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PubMedDate not supplied

Design of an artificial lung compliance chamber for pulmonary replacement.

Matching the impedance of an artificial lung for pulmonary replacement to native pulmonary impedance is important in preventing right ventricular dysfunction. A lumped-parameter theoretical model and bench-top experiments were used to investigate the effect of a prototype compliance chamber on input impedance. The bench-top simulation consisted of a pulsatile flow generator, a prototype compliance chamber, and a low resistance artificial lung connected in series. Effective compliance was varied using pneumatic compression. The theoretical model considered a similar circuit with resistors before and after a compliance element. The bundle flow pulsatility (flow amplitude divided by average flow) and input impedance were calculated in the theoretical and experimental models. More compliance and lower upstream resistance result in lower bundle flow pulsatility and reduced first harmonic impedance. Matching the time scale of the circuit to the period of pulsatile flow also reduces impedance. The bench-top circuit demonstrated an optimal chamber pressure at which first harmonic impedance is reduced by 80%. The prototype compliance chamber in series with the artificial lung more closely matches native pulmonary impedance. The lumped-parameter model and the bench-top simulation will aid in the design and testing of compliance chamber modifications to improve its efficiency.

Artificial Organs↗
PubMed1997

Effects of frictional losses and pulsatile flow on the collapse of stenotic arteries.

High-grade stenosis can produce conditions in which the artery may collapse. A one-dimensional numerical model of a compliant stenosis was developed from the collapsible tube theory of Shapiro. The model extends an earlier model by including the effects of frictional losses and unsteadiness. The model was used to investigate the relative importance of several physical parameters present in the in vivo environment. The results indicated that collapse can occur within the stenosis. Frictional loss was influential in reducing the magnitude of collapse. Large separation losses could prevent collapse outright even with low downstream resistances. However, the degree of stenosis was still the primary parameter governing the onset of collapse. Pulsatile solutions demonstrated conditions that produce cyclic collapse within the stenosis. This study predicts certain physiologic conditions in which collapse of arteries may occur for high-grade stenoses.

Arterial Occlusive Diseases↗
PubMed2002

Cerebral blood flow pulsatility deficits in HIV+ poly substance abusers: differences associated with antiviral medications.

This study examines the influence of HIV-seropositivity and antiviral medications on cerebral blood flow in cocaine abusers. Forty-five HIV negative (HIV-) cocaine abusers, 36 HIV positive (HIV+) cocaine abusers (CD4; mean 378, +/-229) and 27 control HIV- subjects were studied. Blood flow velocity and pulsatility were determined for the anterior and middle cerebral arteries using transcranial Doppler sonography (TCD). Psychological assessments, which included the psychiatric symptom checklist (SCL-90R), hopelessness (Beck) and well-being (Ellison) questionnaires revealed greater psychiatric distress in HIV+ cocaine abusers than the other groups. HIV- cocaine abusers and HIV+ cocaine abusers not receiving antiviral medications (n=25 of 36) had elevated pulsatility values, indicating increased resistance in the cerebral blood vessels in comparison to control subjects. HIV+ cocaine abusers using antiviral medications (n=11 of 36) had pulsatility values similar to HIV- control subjects. Interestingly, there was no significant relationship between intensity of psychiatric distress reported by HIV+ cocaine abusers and perfusion deficits. Our findings suggest that unmedicated HIV+ cocaine abusers have cerebrovascular deficits, which are similar to HIV- cocaine abusers. In addition, the use of antiviral medications appears to be associated with a reduction of these deficits in HIV+ cocaine abusers. Nevertheless, more studies will be needed before any conclusion can be reached regarding possible beneficial effects of these agents on the cerebral vasculature.

Adult↗
PubMed2000

In vitro system to study realistic pulsatile flow and stretch signaling in cultured vascular cells.

We developed a novel real-time servo-controlled perfusion system that exposes endothelial cells grown in nondistensible or distensible tubes to realistic pulse pressures and phasic shears at physiological mean pressures. A rate-controlled flow pump and linear servo-motor are controlled by digital proportional-integral-derivative feedback that employs previously digitized aortic pressure waves as a command signal. The resulting pressure mirrors the recorded waveform and can be digitally modified to yield any desired mean and pulse pressure amplitude, typically 0-150 mmHg at shears of 0.5-15 dyn/cm(2). The system accurately reproduces the desired arterial pressure waveform and cogenerates physiological flow and shears by the interaction of pressure with the tubing impedance. Rectangular glass capillary tubes [1-mm inside diameter (ID)] are used for real-time fluorescent imaging studies (i. e., pH(i), NO, Ca(2+)), whereas silicon distensible tubes (4-mm ID) are used for more chronic (i.e., 2-24 h) studies regarding signal transduction and gene expression. The latter have an elastic modulus of 12.4. 10(6) dyn/cm(2) similar to in vivo vessels of this size and are studied with the use of a benchtop system. The new approach provides the first in vitro application of realistic mechanical pulsatile forces on vascular cells and should facilitate studies of phasic shear and distension interaction and pulsatile signal transduction.

Actins↗
PubMed2001

Hepatic tissue engineering on 3-dimensional biodegradable polymers within a pulsatile flow bioreactor.

BACKGROUND: An optimal method for hepatocyte transplantation is not yet determined. With the principles of tissue engineering in vitro conditioning of hepatocytes on biodegradable polymer in a flow bioreactor before implantation forming spheroids may achieve increased cell mass and function to replace lost organ function in vivo. METHODS: Biodegradable poly-L-lactic (PLLA) polymer discs were seeded with rat hepatocytes in a concentration of 10 x 10(6) cells per ml and exposed to a medium flow of 24 ml/min for 1, 2, 4 and 6 days. The number and diameter of spheroidal aggregates was measured by phase-contrast microscopy. H&E histology was performed. Albumin production as hepatocyte specific function was determined by ELISA. RESULTS: Spheroids of viable hepatocytes of 50-200 microm in diameter were formed. Both the number and diameter of the spheroids increased during the first 2 days and then remained constant until day 6. Albumin production was maintained throughout the culture period. CONCLUSION: Short (2- 3 days) pre-transplant conditioning of hepatocytes in a flow bioreactor on biodegradable PLLA resulted in formation of spheroids with a liver-like morphology and preserved specific metabolic function. Tissue engineered hepatocyte spheroids on polymer may represent a functionally active and easy transplantable neotissue and may serve as an in vivo substitute for lost liver function.

Albumins↗
PubMedDate not supplied

In vitro pulsatile flow evaluation of a stentless porcine aortic bioprosthesis.

Suboptimal hemodynamic performance, tissue calcification, and limitation in long-term durability have been encountered clinically after aortic valve replacement with currently available bioprostheses. It is believed that some of these problems may be caused, directly or indirectly, by the stents of the bioprostheses. To address these deficiencies, the authors undertook the development of the Edwards Prima Stentless Bioprosthesis. This study was designed to evaluate the hemodynamic performance of the Edwards Prima Stentless Bioprosthesis in a pulse duplicator system. The stented Carpentier-Edwards Porcine Bioprosthesis (Baxter Healthcare Corp., Irvine, CA), which has been used in United States clinics for more than 10 years, was used as a control device. The flow fields in the vicinity of the test bioprostheses were inspected with color Doppler flow mapping. The transvalvular pressure gradients were measured invasively with a catheter and calculated with the Doppler determined velocity using a simplified Bernoulli equation. Additionally, the leakage volumes were determined with an electromagnetic flowmeter. In the Doppler flow mapping study, during systole, a central flow was observed distal to the stentless and stented bioprostheses. The central flow distal to the stentless bioprosthesis was broader than that observed distal to its stented counterpart. During diastole, no regurgitation was detected by color Doppler flow mapping in either the stentless or stented groups. The Doppler determined transvalvular pressure gradients correlated well with those measured by catheter (r = 0.990). Moreover, it was learned that the transvalvular pressure gradients of the stentless bioprosthesis were less than those of its stented counterpart, especially for the smaller sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

In vitro pulsatile flow study on effective orifice area of prosthetic mechanical heart valves.

The effective orifice area (EOA) of a mechanical heart valve is an index of how well the valve design utilizes its primary orifice area (POA). In vitro measurements of EOA of aortic valves were maintained by means of pressure drop and root mean square flow rate measurements in a pulse duplicator during systole. Edwards-Duromedics, St. Jude Medical, and Carbomedics aortic valves of sizes 19 19 mm, 21 mm, 25 mm, and 27 mm were analyzed over a cardiac output range of 3 to 7 liters/min. The resultant ratios of EOA/POA were in the range of 0.6-0.8. A simplified equation suggested by the FDA was used in this study to calculate EOAs. To agree with original assumptions of the simplification, the entrance flow area (EFA) where upstream pressure is measured, must be large as compared to that of the test valves. If not, the formula can yield questionable results such as implying that the EOA can be larger than the POA (Walker P et al, 1992) [1]. This paper discusses the limitations in using such an equation. In conclusion, we suggest utilizing the parameter square root of 1-(POA/EFA)2 to evaluate the validity of the data processing, before using the equation. The parameter should be close to one, and in this study it was 0.997.

Biomechanical Phenomena↗
PubMed1990

Experimental analysis of pulsatile flow through elastic collapsible tubes: application to cardiac assist device.

This study presents a simulated analysis of Phased Compression Cardiac Assist Device (PCCAD) and evaluation of its applicability as a non-invasive temporary assist for a failing heart. The new technique is based on the chest pump mechanism for blood flow augmentation during external massage by phased compression of the abdominal and thoracic cavities. A semi-closed hydraulic system to simulate the systemic circulation was constructed; the system includes a left ventricle which functions according to the Starling principle and a pneumatic system which controls the pressures applied to the thoracic and abdominal cavities, in complete synchronization with the beating normal or failing heart. The possibility of manipulating the three pumps in series (venous, heart, and arterial) has been checked, and the principal parameters which effect the efficiency of the PCCAD were evaluated. This in vitro analysis shows the high potential of a non-invasive temporary cardiac assist device. It points to the necessary measures one has to take in order to achieve good synchronization and to interfere externally with the augmentation of cardiac output or with the augmentation of root aortic pressure.

Biomechanical Phenomena↗
PubMedDate not supplied

Pressure peaking in pulsatile flow through arterial tree structures.

An analytical iterative scheme is presented for computing the local characteristics of pressure and flow waves as they progress along a tree structure and become modified by wave reflections. Results are obtained to illustrate the phenomenon of pressure peaking under two different sets of circumstances. In the first case, the propagation of a single harmonic wave along a simple tree is considered, where wave reflections modify the amplitude of the pressure wave as it travels. In the second case, the propagation of a composite wave along a tree with multiple branches is considered, where wave reflections modify the shape of the wave as it travels and cause it to peak. The results demonstrate unambiguously that the root cause of this phenomenon is wave reflections caused by stepwise decreases in admittance, as has been previously suggested, rather than due to nonlinear interactions, as has also been previously suggested. It is shown clearly that even when wave reflections combine linearly, they lead to considerable peaking in the pressure waveform.

Animals↗
PubMed2000

True-lumen collapse in aortic dissection: part II. Evaluation of treatment methods in phantoms with pulsatile flow.

PURPOSE: To discover and evaluate the effective treatment methods to prevent or relieve true-lumen collapse in models of aortic dissection. MATERIALS AND METHODS: Two phantoms were built to simulate type B aortic dissection. After true-lumen collapse was induced, experiments were conducted to evaluate the effectiveness of clinically relevant variables in relieving the collapse. Variables included entry-tear size, branch-vessel flow distribution, distal reentry communication between the true and false limbs, aortic fenestrations, and pump output. To test the effect of closing the entry tear, a stent-graft was deployed over the entry tear under physiologic conditions in a mock-flow loop. The difference in the effect of each variable on the prevention and relief of true-lumen collapse was also investigated. RESULTS: It was more difficult to relieve true-lumen collapse than it was to prevent it. Placement of a stent-graft over the entry tear was the most effective method of relieving true-lumen collapse. Less-effective procedures included opening a false-lumen outflow branch and opening the distal reentry branch. Opening the fenestration-branch loops, meant to simulate the creation of artificial fenestrations in the intimal flap, did not relieve true-lumen collapse. CONCLUSION: The definitive treatment for true-lumen collapse in aortic dissection is direct repair of the entry tear to decrease false-lumen inflow. Otherwise, increasing the false-lumen outflow and/or creating distal fenestrations between the true and false lumina distal to the level of the compromised aortic branch are less-effective alternatives.

Aortic Dissection↗
PubMed2000

True-lumen collapse in aortic dissection: part I. Evaluation of causative factors in phantoms with pulsatile flow.

PURPOSE: To investigate the causative factors in true-lumen collapse in a model of aortic dissection. MATERIALS AND METHODS: Phantoms with an aortic arch, true and false lumina with abdominal branch vessels, and a distal bifurcation were used to model a Stanford type B aortic dissection. The effects of anatomic factors (entry-tear size, branch-vessel flow distribution, fenestrations, distal reentry communication) and physiologic factors (peripheral resistance in the branch vessels, pump output and rate, vascular compliance) on true-lumen collapse were investigated. The morphology of the true lumen was observed. Branch pressures and flow rates were measured. RESULTS: True-lumen collapse was induced and was exacerbated by an increase in the size of the entry tear, a decrease in the false-lumen outflow caused by occluding the false-lumen branch vessels, and an increase in the true-lumen outflow caused by lowering the peripheral resistance in true-lumen branch vessels. Two kinds of true-lumen collapse depended on pump output. With low pump output and low outflow resistance from the true lumen, the true lumen collapsed. With high pump output and low inflow resistance in the false lumen, the true lumen was compressed. Distal reentry communication between the true and false limbs was more effective than aortic fenestrations in preventing true-lumen collapse. CONCLUSION: True-lumen collapse in this dissection model strongly depends on the difference in the ratios of inflow capacity to outflow capacity in the true and false lumina. Both anatomic and physiologic factors can affect true-lumen collapse.

Aortic Dissection↗
PubMedDate not supplied

Effect of synchronous and asynchronous pulsatile flow during left, right, and biventricular bypass.

Ventricular assist devices augment aortic or pulmonary flow while the patient's heart recovers from surgery or infarction. Most are used in the asynchronous full-to-empty mode, but they also may be used in a synchronous counter-pulsation mode. This study examines which assist mode optimally reduces myocardial oxygen consumption (MVO2). Eighteen pigs were instrumented with pulmonary artery, carotid artery, and coronary sinus catheters for determination of MVO2. Pierce-Donachy Ventricular Assist Devices (VAD) were used in left, right, or biventricular assist mode. Fifteen minute periods each of control, synchronous, and asynchronous bypass were randomly instituted. The mid-left anterior descending coronary artery was then ligated, and the sequence repeated. At the end of each period, MVO2 was determined. In comparison with controls, MVO2 was statistically significant in the BIVAD asynchronous mode only. Synchronized counterpulsation did not decrease MVO2. When ventricular assist devices are used to aid in cardiac recovery postoperatively or postmyocardial infarction, biventricular assist should be used.

Animals↗