Showing papers on "Pulsatile flow published in 1994"

Quantitative magnetic resonance flow imaging.

Abstract: Time-of-flight and phase shift methods have both been used for vascular imaging with magnetic resonance. Phase methods, and phase contrast in particular, are well suited to quantitative measurements of velocity and volume flow rate. The most robust methods for measuring flow encode through-plane velocity into phase shift and compute flow by integrating the measured velocity over the vessel lumen. The accuracy of the flow data can be degraded by the effects of acceleration and eddy currents and by partial volume effects, including the effects of finite slice thickness and resolution, pulsatile waveforms, motion, and chemical shift. The reproducibility depends on the signal-to-noise of the data and the strength of the flow encoding and can be degraded by inconsistent definition of the vessel boundary. The adjustable flow sensitivity inherent in this method is a particular asset, allowing phase contrast flow measurement to operate over a dynamic range exceeding 10(5). Recently developed rapid imaging methods are helpful in applications that would be compromised by respiratory motion. With care, excellent quantitative data can be quickly obtained in vivo, and the resulting flow information is valuable for the diagnosis and management of a variety of conditions.

Steady Inspiratory Flow in a Model Symmetric Bifurcation

Abstract: Flow in a bifurcating tube system typifying a major bronchial bifurcation is studied experimentally with a two color, two velocity component laser Doppler anemometer. The flow loop is composed of a pumping station, flow stratifiers and a constant head pressure tank; it can accommodate steady, pulsatile or oscillatory flow. The test section is a symmetric bifurcation of constant cross sectional area and has a branching angle of 70 deg. The test section is a cast of clear silicon rubber in a plexiglass mold that was milled on a numerically controlled milling machine. The flow division ratio from the parent to daughter branches is about unity. Steady flow results that model the inspiratory phase at Reynolds numbers of 518, 1036 and 2089, corresponding to Dean numbers of 98, 196 and 395, show that in the bifurcation plane velocity profiles in the daughter branches are skewed toward the inner wall. In the transverse plane, "m" shaped velocity profiles are found with low velocity at the center. Secondary flow patterns, which are responsible for such phenomena, are first observed at the axial position where the flow begins to turn. Flow separation was not observed at any point in the bifurcation.

Pulsatile velocity measurements in a model of the human abdominal aorta under resting conditions.

Abstract: Oscillations in near-wall velocity direction have been found to correlate with atherosclerotic plaque localization in the carotid sinus bifurcation. However, it remains unproven whether these conditions could account for the localization of the disease at other sites where atherosclerosis forms. The abdominal aorta is an important site of clinical disease in a relatively straight segment of artery. This study was initiated to quantify the velocity field in the abdominal aorta in order to determine if local differences in hemodynamic velocity directions could account for the localization of disease in this segment. Magnetic Resonance Imaging velocimetry was used to measure the pulsatile velocity profiles in an anatomically accurate in vitro model of the abdominal aorta. Velocities measured in the suprarenal aorta were laminar and reversed minimally, comparing well with theoretical solutions of Womersley flow (r = 0.96). The time-averaged velocity was +3.0 cm/s near-wall at a distance of 1 mm away from the wall. In the infrarenal aorta, the maximal velocities were skewed toward the anterior wall. At the posterior wall, velocity oscillated in direction and was retrograde for 82 percent of the cardiac cycle. The time-averaged velocity near the posterior wall was -12.5 cm/s as compared to +3.00 cm/s near the anterior wall. At the aortic bifurcation, the locations of maximal and minimal velocities in this slice were concentrated near the lateral posterior walls. This study quantifies the magnitude of low and oscillatory velocity that may exist in the abdominal aorta and suggests that there is a strong relationship between the velocities in the retrograde direction under resting conditions and the distribution of atherosclerotic plaque.

Flow and stress characteristics in rigid walled and compliant carotid artery bifurcation models.

Abstract: Computer simulation of pulsatile non-Newtonian blood flow has been carried out in different human carotid artery bifurcation models In the first part of the investigation, two rigid walled models are analysed, differing in the bifurcation angle (wide angle and acute angle bifurcation) and in the shape of both the sinus (narrow and larger sinus width) and the bifurcation region (small and larger rounding of the flow divider), in order to contribute to the study of the geometric factor in atherosclerosis The results show a significant difference in the wall shear stress and in the flow separation Flow recirculation in the sinus is much more pronounced in the acute angle carotid An important factor in flow separation is the sinus width In the second part of the study, flow velocity and wall shear stress distribution have been analysed in a compliant carotid artery bifurcation model In the mathematical model, the non-Newtonian flow field and the idealized elastic wall displacement are coupled and calculated iteratively at each time step Maximum displacement of approximately 6% of the diastolic vessel diameter occurs at the side wall of the bifurcation region The investigation demonstrates that the wall distensibility alters the flow field and the wall shear stress during the systolic phase Comparison with corresponding rigid wall results shows that flow separation and wall shear stress are reduced in the distensible wall model

Delivery devices with pulsatile effect

Abstract: Delivery devices which include a beneficial agent and an activating mechanism in separate compartments separated by a moveable partition are designed to deliver the beneficial agent in a pulsatile manner through an orifice. The pulsatile delivery is achieved by a band of resilient material which is placed over the orifice. The resilient material is sufficiently tight to substantially seal the orifice when the pressure within the device is below a threshold level while at the same time being capable of being stretched when the internal pressure exceeds the threshold.

MR imaging and MR angiography in the evaluation of pulsatile tinnitus.

Abstract: PURPOSE 1) To evaluate the scope of imaging findings seen with spin-echo MR and MR angiography (MRA) in patients with pulsatile tinnitus; 2) to determine whether MRA adds additional imaging information (to that provided by spin-echo MR) necessary for determining the cause of pulsatile tinnitus; and 3) to suggest MR and MRA imaging techniques for evaluation of patients with pulsatile tinnitus. METHODS Forty-nine patients with pulsatile tinnitus were evaluated with MR and MRA. Seventeen of these patients had conventional angiography. RESULTS Vascular lesions or paraganglioma were demonstrated in 28 patients. Of these 28 lesions, the majority were seen best (46%) or only (36%) on MRA. The spectrum of lesions detected included dural arteriovenous fistula (nine), extracranial arteriovenous fistula (three), paraganglioma (five), jugular bulb variants (three), aberrant internal carotid artery (one), internal carotid artery stenosis (one), tortuous internal carotid artery (one), carotid dissection with pseudoaneurysm (one), stenosis of the transverse sinus (two), and arteriovenous malformation (two). CONCLUSIONS MRA, in conjunction with spin-echo imaging, markedly enhances the ability of MR to diagnose the lesions responsible for pulsatile tinnitus.

Ocular blood flow measurement.

Abstract: Many techniques have been devised to measure the haemodynamics of the human and animal eye. In this perspective these are outlined and their use in ophthalmic investigation summarised. Some have exploited the ability of an observer to directly visualise the retinal vasculature by optical means, others have been designed to study the haemodynamics ofthe invisible parts of the eye such as the choroid, optic nerve head, and ciliary body. Although useful in ophthalmic investigation, none have satisfied all of the requirements of the researchers in this field and most have not achieved regular use in clinical practice. In any examination of blood flow a multitude of variables must be studied (Table 1). The interrelation ofthese variables must be determined while considering physical or physiological principles (Table 2) which are often not strictly applicable to the vasculature for example, the Hagen Poiseuille law was described for a rigid tube and not for elastic walled tubes such as blood vessels. In the human, study ofthe circulation is further hindered by the requirement for a noninvasive and safe method for obtaining measurements. Study of the haemodynamics must be performed if we are to understand the mechanisms leading to the large variety of vascular diseases which affect the eye. The blood flow to the eye is of particular interest because: (1) Many localised and systemic disorders affect the vasculature of the eye. (2) The eye has unusual haemodynamic properties because the tissues are subjected to a high intraocular pressure. (3) Ocular blood flow is autoregulated for example, during changes in retinal illumination, blood pressure, or posture. (4) Pharmacological agents which are routinely used in systemic and ocular diseases may affect the blood supply of the eye.

Blood flow changes in permanent maxillary canines during retraction

Abstract: The influence of external load on the blood flow of permanent maxillary canine teeth was assessed by laser Doppler flowmetry (LDF). Blood flow readings were obtained from 10 maxillary canines and compared with the contralateral teeth simultaneously. Readings were obtained from the teeth before, during, and after the application of a 50 g force (gf) which was applied using a removable appliance. The probe for LDF measurement was held in place by a splint constructed of a silicone impression material designed to allow movement of the tooth, but prevent instability of the probe. LDF demonstrated a reading from the canine teeth consistent with recordings of blood flow, i.e. the traces were similar to the pulsatile nature of pulse pressure recordings taken from the subjects' ear-lobes. After loading the effect on the canine was (1) a decrease in blood flow as measured with LDF followed by (2) an increase in flow after 32.3 minutes (SD 4.74). These changes were statistically significant (P < 0.05) using Student's t-test. The increase in blood flow was still present after 24 and 48 hours, but returned to preload values within 72 hours. In two cases it was found that the decrease in blood flow remained as long as the load was applied. The response appeared similar to reactive hyperaemia found following placement of a tourniquet. However, the response time was substantially longer for the hyperaemic phase.

The application of breath hold phase velocity mapping techniques to the measurement of coronary artery blood flow velocity: phantom data and initial in vivo results.

Abstract: A segmented kappa-space breath hold phase velocity mapping technique has been developed for the study of coronary artery blood flow velocity. In vitro validation has been performed using a number of pulsatile flow phantoms and the accuracy of the technique for determining the velocity increase at the site of a stenosis demonstrated in several phantom models. Examples of both in-plane and through-plane velocity maps of the left anterior descending and right coronary arteries of normal subjects in early diastole are presented. In one subject, through-plane velocity maps were obtained in the right and left anterior descending arteries throughout the cardiac cycle in order to generate flow velocity time curves. The problems associated with coronary artery velocity mapping are discussed.

Pulsatile kidney perfusion for evaluation of high-risk kidney donors safely expands the donor pool.

Abstract: Pulsatile preservation of cadaveric kidney allografts allows effective organ preservation and the opportunity to obtain information about the quality of the organ during storage. The use of information obtained by measuring FLOW (ml/min) and renal resistance (mean perfusion pressure/FLOW (ml/min)) while kidneys are undergoing pulsatile perfusion offers quantitative data to determine suitability of an organ for transplant. This may be more accurate than the combination of demographic variables with past and current medical information that has been traditionally used to evaluate donor suitability. We have evaluated 82 kidneys that received a period of pulsatile preservation at our institution. Kidneys were from consecutive donors age > or = 42 years and were stratified into three high risk groups: donor age > or = 60 (48%), the presence of hypertension (52%), and IMPORT kidney (48%). All IMPORT kidneys were obtained through UNOS after the local center considered them unacceptable for transplant and none were 6-antigen match organs. Kidneys were discarded if they failed to have a FLOW > or = 70 ml/min or RR or = 60 (p = 0.002). The average FLOW and RR of transplanted kidneys was 103 ml/min and 0.3283 respectively. Six of 69 (8.6%) transplanted organs required dialysis (ATN). No pump parameters predicted the development of ATN although 5 of 6 kidneys were IMPORTs. Recipient outcome data, including 72-hour urine output and day 10 serum Cr, were affected by organ risk group, but these did not impact on patient care.(ABSTRACT TRUNCATED AT 250 WORDS)

Unsteady Entrance Flow Development in a Straight Tube

Abstract: The entrance conditions for pulsatile flow are important in the understanding blood flow out of the heart and in developing regions at branches. The pulsatile entrance flow was solved using a spectral element simulation of the full unsteady Navier-Stokes equations. A mean Reynolds number of 200 and a range of Womersley parameters from 1.8 to 12.5 was used for a sinusoidal inlet flow waveform 1 + sin (omega t). Variations in the entrance length were observed during the pulsatile cycle. The amplitude of the entrance length variation decreased with an increase in the Womersley parameter. The phase lag between the entrance length and the inlet flow waveform increased for Womersley parameter alpha up to 5.0 and decreased for alpha larger than 5.0. For low alpha, the maximum entrance length during pulsatile flow was approximately the same as the steady entrance length for the peak flow. For high varies; is directly proportional to, the pulsatile entrance length was more uniform during the cycle and tended to the entrance length for the mean flow. The wall shear rate reached its far downstream value after only about half of the entrance length and also exhibited a dependence on alpha. The results quantify the entrance conditions typically encountered in studies of the arterial system.

Process of continuous noninvasive hemometry

Abstract: This invention relates to a process of determining continuously and non-invasively (without the withdrawal of blood.) the concentrations of hemoglobin. This is done by measurement of the path length and analysis of the pulsatile component of absorbance of multiple wave lengths of light transmitted through a tissue bed. This invention also relates to the process of simultaneous direct or indirect measurement of the pulsatile arterial width or arterial diameter which is equivalent to the pulsatile path length of the light transmitted across the tissue bed. Measurement of this arterial diameter or pulsatile path length is a prerequisite for non invasive determination of the hemoglobin, hematocrit or pigment concentrations in blood.

Preliminary Analysis of the Effects of Blood Vessel Movement on Blood Flow Patterns in the Coronary Arteries

Abstract: Blood flow patterns are believed to be involved in the formation and progression of arterial diseases. It is possible that the normal physiologic movement of blood vessels during the cardiac cycle affects blood flow patterns significantly. For example, the contraction of the heart in systole and subsequent relaxation in diastole create movements of the coronary arteries, as evidenced in real-time angiography. The effects of this movement on coronary artery flow patterns have never been previously analyzed. This work was undertaken to provide a preliminary estimate of the importance of the effects of such physiologic movements on blood flow patterns in the coronary arteries. A Womersley-type solution was used to determine the effect of axial movement on the wall shear rate in a simplified model of the coronary arteries. The pulsatile pressure gradient was derived from previously published coronary artery flow waveforms. The axial movement function was obtained from a three-dimensional reconstruction of a biplanar coronary angiogram. Significant changes in wall shear rate were noted when the movement was taken into account. The maximum and minimum wall shear rates were 10 percent smaller and 107 percent larger in magnitude respectively, and the Oscillatory Shear Index (OSI) was doubled. Most of the changes in wall shear rate were observed in systole, when the pressure gradient is minimal and the movement is strongest. The results indicate that blood vessel movement during the cardiac cycle has a significant effect on hemodynamic phenomena which have been associated with the development of atherosclerosis.

Pulsatile Velocity Measurements in a Model of the Human Abdominal Aorta Under Simulated Exercise and Postprandial Conditions

Abstract: This study examines the hemodynamics of the abdominal aorta during physiological changes in flow rates and pulse rate that occur under exercise and postprandial conditions. Hemodynamic measurements were performed using an in vitro model which took into account seven major branches, the curvature, and the pulsatile nature of blood flow of the abdominal aorta. Magnetic Resonance Imaging velocimetry employing phase-velocity encoding was used to measure the pulsatile axial velocity profiles for the entire cross-section at three axial locations. Under simulated exercise conditions, the forward velocities were approximately double those seen during rest, and the flow reversal seen for resting conditions was greatly reduced. Near the posterior wall of the infrarenal aorta, the velocities were negative for only 21 percent of the cardiac cycle as compared with 82 percent for resting conditions. Postprandial conditions produced a 25 percent reduction in peak velocity and a 33 percent reduction in mean velocity near the left anterior wall of the aorta just distal to the superior mesenteric artery (in comparison with resting conditions). The changes that can occur in abdominal aorta hemodynamics under different physiologic conditions may affect the rate of progression of atherosclerosis at this site.

Coordinate pulsatile insulin secretion by chronic intraportally transplanted islets in the isolated perfused rat liver

Abstract: In the present studies we sought to address the following questions: do chronically transplanted intrahepatic islets (IHI-Tx) secrete insulin in a coordinate pulsatile manner, and, if so, is reestablishment of this coordinate pulsatility a function of time after transplantation? We studied isolated perfused livers at 10 mM glucose from 27 rats rendered diabetic with streptozotocin and then transplanted with approximately 2 x 10(3) islets, 2 (n = 5), 7 (n = 5), 30 (n = 5), and 200 (n = 12) d after transplantation. 12 out of 12 of the 200-d IHI-Tx secreted insulin in coordinate pulses (frequency 3.9 +/- 0.3 pulses/h, amplitude 15.2 +/- 2.4 nmol/min). In contrast, one out of five 2-d, zero out of five 7-d, and one out of five 30-d IHI-Tx showed pulsatile insulin secretion. Insulin secretion was markedly greater (76 +/- 13 vs 13 +/- 3 nmol/min, P < 0.0001) in the 200-d versus early IHI-Tx. Pentobarbital 25 micrograms/ml had no effect on total (13.9 +/- 3.9 vs 15.9 +/- 3.9 nmol/min), nonpulsatile (12.9 +/- 3.5 vs 14.1 +/- 3.3 nmol/min), or pulsatile (pulse amplitude 17.6 +/- 4.5 vs 20.0 +/- 4.2 nmol/min, pulse frequency 4.1 +/- 0.3 vs 4.0 +/- 0.7 pulses/h) insulin secretion. Using synaptophysin, islet innervation was documented in 12 out of 12 200-d IHI-Tx but in none of the early IHI-Tx. We conclude that established (approximately 200 d) IHI-Tx secrete insulin in a coordinate pulsatile manner and that establishment of coordinate pulsatile insulin secretion by IHI-Tx is accompanied by increased total insulin secretion and is associated with islet reinnervation.

Derivation of shear rates from near-wall LDA measurements under steady and pulsatile flow conditions.

Abstract: Atherosclerosis, thrombosis, and intimal hyperplasia are major forms of cardiovascular diseases in the United States. Previous studies indicate a significant correlation between hemodynamics, in particular, wall shear rate, and pathology of the arterial walls. While results of these studies implicate morphologic and functional changes related to wall shear rate magnitude, a standard technique for wall shear rate measurement has not been established. In this study, theoretical and in-vitro experimental fully developed steady and physiologic pulsatile flow waveforms have been used to obtain velocity profiles in the near-wall region. The estimated wall shear rates from these results are compared to the theoretical value to assess the accuracy of the approximating technique. Experimentally obtained results from LDA suggest that in order to minimize the error in velocity data, and subsequently, the wall shear rate, the first measured velocity has to be 500 microns away from the wall. While a linear approximation did not produce errors larger than 16.4 percent at peak systole, these errors substantially increased as the velocity magnitudes decreased during late systole and diastole. Overall, a third degree polynomial curve fit using four points produced the most accurate estimation of wall shear rate through out the cardiac cycle. Results of higher degree curve-fitting functions can be unpredictable due to potential oscillations of the function near the wall. Hence, based on the results of this study, use of a linear approximation is not recommended; a third degree curve-fitting polynomial, using four points provided the most accurate approximation for these flow waveforms.

MR measurements of pulsatile pressure gradients.

Abstract: A magnetic resonance (MR) imaging method for evaluating pulsatile pressure gradients in laminar blood flow is presented. The technique is based on an evaluation of fluid shear and inertial forces from cardiac-gated phase-contrast velocity measurements. The technique was experimentally validated by comparing MR and manometer pressure gradient measurements performed in a pulsatile flow phantom. Analyses of random noise propagation and sampling error were performed to determine the precision and accuracy of the method. The results indicate that a precision of 0.01-0.03 mmHg/cm and an accuracy of better than 8% can be achieved by using standard clinical pulse sequences in tubes exceeding 6 mm in diameter. The authors conclude that MR measurement of pressure gradients is feasible and that additional hemodynamic information may be derived from conventional phase-contrast imaging studies.