Khosrow Behbehani

Dynamic pressure–flow relationship of the cerebral circulation during acute increase in arterial pressure

The physiological mechanism(s) for the regulation of the dynamic pressure–flow relationship of the cerebral circulation are not well understood. We studied the effects of acute cerebral vasoconstriction on the transfer function between spontaneous changes in blood pressure (BP) and cerebral blood flow velocity (CBFV) in 13 healthy subjects (30 ± 7 years). CBFV was measured in the middle cerebral artery using transcranial Doppler. BP was increased stepwise with phenylephrine infusion at 0.5, 1.0 and 2.0 μg kg–1 min–1. Phenylephrine increased BP by 11, 23 and 37% from baseline, while CBFV increased (11%) only with the highest increase in BP. Cerebrovascular resistance index (BP/CBFV) increased progressively by 6, 17 and 23%, demonstrating effective steady‐state autoregulation. Transfer function gain at the low frequencies (LF, 0.07–0.20 Hz) was reduced by 15, 14 and 14%, while the phase was reduced by 10, 17 and 31%. A similar trend of changes was observed at the high frequencies (HF, 0.20–0.35 Hz), but gain and phase remained unchanged at the very low frequencies (VLF, 0.02–0.07 Hz). Windkessel model simulation suggests that increases in steady‐state cerebrovascular resistance and/or decreases in vascular compliance during cerebral vasoconstriction contribute to the changes in gain and phase. These findings suggest that changes in steady‐state cerebrovascular resistance and/or vascular compliance modulate the dynamic pressure–flow relationship at the low and high frequencies, while dynamic autoregulation is likely to be dominant at the very low frequencies. Thus, oscillations in CBFV are modulated not only by dynamic autoregulation, but also by changes in steady‐state cerebrovascular resistance and/or vascular compliance.

Mechanism of blood pressure and R-R variability: insights from ganglion blockade in humans

Spontaneous blood pressure (BP) and R‐R variability are used frequently as ‘windows’ into cardiovascular control mechanisms. However, the origin of these rhythmic fluctuations is not completely understood. In this study, with ganglion blockade, we evaluated the role of autonomic neural activity versus other ‘non‐neural’ factors in the origin of BP and R‐R variability in humans. Beat‐to‐beat BP, R‐R interval and respiratory excursions were recorded in ten healthy subjects (aged 30 ± 6 years) before and after ganglion blockade with trimethaphan. The spectral power of these variables was calculated in the very low (0.0078‐0.05 Hz), low (0.05‐0.15 Hz) and high (0.15‐0.35 Hz) frequency ranges. The relationship between systolic BP and R‐R variability was examined by cross‐spectral analysis. After blockade, R‐R variability was virtually abolished at all frequencies; however, respiration and high frequency BP variability remained unchanged. Very low and low frequency BP variability was reduced substantially by 84 and 69 %, respectively, but still persisted. Transfer function gain between systolic BP and R‐R interval variability decreased by 92 and 88 % at low and high frequencies, respectively, while the phase changed from negative to positive values at the high frequencies. These data suggest that under supine resting conditions with spontaneous breathing: (1) R‐R variability at all measured frequencies is predominantly controlled by autonomic neural activity; (2) BP variability at high frequencies (> 0.15 Hz) is mediated largely, if not exclusively, by mechanical effects of respiration on intrathoracic pressure and/or cardiac filling; (3) BP variability at very low and low frequencies (< 0.15 Hz) is probably mediated by both sympathetic nerve activity and intrinsic vasomotor rhythmicity; and (4) the dynamic relationship between BP and R‐R variability as quantified by transfer function analysis is determined predominantly by autonomic neural activity rather than other, non‐neural factors.

A controller for regulation of mean arterial blood pressure using optimum nitroprusside infusion rate

An integrating self-tuning control strategy for control of mean arterial blood pressure using sodium nitroprusside is presented. Next to robust performance, the most attractive feature of the controller is its capability to optimize the quantity of infused medication without introducing a bias in the blood pressure level, a problem that exists in some of the other adaptive control strategies that have been proposed previously. Further, the controller design requires only the knowledge of the pure delay and the order of the transfer function describing the patients response to the medication; it does not require that the entire transfer function be specified. The derivation of the controller is not based on the patient response to sodium nitroprusside; indeed, it is a general adaptive control strategy for control of systems with transport delay. The controller performs robustly in the presence of variations in the patient response and successfully controls the pressure at the desired level. The ability of this strategy to reduce the amount of infused medication makes it potentially attractive for use in clinical applications, as large doses or long-term use of sodium nitroprusside can adversely affect central nervous system and hematopoietic tissues.<<ETX>>

A noninvasive technique for detecting obstructive and central sleep apnea

A new noninvasive method to detect obstructive and central sleep apnea [(OSA) and (CSA)] events is described. Data were collected from ten volunteer subjects with a previous diagnosis of OSA while they were titrated for continuous positive airway pressure (CPAP) therapy. Apneic events were identify by analyzing of estimated airway impedance determined from pressure and airflow signals delivered from CPAP. To enhance performance of this technique, a single-frequency (5 Hz with 0.5 cmH/sub 2/O peak-to-peak amplitude) probing signal was superimposed on the applied CPAP pressure. The results indicated that estimated airway impedance during OSA (mean: 17.9, SD: 3.4, N=50) was significantly higher then during CSA (mean: 4.1, SD: 1.7, N=50). When the estimated impedance of OSA and CSA events were compared to a fixed threshold, 100% of all events can be correctly categorized. These results indicate that it may be possible to diagnose OSA and CSA noninvasively based upon this technique. The instrument and the algorithm required are relatively simple and can be incorporated in a home-based device. If this method was used for prescreening apnea patients, it could reduce cost, waiting time, and discomfort associated with traditional diagnostic procedures.

EEG feature extraction for classification of sleep stages

Automated sleep staging based on EEG signal analysis provides an important quantitative tool to assist neurologists and sleep specialists in the diagnosis and monitoring of sleep disorders as well as evaluation of treatment efficacy. A complete visual inspection of the EEG recordings acquired during nocturnal polysomnography is time consuming, expensive, and often subjective. Therefore, feature extraction is implemented as an essential preprocessing step to achieve significant data reduction and to determine informative measures for automatic sleep staging. However, the analysis of the EEG signal and extraction of sensitive measures from it has been a challenging task due to the complexity and variability of this signal. We present three different schemes to extract features from the EEG signal: relative spectral band energy, harmonic parameters, and Itakura distance. Spectral estimation is performed by using autoregressive (AR) modeling. We then compare the performance of these schemes with the view to select an optimal set of features for specific, sensitive, and accurate neuro-fuzzy classification of sleep stages.

Intraoperative bile duct visualization using near-infrared hyperspectral video imaging

BACKGROUND Current methodologies for imaging the biliary system during cholecystectomy are cumbersome and do not eliminate the risk of bile duct injury. We describe an approach to intraoperative biliary imaging that will enable surgeons to see through the hepatoduodenal ligament and visualize the anteriorly placed biliary system. METHODS A laparoscopic-capable, near-infrared, hyperspectral imaging system was built. Reflected light passes through a liquid crystal filter that is continuously tunable in the near-infrared spectrum (650-1,100 nm). Spectroscopic image data are collected from laparoscopic surgery images onto array detectors formatted into a 3-dimensional hyperspectral data cube having spatially resolved images in the x-y plane and wavelength data in the z plane. Deconvoluting and color-coding the spatial and spectral information provides an image representative of inherent chemical properties to the imaged tissue. RESULTS Images of porcine biliary structures were obtained. The common duct-reflected spectra displayed a characteristic lipid shoulder at 930 nm and a strong water peak at 970 nm. Venous structures had absorption peaks at 760 nm (deoxyhemoglobin), 800 nm (oxyhemoglobin), and 970 nm (water). Arterial vessels had absorption peaks at 800 nm and 970 nm that would be expected for oxyhemoglobin and water. CONCLUSIONS We have designed and constructed a device to significantly enhance intraoperative biliary imaging. This system should enable surgeons to see through the hepatoduodenal ligament and image the anteriorly placed biliary system without the need for dissection of the cystic duct, as is needed with intraoperative cholangiography. Because the biliary system can be seen before any dissection is performed, this dimensional imaging technology has the potential for eradicating bile duct injury.

Automatic control of airway pressure for treatment of obstructive sleep apnea

Obstructive sleep apnea (OSA) occurs when airflow ceases because of pharyngeal wall collapse in sleep. Repeated apneic events results in the development of a pathological condition called OSA syndrome. The authors describe the methodology and design of a prosthetic device, named automatic positive airway pressure (APAP), for treatment of this syndrome. HPAP applies a stream of air via a nasal mask at an initial pressure selected by the patient. By sensing specific pressure characteristics of air flow immediately preceding pharyngeal wall collapse, the APAP device automatically raises the applied pressure to maintain a patent upper airway and thus prevent apnea. Conversely, when such conditions are absent, pressure is lowered step wise until a preselected minimum pressure is reached. Performance evaluation of the APAP system in five OSA patients and five normal (asymptomatic for sleep apnea) subjects revealed that it effectively treated OSA syndrome. It lowered the apnea-hypopnea index without disturbing sleep and resulted in a lower mean airway pressure compared to the traditional continuous positive airway pressure (CPAP) therapy. The results also show that the pressure needed to prevent OSA varied significantly throughout the night. For OSA syndrome patients, this pressure ranged from 3 to 18 cm H/sub 2/O. The mean airway pressure for these patients had a sample average of 6.80 cm H/sub 2/O and a standard deviation of 3.17 cm H/sub 2/O. In normal subjects, the device did not raise pressure except in response to pharyngeal wall vibration events.<<ETX>>

Enhanced Functional Brain Imaging by Using Adaptive Filtering and a Depth Compensation Algorithm in Diffuse Optical Tomography

Reflectance diffuse optical tomography (rDOT) of brain function is limited by its high sensitivity to the superficial tissues (i.e., the scalp and skull) and by its severe decrease in measurement sensitivity with increased depth. Significant interference in rDOT results from spontaneous fluctuations that are embedded in both the superficial tissues and brain, such as arterial pulsation and vasomotion. In this study, first we investigate coherence and phase shift of the spontaneous fluctuations in the resting state, within the superficial tissues and at various depths of the brain, respectively. We demonstrate that the spontaneous fluctuations originating from arterial pulsations (~ 1 Hz) are spatially global and temporally coherent, while the fluctuations originating from vasomotion (~ 0.1 Hz) tend to have less coherence with increased depth. Second, adaptive cancellation of spontaneous fluctuations with a frequency-specific strategy is utilized and validated in both resting and activation (evoked by a finger-tapping task) states. Third, improved depth localization of motor activation in reconstructed rDOT images is achieved by combining adaptive cancellation with a depth compensation algorithm that we recently reported.

Identification of abnormal motor cortex activation patterns in children with cerebral palsy by functional near-infrared spectroscopy

We demonstrate the utility of functional near-infrared spectroscopy (fNIRS) as a tool for physicians to study cortical plasticity in children with cerebral palsy (CP). Motor cortex activation patterns were studied in five healthy children and five children with CP (8.4+/-2.3 years old in both groups) performing a finger-tapping protocol. Spatial (distance from center and area difference) and temporal (duration and time-to-peak) image metrics are proposed as potential biomarkers for differentiating abnormal cortical activation in children with CP from healthy pediatric controls. In addition, a similarity image-analysis concept is presented that unveils areas that have similar activation patterns as that of the maximum activation area, but are not discernible by visual inspection of standard activation images. Metrics derived from the images presenting areas of similarity are shown to be sensitive identifiers of abnormal activation patterns in children with CP. Importantly, the proposed similarity concept and related metrics may be applicable to other studies for the identification of cortical activation patterns by fNIRS.

Accommodation of time delay variations in automatic infusion of sodium nitroprusside

A new self-tuning regulator for control of mean arterial blood pressure using sodium nitroprusside is presented. In addition to adapting to various patient response gains and time constants, the controller can accommodate variations in pure time delays, a problem that has not been addressed fully in previously proposed controllers. The control algorithm derivation is based on incorporating a self-turning controller with an adaptive discrete time delay compensator. The derivation, however, is general and can potentially be applied to other pharmacological agents in addition to sodium nitroprusside. The a priori information required for implementation of the controller is the estimate of the order of the patient response transfer function and the range of the pure time delay. An attractive feature of the controller is its capability to optimize the level of the infused drug during the transient phase of the control without creating an output offset. This feature tends to reduce the overall level of the infused drug. Hence, it may be useful when the cost of a drug or its long-term administration side effects is of concern, such as in the case of sodium nitroprusside.<<ETX>>