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S L Dawson

Publications and source records attributed to S L Dawson.

At least 19 recordsLinked to original sources
PubMed2001

Cerebral blood flow velocity response to induced and spontaneous sudden changes in arterial blood pressure.

The influence of different types of maneuvers that can induce sudden changes of arterial blood pressure (ABP) on the cerebral blood flow velocity (CBFV) response was studied in 56 normal subjects (mean age 62 yr, range 23-80). ABP was recorded in the finger with a Finapres device, and bilateral recordings of CBFV were performed with Doppler ultrasound of the middle cerebral arteries. Recordings were performed at rest (baseline) and during the thigh cuff test, lower body negative pressure, cold pressor test, hand grip, and Valsalva maneuver. From baseline recordings, positive and negative spontaneous transients were also selected. Stability of PCO2 was monitored with transcutaneous measurements. Dynamic autoregulatory index (ARI), impulse, and step responses were obtained for 1-min segments of data for the eight conditions by fitting a mathematical model to the ABP-CBFV baseline and transient data (Aaslid's model) and by the Wiener-Laguerre moving-average method. Impulse responses were similar for the right- and left-side recordings, and their temporal pattern was not influenced by type of maneuver. Step responses showed a sudden rise at time 0 and then started to fall back to their original level, indicating an active autoregulation. ARI was also independent of the type of maneuver, giving an overall mean of 4.7 +/- 2.9 (n = 602 recordings). Amplitudes of the impulse and step responses, however, were significantly influenced by type of maneuver and were highly correlated with the resistance-area product before the sudden change in ABP (r = -0.93, P < 0.0004). These results suggest that amplitude of the CBFV step response is sensitive to the point of operation of the instantaneous ABP-CBFV relationship, which can be shifted by different maneuvers. Various degrees of sympathetic nervous system activation resulting from different ABP-stimulating maneuvers were not reflected by CBFV dynamic autoregulatory responses within the physiological range of ABP.

Adult↗
PubMed2000

Designing a computer-based simulator for interventional cardiology training.

Interventional cardiology training traditionally involves one-on-one experience following a master-apprentice model, much as other procedural disciplines. Development of a realistic computer-based training system that includes hand-eye coordination, catheter and guide wire choices, three-dimensional anatomic representations, and an integrated learning system is desirable, in order to permit learning to occur safely, without putting patients at risk. Here we present the first report of a PC-based simulator that incorporates synthetic fluoroscopy, real-time three-dimensional interactive anatomic display, and selective right- and left-sided coronary catheterization and angiography using actual catheters. Significant learning components also are integrated into the simulator.

Cardiac Catheterization↗
PubMed2000

Which parameters of beat-to-beat blood pressure and variability best predict early outcome after acute ischemic stroke?

BACKGROUND AND PURPOSE: In hypertensive populations, increasing blood pressure (BP) levels and BP variability (BPV) are associated with a greater incidence of target organ damage. After stroke, elevated 24-hour BP levels predict a poor outcome, although it is uncertain whether shorter-length BP recordings assessing mean BP levels and BPV have a similar predictive role. The objectives of this study were to compare the different measures of beat-to-beat BP and BPV on outcome after acute ischemic stroke and assess whether these parameters were affected by stroke subtype. METHODS: Ninety-two consecutive admissions with a CT-confirmed diagnosis of acute ischemic stroke were recruited, of whom 54 had cortical infarction, 29 subcortical, and 9 posterior circulation infarction. Casual and two 5-minute recordings of beat-to-beat BP (Finapres, Ohmeda) were made under standardized conditions within 72 hours of ictus, with mean BP levels taken as the average of this 10-minute recording and BPV as the standard deviation. Outcome was assessed at 30 days as dead/dependent or independent (Rankin </=2). The effects of BP, BPV, and stroke subtype on outcome were studied with the use of logistic regression. Stroke subjects were subsequently divided by BP quartiles and within each quartile into low- and high-variability groups; the influence of high BPV on outcome was also assessed. RESULTS: The odds ratio for death/dependency was significantly higher in cortical strokes compared with subcortical and posterior circulation strokes even after controlling for differences in BP and BPV (OR 4.19, P=0.002). Beat-to-beat systolic BP (SBP), diastolic BP (DBP), and mean arterial pressure (MAP +/- SD) levels were higher in the dead/dependent group compared with the independent group (MAP 106+/-20.4 mm Hg vs 97+/-19.1 mm Hg, P<0.02), as was MAP variability: 6.1 (interquartile range 4.5 to 7.4 mm Hg) versus 4.9 (3.8 to 6.4 mm Hg, P=0.02). The odds ratio for a poor outcome was 1. 38 (P=0.014) for every 10-mm Hg increase in MAP and 1.32 (P=0.02) for every 1-mm Hg increase in MAP variability. Casual BP measurements had no prognostic significance. For the group as a whole when separated into BP quartiles, those with a high MAP and DBP but not SBP variability within each quartile had a worse prognosis compared with those with a low BPV. CONCLUSIONS: A poor outcome at 30 days after ischemic stroke was dependent on stroke subtype, beat-to-beat DBP, and MAP levels and variability. Important prognostic information can be readily obtained from a short period of noninvasive BP monitoring in the acute stroke patient. These findings have important implications, particularly regarding the use of hypotensive agents in the acute stroke period.

Acute Disease↗
PubMed2000

ICTS, an interventional cardiology training system.

In this article, we present an Interventional Cardiology Training System developed by the Medical Application Group at Mitsubishi Electric in collaboration with the Center for Innovative Minimally Invasive Therapy. The core of the ICTS is a computer simulation of interventional cardiology catheterization. This simulation integrates clinical expertise, research in learning, and technical innovations to create a realistic simulated environment. The goal of this training system is to augment the training of new cardiology fellows as well as to introduce cardiologists to new devices and procedures. To achieve this goal, both the technical components and the educational content of the ICTS bring new and unique features: a simulated fluoroscope, a physics model of a catheter, a haptic interface, a fluid flow simulation combined with a hemodynamic model and a learning system integrated in a user interface. The simulator is currently able to generate--in real-time--high quality x-ray images from a 3D anatomical model of the thorax, including a beating heart and animated lungs. The heart and lung motion is controlled by the hemodynamic model, which also computes blood pressure and EKG. The blood flow is then calculated according to the blood pressure and blood vessel characteristics. Any vascular tool, such as a catheter, guide wire or angioplasty balloon can be represented and accurately deformed by the flexible tool physics model. The haptics device controls the tool and provides appropriate feedback when contact with a vessel wall is detected. When the catheter is in place, a contrast agent can be injected into the coronary arteries; blood and contrast mixing is computed and a visual representation of the angiogram is displayed by the x-ray renderer. By bringing key advances in the area of medical simulation--with the real-time x-ray renderer for instance--and by integrating in a single system both high quality simulation and learning tools, the ICTS opens new perspectives for computer based training systems.

Angioplasty, Balloon, Coronary↗
PubMed1999

Older subjects show no age-related decrease in cardiac baroreceptor sensitivity.

OBJECTIVE: To examine the relationship between age, blood pressure and cardiac baroreceptor sensitivity derived from spectral analysis, the Valsalva manoeuvre and impulse response function. METHODS: We studied 70 healthy normotensive volunteers who were free from disease and not taking medication with cardiovascular or autonomic effects. We measured beat-to-beat arterial blood pressure and used standard surface electrocardiography to record pulse interval under standardized conditions with subjects resting supine as well as during three Valsalva manoeuvres. We performed single, multiple and stepwise regression of patient characteristics against cardiac baroreceptor sensitivity results. RESULTS: There is a non-linear decline in cardiac baroreceptor sensitivity with advancing age, increasing systolic blood pressure and heart rate values (except for the Valsalva-derived result), but little further decline after the fourth decade. Only age significantly influenced values derived using the Valsalva manoeuvre and impulse response analysis. Using spectral analysis, age, systolic and diastolic blood pressure and heart rate influenced cardiac baroreceptor sensitivity, age contributing to 50% of the variability. Age also influenced the relationship between pulse interval and blood pressure, possibly indicating more non-baroreceptor-mediated changes with advancing age. CONCLUSIONS: Although age is the dominant factor influencing cardiac baroreceptor sensitivity in this normotensive population, there is little change in mean values after 40 years of age. The differences in the relationship between pulse interval and blood pressure with advancing age have implications for the calculation of cardiac baroreceptor sensitivity using spectral analysis.

Adult↗
PubMed1999

Linear and nonlinear analysis of human dynamic cerebral autoregulation.

The linear dynamic relationship between systemic arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) was studied by time- and frequency-domain analysis methods. A nonlinear moving-average approach was also implemented using Volterra-Wiener kernels. In 47 normal subjects, ABP was measured with Finapres and CBFV was recorded with Doppler ultrasound in both middle cerebral arteries at rest in the supine position and also during ABP drops induced by the sudden deflation of thigh cuffs. Impulse response functions estimated by Fourier transfer function analysis, a second-order mathematical model proposed by Tiecks, and the linear kernel of the Volterra-Wiener moving-average representation provided reconstructed velocity model responses, for the same segment of data, with significant correlations to CBFV recordings corresponding to r = 0.52 +/- 0.19, 0.53 +/- 0.16, and 0.67 +/- 0.12 (mean +/- SD), respectively. The correlation coefficient for the linear plus quadratic kernels was 0.82 +/- 0.08, significantly superior to that for the linear models (P < 10(-6)). The supine linear impulse responses were also used to predict the velocity transient of a different baseline segment of data and of the thigh cuff velocity response with significant correlations. In both cases, the three linear methods provided equivalent model performances, but the correlation coefficient for the nonlinear model dropped to 0.26 +/- 0.25 for the baseline test set of data and to 0.21 +/- 0.42 for the thigh cuff data. Whereas it is possible to model dynamic cerebral autoregulation in humans with different linear methods, in the supine position a second-order nonlinear component contributes significantly to improve model accuracy for the same segment of data used to estimate model parameters, but it cannot be automatically extended to represent the nonlinear component of velocity responses of different segments of data or transient changes induced by the thigh cuff test.

Adult↗
PubMed1999

Critical closing pressure explains cerebral hemodynamics during the Valsalva maneuver.

The Valsalva maneuver (VM), a voluntary increase in intrathoracic pressure of approximately 40 mmHg, has been used to examine cerebral autoregulation (CA). During phase IV of the VM there are pronounced changes in mean arterial blood pressure (MABP), pulse interval, and cerebral blood flow (CBF), but the changes in CBF are of a much greater magnitude than those seen in MABP, a finding to date attributed to either a delay in activation of the CA mechanism or the inability of this mechanism to cope with the size and speed of the blood pressure changes involved. These changes in CBF also precede those in MABP, a pattern of events not explained by the physiological process of CA. Measurements of CBF velocity (transcranial Doppler) and MABP (Finapres) were performed in 53 healthy volunteers (aged 31-80 yr). By calculating beat-to-beat values of critical closing pressure (CCP) during the VM, we have found that this parameter suddenly drops at the start of phase IV, providing a coherent explanation for the large increase in CBF. If CCP is included in the estimation of cerebrovascular resistance, a temporal pattern more consistent with an autoregulatory response to the MABP overshoot is also found. CCP is intricately involved in the control of CBF during the VM and should be considered in the assessment of CA.

Adult↗
PubMed1998

Diurnal blood pressure change varies with stroke subtype in the acute phase.

BACKGROUND AND PURPOSE: It is unclear whether acute stroke is associated with a loss of the normal diurnal blood pressure (BP) change and whether stroke type influences this. Some of this confusion results from the use of fixed time definitions of day and night, which can be overcome by the use of cumulative sums analysis (cusums). METHODS: Ninety-eight stroke patients had 24-hour BP monitoring (Spacelabs 90207) performed within 48 hours of ictus. Three subgroups were identified: cortical infarct, n=50; subcortical infarct, n=29; and primary intracerebral hemorrhage [PICH], n= 19. An age-matched control group of 74 subjects was also studied. Diurnal change was assessed by both day-night differences (absolute and percentage) and cusums (cusums plot height [CPH] and circadian alteration magnitude [CDCAM]); ANCOVA was used to compare groups. RESULTS: Compared with control subjects, cortical infarct and PICH subgroups had significantly reduced mean diurnal systolic changes using day-night differences (absolute, -12 and -17 mm Hg; percentage, -10 and -12, respectively; P < 0.0001) and cusums (CDCAM, -6.96 and -8.6 mm Hg; CPH, -32.05 and -46.04 mm Hg, respectively; P < 0.005), only the subcortical infarct subgroup demonstrated reduced percentage differences (-4.4%, P < 0.02). Mean diastolic differences were significantly reduced in all stroke subgroups(CPH, -24.84, -17.31, and -36.92 mm Hg; absolute, -8.26, -4.04, and -11.44 mm Hg; percentage, -10.65, -5.81, and -15.23%, for cortical infarct, subcortical infarct, and PICH subgroups, respectively; P < 0.05), except for CDCAM, which was not reduced in subcortical infarcts (-4.78 and -7.70 mm Hg for cortical infarct and PICH subgroups, respectively; P < 0.001). CONCLUSIONS: Diurnal BP change was reduced in the 3 stroke subgroups studied, especially in patients with cortical infarcts and PICH. This may reflect damage to the central modulation of autonomic BP control. The implications in terms of prognosis and therapy in the acute period require further study.

Acute Disease↗
PubMed1998

Force interactions in laparoscopic simulations: haptic rendering of soft tissues.

Research in the area of computer assisted surgery and surgical simulation has mainly focused on developing 3D geometrical models of the human body from 2D medical images, visualization of internal structures for educational and preoperative surgical planning purposes, and graphical display of soft tissue behavior in real time. Conveying to the surgeon the touch and force sensations with the use of haptic interfaces has not been investigated in detail. We have developed a set of haptic rendering algorithms for simulating "surgical instrument--soft tissue" interactions. Although the focus of the study is the development of algorithms for simulation of laparoscopic procedures, the developed techniques are also useful in simulating other medical procedures involving touch and feel of soft tissues. The proposed force-reflecting soft tissue models are in various fidelities and have been developed to simulate the behavior of elastically deformable objects in virtual environments. The developed algorithms deal directly with geometry of anatomical organs, surface and compliance characteristics of tissues, and the estimation of appropriate reaction forces to convey to the user a feeling of touch and force sensations.

Algorithms↗
PubMed1997

The reproducibility of cardiac baroreceptor activity assessed non-invasively by spectral sequence techniques.

Baroreceptor sensitivity (BRS) is increasingly used as a prognostic indicator in cardiovascular disease. Traditionally it has been measured using invasive techniques with pharmacological manipulation of blood pressure (BP). With the advent of newer methods to measure pulse interval and beat-to-beat changes in BP it is now possible, using sophisticated mathematical modelling techniques, to calculate cardiac BRS non-invasively. However, there are virtually no data on the reproducibility of these newer techniques and what factors may affect the repeatability of these measurements. We studied 39 subjects, aged 22-82 years, with a supine systolic BP range 97-160 mmHg and a diastolic BP range 57-94 mmHg on two occasions between 1 week and 6 months apart. Cardiac BRS was measured by power spectral analysis using Fast Fourier Transformation (FFT), sequence analysis (using up, down and combined sequences) and from phase IV of the Valsalva manoeuvre. There was no significant difference between visits for any of the methods for measuring cardiac BRS. Mean BRS values were similar for FFT (16.7 +/- 11.2 ms/mmHg) and sequence analysis (15.8 +/- 11.4 ms/mmHg); however, results using phase IV of the Valsalva manoeuvre were significantly lower (8.1 +/- 2.9 ms/mmHg, p < 0.0001). The coefficient of variation for the five measures of cardiac BRS varied from 16.8% for Valsalva-derived values to 26.1% for 'down' sequence analysis. However, in ten subjects BRS could not be calculated from the Valsalva manoeuvre. None of the independent variables tested (including age, BP levels and time between testing) significantly influenced the degree of repeatability. In summary, there appears to be little difference between these non-invasive methods in their degree of reproducibility. These techniques would seem suitable for longitudinal studies of changes in cardiac BRS and overcome many of the problems associated with the invasive pharmacological methods.

Adult↗
PubMed1996

Attenuation effects of biliary endoprostheses on therapeutic radiation.

Seven commercially available biliary stents were tested to determine if they caused any clinically important attenuation of therapeutic radiation. Attenuation effects were evaluated in models of brachytherapy (iridium-192 source) and external beam radiation therapy (cobalt-60 source), with use of parameters that mimicked those used in patient treatment for biliary neoplasms. No stent demonstrated clinically important attenuation or scatter of therapeutic radiation, so local dosimetry was not affected.

Bile Duct Neoplasms↗
PubMed1996

Evaluation of the abdomen in sepsis of unknown origin.

The radiologic evaluation of sepsis of unknown origin has changed dramatically since the introduction of cross-sectional imaging. Interventional procedures such as abscess drainage, cholecystostomy, biliary drainage, nephrostomy, and fluid aspiration have reduced the morbidity and mortality associated with occult sources of sepsis. This article examines some of the common etiologies and treatments of sepsis in the hospitalized patient.

Abdominal Abscess↗
PubMed1995

Tissue ablation with radiofrequency: effect of probe size, gauge, duration, and temperature on lesion volume.

RATIONALE AND OBJECTIVES: We evaluated the parameters affecting the size and distribution of thermal tissue damage produced by radiofrequency electrodes. METHODS: Thermal lesions were produced by electrodes connected to a radiofrequency generator in specimens of liver (n = 143) and muscle (n = 20). Various combinations of probe tip exposure (0.5-8 cm), gauge (12-24 gauge), duration of treatment (0.5-12 min), and temperature (80-90 degrees C) were studied. The resulting volumes of tissue coagulation were measured and compared. RESULTS: Lesions equal to or less than 1.6 cm in diameter were symmetrically distributed around the electrode. Lesion diameter (but not length) increased with probe gauge and duration of treatment to a maximum of 6 min. However, lesions with mean diameters larger than 1.6 cm could not be produced using a single probe with any technique. Lesion length correlated with probe tip exposure from 1 to 8 cm (r2 = .996). Over the limited range investigated, increased temperature had minimal effects, except for tip exposures greater than 5 cm, in which larger and more uniform lesions resulted. Lesions varied equal to or less than 3 mm in diameter and equal to or less than 5 mm in length for each combination of variables. CONCLUSION: Radiofrequency ablation can accurately and reproducibly cause coagulative tissue necrosis. Necrosed tissue volume increases with length of exposed probe tip, larger probes, and sessions lasting at least 6 min.

Animals↗
PubMed1995

Percutaneous ethanol injection for the treatment of hepatic tumors: indications, mechanism of action, technique, and efficacy.

Percutaneous injection of ethanol for treatment of hepatic tumors was first described by Sugiura et al. [1] in 1983. To date, reported experience with the procedure has focused mainly on therapy of hepatocellular carcinoma (HCC) [2-15], with little information available regarding its use in treating metastatic liver lesions [16]. Interest in this technique has grown recently as a possible means of treating patients with metastatic liver disease. The purpose of this perspective is to review the indications, mechanism of action, technique, complications, and efficacy of percutaneous ethanol injection for liver tumor treatment.

Carcinoma, Hepatocellular↗
PubMed1995

Percutaneous drainage of subphrenic fluid collections that occur after splenectomy: efficacy and safety of transpleural versus extrapleural approach.

OBJECTIVE: The purpose of this study was to compare the safety and efficacy of transpleural and extrapleural approaches for draining left subphrenic abscesses in patients who have had splenectomy. MATERIALS AND METHODS: Twenty-five patients who had percutaneous catheter drainage (28 drainages) of postsplenectomy fluid collections were studied. Twenty drainages were transpleural and eight were extrapleural. In eight of the 20 transpleural drainages, it was elected to place the catheter transpleurally. In the remaining 12 patients, catheter drainage was judged to be transpleural on review, despite efforts to use an extrapleural approach. RESULTS: Sixteen of 18 patients who had transpleural percutaneous drainage and six of seven patients who had true extrapleural (subcostal) percutaneous drainage were cured by catheter drainage requiring no further intervention. The mean number of days of drainage was not significantly different (p > .05) for the group drained transpleurally (mean, 18 days; range, 1-90 days) versus the group drained extrapleurally (mean, 20 days; range, 6-43 days). Complications (pneumothorax) requiring treatment were seen in two patients in whom a transpleural approach was used and in none of the patients in whom an extrapleural approach was used. Complications that did not require treatment were seen in four further patients drained transpleurally. These were inadvertent placement of catheter into pleural space (two patients) and pneumothorax not requiring treatment (two patients). No patient had an empyema. CONCLUSION: Transpleural drainage of left subphrenic collections occurring after splenectomy is associated with a slightly increased complication rate but has a success rate similar to that of extrapleural drainage.

Adult↗
PubMed1995

Tissue ablation with radiofrequency using multiprobe arrays.

RATIONALE AND OBJECTIVES: We studied the feasibility of increasing the volume of tissue destroyed by radiofrequency tissue coagulation using multiprobe arrays and defined parameters that determine lesion size and shape. METHODS: Radiofrequency was applied to ex vivo calf liver using arrays of two to five 18-gauge probes for 6 min at 70-90 degrees C. Probe spacing (1-3 cm) and arrangement, as well as the method of radiofrequency application (simultaneous or sequential), were varied. The resulting areas of tissue coagulation were measured and compared. RESULTS: Uniform tissue necrosis was observed with simultaneous radiofrequency application for probes 1.5 cm or less apart. At 1.5 cm, arrays of three equidistant probes produced spheroid lesions approximately 3.0 +/- 0.2 cm in diameter. Arrays of four equidistant probes produced cuboid lesions of 3.2 +/- 0.1 cm per side. However, probes placed 2 cm or more apart produced independent lesions 1.4 cm in diameter, with incomplete necrosis between probes. In the trials using five-probe arrays, a central region 4mm in diameter showed no visible evidence of tissue necrosis. With each array, lesion size varied less than 3 mm in any direction. Greater necrosis was accomplished when radiofrequency was applied simultaneously rather than sequentially. CONCLUSION: Multiprobe radiofrequency arrays permit the destruction of more tissue in a single treatment session than is possible with multiple individual probes operating alone. Probes spaced 1.5 cm or less apart act synergistically, producing a total volume of coagulated tissue that is greater than when the individual probes are operated sequentially.

Animals↗
PubMed1994

Failed metallic biliary stents: causes and management of delayed complications.

OBJECTIVE: To describe the incidence, management and long-term outcome of metal stent failure in patients with malignant biliary obstruction. SUBJECTS AND METHODS: Sixty-nine patients received a total of 93 metallic biliary stents for relief of malignant biliary obstruction. Twenty-nine patients had hilar tumours; 40 had common bile duct tumours. RESULTS: Ten of 69 patients (14%) presented with stent occlusion at a mean interval of 4 months after stent insertion. Five of 29 patients (17%) with hilar lesions and five of 40 patients (12%) with common bile duct lesions had stent occlusion. Occlusion was due to tumour overgrowth in eight patients and to occlusion by debris in two. The eight patients with tumour overgrowth were treated with internal/external catheters (5 patients), no therapy (2 patients), and further metal stents (1 patient). These eight patients with tumour overgrowth had a limited lifespan after tumour overgrowth occurred with a mean survival of 2.6 months. The two patients with occlusion due to debris were treated by sweeping the stent with a balloon catheter and these patients survived 26 and 27 months, respectively. CONCLUSION: Adequate peripheral purchase in the biliary tree and overstenting are necessary to prevent tumour overgrowth when stenting hilar lesions. The development of stent occlusion due to tumour overgrowth heralds a limited survival and internal/external catheters are preferred over further metal stents for palliation.

Adult↗