Dynamic heterogeneity of cerebral hypoperfusion after prolonged cardiac arrest in dogs measured by the stable xenon/CT technique: a preliminary study.

After prolonged cardiac arrest and reperfusion, global cerebral blood flow (gCBF) is decreased to about 50% normal for many hours. Measurement of gCBF does not reveal regional variation of flow or permit testing of hypotheses involving multifocal no-flow or low-flow areas. We employed the noninvasive stable Xenon-enhanced Computerized Tomography (Xe/CT) local CBF (LCBF) method for use in dogs before and after ventricular fibrillation (VF) cardiac arrest of 10 min. This was followed by external cardiopulmonary resuscitation (CPR) and control of cardiovascular pulmonary variables to 7 h postarrest. In a sham (no arrest) experiment, the three CT levels studied showed normal regional heterogeneity of LCBF values, all between 10 and 75 ml/100 cm3 per min for white matter and 20 and 130 ml/100 cm3 per min for gray matter. In four preliminary CPR experiments, the expected global hyperemia at 15 min after arrest, was followed by hypoperfusion with gCBF reduced to about 50% control and increased heterogeneity of LCBF. Trickle flow areas (LCBF less than 10 ml/100 cm3 per min) not present prearrest, were interspersed among regions of low, normal, or even high flow. Regions of 125-500 mm3 with trickle flow or higher flows, in different areas at different times, involving deep and superficial structures migrated and persisted to 6 h, with gCBF remaining low. These preliminary results suggest: no initial no-reflow foci (less than 10 ml/100 cm3 per min) larger than 125 mm3 persisting through the initial global hyperemic phase; delayed multifocal hypoperfusion more severe than suggested by gCBF measurements; and trickle flow areas caused by dynamic factors.

Electrochemical glucose sensing at low potentials.

An electrochemical sensing method was studied, employing cyclic voltammetry at smooth Pt-electrodes in the low potential region. It has certain advantages over methods that employ higher potentials or constant voltage amperometry. There are also advantages over methods employing active enzymes or other unstable elements. In addition to a specific scanning potential we have studied voltage-delay-pulsing techniques for electrode rejuvenation and innovative methods of creating analyte-selective covering membrane. At potentials below -0.6 V vs Ag/AgCl, peaks of adsorbed glucose species have been observed in cyclic voltammograms. These peaks at about -0.7 V (oxidation peak) and -0.8 V (reduction peak) have yielded reproducible current-glucose concentration relationships, linear through the region of clinically important glucose concentrations (50-300 mg/dl) and stable over time. Since these potentials are not in the redox range of many potentially interfering substances, selectivity is enhanced.

Glucose concentration at possible sensor tissue implant sites.

It is generally acknowledged that the ideal automatic insulin infusion system would be a closed loop that metered insulin delivery in response to a feedback sensor such as an implantable glucose detector. Most current efforts are aimed at a transducer located within the blood vascular tree. We believe that the blood constitutes an especially hostile environment for such a device. The possibility of placing the sensor with a reasonably rapid response to dynamic alterations in glucose metabolism in a space containing fluid outside the bloodstream was studied. The subcutaneous extracellular space, peritoneum, pleura, and pericardium were included. Generally, the concentration of glucose ranged between 50 and 115 mg/dl under steady-state conditions. The experimental design did not permit monitoring rapid responses to artificially induced dynamic changes. There were several situations where lower values were recorded, suggesting that a wide range of concentrations might occur. The authors have concluded that these experimental results are compatible with the possibility of a suitable locus for the glucose sensor in the extracellular, extravascular space.