Guidelines and standards
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9 who reported normal root dimensions for three age groups ( Figure 3
). The upper limit of normal aortic diameter has been defined as 2 SDs greater than the mean predicted diameter. The Z score (the num- ber of SDs above or below the predicted mean normal diameter) is a useful way to quantify aortic dilatation. Among normal subjects, 95.4% have Z scores between À2 and 2. Therefore, an aortic diam- eter can be considered dilated when the Z score is $2. Using the Z score allows comparison of a given patient’s aortic size at different time points, accounting for the effects of advancing age and increasing Figure 2
Transthoracic echocardiogram in the parasternal long-axis view (zoomed on aortic root and ascending aorta) illus- trating measurement of the aortic root diameter at sinus of Valsalva level at end-diastole using the leading edge–to–leadin- g-edge method. asc Ao, Ascending aorta; LVOT, left ventricular outflow tract. Figure 1 CT reconstruction of a normal aorta illustrating its segmentation as follows: segment I = aortic root; segment II = tubular ascending aorta (subdivided into IIa [STJ to the pulmo- nary artery level] and IIb [from the pulmonary artery level to the brachiocephalic artery]); segment III = aortic arch; segment IV = descending thoracic aorta (subdivided into IVa [from the left subclavian artery to the level of the pulmonary artery] and IVb [from the level of the pulmonary artery to the diaphragm]); and segment V = abdominal aorta (subdivided into Va [upper abdominal aorta from the diaphragm to the renal arteries] and Vb [from the renal arteries to the iliac bifurcation]). 122 Goldstein et al Journal of the American Society of Echocardiography February 2015
body size, thus distinguishing normal from pathologic growth. The Z score is therefore particularly useful for evaluating growing children. It should be mentioned that aortic root dimensions may be increased by the hemodynamic effects of both endurance and strength exercise training in competitive athletes. 17-19
This aortic root enlargement appears to be greater at the sinuses of Valsalva than at the aortic annulus or STJ. However, it should be emphasized that the effects of exercise training on aortic diameters are relatively small and that marked enlargement should suggest a pathologic process. 17,18 Recently, making use of a database consisting of a multiethnic pop- ulation of 1,207 apparently normal adolescents and adults $15 years of age, investigators devised equations to predict mean normal aortic root diameter and its upper limit by age, body size (BSA or height), and gender 6 ( Table 1 for men and Table 2 for women). These equa- tions have been used graphically to depict the upper limits of the 95% confidence interval for normal aortic root diameter using surfaces to depict the interacting effects of age and body size (see Figure 4
for men and
Figure 5 for women). A noncontrast gated cardiac computed tomographic study, 20 including 4,039 adult patients, showed age, BSA, gender, and hyper- tension to be directly associated with thoracic aortic diameters perpendicular to the long axis of the aorta. These associations are concordant with those from echocardiographic studies. In another recent large study using similar methodology, the mean value of the diameters of the ascending aorta was 1.8 6 0.2 cm/m 2 and of the de- scending thoracic aorta was 1.4 6 0.2 cm/m 2 , with the upper limits of normal being 2.1 and 1.8 cm/m 2 , respectively. 21 However, more ac- curate normal values of thoracic aortic diameters may be obtained by anatomically correct double-oblique short-axis images using electro- cardiographically gated multidetector CT or by MRI of axially ori- ented aortic segments. The upper limits of normal are 3.7 cm for the aortic root at the sinuses, 3.6 cm for the ascending aorta and 2.5 cm for the descending thoracic aorta by CT, 8 and 2.5 cm for the descending thoracic aorta and 2.0 cm for the upper abdominal aorta by MRI. 22 As with echocardiography, aortic root and ascending aortic diameters increase significantly with age and BSA on CT and MRI. Aortic root diameters increase 0.9 mm per decade in men and 0.7 mm per decade in women. 4 The establishment of normative values and reference ranges, tak- ing into account aging and gender, is of great importance for diag- nosis, prognosis, serial monitoring, and determining the optimal timing for surgical intervention. Normal values and proximal aortic di- ameters have been reported using different imaging techniques, from the pioneer studies based on M-mode and 2D echocardiography 9,10
to more recent studies obtained using CT 7,8,20,23-25 and MRI. 5,26
Despite differences in image acquisition methods, temporal and spatial resolution, and signal-to-noise ratios, CT, MRI, TTE, and trans- esophageal echocardiography (TEE) have evolved as near equal stan- dards for assessing aortic root size. Each of these modalities has Table 2 Normal aortic root diameter by age for women with BSA of 1.7 m 2 Age (y) 15–29 30–39
40–49 50–59
60–69 $70
Mean normal (cm) 2.9
3.0 3.2
3.2 3.3
3.4 Upper limit of normal (cm) 3.3
3.4 3.6
3.6 3.7
3.9 Add 0.5 mm per 0.1 m 2 BSA above 1.7 m 2 or subtract 0.5 mm per 0.1 m 2
2 . 6 Figure 3 Aortic root diameter (vertical axis) in relation to BSA (horizontal axis) in apparently normal individuals aged 1 to 15 (left panel, blue), 20 to 39 (center panel, green), and $40 (right panel, pink) years. For example, an individual between the ages of 20 and 39 years (center panel, green) who has a BSA of 2.0 m 2 (vertical green line) has a normal root diameter range (2 SDs) between 2.75 and 3.65 cm, as indicated by the intersections of the two horizontal green lines with the green-shaded parallelogram. Table 1
Normal aortic root diameter by age for men with BSA of 2.0 m
2 Age (y)
15–29 30–39
40–49 50–59
60–69 $70
Mean normal (cm) 3.3
3.4 3.5
3.6 3.7
3.8 Upper limit of normal (cm) (95% CI) 3.7
3.8 3.9
4.0 4.1
4.2 Add 0.5 mm per 0.1 m 2 BSA above 2.0 m 2 or subtract 0.5 mm per 0.1 m 2
2 . 6 CI, Confidence interval. Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 123 advantages and disadvantages, which have been discussed. It should be emphasized that normal aortic diameters vary systematically by age, gender, and body size, and reference values indexed to those pa- rameters have been provided. Last, it is critically important to empha- size not only methodologic variance but also inter- and intraobserver variability. In several studies, variability of measurement of proximal aortic diameters ranges from 1.6 to 5 mm. 8,23,24,27,28 Given this degree of variability, apparent small changes in proximal aortic diameters on serial computed tomographic examinations may be within the range of measurement error. Accordingly, for all imaging techniques, we recommend that changes
of #3 mm by
electrocardiographically gated
CT and
#5 mm without electrocardiographic gating be viewed with caution and skepticism. B. How to Measure the Aorta Accurate and reproducible measurements of aortic dimensions are necessary for the detection and classification of aortic disease and for guiding therapeutic decisions. Modern imaging modalities enable one to make measurements far more accurately than did invasive contrast angiography, the only tool originally available. Echocardiography, CT, and MRI each has particular strengths and limitations but can be adapted for the acquisition of views that allow measurement of the diameter or cross-sectional area of different seg- ments of the aorta ( Figure 1 ). 1. Interface, Definitions, and Timing
of Aortic
Measurements. The American Society of Echocardiography (ASE) proposed standards for measurement of the aortic root in 1978.
29 The ASE recommended measurement at end-diastole from the leading edge of the anterior root wall to the leading edge of the posterior aortic root wall. This technique was believed to minimize the impact of ‘‘blooming’’ of bright reflectors on this measurement. The ASE-recommended method was followed in many important clinical and epidemiologic studies 10,13
that have reported normal limits for individuals of differing body size and age, and these normal limits have been incorporated into multiple guidelines for imaging in adults ( Figures 4 and 5 ). 1,9,16 As a consequence, much of the available data on normal aortic root size as well as on the prevalence and prognostic significance of aortic dilatation in adults have emerged from echocardiography. 6,10,13 Societal guidelines for measurement by CT or MRI are not currently available. Consequently, uniformity in measurement methods is lacking. Many research and clinical studies using these mo- dalities have reported aortic measurements made from inner edge to inner edge on electrocardiographically gated or nongated images. The 2010 guidelines for the diagnosis and management of thoracic aortic disease took the opposite approach, recommending measurement of aortic diameter between external surfaces to avoid confounding by intraaortic thrombus or atheroma, as is commonly found in the abdominal but not in the ascending aorta. 1 Furthermore, there is no standardized ‘‘trigger time’’ (end-systole vs end-diastole) for image acquisition. Thus, the use of multiple imaging modalities such as CT, MRI, and 2D and three-dimensional (3D) echocardiography has led to nonuniformity in measurement techniques. Moreover, there is currently no standardized approach for reconciling aortic measurements across imaging modalities (echocardiography, CT, MRI, aortography) by trigger time (end-systole vs end-diastole) or by edge selection (leading edge, inner-inner, outer-outer). This writing committee had hoped to recommend a uniform and consistent mea- surement technique to minimize differences among these various im- aging modalities. However, after much consideration, the group recommends that echocardiographic measurements continue to be made in the standard fashion from leading edge to leading edge, at end-diastole, and perpendicular to the long axis of the aorta. The advantages of end-diastolic measurements include greater reproduc- ibility (because aortic pressure is most stable in late diastole) and the ease of identification of end-diastole by the onset of the QRS Figure 5
Surfaces representing aortic diameters 1.96 Z score (95% confidence interval) above the predicted mean value of aortic diameter for age and BSA in female subjects $15 years of age. (Adapted from Devereux et al. 6 ) Figure 4 Surfaces representing aortic diameters at a 1.96 Z score (95% confidence interval) above the predicted mean for age and BSA in male subjects $15 years of age. (Adapted from Devereux et al. 6 )
Journal of the American Society of Echocardiography February 2015 Figure 6 Models of the thoracic aorta showing the cut planes of the aortic annulus for each applied imaging modality. (A) Angiog- raphy in the 90
left anterior oblique (LAO) projection with an orange arrow indicating the sagittal annulus diameter (left) and in the 0
posteroanterior (p.a.) projection with a blue arrow indicating the coronal annulus diameter (right). (B) TTE (left) and 2D TEE (right) left ventricular outflow tract (LVOT) view of the aortic annulus. The cut planes slightly differ because parasternal and midesophageal acoustic are not quite comparable. Both the transthoracic and 2D transesophageal echocardiographic LVOT views resemble a sagittal view (bright and dark yellow arrows, respectively). The direction of the arrows in the aortic arch model and the echocardio- graphic images indicate the scanning direction. Individual adjustments in scan plane direction are shown in the model. (C) Three-dimensional transesophageal echocardiographic cropped images of a sagittal (left) and coronal (right) view with the corre- sponding diameters (orange and blue arrows). The sagittal and coronal cut planes are depicted in the aortic arch model and the anatomic short-axis view (middle). (D) Dual-source computed tomographic (DSCT) reconstructed images of a sagittal (left) and cor- onal (right) view with the corresponding diameters (orange and blue arrows). The sagittal and coronal cut planes are depicted in the aortic arch model and the anatomic short-axis view (middle). AO, Ascending aorta; LA, left atrium. (From Altiok et al. 418
) Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 125 complex. Although other techniques use the inner edge–to–inner edge approach, there are currently insufficient data to warrant a change for echocardiography. Available data suggest that the echocar- diographic leading edge–to–leading edge approach produces values comparable with those produced by the inner edge–to–inner edge approach on CT and MRI, is reproducible, and links to a large body of historical and prognostic data that have long guided clinical deci- sion making. For all modalities, it is desirable, whenever possible, to specify the locations of measurements, by referencing them to a given landmark. For example, with TEE, a measurement of the maximal diameter of the ascending aorta may be reported by its distance from the STJ. In the descending thoracic aorta, reference to the location of a mea- surement or abnormality is usually made by its distance from the in- cisors. Similar attempts should be made for measurements and findings with CT and MRI. 2. Geometry of Different Aortic Segments: Impact on Measurements. Accurate and reproducible measurement of aortic diameter or cross-sectional area in a given segment requires three measurements of its diameter perpendicular to the long axis. In most cases, the largest correctly oriented measurement is reported. a. Aortic Annulus.–Although the aortic annulus is approximately cir- cular in children and young adults, it may become elliptical in older adults. Thus, 3D imaging by CT or echocardiography or 2D imaging in multiple planes (e.g., long-axis or sagittal and coronal planes) is required to measure a diameter that is accurate enough to be used when selecting patients for transcatheter aortic valve replacement ( Figure 6
). b. Sinuses of Valsalva and STJ.–Aortic root diameter can be measured perpendicular to its long axis by 2D echocardiography or in analogous nontrue coronal and sagittal plane by MRI or CT. The variability in this measurement resulting from the orientation of the aortic root is overcome by choosing the largest diameter measured from the right coronary sinus of Valsalva to the posterior (usually non- coronary) sinus, parallel to the aortic annulus and perpendicular to the long axis of the proximal aorta in several slightly differently oriented long-axis views. Failure to search for the largest correctly oriented measurement can lead to underestimation of aortic root diameter. Aortic root diameter is commonly measured by CT or MRI between the inner edges from commissure to opposite sinus ( Figure 7 ). Diameters measured using the sinus-to-sinus method are generally a mean of 2 mm larger than those measured by the sinus-to- commissure method 4,7 (
). However, using the sinus-to-sinus method has several advantages, including the ease of detecting cusp margins in computed tomographic or MRI transverse planes, close agreement with echocardiographic measurements, and greater feasi- bility in bicuspid valves. Thus, for aortic measurements by CT and MRI, it is recommended to average the three sinus-to-sinus measure- ments in end-diastole in the sinus-of-Valsalva plane. When the sinuses are unusually asymmetric, it may be preferable to report the three measurements individually.
principles apply to obtaining correct measurements of the other aortic segments. Conventional imaging by all modalities and techniques can be used to measure the diameter of aortic segments that are oriented along the long axis of the body. However, the necessity to avoid ob- lique imaging that can overestimate the aortic diameter applies to the Figure 7 Aortic root measurements by CT. The aortic root diameter is commonly measured between the inner edges from one commissure to opposite sinus (yellow line) or from one sinus to another sinus (red line), as shown in the large image (left), which is a zoomed cross-sectional view of the aortic root at the sinus of Valsalva level using a double oblique image for orientation (shown in the right panel). 126 Goldstein et al Journal of the American Society of Echocardiography February 2015 aortic arch and to portions of the descending thoracic and abdominal aorta that may take a tortuous course ( Figure 9 ). We emphasize that there is no standardized method for measuring the aorta across imaging modalities (echocardiography, CT, MRI, aortography). Although one of the major goals of this writing commit- tee was to provide a uniform and universally accepted method to minimize differences among these various imaging modalities, no consensus could be reached. After much consideration, it is recom- mended that echocardiographic measurements continue to be made from leading edge to leading edge. Although other techniques use inner edge–to–inner edge or outer edge–to–outer edge approaches, there are currently insufficient data to warrant a change for echocardiography. Available data suggest that the echocardio- graphic leading edge–to–leading edge approach gives larger measure- ments compared with the inner edge–to–inner edge approach on CT (average difference, 2 mm), and the leading edge–to–leading edge method links to a large body of historical and, more important, prog- nostic data that influence decision making. 8 Out of concern that pa- tient management might be adversely affected (i.e., intervention might be delayed, leading to a catastrophic complication such as rupture or dissection) by switching to a new protocol that would lead to a smaller measurement, it was decided to continue to recom- mend the leading edge–to–leading edge approach. C. Aortic Physiology and Function The aorta functions as both a conduit and a reservoir. Its elastic prop- erties allow it to expand in systole and recoil during diastole. Thus, under normal conditions, a large proportion (up to 50%) of the left ventricular stroke volume is stored in the aorta (mainly in the ascending aorta) at end-systole, and the stored blood is then propelled forward during diastole into the peripheral circulation. This reservoir function is important for maintaining blood flow and arterial pressure throughout the cardiac cycle. The thoracic aorta is more distensible than the abdominal aorta because its media contains more elastin. Aortic distensibility declines with age and as a result of premature degeneration in elastin and collagen associated with some disease states.
30 During left ventricular systole, this loss of aortic wall compli- ance results in increased systolic pressure and pulse pressure and, in turn, aortic dilatation and lengthening. The compliance of the aortic wall may be estimated by assessing change in aortic volume in relation to the simultaneous change in aortic pressure. This may be assessed locally by diameter or area change through the cardiac cycle in rela- tion to pressure change (e.g., distensibility) or regionally by determina- tion of the velocity of the pulse wave. 1. Local Indices of Aortic Function. Techniques that provide accurate definition of the aortic diameter or volume in systole and diastole can be used to evaluate the elastic properties of the aorta. The most commonly applied indices for clinical purposes are aortic distensibility and the stiffness index, which is less dependent on Yüklə 9,02 Mb. Dostları ilə paylaş:
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