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Biomedical subjects

K Wayne Johnston

Publications and source records attributed to K Wayne Johnston.

At least 19 recordsLinked to original sources
PubMed2005

Human saphenous vein coronary artery bypass graft morphology, geometry and hemodynamics.

Coronary artery bypass graft (CABG) failure has been linked to graft hemodynamics, in particular wall shear stress. This study characterizes the morphology, geometry and wall shear stress patterns in human CABGs. The intimal thickness (IT) in 49 human saphenous vein CABGs was measured by digital light microscopy. The geometry of six saphenous vein CABGs was replicated by post-mortem infusion of Batson's #17 anatomical corrosion casting compound. Graft hemodynamics were evaluated in two flow models, fabricated from the casts, under steady (Re = 110) and pulsatile flow (Re = 110, alpha = 2) conditions. Saphenous vein CABGs in situ for more than 2 months had, on average, the greatest IT on the hood and suture sites of the distal anastomosis. Floor thickening was highly variable and significantly less than IT at the hood, suture site and graft body. All casts showed an indentation along the floor and 5/6 casts displayed a sharp local curvature on the hood. In both flow models, a large increase in wall shear rate occurred on the hood, just proximal to the toe. The local geometry of the hood created this large spatial gradient in wall shear stress which is a likely factor in hood intimal hyperplasia.

Aged↗
PubMed2005

Human factors as a source of error in peak Doppler velocity measurement.

OBJECTIVE: The study was conducted to assess the error and variability that results from human factors in Doppler peak velocity measurement. The positioning of the Doppler sample volume in the vessel, adjustment of the Doppler gain and angle, and choice of waveform display size were investigated. We hypothesized that even experienced vascular technologists in a laboratory accredited by the Intersocietal Commission for Accreditation of Vascular Laboratories make significant errors and have significant variability in the subjective adjustments made during measurements. METHODS: Problems of patient variability were avoided by having the four technologists measure peak velocities from an in vitro pulsatile flow model with unstenosed and 61% stenosed tubes. To evaluate inaccurate angle and sample volume positioning, a probe holder was used in some of the experiments to fix the Doppler angle at 60 degrees. The effect of Doppler gain was studied at three settings--low, ideal, and saturated gains--that were standardized from the ideal level chosen by consensus amongst the technologists. Two waveform display sizes were also investigated. Peak velocity measurement was assessed by comparison with true peak velocities. For each variable studied, average peak velocities were calculated from the 10 measurements made by each technologist and used to find the percent error from the true value, and the coefficient of variation was used to measure the variability. RESULTS: Doppler angle, sample volume placement, and the Doppler gain were the most significant sources of error and variability. Inaccurate angle and placement increased the variability in measurements from 1% to 2% (range) to 4% to 6% for the straight tube and from 1% to 2% to 3% to 9% for the 61% stenosis. The peak velocity error was increased from 9% to 13% to 7% to 28% for the stenosis. Both measurement error and variability were strongly dependent on the Doppler gain level. At low gain, the error was approximately 10% less than the true value and at saturated gain, 20% greater. The display size only affected measurements from the stenosed tube, increasing the error from 9% to 13% to 15% to 24%. CONCLUSIONS: Major factors affecting Doppler peak velocity measurement error and variability were identified. Inaccurate angle and sample volume placement increased the variability. The presence of a stenosis was found to increase the measurement errors. The error was found to depend on the Doppler gain setting, with greater variability at low and saturated gains and on the display size with a stenosis. CLINICAL RELEVANCE: Doppler ultrasound peak velocity measurements are widely used for the diagnostic assessment of the severity of arterial stenoses. However, it is known that these measurements are often in error. We have identified subjective human factors introduced by the technologist and assessed their contribution to peak velocity measurement error and variability. It is to be hoped that by understanding this, improvements in the machine design and measurement methods can be made that will result in improved measurement accuracy and reproducibility.

Artifacts↗
PubMed2005

Effects of beam steering in pulsed-wave ultrasound velocity estimation.

Experimental and computer simulation methods have been used to investigate the significance of beam steering as a potential source of error in pulsed-wave flow velocity estimation. By simulating a typical linear-array transducer system as used for spectral flow estimation, it is shown that beam steering can cause an angle offset resulting in a change in the effective beam-flow angle. This offset primarily depends on the F-number and the nominal steering angle. For example, at an F-number of 3 and a beam-flow angle of 70 degrees , the velocity error changed from -5% to + 5% when the steering angle changed from -20 degrees to + 20 degrees . Much higher errors can occur at higher beam-flow angles, with smaller F-numbers and greater steering. Our experimental study used a clinical ultrasound system, a tissue-mimicking phantom and a pulsatile waveform to determine peak flow velocity errors for various steering and beam-flow angles. These errors were found to be consistent with our simulation results.

Blood Flow Velocity↗
PubMed2004

Predictors of repair and effect of gender on treatment of ruptured abdominal aortic aneurysm.

OBJECTIVE: The purpose of this study was to determine factors associated with increased likelihood of patients undergoing surgery to repair ruptured abdominal aortic aneurysms (AAAs). Specifically, we investigated whether men were more likely than women to be selected for surgery after rupture of AAAs. METHODS: All patients with a ruptured AAA who came to a hospital in Ontario between April 1, 1992, and March 31, 2001, were included in this population-based retrospective study. Administrative data were used to identify patients, patient demographic data, and hospital variables. RESULTS: Crude 30-day mortality for the 3570 patients who came to a hospital with a ruptured AAA was 53.4%. Of the 2602 patients (72.9%) who underwent surgical repair, crude 30-day mortality was 41.0%. Older patients (odds ratio [OR], 0.649 per 5 years of age; P<.0001), with a higher Charlson Comorbidity Index (OR, 0.848; P<.0001), were less likely to undergo AAA repair. Patients treated at high-volume centers (OR, 2.674 per 10 cases; P<.0001) and men (OR, 2.214; P<.0001) were more likely to undergo AAA repair. CONCLUSION: Men are more likely to undergo repair of a ruptured AAA than women are, for reasons that are unclear. Given the large magnitude of the effect, further studies are clearly indicated.

Aged↗
PubMed2004

Interobserver variability of carotid Doppler peak velocity measurements among technologists in an ICAVL-accredited vascular laboratory.

OBJECTIVE: This study was designed to investigate interobserver variability in the measurement of internal carotid artery (ICA) peak systolic velocity (PSV). We hypothesize that the reproducibility of repeated duplex scanning parameters, in the hands of very experienced vascular technologists in a laboratory accredited by the Intersocietal Commission for Accreditation of Vascular Laboratories, would be excellent. METHODS: Thirty-one patients underwent carotid duplex scanning by three vascular technologists using the same duplex scanning system. They examined patients with the laboratory's standard protocol. Statistical analysis of the sources of variation was carried out with two-way analysis of variance. The Altman-Bland method was used to detect bias and evaluate the interval of agreement between technologists for the ICA PSV on a continuous scale. The kappa statistic enabled measurement of agreement for ICA PSV on a categorical scale of stenosis (<50%, 50%-70%, >70%). RESULTS: Patient variability was responsible for 97.2% of the total variance, with only 0.58% (P<.005) attributed to the technologists. The level of agreement on a continuous scale between the measurements of ICA PSV by our technologists is wide. For individual patients it ranged from -25% to 43% between technologists A and B, -27% to 43% between technologists A and C, and -27% to 31% between technologists B and C. When we compared the three technologists, no systematic overestimation or underestimation of the ICA PSV was found (ie, no fixed bias). The level of agreement between the technologists did not depend on the value of the PSV (ie, no proportional bias). However, analysis of ICA PSV agreement on a categorical scale revealed almost perfect agreement (kappa>0.8). CONCLUSION: From measurements of PSV, the severity of carotid stenosis can be reproducibly categorized into ranges (<50%, 50%-70%, >70). However, the unacceptably wide interobserver variation of ICA PSV on a continuous scale makes the interchangeability of our technologists' measurements problematic for clinical use, as in determination of progression of severity of stenosis. When an ICA PSV measurement is in the vicinity of a cutoff value, the diagnostic accuracy may be improved with the use of additional diagnostic testing.

Blood Flow Velocity↗
PubMed2004

Survival after ruptured abdominal aortic aneurysm: effect of patient, surgeon, and hospital factors.

OBJECTIVE: The purpose of this study was to determine the effects of patient, surgeon, and hospital factors on survival after repair of ruptured abdominal aortic aneurysm (AAA) and to compare them with risk factors for survival after elective AAA repair. It was hypothesized that patients operated on by high-volume surgeons with subspecialty training would have better outcomes, which might argue for regionalization of AAA surgery. METHODS: In this population-based retrospective cohort study, surgeon billing and administrative data were used to identify all patients who had undergone AAA repair between April 1, 1992, and March 31, 2001, in Ontario, Canada. Demographic information was collected for each patient, as well as numerous variables related to the surgeons and hospitals. RESULTS: There were 2601 patients with ruptured AAA repair, with an average 30-day mortality rate of 40.8%. Significant independent predictors of lower survival were older age, female gender, lower patient income quintile, performance of surgery at night or on weekends, repair in larger cities, surgeons with lower annual volume of ruptured AAA operations, and surgeons without vascular or cardiothoracic fellowship training. There were 13,701 patients with elective AAA repair, with an average 30-day mortality rate of 4.5%. Significant independent predictors of lower survival were similar, except gender was not significant, but the Charlson Comorbidity Index was. When the hazard ratios associated with predictive factors were compared, surgeon factors appeared to be more important in ruptured AAA repair, and patient factors appeared more important in elective AAA repair. CONCLUSION: For elective AAA repair, and even more so for ruptured AAA repair, high-volume surgeons with subspecialty training conferred a significant survival benefit for patients. Although this would seem to argue in favor of regionalization, decisions should await a more complete understanding of the relationship between transfer time, delay in treatment, and outcome.

Aged↗
PubMed2004

Sample volume shape for pulsed-flow velocity estimation using a linear array.

Various definitions of the sample volume (SV) shape have been proposed, but they are mostly based on transducers with axisymmetrical geometry. We have defined the SV as that spatial region in which scatterers contribute a component to the total gated received-signal energy above a defined threshold. This definition is consistent with modern pulsed transducer arrays and accounts for the need to impose a signal/noise threshold. Based on this definition, SVs for a typical linear phased-array transducer were simulated using custom-designed software. The effects of different transmit pulses, receive gates, apertures, SV depths and lateral foci were studied using a one-dimensional (1-D) beam-forming array, with a fixed lens in the elevation direction. Based on a simplified method of analysis, the features of the beam-steered SV are qualitatively similar to those of the nonsteered SV, when compared at the same beam-flow angle. These studies have helped provide a clearer understanding of the manner in which the SV energy distribution is affected by various parameters. The results can have potentially significant implications in the use of ultrasound (US) for blood velocity estimation, specifically with respect to locating the SV within the blood vessel and the origin of the velocity spectrum.

Computer Simulation↗
PubMed2004

Hemodynamic effects of clot entrapment in the TrapEase inferior vena cava filter.

PURPOSE: The TrapEase vena cava filter has a symmetric design. Emboli can be trapped in the outlet conical section (superior cone) or between the filter and vessel wall at the inlet end (inferior cone). The purpose of this in vitro study is to investigate the hemodynamic effects of clot entrapment by the TrapEase filter and to examine the possibility of flow-induced filter thrombosis. MATERIALS AND METHODS: Velocity and wall shear stress maps were determined for steady flow with use of the photochromic flow visualization technique. Experiments were done for a filter without clot and for three other cases: an asymmetric clot in the inferior zone, a symmetric clot in the superior zone, and a filter with both zones partially occluded. Each simulated clot was 1500 mm(3) and the vessel diameter was 2 cm. RESULTS: The unoccluded filter did not significantly affect the flow field. However, for a partially occluded filter, flow stagnation/recirculation and turbulence developed downstream from the clot. The greatest effect was noted when the clot was against the vessel wall in the inferior trapping region. CONCLUSIONS: The tendency for clots to be trapped between the filter and the vessel wall in the inferior (inlet) region may play an important role in the performance of the TrapEase filter. A clot in this configuration will generate a large region of flow stagnation/recirculation that is considered to be prothrombotic. In addition, a significant amount of the filter wire will be embedded in this region, which may also promote thrombosis.

Blood Flow Velocity↗
PubMed2004

Experimental study on local mass transfer in a simplified bifurcation model: potential role in atherosclerosis.

Local mass transfer coefficients and flow patterns were examined in an idealized human aortic bifurcation model. The objectives of this study are to gain further insights on the convective mass transfer process and its possible role in the localization of atherosclerotic lesions. The laser photochromic tracer method provided velocity and wall shear stress estimates in the plane of symmetry of a UV-transparent Plexiglas bifurcation model. Steady flow data were acquired at Reynolds numbers of 500, 600, and 750. A novel copper electrodeposition technique was used to obtain time-averaged convective local mass transfer coefficients in a model identical to that used in the flow experiments. The laminar flow mass transfer data for the trunk of the bifurcation are in good agreement with the analytical Levesque solution. At the bifurcation, higher mass transfer coefficients along the inner wall and lower ones along the outer wall were observed. Further, mass transfer and wall shear stress follow similar patterns both on the inner and outer walls in that StSc 2/3 and C(f)/2 demonstrate analogous behavior. Lower transfer rates of momentum and mass occurred along the outer wall of the branches where lesions tend to develop.

Aorta, Abdominal↗
PubMed2003

The relationship between wall shear stress distributions and intimal thickening in the human abdominal aorta.

PURPOSE: The goal of this work was to determine wall shear stress (WSS) patterns in the human abdominal aorta and to compare these patterns to measurements of intimal thickness (IT) from autopsy samples. METHODS: The WSS was experimentally measured using the laser photochromic dye tracer technique in an anatomically faithful in vitro model based on CT scans of the abdominal aorta in a healthy 35-year-old subject. IT was quantified as a function of circumferential and axial position using light microscopy in ten human autopsy specimens. RESULTS: The histomorphometric analysis suggests that IT increases with age and that the distribution of intimal thickening changes with age. The lowest WSS in the flow model was found on the posterior wall inferior to the inferior mesenteric artery, and coincided with the region of most prominent IT in the autopsy samples. Local geometrical features in the flow model, such as the expansion at the inferior mesenteric artery (common in younger individuals), strongly influenced WSS patterns. The WSS was found to correlate negatively with IT (r2 = 0.3099; P = 0.0047). CONCLUSION: Low WSS in the abdominal aorta is co-localized with IT and may be related to atherogenesis. Also, rates of IT in the abdominal aorta are possibly influenced by age-related geometrical changes.

Adult↗
PubMed2003

Use of interventional procedures for peripheral arterial occlusive disease in Ontario between 1991 and 1998: a population-based study.

PURPOSE: Although peripheral arterial occlusive disease (PAOD) is a public health issue in the elderly population, limited population-based data are available on use of interventional procedures in Canada. We describe trends in use of interventional procedures to treat PAOD in Ontario over the past decade. METHODS: A retrospective population-based cohort study was conducted for fiscal years 1991 to 1998 with Ontario administrative databases to identify all arterial bypass surgeries, angioplasty procedures, and amputations performed. RESULTS: A total of 19,332 bypass operations, 16,334 angioplasty procedures, and 17,534 amputations were identified. Population-based rates showed that angioplasty use peaked at about 110 per 100,000 at age 65 to 74 years, arterial bypass surgery use peaked at 129 per 100,000 at age 75 to 84 years, and amputation use peaked at 138 per 100,000 at age 85 years or older. All types of interventional procedures to treat PAOD were performed more frequently in men than in women. Age-adjusted and sex-adjusted rate of arterial bypass surgery decreased significantly, from 77 to 61 per 100,000 population aged 45 years or older (P =.0002, linear regression analysis), whereas rate for PTA increased significantly, from 59 to 75 per 100,000 population aged 45 years or older (P =.0005). The overall major amputation rate declined slightly over the study period, influenced by the decreased rate in patients aged 85 years or older. The revascularization rate in patients aged 85 years or older increased (P =.055). CONCLUSION: Reduced use of arterial bypass surgery and increased use of angioplasty procedures has occurred over the past decade and may reflect a change in the practice pattern of vascular surgeons in Ontario, who have become more conservative in treating localized disease and reserve surgical interventions for more severe forms of PAOD. The slight reduction in overall major amputation rate, driven by decreased rate in patients aged 85 years or older, may reflect a trend toward a more aggressive revascularization approach in this age group.

Age Distribution↗
PubMed2003

Outcome of revascularization procedures for peripheral arterial occlusive disease in Ontario between 1991 and 1998: a population-based study.

PURPOSE: We describe the outcome of revascularization procedures used to treat peripheral arterial occlusive disease (PAOD), using population-based administrative data. METHODS: A retrospective population-based cohort study utilizing administrative databases in Ontario, Canada, was conducted for fiscal years 1991 to 1998 to identify patients who underwent arterial bypass surgery and percutaneous transluminal angioplasty to treat PAOD. The Kaplan-Meier method was used to calculate cumulative survival rate and amputation-free survival rate. To analyze factors that affect these rates, multivariate analysis was performed with Cox proportional hazard models. RESULTS: Over the study period 15,824 patients underwent bypass operations and 11,548 underwent angioplasty. For patients who underwent bypass surgery, 5-year cumulative survival rate was 61.5% and major amputation-free survival rate was 83.4%, compared with 69% and 92.2%, respectively, for patients who underwent angioplasty. Male sex, older age, diabetes, and heart disease were associated with increased risk for death after revascularization procedures. Increased risk for major amputation after revascularization procedures was associated with male sex, older age, and diabetes, whereas hypertension was linked to decreased risk. CONCLUSION: To evaluate the long-term outcome of revascularization procedures for PAOD at the population level, survival and major amputation-free survival rates should be used, because they provide more clinically accepted estimates compared with the correlation between utilization rates for revascularization and amputation procedures, which have been used to describe outcome in previously published reports in the literature.

Amputation, Surgical↗
PubMed2003

Guidelines for the treatment of abdominal aortic aneurysms. Report of a subcommittee of the Joint Council of the American Association for Vascular Surgery and Society for Vascular Surgery.

Decision-making in regard to elective repair of abdominal aortic aneurysms (AAA) requires careful assessment of factors that influence rupture risk, operative mortality, and life expectancy. Individualized consideration of these factors in each patient is essential, and the role of patient preference is of increasing importance. It is not possible or appropriate to recommend a single threshold diameter for intervention which can be generalized to all patients. Based upon the best available current evidence, 5.5 cm is the best threshold for repair in an "average" patient. However, subsets of younger, good-risk patients or aneurysms at higher rupture risk may be identified in whom repair at smaller sizes is justified. Conversely, delay in repair until larger diameter may be best for older, higher-risk patients, especially if endovascular repair is not possible. Intervention at diameter <5.5 cm appears indicated in women with AAA. If a patient has suitable anatomy, endovascular repair may be considered, and it is most advantageous for older, higher-risk patients or patients with a hostile abdomen or other technical factors that may complicate standard open repair. With endovascular repair, perioperative morbidity and recovery time are clearly reduced; however, there is a higher reintervention rate, increased surveillance burden, and a small but ongoing risk of AAA rupture. There is no justification at present for different indications for endovascular repair, such as earlier treatment of smaller AAA. Until long-term outcome of endoluminal repair is better defined and results of randomized trials available, the choice between endovascular and open repair will continue to rely heavily on patient preference.

Aortic Aneurysm, Abdominal↗