Biomedical subjects
G Johnson
Publications and source records attributed to G Johnson.
Effects of instructional design variables on vocabulary acquisition of LD students: a study of computer-assisted instruction.
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Effect of hemorrhagic shock upon spinal cord blood flow and evoked potentials.
Although somatosensory evoked potentials are being used in the evaluation of spinal cord injury and the monitoring of cord function during surgical procedures, their limitations in the face of fluctuations in blood pressure await further clarification. To study the effect of hypotension upon spinal cord blood flow (SCBF) and spinal evoked potentials (SEPs), we subjected five anesthetized lambs to graded hypotension to a mean arterial pressure (MAP) of 80, 60, 40, and less than or equal to 30 mm Hg. Five animals served as controls. Maximal hypotension was associated with a significant decrease in renal and sciatic nerve blood flow of 83% and 77%, respectively. SCBF, on the other hand, showed no decrease with hypotension down to a MAP of 40 mm Hg. As hypotension progressed, a gradual but significant slowing in nerve conduction velocity was noted without alteration in the cord conduction velocity from L7 to L1. Control animals demonstrated a decline in sciatic nerve blood flow of 48%, without any change in latency or amplitude of nerve action potentials. These findings suggest that, in hypotension, the peripheral nerve (lacking autoregulation) becomes ischemic, resulting in slowing of nerve conduction velocity with an increase in latency and decrease in amplitude of SEPs. Cord conduction velocity, on the other hand, remains unchanged. Caution is advised in relying upon SEPs generated by peripheral nerve stimulation to monitor cord function in situations where profound hypotension is anticipated. Variations in SEPs may reflect alterations in the peripheral nerve and not the spinal cord.
In vitro senescence of human keratinocyte cultures.
Human keratinocytes have been serially cultivated in low (0.015 mM) and high (1.8 mM) calcium containing medium. The calcium concentration of the growth medium significantly influenced the cell growth period in vitro. Cells grown in low calcium medium underwent 35-40 population doublings over 16-17 passages, while cells grown in high calcium medium ceased to proliferate after 20 population doublings over 7 passages. Changing the keratinocytes from one in vitro environment to the other drastically altered the lifespan in culture of populations derived from the same primary tissue. The degree of DNA methylation of human keratinocytes was shown to decrease with age in both high and low calcium culture conditions but does not appear to be associated with differentiation.
Accuracy and precision in the measurement of relaxation times from nuclear magnetic resonance images.
The accuracy (proximity to the true value) and precision (reproducibility) of relaxation times derived from nuclear magnetic resonance images were investigated. Two methods of deriving relaxation times were considered. A patient scanning protocol in which the minimum number of scans necessary for the calculation (three) were performed. Calculated T1 and T2 images were then formed. An animal (cat) protocol in which many more scans were performed. The data were read from the display and fitted by computer to the theoretical curves. The accuracy of the measurements was determined by an empirical method. A series of bottles with different concentrations of MnCl2 and CuSO4 in water were prepared and their relaxation times determined using the imager as a simple pulsed spectrometer. These values were compared with those derived from images. Over the normal range of tissue values (T1 less than 700 ms, T2 less than 200 ms) the animal protocol gave values of T1 up to 1% shorter than the true values. The T2 values were up to 5% shorter. Patient protocol values were up to 7% shorter for T1 and up to 20% shorter for T2. There was some difference between results for MnCl2 and for CuSO4 (particularly for patient T2s), suggesting that the results depend to a small extent on the T1/T2 ratio. The precision of the values was investigated by considering the standard deviations (SDs) of brain tissue measurements over populations of cats (animal protocols) and normal control subjects and multiple sclerosis patients (patient protocols). These were compared with the SDs of measurements of calibration bottles scanned with the patients. Standard deviations of 3% for T1 and 6% for T2 were found over 19 cats using the animal protocols; SDs of 7% for T1 and 14% for T2 were found over 15 normal control subjects using the patient protocols. Standard deviations of bottle measurements were similar to these figures. There are also variations between different subjects and different regions of the brain. There was no significant change between readings on the same patient in follow-up studies. Other sources of variation in the measurements made with the patient protocols were investigated by scanning phantoms. Noise in T1 and T2 images is about 2%. Spatial non-uniformity within slices is about 1% for T1 and 10% for T2. Non-uniformity between slices in multislice sets is 4% for T1 and 14% for T2. There is no long-term variation in measured values over 9 months; short-term variation is approximately 1%.
The use of angiodynography to quantify blood flow in the canine aorta.
Previous attempts to quantify blood flow by means of ultrasound have been hampered by inaccurate diameter measurements and failure to account for the parabolic nature of the cross-sectional velocity profile. Angiodynography (ADG) provides a B-mode image of vessel walls, and the outer flow envelope is constantly monitored to provide phasic diameter measurements throughout the cardiac cycle. Frequency shifts are color-coded to provide a real-time visual display of varying velocity patterns across the vessel lumen throughout the cardiac cycle as well as calculation of average flow velocity and mean blood flow. Simultaneous flow values obtained with ADG were compared with electromagnetic flow probe (EMF) measurements in 12 open-chest dogs. Flow was varied with volume administration and exsanguination, and 201 data points were obtained. The average flow determined by EMF was 2.0 +/- 0.9 L/min compared with 2.04 +/- 0.71 L/min for ADG. The correlation coefficient was 0.88, the y-intercept was close to zero, and the slope approached unity, confirming the ability of ADG to accurately determine volume blood flow rather than merely to establish a flow trend. It is concluded that ADG offers an accurate way of quantifying volume blood flow.
Cognitive function, cognitive style and life satisfaction in a 68-year-old male population.
A representative sample of 68-year-old men living in the city of Malmö, Sweden, was examined by means of psychological tests and questionnaires regarding cognitive capacity, cognitive style as an expression of personality, and life satisfaction. Reference values for these parameters are presented, and a continuous cohort increase of verbal ability could be identified in this age group. That the prevalence of cognitive reduction was found to be surprisingly high is tentatively interpreted as reflecting an underdiagnosis of these symptoms as concomitant to other diseases in the general population. Earlier results supporting the concept of terminal decline could not be replicated. The reason for this might be that the present population was somewhat younger than those earlier investigated. Contrary to expectation, life satisfaction did not correlate to either cognitive reduction or cognitive style. It had, however, a positive correlation to measures of fluid intelligence.
Efficacy of military antishock trousers in compensatory and decompensatory hemorrhagic hypotension.
The compensatory cardiovascular response to hemorrhage includes a baroreceptor-induced activation of the sympathetic nervous system resulting in an attempt to reestablish MAP through peripheral vasoconstriction. If the hypotension is not reversed this compensatory vasoconstriction will progress to a loss of vascular tone known as vascular decompensation. The primary purpose of the present study was to compare the effectiveness of military antishock trousers (MAST) applied during the compensatory and decompensatory stages of hemorrhagic hypotension. MAST pressures of 30, 50, 70, and 90 mm Hg were applied during control, compensation, and decompensation. The results showed that MAST pressures up to 90 mm Hg were ineffective at raising mean arterial blood pressure (MAP) when applied to normotensive dogs; MAP increased 62% when MAST were applied during compensation as the result of a significant augmentation of cardiac output (stroke volume and heart rate) with no change in TPR; and a modest increase in MAP from 40 to 55 mm Hg occurred when MAST pressure was increased to 70 mm Hg during decompensation, which was accounted for entirely on the basis of an increased total peripheral resistance with no significant change in CO.
Mechanical stress stimulates aortic endothelial cells to proliferate.
The effects of applied cyclic tensional deformation and relaxation on cultured bovine aortic endothelial cells were examined. Endothelial cells from passages 3 to 9 were seeded in flexible-bottomed plates and allowed to attach for 24 hours. Endothelial cells in the experimental group (n = 6 wells per time point) were placed in a vacuum-operated stress-providing instrument that exerted an average elongation of 10% at maximum downward deflection of the culture plate bottom. The stretched endothelial cells were subjected to repeating cycles of 10 seconds elongation and 10 seconds relaxation from days 1 through 7 in culture. Endothelial cells in the control group (n = 6 wells per time point) were subjected to similar incubation conditions as the experimental group but without tensional deformation. Tritiated thymidine was added to cells 24 hours before harvesting. On days 0, 1, 3, 5, and 7 cells were counted and analyzed for trichloroacetic acid-precipitable tritiated thymidine incorporation. The results showed that 3 cycles/min mechanical stretching stimulated deoxyribonucleic acid synthesis and endothelial cell division. We conclude that cyclic tensional deformation may stimulate endothelial cell proliferation. It is possible that naturally occurring cyclic mechanical deformation in vivo, such as the repetitive stretching and relaxation of aortic tissue by the heart, may invoke a particular pattern of synthesis and division in endothelial cells.
Vascular surgery in the United States. Report of the Joint Society for Vascular Surgery--International Society for Cardiovascular Surgery Committee on Vascular Surgical Manpower.
The Joint Committee on Vascular Surgical Manpower was established in 1985 by the Society for Vascular Surgery and the North American Chapter of the International Society for Cardiovascular Surgery. It was charged to provide recommendations regarding vascular surgical manpower requirements for the next 15 years. Analysis of National Center for Health Statistics vascular operative rate data and 1690 questionnaire responses from vascular surgeons documented that vascular surgeons performed 235,400 (41%) of the total of 571,000 vascular operations undertaken in 1985. Vascular surgeons performed 87% of 30,000 aortoiliofemoral reconstructions, 77% of 72,000 peripheral vessel bypasses, 75% of 33,000 abdominal aortic aneurysm repairs, 59% of 55,000 angioaccess procedures, and 50% of 107,000 carotid endarterectomies. However, lack of accurate data on caseloads of surgeons who were not vascular specialists precludes precise prediction of manpower requirements for vascular surgery. It is important that surgical leaders and policy makers define the types of vascular surgical procedures that may be undertaken by vascular and other surgeons. Ongoing analyses must include such determinations to establish accurate data for the prediction of future manpower needs for vascular surgery. Furthermore, future manpower studies should be linked to outcome studies to assess not only numbers of surgeons and operations but quality of care as well.
Detection of optic nerve lesions in optic neuritis with magnetic resonance imaging.
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Relationship of staphylococcal tolerance, teichoic acid antibody, and serum bactericidal activity to therapeutic outcome in Staphylococcus aureus bacteremia.
A randomized cooperative study of therapy for Staphylococcus aureus bacteremia was conducted in which nafcillin was given for four or six weeks to patients with clinical endocarditis and for two or four weeks to those without evidence of endocarditis. Eighty-four patients were enrolled, and 32 completed treatment, all of whom had bacteriologic cures. Three patients, treated for two weeks, had complications that were undetectable by assay of serum teichoic acid antibody. Data were insufficient to allow conclusions regarding the optimal duration of therapy for patients with or without endocarditis. However, the results suggest that neither clinical nor immunologic methods can reliably detect complications in patients treated for two weeks only. In addition, patients infected with tolerant organisms remained febrile longer than those infected with nontolerant strains but did not require additional antibiotics for cure. Peak serum bactericidal activity at a dilution of 1:8 or greater was present in all patients. Serum bactericidal activity of 1:8 prior to an antibiotic dose was not necessary for cure.
Effect of the acute phase reaction on blood viscosity after infrainguinal arterial bypass.
A new method of determining apparent blood viscosity at low shear rates using a porous bed viscometer has been validated in an experimental study of canine blood altered by isovolemic hemodilution. The data confirm the ability of the technique to detect the non-Newtonian behavior of blood at higher hematocrit and fibrinogen values. The technique was then employed in a limited clinical study of nine patients undergoing infrainguinal vascular reconstruction to evaluate the effect of operation and the associated increase in serum fibrinogen levels, resulting from the acute phase reaction, on blood viscosity. Although there was a substantial change in serum glycoprotein levels, the concurrent relative decrease in red cell mass after operation resulted in a decrease in whole blood viscosity. These findings support the beneficial effect of moderate hemodilution as a means of compensating for the potentially detrimental effects of the acute phase reaction on blood rheology.
Frequency, severity, and physiologic importance of carotid siphon lesions.
Atherosclerotic stenosis of the intracranial portion of the internal carotid artery (SIPHON) may be the cause of neurologic symptoms. The frequency and severity of these lesions were studied in 226 vessels subjected to conventional two-plane carotid angiography, and the impact of sequential or tandem lesions on cerebral flow was assessed in the 167 arteries from this group that also underwent Gee oculoplethysmography (OPG-Gee), a commonly accepted indicator of the physiologic importance of internal carotid stenosis. Atherosclerotic involvement of the SIPHON was present in 46% (103/226) of the vessels studied, including six total occlusions, 11 severe stenoses (severe, 80% to 99% area reduction [AR]), 27 moderate stenoses (moderate, 50% to 79% AR), 40 mild stenoses (mild, 10% to 49% AR), and 19 instances of mural disease (MUR). This was compared with a 73% rate (166 of 226) of proximal internal carotid artery (PICA) stenosis in the neck, including six to 43 severe, 29 moderate, 50 mild, and 38 MUR. There was little correlation (r = 0.33) between the degree of angiographic stenosis in the PICA and the SIPHON. All of the totally occluded arteries had a positive OPG-Gee, whereas a positive study was noted in only 6% (7 of 109) of the arteries with less than moderate stenosis in the PICA. Twenty-one vessels had moderate stenosis of the PICA. Eight of these arteries had an associated moderate or severe lesion in the SIPHON, and seven of eight (88%) had a positive OPG-Gee. The remaining 13 of this subset had had a normal or mildly diseased SIPHON, and 12 of 13 (92%) had a normal OPG-Gee.(ABSTRACT TRUNCATED AT 250 WORDS)
Rifampin protection against experimental graft sepsis.
The risk of infection in vascular prosthetic conduits appears to be greatest in the perioperative period and the organism most frequently found is Staphylococcus aureus. Previous work suggests that antibiotics must be chemically bonded to the material to resist rapid washout caused by the flow of blood through the graft. The exception to this is rifampin, which remains fixed in Dacron prostheses after passive addition of the agent to aliquots of blood used to clot the interstices of porous Dacron grafts. This characteristic of rifampin is presumed to be caused by its poor water solubility. This potential infection resistance was challenged in a standard model of a canine infrarenal aortic graft by intravenous infusion of S. aureus organisms (10(7)) in the perioperative period. The grafts of five animals were preclotted with 9 ml of autogenous blood plus 1 ml of rifampin (60 mg/ml). A second group had similar procedures with 1 ml of cefazolin (238 mg/ml) substituted for the rifampin, and a control group had 1 ml of saline solution added to the 9 ml aliquot of blood. The animals were killed at 3 weeks and examined for clinically apparent infection. Rings of the graft material were also removed aseptically and cultured. All five grafts preclotted with cefazolin had clinical and culture evidence of infection (S. aureus), as did the grafts of three of the five control dogs. None of the grafts preclotted with rifampin was infected (p less than 0.05). Addition of rifampin to the blood used to clot the graft interstices appears to be a simple way of imparting graft resistance to perioperative sepsis.
Should ablative operations be used for bleeding esophageal varices?
To evaluate the long-term success of an ablative procedure for esophageal varices, the clinical results of 60 standardized, non-shunt (Womack) operations performed from 1953-1974 were reviewed. The overall operative mortality in this series was 35%. The 39 patients surviving operation have been followed from 5 to 21 years (mean: 13.3 years). Excluding operative mortality, the absolute 5-year survival rates for Child's classes A, B, and C patients were 100%, 63%, and 33%, respectively. The actuarial survival for all patients was 40% at 5 years, 24% at 10 years, and 15% at 15 years. Although the incidence of recurrent bleeding was 54%, clinical factors predictive for rebleeding could not be identified. A review of a collected series of other ablative operations, with and without esophageal transection, generally reveals unacceptable mortality and rebleeding rates. It is concluded that an ablative operation without esophageal transection should be used only in highly selected patients who do not have appropriate veins suitable for venous shunt.
Nuclear magnetic resonance. Its implications for the anaesthetist.
The theory and practice of clinical nuclear magnetic resonance (NMR) imaging is reviewed. Problems which the anaesthetist will encounter are considered, and recommendations are proposed. Possible uses of NMR in anaesthesia are discussed.
Magnetic resonance imaging of experimental cerebral oedema.
Triethyl tin(TET)-induced cerebral oedema has been studied in cats by magnetic resonance imaging (MRI), and the findings correlated with the histology and fine structure of the cerebrum following perfusion-fixation. MRI is a sensitive technique for detecting cerebral oedema, and the distribution and severity of the changes correlate closely with the morphological abnormalities. The relaxation times, T1 and T2 increase progressively as the oedema develops, and the proportional increase in T2 is approximately twice that in T1. Analysis of the magnetisation decay curves reveals slowly-relaxing and rapidly-relaxing components which probably correspond to oedema fluid and intracellular water respectively. The image appearances taken in conjunction with relaxation data provide a basis for determining the nature of the oedema in vivo.