Exploring smooth muscle phenotype and function in a bioreactor model of abdominal aortic aneurysm.

BACKGROUND: Vascular smooth muscle cells (SMC) are central to arterial structure and function yet their involvement in the progression of abdominal aortic aneurysm (AAA) disease is not well studied. The progressive and silent nature of AAA in man essentially restricts research to the use of "end-stage" tissue recovered during surgical repair. This study aimed to generate an ex vivo model of AAA using protease-treated porcine carotid arteries maintained in a novel bioreactor, and to compare the structural and functional changes in SMC cultured from the recovered vessels with those from human tissue acquired at elective surgical repair. METHODS: Freshly isolated porcine arteries were pretreated with collagenase and/or elastase before culturing under flow in a bioreactor for 12 days. Human end-stage aneurysmal tissue and saphenous veins from age-matched controls were collected from patients undergoing surgery. SMC were cultured and characterised (immunocytochemistry, measurement of spread cell area) and assessed functionally at the level of proliferation (cell-counting) and matrix-metalloproteinase (MMP) secretion (gelatin zymography). Cellular senescence was investigated using ß-galactosidase staining and apoptosis was quantified using a fluorescence-based caspase 3 assay. RESULTS: Co-expression of alpha-smooth muscle actin and smooth muscle myosin heavy chain confirmed all cell populations as SMC. Porcine SMC harvested and cultivated after collagenase/elastase pretreatment displayed a prominent "rhomboid" morphology, increased spread area (32%, P < 0.01), impaired proliferation (47% reduction, P < 0.05), increased senescence (52%, P < 0.001), susceptibility to apoptosis and reduced MMP-2 secretion (60% decrease, P < 0.01) compared with SMC from vehicle, collagenase or elastase pre-treated vessels. Notably, these changes were comparable to those observed in human AAA SMC which were 2.4-fold larger than non-aneurysmal SMC (P < 0.001) and exhibited reduced proliferation (39% reduction, P < 0.001), greater apoptosis (4-fold increase, P < 0.001), and increased senescence (61%, P < 0.05). CONCLUSIONS: Combined collagenase/elastase exposure of porcine artery maintained in a bioreactor under flow conditions induced a SMC phenotype characteristic of those cultured from end-stage AAA specimens. This model has potential and versatility to examine temporal changes in SMC biology and to identify the molecular mechanisms leading to early aberrancies in SMC function. In the longer term this may inform new targets to maintain aortic SMC content and drive cells to a "reparative" phenotype at early stages of the disease.

Amplitude modulation drive to rectangular-plate linear ultrasonic motors with vibrators dimensions 8 mm x 2.16 mm X 1 mm.

In this paper, to exploit the contribution from not only the stators but also from other parts of miniature ultrasonic motors, an amplitude modulation drive is proposed to drive a miniature linear ultrasonic motor consisting of two rectangular piezoelectric ceramic plates. Using finite-element software, the first longitudinal and second lateral-bending frequencies of the vibrator are shown to be very close when its dimensions are 8 mm x 2.16 mm x 1 mm. So one single frequency power should be able to drive the motor. However, in practice the motor is found to be hard to move with a single frequency power because of its small vibration amplitudes and big frequency difference between its longitudinal and bending resonance, which is induced by the boundary condition variation. To drive the motor effectively, an amplitude modulation drive is used by superimposing two signals with nearly the same frequencies, around the resonant frequency of the vibrators of the linear motor. When the amplitude modulation frequency is close to the resonant frequency of the vibrator's surroundings, experimental results show that the linear motor can move back and forward with a maximum thrust force (over 0.016 N) and a maximum velocity (over 50 mm/s).

Hydrodynamic function of the second-generation mitroflow pericardial bioprosthesis.

BACKGROUND: The hydrodynamic function of the smaller size Mitroflow Synergy stented pericardial bioprostheses has been studied in an in vitro fresh tissue aortic root model and compared with previous studies of free-sewn bioprostheses. METHODS: Three valves of each of the sizes 19, 21, and 23 mm were sutured into fresh tissue aortic roots and tested in a pulsatile flow simulator using two different ventricular input impedance conditions. A high-speed camera was used to study the leaflet opening and closing configurations. Mean pressure difference as a function of root mean square forward flow, effective orifice area, regurgitant volumes, and total energy loss across the valves was measured. RESULTS: Mean pressure difference with respect to root mean square forward flow decreased as the valve size increased. Thus effective orifice area increased as the valve size increased. The open leaflet configuration images showed that all three sizes of Mitroflow valves had a large circular orifice with minimal open leaflet deformation. All valves closed competently with no visible leakage and no closed regurgitant volume. The Mitroflow valves showed better effective orifice areas compared with previously tested frame-mounted porcine bioprostheses but lower effective orifice areas compared with porcine stentless bioprostheses; however, the open leaflet bending deformation was better than for any of the previously tested bioprosthetic valves. CONCLUSIONS: The hydrodynamic function of the Mitroflow Synergy stented pericardial bioprosthesis shows potential for good in vivo hemodynamic performance. The good hemodynamic performance combined with relative ease of implantation technique makes the pericardial valve a good valve in the aortic position, particularly in older patients with small annuli.

Quantification of pulmonary autograft characteristics using magnetic resonance imaging.

BACKGROUND AND AIM OF THE STUDY: The diameters and distensibility of the native pulmonary root and their effect on pulmonary autograft performance were examined pre- and postoperatively using cardiac ultrasound and magnetic resonance imaging (MRI). METHODS: Eight patients undergoing the Ross procedure were prospectively involved. The diameters of the native aortic, native pulmonary and autograft roots were measured at the level of the annulus, sinus, sinotubular junction and in the main root using MRI through the cardiac cycle. Ultrasound was also used to estimate the degree of regurgitation, both pre- and postoperatively. RESULTS: The pulmonary root implanted into the systemic circulation increased in size but decreased in distensibility significantly at the sinus, sinotubular junction and main root, but not at the annulus. Postoperatively, the pulmonary autograft annulus showed a similar size and distensibility to that of the native aortic annulus. A wide range of aortic annular sizes (22-30 mm) produced clinically competent valves postoperatively. All undersized pulmonary valves showed only trivial regurgitation postoperatively. Although there was no clear correlation between root shape and valve insufficiency, two patients with mild and moderate autograft regurgitation both had divergent pulmonary roots (diameter at sinotubular junction > annulus diameter) preoperatively. CONCLUSION: The pulmonary autograft using the root replacement technique functioned well in all but one case. The shape of the native pulmonary root may be a determinant of early autograft regurgitation, as well as the diameter and the size mismatch between the great arteries.

Performance improvement of rectangular-plate linear ultrasonic motors using dual-frequency drive.

To improve the performances of a rectangular-plate linear ultrasonic motor for specific applications, a dual-frequency drive has been proposed and investigated. Through careful design of the rectangular piezoelectric ceramic plate, its first longitudinal resonant frequency coincides with its second lateral bending resonant frequency and is one-third of its higher lateral bending resonant frequency. When a square-wave voltage is used to drive the motor, its first longitudinal and second bending and the higher bending vibration modes are excited. Experimental results show that the maximum thrust force and maximum velocity of the motor are over 170% of those obtained from the single-frequency sine-wave drive when the voltage performance of the motor becomes saturated.

The circumferential loading of a direct cardiac compression assist device.

Heart disease is the developed world's largest killer. Transplantation of the failing heart remains the most effective treatment currently employed, but demand far exceeds donor supply. In a bid to address this imbalance, the use of mechanical circulatory support has been explored since the mid-1960s. This paper utilizes one such device, which achieves assistance by mechanically compressing the epicardial surface of the failing heart. The circumferential normal loading of the device is investigated, showing how frictional effects inherent to the device's operation affect localized surface pressure. Results showed that as distance from the device's actuator increased, assistive systolic force reduced, whilst device constriction to ventricular filling detrimentally increased. Active device relaxation was shown to limit the diastolic effect outlined above, providing the simulated diseased heart with an improved cardiac output. The results also raise questions concerning device in-vivo positioning and short-comings with the current heart simulator.

The effects of time varying curvature on species transport in coronary arteries.

Alterations in mass transport patterns of low-density lipoproteins (LDL) and oxygen are known to cause atherosclerosis in larger arteries. We hypothesise that the species transport processes in coronary arteries may be affected by their physiological motion, a factor which has not been considered widely in mass transfer studies. Hence, we numerically simulated the mass transport of LDL and oxygen in an idealized moving coronary artery model under both steady and pulsatile flow conditions. A physiological inlet velocity and a sinusoidal curvature waveform were specified as velocity and wall motion boundary conditions. The results predicted elevation of LDL flux, impaired oxygen flux and low wall shear stress (WSS) along the inner wall of curvature, a predilection site for atherosclerosis. The wall motion induced changes in the velocity and WSS patterns were only secondary to the pulsatile flow effects. The temporal variations in flow and WSS due to the flow pulsation and wall motion did not affect temporal changes in the species wall flux. However, the wall motion did alter the time-averaged oxygen and LDL flux in the order of 26% and 12% respectively. Taken together, these results suggest that the wall motion may play an important role in coronary arterial transport processes and emphasise the need for further investigation.

Disturbed flow promotes deposition of leucocytes from flowing whole blood in a model of a damaged vessel wall.

Departure from simple laminar flow in arteries may promote the local attachment of leucocytes either to intact endothelium or platelet thrombi. We perfused blood through a chamber with a backward facing step, to observe whether adhesion from whole blood to P-selectin was indeed localized to a region of recirculating flow, and whether platelets binding to collagen in such a region could capture leucocytes. Blood flowing over the step established a stable vortex, a reattachment point where forward and backward flow separated, and a simple laminar flow with wall shear rate c. 400/s further downstream. Fluorescently labelled leucocytes were observed to attach to P-selectin immediately upstream or downstream of the reattachment point, and to roll back towards the step or away from it, respectively. There was negligible adhesion further downstream. When a P-selectin-Fc chimaera was used to coat the chamber, stable attachment occurred, again preferentially in the disturbed flow region. Numerous platelets adhered to a collagen coating throughout the chamber, although there were local maxima either side of the reattachment point. The adherent platelets captured flowing leucocytes in these regions alone. Leucocytes may adhere from flowing blood in vessels with high shear rate if the flow is disturbed. While platelets can adhere over a wider range of shear rates, their ability to capture leucocytes may be restricted to regions of disturbed flow.

The quantification of pulmonary valve haemodynamics using MRI.

The optimum slice location within the pulmonary root to quantify pulmonary valve haemodynamics was examined using magnetic resonance (MR) phase velocity mapping. MRI was carried out on 15 patients with congenital aortic valve disease. Although the patients had aortic valve disease, all measurements were made on the pulmonary valve. Systolic (Q(SYS)) and diastolic (Q(DIAS)) blood flow volume and cardiac index (CI) were determined at four pulmonary artery locations. The change in diastolic flow volume relative to slice 1, closest to the pulmonary valve, was also calculated. For a change in axial position of 1.5 cm, i.e. from 0.5 to 2 cm from the annulus, there was a change in diastolic flow volume of 4.4 ml. There was a significant increase in the mean diastolic flow from 3.4 to 7.7 ml (p = 0.01 between slice positions 0.5 and 2 cm. However, there was no significant change in CI, 3.4-3.7 l/min/m2 (p = 0.14) over the same distance. We believe that two factors are responsible for these results. The first is that of compliance, whose effects can be minimized by placing the MR slice close to the valve, however, this will not account for the second factor, being that of valve motion, and hence diastolic pulmonary valve flow or regurgitant volume will be underestimated. The degree of underestimation may only be important at mild and moderate levels of regurgitation or if changes in regurgitation are to be temporally measured.