Prison prescribing practice: practitioners' perspectives on why prison is different.

The current prison population in England and Wales has multiple, complex healthcare needs, presenting unique challenges to those caring for prisoners. Prison numbers have increased dramatically in the last 10 years. There are now approximately 84,000 prisoners in England and Wales and 120,000 new episodes of imprisonment each year . The authors all contribute to prison healthcare. Below, we discuss a key issue arising from first-hand experience of prisoners' health and social care needs, the prescription of psycho-active drugs by primary and secondary care practitioners. This is a core medical task, but beset with difficulties. These difficulties are not necessarily encountered in other areas of prison healthcare. However, they do illustrate how providing healthcare to prisoners is complex, often lacking a research base and can have pitfalls that are not obvious to the outsider.

The vertex-positive scalp potential evoked by faces and by objects.

The influence of stimulus form on the scalp-recorded "vertex positive peak" (VPP) evoked by images of faces (Jeffreys 1989a) was studied in seven subjects. In separate experiments, we recorded the responses to 2D images of: (1) many different depictions of human faces; (2) the heads of several different species; (3) many familiar non-face objects; and (4) stimuli where the configuration of objects were modified to produce an "illusory" or "non-contextual" subjective impression of a face. The results showed that every facial representation, including the "illusory" stimuli, and most of the non-face objects, evoked a VPP of corresponding form and scalp distribution. The object-evoked VPPs, however, were always smaller and usually later than those evoked by the faces. VPPs of longer latency but often comparable amplitude were also recorded for impoverished compared to well-defined facial representations; and for most non-human compared to human faces. Very consistent responses were recorded to repeated presentations of the same stimulus for the same subject, but there was considerable variation in latency as well as amplitude (but not form) of the VPP evoked under identical experimental conditions for different subjects. These response properties of the VPP, suggest that its underlying physiological generators are sensitive to basic configural properties of the visual stimulus; and also the face- and object-related information are processed in the same brain area(s), although not necessarily by the same physiological mechanisms.

Evoked potential evidence for human brain mechanisms that respond to single, fixated faces.

The influence of visual fixation position and stimulus size on the scalp-recorded "vertex positive peak" (VPP) evoked by images of faces was studied in three subjects. Responses were recorded, in turn, for line-drawn, frontal-view faces of approximately 8, 4, 2, and 1 deg length, fixated at the centre (bridge of the nose), and at points 1, 2, 3, and 4 deg to the left and right, and above and below, centre. The results showed that central fixation produced VPPs of similar, maximal amplitude for all face sizes. By comparison, "on-face" eccentric viewing yielded attenuated and delayed responses, and the degree of response attenuation as a function of eccentricity was directly related to the face size, with similar amplitude responses being evoked for corresponding fixation locations on each face. Very small or no VPPs were recorded for most "off-face" fixations. Similar results were observed for profile faces, except that the maximal VPP was recorded for fixations near the eyes and not in the centre of the head, and almost identical VPPs were evoked by a centrally fixated face presented with and without an adjacent face or object. These response properties, which correspond to the subjective perception of the facial stimuli, suggest that the VPP reflects brain mechanisms optimized to respond to single, fixated faces, irrespective both of facial image size and of the presence of neighbouring figures.

Input impedance of the canine vertebral artery.

In experiments on six anaesthetized dogs it was found that the vertebral artery contributed 69.2 +/- 1.7% of cerebral and 41.5 +/- 2.5% of cephalic flow. The input characteristics were that the vertebral artery had a flow of 0.71 +/- 0.05 ml X s-1 into an input impedance of 23.7 +/- 1.9 GN X m-5 X s. This compared to external and internal carotid flows of 0.69 +/- 0.05 and 0.31 +/- 0.02 ml X s-1 into input impedances of 23.9 +/- 2.1 and 52.1 +/- 2.7 GN X m-5 X s respectively. Standardized per 100 g of tissue the impedances of the vertebral and internal carotid arteries were similar at 18.6 +/- 2.0 and 19.1 +/- 1.5 GN X m-5 X s X 100 g-1 respectively, while that for the external carotid was higher at 129.9 +/- 10.2 GN X m-5 X s X 100 g-1. The impedance spectrum for the vertebral artery was of low impedance type, settling about a characteristic impedance of 2.01 +/- 0.28 GN X m-5 X s without significant oscillation, associated with the phase only becoming slightly positive above 10 Hz and reflexion coefficients of less than 0.25 by a frequency of 6 Hz. This was similar to the spectrum for the internal carotid artery and contrasted with the high impedance spectrum with major reflexion found for the external carotid artery.

Contribution of internal and external carotid beds to common carotid artery input impedance in the dog.

The input impedance of the canine common carotid artery has been obtained using high fidelity pressure and flow recordings at the level of the superior thyroid artery. From the impedance spectra initially, and following internal and external carotid artery occlusion, separate estimations of internal carotid, external carotid and external/internal collateral bed impedances were derived by the use of network theory. Studies were carried out on six dogs. The common carotid input resistance was 10.98 +/- 1.42 (S.E.M.) X 10(6) kN.m-5.s, while those for the internal, external and collateral beds were 48.80 +/- 7.12, 14.94 +/- 2.37 and 171.42 +/- 46.83 X 10(6) kN.m-5.s respectively. The impedance spectrum of the common carotid, external carotid and collateral beds were of a high impedance type with major reflexions, while that for the internal carotid bed was of low impedance type with much less reflexion. On standardization of impedance for tissue mass the resistance levels became 17.44 +/- 2.91 X 10(6) kN.m-5.s. 100 g-1 for internal and 85.41 +/- 4.75 X 10(6) kN.m-5.s. 100 g-1 for external carotid beds. Thus, common carotid input impedance differences between dog and man are due to differences in relative mass of tissue perfused by internal and external carotid arteries, rather than to different peripheral-bed characteristics in the two species.

Method of assessing the contribution of components of an anastomosing vascular network to total vascular impedance.

The cerebral circulation has extensive communications between its four feeder arteries, their tributaries, and the extracerebral arteries. A theoretically derived method and its experimental verification is described by which the various impedances in a network can be quantified. It involved serial observations of pulsatile pressure, flow, and input impedance, in response to a pattern of input vessel occlusions. Fourier analysis permitted the response of the network to be studied for the component frequencies, so that linear and phasor algebra could be used to obtain a solution. Possible errors introduced by assumptions in the original model are minimal, and do not invalidate the method in practice.