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At least 1,207 records · Page 67Linked to original sources
PubMed2005

Generation of parametric image of regional myocardial blood flow using H(2)(15)O dynamic PET and a linear least-squares method.

UNLABELLED: Although a parametric image of myocardial blood flow (MBF) can be obtained from H(2)(15)O PET using factor and cluster analysis, this approach is limited when factor analysis fails to extract each cardiac component. In this study, a linear least-squares (LLS) method for estimating MBF and generating a MBF parametric image was developed to overcome this limitation. The computer simulation was performed to investigate the statistical properties of the LLS method, and MBF values obtained from the MBF parametric images in dogs were compared with those obtained using the conventional region of interest (ROI) and invasive microsphere methods. METHODS: A differential model equation for H(2)(15)O in the myocardium was modified to incorporate the partial-volume and spillover effect. The equation was integrated from time 0 to each PET sampling point to obtain a linearlized H(2)(15)O model equation. The LLS solution of this equation was estimated and used to calculate the MBF, the perfusable tissue fraction (PTF), and the arterial blood volume fraction (V(a)). A computer simulation was performed using the input function obtained from canine experiments and the tissue time-activity curves contaminated by various levels of Poisson noise. The parametric image of the MBF, PTF, and V(a) was constructed using the PET data from dogs (n = 7) at rest and after pharmacologic stress. The regional MBF from the parametric image was compared with those produced by the ROI method using a nonlinear least-squares (NLS) estimation and an invasive radiolabeled microsphere technique. RESULTS: The simulation study showed that the LLS method was better than the NLS method in terms of statistical reliability, and the parametric images of the MBF, PTF, and V(a) using the LLS method had good image quality and contrast. The regional MBF values using the parametric image showed a good correlation with those using the ROI method (y = 0.84x + 0.40; r = 0.99) and the microsphere technique (y = 0.95x + 0.29; r = 0.96). The computation time was approximately 10 s for the 32 x 32 x 6 x18 (pixel x pixel x plane x frame) matrix. CONCLUSION: A noninvasive, very fast, and accurate method for estimating the MBF and generating a MBF parametric image was developed using the LLS estimation technique and H(2)(15)O dynamic myocardial PET.

Animals↗
PubMed2005

The effect of dissolved carbon dioxide on cavitation intensity in mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Mechanical heart valves (MHVs) are known to induce cavitation during closure and rebound. Cavitation may lead to blood element damage and stable bubble formation, with the latter introducing emboli into the cranial circulation and increasing the risk of stroke. Previous research has suggested that CO2 is the primary blood gas involved in stable bubble growth, due to its high solubility compared to that of oxygen or nitrogen. The primary objective of this study is to determine the role that CO2 plays in MHV-induced cavitation bubble formation. METHODS: Degassed water (5 ppm) was supplemented with CO2 at partial pressures of 0, 40 and 100 mmHg. Cavitation was visualized using high-speed videography for 29 mm Björk-Shiley Monostrut and Medtronic Hall MHVs in the mitral position. Experimental parameters (heart rate, systolic duration, and left ventricular pressure) were adjusted to provide dp/dt values of 500, 2,500 and 4,500 mmHg/s. High-frequency pressure fluctuations of cavitation bubble collapse were detected using a hydrophone. RESULTS: Root-mean square (RMS) values were calculated to quantify the cavitation intensity for both MHVs at the three loading conditions. The images of cavitation bubble formation and collapse were correlated to their respective RMS values. This study revealed no statistical difference between the cavitation intensities produced by either of the MHVs for the range of CO2-supplemented degassed water tested. For example, at the most physiologic loading condition of 2,500 mmHg/s, the RMS values for the Björk-Shiley Monostrut valve in degassed water containing 0 and 100 mmHg CO2 were 32.7 +/- 3.5 and 34.3 +/- 6.1 mmHg, respectively. CONCLUSION: The results of this in-vitro study show that, despite affecting stable bubble growth, the presence and quantity of dissolved CO2 does not affect the intensity of the cavitation events occurring during impact of the valve occluder with its housing. Therefore, the role of CO2 is limited to stable bubble development.

Carbon Dioxide↗
PubMed2006

In vitro evaluation of 2D-digital subtraction angiography versus 3D-time-of-flight in assessment of intracranial cerebral aneurysm filling after endovascular therapy.

BACKGROUND AND PURPOSE: The aim of this study was to evaluate 2D-digital subtraction angiographic (DSA) and 3D-time-of-flight (TOF) MR imaging in assessment of aneurysmal residue by using a pulsating silicon aneurysm model. For each imaging system, we studied intra- and interobserver reproducibility and the agreement between interpretations and reference measurements. We also examined how each imaging technique affected the operator's therapeutic decision. METHODS: Two silicon aneurysm models depicting subarachnoidal aneurysms were used, one with a wide neck and one with a narrow neck. Each aneurysm model was placed in series on a pulsed flow circuit and was filled with Guglielmi detachable coils to simulate a clinical case. Each aneurysm was then gradually filled with silicon gel in increments of 10%, up to 100% to simulate different levels of occlusion (residual neck or dog ear, partial, complete) at each filling level. For each level of filling, we performed conventional 2D-DSA and 3D-TOF MR imaging. We submitted the images for examination by 2 senior medical staff with 2 readings per image. A combined reading of the 2 images was submitted to each expert to determine whether the 2 examinations were complementary. RESULTS: The 2D-DSA analysis showed good reproducibility (k = 0.8 and k = 0.57) and agreement (k = 0.71) in describing "complete" treatments. The distinction between a "residual neck" and "partial treatment," however, was not reliable. The 2D-DSA provided a good description of the coil and silicon protrusion into the parent artery. The 3D-TOF analysis of the residual aneurysm, however, was not reproducible, though it was more effective than the 2D-DSA in evaluation of partially wide-necked aneurysms (k = 0.68 MR imaging vs k = 0.041 2D-DSA; P = .018). At the same filling level, the 2D-DSA analysis indicated repeat treatment more often than 3D-TOF analysis (P = .059). CONCLUSION: The 2D-DSA remains the gold standard, but MR imaging is more effective in evaluating a "partial treatment." The 2D-DSA analysis indicated repeat treatment more often than the 3D-TOF for the same occlusion level. The distinction between "partial treatment" and a "residual neck" was not reliable with either method of evaluation.

Angiography, Digital Subtraction↗
PubMed1992

Mathematical modelling of post-exercise heart rate recovery in ponies.

Using a recently developed nonlinear algorithm which identifies reference points on left ventricular pressure waveforms and other waveforms, we analyzed heart rate recovery in ponies after strenuous treadmill exercise. Each pony performed exercise tests with and without premedication with atropine methyl nitrate. Resting and exercise heart rates were 54 +/- 3 and 209 +/- 6 beats/min, respectively, without premedication and 85 +/- 8 and 212 +/- 5 beats/min, respectively, following atropine premedication. Post-exercise heart rate recovery without premedication was best described by a biexponential curve with a rapid initial period (duration = 55 +/- 6 sec, Tl/e = 15 +/- 2 sec) and a slower secondary period (duration = 267 +/- 5 sec, Tl/e = 455 +/- 78 sec). After atropine premedication, heart rate recovery followed a monoexponential curve (Tl/e = 174 +/- 46 sec) in 4 ponies, but remained biexponential (Part 1 duration = 74 sec, Tl/e = 20 sec; Part 2 duration = 243 sec, Tl/e = 342 sec) in the other ponies. The calculated time required for heart rate to return to resting levels after atropine premedication was 11.3 +/- 2.2 min, compared to 26.3 +/- 3.9 min without premedication. However, without premedication, only 6.6 +/- 1.6 min were required to reach the post-atropine resting heart rate, indicating that the time difference was due to the higher resting heart rate after atropine premedication. Thus, heart rate recovery in the non-premedicated pony follows a biexponential curve similar to that observed in other species.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMed2005

Impact of the volume of aneurysmal contents on intraaneurysmal pressure after endovascular grafting (experimental studies).

With the purpose of clarifying the nature and outlining certain constituents of such an undesirable condition designated as "endotension", that emerges after transluminal grafting of the aneurysmally changed aorta, a basic theoretical model was worked out for explanation of "endotension". Also, there was designed and constructed in vitro an original experimental model using which the authors carried out a study into the relationship between the pressure in the aneurysm after its complete exclusion from the blood flow by an endovascular graft and the volume of the aneurysmal contents. Some factors described in the literature as influencing the pressure level in the aneurysm were at the given stage purposefully excluded. The volume of the "aneurysm" reproduced in our model was equal to 675 ml; each of five vascular grafts implanted into the aneurysm measured 110 mm in length. The pulsating liquid flow was reproduced in the system using a serial appliance for extracorporeal circulation. The liquid was aspirated from the aneurysm by means of a syringe with concurrent pressure guidance in the sac. To start pressure lowering, it was necessary to evacuate 0.4-1.6 ml of the liquid. The real clinical situations were considered from the standpoint of our results.

Aortic Aneurysm↗
PubMed2006

Survey of Poincaré indices for measuring heart rate variability.

Multiple indices have been developed to characterise the nonperiodic behaviour of heart rate variability displayed in Poincaré plots, particularly with R-R interval data collected over 24hr periods. Some of these indices have been compared to statistical time-domain indices and some to spectral indices, while others have been used to separate data sets such as cardiac abnormalities. A survey of published indices for Poincaré plot analysis was made covering literature through the 1990's to the present day. Statistical time-domain indices and the surveyed Poincaré indices were used to analyse the same data set of R-R intervals that included: supine, standing, deep breathing, light exercise and recovery. This study examined these Poincaré indices and determined their correlations with time-domain indices, and investigated how the correlations are affected by analysing shorter time periods. Further investigation of the Poincaré indices that do not correlate strongly with SD or rMSSD may prove interesting in determining dynamic characteristics of cardiac nervous system activity.

Algorithms↗
PubMed2005

Growth, anisotropy, and residual stresses in arteries.

A simple phenomenological theory of tissue growth is used in order to demonstrate that volumetric growth combined with material anisotropy can lead to accumulation of residual stresses in arteries. The theory is applied to growth of a cylindrical blood vessel with the anisotropy moduli derived from experiments. It is shown that bending resultants are developed in the ring cross-section of the artery. These resultants may cause the ring opening or closing after cutting the artery in vitro as it is observed in experiments. It is emphasized that the mode of the arterial ring opening is affected by the parameters of anisotropy.

Animals↗
PubMed2005

The effect of longitudinal pre-stretch and radial constraint on the stress distribution in the vessel wall: a new hypothesis.

It is well known that blood vessels shorten axially when excised. This is due to the perivascular tethering constraint by side branches and the existence of pre-stretch of blood vessels at the in situ state. Furthermore, vessels are radially constrained to various extents by the surrounding tissues at physiological loading. Our hypothesis is that the axial pre-stretch and radial constraint by the surrounding tissue homogenizes the stress and strain distributions in the vessel wall. A finite element analysis of porcine coronary artery and rabbit thoracic aorta based on measured material properties, geometry, residual strain and physiological loading is used to compute the intramural stresses and strains. We systematically examined the effect of pre-stretch and external radial constraint in both vessels. Our results show that both stretching in the axial direction and compression in the radial direction lead to a more homogeneous strain and stress state in the blood vessel wall. A "uniform biaxial strain" hypothesis is proposed for the blood vessel wall and the ramifications are discussed.

Animals↗
PubMed2006

[In vitro response time of continuous cardiac output measurement in STAT mode of Vigilance after controlled flow change].

BACKGROUND: Continuous cardiac output measurement in STAT mode (STAT CCO) equipped with Vigilance displays cardiac output every 30 to 60 seconds. The aim of this study is to verify the hypothesis that each value with this system is computed only from the data collected in one update period. METHODS: The circuit was filled with normal saline and flowed by a roller pump in in vitro setting. The flow rate was set at either 2.5 l x min(-1) or 5.0 l x min(-1) and changed quickly to another state after each state had been maintained for 25 minutes. The change operation was repeated 10 times. The maximum difference was defined as the difference between the value at the start and the maximum change value. The response time was defined as the time from the start to the time to reach 80% of the maximum difference. In each operation, the response time of STAT CCO was calculated. RESULTS: The response time of STAT CCO was 9.7 +/- 1.3 min (mean +/- SD). CONCLUSIONS: The response time of STAT CCO was about 10 times longer than one update period. This result suggests that STAT CCO values are not computed only from the data collected in one update period.

Cardiac Output↗
PubMed2006

An analytical model for the investigation of axial wall forces generated by an arterial stenosis.

The causes of arteriosclerosis are intensively investigated since many decades. While circumferential wall stress has received a lot of attention, axial stress (also called "longitudinal" stress) has been largely neglected, and practically never incriminated. However, it has been suggested in 2003 that moderate and severe arterial stenoses may induce non negligible axial forces cyclically in the vessel segment just proximal to the constriction cone. In the present contribution, we describe a simple analytical model that allows to study the distribution of these forces along the vessel in dependence of the respective axial elasticities of vessel and surrounding tissues, and of the stenosis length.

Arteries↗
PubMedDate not supplied

ASPD: A prospective study of adequacy in Asian patients on long term, small volume, continuous ambulatory peritoneal dialysis.

BACKGROUND: The impact of small solute clearance on patient survival in continuous ambulatory peritoneal dialysis (CAPD) is not yet solidified. Previously, we demonstrated that CAPD using small volume (6 L) daily exchanges provides adequate dialysis for most Asian patients. METHODS: We conducted a prospective, long-term observational study to determine the optimal dialysis adequacy that may provide better patient survival for Asian patients who receive small-volume CAPD. We recruited 294 patients. The initial CAPD regime was 3 x 2-L exchanges daily. The same regime was maintained unless there was significant loss of ultrafiltration or fluid retention despite the use of hypertonic dialysate. RESULTS: Median study period was 38.9 (range 5 - 76.5) months, with 81% and 27% of patients remaining in the study at 24 and 48 months respectively. The overall survival rates at 2 and 4 years were 94.0% and 74.8% respectively. Our long-term data revealed that survival rate was related to Kt/V values. Survival rates were significantly higher for patients with total Kt/V > 2.0 than for patients with Kt/V < 1.7 (p = 0.02). The former group had lower body mass index and higher residual renal function and peritoneal Kt/V than the Latter group. On analysis using Cox proportional hazards regression models, cardiovascular disease (CVD), lower urine volume, and higher body mass index were independent predictors of mortality. Patients with higher renal Kt/V had a significantly lower risk of mortality (RR = 0.018, p = 0.01) after adjusting for the effects of CVD and diabetes mellitus. CONCLUSION: Our data recommend that 1.7 be the minimal target for total Kt/V in patients on long-term CAPD. Patients with high body mass index, low residual urine volume, and significant CVD need close monitoring.

Adult↗
PubMed1991

In vitro hemodynamic analysis of flexible artificial ventricles.

An in vitro fluid dynamic study was performed to compare the hemodynamic characteristics of a rigid and a flexible total artificial heart. The artificial ventricles were incorporated into a mock circulatory system, and pressure signals within the ventricular chamber, proximal to the inflow valve and distal to the outflow valve, were obtained. The instantaneous flow rate through the inflow and outflow valves was measured with electromagnetic flow probes. Flow visualization studies performed on the flexible ventricle suggested a vortical motion within the chamber with a smooth washout of fluid in the next pumping phase, but flow disturbances were observed near the wall of the ventricle as well as near the outflow valve. The rate of pressure increase (dP/dt) was smaller in the flexible ventricle as compared with the rigid ventricle for comparable flows and heart rates. The results of the present study indicated that the flexible ventricle with polyurethane valves, having the advantage of ease of implantation and cost savings, can be a viable alternative as a bridge to transplant.

Heart, Artificial↗
PubMedDate not supplied

Application of finite element analysis for assessing biocompatibility of intra-arterial catheters and probes.

A commercial finite element modeling program (FIDAP) was adapted to compute the fluid dynamics of laminar blood flow around an intra-arterial catheter and/or sensor probe. The model provided an accurate transient solution to the Navier-Stokes equations under pulsatile blood flow conditions. To simulate the compliance in the catheter tubing set, a second order convolution integral was incorporated into the boundary conditions. The saline drip rate and catheter compliance could be specified, and the bulk blood flow, blood pressure, and heart rate were varied to simulate specific patient conditions. Analysis of the transient solution was used to assess probable sites for thrombus activation and deposition. The transient velocity and pressure fields identified regions of separated flow and recirculation. The computed shear rates and stresses were used to predict hemolysis, platelet activation, and thrombus formation. Analysis of particle paths provided an estimate of residence times and thrombus deposition sites.

Biocompatible Materials↗
PubMedDate not supplied

A sophisticated electromechanical ventricular simulator for ventricular assist system testing.

With the advent of chronically implanted left ventricular assist systems, new adaptive control algorithms are being developed to allow automatic device control under a variety of hemodynamic conditions. An electromechanical pump is described that accurately mimics ventricular function. A pusher-plate sac pump with tri-leaflet valves is coupled to a high-speed linear motor. Pump chamber volume and pressure are constrained to follow the time-varying elastance model of ventricular function by use of a microprocessor feedback loop.

Computer Simulation↗
PubMedDate not supplied

Dynamic internal compliance of a vascular prosthesis.

A new technique was used to measure dynamic internal compliance of a blood vessel or vascular graft subjected to dynamic internal pressure. The internal compliance can be broken into three categories: the volumetric compliance (CV), defined as (dV/V)/dP; the longitudinal compliance (CL), defined as (dL/L)/dP; and radial compliance (CR), defined as (dR/R)/dP. It can be shown mathematically that CV = 2 CR + CL. Thus, measuring any two of the three entities will also give the value for the third. A Dynatek DCT1 dynamic compliance tester was used for measuring the compliance of DenaflexTM biologic grafts and fresh bovine internal thoracic arteries, from which the Denaflex grafts were obtained by fixation. Volumetric compliance was obtained with the test sample mounted in a loose loop that allowed the sample to move both radially and longitudinally. By mounting the sample in a straight fashion that limited longitudinal movement, the radial compliance was determined. The longitudinal compliance was then calculated from the above relationship. Test results show that the fresh bovine artery had an average volumetric compliance of 26.1%/100 mmHg, radial compliance of 9.5%/100 mmHg, and longitudinal compliance of 7.2%/100 mmHg. The Denaflex vascular graft showed a reduction in longitudinal and radial compliance, compared to the fresh raw artery, as a result of extensive fixation.

Animals↗
PubMed1991

Shear stress related blood damage along the cusp of a tri-leaflet prosthetic valve.

Blood flowing through a prosthetic heart valve can be damaged by flow-induced shear forces. Fluid dynamics variables and geometric factors play an important role in the evaluation of shear-stress-related blood damage. Central-flow prosthetic valves have been considered as an optimal replacement for mechanical and biological valves. Recently it was shown that shear stress distribution along the surface of a polyurethane cusp reaches values that can damage the blood elements. A mathematical model correlating the effects of shear stresses on blood corpuscles with clinical findings was employed in vitro. The model can be applied to the effects of blood-surface interaction and is of clinical relevance.

Blood Cells↗
PubMedDate not supplied

Comparison of pressure-volume-flow relationships in centrifugal and roller pump extracorporeal membrane oxygenation systems for neonates.

Theoretical advantages and risks exist for the use of both the centrifugal and roller pump systems in neonatal extracorporeal membrane oxygenation (ECMO). The authors studied the pressure-volume-flow relationships in clinically configured ECMO systems using these two pumps and a simulated patient to characterize differences in the circuit mechanics of the two systems, and thereby improve the design of subsequent clinical comparative trials of the pumps themselves. The relationship between flow and pressure generated across the pump was identical for the two systems. Within the range of clinically used flows, there was a direct relationship between pump revolution and flow with the roller pump, and between pump revolution and pressure generated for the centrifugal pump. Flow was limited in both systems by restrictions on negative pressure generating capacity. In the roller pump circuit, the venous reservoir (bladder box) assembly interrupted flow when negative pressure exceeded -20 mmHg; in the centrifugal pump system, forward flow stopped when negative pressure exceeded -100 mmHg. Volume had no detectable effect on the patient-pump inlet pressure gradient until critically low volumes were reached. At that point, removal of a few milliliters of volume led to large increases in the pressure gradient. The authors conclude that differences in pressure-volume-flow relationships between roller and centrifugal pump ECMO systems are due to the presence of the bladder box in the roller pump circuit. The advantages and disadvantages of the greater negative pressure in the centrifugal pump system require further study.

Equipment Design↗