Heinrich Schima

“overestimation” of catheter gradients by doppler ultrasound in patients with aortic stenosis: a predictable manifestation of pressure recovery

OBJECTIVES This study sought to evaluate whether pressure recovery can cause significant differences between Doppler and catheter gradients in patients with aortic stenosis, and whether these differences can be predicted by Doppler echocardiography. BACKGROUND Pressure recovery has been shown to be a source of discrepancy between Doppler and catheter gradients across aortic stenoses in vitro. However, the clinical relevance of this phenomenon for the Doppler assessment of aortic stenosis has not been evaluated in patients. METHODS Twenty-three patients with various degrees of aortic stenosis were studied with Doppler echocardiography and catheter technique within 24 h. Using an equation previously validated in vitro, pressure recovery was estimated from peak transvalvular velocity, aortic valve area and cross-sectional area of the ascending aorta and compared with the observed differences between Doppler and catheter gradients. Doppler gradients were also corrected by subtracting the predicted pressure recovery and then were compared with the observed catheter gradients. RESULTS Predicted differences between Doppler and catheter gradients due to pressure recovery ranged from 5 to 82 mm Hg (mean +/- SD, 19 +/- 16 mm Hg) and 3 to 54 mm Hg (12 +/- 11 mm Hg) for peak and mean gradients, respectively. They compared well with the observed Doppler-catheter gradient differences, ranging from -5 to 75 mm Hg (18 +/- 18 mm Hg) and -7 to 48 mm Hg (11 +/- 13 mm Hg). Good correlation between predicted pressure recovery and observed gradient differences was found (r = 0.90 and 0.85, respectively). Both the noncorrected and the corrected Doppler gradients correlated well with the catheter gradients (r = 0.93-0.97). However, noncorrected Doppler gradients significantly overestimated the catheter gradients (slopes, 1.36 and 1.25 for peak and mean gradients, respectively), while Doppler gradients corrected for pressure recovery showed good agreement with catheter gradients (slopes, 1.03 and 0.96; standard error of estimate [SEE] 8.1 and 6.9 mm Hg; mean difference +/- SD 0.4 +/- 8.0 mm Hg and 1.1 +/- 6.8 mm Hg for peak and mean gradients, respectively). CONCLUSIONS Significant pressure recovery can occur in patients with aortic stenosis and can cause discrepancies between Doppler and catheter gradients. However, pressure recovery and the resulting differences between Doppler and catheter measurements may be predicted from Doppler velocity, aortic valve area and size of the ascending aorta.

Numerical study of wall mechanics and fluid dynamics in end-to-side anastomoses and correlation to intimal hyperplasia

In order to analyse the wall mechanics and the flow dynamics in compliant vascular distal end-to-side anastomoses, computer simulation has been performed. In a model study the effect of compliance mismatch on the wall displacements and on the intramural stresses as well as the influence of wall distensibility on the flow patterns are demonstrated applying two distensible models with different graft elasticity. In addition, the flow in a rigid model simulating a vein graft without adaption of the venous lumen has been investigated. The geometries for these models were obtained from a concurrent experimental study, where the formation of distal anastomotic intimal hyperplasia (DAIH) was studied in untreated and externally stiffened autologous venous grafts in sheep. In the flow study the time-dependent, three-dimensional Navier-Stokes equations describing the motion of an incompressible Newtonian fluid are applied. The vessel wall is modelled using a geometrically non-linear shell structure. In an iteratively coupled approach the transient shell equations and the governing fluid equations are solved numerically using the finite element method. In both compliant models maximum displacement and areas of steep stress gradients are observed in the junction region along the graft-artery intersection. The comparison of the normal deformations and the distribution and magnitude of intramural stress shows quantitative differences. The graft elasticity acts as a regulating factor for the deformability and the stress concentration in the junction area: In the model with high graft-elasticity maximum normal deformation at the side wall is 17%. This is twice as large as in the stiff graft model and maximum principle stress at the inner surface differs by one order of magnitude. The numerical results concerning the flow patterns indicate strongly skewed axial velocity profiles downstream of the junction, large secondary motion, flow separation and recirculation on the artery floor opposite the junction and at the inner wall downstream of the toe. In these regions a correlation between the time-averaged fluid wall shear stress and intimal thickening found in the animal experiment can be observed, whereas the pronounced formation of DAIH at the suture line seems to be mainly dependent on wall mechanical factors such as intramural stress and strain.

Numerical study of hemodynamics and wall mechanics in distal end-to-side anastomoses of bypass grafts

The development and progress of distal anastomotic intimal hyperplasia seems to be promoted by altered flow conditions and intramural stress distributions at the region of the artery-graft junction of vascular bypass configurations. From clinical observations, it is known that intimal hyperplasia preferentially occurs at outflow anastomoses of prosthetic bypass grafts. In order to gain a deeper insight into post-operative disease processes, and subsequently, to contribute to the development of improved vascular reconstructions with respect to long term patency rates, detailed studies are required. In context with in vivo experiments, this study was designed to analyze the flow dynamics and wall mechanics in anatomically correct bypass configurations related to two different surgical techniques and resulting geometries (conventional geometry and Miller-cuff). The influence of geometric conditions and of different compliance of synthetic graft, the host artery and the interposed venous cuff on the hemodynamic behavior and on the wall stresses are investigated. The flow studies apply the time-dependent, three-dimensional Navier-Stokes equations describing the motion of an incompressible Newtonian fluid. The vessel walls are described by a geometrically non-linear shell structure. In an iterative coupling procedure, the two problems are solved by means of the finite element method. The numerical results demonstrate non-physiological flow patterns in the anastomotic region. Strongly skewed axial velocity profiles and high secondary velocities occur downstream the artery-graft junction. On the artery floor opposite the junction, flow separation and zones of recirculation are found. The wall mechanical studies show that increased compliance mismatch leads to increased intramural stresses, and thus, may have a proliferative influence on suture line hyperplasia, as it is observed in the in vivo study.

Renal Function and Outcome After Continuous Flow Left Ventricular Assist Device Implantation

BACKGROUND Renal dysfunction as a risk factor with the use of left ventricular assist devices (LVAD) is controversial. We determined the effect of renal function on outcomes after continuous flow LVAD implantation. METHODS Eighty-six patients with advanced heart failure undergoing continuous flow LVAD implantation as bridge to transplantation from November 1998 to July 2007 were retrospectively analyzed. Renal function was assessed using the Modification of Diet in Renal Disease study-derived glomerular filtration rates (GFR [mL x min(-1) x 1.73 m(-2)]). Patients were categorized into two groups based on pre-LVAD GFR: those with normal renal function (GFR > 60, n = 46), and those with renal dysfunction (GFR < 60, n = 40). RESULTS Post-LVAD survival at 1, 3, and 6 months for GFR greater than 60 was 91.3%, 79.9%, 72.6%, respectively, and for GFR less than 60, it was 92.5%, 66.5%, 47.9%, respectively (p = 0.038). Bridge-to-transplant rate was lower for GFR less than 60 than for GFR greater than 60 (40.0% versus 63.0%, p = 0.033). For GFR less than 60, GFR improved on LVAD support: implant to month 6, 41.7 +/- 11.5 to 62.7 +/- 25.0 (p = 0.021). Post-LVAD survival was improved in GFR less than 60 patients who after LVAD implantation recovered renal function to GFR greater than 60 (p < 0.001). Patients with post-LVAD renal failure had significantly lower post-LVAD survival regardless of pre-LVAD renal function (p < 0.001). CONCLUSIONS Patients with renal dysfunction have poorer outcomes after continuous flow LVAD implantation. However, renal function improves after LVAD implantation and is associated with improved survival. Our data underscore the importance of end-organ function in patient selection for LVAD therapy.

Importance of Pressure Recovery for the Assessment of Aortic Stenosis by Doppler Ultrasound Role of Aortic Size, Aortic Valve Area, and Direction of the Stenotic Jet In Vitro

BACKGROUND Pressure recovery has been shown to occur distal to aortic stenoses in experimental and clinical studies. However, its clinical relevance in this setting has not yet been evaluated. METHODS AND RESULTS To address the hypothesis that pressure recovery can cause significant differences between Doppler and catheter gradients in aortic stenosis and to examine the effects of aortic size, aortic valve area, and direction of the stenotic jet on these differences, stenoses with valve areas from 0.5 to 1.25 cm2 and aortic diameters from 1.8 to 5.0 cm were studied in a pulsatile flow model. Jets entered the aorta centrally or eccentrically with angles of 15 degrees, 30 degrees, or 45 degrees. Overall, good correlation was found between Doppler and catheter gradients. However, when the various combinations of orifices and aortas were analyzed separately, slopes varied from 1.0 to 1.86, and the Doppler-catheter gradient differences ranged from -2 (small valve area with a large aorta) to 66 mm Hg (80% overestimation by Doppler echocardiography) when the stenosis was moderate and the aorta was small. Mild eccentricity of the jet did not significantly alter the results. However, overestimation by Doppler decreased with increasing jet eccentricity. Finally, differences between Doppler and catheter gradients could be predicted by estimating pressure recovery from Doppler measurements. CONCLUSIONS Significant pressure recovery can occur in aortic stenosis and can cause differences between Doppler and catheter gradients. These differences may reach clinical relevance, particularly when the stenosis is moderate and the aorta is small and can be predicted from Doppler measurements.

Value and Limitations of Proximal Jet Dimensions for the Quantitation of Valvular Regurgitation: An In Vitro Study Using Doppler Flow Imaging

To evaluate usefulness and limitations of proximal jet dimensions for the quantitation of valvular regurgitation, fluid jets were created in a pulsatile flow model, and proximal jet width and cross-sectional area were measured by means of color Doppler flow imaging. When circular orifices with diameters from 1 to 6 mm were studied, jet width and cross-sectional jet area measured as close as possible to the orifice were directly related to the orifice diameter (r = 0.97; standard error of the estimate, 0.4 mm; y = 0.5 + 0.97x) and the orifice area (r = 0.97; standard error of the estimate, 5.7 mm2; y = 0.22 + 2.47x), respectively. No significant dependence on flow rate or pressure gradient was found for these measurements. Jet width was measured with color M-mode Doppler by use of the smallest sample volume size (1 mm) and was slightly greater than the orifice diameter (4.2 +/- 1.7 mm versus 3.8 +/- 1.7 mm). However, cross-sectional areas were approximately fourfold the orifice areas, on average (52.5 +/- 24.6 mm2 versus 12.3 +/- 9.7 mm2), with a range of twelvefold (smallest orifice) to threefold (largest orifice). When the sample volume size was reduced from 2.4 to 1.0 mm, color areas decreased by 25.6 +/- 6.0%. Slit-shaped orifices were studied with two different orientations of the slit-parallel and perpendicular to the ultrasound beam: Color M-mode measurements were again slightly greater than length and width of the slit, but cross-sectional areas were substantially larger than the orifice areas and increased between 44% and 115% when changing the orientation of the slit from perpendicular to parallel. This, again, reflected problems with lateral resolution. When cross-sectional areas were measured at increasing distances from the orifice, the cross-sectional jet area increased significantly within a few millimeters. This increase was greater with higher gradients and smaller orifice sizes. In case of a small orifice (2 mm) and a high gradient (130 to 160 mm Hg), increase in area was as great as 122% within a distance of only 5 mm. Thus, proximal jet width and cross-sectional area were directly related to the orifice size, which could be a valuable parameter for the evaluation of valvular regurgitation. Measurements of jet width by color M-mode seemed to be most accurate but are limited by the fact that in vivo valvular defects may be irregular rather than of circular shape.(ABSTRACT TRUNCATED AT 400 WORDS)

Compliance mismatch and formation of distal anastomotic intimal hyperplasia in externally stiffened and lumen-adapted venous grafts.

OBJECTIVE Compliance and formation of distal anastomotic intimal hyperplasia (DAIH) were investigated in externally stiffened venous grafts of varying calibers. METHODS 36 femoropopliteal reconstructions were performed in 18 sheep. The autologous venous grafts were inserted into tubes made of Dacron mesh to achieve compliance-mismatch and lumen adaptation. Compliance was measured by echotracked ultrasonography and profiles of DAIH were generated from histologic sections harvested after 8.3 months. MAIN RESULTS The external mesh tube significantly lowered the local compliance of graft and host artery. DAIH appeared extensively in those groups where mesh tube constricted venous grafts met untreated host arteries (p = 0.002). No differences in compliance and DAIH formation were observed when grafts with large and adapted diameters were compared. CONCLUSIONS For prevention of DAIH the distal venous graft diameter is not important, while the local compliance of an autologous vein is a predictive factor for DAIH formation and thus long-term patency.

Renal Function After Implantation of Continuous Versus Pulsatile Flow Left Ventricular Assist Devices

BACKGROUND This study was designed to determine the effect of continuous vs pulsatile flow devices on renal function after left ventricular assist device (LVAD) implantation. METHODS Ninety-two patients undergoing LVAD implantation as bridge-to-transplant therapy were retrospectively analyzed. Patients receiving continuous flow devices (n = 63, 68.5%) were compared with patients receiving pulsatile flow devices (n = 29, 31.5%). Renal function was assessed by 2 calculated glomerular filtration rates (GFR) using the Modification of Diet in Renal Disease (MDRD)-derived GFR (ml/min/1.73 m(2)) and the Cockcroft-Gault-derived creatinine clearance (CrCl, ml/min). RESULTS Mean GFR/CrCl was comparable between the groups at LVAD implantation, in the post-implantation period, and at transplantation. Both groups had a significant increase in mean GFR at Week 1 post-implantation (continuous, 59.4 +/- 22.8 to 76.4 +/- 38.6, p = 0.001; pulsatile, 52.5 +/- 21.1 to 69.2 +/- 34.7; p = 0.007), Week 4 (continuous, 59.9 +/- 23.0 to 84.3 +/- 32.9; p < 0.001; pulsatile, 50.3 +/- 21.1 to 79.9 +/- 38.7, p = 0.007), and Week 12 (continuous, 60.3 +/- 23.1 to 75.3 +/- 30.2, p = 0.004; pulsatile, 55.5 +/- 23.1 to 74.2 +/- 27.2, p = 0.037) that was also seen with the Cockcroft-Gault-calculated CrCl. No significant increase occurred in mean GFR/CrCl to transplantation. Incidence of post-implantation renal failure was comparable between the groups (continuous, 38.1%; pulsatile, 31.0%; p = 0.512). CONCLUSIONS After LVAD implantation, patients with continuous flow devices and patients with pulsatile flow devices have comparable renal function.

Fluid dynamics, wall mechanics, and oxygen transfer in peripheral bypass anastomoses.

AbstractIntimal hyperplasia at vascular anastomoses seems to be promoted by altered flow conditions and stress distributions within the anastomotic region. In order to gain deeper insight into postoperative disease processes, and subsequently, to contribute to the development of improved vascular reconstructions, detailed studies, also on local flow dynamics and related mass transport and wall mechanical effects, are required. In context with in vivo studies, computer simulation based on casts of femoro-popliteal bypasses implanted into sheep were performed to analyze the flow dynamics, the oxygen transport, and the wall and suture mechanics in anatomically correct bypass configurations related to three established surgical techniques and resulting geometries (conventional type anastomosis, Taylor-patch and Miller-cuff anastomoses with venous interposition grafts of different modifications). The influence of geometry, compliance of the graft, the interponated vein patch and vein cuff, and of the artery was included. Time-dependent, three-dimensional Navier–Stokes equations describing the flow field, and a nonlinear shell structure for the vessel walls were coupled using finite element methods. The numerical results demonstrate nonphysiological flow patterns in the anastomotic region. Strongly skewed axial velocity profiles and secondary velocities occur in the junction region. In the Miller-cuff a vortex may induce a wash-out effect which protects the host artery. On the artery floor opposite the junction flow separation and zones of recirculation were found. The analysis of oxygen transport illustrates a correlation between zones of low wall shear stress and reduced oxygen flux into the wall. Wall mechanics show that increased compliance mismatch leads to increased and discontinuous intramural stresses. Comparison to histomorphological findings on intimal hyperplasia shows certain correlations, particularly increased compliance mismatch has a proliferate influence on suture line hyperplasia. The reduction of compliance mismatch using vein interposition results in decreased generation of intimal hyperplasia, and therefore, contributes to improvement of patency rates, while the geometrical modification and the resulting change of the flow pattern seems to be less important for the growth of anastomotic intimal hyperplasia.

Electrospinning of aligned fibers with adjustable orientation using auxiliary electrodes

Abstract A conventional electrospinning setup was upgraded by two turnable plate-like auxiliary high-voltage electrodes that allowed aligned fiber deposition in adjustable directions. Fiber morphology was analyzed by scanning electron microscopy and attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR). The auxiliary electric field constrained the jet bending instability and the fiber deposition became controllable. At target speeds of 0.9 m s−1 90% of the fibers had aligned within 2°, whereas the angular spread was 70° without the use of auxiliary electrodes. It was even possible to orient fibers perpendicular to the rotational direction of the target. The fiber diameter became smaller and its distribution narrower, while according to the FTIR-ATR measurement the molecular orientation of the polymer was unaltered. This study comprehensively documents the feasibility of directed fiber deposition and offers an easy upgrade to existing electrospinning setups.