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Biomedical subjects

L Symon

Publications and source records attributed to L Symon.

At least 235 records · Page 13Linked to original sources
PubMed1977

The radiological investigation of intramedullary spinal tumours.

The place of the various radiological techniques in the diagnosis of spinal cord tumours is described. With appropriate regard to the clinical picture, the choice of selective angiography. Metrizamide or Myodil myelography, and possible cyst puncture and endomyelogram, may be made with accuracy. Air myelography is less useful and less frequently employed. With these radiological aids, accurate localisation of tumour nodules for biopsy or excision is usually possible, and the necessity for long exploratory laminectomies avoided.

Angiography↗
PubMed1976

The effect of intracranial hypertension on cerebrovascular resistance--an experimental study.

Pulse transmission through cortex between a vein and artery in the middle cerebral distribution has been used as an index of vascular resistance in the middle cerebral field in baboons. Following catheterisation of these small vessels, the skull has been reconstituted and the effect of intracranial hypertension produced by cisternal infusion has been studied. An earlier linear relaxation of resistance in relation to rising intracranial pressure has been succeeded by a second phase in which pulse transmission increases quite profoundly despite minimal further increase in intracranial pressure. This appears to represent the exhaustion of the compliance of the vascular compartment, and corresponds closely to levels of intracranial pressure at which failure of autoregulation to increased intracranial pressure has been observed in previous experiments. Spontaneous bursts of transient increased pressure made their appearance in a number of animals at this level of pressure. The possible significance of increased pulse transmission following exhaustion of compliance in relation to breakdown of the blood brain barrier and potential spreading oedema is discussed.

Animals↗
PubMed1976

Measurements of oxygen tension in the cerebral cortex of baboons.

A polystyrene-covered platinum electrode (100-150 mum diameter) has been used to measure cortical tissue oxygen tension in baboon brains. The method of preparation, calibration, and the importance of small residual current (less than 40 nA) as an attribute of a reliable electrode, are described. With electrodes of this size, there was a large (16 +/- 12nA/torr) and linear current output with pO2 changes. The effect of avrious gases in addition to oxygen is described; halothane inhalation increases the apparent pO2 and hydrogen, used for blood flow estimations, reduces the recorded pO2. In 48 separate electrode placements in 13 baboons, the mean cortical qo2 was 23.8 +/- 12 mm Hg, with a range from 1-79 mm Hg; following occlusion of one middle cerebral artery, 37 electrodes recorded a pO2 of less then 5 mm Hg pO2 Oscillations were invariably noted in control conditions, independent of blood pressure; these waves disappeared during MCA occlusion and appeared to be augmented following release of the clip. Blood pressure "spikes" produce immediate and synchronous changes in all electrodes entirely different from the spontaneous waves. Such blood pressure changes may mask the true effect of hypercapnia on tissue pO2 and, if ignored, may lead to erroneous assumptions regarding local neural control of the circulation, the increased pO2 secondary to hypertension being regarded as evidence of regional vasodilation. A SUdden change in inspired pO2-the "air test"-was performed in control conditions and following the ischaemic insult, and the rate of change of cortical pO2 compared. The gradient was significantly greater (P less than 0.05) following ischaemia, suggesting a changed ratio in the tissue's flow to oxygen requirements and/or a persisting vasodilatation.

Animals↗
PubMed1976

Changes in regional cortical tissue oxygen tension and cerebral blood flow during temporary middle cerebral artery occlusion in baboons.

Cortical tissue oxygen measured by a platinum cathode, and cerebral blood flow recorded by a hydrogen clearance technique, were measured in 13 baboons before, during and after temporary occlusion of the middle cerebral artery. Mean control pO2 was 23.8 +/- 14 mm Hg and mean flow 51.3 +/- 12 ml/100g/min. During the occlusion, there was a gradation in pO2 from values in the opercular area of 3.6 +/- 5.9 mm Hg, to values in the high parietal area of 11.9 +/- 11.7, these being statistically different (P less than 0.05) from each other. The corresponding flow values were 5.5 +/- 7.5 (opercular) and 22.3 +/- 21.7 ml/100 g/min parietal (P less than 0.01). Following removal of the MCA clip, between 20% and 30% of the electrodes registered an early hyperoxia and hyperaemia, which lasted up to 5 min. A late and prolonged hyperoxia, with less evidence of hyperaemia, was also noted in about 20%. The mean tissue pO2, however, at 5-min intervals up to 40 min following the removal of the clip only reached 60-80% of control values in the most ischaemic areas. Only the parietal region showed a mean pO2 above control levels. The mean flow data were uniformly reduced in all regions to about 80% of control values. During and after a second occlusion in 6 animals, similar changes were noted but with even fewer instances of hyperoxia. The mean oxygen and flow results were lower than with the first occlusion, but the reduction was not significant. There was no overall effect of hypercapnia on cortical tissue pO2 during the control period, but there was a significant (P less than 0.05) reduction during the same procedure after the period of ischaemia. An increase in pO2 during hypercapnia could be observed if there were arousal responses of blood pressure "spikes".

Animals↗
PubMed1976

Cerebrovascular carbon dioxide reactivity and conductance in patients awake and under general anesthesia.

Paired cerebral blood flow measurements at two levels of arterial carbon dioxide (CO2) tension were made in 36 awake patients and 85 patients under general anesthesia to measure the percent change in flow per mm Hg change in PaCO2 (CO2 reactivity). CO2 reactivity was significantly greater in the generally anesthetized (5.96) than in the awake group (3.65). In both groups, a linear correlation was found between mean percent CO2 reactivity and conductance (the reciprocal of resistance), up to a level of conductance of 0.85 in awake and 0.95 in unconscious patients. When the CO2 reactivity of 18 awake patients and 59 unconscious patients was compared with that of respective control groups, only 11 patients were found to have abnormal CO2 reactivities for their conductance levels.

Anesthesia, General↗
PubMed1975

The vasodilator action of angiography with Urografin 60% on the basal vessels of the brain of the baboon.

Angiography under experimental conditions shows that Urografin has a vasodilator action of the order of 8% upon the basal cerebral arteries of the baboon. This action is not seen within the first 4 sec of the angiogram and the dilator effect of a single injection has passed off completely 15 min later. Three successive injections within 7 min show little evidence of cumulative vasodilatation. The vasodilator action of three successive angiograms has disappeared 22 to 29 min later. Because of wide variations in the degree of vasodilatation shown by individual baboons it may be necessary to use each baboon as its own control in subsequent work.

Angiography↗
PubMed1975

Cerebral blood flow in dementia.

Twenty-four patients of comparable age, blood pressure, and degree of dementia were classified by an "Ischemic Score" based on clinical features into "multi-infarct" and "primary degenerative" dementia. Regional cerebral blood flow (CBF) was measured by the intracarotid xenon 133 method. Both groups showed a decreased proportion of rapidly clearing brain tissue (largely gray matter). Cerebral blood flow per 100 gm brain per minute was normal in the primary degenerative group but low in the multi-infarct group. This suggests the blood flow is adequate for metabolic needs of the brain in patients with primary degenerative dementia but inadequate for those with multi-infarct dementia. There was no correlation between degree of dementia and CBF in the primary degenerative group but an inverse relationship existed in the multi-infarct group. Reactivity of blood vessels to reduction of arterial carbon dioxide pressure was normal in both groups.

Aged↗
PubMed1974

Effect of supratentorial space-occupying lesions on regional intracranial pressure and local cerebral blood flow: an experimental study in baboons.

Cortical blood flow and epidural intracranial pressure have been measured in the two supratentorial compartments of the intracranial space in experimental baboons during the acute expansion of a parieto-occipital epidural balloon. Differential pressures between the two halves of the supratentorial space have been found, and these have been associated with evidence that flow has fallen more quickly in the hemisphere most compressed. The evidence points to a more rapid exhaustion of the autoregulatory capacity in the hemisphere subjected to greater compression, a fall in perfusion pressure to below critical autoregulatory levels occurring slightly before that in the opposite hemisphere, and the establishment of a differential flow pattern for a short time during a critical phase of compression. The displacements induced by inflation of the parieto-occipital balloon have been described.

Animals↗
PubMed1974

Effect of changes in cerebral blood flow on proportion of high and low flow tissue in the brain.

Regional cerebral blood flow was measured by injection of (133)Xenon into the internal carotid artery in 11 patients with cerebrovascular disease. All patients were studied under general anaesthesia, first at normocapnia and then at hypocapnia. The 15 minute isotope clearance curves were analysed by computer by two-compartmental analysis and regional changes in flow and the proportions of fast and slow clearing tissue obtained at two levels of arterial CO(2) tension. Hypocapnia caused a fall in blood flow which was consistently accompanied by a decrease in the proportion of fast clearing tissue. Regional changes were not significantly different from the hemisphere mean changes. There was no correlation between changes in blood flow through grey matter and the proportion of fast clearing tissue on a hemisphere mean basis, but on regional analysis the data from 10 out of the 11 patients showed that in areas where blood flow through grey matter changed most the proportion of fast clearing tissue changed least and vice versa. A hypothesis has been proposed to explain this phenomenon.

Adult↗