Lee J. Frazin

Numerical study on the effect of secondary flow in the human aorta on local shear stresses in abdominal aortic branches.

Flow in the aortic arch is characterized primarily by the presence of a strong secondary flow superimposed over the axial flow, skewed axial velocity profiles and diastolic flow reversals. A significant amount of helical flow has also been observed in the descending aorta of humans and in models. In this study a computational model of the abdominal aorta complete with two sets of outflow arteries was adapted for three-dimensional steady flow simulations. The flow through the model was predicted using the Navier-Stokes equations to study the effect that a rotational component of flow has on the general flow dynamics in this vascular segment. The helical velocity profile introduced at the inlet was developed from magnetic resonance velocity mappings taken from a plane transaxial to the aortic arch. Results showed that flow division ratios increased in the first set of branches and decreased in the second set with the addition of rotational flow. Shear stress varied in magnitude with the addition of rotational flow, but the shear stress distribution did not change. No regions of flow separation were observed in the iliac arteries for either case. Helical flow may have a stabilizing effect on the flow patterns in branches in general, as evidenced by the decreased difference in shear stress between the inner and outer walls in the iliac arteries.

Functional chiral asymmetry in descending thoracic aorta.

To determine whether rotational blood flow or chiral asymmetry exists in the human descending thoracic aorta, we established the ability of color Doppler ultrasound to detect rotational flow in a tornado tube model of a vortex descending fluid column. In a model of the human aortic arch with a pulse duplicator, color Doppler was then used to demonstrate that rotational flow occurs first in the transverse arch and then in the proximal descending thoracic aorta. With the use of color Doppler esophageal echocardiography, 53 patients (age range, 25-78 years; mean age, 56.4 years) were prospectively examined for rotational flow in the descending thoracic aorta. At 10 cm superior to retro-left ventricular position, 22 of 38 patients (58%) revealed rotational flow with obvious diastolic counterclockwise rotation but less obvious systolic clockwise rotation. At 5 cm superior to retro-left ventricular position, 29 of 46 patients (63%) revealed rotational flow with a tendency toward systolic clockwise and diastolic counterclockwise rotation. At the retro-left ventricular position, 47 of 53 patients (89%) revealed rotational flow, usually of a clockwise direction, occurring in systole. Our data suggest that aortic flow is not purely pulsatile and axial but has a rotational component. Rotational flow begins in the aortic arch and is carried through to the descending thoracic aorta, where flow is chirally asymmetric with systolic clockwise and diastolic counterclockwise components. These data demonstrate an aortic rotational flow component that may have physiological implications for organ perfusion.

Confirmation and initial documentation of thoracic and abdominal aortic helical flow : An ultrasound study

&NA; Aortic helical flow may play an important role in plaque deposition, dissection formation, and organ perfusion. The authors have previously demonstrated, using in vitro flow models and transesophageal echocardiography, that helical flow begins in the mammalian aortic arch and continues into the descending thoracic aorta. The purpose of this study was to confirm thoracic aortic helical flow and document its extent into the abdominal aorta using direct measurements. Twelve mongrel dogs underwent surgery with exposure of the abdominal aorta up to the diaphragm. Six of the 12 underwent further thoracotomy with thoracic aorta exposure. Color Doppler ultrasound images were obtained using a 5 megaHz esophageal transducer, hand held, directly applied, and visually aligned for transverse aortic imaging. Helical flow was considered present with the appearance of red/blue hemicircles during a systolic wave when the aorta was imaged transversely. All six dogs that had thoracotomy showed clockwise thoracic aortic helical flow (along the direction of blood flow) at the retro left ventricular region. In all dogs, clockwise helical flow was demonstrated to and immediately beyond the renal arteries. In 11 of 12 dogs, clockwise helical flow was demonstrated 7 cm below the renal arteries. The study confirms the presence of helical flow in the thoracic aorta and documents its extent into the abdominal aorta below the level of the renal arteries. The teleologic flow pattern of mammals may extend to other classes of vertebrates and must be accounted for in studies of endothelial shear and flow separation. In addition, tangential velocities imparted by helical flow may affect organ perfusion. ASAIO Journal 1996;42:951‐956.

Numerical study on the effect of steady axial flow development in the human aorta on local shear stresses in abdominal aortic branches

The three-dimensional flow through a rigid model of the human abdominal aorta complete with iliac and renal arteries was predicted numerically using the steady-state Navier Stokes equations for an incompressible. Newtonian fluid. The model adapted for our purposes was determined from data obtained from cine-CT images taken of a glass chamber that was constructed based on anatomical averages. The iliac arteries had a bifurcation angle of approximately 35 and a branch-to-trunk area ratio of 1.27. whereas the renal arteries had left and right branch angles of 40 and an area ratio of 0.73. The numerical tool FLOW3D (AEA Industrial Technology, Oxfordshire, UK) utilized body-fitted coordinates and a finite volume discretization procedure. Purely axial velocity profiles were introduced at the entrance of the model for a range of cardiac outputs. The four-branch numerical model developed for this investigation produced flow and shear conditions comparable to those found in other reported works. The total wall shear stress distribution in the iliac and renal arteries followed standard trends. with maximum shear stresses occurring in the apex region and lower shear stresses occurring along the lateral walls. Shear stresses and flow rate ratios in the downstream arteries were more effected by inlet Re than the upstream arteries. These results will be used to compare further simulations which take into effect the rotational component of flow which is present in the aortic arch.

Plaque and Structural Characteristics of the Descending Thoracic Aorta Using Transesophageal Echocardiography

The in vivo acoustic and structural characteristics of atherosclerosis in the descending thoracic aorta have not been well delineated. We prospectively evaluated the descending thoracic aorta of 147 patients (35 women and 112 men; age, 61 +/- 14 years) who underwent clinically indicated transesophageal echocardiography. Patients with suspected disease of the aorta were excluded. Thirty-eight patients (26%) had protruding plaques (men, 25%; women, 29%). Six patients had mobile intimal densities with the mobile area ranging up to 1 cm2. As expected, aortic lumen area was decreased (plaque-free, 3.53 cm2; plaque, 3.19 cm2; p less than 0.05) and wall area was increased (plaque-free, 1.51 cm2; plaque, 1.92 cm2; p less than 0.05) in the regions of the plaque. However, total arterial area was not increased (plaque-free, 5.04 cm2; plaque, 5.09 cm2; difference not significant) in a compensatory manner as observed in other arterial beds. Plaque gray scale was less than the gray scale of plaque-free wall (plaque-free, 141.2; plaque, 122.7; p less than 0.05) when compared at the same level of the descending thoracic aorta or with a second aortic plaque-free level (plaque-free, 150.4; plaque, 122.7; p less than 0.05). Standard deviation of gray scale level was similar between plaque and normal regions. Unsuspected protruding plaques in the descending thoracic aorta occurred in one quarter of the patients referred for routine transesophageal examination. Plaques tended to have lower echogenicity and were differentiated from plaque-free walls within patients. Plaque formation did not result in increased total arterial area. These data suggest that the degree or character of compensatory atherosclerotic remodeling in the highly elastic descending thoracic aorta may differ from other arterial beds.

Atherosclerotic plaque evolution in the descending thoracic aorta in familial hypercholesterolemic patients. A transesophageal echo study.

We explored the concept that transesophageal echocardiography can be used as a tool to detect, characterize, and study plaque morphology in the descending thoracic aorta. The pattern of atherosclerotic plaques in the descending thoracic aorta in familial hypercholesterolemic (FH) patients was evaluated. Additionally, evolution of plaque characteristics as a result of therapy was analyzed. In a randomized prospective protocol, eight FH patients (five men and three women, aged 23 to 65 years [mean +/- SD, 42 +/- 14 years]) receiving standard therapy (n = 3; baseline low-density lipoprotein [LDL] cholesterol, 222 +/- 71 mg/dL, mean +/- SD) or LDL apheresis (n = 5; baseline LDL cholesterol, 262 +/- 51 mg/dL) were studied. Baseline and follow-up (mean, 12 months) transesophageal echocardiographic studies were performed. Measurements obtained were atherosclerotic plaque area (PA), aortic wall area (WA), total arterial area (TAA), and plaque-to-wall area ratio (PWR). LDL cholesterol decreased in both groups. The greatest severity of plaque was detected at 30 to 35 cm from the incisors (approximately 15 to 20 cm from the aortic arch). The smallest plaques were present at the arch and more distal descending aorta. In the control group, TAA, PA, and PWR did not change significantly (P = NS versus baseline). In the LDL-apheresis group, TAA increased (P < .05 versus baseline), PA decreased in three of five patients (P = NS versus baseline), and PWR fell (P < .05 versus baseline).(ABSTRACT TRUNCATED AT 250 WORDS)

Feasibility of transgastric ultrasonography of the abdominal aorta

PURPOSE Transesophageal echocardiography provides detailed images of the thoracic aorta, but imaging of the abdominal aorta and its branches does not occur routinely when the transesophageal echocardiography transducer is advanced into the stomach. Transgastric aortic ultrasonography (TAUS) was investigated as an intraoperative procedure to determine whether transgastric imaging of the abdominal aortic, mesenteric, and renal arteries could be obtained and whether pathologic lesions of these arteries could be identified. METHODS Twelve patients with diagnoses of aortic aneurysmal or occlusive disease, chronic mesenteric ischemia, or renal artery stenosis that required operative treatment were examined. Preoperative transabdominal duplex imaging was performed in all 12 patients. Transgastric B-mode and color-flow ultrasonography of the abdominal aorta and branches was performed as an intraoperative procedure with the patients under general anesthesia. After the abdomen was opened but before the vascular reconstruction was performed, the transesophageal transducer was advanced into the stomach and directed by the surgeons hand to obtain an image of the underlying aorta and branches. TAUS images were compared with those obtained by standard transabdominal duplex imaging. RESULTS TAUS provided high-resolution images of both aneurysmal and occlusive aortic disease. In all 12 cases the images of the aortic lumen, plaque, and thrombus obtained with TAUS had greater detail and better resolution than those obtained with transabdominal duplex imaging. The origins of the renal arteries were seen with TAUS in 23 of 24 cases, whereas transabdominal ultrasonography obtained images of the origins in only 6 of 24 cases (p < 0.01). In the five renal arteries and the one mesenteric artery with hemodynamically significant stenoses in which transabdominal ultrasonography identified the stenoses based on velocity criteria alone, TAUS visualized the occlusive plaque at the origin of the renal and mesenteric arteries. CONCLUSION Intraoperative TAUS is feasible and may be useful for evaluating atherosclerotic disease of the abdominal aorta and renal arteries.

Transesophageal echocardiographic detection of thoracic aorta dissection extending into the celiac artery

Special positioning of a transesophageal echocardiography probe allows visualization of the superior portion of the abdominal aorta including the celiac artery branch. This case report describes a thoracic aorta dissection that extended into the abdomen and involved the celiac artery.

Cannulation of the aortic branches using ultrasound guidance : An animal study

Catheter placement by ultrasound may reduce radiation, improve positioning, and allow the use of echo contrast agents for diagnostic and therapeutic procedures. To evaluate its utility in the peripheral and coronary vascular beds, a preshaped 20 MHz Doppler catheter was inserted into the femoral artery for renal artery, or into the right carotid artery for left coronary artery cannulation in five dogs. Ultrasonic imaging of the vascular structure and catheter was provided by either transabdominal or transesophageal ultrasound. Using Doppler waveform polarity for retrograde guidance, the catheter was advanced to the region of the left renal or left coronary ostia. Ultrasonic emissions from the Doppler catheter were identified by color Doppler mode of the ultrasound machine and allowed the catheter tip to be identified within the beam width of the scanning transducer, providing the depth dimension. In the two animals in which left renal artery cannulation was attempted, the catheter was successfully manipulated into the ostium. In two of the three animals in which left coronary artery cannulation was attempted, the catheter was successfully manipulated into the ostium, followed by saline contrast injections revealing myocardial perfusion. In addition, in one animal, a Doppler flow wire was identified as it was advanced into the mid circumflex coronary. In conclusion, ultrasonically guided cannulation of aortic branches may be possible without x-ray, and this technique may lead to further use of ultrasound in diagnostic and therapeutic procedures.

980-86 Abdominal Aortic Ultrasound: A Comparison Between Transabdominal and Transgastric Approaches

Transgastric ultrasound may allow enhanced resolution of the abdominal aorta and branches. This study compares the morphological findings of abdominal aortic angiography andlor CT with results of transabdominal and transgastric two-dimensional (2D) and Doppler ultrasound. In 11 patients with abdominal aortic and/or renal artery disease (average age 63 yrs; range 43–75 yrs), preoperative angiography and/or CT, and transabdominal ultrasound was performed followed by transgastric ultrasound. Transgastric ultrasound was performed by manually plating a transesophageal transducer upon the abdominal aorta during reparative surgery. Results Angiography andlor CT Transabdominal Ultrasound Transgastric Ultrasound n = patients Aorta visualized 11 10 11 Aortic Aneurysm 8 8 8 Aortic PlaquelThrombus 7 7 11 Dissection Flap 2 0 2 n = vessels Renal Arteries Visualized 22 6 22 Renal Artery Obstruction 3 0 3 Conclusion This study suggests that transgastric ultrasound offers better visualization of the abdominal aorta and renal artery branches than transabdominal ultrasound. Imagery may be at least as accurate as that obtained with angiography or CT. Fiberoptic guidance for a transesophageal transducer may allow routine abdominal aorta and renal artery imaging from the transgastric approach and reduce the need for angiography or CT.