- Email: [email protected]
Flow volume asymmetry in the right aortic arch in children with magnetic resonance phase encoded velocity mapping
Flow volume asymmetry in the right aortic arch in children with magnetic resonance phase encoded velocity mapping Mark A. Fogel, MD,a,b Paul M. Weinberg, MD,a,b and John Haselgrove, PhDb Philadelphia, Pa
Background The right aortic arch is not uncommon in pediatrics. Flow dynamics in this type of aortic arch, which is important for cardiac energetics, organ perfusion, and Doppler flow calculations, have not been defined. Although there are complex secondary flow patterns, bulk axial flow makes up most of the energy use.
Methods We examined 14 children with a right aortic arch by using through-plane phase-encoded magnetic resonance velocity mapping in the ascending and descending aorta to determine flow volume symmetry and velocity. The aortic cross section was divided into 4 quadrants aligned along the long axis of the aorta. Significance was defined as a P value ⬍.05. Results In the ascending aorta, the posterior right quadrant demonstrated significantly greater blood flow than the other quadrants across the entire cardiac cycle (28% vs 23%-25%) and at the point of maximum flow (29% vs 22%-25%). Flow asymmetry was also present in the descending aorta; there was significantly more flow in the posterior quadrants than the anterior quadrants in total flow across the cardiac cycle (28% vs 21%-23%) and at the point of maximum flow (27%-28% vs 20%-24%). The time to maximum flow was significantly shorter in the ascending than the descending aorta (18% vs 24% of the cardiac cycle). In 10 of 14 patients, maximum velocity occurred in the right half of both the ascending and descending aorta. Flow reversal at end-systole was haphazard, occurring in all quadrants.
Conclusion Flow volume asymmetry exists in the ascending and descending portions of the right aortic arch, which has implications for cardiac energetics, organ perfusion, and Doppler scanning flow calculations. This information may be useful in designing improved aortic surgical reconstructions in cases of congenital heart disease. (Am Heart J 2003; 145:154-61.)
Right aortic arches (RAos) occur not infrequently in congenital heart disease and can be associated with vascular rings or with such congenital heart diseases as truncus arteriosus, double outlet right ventricle, or tetralogy of Fallot.1 The RAo is defined as an arch that passes to the right of the trachea over the right mainstem bronchus and generally continues on as the descending aorta to the right (although it also more rarely can descend on the left) of the spine.1 Blood flow dynamics in the aorta are very complex because it has been demonstrated to contain both primary and secondary flow patterns (in left aortic arches). Many studies performed with various techniques
From the aDivision of Cardiology, Department of Pediatrics, and the bDepartment of Radiology, The Children’s Hospital of Philadelphia, Philadelphia, and the Departments of Pediatrics and Radiology, The University of Pennsylvania School of Medicine, Philadelphia, Pa. Submitted August 13, 2001; accepted May 3, 2002. Reprint requests: Mark A. Fogel, MD, The Children’s Hospital of Philadelphia, Division of Cardiology, 34th St and Civic Center Blvd, Philadelphia, PA 19104. E-mail: [email protected] Copyright 2003, Mosby, Inc. All rights reserved. 0002-8703/2003/$30.00 ⫹ 0 doi:10.1067/mhj.2003.28
have shown multiple flow profiles, including both axisymmetric and skewed geometries.2-14 Part of this discrepancy lies in the flow profiles being a function of many factors. For example, in pathologic states such as aging,11,15,16 hypertension,15,17,18 or surgical reconstruction,19 this pattern is markedly altered and demonstrates a skewed flow profile. Previous studies have been performed only in left aortic arches and, with few exceptions, in adults. Patients with RAos have not been studied. Understanding the mechanics of flow in the RAo is important in fluid dynamics for efficient mass transfer of blood and for the use of Doppler techniques in studying this flow (a skewed flow profile would yield an erroneous result).20,21 We used magnetic resonance imaging (MRI) through-plane, phase-encoded velocity mapping to examine the flow profile (flow volume) in cross section in the ascending aorta (AAo) and descending aorta (DAo) of patients with an RAo. Although there are complex secondary flow patterns in the aorta and the “through-plane” data presented resolve only 1 of the axes of the 3-dimensional flow vector, this forward flow component represents most of
American Heart Journal Volume 145, Number 1
Fogel, Weinberg, and Haselgrove 155
Figure 1
Anatomic location of quadrants. The image on the right is a typical candy cane view of the aortic arch (Ao). The white lines represent the region the ascending aorta (AAo) and descending aorta (DAo) in which the velocity measurements were obtained. The graphics on the upper left represent how the AAo and DAo are viewed in cross section (in an magnetic resonance imaging axial view, as shown in the lower left) and the anatomic location of the quadrants. The legend appears at the extreme left. AL, Anterior and left quadrant; AR, anterior and right quadrant; PL, posterior and left quadrant; PR, posterior and right quadrant.
the energy use by the cardiovascular system. This simplified approach, although not taking into account these secondary flow patterns, highlights the major source of bulk flow in the body. The hypothesis to be tested (null hypothesis) is that flow distribution (flow volume) in the RAo in children is axisymmetric around the center of the vessel.
Material and methods Patients We studied 14 children, with a mean age (⫾ SD) of 1.1 ⫾ 0.8 years, who underwent cardiac MRI as a means of evaluating their aortic arch at our institution between January and December 2000. All patients had an RAo with a right DAo and had normal cardiovascular anatomy otherwise. Velocity mapping across the aorta was performed as a routine part of the MRI examination. All values are given as the mean ⫾ SD. Heart rates were 110 ⫾ 15 beats per minute. All studies were adequate for interpretation. No patient had exclusionary arrhythmias that precluded their imaging in the scanner.
Magnetic resonance imaging All patients were sedated before imaging with either pentobarbital, chloral hydrate, or meperidine. All patients were monitored by use of pulse oxymetry, electrocardiography, and direct visualization via television. All patients tolerated the procedure without incident.
We used a Siemens 1.5 Tesla Vision and Sonata MRI system (Siemens Medical Systems, Iselin, NJ). The following scanning protocol was used (Figure 1): 1. After localizers, T1-weighted, spin-echo axial images were acquired spanning the entire thorax. These images were a means of evaluating cardiovascular anatomy and were used in multiplanar reconstruction (see below). The effective repetition time (TR) was the R-R interval (range, 400-700 ms); the echo time (TE) was 15 ms; the number of excitations (NEX) was 3; the image matrix size was 128 ⫻ 256 pixels interpolated to 256 ⫻ 256; the field of view ranged from 160 to 250 mm (a rectangular field of view was used when appropriate); and slice thicknesses were 3 to 8 mm. 2. We used “multiplanar reconstruction,” a software package resident on the Siemens MRI system, on the transverse images to calculate the exact slice position and double-oblique angles to obtain a “candy-cane” view of the aorta. An imaging plane perpendicular to flow was then determined: • for the AAo, at the level of the pulmonary artery • for the DAo, distal to the point at which the Ao arch ends it curve, at the level of the left atrium. By obtaining views of the AAo from a left anterior oblique view and the DAo from off-axis coronal images, we then confirmed the slice position and double-oblique angles were correct and were perpendicular to blood flow. The multiplanar reconstruction software stacks the
American Heart Journal January 2003
156 Fogel, Weinberg, and Haselgrove
Table I. Flow and velocity parameters used in this study Flow parameters Total flow in quadrant Time to max flow Distribution of flow at max flow Max flow in each quadrant Range of flow Max range of flow Time to max range of flow Velocity Parameters Max velocity Time to max velocity Quadrant of max velocity Range of velocities Max range of velocities Time to max range of velocities
Total amount of flow across the entire cardiac cycle, calculated by the sum of flow in each imaged phase Percent of cardiac cycle from R wave to the point of maximum flow in the AAo or DAo Percent of flow in each quadrant at the point of maximum flow in the AAo or DAo Maximum flow (in %) in each quadrant during the entire cardiac cycle Difference (in %) between the quadrant of maximum flow and quadrant of minimum flow at a given phase. The maximum range of flows across the entire cardiac cycle Percent of cardiac cycle from R wave to the point of maximum range of flow Highest velocity measured in any quadrant Percent of cardiac cycle from R wave to the point of maximum velocity in the AAo or DAo Quadrant where the max velocity is located Difference between the maximum and minimum velocity at a given phase The maximum range of velocities across the entire cardiac cycle Percent of cardiac cycle from R wave to the point of maximum range of velocities
transverse images and reconstructs the data into 3 other orthogonal planes. 3. Through-plane phase-encoded velocity mapping was then performed in the AAo and DAo on the basis of the reconstructed images in the second step. The velocity encoding (VENC) used was 150 cm per second. The TR was 25 ms, which yielded approximately 15 to 28 images. The TE was 7.3 ms; the NEX was 2; the image matrix size was 256 ⫻ 256 pixels. The field of view (FOV) ranged from 160 to 270 mm, depending on the size of the patient, and a rectangular FOV was used when appropriate. The slice thickness ranged from 4 to 6 mm, depending on the size of the patient.
Image and data analysis Images were downloaded to on a Sun Ultra SPARC workstation (Sun Microsystems, Palo Alto, Calif) and were analyzed with software written in the IDL programming language. On each image, the AAo and DAo were identified in cross section, and the borders of the vessel were traced. The circles created were then divided into 4 equal quadrants. A reference diameter was chosen for both vessels by means of joining the center of both circles. The other diameter used as a means of completing the 4 quadrants was perpendicular to the reference diameter (Figure 1). These diameters divided the cross section of the AAo and DAo into right and left halves and anterior and posterior halves. The resulting 4 quadrants were designated (1) anterior and right (AR), (2) posterior and right (PR), (3) posterior and left (PL), and (4) anterior and left (AL). The quadrants were divided in this manner because if blood is expected to follow the aortic arch, flow in the anterior quadrants in the AAo should become flow in the posterior quadrants in the DAo. Similarly, flow in the posterior quadrants in the AAo should become flow in the anterior quadrants in the DAo. For each quadrant at each imaged phase, flow was calculated by (1) measuring the signal intensity in each pixel of the quadrant and deriving a velocity, (2) multiplying the velocity by the pixel size, and (3) summing the flows in each pixel over the entire quadrant. Because of differing heart rates and a fixed temporal resolution of 25 ms, the phase of cardiac cycle was standardized as a percentage of the cardiac
cycle. In addition, because flow varies at each phase of the cardiac cycle, the percentage of flow in each quadrant (of the total flow in the AAo or DAo at a given phase, defined as the sum of flow in all 4 quadrants) was used as a means of gauging the degree of flow in each quadrant. The parameters used in this study (both flow and velocity) are defined in Table I.
Statistical analysis Analysis of variance with repeated measures was used as a means of determining significant differences in flows between each quadrant. Pairwise comparison was made by use of the Student-Newman-Keuls test. The 2-way, paired Student t test was used as a means of comparing means of 2 groups (eg, anterior and posterior halves of the aorta). Statistical analysis was performed with JMP software version 3.1.4 (SAS Institute, Cary, NC) or Sigma Stat software (Jandel Corporation, San Rafael, Calif). All values are given as the mean ⫾ SD. Significance was defined as a P value ⬍.05.
Results Results are displayed in a 3-dimensional bar graph format to give a sense, anatomically, of where each quadrant is located.
Flow parameters Figures 2, A and B, demonstrate the amount of total flow in each quadrant in the entire cardiac cycle in patients with an RAo in the AAo and DAo, respectively. In the AAo, the PR demonstrated significantly greater blood flow throughout the cardiac cycle than the other 3 quadrants (28% vs 23%-25%, P ⫽ .007). Flow volume asymmetry was also demonstrated in the DAo; the posterior quadrants demonstrated greater flow than the anterior quadrants (28% vs 21%-23%, P ⬍ .05). This translated into the posterior quadrants carrying 56% of the flow, whereas the anterior quadrants carried 44% of the flow.
American Heart Journal Volume 145, Number 1
Figure 2
Total flow by quadrant across the entire cardiac cycle (Total Flow) in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The graph is created in 3 dimensions to better visualize anatomically where the quadrants are located. Anatomic orientation is in the upper left hand corner; the legend is in the lower left hand corner. A, AAo: Significantly greater flow occurs in the posterior and right quadrant (PR) than in the other 3 quadrants (asterisk, P ⫽ .007). B, DAo: There is significantly greater flow in the posterior quadrants than the anterior quadrants (asterisk, P ⬍ .05). A, Anterior; AL, anterior and left; AR, anterior and right; L, left; P, posterior; PL, posterior and left; PR, posterior and right; R, right.
The time to maximum flow in the AAo was 18% ⫾ 2.8% of the cardiac cycle from the R wave, whereas in the DAo, it was 24% ⫾ 5.5% (P ⫽ .02). This was an expected finding, with the elasticity of the aortic wall and the presence of the AAo branch vessels, etcetera, coming into play. At this point in the cardiac cycle, the distribution of flow among the 4 quadrants demonstrated a flow asymmetry (Figure 3, A and B); in the AAo, similar to the total flow across the entire cardiac cycle, the PR carried significantly more blood than the other quadrants (29% vs 22%-25%, P ⫽ .0006). In the DAo, similar to the findings for total flow, the posterior quadrants carried significantly more blood than the anterior quadrants (27%-28% vs 20%-24%, P ⫽ .04) in each quadrant throughout the entire cardiac cycle.
Fogel, Weinberg, and Haselgrove 157
Figure 3
Distribution of flow at the point of maximum flow (Max Flow) by quadrant in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The graph is created in 3 dimensions to better visualize anatomically where the quadrants are located. Anatomic orientation is in the upper left hand corner; the legend is in the lower left hand corner. A, AAo: Significantly greater flow occurs in the posterior and right quadrant (PR) than in the other 3 quadrants (asterisk, P ⫽ .0006). B, DAo: There is significantly greater flow in the posterior quadrants than the anterior quadrants (asterisk, P ⫽ .04). A, Anterior; AL, anterior and left; AR, anterior and right; L, left; P, posterior; PL, posterior and left; PR, posterior and right; R, right.
A short reversal of flow was observed at end-systole in the AAo and DAo. However, there was no reproducible pattern observed for the quadrant in which this occurred; flow reversal among the various patients with RAo was found in every quadrant, and in 2 patients, it was not found at all (although flow deceleration close to zero was present). Figures 4, A and B, are representative flow curves of the AAo and DAo from the RAo. From these tracings, the flow reversal is apparent in both the AAo and DAo. In addition, it is clear that the flows in the anterior half of the DAo (AL, AR) are considerably smaller than the ones in the posterior half (PL, PR). The maximum flow at any point in the cardiac cycle in each quadrant did not differ whether in the AAo or
158 Fogel, Weinberg, and Haselgrove
Figure 4
Time course of flow in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The legend is in on the right. A, AAo: 4 curves represent the time course of all quadrants. Flow reversal, consistent with the dicrotic notch, occurs 275 ms after the R wave in this patient in all but the anterior and left (AL) quadrants. B, DAo: 4 curves represent the time course of all quadrants. Both posterior quadrants of the DAo maximally separate from the anterior quadrants in peak systole.
DAo. In addition, the maximum range of flow and the time to the maximum range of flow did not differ between the AAo and DAo.
Velocity parameters The maximum velocity of 100 ⫾ 14 cm per second in the AAo differed significantly from that in the DAo (119 ⫾ 13 cm/s); however, the time to this maximum velocity did not. The maximum velocity in both the AAo and DAo was located in the right quadrants in 10 of 14 patients (71%). The maximum range of velocities and the time to the maximum range of velocities were not significantly different between the AAo and DAo.
Discussion The development of an RAo occurs in utero when the left fourth branchial arch dissolves but the right fourth branchial arch does not.1 The RAo can potentially form a vascular ring or be associated with congenital heart disease, such as double outlet right ventri-
American Heart Journal January 2003
cle, truncus arteriosus, or tetralogy of Fallot.1 It is defined as an aortic arch that courses over the right mainstem bronchus and descends on the right (as in all our patients) or the left of the spine. Reports in the literature on flow profiles in the aorta have been in left aortic arches, and the importance of understanding flow profiles has been discussed.19 Flow profiles have implications in the energetics of the cardiovascular system and reflect the integrated function of the mechanics of ventricular ejection,22 aortic wall mechanics,11,17,18 wave reflections,14,17 and other factors. For example, the twisting motion of the left ventricle and the compliance of the aorta all play into the final formation of the flow volume in various quadrants of the aorta. Alterations in these factors may result in changes in the twisting pattern of secondary helical flows or skewed velocity profiles.11,15,16,18,19 To explain how the flow profile is important in cardiovascular energetics, one needs to think about flow as an infinite amount of infinitesimally small streamlines. A flat flow profile, for example, would have all streamlines moving at the same velocity, with little shear forces experienced between them. The shear forces between streamlines is a direct function of blood viscosity and the slope of the axial velocity profile with respect to the radius. It follows that as the slope decreases (ie, a more flat profile), the less shear forces there are and the less energy loss there is. In addition, knowing the shape of the flow profile is crucial in understanding Doppler flow calculations because it is based on an axisymmetric flow profile (eg, placing a sample volume over the larger velocity of the skewed profile would overestimate flow).20,21 With the demonstration of flow volume asymmetry in patients with RAo, our study has confirmed that simply placing the sample volume in the aorta does not guarantee a valid result in this type of aortic arch. Indeed, the study by Lucas et al21 demonstrated that there is a “most reliable position for estimating mean velocity.” On a more fundamental level, the mechanisms of flow in the aorta have implications for organ perfusion, the development of atherosclerosis, and aortic dissection.23 This is important not only when the pediatric population grows to adulthood, but altered organ perfusion may also affect growth and development. Finally, the importance in understanding the mechanics of flow is that something can be done about it (ie, surgery). There are a number of diseases that occur in association with an RAo that require surgical intervention on the aorta. For example, patients with truncus arteriosus need surgical reconstruction as a means of separating the aorta from the pulmonary arteries. Patients with heterotaxy syndrome may have a myriad of aortic diseases, including RAo, all of which require an intervention. Coarctation is also known to occur with an RAo. To minimize the impact on the
American Heart Journal Volume 145, Number 1
cardiovascular energetics (which may be more apparent during exercise), it would be important to mimic normal aortic flow profiles. The surgeon can manipulate the RAo flow volume distribution to replicate normal aortic flow by changing the biomaterial used or altering the aortic arch geometry that is created. This may aid in organ perfusion, in the prevention of future atherosclerosis, and potentially in optimizing cardiovascular energetics. Our study is a first step in this regard. Because most energy use in propelling blood through the body is in forward flow along the long axis of the blood vessels, our study focused on this bulk flow. There are, however, complex secondary flow patterns described by many investigators who used both in vitro models and echocardiography.23-26 Although resolving the other 2 dimensions of the 3-dimensional velocity fields would be very useful and interesting, the flow patterns in the aorta are very complex and would require much more extensive imaging and image analysis. In addition, the secondary flow patterns that are present contribute much less to the energy used in ventricular ejection. This is why we used our simplified approach: to concentrate on this bulk transport phenomenon. We do recognize, however, that these secondary flow patterns may contribute to the phenomenon that we have observed in this study. Our investigation tested the hypothesis that the flow pattern in the aorta of children with an RAo is axisymmetric, and we have demonstrated that flow volume asymmetry exists in both the AAo and the DAo. In the AAo, across the entire cardiac cycle and at the point of maximum flow, the PR quadrant carried significantly greater blood flow than the other 3 quadrants. This finding is consistent with the study of Klipstein et al27 in adults with left aortic arches, in whom the velocity profile was “skewed toward the posterior left wall.” It is in contrast, however, to other studies that have demonstrated increased velocities in the left ventricular outflow tract anteriorly,28-31 which would be expected to continue into the AAo. In the DAo, the posterior quadrants carried significantly more blood than the anterior quadrants across the entire cardiac cycle and at maximum flow. This is even more interesting because the quadrants were designed to follow the curve of the aortic arch: ●
●
If blood is expected to follow the aortic arch, flow in the anterior quadrants in the AAo should become flow in the posterior quadrants in the DAo. Similarly, flow in the posterior quadrants in the AAo should become flow in the anterior quadrants in the DAo.
This most likely is the result of the complex secondary flow patterns that are known to exist in the aorta.23-26 This may also be the result of the curvature of
Fogel, Weinberg, and Haselgrove 159
the aortic arch (RAo curvature is different from that of the left aortic arch) or the aortic branches “siphoning off” blood. In the simplest approximation of the aorta, steady, fully developed laminar flow in a curved pipe, it is well established that the highest velocity is on the outside of the bend and the lowest velocity is on the inside.32 Our findings in the DAo are consistent with this. Indeed, Falsetti et al27 have suggested that the curvature of the aortic arch and its branch vessels may play a role in forming the velocity profile of the DAo. Flow reversal observed at end-systole was found to be nonuniform in the RAo, occurring in every quadrant of the aorta and sometimes not at all (although flow deceleration close to zero was present). In the work of Klipstein et al27 and Chandran,4 this reversal of flow in the AAo of left aortic arches in adults occurred in the anterior quadrants. It is unclear why this pattern was haphazard, and it may be the result of the geometry of the RAo or the aortic branching pattern. The location of maximum velocity occurred in the right quadrants in 71% of patients with an RAo. Because the location of maximum velocity was defined as the voxel with the greatest velocity at any point in the cardiac cycle, it does not necessarily follow the “flow” findings because flow is the integrated velocity over a much larger area. The skewing of the velocity profiles found in other studies can account for this.2,6,9 Indeed, Bogren et al33 found “particle paths that are spatially close to each other may have different velocities.” As aforementioned, Doppler flow calculations are made on the basis of an axisymmetric flow profile.20,21 With the demonstration of flow volume asymmetry in patients with RAo, our study has confirmed that simply placing the sample volume in the aorta does not guarantee a valid result in this type of aortic arch. Indeed, the study by Lucas et al21 demonstrated that there is a “most reliable position for estimating mean velocity.” We are not aware of any study that describes the flow volume distribution in the RAo, and, with one exception, we are aware of no studies of “spatial” velocity profiles in pediatrics. Earlier studies have examined the left aortic arch in either 1 or 2 dimensions in in vitro models,23,24 adults,2-8,10,11 or animals.21,25,34,35 Some studies have shown axisymmetric flow, whereas others have shown a skewed profile. The secondary helical flow patterns known to exist in the aorta24-26 most likely play a role in this discrepancy. The one study published in children in 1 dimension (single plane in the center of the aorta) investigated patients who had undergone an aortic-to-pulmonary anastomosis and a Fontan procedure; they all had left aortic arches.19 That study demonstrated a skewed profile compared with patients who did not have an aortic reconstruction. Patients who did not have an aortic
160 Fogel, Weinberg, and Haselgrove
reconstruction had a “flat” profile in the plane down the center of the aorta.
Limitations Temporal resolution in this study was 25 ms (40 Hz). Any flow-related phenomenon that took place quicker than this was not recorded. Similarly, we only took samples at 2 points in the aorta, and any flowrelated events in other regions of the vessel were not imaged. We doubt that this has substantially affected our results or conclusions. Our study population was young children, although we did not separate our findings by means of age. It is possible that there are subtle age-related changes in children as they grow that may alter the flow profile in the aorta and that we did not report. For this group as a whole, however, our study is convincing. We used a square matrix of 256 ⫻ 256 pixels for each patient, which yielded different pixel sizes for each patient because the FOV varied with patient size. Although these variations are small, they do represent a different resolution for the accuracy in various patients. Again, we do not believe this affected the results or conclusions. Finally, as noted, our study was a simplified approach to this issue, focusing on the axial flow that accounts for the bulk of energy used by the cardiovascular system. This study did not resolve the rotational components demonstrated to be present in the aorta and, indeed, may have contributed to our findings. Although we did not delve into these secondary flow patterns, our findings are not negated and are a first step in this line of study in geometric considerations in children.
Conclusion Flow volume asymmetry exists in the right aortic arch in children. In the AAo, the PR quadrant carried the most flow across the entire cardiac cycle and at the point of maximum flow. In the DAo, the posterior quadrants carried the most flow across the entire cardiac cycle and at the point of maximum flow. In addition, the location of maximum velocity in the right aortic arch for most patients is located in the right half of the arch. These results improve our understanding of aortic flow and will aid in improving aortic surgery in the future.
References 1. Shufford WH, Sybers RG. The aortic arch and its malformations. Springfield: Charles C Thomas Publishers; 1974. p. 12-92. 2. Segadal L, Matre K. Blood velocity distribution in the human ascending aorta. Circulation 1987;76:90-100. 3. Mathison M, Furuse A, Asano K. Doppler analysis of flow velocity profile at the aortic root. J Am Coll Cardiol 1988;12:947-54.
American Heart Journal January 2003
4. Chandran KB. Flow dynamics in the human aorta. J Biomech Eng 1993;115:611-6. 5. Kilner PJ, Yang GZ, Mohiaddin RH, et al. Helical and retrograde secondary flow patterns in the aortic arch studied by three-directional magnetic resonance velocity mapping. Circulation 1993;88: 2235-47. 6. Rieu R, Friggi A, Pellisier R. Velocity distribution along an elastic model of human arterial tree. J Biomech 1985;18:703-15. 7. Paulsen PK. The hot-film anemometer—a method for blood velocity determination: II: in vivo comparison with the electromagnetic blood flowmeter. Eur Surg Res 1980;12:149-58. 8. Segadal L. Velocity distribution model for normal blood flow in the human ascending aorta. Med Biol Eng Comput 1991;29:489-92. 9. Seed WA, Wood NB. Velocity patterns in the aorta. Cardiovasc Res 1971;5:319-30. 10. Matsuda T, Shimizu K, Sakurai T, et al. Measurement of aortic blood flow with MR imaging: comparative study with Doppler US. Radiology 1987;162:857-61. 11. Reichek N, Hu YL, Kramer CM, et al. Normal aging profoundly alters ascending aortic flow profile [abstract]. Circulation 1999; 100(Suppl):I-457. 12. O’Rourke MF. Pressure and flow waves in systemic arterial arteries and the anatomic design of the arterial system. J Appl Physiol 1967;23:139-49. 13. O’Rourke MF, Yaginuma T. Wave reflections and the arterial pulse. Arch Internal Med 1984;144:366-71. 14. O’Rourke MF, Kelly RP. Wave reflections in the systemic circulation and its implications in ventricular function. J Hypertens 1993; 11:327-37. 15. Dahan M, Paillole C, Ferreira B, et al. Doppler echocardiographic study of the consequences of aging and hypertension on the left ventricle and aorta. Eur Heart J 1990;11:39-45. 16. O’Rourke MF, Blazek JV, Morreels CL, et al. Pressure wave transmission along the human aorta: changes with age and in arterial degenerative disease. Circ Res 1968;23:567-79. 17. O’Rourke MF. Systolic blood pressure, arterial compliance and early wave reflection, and their modification by antihypertensive therapy. J Hum Hypertens 1989;3:47-52. 18. O’Rourke MF. Arterial stiffness, systolic blood pressure and logical treatment of hypertension. Hypertension 1990;15:339-47. 19. Fogel MA, Weinberg PM, Hoydu A, et al. Effect of surgical reconstruction on flow profiles in the aorta using magnetic resonance blood tagging. Ann Thorac Surg 1997;63:1691-700. 20. Fisher DC, Sahn DJ, Friedman MJ, et al. The effect of variations on pulse Doppler sampling site on calculation of cardiac output: an experimental study in open-chest dogs. Circulation 1983;67:370-6. 21. Lucas Cl, Keagy BA, Hsiao HS, et al. The velocity profile in the canine ascending aorta and its effects on the accuracy of pulsed Doppler determinations of mean blood velocity. Cardiovasc Res 1984;18:282-93. 22. Nichols WW, O’Rourke MF. McDonald’s blood flow in arteries. Theoretical, experimental and clinical principles. New York: Oxford University Press; 1998. p. 171. 23. Frazin LJ, Lanza G, Vonesh M, et al. Functional chiral asymmetry in descending thoracic aorta. Circulation 1990;82:1985-94. 24. Shipkowitz T, Rodgers VGJ, Frazin LJ, et al. Numerical study on the effect of secondary flow in the human ascending aorta on local shear stresses in the abdominal aortic branches. J Biomech 2000;33:717-28.
American Heart Journal Volume 145, Number 1
Fogel, Weinberg, and Haselgrove 161
25. Farthing S, Peronneau P. Flow in the thoracic aorta. Cardiovasc Res 1979;13:607-20. 26. Frazin LJ, Vanesh MJ, Chandran KB, et al. Confirmation and initial documentation of thoracic and abdominal aortic helical flow. An ultrasound study. ASAIO J 1996;42:951-6. 27. Klipstein RH, Firmin DN, Underwood SR, et al. Blood flow patterns in the human aorta studied by magnetic resonance. Br Heart J 1987;58:316-23. 28. Kupari M, Hekali P, Poutanen V-P. Cross sectional profiles of systolic flow velocities in left ventricular outflow tract of normal subjects. Br Heart J 1995;74:34-9. 29. Rossvoll O, Samstad S, Torp HG. The velocity distribution in the aortic annulus in normal subjects: a quantitative analysis of two-dimensional velocity maps. J Am Soc Echocardiogr 1991;4:367-78. 30. Kupari M, Koskinen P. Systolic flow velocity profile in the left ven-
31. 32. 33.
34.
35.
tricular outflow tract in persons free of heart disease. Am J Cardiol 1993;72:1172-8. Mathison M, Furuse A, Asano K. Doppler analysis of flow velocity profile at the aortic root. J Am Coll Cardiol 1988;12:947-54. Thomas JD. Flow in the descending aorta: a turn of the screw of a sideways glance? Circulation 1990;82:2263-5. Bogren HG, Buonocore MH. 4D magnetic resonance velocity mapping of blood flow patterns in the aorta in young vs elderly normal subjects. J Magn Reson Imaging 1999;10:861-9. Falsetti HL, Kiser KM, Francis GP, et al. Sequential velocity development in the ascending and descending aorta of the dog. Circ Res 1972;31:328-38. Hasenkam JM, Ostergaard JH, Pedersen EM, et al. A model for acute haemodynamic studies in the ascending aorta in pigs. Cardiovasc Res 1988;22:464-71.
The following article is an AHJ Online Exclusive. Full text of this article is available at no charge at our website: www.mosby.com/ahj.
Home-based versus hospital-based exercise programs in patients with coronary artery disease: Effects on coronary vasomotion Stephan Gielen, MD, Sandra Erbs, MD, Axel Linke, MD, Sven Mo ¨ bius-Winkler, MD, Gerhard Schuler, MD, and Rainer Hambrecht, MD Leipzig, Germany
Background In a randomized study, we recently documented that a
measured with a Doppler wire. Coronary blood flow (CBF) was calculated
vigorous, hospital-based exercise training (ET) program improves coronary
by multiplying vascular cross-sectional area and APV.
endothelial function in coronary artery disease. The aim of this consecutive
Results CBF increased in response to 7.2 g/min acetylcholine, from
study was to assess whether a home-based exercise program with re-
27% ⫾ 11% at the beginning of the study to 110% ⫾ 24% after 4 weeks
duced average training duration can sustain previously achieved effects
(P ⬍ .01 vs control group). After 6 months, the increase in CBF was lower
on coronary endothelial function.
versus inhospital training (67% ⫾ 18%, P ⬍ .05 vs 4 weeks). Changes in
Methods Nineteen patients with coronary endothelial dysfunction doc-
APV between 1 and 6 months correlated with daily training durations (r ⫽
umented by acetylcholine-induced vasoconstriction were randomly as-
0.65, P ⫽ .03).
signed to a training group (n ⫽ 10) or a control group (n ⫽ 9). After 4
Conclusions Home-based ET sustained part of the effects of hospital-
weeks of inhospital training (60 min of bicycle ergometry per day), all
based ET on endothelium-dependent vasodilation in coronary artery dis-
training patients were enrolled in a 5-month home-based program of 20
ease. However, acetylcholine-induced increases in CBF were lower after
minutes’ ergometry training per day and 1 group training session per
home-based ET, suggesting a relation between daily training duration and
week. At baseline, after 4 weeks and 6 months, endothelium-mediated
improvement of coronary vasomotion. (Am Heart J 2003;145:e3.)
vasodilation was assessed by quantitative coronary angiography after intracoronary infusions of acetylcholine. Average peak velocity (APV) was
30
Background The right aortic arch is not uncommon in pediatrics. Flow dynamics in this type of aortic arch, which is important for cardiac energetics, organ perfusion, and Doppler flow calculations, have not been defined. Although there are complex secondary flow patterns, bulk axial flow makes up most of the energy use.
Methods We examined 14 children with a right aortic arch by using through-plane phase-encoded magnetic resonance velocity mapping in the ascending and descending aorta to determine flow volume symmetry and velocity. The aortic cross section was divided into 4 quadrants aligned along the long axis of the aorta. Significance was defined as a P value ⬍.05. Results In the ascending aorta, the posterior right quadrant demonstrated significantly greater blood flow than the other quadrants across the entire cardiac cycle (28% vs 23%-25%) and at the point of maximum flow (29% vs 22%-25%). Flow asymmetry was also present in the descending aorta; there was significantly more flow in the posterior quadrants than the anterior quadrants in total flow across the cardiac cycle (28% vs 21%-23%) and at the point of maximum flow (27%-28% vs 20%-24%). The time to maximum flow was significantly shorter in the ascending than the descending aorta (18% vs 24% of the cardiac cycle). In 10 of 14 patients, maximum velocity occurred in the right half of both the ascending and descending aorta. Flow reversal at end-systole was haphazard, occurring in all quadrants.
Conclusion Flow volume asymmetry exists in the ascending and descending portions of the right aortic arch, which has implications for cardiac energetics, organ perfusion, and Doppler scanning flow calculations. This information may be useful in designing improved aortic surgical reconstructions in cases of congenital heart disease. (Am Heart J 2003; 145:154-61.)
Right aortic arches (RAos) occur not infrequently in congenital heart disease and can be associated with vascular rings or with such congenital heart diseases as truncus arteriosus, double outlet right ventricle, or tetralogy of Fallot.1 The RAo is defined as an arch that passes to the right of the trachea over the right mainstem bronchus and generally continues on as the descending aorta to the right (although it also more rarely can descend on the left) of the spine.1 Blood flow dynamics in the aorta are very complex because it has been demonstrated to contain both primary and secondary flow patterns (in left aortic arches). Many studies performed with various techniques
From the aDivision of Cardiology, Department of Pediatrics, and the bDepartment of Radiology, The Children’s Hospital of Philadelphia, Philadelphia, and the Departments of Pediatrics and Radiology, The University of Pennsylvania School of Medicine, Philadelphia, Pa. Submitted August 13, 2001; accepted May 3, 2002. Reprint requests: Mark A. Fogel, MD, The Children’s Hospital of Philadelphia, Division of Cardiology, 34th St and Civic Center Blvd, Philadelphia, PA 19104. E-mail: [email protected] Copyright 2003, Mosby, Inc. All rights reserved. 0002-8703/2003/$30.00 ⫹ 0 doi:10.1067/mhj.2003.28
have shown multiple flow profiles, including both axisymmetric and skewed geometries.2-14 Part of this discrepancy lies in the flow profiles being a function of many factors. For example, in pathologic states such as aging,11,15,16 hypertension,15,17,18 or surgical reconstruction,19 this pattern is markedly altered and demonstrates a skewed flow profile. Previous studies have been performed only in left aortic arches and, with few exceptions, in adults. Patients with RAos have not been studied. Understanding the mechanics of flow in the RAo is important in fluid dynamics for efficient mass transfer of blood and for the use of Doppler techniques in studying this flow (a skewed flow profile would yield an erroneous result).20,21 We used magnetic resonance imaging (MRI) through-plane, phase-encoded velocity mapping to examine the flow profile (flow volume) in cross section in the ascending aorta (AAo) and descending aorta (DAo) of patients with an RAo. Although there are complex secondary flow patterns in the aorta and the “through-plane” data presented resolve only 1 of the axes of the 3-dimensional flow vector, this forward flow component represents most of
American Heart Journal Volume 145, Number 1
Fogel, Weinberg, and Haselgrove 155
Figure 1
Anatomic location of quadrants. The image on the right is a typical candy cane view of the aortic arch (Ao). The white lines represent the region the ascending aorta (AAo) and descending aorta (DAo) in which the velocity measurements were obtained. The graphics on the upper left represent how the AAo and DAo are viewed in cross section (in an magnetic resonance imaging axial view, as shown in the lower left) and the anatomic location of the quadrants. The legend appears at the extreme left. AL, Anterior and left quadrant; AR, anterior and right quadrant; PL, posterior and left quadrant; PR, posterior and right quadrant.
the energy use by the cardiovascular system. This simplified approach, although not taking into account these secondary flow patterns, highlights the major source of bulk flow in the body. The hypothesis to be tested (null hypothesis) is that flow distribution (flow volume) in the RAo in children is axisymmetric around the center of the vessel.
Material and methods Patients We studied 14 children, with a mean age (⫾ SD) of 1.1 ⫾ 0.8 years, who underwent cardiac MRI as a means of evaluating their aortic arch at our institution between January and December 2000. All patients had an RAo with a right DAo and had normal cardiovascular anatomy otherwise. Velocity mapping across the aorta was performed as a routine part of the MRI examination. All values are given as the mean ⫾ SD. Heart rates were 110 ⫾ 15 beats per minute. All studies were adequate for interpretation. No patient had exclusionary arrhythmias that precluded their imaging in the scanner.
Magnetic resonance imaging All patients were sedated before imaging with either pentobarbital, chloral hydrate, or meperidine. All patients were monitored by use of pulse oxymetry, electrocardiography, and direct visualization via television. All patients tolerated the procedure without incident.
We used a Siemens 1.5 Tesla Vision and Sonata MRI system (Siemens Medical Systems, Iselin, NJ). The following scanning protocol was used (Figure 1): 1. After localizers, T1-weighted, spin-echo axial images were acquired spanning the entire thorax. These images were a means of evaluating cardiovascular anatomy and were used in multiplanar reconstruction (see below). The effective repetition time (TR) was the R-R interval (range, 400-700 ms); the echo time (TE) was 15 ms; the number of excitations (NEX) was 3; the image matrix size was 128 ⫻ 256 pixels interpolated to 256 ⫻ 256; the field of view ranged from 160 to 250 mm (a rectangular field of view was used when appropriate); and slice thicknesses were 3 to 8 mm. 2. We used “multiplanar reconstruction,” a software package resident on the Siemens MRI system, on the transverse images to calculate the exact slice position and double-oblique angles to obtain a “candy-cane” view of the aorta. An imaging plane perpendicular to flow was then determined: • for the AAo, at the level of the pulmonary artery • for the DAo, distal to the point at which the Ao arch ends it curve, at the level of the left atrium. By obtaining views of the AAo from a left anterior oblique view and the DAo from off-axis coronal images, we then confirmed the slice position and double-oblique angles were correct and were perpendicular to blood flow. The multiplanar reconstruction software stacks the
American Heart Journal January 2003
156 Fogel, Weinberg, and Haselgrove
Table I. Flow and velocity parameters used in this study Flow parameters Total flow in quadrant Time to max flow Distribution of flow at max flow Max flow in each quadrant Range of flow Max range of flow Time to max range of flow Velocity Parameters Max velocity Time to max velocity Quadrant of max velocity Range of velocities Max range of velocities Time to max range of velocities
Total amount of flow across the entire cardiac cycle, calculated by the sum of flow in each imaged phase Percent of cardiac cycle from R wave to the point of maximum flow in the AAo or DAo Percent of flow in each quadrant at the point of maximum flow in the AAo or DAo Maximum flow (in %) in each quadrant during the entire cardiac cycle Difference (in %) between the quadrant of maximum flow and quadrant of minimum flow at a given phase. The maximum range of flows across the entire cardiac cycle Percent of cardiac cycle from R wave to the point of maximum range of flow Highest velocity measured in any quadrant Percent of cardiac cycle from R wave to the point of maximum velocity in the AAo or DAo Quadrant where the max velocity is located Difference between the maximum and minimum velocity at a given phase The maximum range of velocities across the entire cardiac cycle Percent of cardiac cycle from R wave to the point of maximum range of velocities
transverse images and reconstructs the data into 3 other orthogonal planes. 3. Through-plane phase-encoded velocity mapping was then performed in the AAo and DAo on the basis of the reconstructed images in the second step. The velocity encoding (VENC) used was 150 cm per second. The TR was 25 ms, which yielded approximately 15 to 28 images. The TE was 7.3 ms; the NEX was 2; the image matrix size was 256 ⫻ 256 pixels. The field of view (FOV) ranged from 160 to 270 mm, depending on the size of the patient, and a rectangular FOV was used when appropriate. The slice thickness ranged from 4 to 6 mm, depending on the size of the patient.
Image and data analysis Images were downloaded to on a Sun Ultra SPARC workstation (Sun Microsystems, Palo Alto, Calif) and were analyzed with software written in the IDL programming language. On each image, the AAo and DAo were identified in cross section, and the borders of the vessel were traced. The circles created were then divided into 4 equal quadrants. A reference diameter was chosen for both vessels by means of joining the center of both circles. The other diameter used as a means of completing the 4 quadrants was perpendicular to the reference diameter (Figure 1). These diameters divided the cross section of the AAo and DAo into right and left halves and anterior and posterior halves. The resulting 4 quadrants were designated (1) anterior and right (AR), (2) posterior and right (PR), (3) posterior and left (PL), and (4) anterior and left (AL). The quadrants were divided in this manner because if blood is expected to follow the aortic arch, flow in the anterior quadrants in the AAo should become flow in the posterior quadrants in the DAo. Similarly, flow in the posterior quadrants in the AAo should become flow in the anterior quadrants in the DAo. For each quadrant at each imaged phase, flow was calculated by (1) measuring the signal intensity in each pixel of the quadrant and deriving a velocity, (2) multiplying the velocity by the pixel size, and (3) summing the flows in each pixel over the entire quadrant. Because of differing heart rates and a fixed temporal resolution of 25 ms, the phase of cardiac cycle was standardized as a percentage of the cardiac
cycle. In addition, because flow varies at each phase of the cardiac cycle, the percentage of flow in each quadrant (of the total flow in the AAo or DAo at a given phase, defined as the sum of flow in all 4 quadrants) was used as a means of gauging the degree of flow in each quadrant. The parameters used in this study (both flow and velocity) are defined in Table I.
Statistical analysis Analysis of variance with repeated measures was used as a means of determining significant differences in flows between each quadrant. Pairwise comparison was made by use of the Student-Newman-Keuls test. The 2-way, paired Student t test was used as a means of comparing means of 2 groups (eg, anterior and posterior halves of the aorta). Statistical analysis was performed with JMP software version 3.1.4 (SAS Institute, Cary, NC) or Sigma Stat software (Jandel Corporation, San Rafael, Calif). All values are given as the mean ⫾ SD. Significance was defined as a P value ⬍.05.
Results Results are displayed in a 3-dimensional bar graph format to give a sense, anatomically, of where each quadrant is located.
Flow parameters Figures 2, A and B, demonstrate the amount of total flow in each quadrant in the entire cardiac cycle in patients with an RAo in the AAo and DAo, respectively. In the AAo, the PR demonstrated significantly greater blood flow throughout the cardiac cycle than the other 3 quadrants (28% vs 23%-25%, P ⫽ .007). Flow volume asymmetry was also demonstrated in the DAo; the posterior quadrants demonstrated greater flow than the anterior quadrants (28% vs 21%-23%, P ⬍ .05). This translated into the posterior quadrants carrying 56% of the flow, whereas the anterior quadrants carried 44% of the flow.
American Heart Journal Volume 145, Number 1
Figure 2
Total flow by quadrant across the entire cardiac cycle (Total Flow) in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The graph is created in 3 dimensions to better visualize anatomically where the quadrants are located. Anatomic orientation is in the upper left hand corner; the legend is in the lower left hand corner. A, AAo: Significantly greater flow occurs in the posterior and right quadrant (PR) than in the other 3 quadrants (asterisk, P ⫽ .007). B, DAo: There is significantly greater flow in the posterior quadrants than the anterior quadrants (asterisk, P ⬍ .05). A, Anterior; AL, anterior and left; AR, anterior and right; L, left; P, posterior; PL, posterior and left; PR, posterior and right; R, right.
The time to maximum flow in the AAo was 18% ⫾ 2.8% of the cardiac cycle from the R wave, whereas in the DAo, it was 24% ⫾ 5.5% (P ⫽ .02). This was an expected finding, with the elasticity of the aortic wall and the presence of the AAo branch vessels, etcetera, coming into play. At this point in the cardiac cycle, the distribution of flow among the 4 quadrants demonstrated a flow asymmetry (Figure 3, A and B); in the AAo, similar to the total flow across the entire cardiac cycle, the PR carried significantly more blood than the other quadrants (29% vs 22%-25%, P ⫽ .0006). In the DAo, similar to the findings for total flow, the posterior quadrants carried significantly more blood than the anterior quadrants (27%-28% vs 20%-24%, P ⫽ .04) in each quadrant throughout the entire cardiac cycle.
Fogel, Weinberg, and Haselgrove 157
Figure 3
Distribution of flow at the point of maximum flow (Max Flow) by quadrant in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The graph is created in 3 dimensions to better visualize anatomically where the quadrants are located. Anatomic orientation is in the upper left hand corner; the legend is in the lower left hand corner. A, AAo: Significantly greater flow occurs in the posterior and right quadrant (PR) than in the other 3 quadrants (asterisk, P ⫽ .0006). B, DAo: There is significantly greater flow in the posterior quadrants than the anterior quadrants (asterisk, P ⫽ .04). A, Anterior; AL, anterior and left; AR, anterior and right; L, left; P, posterior; PL, posterior and left; PR, posterior and right; R, right.
A short reversal of flow was observed at end-systole in the AAo and DAo. However, there was no reproducible pattern observed for the quadrant in which this occurred; flow reversal among the various patients with RAo was found in every quadrant, and in 2 patients, it was not found at all (although flow deceleration close to zero was present). Figures 4, A and B, are representative flow curves of the AAo and DAo from the RAo. From these tracings, the flow reversal is apparent in both the AAo and DAo. In addition, it is clear that the flows in the anterior half of the DAo (AL, AR) are considerably smaller than the ones in the posterior half (PL, PR). The maximum flow at any point in the cardiac cycle in each quadrant did not differ whether in the AAo or
158 Fogel, Weinberg, and Haselgrove
Figure 4
Time course of flow in the ascending aorta (AAo) and descending aorta (DAo) of the right aortic arch (RAo). The legend is in on the right. A, AAo: 4 curves represent the time course of all quadrants. Flow reversal, consistent with the dicrotic notch, occurs 275 ms after the R wave in this patient in all but the anterior and left (AL) quadrants. B, DAo: 4 curves represent the time course of all quadrants. Both posterior quadrants of the DAo maximally separate from the anterior quadrants in peak systole.
DAo. In addition, the maximum range of flow and the time to the maximum range of flow did not differ between the AAo and DAo.
Velocity parameters The maximum velocity of 100 ⫾ 14 cm per second in the AAo differed significantly from that in the DAo (119 ⫾ 13 cm/s); however, the time to this maximum velocity did not. The maximum velocity in both the AAo and DAo was located in the right quadrants in 10 of 14 patients (71%). The maximum range of velocities and the time to the maximum range of velocities were not significantly different between the AAo and DAo.
Discussion The development of an RAo occurs in utero when the left fourth branchial arch dissolves but the right fourth branchial arch does not.1 The RAo can potentially form a vascular ring or be associated with congenital heart disease, such as double outlet right ventri-
American Heart Journal January 2003
cle, truncus arteriosus, or tetralogy of Fallot.1 It is defined as an aortic arch that courses over the right mainstem bronchus and descends on the right (as in all our patients) or the left of the spine. Reports in the literature on flow profiles in the aorta have been in left aortic arches, and the importance of understanding flow profiles has been discussed.19 Flow profiles have implications in the energetics of the cardiovascular system and reflect the integrated function of the mechanics of ventricular ejection,22 aortic wall mechanics,11,17,18 wave reflections,14,17 and other factors. For example, the twisting motion of the left ventricle and the compliance of the aorta all play into the final formation of the flow volume in various quadrants of the aorta. Alterations in these factors may result in changes in the twisting pattern of secondary helical flows or skewed velocity profiles.11,15,16,18,19 To explain how the flow profile is important in cardiovascular energetics, one needs to think about flow as an infinite amount of infinitesimally small streamlines. A flat flow profile, for example, would have all streamlines moving at the same velocity, with little shear forces experienced between them. The shear forces between streamlines is a direct function of blood viscosity and the slope of the axial velocity profile with respect to the radius. It follows that as the slope decreases (ie, a more flat profile), the less shear forces there are and the less energy loss there is. In addition, knowing the shape of the flow profile is crucial in understanding Doppler flow calculations because it is based on an axisymmetric flow profile (eg, placing a sample volume over the larger velocity of the skewed profile would overestimate flow).20,21 With the demonstration of flow volume asymmetry in patients with RAo, our study has confirmed that simply placing the sample volume in the aorta does not guarantee a valid result in this type of aortic arch. Indeed, the study by Lucas et al21 demonstrated that there is a “most reliable position for estimating mean velocity.” On a more fundamental level, the mechanisms of flow in the aorta have implications for organ perfusion, the development of atherosclerosis, and aortic dissection.23 This is important not only when the pediatric population grows to adulthood, but altered organ perfusion may also affect growth and development. Finally, the importance in understanding the mechanics of flow is that something can be done about it (ie, surgery). There are a number of diseases that occur in association with an RAo that require surgical intervention on the aorta. For example, patients with truncus arteriosus need surgical reconstruction as a means of separating the aorta from the pulmonary arteries. Patients with heterotaxy syndrome may have a myriad of aortic diseases, including RAo, all of which require an intervention. Coarctation is also known to occur with an RAo. To minimize the impact on the
American Heart Journal Volume 145, Number 1
cardiovascular energetics (which may be more apparent during exercise), it would be important to mimic normal aortic flow profiles. The surgeon can manipulate the RAo flow volume distribution to replicate normal aortic flow by changing the biomaterial used or altering the aortic arch geometry that is created. This may aid in organ perfusion, in the prevention of future atherosclerosis, and potentially in optimizing cardiovascular energetics. Our study is a first step in this regard. Because most energy use in propelling blood through the body is in forward flow along the long axis of the blood vessels, our study focused on this bulk flow. There are, however, complex secondary flow patterns described by many investigators who used both in vitro models and echocardiography.23-26 Although resolving the other 2 dimensions of the 3-dimensional velocity fields would be very useful and interesting, the flow patterns in the aorta are very complex and would require much more extensive imaging and image analysis. In addition, the secondary flow patterns that are present contribute much less to the energy used in ventricular ejection. This is why we used our simplified approach: to concentrate on this bulk transport phenomenon. We do recognize, however, that these secondary flow patterns may contribute to the phenomenon that we have observed in this study. Our investigation tested the hypothesis that the flow pattern in the aorta of children with an RAo is axisymmetric, and we have demonstrated that flow volume asymmetry exists in both the AAo and the DAo. In the AAo, across the entire cardiac cycle and at the point of maximum flow, the PR quadrant carried significantly greater blood flow than the other 3 quadrants. This finding is consistent with the study of Klipstein et al27 in adults with left aortic arches, in whom the velocity profile was “skewed toward the posterior left wall.” It is in contrast, however, to other studies that have demonstrated increased velocities in the left ventricular outflow tract anteriorly,28-31 which would be expected to continue into the AAo. In the DAo, the posterior quadrants carried significantly more blood than the anterior quadrants across the entire cardiac cycle and at maximum flow. This is even more interesting because the quadrants were designed to follow the curve of the aortic arch: ●
●
If blood is expected to follow the aortic arch, flow in the anterior quadrants in the AAo should become flow in the posterior quadrants in the DAo. Similarly, flow in the posterior quadrants in the AAo should become flow in the anterior quadrants in the DAo.
This most likely is the result of the complex secondary flow patterns that are known to exist in the aorta.23-26 This may also be the result of the curvature of
Fogel, Weinberg, and Haselgrove 159
the aortic arch (RAo curvature is different from that of the left aortic arch) or the aortic branches “siphoning off” blood. In the simplest approximation of the aorta, steady, fully developed laminar flow in a curved pipe, it is well established that the highest velocity is on the outside of the bend and the lowest velocity is on the inside.32 Our findings in the DAo are consistent with this. Indeed, Falsetti et al27 have suggested that the curvature of the aortic arch and its branch vessels may play a role in forming the velocity profile of the DAo. Flow reversal observed at end-systole was found to be nonuniform in the RAo, occurring in every quadrant of the aorta and sometimes not at all (although flow deceleration close to zero was present). In the work of Klipstein et al27 and Chandran,4 this reversal of flow in the AAo of left aortic arches in adults occurred in the anterior quadrants. It is unclear why this pattern was haphazard, and it may be the result of the geometry of the RAo or the aortic branching pattern. The location of maximum velocity occurred in the right quadrants in 71% of patients with an RAo. Because the location of maximum velocity was defined as the voxel with the greatest velocity at any point in the cardiac cycle, it does not necessarily follow the “flow” findings because flow is the integrated velocity over a much larger area. The skewing of the velocity profiles found in other studies can account for this.2,6,9 Indeed, Bogren et al33 found “particle paths that are spatially close to each other may have different velocities.” As aforementioned, Doppler flow calculations are made on the basis of an axisymmetric flow profile.20,21 With the demonstration of flow volume asymmetry in patients with RAo, our study has confirmed that simply placing the sample volume in the aorta does not guarantee a valid result in this type of aortic arch. Indeed, the study by Lucas et al21 demonstrated that there is a “most reliable position for estimating mean velocity.” We are not aware of any study that describes the flow volume distribution in the RAo, and, with one exception, we are aware of no studies of “spatial” velocity profiles in pediatrics. Earlier studies have examined the left aortic arch in either 1 or 2 dimensions in in vitro models,23,24 adults,2-8,10,11 or animals.21,25,34,35 Some studies have shown axisymmetric flow, whereas others have shown a skewed profile. The secondary helical flow patterns known to exist in the aorta24-26 most likely play a role in this discrepancy. The one study published in children in 1 dimension (single plane in the center of the aorta) investigated patients who had undergone an aortic-to-pulmonary anastomosis and a Fontan procedure; they all had left aortic arches.19 That study demonstrated a skewed profile compared with patients who did not have an aortic reconstruction. Patients who did not have an aortic
160 Fogel, Weinberg, and Haselgrove
reconstruction had a “flat” profile in the plane down the center of the aorta.
Limitations Temporal resolution in this study was 25 ms (40 Hz). Any flow-related phenomenon that took place quicker than this was not recorded. Similarly, we only took samples at 2 points in the aorta, and any flowrelated events in other regions of the vessel were not imaged. We doubt that this has substantially affected our results or conclusions. Our study population was young children, although we did not separate our findings by means of age. It is possible that there are subtle age-related changes in children as they grow that may alter the flow profile in the aorta and that we did not report. For this group as a whole, however, our study is convincing. We used a square matrix of 256 ⫻ 256 pixels for each patient, which yielded different pixel sizes for each patient because the FOV varied with patient size. Although these variations are small, they do represent a different resolution for the accuracy in various patients. Again, we do not believe this affected the results or conclusions. Finally, as noted, our study was a simplified approach to this issue, focusing on the axial flow that accounts for the bulk of energy used by the cardiovascular system. This study did not resolve the rotational components demonstrated to be present in the aorta and, indeed, may have contributed to our findings. Although we did not delve into these secondary flow patterns, our findings are not negated and are a first step in this line of study in geometric considerations in children.
Conclusion Flow volume asymmetry exists in the right aortic arch in children. In the AAo, the PR quadrant carried the most flow across the entire cardiac cycle and at the point of maximum flow. In the DAo, the posterior quadrants carried the most flow across the entire cardiac cycle and at the point of maximum flow. In addition, the location of maximum velocity in the right aortic arch for most patients is located in the right half of the arch. These results improve our understanding of aortic flow and will aid in improving aortic surgery in the future.
References 1. Shufford WH, Sybers RG. The aortic arch and its malformations. Springfield: Charles C Thomas Publishers; 1974. p. 12-92. 2. Segadal L, Matre K. Blood velocity distribution in the human ascending aorta. Circulation 1987;76:90-100. 3. Mathison M, Furuse A, Asano K. Doppler analysis of flow velocity profile at the aortic root. J Am Coll Cardiol 1988;12:947-54.
American Heart Journal January 2003
4. Chandran KB. Flow dynamics in the human aorta. J Biomech Eng 1993;115:611-6. 5. Kilner PJ, Yang GZ, Mohiaddin RH, et al. Helical and retrograde secondary flow patterns in the aortic arch studied by three-directional magnetic resonance velocity mapping. Circulation 1993;88: 2235-47. 6. Rieu R, Friggi A, Pellisier R. Velocity distribution along an elastic model of human arterial tree. J Biomech 1985;18:703-15. 7. Paulsen PK. The hot-film anemometer—a method for blood velocity determination: II: in vivo comparison with the electromagnetic blood flowmeter. Eur Surg Res 1980;12:149-58. 8. Segadal L. Velocity distribution model for normal blood flow in the human ascending aorta. Med Biol Eng Comput 1991;29:489-92. 9. Seed WA, Wood NB. Velocity patterns in the aorta. Cardiovasc Res 1971;5:319-30. 10. Matsuda T, Shimizu K, Sakurai T, et al. Measurement of aortic blood flow with MR imaging: comparative study with Doppler US. Radiology 1987;162:857-61. 11. Reichek N, Hu YL, Kramer CM, et al. Normal aging profoundly alters ascending aortic flow profile [abstract]. Circulation 1999; 100(Suppl):I-457. 12. O’Rourke MF. Pressure and flow waves in systemic arterial arteries and the anatomic design of the arterial system. J Appl Physiol 1967;23:139-49. 13. O’Rourke MF, Yaginuma T. Wave reflections and the arterial pulse. Arch Internal Med 1984;144:366-71. 14. O’Rourke MF, Kelly RP. Wave reflections in the systemic circulation and its implications in ventricular function. J Hypertens 1993; 11:327-37. 15. Dahan M, Paillole C, Ferreira B, et al. Doppler echocardiographic study of the consequences of aging and hypertension on the left ventricle and aorta. Eur Heart J 1990;11:39-45. 16. O’Rourke MF, Blazek JV, Morreels CL, et al. Pressure wave transmission along the human aorta: changes with age and in arterial degenerative disease. Circ Res 1968;23:567-79. 17. O’Rourke MF. Systolic blood pressure, arterial compliance and early wave reflection, and their modification by antihypertensive therapy. J Hum Hypertens 1989;3:47-52. 18. O’Rourke MF. Arterial stiffness, systolic blood pressure and logical treatment of hypertension. Hypertension 1990;15:339-47. 19. Fogel MA, Weinberg PM, Hoydu A, et al. Effect of surgical reconstruction on flow profiles in the aorta using magnetic resonance blood tagging. Ann Thorac Surg 1997;63:1691-700. 20. Fisher DC, Sahn DJ, Friedman MJ, et al. The effect of variations on pulse Doppler sampling site on calculation of cardiac output: an experimental study in open-chest dogs. Circulation 1983;67:370-6. 21. Lucas Cl, Keagy BA, Hsiao HS, et al. The velocity profile in the canine ascending aorta and its effects on the accuracy of pulsed Doppler determinations of mean blood velocity. Cardiovasc Res 1984;18:282-93. 22. Nichols WW, O’Rourke MF. McDonald’s blood flow in arteries. Theoretical, experimental and clinical principles. New York: Oxford University Press; 1998. p. 171. 23. Frazin LJ, Lanza G, Vonesh M, et al. Functional chiral asymmetry in descending thoracic aorta. Circulation 1990;82:1985-94. 24. Shipkowitz T, Rodgers VGJ, Frazin LJ, et al. Numerical study on the effect of secondary flow in the human ascending aorta on local shear stresses in the abdominal aortic branches. J Biomech 2000;33:717-28.
American Heart Journal Volume 145, Number 1
Fogel, Weinberg, and Haselgrove 161
25. Farthing S, Peronneau P. Flow in the thoracic aorta. Cardiovasc Res 1979;13:607-20. 26. Frazin LJ, Vanesh MJ, Chandran KB, et al. Confirmation and initial documentation of thoracic and abdominal aortic helical flow. An ultrasound study. ASAIO J 1996;42:951-6. 27. Klipstein RH, Firmin DN, Underwood SR, et al. Blood flow patterns in the human aorta studied by magnetic resonance. Br Heart J 1987;58:316-23. 28. Kupari M, Hekali P, Poutanen V-P. Cross sectional profiles of systolic flow velocities in left ventricular outflow tract of normal subjects. Br Heart J 1995;74:34-9. 29. Rossvoll O, Samstad S, Torp HG. The velocity distribution in the aortic annulus in normal subjects: a quantitative analysis of two-dimensional velocity maps. J Am Soc Echocardiogr 1991;4:367-78. 30. Kupari M, Koskinen P. Systolic flow velocity profile in the left ven-
31. 32. 33.
34.
35.
tricular outflow tract in persons free of heart disease. Am J Cardiol 1993;72:1172-8. Mathison M, Furuse A, Asano K. Doppler analysis of flow velocity profile at the aortic root. J Am Coll Cardiol 1988;12:947-54. Thomas JD. Flow in the descending aorta: a turn of the screw of a sideways glance? Circulation 1990;82:2263-5. Bogren HG, Buonocore MH. 4D magnetic resonance velocity mapping of blood flow patterns in the aorta in young vs elderly normal subjects. J Magn Reson Imaging 1999;10:861-9. Falsetti HL, Kiser KM, Francis GP, et al. Sequential velocity development in the ascending and descending aorta of the dog. Circ Res 1972;31:328-38. Hasenkam JM, Ostergaard JH, Pedersen EM, et al. A model for acute haemodynamic studies in the ascending aorta in pigs. Cardiovasc Res 1988;22:464-71.
The following article is an AHJ Online Exclusive. Full text of this article is available at no charge at our website: www.mosby.com/ahj.
Home-based versus hospital-based exercise programs in patients with coronary artery disease: Effects on coronary vasomotion Stephan Gielen, MD, Sandra Erbs, MD, Axel Linke, MD, Sven Mo ¨ bius-Winkler, MD, Gerhard Schuler, MD, and Rainer Hambrecht, MD Leipzig, Germany
Background In a randomized study, we recently documented that a
measured with a Doppler wire. Coronary blood flow (CBF) was calculated
vigorous, hospital-based exercise training (ET) program improves coronary
by multiplying vascular cross-sectional area and APV.
endothelial function in coronary artery disease. The aim of this consecutive
Results CBF increased in response to 7.2 g/min acetylcholine, from
study was to assess whether a home-based exercise program with re-
27% ⫾ 11% at the beginning of the study to 110% ⫾ 24% after 4 weeks
duced average training duration can sustain previously achieved effects
(P ⬍ .01 vs control group). After 6 months, the increase in CBF was lower
on coronary endothelial function.
versus inhospital training (67% ⫾ 18%, P ⬍ .05 vs 4 weeks). Changes in
Methods Nineteen patients with coronary endothelial dysfunction doc-
APV between 1 and 6 months correlated with daily training durations (r ⫽
umented by acetylcholine-induced vasoconstriction were randomly as-
0.65, P ⫽ .03).
signed to a training group (n ⫽ 10) or a control group (n ⫽ 9). After 4
Conclusions Home-based ET sustained part of the effects of hospital-
weeks of inhospital training (60 min of bicycle ergometry per day), all
based ET on endothelium-dependent vasodilation in coronary artery dis-
training patients were enrolled in a 5-month home-based program of 20
ease. However, acetylcholine-induced increases in CBF were lower after
minutes’ ergometry training per day and 1 group training session per
home-based ET, suggesting a relation between daily training duration and
week. At baseline, after 4 weeks and 6 months, endothelium-mediated
improvement of coronary vasomotion. (Am Heart J 2003;145:e3.)
vasodilation was assessed by quantitative coronary angiography after intracoronary infusions of acetylcholine. Average peak velocity (APV) was
30