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K W Johnston

Publications and source records attributed to K W Johnston.

At least 19 recordsLinked to original sources
PubMed1992

Visualization of complex flow fields, with application to the interpretation of colour flow Doppler images.

The interpretation of colour flow Doppler images in regions of complex flow can be difficult. To aid in such interpretations, we have developed software which accepts as input the results of a fluid flow simulation program and produces simulated colour flow Doppler images on a workstation. This allows direct comparison between well-described flow fields and colour flow Doppler images. Application of this approach is demonstrated for flow in a two-dimensional model of a distal graft-to-vessel anastomosis, and the interpretation of the results for three-dimensional flows is briefly discussed. In addition, a method for presenting two-dimensional (vectorial) velocity data via colour coding is presented.

Algorithms↗
PubMed1992

Factors that influence the outcome of aortoiliac and femoropopliteal percutaneous transluminal angioplasty.

In the past, patients with peripheral arterial occlusive disease were managed by conservative treatment or by vascular reconstructive surgery. Now, percutaneous transluminal angioplasty and other endovascular methods provide an important alternative for managing selected patients with peripheral arterial occlusive disease. Overall, the 5-year success rate after iliac angioplasty is 53.4%, but the success rate is higher if percutaneous transluminal angioplasty is performed on the common iliac artery or on a stenosed artery. In contrast, percutaneous transluminal angioplasty of the femoral and popliteal arteries has a relatively poor long-term success rate except for the treatment of patients with stenoses with good run-off. When the run-off is poor or an arterial occlusion is present, the role of femoropopliteal angioplasty is limited, and the procedure should be considered only for high-risk patients who do not have autogenous tissue for reconstructive surgery.

Angioplasty, Balloon↗
PubMed1992

Femoral and popliteal arteries: reanalysis of results of balloon angioplasty.

The author presents an alternative statistical analysis of the results of the University of Toronto series of percutaneous transluminal angioplasty (PTA) of the femoral and popliteal arteries (n = 254). After recalculation of the data with the Kaplan-Meier method, the postprocedure success rate ranged from 88.8% +/- 2.0 at 1 month to 35.7% +/- 4.8 at 6 years. With Cox multiple regression analysis, the type of femoropopliteal lesion and the runoff were the variables that were useful to predict late results. For stenoses with good runoff, the success rate was 53% at 5 years; with poor runoff, 31% at 5 years. For occlusions with good runoff, the success rate was 36% at 5 years; with poor runoff, 16% at 5 years. In initially successful cases, ongoing clinical success at 1, 3, and 5 years was better in patients with good runoff at the time of PTA than in those with poor runoff. Now that more recent studies have documented improved technical success in femoropopliteal PTA, a comparative study of the relative safety, long-term clinical efficacy, and cumulative cost of PTA versus surgery seems warranted.

Angioplasty, Balloon↗
PubMed1991

Cerebral blood flow velocities by transcranial Doppler during parabolic flight.

Microgravity is produced for 20 to 30 seconds in NASA's KC-135 aircraft at the end of a 2 G pullup for each of 40 parabolas per flight. Continuous transcranial Doppler ultrasound, arterial blood pressure, and acceleration levels were recorded for 12 male and 8 female healthy subjects without known cardiovascular or cerebrovascular disease. Recordings were made throughout 10 parabolas per subject in each of the supine, sitting, and standing postures. The data were digitized for off-line analysis using Fast Fourier Transform and other signal processing methods. A phase lag in changes to transcranial Doppler waveforms from the onset of acceleration was more pronounced in the standing position than in the sitting position. There was less of a phase lag in the supine position. These ultrasound changes preceded the more delayed variations in arterial blood pressure. The KC-135 provides a unique short-term environment that allows measurement of the human response to variations in acceleration but limits physiological monitoring of responses to a steady state of microgravity.

Acceleration↗
PubMed1991

Experimental study of the effects of pulsed Doppler sample volume size and position on the Doppler spectrum.

With a pulsed Doppler system, the recorded Doppler spectrum is expected to vary depending upon the sample volume size relative to the diameter of the vessel, the position of the sample volume in the vessel and the velocity profile. In the in vitro experiments described in this paper, the velocity profile was kept constant by using steady parabolic flow in a flow model. As the Doppler sample volume size and position were changed, the maximum variations of quantitative measurements from the Doppler spectrum were determined. The maximum, mean and mode frequencies and spectral broadening index (SBI) were affected by the position of the sample volume but to a lesser degree by its length (1.5-5.0 mm) relative to the 9.5 mm beam path length across the tube. When the centre of the Doppler sample volume was moved within the central 25% of the tube, the maximum variations were as follows: maximum frequency 3-5%, mean frequency 8-9%, mode frequency 8-9% and SBI 16-18%, where the range indicates the effect of increasing the sample volume size. Based on these results obtained under steady flow conditions in vitro, it is concluded that quantification of pulsed Doppler spectra may be feasible if the sample volume is positioned within the central 25% of the vessel.

Blood Flow Velocity↗
PubMed1991

Quantitative study of steady flow using color Doppler ultrasound.

The use of color Doppler flow mapping systems for quantitative in vitro studies of flow fields is examined and illustrated. A 5-MHz color Doppler system was used, and the resolution was determined by comparing the results of flow-field measurement for steady parabolic pipe flow with calculated values. The velocity accuracy was about 6% of the velocity corresponding to half the pulse repetition frequency, and the spatial resolution was better than 1 mm. Frame frequency limitations permitted only partial tracking of fast temporal changes in the flow field. However, detection of vortices downstream from a small cylinder placed in the flow tube was significantly enhanced by synchronizing the frame frequency with the vortex shedding frequency and using a velocity-variance mode. Color Doppler aliasing was found to be useful to define streamlines and determine whether the flow was laminar or turbulent. The color Doppler system clearly imaged Poiseuille, transitional and turbulent flow and vortex shedding in vitro. It is concluded that color Doppler ultrasound flow mapping can enable large, complex flow fields to be quantitatively studied in vitro.

Blood Flow Velocity↗
PubMed1991

Suggested standards for reporting on arterial aneurysms. Subcommittee on Reporting Standards for Arterial Aneurysms, Ad Hoc Committee on Reporting Standards, Society for Vascular Surgery and North American Chapter, International Society for Cardiovascular Surgery.

The literature on arterial aneurysms is subject to potential misinterpretation because of inconsistencies in reporting standards. The joint councils of the Society for Vascular Surgery and the North American Chapter of the International Society for Cardiovascular Surgery appointed an ad hoc committee to address this issue. This communication, prepared in response to the need for standardized reporting, defines and classifies arterial aneurysms and recommends standards for describing the causes, manifestations, treatment, and outcome criteria that are important when publishing data on aneurysmal disease.

Aneurysm↗
PubMed1991

Computer simulation of blood flow patterns in arteries of various geometries.

The purpose of this study is to illustrate the application of computer simulation to the study of blood flow through arteries and to demonstrate the relationship between geometry of the vessels and local flow patterns. A finite element computer program was developed to simulate steady and pulsatile blood flow by solving the continuity and Navier-Stokes equations. The accuracy of the computational method has been confirmed by comparing the numeric results to analytic solutions and to published experimental data from physical models. The results are presented as plots of the velocity vectors, streamlines, and pressure contours. The computational model has been applied to illustrate flow patterns in the following situations: pulsatile flow in a cylindric artery and an artery with an axisymmetric stenosis, steady flow in cylindric arteries with stenoses of varying severity and with different flow rates, steady flow in an artery containing a fusiform aneurysm, steady flow in a two-dimensional model of a symmetric Y-shaped bifurcation, and steady flow in a two-dimensional model of the carotid bifurcation. Regions that are commonly associated with arterial disease often coincide with zones of reversed or stagnant flow. In conclusion, the versatility and feasibility of computational simulation of blood flow is illustrated by this study. Although this mathematic model is a simplification of the real flow phenomena, it yields results that provide useful insights into the understanding of local blood flow patterns for a variety of complex geometries.

Aneurysm↗
PubMed1990

Transcranial Doppler studies of flow velocity in middle cerebral artery in weightlessness.

Physiological adaptation to weightlessness requires changes in cardiovascular system parameters to maintain homeostasis in the presence of cephalic fluid shifts. The cerebral circulation must respond immediately to these systemic changes or impairment of cerebral function will occur. Blood flow velocities of the middle cerebral artery were measured by transcranial Doppler (TCD) ultrasound in NASA's KC-135 aircraft from four healthy subjects in the supine position. Transcranial Doppler data with accompanying acceleration information were analyzed in three segments in each parabola. Cardiac cycles for each segment of all 20 parabolas were pooled for individual subjects. A Student's t test on the data revealed statistically significant differences in the mean and peak frequencies, systolic/diastolic ratios and Doppler power between positive 2 gz and microgravity (10(-2) gz) as well as between positive 2 gz and negative 2 gz. Velocity waveform profiles differed for the first one third of each segment, with a more resistive waveform developing during the last two thirds. Changes in systemic arterial resistance and/or raised intracranial pressure may contribute to these TCD waveform changes.

Adaptation, Physiological↗
PubMed1990

Development of methods to analyse transcranial Doppler ultrasound signals recorded in microgravity.

During space flights, several clinical syndromes may be the result of changes in cerebral circulation. The purpose of the paper is to describe the development and initial evaluation of a system for recording, processing and displaying transcranial Doppler ultrasound (TCD) waveforms from the middle cerebral artery (MCA) in microgravity. Volunteers were repeatedly subjected to 15-20 s intervals of microgravity ('near zero gravity') during flights on the KC-135 military aircraft. Continuous TCD recordings from the MCA were stored on magnetic tape. The paper describes the system that was developed to digitise the Doppler ultrasound data and markers that corresponded to the various levels of microgravity, obtain the maximum and mean Doppler waveforms, identify the waveforms and quantify them. The results demonstrate the feasibility of making TCD recordings in a microgravity environment and illustrate excellent performance of the system and its ease of operation. Quantitative waveform analysis of the recordings from the first subject studied in the supine position showed statistically significant changes in MCA velocity waveforms during microgravity.

Cerebral Arteries↗
PubMed1990

Physiological changes in the somatosensory forepaw cerebral cortex of adult raccoons following lesions of a single cortical digit representation.

The purpose of this study was to determine whether restricted lesions within primary somatosensory (SmI) cortex cause changes in the functional organization of cortical areas bordering on the site of injury. Focal ablations of cortical tissue were made in the representational area for digit 3 within the SmI forepaw cortex of adult raccoons. Electrophysiological mapping experiments done 15-17 weeks later showed that significant alterations had occurred in the response properties of clusters of neurons within those representational zones adjoining the lesion--the zones for digit 2, digit 4, and the palmar pads. These three cortical areas were modified by the appearance of new, usually weaker secondary inputs and changes in some properties of the normal primary inputs from the forepaw. (i) Many neurons responded to stimulation of previously ineffective skin regions; the new inputs often originated from digit 3 but frequently involved other digits or the pads as well. (ii) Neuronal receptive fields (RFs), mapped at a standard suprathreshold stimulus intensity, were larger than normal. (iii) Skin type and submodality sensitivity typically were less specific than normal; more neurons had RFs that included both glabrous and hairy skin or claws and displayed mixtures of responsiveness to skin touch, hair deflection, or claw touch. (iv) The representation of RF location, skin type, and submodality sensitivity was more variable as a function of horizontal and vertical distance through the cortex. In general, the physiological changes were found to degrade the somatotopic order and response specificity of the intact cortical areas adjoining the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Detailed visualization of pulsatile flow fields produced by modelled arterial stenoses.

A multiple trace photochromic method was used to visualize the pulsatile flow field created by modelled arterial stenoses of 38% and 65% area reductions. Using flow parameters similar to those of a medium sized artery in man, the flow patterns at seven axial locations in relation to the stenosis were simultaneously photographed at various times throughout the flow cycle. With the 65% stenosis, the wall shear stress in the vicinity of the reattachment point was found to fluctuate quite strongly during the turbulent phase of the flow cycle, giving rise to instantaneous shear stresses that were at least eight times larger than those measured upstream. For the 38% stenosis, much smaller shear stresses were observed. These and other results are described in detail.

Arterial Occlusive Diseases↗
PubMed1990

Intraabdominal hemorrhage as a result of segmental mediolytic arteritis of an omental artery: case report.

This article describes the fifth reported case of segmental mediolytic arteritis and the second in a survivor. The patient had intraabdominal bleeding as a result of a ruptured omental artery. The pathologic and arteriographic findings are described. The pathology is characterized by segmental disruption of the medial smooth muscle cells and the initiation of mediolysis. Mediolysis is associated with marked segmental thinning of the vessel wall, often with only the adventitia intact. Fibrin is deposited at the adventitial and medial surfaces, and hemorrhage into the media may occur. As in this reported case, lysis of the adventitia leads to sudden, often catastrophic intraabdominal hemorrhage. Little associated adventitial inflammation occurred. Segmental mediolytic arteritis seems to involve the intra-abdominal muscular arteries in elderly patients with nonspecific abdominal symptoms. An angiogram showed patchy areas of narrowing involving ileal, gastroduodenal, and renal arteries that correlated with the pathologic findings observed in the excised omental arteries.

Abdomen↗
PubMed1990

Steady and pulsatile flow fields in an end-to-side arterial anastomosis model.

We investigated the flow field within a rigid-walled in vitro model of an end-to-side 45 degree anastomosis in an attempt to identify possible hemodynamic factors that may contribute to the pathogenesis of distal anastomotic intimal hyperplasia. A high-resolution photochromic tracer technique was used to visualize the flow in orthogonal planes and to determine the axial wall shear stress profiles for both steady and pulsatile flows over a range of physiologically relevant conditions. The flow field showed qualitative similarities to those seen in curved vessel: rapidly moving fluid from the graft section affects the bed of the host vessel, that is, the wall opposite the anastomosis, eventually advancing down the host vessel in a spiraling motion. A small mobile separation zone was noted at the toe of the anastomosis. Comparison of wall shear stress profiles with previously reported preferential sites for the development of intimal hyperplasia supported a low wall shear stress and/or flow separation pathogenesis hypothesis. One notable exception was the bed of the host artery that appeared to be subjected to a complex hemodynamic environment.

Arteriovenous Shunt, Surgical↗
PubMed1990

Evidence of a possible link between poststenotic dilation and wall shear stress.

The effects of an axisymmetric 65% area reduction stenosis on a pulsatile flow were investigated by use of an in vitro model that permits simultaneous visualization of the flow velocity profiles at seven sites. By use of seven lenses to focus the ultraviolet light from a nitrogen laser, seven thin blue lines were produced in the photochromic solution flowing through the tube. The displacement profiles of the dye traces were photographed, resulting in the acquisition of the velocity profiles. From these traces, the flow pattern was determined, and the wall shear stresses were measured. Turbulence was generated 3.3 to 6.5 tube diameters downstream from the edge of the stenosis, depending on the time in the pulsatile flow cycle. Maximum wall shear stress fluctuations between positive and negative values appeared to lie within 1.6 to 3.3 tube diameters downstream of the stenosis. In several illustrative clinical cases of thoracic outlet arterial compression, the poststenotic dilation was maximum at 2.0 +/- 0.3 vessel diameters downstream. Based on these observations, it is postulated that wall shear stress fluctuations may be important in the development of poststenotic dilation.

Arteries↗