Description of a closed window technique for in vivo study of the feline basilar artery

Modelling vascular reactivity to investigate the basis of the relationship between cerebral blood volume and flow under CO2 manipulation

Changes in cerebral blood flow (f) and vascular volume (v) are of major interest in mapping cerebral activity and metabolism, but the relation between them currently lacks a sufficient theoretical basis. To address this we considered three models: a uniform reactive tube model (M1); an extension of M1 that includes passive arterial inflow and venous volume (M2); and a more anatomically plausible model (M3) consisting of 19 compartments representing the whole range of vascular sizes and respective CO2 reactivities, derived from literature data. We find that M2 cannot be described as the simple scaling of a tube law, but any divergence from a linear approximation is negligible within the narrow physiological range encountered experimentally. In order to represent correctly the empirically observed slope of the overall v-f relationship, the reactive bed should constitute about half of the total vascular volume, thus including a significant fraction of capillaries and/or veins. Model M3 demonstrates systematic variation of the slope of the v-f relationship between 0.16 and 1.0, depending on the vascular compartment under consideration. This is further complicated when other experimental approaches such as flow velocity are used as substitute measurements. The effect is particularly large in microvascular compartments, but when averaged with larger vessels the variations in slope are contained within 0.25 to 0.55 under conditions typical for imaging methods. We conclude that the v-f relationship is not a fixed function but that both the shape and slope depend on the composition of the reactive volume and the experimental methods used.

Transcranial Doppler sonography and internal jugular bulb saturation during hyperventilation in patients with fulminant hepatic failure

Mechanical hyperventilation is often used to postpone or ameliorate intracranial hypertension in patients with fulminant hepatic failure (FHF). Because such treatment may critically reduce cerebral blood flow (CBF), bedside techniques to monitor CBF are warranted. In this study, we evaluated the efficacy of transcranial Doppler (TCD) sonography of the middle cerebral artery (MCA) and internal jugular bulb saturation (svJO(2)) to determine relative changes in CBF during mechanical hyperventilation in 8 patients with FHF (median age, 40 years; range, 20 to 54 years). We found that TCD and svJO(2) decreased during hyperventilation in parallel with CBF, determined by the xenon 133 ((133)Xe) washout technique. Quantitatively, the TCD method was less accurate to determine carbon dioxide (CO(2)) reactivity compared with svJO(2) and the (133)Xe technique. This indicates a slight change in MCA diameter during hyperventilation. We conclude that TCD and svJO(2) monitoring may give valuable information on relative changes in CBF during hyperventilation. However, the TCD method appears less accurate for quantitative estimation of CO(2) reactivity in patients with FHF.

A new model for in vivo observation of the feline spinal microcirculation: the closed spinal window

A new experimental model is described that uniquely allows the in vivo observation and quantification of vascular caliber changes on the dorsal surface of the feline spinal cord. The model consists of a rectangular Plexiglas window that is sutured to the lumbar dura and is supported by a special holder. Inlet and outlet tubes attached to the window serve for topical applications of mock cerebrospinal fluid or vasoactive agents to the surface of the cord and for continuous monitoring of intrathecal pressure. Pial vessels below the window were observed at 200-fold magnification with the aid of a microvideo camera. Spinal arterioles reacted to hypercarbia and superfusion with acetylcholine solution in a manner similar to cerebral arterioles. Tests with increased intrathecal pressure showed that the window remained watertight between 25 and 130 mm Hg, with an average leakage pressure of 57.8 +/- 33.5 mm Hg. To promote the use of this model in other laboratories, the authors give a detailed description of the closed spinal window preparation and report their experiences gained from 50 experiments. It is concluded that the closed spinal window is a highly reproducible model, suitable for the study of the feline spinal microcirculation for several hours in vivo.