Photothrombosis in rabbit brain cortex: follow up by continuous pO2 measurement.

Continuous recording of changes in local pO2 during and after brain infarction in surviving animals which can be followed for months or years, may provide interesting information concerning pathophysiology and treatment of stroke and thrombosis. We performed such measurements before, during and till 4 weeks after photochemical induction of a cerebrocortical infarction in three rabbits. Rose bengal--a photosensitive dye which sticks to endothelial cells and gives rise to endothelial damage and thrombosis when illuminated--was injected intravenously. After injection, a circular area (diameter 3 mm) of the brain cortex was illuminated using an optic fiber conducting light from a halogen lamp, whether or not filtered by heat and colour filters. In order to enable pO2 measurement in and near the infarct zone, we constructed a transparent plastic frame in which pO2 electrodes were fixed beneath and 1 mm besides a shaft permitting mounting of the optic fiber. A black adhesive ring (inner diameter 3 mm) was attached to the bottom of the frame providing a perfectly demarcated illumination area. After fixation the electrodes were calibrated and the frame was implanted in the rabbit's skull. Ten days later an infarction was induced; pO2 was monitored continuously before, during and till 4 hours after this induction. Furthermore, pO2 was recorded 24 hours, 48 hours, 5 days, 14 days and 4 weeks after infarction. Parameters describing the time course of pO2 were determined. In the illuminated area pO2 decreased after a certain latency time to reach a very low level, probably zero level, where it remained for at least 24 hours. Gradually, recovery was observed during the following days, and four weeks after infarction both level and pattern of electrode current appeared to be normal again. In the border zone pO2 decreased but did not reach zero level. Recovery was observed earlier than in the illuminated area.

Construction, calibration and evaluation of pO2 electrodes for chronical implantation in the rabbit brain cortex.

Aiming at continuous polarographic measurement of the mean pO2 in the rabbit brain cortex before, during and after photochemically induced infarction, we designed and constructed monopolar platinum oxygen electrodes of the open type for chronical implantation. The measuring tip (length 1 mm, diameter 0.1 mm) is covered with a homogenous membrane of cellulose acetate. The electrode currents are measured by a four-channel amplifier of proper design; the device permits accurate and stable polarisation, identical for each channel. Moreover, a calibration device has been constructed. It consists of a Buchner funnel filled with Ringer solution and mounted in a temperature-controlled bath. In order to create a specific partial pressure of oxygen in the calibration chamber, predetermined gasmixtures are bubbled through the solution using computer controlled mass flow regulators. The calibration device thus permits the determination of primary and secondary electrode parameters, i.e. linearity, oxygen sensitivity and residual current, and polarisation dependency, temperature dependency, sensitivity to CO2, electrode stability, dynamic behaviour and oxygen consumption. Three groups, each of them containing ten electrodes, have been tested with regard to electrode parameters: the first group contains bare electrodes, the second and the third group contain membrane covered electrodes, with a membrane thickness of 10 and 20 microns respectively. In order to evaluate acute and long-term effects of implantation on the brain cortical tissue and on the sensors' measuring qualities, electrodes have been implanted for different time periods (51 days, 30 days, 9 days, 5 min). pO2 was recorded regularly and polarograms have been registered. The effects on cortical tissue have been studied with the aid of light microscopy.

Technical aspects and clinical results of an esophageal pick-up for better recording of posteriorly generated heart sounds.

A miniature esophageal microphone has been constructed. It has a cylindrical shape with 5-mm outer diameter and 25-mm length. It is a type of accelerometer that detects vibration from the esophageal wall. It has a resonance frequency of 2.1 kHz and an acceleration charge sensitivity of 2.8 pCm-1s2. The upper limit of the useful frequency range is about 1 kHz. It has directional sensitivity which reaches a maximum perpendicular to the plane of the crystal. With the esophageal microphone cardiovascular sound is recorded simultaneously with other tracings: external phonocardiogram, E.C.G., carotid pulse and M-mode echocardiogram. The great advantage of the application of the esophageal microphone is that it can be positioned in the esophagus close to the mitral valve and the left atrium by use of fluoroscopy, so that sounds and murmurs originating from that part of the heart are recorded in detail. In a case of a shunt between aorta and pulmonary artery the microphone was used at different levels in the esophagus, showing different amplitudes of a continuous murmur. In all other cases the probe was situated in the esophagus just above the level of the mitral valve behind the left atrium. In 2 cases the advantage of the esophageal microphone (revealing signs of slight mitral incompetence) over the body surface microphone is shown. In 3 patients with artificial valves in the aortic and mitral orifices (where left heart catheterisation and left ventricular angiography is a considerable risk), the esophageal microphone also improved the postoperative evaluation and diagnosis of mitral valve regurgitation considerably.

Standardization of a device for continuous observation of local flow in tissue.

As our experimental set-up for continuous recording of local blood flow in the cerebral cortex of a laboratory animal with chronically implanted miniature thermistors (based on the heat clearance principle) gave satisfactory results for routine tests of pharmacological agents during anoxia, hypoxia, hypercapnia, etc., experiments, we intended to standardize the apparatus, to increase accuracy, to facilitate calibration and to enhance flexibility with respect to the operator. The exponential aspect in the thermistor resistance/temperature characteristic is linearized by applying a logarithmic converter in the thermistor amplifier. Calibration to the centigrade temperature scale is performed by a three digit numerical adaptation of two thermistor constants determined in a thermostatic-cryostatic bath (zero and slope). A heating power measuring circuit is provided so that the dissipation constant of the thermistor implanted in tissue can be obtained and the thermal conductivity of the tissue can be estimated. Linearity of the relation between cooling of the heated thermistor and local flow, for small cooling values as they are registered in vivo, is still being investigated.

Assessment of the in vivo recording of local cerebral blood flow using a thermistor device.

In order to obtain a continuous measurement of local blood flow in the cerebral cortex of a laboratory animal using chronically implanted sensors, we have developed a device based on the heat clearance principle. Flow information is obtained from temperature measurement by means of two thermistors one of them being heated at a defined level above ambient tissue temperature; as such, cooling of the heated thermistor caused by convection phenomena in its vicinity, can be related to local perfusion rate. In a first step "in vitro" measurements were performed in order to study the behaviour, sensitivity and reliability of the device; a physical model was established explaining the results. In this paper we describe "in vivo" tests in the rabbit's brain cortex with the miniature thermistors (0.5 mm diameter) chronically implanted (at the cortical surface). Results are correlated with oxygen tension measurements using (smaller) pO2 electrodes inserted into the cortical tissue. We have observed that all sensors are well tolerated by the animals who remain symptom free. Test experiments, inducing a well known physiological effect on local blood flow, such as arterial clamping, inhalation of CO2 gas mixtures, etc., are performed. The phenomena during induced anoxic anoxia are also shown. These preliminary investigations are essential in order to attempt by future experiments the establishment of a correlation between "in vivo" recorded flow signals and the "in vitro" measured characteristics.

Discriminant parameters representing cerebral cortical function during anoxic anoxia investigations.

In order to quantify the effect of specific drugs on the cerebral cortex an "in vivo" model has been developed for the induction and the observation of anoxic anoxia. Rabbits are used as test animals. Sensors for the assessment of local parameters are chronically implanted: ECoG electrodes are applied; pO2 electrodes are inserted into the cortical tissue. The derived somatosensory evoked potentials are used for evaluating the cerebral cortical function. Animals are cannulated with a tracheal tube, curarized and artificially ventilated. Anoxic anoxia, controlled by a special purpose microprocessor system, may then be induced and repeated in a reproducible way. During the experiments local (pO2, ECoG, SEP) as well as general parameters (ECG, heart rate, systemic blood pressure, rectal temperature) are recorded and stored on analog magnetic tape as well as digitized with the microprocessor system. The cortical pO2 is measured with a polarographic method, the SEP's are obtained on-line by time coherent averaging and the ECoG states (e.g. epilepsy during anoxia) are derived by using band-pass filters and rms detectors. Off-line the signals (pO2, heart rate, mean systemic blood pressure, temperature) are standardized and represented together with parameters derived from ECoG and SEP. SEP-waveform parameters indicating intensity (norm) and similarity with a reference SEP signal (correlation value) are used. The measuring and processing method is still being optimized; special attention is being paid towards the quality of the calculated SEP's which are to be used for the quantification of the cortical function during reference, anoxia and recovery period. As such, in order to improve the signal-to-noise ratio of the SEP's and, consequently, of the derived parameters, ECoG signals are digitized off-line and subjected to a preprocessing, implying filtering and spectral analysis procedures.

A standardized 'in vivo' model for the study of experimental arterial thrombosis: description of a method.

A method for "in vivo" induction and registration of arterial platelet thrombosis has been developed and standardized in a branch of the mesenteric artery of the white Wistar rat. It consists in local deendothelialization by electrical current. Thrombus induction is performed by topical superfusion with ADP; when the superfusion is discontinued the thrombus disappears but can again be induced reproducibly by renewal of the ADP superfusion after a time interval not exceeding 15 minutes. Registration of the thrombotic phenomenon is made possible by microprojection of the investigated arterial segment. An appropriate optoelectronic device allows the on-line derivation of discriminating parameters. Off-line storage and processing of the experimental data by computer is provided. The method is automatized and can easily be applied to other species of laboratory animals.

A thermistor device for the continuous recording of mass transport velocity in tissue based on the heat clearance principle.

A method for the recording of mass transport velocity in tissue, related to the relative local mass flow, has been developed. It is based on the heat clearance principle, applied in the continuous mode. Thermistors are used as temperature sensors; heating is performed by a high frequency square wave, thus avoiding an additional heating coil. The cooling of the heated thermistor, with as a reference a non-heated one, is related to the local mass transport velocity. A test equipment has been built for the calibration of the device and a set of results have been obtained in test fluids with a thermal conductivity approaching the one of brain tissue (glycerin and water). A mathematical model has been set up which gave results consistent with the recorded data. From this model significant parameters could be derived which, introduced in a compensation circuit, may be used in order to linearize the relation between the measuring value and the velocity variable. In vivo results in the brain cortex of the rabbit have been obtained.