Cerebrovascular Responses To Hypotension And Hypercapnia In The Rabbit: Effect Of Alpha-receptor Blockade And Carotid Artery Occlusion

In this study, the responsiveness of the cerebrovasculature to hypercapnia provided an indication of the residual capacity for dilation available (ie. the dilative reserve). The relationship between cerebral blood flow (CBF) and pial vessel caliber responses and the dependence of these responses on the dilative reserve were examined in three groups of rabbits: 31 control animals; 21 phenoxybenzamine treated animals (alpha adrenoreceptor blockade); and 21 animals subjected to carotid artery occulusion. The effects of hemorrhagic hypotension on both CBF (H(,2) clearance) and pial vessel caliber (image splitting technique) and on the cerebrovascular responsiveness to hypercapnia (the CO(,2) response) were investigated in each group.;Three characteristic autoregulatory regions were identified. (1) During moderate reductions in perfusion pressure (PP) the pial vessels (\u3c 200 (mu)m) dilated progressively and CBF autoregulation was complete. The dilative reserve was functionally intact as the CBF CO(,2) response was constant and that of the pial vessels increased. (2) At intermediate PP, autoregulatory pial vessel dilation continued and CBF declined gradually (incomplete autoregulation). Both the pial vessel and CBF CO(,2) responses decreased, indicating a reduced dilative reserve. (3) The lower limit of autoregulation occurred at a cerebral PP of approx. 35 mm Hg. Both CBF and pial vessel caliber decreased pressure passively and there was a complete loss of CO(,2) reactivity--a depletion of the dilative reserve. Phenoxybenzamine infusion resulted in an increase in the dilative reserve and a relative improvement in CBF autoregulation. These changes were related to the decrease in CBF and metabolism caused by phenoxybenzamine. Carotid artery occulsion reduced the pressure in the Circle of Willis. This decreased the dilative reserve and shifted the lower limit of autoregulation to a PP of 45 mm Hg. The dilative reserve and the efficiency of CBF autoregulation were strongly interrelated. Changes in total precapillary resistance closely paralleled alterations in pial vascular resistance whereas the large inflow arteries were unreactive. The pial vessel responses appeared to be qualitatively representative of those occurring in the intraparenchymal vasculature

The puzzle of non-participation in continuing training : an empirical study of chronic vs. temporary non-participation

"Although participation in continuing vocational training is often found to be associated with considerable individual benefits, a puzzlingly large number of people still do not take part in training. In order to solve the puzzle we distinguish between temporary and chronic non-participants. Previous studies have shown that training participants and non-participants differ in unobservable characteristics and therefore self-select into training or not. We show that even non-participants cannot be treated as a homogeneous group: there are those who never take part in training (chronic non-participants) and those who are not currently taking part (temporary (non-)participants). Using a unique data set of non-participants commissioned by the German 'Expert Commission on Financing Lifelong Learning' and covering a very large number of individuals not taking part in training, we separate and compare chronic and temporary non-participants. By estimating a sample selection model using maximum likelihood estimation we take potential selection effects into account: temporary (non-)participants may be more motivated or may have different inherent skills than chronic nonparticipants. We find that chronic non-participants would have higher costs than temporary (non-)participants and their short-term benefits associated with their current jobs would be lower. However, in the long run even chronic non-participants would benefit similarly from participation due to improved prospects on the labor market. The results indicate that chronic non-participants either misperceive future developments or suffer from an exceptionally high discount rate, which in turn leads in their view to a negative cost-benefit ratio for training." (Author's abstract, IAB-Doku) ((en)) Additional Information Kurzfassung (deutsch) Executive summary (English)Weiterbildung, Teilnehmer, Bildungsbeteiligung, Bildungsinvestitionen, Bildungsertrag, Kosten-Nutzen-Analyse, Bildungsökonomie

The cerebrovascular effects of adrenaline, noradrenaline and dopamine infusions under propofol and isoflurane anaesthesia in sheep

Publisher's copy made available with the permission of the publisher © Australian Society of AnaesthetistsInfusions of catecholamines are frequently administered to patients receiving propofol or isoflurane anaesthesia. Interactions between these drugs may affect regional circulations, such as the brain. The aim of this animal (sheep) study was to determine the effects of ramped infusions of adrenaline, noradrenaline (10, 20, 40 µg/min) and dopamine (10, 20, 40 µg/kg/min) on cerebral blood flow (CBF), intracranial pressure (ICP), cerebrovascular resistance (CVR) and cerebral metabolic rate for oxygen (CMRO₂). These measurements were made under awake physiological conditions, and during continuous propofol (15 mg/min) or 2% isoflurane anaesthesia. All three catecholamines significantly and equivalently increased mean arterial pressure from baseline in a dose-dependent manner in the three cohorts (P0.05). Under propofol (n=6) and isoflurane (n=6), all three catecholamines significantly increased CBF (P<0.001). Dopamine caused the greatest increase in CBF, and was associated with significant increases in ICP (awake: P<0.001; propofol P<0.05; isoflurane P<0.001) and CVR (isoflurane P<0.05). No significant changes in CMRO₂ were demonstrated. Under propofol and isoflurane anaesthesia, the cerebrovascular effects of catecholamines were significantly different from the awake, physiological state, with dopamine demonstrating the most pronounced effects, particularly under propofol. Dopamine-induced hyperaemia was associated with other cerebrovascular changes. In the presence of an equivalent effect on mean arterial pressure, the exaggerated cerebrovascular effects under anaesthesia appear to be centrally mediated, possibly induced by propofol- or isoflurane-dependent changes in blood-brain barrier permeability, thereby causing a direct influence on the cerebral vasculature.http://www.aaic.net.au/Article.asp?D=200205