excitatory amino acid glutamate both dilate cerebral arterioles in newborn pigs. The key enzyme in CO synthesis is heme oxygenase, which is highly expressed in neurons with glutamatergic receptor activity as well as cerebral microvessels. During seizures the extracellular level of glutamate is increased, which results in excessive depolarization of neurons. We hypothesized that CO is a mediator of excitatory amino acid–induced dilation of the cerebral microvasculature during seizures. Three groups of piglets were examined:1) i.v. normal saline (sham control), 2) topical chromium mesoporphyrin (Cr-MP, 15 × 10−6 M), and 3) i.v. tin-protoporphyrin (Sn-PP, 4 mg/kg). Synthetic metalloporphyrins (Cr-MP and Sn-PP) are heme oxygenase inhibitors, thereby reducing CO synthesis. Implanted closed cranial windows were used to monitor changes in pial arteriolar diameters. Seizures were induced by administration of i.v. bicuculline. Changes in pial arteriolar diameters were monitored during 30 min of status epilepticus. The percent increase in pial arteriolar dilation in the saline group during seizures was 68 ± 3%. In the metalloporphyrin groups, the pial arteriolar dilation was markedly reduced (35 ± 3% and 13 ± 1%, for Cr-MP and Sn-PP, respectively;p < 0.05, compared with the saline group). We conclude that metalloporphyrins by inhibition of heme oxygenase and prevention of CO synthesis attenuate pial arteriolar dilation during seizures. Therefore, CO appears to be involved in cerebral vasodilation caused by glutamatergic seizures.

Influence of hypercapnic vasodilation on cerebrovascular autoregulation and pial arteriolar bed resistance in piglets

Changes in both pial arteriolar resistance (PAR) and simulated arterial-arteriolar bed resistance (SimR) of a physiologically based biomechanical model of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure and intracranial pressure, are used to test the hypothesis that hypercapnia disrupts autoregulatory reactivity. To evaluate pressure reactivity, vasopressin-induced acute hypertension was administered to normocapnic and hypercapnic (N = 12) piglets equipped with closed cranial windows. Pial arteriolar diameters were used to compute arteriolar resistance. Percent change of PAR (%DeltaPAR) and percent change of SimR (%DeltaSimR) in response to vasopressin-induced acute hypertension were computed and compared. Hypercapnia decreased cerebrovascular resistance. Indicative of active autoregulatory reactivity, vasopressin-induced hypertensive challenge resulted in an increase of both %DeltaPAR and %DeltaSimR for all normocapnic piglets. The hypercapnic piglets formed two statistically distinct populations. One-half of the hypercapnic piglets demonstrated a measured decrease of both %DeltaPAR and %DeltaSimR to pressure challenge, indicative of being pressure passive, whereas the other one-half demonstrated an increase in these percentages, indicative of active autoregulation. No other differences in measured variables were detectable between regulating and pressure-passive piglets. Changes in resistance calculated from using the model mirrored those calculated from arteriolar diameter measurements. In conclusion, vasodilation induced by hypercapnia has the potential to disrupt autoregulatory reactivity. Our physiologically based biomechanical model of cerebrovascular pressure transmission accurately estimates the changes in arteriolar resistance during conditions of active and passive cerebrovascular reactivity.

Correlation Coefficient between Intracranial and Arterial Pressures: A Gauge of Cerebral Vascular Dilation

With the use of a laboratory model, arterial and intracranial pressure signals were obtained under conditions of varying depths of hypercapnia ranging from normocapnia to deep hypercapnia. Also, with the use of a closed cranial window, measures of cerebral arteriolar diameter and estimates of cerebral venous flow were obtained. The correlation of the intracranial and arterial pressure signals, arteriolar diameter, and estimates of venous flow exhibit a dose-dependent characteristic by increasing monotonically with increasing progressive states of increasing hypercapnia. These results indicate that the correlation between intracranial and arterial pressure signals provides an estimate of the gauge of the cerebral vasculature.

Design of finite word length FIR digital filter using a parallel genetic algorithm

The application of a parallel genetic algorithm implemented with a Hypercube computer to design optimal finite-wordlength finite-impulse-response digital filters is described. Two examples of this application to the design of a Chebyshev low-pass digital filter are given. One example, describing a filter with a length of 40 coefficients and 10-b wordlength, is used for comparison to a previously reported identical design problem which was solved with a general-purpose integer programming algorithm. This comparison reveals that the genetic algorithm produces a better Chebyshev approximation to the desired frequency response. The second example describes the results of the application of the genetic algorithm to the design of a high-order filter developed for the processing of seismic signals. The characteristics of this second filter with 140 coefficients and a 16-b wordlength are compared to those obtained by a rounded-off method.<<ETX>>

Intracranial pressure monitoring: modeling cerebrovascular pressure transmission.

OBJECTIVES To examine changes in cerebrovascular pressure transmission derived from arterial blood pressure (ABP) and intracranial pressure (ICP) recordings by autoregressive moving average modeling technique. METHODS Digitized ICP and ABP recordings were obtained from patients with brain injury. Two groups were defined: Group A with 4 patients who demonstrated plateau waves, and Group B with 4 intracranial hypertensive, hypoperfused patients. For each 16.5 s interval, mean values of ICP, ABP, cerebral perfusion pressure (CPP), and corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were computed. RESULTS Mean values of CPP and HMF of 56.2 mmHg and 2.0 Hz for Group A were significantly higher (p < 0.005) than corresponding mean values of 31.9 mmHg and 0.744 Hz for Group B. The mean value of the slope of the regression line between HMF and CPP for group A of -0.034 Hz/mmHg was significantly different (p < 0.025) than the mean value of 0.0077 Hz/mmHg for Group B. Computations of HMF, pressure reactivity, and correlation pressure reactivity index on continuous pressure recordings are illustrated. CONCLUSIONS Values of HMF of cerebrovascular pressure transmission are inversely related to CPP when pressure regulation is thought to be intact, and directly related when regulation is likely lost.

Modeling cyclic variation of intracranial pressure

To test the theoretical feasibility that low frequency baseline changes of the intracranial pressure (ICP) recording during mechanical ventilation are due to cyclic extravascular compressional modulation primarily of the cerebral venous bed, an established isovolumetric model of cerebrospinal fluid dynamics proposed by M. Ursino (1988) was modified. These modifications were made to address the hypothesis that: (1) cyclic extravascular compressional modulation of the cerebral venous bed occurs during positive pressure inhalation; and (2) the degree of modulation is diminished with increasing vascular dilation induced by increasing the level of the partial pressure of carbon dioxide (PCO/sub 2/) within the arterial blood. Modification of the isovolumetric model was accomplished by introducing a cyclic modulation of the resistance of the cerebral venous bed synchronized with ventilation. Simulated model recordings demonstrated that the correlation index between arterial blood pressure and ICP progressively increased monotonically as the level of PCO/sub 2/ increased from 30 mmHg to 80 mmHg. These results support the premise that during positive pressure ventilation, cyclic extravascular compressional modulation of primarily the cerebral venous bed produces a cyclic variation of ICP and the degree of modulation is dependent on the state of vascular dilation.