Biomedical subjects

B B Lieber

Publications and source records attributed to B B Lieber.

28 records · Page 2Linked to original sources

PubMed1992

Compliance of the main pulmonary artery during the ventilatory cycle.

Transmural pulmonary arterial pressure (Ppa), diameter (D), and length (L) of a segment of the main pulmonary artery (MPA) were measured simultaneously in anesthetized open-chest dogs. The instantaneous volume was calculated from D and L. Pulmonary arterial elasticity for diameter (EpD) was calculated as the ratio of the amplitude of Ppa to D oscillation normalized by the mean D. Similar indexes were calculated for L (EpL) and V (Epv). Compliance per unit length was calculated from the dimensions and elasticity of the MPA. Under control conditions with 5 cmH2O positive end-expiratory pressure, EpD, EpL, and Epv at cardiac frequency were 175 +/- 27, 147 +/- 27, and 55 +/- 7 cmH2O, respectively. EpD increased with positive end-expiratory pressure, but EpL decreased and Epv was unaffected. EpD, EpL, Epv, and compliance per unit length were not significantly different between the start of inspiration and the start of expiration. In addition, there were no significant phase differences between the oscillations of Ppa and V at respiratory frequency. We conclude that the previously reported time variation of pulmonary arterial compliance during the ventilatory cycle is not due to time-varying properties of the MPA.

Animals↗

PubMed1991

Time-varying pulmonary arterial compliance.

We tested the hypothesis that pulmonary arterial compliance (Ca) varies during the ventilatory cycle. Pressure and flow in the main pulmonary artery were measured in open-chest dogs under chloralose anesthesia (n = 12) with a positive-pressure volume-cycled ventilator. Input impedance was calculated from the pressure and flow waves of heart cycles obtained immediately after the start of inspiration (SI) and immediately after the start of expiration (SE). A lumped parameter model was used to calculate Ca from the input impedance spectrum of the main pulmonary artery. Three levels of positive end-expiratory pressure (PEEP) were used before and after meclofenamate (n = 6) or vagotomy (n = 6). Ca was significantly greater at SE than at SI at each level of PEEP. PEEP increased Ca at SE but not at SI. None of these changes was altered by meclofenamate or vagotomy, suggesting that these differences of Ca were due to passive mechanical effects rather than an active neurohumoral mechanisms. We conclude that Ca is time varying during the ventilatory cycle because it is altered by the dynamic increase of lung volume between SI and SE, but not with the quasi-static increase of lung volume induced by raising the level of PEEP. These changes of Ca were unaffected by vagal feedback or inhibition of cyclooxygenase. We suggest that the increased Ca just after the start of expiration may result from dynamic shifts of blood volume from the extra-alveolar to the alveolar vessels.

Animals↗

PubMed1990

Post-stenotic core flow behavior in pulsatile flow and its effects on wall shear stress.

Arteries of several species, including man, tend to adjust their diameters such that the mean wall shear stress is in the range of 10-20 dynes cm-2. Additionally, intimal thickening in the human carotid bifurcation correlates well with the reciprocal of wall shear stress as determined in model studies. The correlation indicates that wherever the local mean wall shear stress exceeds approximately 10 dynes cm-2, the artery tends to be spared from intimal thickening. However, it is not known whether mean shear stress, i.e. the time-averaged value, or the instantaneous shear stress is the appropriate correlative variable. Each of these variables suggests different mechanisms for the reaction of the artery wall to its hemodynamic environment. It is therefore important to devise means by which the effects of mean shear and pulsatile shear can be separated in the study of atherogenesis. The present investigation examines the post-stenotic flow field in Plexiglas models under pulsatile conditions approximating those in the aortas of the cynomolgus monkey, an animal often employed in atherogenesis research. Behavior of the core flow and its effects on wall shear stress are studied for stenoses of 75 and 90% area reductions using laser velocimetry. The results show that the post-stenotic field contains regions in which the mean wall shear stress is low, but the pulsatile excursions are large.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

The decomposition of apparent stresses in disturbed pulsatile flow in the presence of large scale organized structures.

Flow disturbance phenomena that occur in unsteady-in-the-mean flows (i.e. pulsatile or oscillating) at moderate Reynolds numbers are analyzed in both the time domain and the frequency domain. The analysis utilizes variable decomposition into a time-varying underlying waveform and flow disturbances which are composed of large scale organized structures and random fluctuations. A practical technique which incorporates time domain phase conditioning, trend removal, and frequency domain matched filtering, is presented and examined using simulated data of known statistical behavior. The applicability of the method is shown by the decomposition of the simulated data and the technique is then applied to experimental data obtained in pulsatile flow through a constricted tube by means of a laser Doppler anemometer. The cross-sectional area reduction at the constriction throat was 90%. The Womersley parameter in the experiments was 5.3 and the Reynolds number based on the average flow rate per cycle was 300 with a minimum/maximum value of 55/600 based on the instantaneous flow rate. Measurements were taken in the flow region downstream of the constriction throat which included several interesting flow disturbance phenomena. The results of the decomposed flow phenomena demonstrate the significant role of large scale organized structures in such flows. This is particularly important when analyzing blood flow in the large arteries in the presence of severe stenosis or behind prosthetic devices in an attempt to estimate the 'turbulent' stress which act on cellular elements. Estimation of the apparent stress tensor is of importance in an effort to elucidate the mechanical factors which influence the durability of red blood cells under abnormal conditions.

Biomechanical Phenomena↗

PubMed1989

On the discrimination between band-limited coherent and random apparent stresses in transitional pulsatile flow.

A frequency domain approach that incorporates a matched filter was examined for discriminating between ordered velocity fluctuations with band-limited frequency content and random velocity variations in pulsatile disturbed flows. Fluctuations at pseudo-discrete frequencies may yield a significant contribution to the apparent stress tensor computed from the unsteady Navier Stokes equations, and an estimate of the stresses arising from these ordered structures can be obtained once the velocity variations have been decomposed. This type of decomposition permits the estimation of the apparent stresses in turbulent flows, consisting of coherent and random parts, in blood flow applications such as diseased constricted arteries or downstream of artificial heart valves.

Blood Flow Velocity↗

PubMed1988

Apparent stresses in disturbed pulsatile flows.

Traditional attempts at decomposing measured velocities into repeatable and random components are examined for a set of velocity data measured under pulsatile flow conditions distal to a 90% axisymmetric constriction. The Reynolds numbers, which are typical of those found in the human carotid artery, are such that transitional phenomena occur during portions of the pulsatile cycle at several axial stations. The implications of the method selected for velocity decomposition upon the computation of fluctuating or 'apparent' stresses is a point of major focus. It is shown that the usual estimation of Reynolds stresses in a pulsatile flow by subtracting the ensemble-averaged velocity from the instantaneous velocity leads to an underestimation of the apparent stress when coherent or repeatable disturbances exist in the flow. An alternative decomposition using a frequency domain approach is presented which combines both random and coherent stresses into a single apparent stress, and it is proposed that this approach is preferable to the traditional ensemble averaging method when estimating fluctuating stresses in arterial flows.

Blood Flow Velocity↗

PubMedDate not supplied

Alteration of hemodynamics in aneurysm models by stenting: influence of stent porosity.

Recent developments in minimally invasive approach to cerebrovascular diseases include the placement of stents in arteries for treatment of aneurysms. Preliminary clinical observations and experimental studies have shown that intravascular stents traversing the orifice may lead to thrombosis and subsequent occlusion of the aneurysm. The alterations in vessel local hemodynamics due to the introduction of a stent are not yet well understood. We investigated changes in local hemodynamics resulting from stent implantation. Pulsatile flow patterns in an experimental flow apparatus were visualized using laser-induced fluorescence of rhodamine dye. The test cells were constructed in a rectangular shape to facilitate an undisturbed longitudinal view of flow patterns in parent vessel and aneurysm models with and without porous stents. Woven nitinol stents of various porosities (76%, 80%, 82%, and 85%) were investigated. The selected fluid dynamic similarity parameters (Reynolds and Womersley numbers) represented conditions usually found in high-flow, larger arteries in humans (such as the carotid artery) and low-flow, smaller arteries (such as the vertebral artery). The mean Reynolds number for the larger arteries was 180, with maximum/minimum values of 490/-30 and the Womersley number was 5.3. The mean Reynolds number for the smaller arteries was 90, with maximum/minimum values of 230/2, and the Womersley number was 2.7. For the larger arteries modeled, placement of a stent of the lowest porosity across the aneurysm orifice resulted in reduction of aneurysmal vortex speed and decreased interaction with parent vessel flow. For smaller arteries, a stent of the same porosity led to a substantial reduction of parent vessel/aneurysmal flow interaction and the appearance of a nonrecirculating crescent of fluid rich in rhodamine dye in the aneurysm dome. Our results can help explain in vivo thrombus formation within an aneurysm after placement of a stent that is compatible with local hemodynamics.

Fluorescent Dyes↗

PubMedDate not supplied

The utility of neural network in the diagnosis of Cheyne-Stokes respiration.

The aim of this study was to design a diagnostic model to identify patients with Cheyne-Stokes respiration (CSR-CSA) based on indices of oximetric spectral analysis. A retrospective analysis of oximetric recordings of 213 sleep studies conducted over a one-year period at a Veterans Affairs medical facility was performed. A probabilistic neural network (PNN) was developed from salient features of the oximetric spectral analysis, desaturation events and the delta index. A fivefold cross-validation was used to assess the accuracy of the neural network in identifying CSR-CSA. When compared to overnight polysomnography, the PNN achieved a sensitivity of 100% (95% confidence interval [CI] 85%-100%) and a specificity of 99% (95% 97%-100%) with a corresponding area under the curve of 99% (95% CI 99%-100%). When combined with overnight pulse oximetry, PNN offers an accurate and easily applicable tool to detect CSR-CSA.

Cheyne-Stokes Respiration↗

PubMedDate not supplied

Oscillatory flow in a symmetric bifurcation airway model.

Flow in a symmetric bifurcation model of analytically known geometry was investigated experimentally under oscillatory flow conditions. The duration of the inspiratory and expiratory phases were set to be equal during the oscillatory period. A two velocity component laser Doppler anemometer was used to interrogate the flow field. Three different flow rates through the bifurcation were investigated. The peak Reynolds numbers, based on peak flow rates, were 700, 1278, and 2077. The Womersley number was set to 4.3 and it was kept at the same value for the three different flow rates. The results suggest that under the conditions studied a quasisteady flow assumption for oscillatory flow is valid for only about 50% of the oscillatory period, or it is limited to represent the oscillatory flow only in the vicinity of peak inspiration and peak expiration. Complex transport phenomena that occur during the transition between the respiratory phases cannot be elucidated and analyzed by quasisteady equivalents.

Acceleration↗

PubMedDate not supplied

Microdroplet tracking using biplane digital subtraction angiography for cerebral arteriovenous malformation blood flow path and velocity determinations.

High-speed biplane angiography is used to determine the path and velocity of microdroplets of contrast material in three dimensions. By allowing more accurate determination of detailed blood flow in feeding vessels and draining veins of cerebral arteriovenous malformations than available with standard angiography, the new method offers the potential for more accurate treatment and further study of neurovascular/cerebrovascular hemodynamics. The first study of the method is presented.

Angiography, Digital Subtraction↗