Flow-induced responses in piglet isolated cerebral arteries.

Although cerebral hemorrhage is a widely occurring neurologic disorder thought to be caused by fluctuating blood flow, the response to flow in the neonatal cerebrovasculature has not been characterized. In the present study, we examined the effect of changing flow on middle cerebral artery diameter and pathways by which flow modulates cerebrovascular tone. Arteries from 2-14-d-old piglets were mounted on cannulas and bathed in and perfused with physiologic saline solution. An electronic system controlled pressure and a syringe pump provided constant flow. The transmural pressure was held constant at 20 mm Hg, and changes in vessel diameter were measured as flow was increased in steps from 0 to 1.60 mL/min (flow/diameter curves). Increasing flow at constant pressure resulted in constriction at flows from 0.077 to 0.152 mL/min and dilation at flows from 0.212 to 1.60 mL/min. The flow/diameter curves were repeated in arteries bathed in Na(+)-reduced or Ca(2+)-free physiologic saline solution; denervated with 6-hydroxydopamine; or treated with indomethacin, N-nitro-L-arginine methyl ester, N omega-nitro-L-arginine (NLA), and L-arginine), ryanodine, or glutaraldehyde. In Na(+)-reduced and in Ca(2+)-free physiologic saline solution, flow constriction was eliminated. Neither indomethacin nor 6-hydroxydopamine affected the biphasic response. N-Nitro-L-arginineL, NLA, and ryanodine blocked dilation, whereas L-arginine restored dilation in NLA-treated arteries. These data suggest that neither prostaglandins nor adrenergic nerve endings participate in flow-induced responses in piglet cerebral arteries. Elimination of flow-constriction by Na+ reduction or Ca2+ removal is consistent with findings in other artery types. The elimination of dilation by N-nitro-L-arginine methyl ester, NLA, and ryanodine suggests that dilation is mediated by nitric oxide and intracellular Ca2+. Whereas the contractile and dilatory responses to agonists remained intact after glutaraldehyde perfusion, both flow-induced constriction and dilation were eliminated, indicating that both types of flow responses result from endothelial cell deformation.

Identification algorithm for systemic arterial parameters with application to total artificial heart control.

A new algorithm for estimating systemic arterial parameters from systolic pressure and flow measurements at the root of the aorta is developed and tested through a systems identification approach. The resulting procedure has direct application to a total artificial heart (TAH) control system currently under development. Identification models, representing the systemic arterial system, are developed from existing work in the area of cardiovascular modeling. The resistive and compliance components of these models are physically significant, representing overall hydraulic properties of the systemic arterial system. A unique method of parameterizing the identification models is designed which operates on the basis of aortic pressure and flow measurements taken exclusively during systole. The estimator is a modified recursive least squares algorithm which utilizes covariance modification to track time-varying parameters and a dead-zone to improve the robustness. Performance of the estimation algorithm was tested on data generated by a higher-order distributed model of the systemic arterial bed using normal canine parameters. Results from model-to-model experiments verify the consistency of the estimates and the ability of the estimator to converge quickly and track dynamically varying parameters.

Design of a radio-linked implantable cochlear prosthesis using surface acoustic wave devices.

Cochlear prosthesis systems for postlingually deaf individuals (those who have become deaf due to disease or injury after having developed mature speech capability) are considered. These systems require the surgical implantation of an array of electrodes within the cochlea and are driven by processed sound signals from outside the body. A system that uses an analog signal approach for transcutaneous transfer of six processed speech data channels using frequency multiplexing is described. The system utilizes a filterbank of six narrowband surface acoustic wave (SAW) filters in the range 72-78 MHz with a 1.2-MHz channel spacing to multiplex the six carrier signals, frequency modulated, by the processed speech signals, onto a composite signal. The same SAW filters are used in the receiver filterbank for signal separation, but are housed in a miniaturized package. The system includes a portable transmitter and a receiver package which is to be implanted in the patient. The implanted circuits are supplied exclusively from power transferred from outside the body via a separate 10-MHz transcutaneous link.

A multichannel PWM telemetry system for kinematic gait analysis.

A multichannel biotelemetry system using pulse-width modulation-frequency modulation (PWM-FM) is described in detail for laboratory construction. Its application in a kinematic gait-analysis system is demonstrated, employing minimally encumbering electrogoniometry and foot-contact switches. The triaxial electrogoniometers sense rotational joint motion, and four foot-switches under the sole of each foot provide information on placement and temporal contact. Signals from the multiple sensors are amplified, encoded by pulse-width modulation, and transmitted at an FM radio frequency of 107 MHz. Received data are decoded and then sampled by a minicomputer for analysis. Results from a comparative study of kinematic gait in five normal subjects and five children with cerebral palsy demonstrate system effectiveness in providing quantitative data and various intrasubject and intersubject gait differences. Factors reviewed in the analysis include swing and stance times; cadence; hip-joint motion in sagittal, coronal, and transverse planes; and sequence of foot placement.

Determination of cardiac output using ensemble-averaged impedance cardiograms.

Although impedance cardiography provides safe and reliable noninvasive estimates of stroke volume in humans, its usefulness is limited by the necessity for subjects to be apneic and motionless. In an effort to circumvent this restriction we studied the validity of ensemble-averaging of impedance data in exercising normal subjects and in intensive-care patients. The correlation coefficient (r value) between 128 ensemble-averaged and standard hand-digitized determinations of stroke volume index from the same records taken during rest and exercise in six normal male subjects was +0.97 (P less than 0.001). The r value for ensemble-averaged stroke volume indices during free breathing and breath hold in the same subjects was +0.92 (P less than 0.001), suggesting that breath hold did not significantly affect the stroke volume estimation. In 14 freely breathing hospital intensive-care patients the r value between simultaneous thermodilution cardiac output readings and ensemble-averaged impedance determinations was +0.87 (P less than 0.01). The results indicate that ensemble-averaging of transthoracic impedance data provides waveforms from which reliable estimates of cardiac output can be made during normal respiration in healthy human subjects at rest and exercise and in critically ill patients.

Biomedical telemetry techniques.

The landmarks and important state-of-the-art work in biomedical telemetry are reviewed. The advantages provided by transmission of physiologic data from minimally restrained to completely unencumbered subjects with biomedical telemetry systems is discussed both from the standpoint of the wide variety of physiologic parameters and subjects that can be monitored and in terms of the various proven electronic techniques for implementing this mode of physiologic data transmission and recovery. Monitored parameter capabilities now include biopotentials (ECG, EMG, EEG), temperature, mechanical events (muscle force, limb motion), pH, pressure (ICP, blood pressure), and others. The system developments in biomedical telemetry have had the aims of minimizing size, weight, and volume while extending the operating capabilities of the systems in terms of minimum required power, multiplicity of data channels, and reliable operation in the intended operating environment. These developments have progressed from discrete transistor to integrated microcircuit implementations in systems which can encode the physiologic data as FM, PAM, PWM, or PCM. Biotelemeters can be utilized as external (backpack) or completely implantable devices and today may be found in the research laboratory, the intensive care unit, portable patient care units, and in the freely ranging animal in the field usually employing a radio link. Also discussed are considerations for power sources and power source recharging through the tissues of implanted subjects. There are advantages and disadvantages to using biomedical telemetry, to using implanted or external biotelemeters, and to each of the various encoding techniques. These are considered, and the future of biomedical telemetry is projected.

The effect of an extraluminal transducer on the reproductive capability of the rabbit.

An extraluminally attached microminiature force transducer, designed for reproductive tract contractility measurements, was investigated in terms of its effects on the physiology of the uterotubal junction in the rabbit. The presence or attachment process of the extraluminal force transducer (EFT) did not affect the rate of pregnancy or the number of embryonic implantation sites whether the attached device was silicone rubber or polyethylene-encased. The uterotubal junction was able to retain the blastocyst for the required time after mating, while an examination of postembryonic mortality revealed a rate of one resorption in some experimental groups. The gestation period was unaffected by the EFT, ranging from 30 to 40 days with a mode of 32, while histologic examinations revealed formation of a thin fibroblastic layer, some increased vascularity, and no abnormal leukocytic accumulation.

Studies of rabbit ovarian contractility using chronically implanted transducers.

A chronically implanted, highly sensitive force transducer was used to study ovarian contractions in the rabbit. The transducer is implanted into the medulla of the ovary through its long axis. The leads are then drawn through the abdominal wall and directed subcutaneously to the back of the neck. The miniature pin connector is fixed into place beneath the skin to facilitate accessibility for repeated recordings, obviating the necessity for anesthesia or serial laparotomies. Intraovarian transducers were implanted in isolated rabbits. Serial recordings of ovarian contractions were made at weekly intervals for 3 weeks, following which ovulation was induced with 100 IU of human chorionic gonadotropin (HCG). Extended recordings were made from 3 through 13 hours after HCG administration to six animals. Recordings revealed increased ovarian contractile activity beginning just before the anticipated time of ovulation. The observations support those of previous acute experiments and demonstrate a relationship between ovarian contractile activity and the process of follicle rupture.