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Biomedical subjects

D Loyd

Publications and source records attributed to D Loyd.

At least 19 recordsLinked to original sources
PubMed2001

Analysis of temperature measurement for monitoring radio-frequency brain lesioning.

During ablative neurosurgery of movement disorders, for instance therapy of Parkinson's disease, temperature monitoring is crucial. This study aims at a quantitative comparison of measurement deviations between the maximum temperature located outside the lesioning electrode and two possible thermocouple locations inside the electrode. In order to obtain the detailed temperature field necessary for the analysis, four finite element models associated with different surroundings and with different power supplies are studied. The results from the simulations show that both the power level and the power density as well as the surrounding medium affect the temperature measurement and the temperature field in general. Since the maximum temperature is located outside the electrode there will always be a deviation in time and level between the measured and the maximum temperature. The deviation is usually 2-7 s and 3-12 degrees C, depending on, for example, the thermocouple location and surrounding medium. Therefore, not only the measured temperature but also the relation between measured and maximum temperature must be accounted for during therapy and device design.

Catheter Ablation↗
PubMed1999

A comparison between in vitro studies of protein lesions generated by brain electrodes and finite element model simulations.

The aim of this study was to develop a finite element model for simulation of the thermal characteristics of brain electrodes and to compare its performances with an in vitro experimental albumin model. Ten lesions were created in albumin using a monopolar electrode connected to a Leksell Neuro Generator and a computer-assisted video system was used to determine the size of the generated lesions. A finite element model was set up of the in vitro experiments using the same thermal properties. With a very simple heat source applied to the finite element model in the proximity of the upper part of the tip, a good agreement (no deviations in width and distance from tip but a deviation in length of -1.6 mm) with the in vitro experiments (width 4.6 +/- 0.1 mm and length 7.4 +/- 0.1 mm) was achieved when comparing the outline of the lesion. In addition, a gelatinous albumin-model was set up and compared to computer simulations resulting in deviations in width of -0.4 mm, length of -2.2 mm and distance from the tip of -0.1 mm. Hence, the utilisation of finite element model simulations may be a useful complement to in-vitro experiments.

Albumins↗
PubMed1997

Thermal injury during TUMT.

A 71-year-old man was treated with transurethral microwave thermotherapy because of symptoms of benign prostatic hyperplasia. The treatment session was performed without any abnormal complaints from the patient. Two hours post-treatment the patient felt pain in his penile shaft and noticed a wound. A thorough investigation revealed that the only possible explanation for the injury was a dislocation of the catheter.

Aged↗
PubMed1996

Comparison of angioscopy, intravascular ultrasound imaging and quantitative coronary angiography in predicting clinical outcome after coronary intervention in high risk patients.

OBJECTIVES: The purpose of this study was to identify qualitative or quantitative variables present on angioscopy, intravascular ultrasound imaging or quantitative coronary arteriography that were associated with adverse clinical outcome after coronary intervention in high risk patients. BACKGROUND: Patients with acute coronary syndromes and complex lesion morphology on angiography are at increased risk for acute complications after coronary angioplasty. Newer devices that primarily remove atheroma have not improved outcome over that of balloon angioplasty. Intravascular imaging can accurately identify intraluminal and intramural histopathologic features not adequately visualized during coronary arteriography and may provide mechanistic insight into the pathogenesis of abrupt closure and restenosis. METHODS: Sixty high risk patients with unstable coronary syndromes and complex lesions on angiography underwent angioscopy (n = 40) and intravascular ultrasound imaging (n = 46) during interventional procedures. In 26 patients, both angioscopy and intravascular ultrasound were performed in the same lesion. All patients underwent off-line quantitative coronary arteriography. Coronary interventions included balloon (n = 21) and excimer laser (n = 4) angioplasty, directional (n = 19) and rotational (n = 6) atherectomy and stent implantation (n = 11). Patients were followed up for 1 year for objective evidence for recurrent ischemia. RESULTS: Patients whose clinical presentation included rest angina or acute myocardial infarction or who received thrombolytic therapy within 24 h of procedure were significantly more likely to experience recurrent ischemia after intervention. Plaque rupture or thrombus on preprocedure angioscopy or angioscopic thrombus after intervention were also significantly associated with adverse outcome. Qualitative or quantitative variables on angiography, intravascular ultrasound or off-line quantitative arteriography were not associated with recurrent ischemia on univariate analysis. Multivariate predictors of recurrent ischemia were plaque rupture on preprocedure angioscopy (p < 0.05, odds ratio [OR] 10.15) and angioscopic thrombus after intervention (p < 0.05, OR 7.26). CONCLUSIONS: Angioscopic plaque rupture and thrombus were independently associated with adverse outcome in patients with complex lesions after interventional procedures. These features were not identified by either angiography or intravascular ultrasound.

Angioplasty, Balloon, Coronary↗
PubMed1996

Heat transfer evaluation of the nasal thermistor technique.

When analyzing transvalvular and venous flow velocity patterns, it is important to relate them to respiration. An accurate recording of respiratory phase can be carried out with different methods. One of these methods is the use of a thermistor, which reacts to the variation in air temperature, placed in the noise of the patient. The thermistor used has a diameter for 1.0 mm and is of standard bead type. Although small, it has a considerable long time-constant and a long time-delay. The high time-constant gives a low cutoff frequency, well below the respiratory frequency and thereby causing a large phase difference. The thermistor was analyzed with the lumped heat capacity method, where it was easy to study the influence from design parameters, time-dependent air temperature, and velocity. The analysis was extended using the finite element method and the temperature field in the thermistor and the probe was calculated as a function of space and time. These calculations confirmed the result from the lumped model. The result showed that timing of respiration was not accurately obtained with the thermistor analyzed. To improve the timing, it was necessary either to change the measuring method or to use signal processing in order to achieve faster response.

Blood Flow Velocity↗
PubMed1995

Successful treatment of chronic total peripheral occlusions that failed conventional techniques using the stiff backend of the Glidewire.

The vast majority of failures of transcatheter interventions in patients with peripheral vascular disease are due to inability to cross the lesion with a guidewire. Although the use of the Glidewire has clearly improved the success rate, failures with especially chronic total occlusions still occur. We describe a new technique using the very stiff backend of the Glidewire, which we have found very successful, when conventional techniques fall in crossing highly resistant lesions.

Aged↗
PubMed1995

Bio-acoustic signals from stenotic tube flow: state of the art and perspectives for future methodological development.

To study the degree of stenosis from the acoustic signal generated by the turbulent flow in a stenotic vessel, so-called phonoangiography was first suggested over 20 years ago. A reason for the limited use of the technique today may be that, in the early work, the theory of how to relate the spectrum of the acoustic signal to the degree of the stenosis was not clear. However, during the last decade, the theoretical basis for this and other biological tube flow applications has been clarified. Now there is also easy access to computers for frequency analysis. A further explanation for the limited diagnostic use of bio-acoustic techniques for tube flow is the strong competition from ultrasound Doppler techniques. In the future, however, applications may be expected in biological tube flow where the non-invasive, simple and inexpensive bio-acoustic techniques will have a definite role as a diagnostic method.

Acoustics↗
PubMed1995

Regurgitant heart valve flow from 2-D proximal velocity field: continued search for the ideal method.

It has been suggested that flow through a leaking heart valve can be determined by studying the proximal velocity field. Normally, only the centre-line velocity is studied as a potential method. The aim of the study is to improve this method by using information from the entire reconstructed proximal velocity field. Four methods are compared: use of the centre-line velocity; use of velocities at three different angles; integration of velocities over a hemisphere; and integration of velocities over an estimated hemi-elliptical isovelocity line. Measurements are performed in a hydraulic model with 4, 6 and 8 mm circular orifices, and these are compared with those from computer simulation. From the results presented in the study, it is suggested that the velocities should be integrated over a hemisphere within a best zone. This zone is dependent on the instrument settings, but in this case it is positioned 1.2-1.4 orifice diameters from the orifice inlet, with an angle of up to +/- 45 degrees from the centre axis, and contains velocities in the range 0.15-0.45 ms-1.

Blood Flow Velocity↗
PubMed1995

A bioacoustic method for timing of respiration at cardiac investigations.

At cardiac investigations Doppler echocardiography is an established technique for the recording of intracardial and intravascular flow velocities. Transvalvular and venous flows are, however, markedly influenced by respiration. Since the start of inspiration is the important time of reference, accurate recording of the respiratory phase is important when analysing these flow velocities. A bioacoustic technique was therefore adopted to meet this demand and was tested in 10 normal subjects. Oesophageal balloon technique was used as a reference, and the bioacoustic technique was compared with a respiratory inductive plethysmograph. The average delay for the acoustic sensor signal compared to the oesophageal pressure changes was 205 +/- 46 (SD) ms, ranging from 160 to 320 ms for normal breathing, and is comparable to that of the respiratory inductive plethysmograph (210 +/- 90 ms). The bioacoustic technique is easier to apply than the respiratory inductive plethysmograph. The sensitivity to disturbances can be further reduced by signal processing and the method has clinical potential for the future.

Adult↗
PubMed1995

Mathematical model that characterizes transmitral and pulmonary venous flow velocity patterns.

The transmitral and pulmonary venous flow velocity (TMFV and PVFV, respectively) patterns are related to the physiological state of the left heart by use of an electrical analog model. Filling of left ventricle (LV) through the mitral valve is characterized by a quadratic Bernoulli's resistance in series with an inertance. Filling of the left atrium (LA) through the pulmonary veins is represented by a lumped network of linear resistance, capacitance, and inertance. LV and LA are each represented by a time-varying elastance. A volume dependency is incorporated into the LV model to produce physiological pressure-volume loops and Starling curves. The state-space representation of the analog model consists of 10 simultaneous differential equations, which are solved by numerical integration. Model validity is supported by the following. First, the expected effects of aging and decreasing LV compliance on TMFV and PVFV are accurately represented by the model. Second, the model-generated TMFV and PVFV waveforms fit well to pulsed-Doppler recordings in normal and postinfarct patients. It is shown that the TMFV deceleration time is prolonged by the increase in LV compliance and, to a lesser extent, by the increase in LA compliance. A shift from diastolic dominance to systolic dominance in PVFV occurs when LA compliance or pulmonary perfusion pressure increases or when LV compliance or mitral valve area decreases. The present model should serve as a useful theoretical basis for echocardiographic evaluation of LV and LA functions.

Animals↗
PubMed1994

Importance of collateral vessels in aortic coarctation: computer simulation at rest and exercise using transmission line elements.

Coarctation of the aorta causes arterial hypertension in the upper body and a low blood pressure downstream. Collateral blood vessels compensate by reducing the downstream pressure drop. To study the effect of various coarctation and collateral properties, we designed a computer model of the arterial circulation. The model contains a flow source and a library of subroutines for the lines and connectors. Distributed friction and wall viscoelasticity effects are included. Computer simulation was performed, using published values for vessel dimensions, in an arterial model with a coarctation and one lumped collateral. Rest and two levels of exercise (by increased heart rate) were studied. Without a collateral, we found the downstream pressure of the model was extremely dependent on the size of the coarctation. A collateral vessel reduced the pressure difference between the up- and downstream circulations. For a severe coarctation, the length and the diameter of the collateral were the main factors determining the downstream pressure and flow, whereas wall stiffness of the collateral had little influence. The relationship between mean pressure drop and cardiac output in coarctation was also dependent on the peripheral resistance in different flow beds, especially during exercise.

Aortic Coarctation↗
PubMed1993

Vascular characteristics influence the aortic ultrasound Doppler signal: computer and hydraulic model simulations.

There is an increasing demand for non-invasive methods for the assessment of left ventricular function. Ultrasound Doppler methods are promising, and the early systolic flow velocity signal immediately distal to the aortic valve has been used clinically for this purpose. However, the signal is influenced not only by left ventricular ejection but also by systemic vascular characteristics. Their relative contribution to the time-velocity signal has not been analysed in depth previously. A theoretical analysis, based on a three-element Windkessel model, neglecting peripheral outflow in early systole and assuming linear pressure rise, was therefore tested in computer and hydraulic model simulations where peripheral outflow was included. Significant changes in early aortic flow velocity parameters were found when vascular characteristics were altered. As predicted by the theory, with a standardized aortic valve area and aortic pressure change, the simulations confirmed that maximal flow velocity is related to compliance of the aorta and the large arteries, and that maximal acceleration is inversely related to the characteristic impedance of the aorta. Therefore, maximal velocity and acceleration can be used for assessment of left ventricular function only in situations where vascular characteristics can be considered relatively constant or where they can be estimated.

Aorta↗
PubMed1992

Brain SPECT and the effect of cerebral angioplasty in delayed ischemia due to vasospasm.

Cerebral vasospasm is a major determinant of outcome after subarachnoid hemorrhage (SAH). Brain SPECT with 99mTc-HMPAO was obtained before and after cerebral angioplasty in 10 patients with delayed ischemia due to vasospasm. Eight patients had clinically evident neurologic improvement after the procedure. Visual interpretation and an internal-reference (cerebellum), manual, semi-quantitative region of interest (ROI) analysis revealed improvement of regional cerebral blood flow (rCBF) in 9 out of 10. There were disagreements between the visual and ROI analysis in the two that did not improve clinically. For all 10, the average increase per anterior circulation vessel dilated (n = 17) was 8.8% by comparison of the corticocerebellar ratios. For the eight that improved, the average increase was 10.5%. Brain SPECT is valuable for evaluating delayed cerebral ischemia caused by vasospasm after SAH and is useful to document the changes in rCBF induced by angioplasty. It is possible that SPECT may be useful to detect critical reductions in perfusion before clinical deficits develop, thereby offering the potential to identify candidates for early treatment with angioplasty.

Adult↗
PubMed1991

Coarctation of the aorta--a theoretical and experimental analysis of the effects of a centrally located arterial stenosis.

Aortic coarctation is a local constriction of the aorta that may severely affect haemodynamics. It is therefore important to quantify these effects. Using Bernoulli's equation and the momentum theorem, the pressure drop is described including the pressure recovery distal to the coarctation and the effects of collateral flow; both laminar and turbulent. Assuming the coarctation and collaterals to be stiff, a quadratic relationship between flow and pressure drop is expected for flow through the coarctation and for turbulent collateral flow. For laminar collateral flow, a linear relationship is expected. The coarctation flow was studied in a model consisting of a rigid tube with local constriction, connected to a flooded-level tank, containing a 36 per cent by weight solution of sucrose, with a viscosity equivalent to that of blood at body temperature. The pressure drop across the constriction showed a quadratic relationship to flow in agreement with theoretical expectations. Pressure recovery in this model was very slight (0-4 mm Hg). Nine patients with aortic coarctation were catheterised. Cardiac output and pressure drop across the coarctation were measured at rest and during supine cycle exercise at two different workloads. The relationship between mean pressure drop and cardiac output tended to be either 'parabolic' or, in some cases, approximately linear, suggesting that the flow situation in aortic coarctation can be quantified by expressions that either linearly or quadratically relate pressure and flow.

Adult↗
PubMed1990

Analysis of different methods of assessing the stenotic mitral valve area with emphasis on the pressure gradient half-time concept.

There are 2 different theoretical models that analyze factors influencing the transmitral pressure gradient half-time (T1/2), defined as the time needed for the pressure gradient to reach half its initial value. In this report the models and the assumptions inherent in them were summarized. One model includes left heart chamber compliance, the other does not. Although the models at a superficial glance seem to be contradictory, the conclusions drawn from them are similar: i.e., T1/2 is influenced not only by valve area, but also by initial maximal pressure gradient and by flow. Different clinical situations in which the T1/2 method for valve area estimation has been shown not to work are analyzed in the 2 models. It is concluded that these models have contributed to our understanding of the T1/2 concept and when it should not be used. We also advocate use of the continuity equation in these situations, since no assumptions then need be made.

Blood Flow Velocity↗
PubMed1987

Problems related to the assessment of fluid velocity and volume flow in valve regurgitation using ultrasound Doppler technique.

Understanding of the factors affecting regurgitant flow through a heart valve and of the inherent limitations of the Doppler technique is needed to interpret correctly the information obtained during an ultrasound Doppler examination. This paper describes the flow conditions at the leaking valve and limitations of the Doppler technique which become important in the case of valve regurgitation. The flow conditions can be described in the following terms: contraction of the flow, core flow dimensions, friction, and intrusion and width of the jet flow. Contraction occurs at the entrance to the orifice and causes the width of the jet at the orifice to be smaller than the orifice itself. This contraction should be taken into account when calculating volume flow. The jet reaches a minimal area at the vena contracta where the flow velocity is close to that expected from the Bernoulli equation. The area of the vena contracta relative to the area of the hole can vary between 0.6 and 1.0; the lowest value is seen at a sharp-edged orifice and the highest value, at a hole with an ideally rounded inlet. Friction has a marginal role on flow velocity at the vena contracta. The velocity at the vena contracta persists in a region called the core flow region. This region has a length of 4-8 hole diameters. The total jet intrusion and the width of the jet are related to both the flow velocity at the hole and the diameter of the hole.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Flow Velocity↗