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A dynamic view on the diameter of abdominal aortic aneurysms
Eur J Vasc Endovasc Surg 15, 308-312 (1998)
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms 1". L&nne*, T. Sandgren 1 and B. Sonesson Departments of Vascular and Renal Diseases, and 1Surgery, MalmO University Hospital, Maim& Sweden Objectives: To study 40-55 mm aneurysms and calculate their size in relation to the individual predicted normal aortic diameter to see if this might add anything in the evaluation of treatment. Material and methods: The anteroposterior diameter of 40-55 ,mm AAAs was measured with an echo-tracking ultrasonic technique in 147 consecutive patients. The weight and height were registered and body surface area calculated. The predicted normal aortic diameters were defined according to nomograms and the diameter increase from the predicted normal aortic size in the individual aneurysms calculated. Results: The median A A A diameter was 48 mm (range 40-55), the BSA 1.85 m 2 (1.42-2.37), and the predicted AO size 19.4 mm (14.3-21.6). The calculated increase of size in the individual aneurysms was 2.51 (1.9-3.53), that is the spread of data doubled as compared to conventional diameter measurements. When females and males were studied separately the A A A diameter was 46.5 mm (40-55) and 48 mm (40-55), respectively (NS). Since the BSA was significantly lower in women than in men, 1.63 (1.42-1.95) and 1.89 (1.47-2.37), respectively (p
Introduction The incidence of abdominal aortic aneurysms is increasing and the fate of an individual a n e u r y s m is unclear. However, the risk of growth and rupture is related to increasing size and most surgeons are in favour of treating aneurysms above 50-55 m m surgically. Smaller aneurysms are usually treated conservatively with follow-up b y ultrasonography. Since rupture m a y occur even in asymptomatic and relatively small aneurysms, early detection and validation are considered to be indispensable for further i m p r o v e m e n t in the m a n a g e m e n t of the disease. H o w ever, several definitions of abdominal aortic aneurysms are used at present, which p r o d u c e v e r y different results in evaluating the incidence and prevalence of the disease. 1 One of the basic problems encountered is defining the normal diameter of the abdominal aorta, which makes it impossible to follow the advice of the Society for Vascular Surgery and the International * Please address all correspondence to: T. L~nne, Department of Vascular and Renal Diseases, Malm~5University Hospital, S-205 02 MalmO, Sweden.
Society for Cardiovascular Surgery (SVS/ISCVS), which defines an a n e u r y s m as a 50% dilatation of a normal artery. 2 Recently, studies on the diameter of the abdominal aorta have s h o w n an increasing diameter with age 3'4 and the possibility n o w exists to define the extent of pathological dilatation using nomograms for the normal diameter on the infrarenal aorta. B The aim of this s t u d y was to evaluate AAAs between 40-55 ram, where there is no consensus in favour of surgical treatment, and to see whether this new w a y of defining the a n e u r y s m size might aid in the decision making.
Material and Methods The study was p e r f o r m e d on 147 consecutive patients, (49-92 years old) with 4 0 - 5 5 m m abdominal aortic aneurysms (113 males and 34 females). All investigations were done with the subject in the supine position. The maximal anteroposterior diameter was registered with an echo-tracking ultrasonic e q u i p m e n t (Diamove, Teltec AB, Lund, Sweden) with the use of
1078-5884/98/040308+05 $12.00/0 © 1998 W.B. Saunders Company Ltd.
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms
309
two electronic markers, each representing one tracking gate, are automatically aligned w i t h a n d locked to the B m o d e echo i m a g e of the inner surface of the anterior a n d posterior arterial wall. The echo-tracker m e a s u r e s the distance b e t w e e n the walls of the vessel. The d i a m e t e r w a s calculated as:
125 -
diastolic diameter + systolic d i a m e t e r
100-
75-
mm
Predicted
Fig. 1. Differences in spread of data (ranges) when calculations of the aneurysm size are based on the diameter (ram) or the number of times by which the actual aneurysm diameter exceeds the predicted normal aortic diameter. a 3.5 M H z B m o d e real-time linear array scanner. The technique has earlier b e e n described in detail. 5 In short,
All d i a m e t e r registrations w e r e carried out b y one or two experienced technicians. The variability of m e a s u r e m e n t s are 3%. 6 The aortic a n e u r y s m w a s visualised longitudinally in a real-time picture a n d the m a x i m u m diameter p e r p e n d i c u l a r to the longitudinal axis defined. The echoes f r o m the walls w e r e o p t i m i s e d a n d after locking the echo-tracker to the inner surface of the a n e u r y s m walls the d i a m e t e r w a s calculated as the m e a n of three consecutive registrations. The height a n d b o d y w e i g h t w e r e m e a s u r e d in each patient a n d b o d y surface area calculated according to d u Bois' f o r m u l a (body surface area, c m 2 = w e i g h t °425 kg x height °72~, c m x 71.84). The a b d o m i n a l aortic size is d e p e n d e n t on age, b o d y surface area a n d sex. By using these variables the n o r m a l aortic d i a m e t e r could be calculated according to the n o m o g r a m s of Sonesson et al. (1994). 3 The n u m b e r of times b y w h i c h the actual a n e u r y s m diameter exceeds the predicted n o r m a l aortic d i a m e t e r w a s defined. M e d i a n (range) values are given, a n d differences b e t w e e n g r o u p s w e r e assessed
50
4O
30 ~9
E
10
M
F
(a)
~
M
F
(b)
Fig. 2a. The aneurysm diameter (mm) in males (m) and females (f). Fig. 2b. The aneurysm size calculated as the number of times by which the actual aneurysm diameter exceeds the predicted normal aortic diameter. Note the larger size in females. Eur J Vasc Endovasc Surg Vol 15, April 1998
T. L~nne e t a l .
310
Table 1. Size evaluation of abdominal aortic aneurysms.
Sex
n
Age (years)
AAA BSA diameter (ram) (m 2)
Predicted Predicted AO diameter3 AAA size
Males and females
147
Males
113
72 49-92 72 53-92 74 49-85 NS
48 40-55 48 40-55 46.5 40-55 NS
19.4 14.3-21.6 19.7 18-21.6 16.4 14.3-17.8 p<0.0001
Females
34
Significance males/females
1.85 1.42-2.37 1.89 1.47-2.37 1.63 1.42-1.95 p<0.0001
2.51 1.90-3.53 2.46 1.90-2.94 2.93 2.25-3.53 p<0.0001
Patients with abdominal aortic aneurysms, 40-55 mm (median)/range. using the M a n n - W h i t n e y U-Test at the level of significance p<0.05.
Results
The AAA diameter was 48 m m (40-55). The BSA was 1.85 m 2 ( 1 . 4 2 - 2 . 3 7 ) and the predicted AO size 19.4 m m (14.3-21.6). The calculated increase of size from the predicted AO diameter in the individual aneurysms was 2.51 (1.9-3.53), that is the spread of data doubled as c o m p a r e d to conventional diameter measurements. W h e n females and males were studied separately the AAA diameter was 46.5 and 48 mm, respectively (NS). Since the BSA was significantly lower in w o m e n than in men, 1.63 (1.42-1.95) and 1.89 (1.47-2.37), respectively (p<0.0001), also the predicted normal aortic size was lower, 16.4 (14.3-17.8) vs. 19.7 (18-21.6) (p<0.0001). Thus, the AAA diameter increase from the predicted size was larger in w o m e n than in men; 2.93 (range 2.25-3.53) vs. 2.46 (range 1.90-2.94), respectively (p<0.0001) (Table 1). Figure 1 shows the a n e u r y s m diameter normalised to 100% and the differences in spread of data (ranges) w h e n calculations were based either on the diameter in m m or the n u m b e r of times b y which the actual a n e u r y s m diameter exceeded the predicted normal aortic diameter. It is seen that the spread of data increases from 30 to 65%, i.e. b y a factor of 2 using the latter definition. Figure 2a shows the a n e u r y s m diameter in m m in the males and females respectively. No differences are seen. In Fig. 2b the a n e u r y s m size is calculated as the n u m b e r of times b y which the actual a n e u r y s m diameter exceeds the predicted normal aortic diameter. Note the significantly larger size in females as c o m p a r e d to males (p<0.0001).
Discussion
This study shows clear differences in the size of abdominal aortic aneurysms (AAA) w h e n evaluated in Eur J Vasc Endovasc Surg Vol 15, April 1998
m m as c o m p a r e d to the increase in size from the predicted value of the normal aorta in each individual. The latter m e t h o d m a y be preferred from a pathophysiological point of view and might add information concerning the growth and rupture patterns of AAAs. The prevalence of AAA has s h o w n an increasing trend within the last decades. This does not seem to be related to more reliable diagnostic methods alone. 7'8 Recent screening studies indicate a prevalence of AAA in m e n over 65 years of about 5%. 9'1° Further, there is a genetic influence with about one-quarter to one-third of the male near relatives to patients with AAA, also having AAA. 1~'12Several studies have s h o w n that aneurysms with a diameter >55-60 m m tend to increase in diameter and the n u m b e r of ruptures also increase in an exponential fashion. This varies, of course, between individual aneurysms ~3 and is w h y vascular surgeons r e c o m m e n d surgical treatment in AAAs >55-60 mm. In the same manner, AAAs less than 40 m m are treated conservatively. However, between 40 and 55 m m there is no consensus in favour of surgical treatment or not. Two large prospective r a n d o m i s e d studies are addressing this issue at the m o m e n t and will hopefully show the preferable treatment. ~4 However, several definitions of AAA are currently used. The Society for Vascular Surgery and the International Society of Cardiovascular Surgery (SVS/ISCVS) define an ane u r y s m as a 50% dilatation of the normal artery adjusted for gender and radiological modality. 2 Sterpetti et al. 1~ suggest a ratio of infrarenal to suprarenal diameters of > 1.5. Collin ~6defines an abdominal aortic a n e u r y s m as a maximal infrarenal diameter of 40 m m or more, or exceeding the maximal diameter of the aorta between the origin of the superior, mesenteric and left renal arteries b y at least 5 mm. W h e n comparing these definitions on p o p u l a t i o n data concerning frequency and prevalence, large differences are seen. Therefore, until a clear uniform definition is used, the true epidemiology of aortic a n e u r y s m s will not be elucidated. The SVS/ISCVS definition seems the most reasonable from a pathophysiological point of view. H o w ever, until recently there has been no knowledge
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms
concerning the normal infrarenal aortic size. 1 In 1994 Sonesson et al. 3 published a paper that defined the normal aortic diameter in relation to sex, age and body surface area with nomograms to be used in the prediction of an individual aortic size. For example, if the maximum diameter of an abdominal aortic aneurysm is 40 mm in a 75-year-old male with a BSA of 2.3 m 2, the expected aortic diameter obtained is 21.9 mm. 3 By dividing 40 m m by 21.9 it is shown that the aneurysm in this patient has increased 1.83 times the expected diameter at this age and body size. On the other hand, if an abdominal aortic aneurysm of 40 mm is encountered in a 50-year-old female with a BSA of 1.4 m 2, the normal aortic diameter in this patient is defined to be 13.9 mm 3 and the aneurysm of this patient has enlarged by 2.88 times. Thus, a discrepancy of almost 100% in enlargement is seen. Stringfellow et a]. 17 used finite element analyses to determine the wall stress distribution in AAA. They found that an aneurysm of similar size arising from a small aorta had much greater wall stress than had an aneurysm arising from a larger aorta. Further, from a biological point of view it seems reasonable to assume that a 40 mm large aneurysm that has expanded 2.9 times from the original size has a more diseased arterial wall than the 40 mm large aneurysm that has expanded only 1.9 times. Similar reasoning was used by Ouriel et al. I8, who standardised the aneurysm diameter to the transverse diameter of the third lumbar vertebrae body as an index of body size. They found this to be a predictor of rupture when the threshold ratio of 1.0 was used. The study by Limet et aI. 19 shows that the risk of rupture is closely related to a relative change in size of an aneurysm more than the change in mm. However, they related the diameter change to the initial aneurysm size at diagnosis and not the predicted normal aortic size. It is possible that their figures would improve, if this had been performed. Another point of concern is the sex-specific frequency of AAA. Several studies have shown a 2-4 times higher AAA frequency in men than in women and that aneurysms start to appear later in women. However, these data are based on a standard diameter definition without any differences between sexes taken into account. With the differences in normal aortic diameter between males and females born in mind, this might indicate an underestimation of the number of aneurysms in women. Further, this might also be the case in the gender-specific inheritance patterns with male near relatives more prone to aneurysm formation than corresponding females. 11'2° To our knowledge no study so far has addressed the prognosis of small aortic aneurysms in relation to sex. However,
311
several large studies, such as The UK Small Aneurysm Trial, The Veteran Aneurysm Detection and Management Study and The Canadian Small Aneurysm Treatment Trial, may elucidate this factor.21The evaluation of our AAAs showed that women had significantly larger AAAs than males, when the size of the aneurysm was related to the predicted normal aortic diameter in the individual patients (Fig. 1). Whether this fact may be of clinical importance cannot be elucidated by this study, since the number of AAAs were relatively small and therefore no outcome was studied. However, Katz et al. (1997)22 showed, in a study on 41 358 patients with AAA, that women were less likely to undergo aortic reconstruction and when they did were less likely than men to survive to discharge. 22 The reliance on AAA diameter as the primary indication for surgery might have resulted in referral for surgery at a later stage of disease among women. This indicates a need for individualisation in the evaluation of an aneurysmal size. In summary, no consensus so far has been reached concerning diagnostic criteria for AAA. A reasonable point of view is to define an aneurysm as a permanent, localised dilatation of an artery exceeding 50% of the expected normal diameter, this now being possible due to knowledge of the normal aortic size. This may facilitate the question of how to treat aneurysm of smaller size (i.e. 40-55 mm) that in fact may be large in relation to the individual's normal predicted aortic diameter.
Acknowledgements This study was supported by grants from the Medical Faculty, Lund University, the Medical Research Council (no. 12661), the Funds of Malta6 University Hospital, and the Swedish Society of MedicIne. We thank the staff of the Department of Clinical Physiology for skilled assistance.
References 1 MOILER D, COLE CW, HILL GB. Epidemiology of abdominal aortic aneurysms: the effect of differing definitions. Eur J Vasc Surg 1992; 6: 647-650. 2 JOHNSTONEKW, RUTHERFORDRB, TILSONMD, SHAI~DM, HOLLIER L, STANLEYJC. Suggested standards for reporting on arterial aneurysms. J Vasc Surg 1991; 13: 444-450. 3 SONESSONB, L;iNNE T, HANSEN FI SANDGRENT. Infrarenal aortic diameter in the healthy person. Eur J Vasc Surg 1994; 8: 89-95. 4 KANAOKa Y, OHCI S, Mol~I T. Age-related changes in the abdominal aortic diameter. Vasc Surg 1994; 28: 349-358. 5 LINDSTROMK, GENNSER G, SINDBERGERIKSEN P, BENTHIN M, DAHL P. An improved echo-tracker for studies on pulse waves in the fetal aorta. In: Rolfe P, ed. Fetal Physiological Measurements. London: Butterworths, 1987: 217-226. Eur J Vasc Endovasc Surg Vol 15, April 1998
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6 LA_NNE T, SANDGREN T, MANGELL Pz SONESSON B, HANSEN E Improved reliability of ultrasonic surveillance of abdominal aortic aneurysms. Eur J Vasc Endovasc Surg 1997; 13: 149-153. 7 MELTON LJ, BICKERSTAEFLK, HOLLIER LH. Changing incidence of abdominal aortic aneurysms: a population based study. Am J Epidemiol 1984; 120: 379-386. 8 CASTLEDENWM, MERCERJC, MEMBERSOF THE WEST AUSTRALIAN VASCULAR SERVICE. Abdominal aortic aneurysms in Western Australia: descriptive epidemiology patterns of rupture. Br J Surg 1985; 72: 109-112. 9 BENGTSSONH, BERGQVISTD, STERNBYN-H. Increasing prevalence of abdominal aortic aneurysms - an autopsy-based study. Eur J Surg 1992; 158: 19-23. 10 COLLIN J, ARAUJO L, WALTON J, LINDSELL D. Oxford screening programme for abdominal aortic aneurysm in men aged 64 to 74 years. Lancet 1988; ii: 613-615. 11 BENGTSSON H, SONESSON B, LNNNE T, LOREN I, BERGQVIST D. Prevalence of abdominal aortic aneurysm in the offspring of patients dying from aneurysm rupture. Br J Surg 1992; 79: 1142-1143. 12 BENGTSSONH, NORRGARD O, ANGQUIST KA, EKBERG O, (~BERG L, BERGQVIST D. Ultrasonographic screening of the abdominal aorta among siblings of patients with abdominal aortic aneurysms. Br J Surg 1989; 76: 589-591. 13 BENGTSSON H, NILSSON P, BERGQVIST D. Natural history of abdominal aortic aneurysm detected by screening. Br J Surg 1993; 80: 718-720. 14 TIrE U.K. SMALLANEURYSMTRIAL PARTICIPANTS. The U.K. small
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15 16 17 18
19 20 21
22
aneurysm trial: design, methods and progress. EurJ Vasc Endovasc Surg 1995; 9: 42-48. STEREETTIAV, SCHULTZ RV, FELDHAUS RI, CHENG SE, PEETZ DJ. Factors influencing enlargement rate of small abdominal aortic aneurysms. J Surg Res 1987; 43: 211-219. COLLIN J. A proposal for a precise definition of abdominal aortic aneurysm: a personal view. J Cardiovasc Surg 1990; 31: 168-169. STRINGFELLOW MM, LAWRENCE PE STRINGTELLOW RG. The influence of aorta - aneurysm geometry upon stress in the aneurysm wail. J Surg Res 1987; 42: 425-433. OURIEL K, GREEN RM, DONAYRE C, SHORTELL CK, ELLIOTT J, DEWEEsE JA. An evaluation of new methods of expressing aortic aneurysm size: relationship to rupture. J Vasc Surg 1992; 15: 12-20. LIMET R. Determination of level of expansion and incidence of rupture of abdominal aortic aneurysms. Bull Mere Acad R Med Belg 1992; 147: 253-261. ADAMS DCR, TULLOH BR, GALLOWAYSW, SHAW E, TULLOH AJ, POSKITT KR. Familial abdominal aortic aneurysm: prevalence and implications for screening. Eur J Vasc Surg 1993; 7: 709-712. POWELL JG MACSWEENEY STR, GREENHALGH R2¥L The spontaneous course of small aortic aneurysm. In: Yao JST, Pearce WH, eds. Aneurysms. New Findings and Treatments. Norwalk: Appleton & Lange, 1994: 71-77. KATZ DJ, STANLEY JC, ZELENOCK GB. Gender differences in abdominal aortic aneurysm prevalence, treatment, and outcome. J Vasc Surg 1997; 25: 561-568.
Accepted 15 October 1997
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms 1". L&nne*, T. Sandgren 1 and B. Sonesson Departments of Vascular and Renal Diseases, and 1Surgery, MalmO University Hospital, Maim& Sweden Objectives: To study 40-55 mm aneurysms and calculate their size in relation to the individual predicted normal aortic diameter to see if this might add anything in the evaluation of treatment. Material and methods: The anteroposterior diameter of 40-55 ,mm AAAs was measured with an echo-tracking ultrasonic technique in 147 consecutive patients. The weight and height were registered and body surface area calculated. The predicted normal aortic diameters were defined according to nomograms and the diameter increase from the predicted normal aortic size in the individual aneurysms calculated. Results: The median A A A diameter was 48 mm (range 40-55), the BSA 1.85 m 2 (1.42-2.37), and the predicted AO size 19.4 mm (14.3-21.6). The calculated increase of size in the individual aneurysms was 2.51 (1.9-3.53), that is the spread of data doubled as compared to conventional diameter measurements. When females and males were studied separately the A A A diameter was 46.5 mm (40-55) and 48 mm (40-55), respectively (NS). Since the BSA was significantly lower in women than in men, 1.63 (1.42-1.95) and 1.89 (1.47-2.37), respectively (p
Introduction The incidence of abdominal aortic aneurysms is increasing and the fate of an individual a n e u r y s m is unclear. However, the risk of growth and rupture is related to increasing size and most surgeons are in favour of treating aneurysms above 50-55 m m surgically. Smaller aneurysms are usually treated conservatively with follow-up b y ultrasonography. Since rupture m a y occur even in asymptomatic and relatively small aneurysms, early detection and validation are considered to be indispensable for further i m p r o v e m e n t in the m a n a g e m e n t of the disease. H o w ever, several definitions of abdominal aortic aneurysms are used at present, which p r o d u c e v e r y different results in evaluating the incidence and prevalence of the disease. 1 One of the basic problems encountered is defining the normal diameter of the abdominal aorta, which makes it impossible to follow the advice of the Society for Vascular Surgery and the International * Please address all correspondence to: T. L~nne, Department of Vascular and Renal Diseases, Malm~5University Hospital, S-205 02 MalmO, Sweden.
Society for Cardiovascular Surgery (SVS/ISCVS), which defines an a n e u r y s m as a 50% dilatation of a normal artery. 2 Recently, studies on the diameter of the abdominal aorta have s h o w n an increasing diameter with age 3'4 and the possibility n o w exists to define the extent of pathological dilatation using nomograms for the normal diameter on the infrarenal aorta. B The aim of this s t u d y was to evaluate AAAs between 40-55 ram, where there is no consensus in favour of surgical treatment, and to see whether this new w a y of defining the a n e u r y s m size might aid in the decision making.
Material and Methods The study was p e r f o r m e d on 147 consecutive patients, (49-92 years old) with 4 0 - 5 5 m m abdominal aortic aneurysms (113 males and 34 females). All investigations were done with the subject in the supine position. The maximal anteroposterior diameter was registered with an echo-tracking ultrasonic e q u i p m e n t (Diamove, Teltec AB, Lund, Sweden) with the use of
1078-5884/98/040308+05 $12.00/0 © 1998 W.B. Saunders Company Ltd.
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms
309
two electronic markers, each representing one tracking gate, are automatically aligned w i t h a n d locked to the B m o d e echo i m a g e of the inner surface of the anterior a n d posterior arterial wall. The echo-tracker m e a s u r e s the distance b e t w e e n the walls of the vessel. The d i a m e t e r w a s calculated as:
125 -
diastolic diameter + systolic d i a m e t e r
100-
75-
mm
Predicted
Fig. 1. Differences in spread of data (ranges) when calculations of the aneurysm size are based on the diameter (ram) or the number of times by which the actual aneurysm diameter exceeds the predicted normal aortic diameter. a 3.5 M H z B m o d e real-time linear array scanner. The technique has earlier b e e n described in detail. 5 In short,
All d i a m e t e r registrations w e r e carried out b y one or two experienced technicians. The variability of m e a s u r e m e n t s are 3%. 6 The aortic a n e u r y s m w a s visualised longitudinally in a real-time picture a n d the m a x i m u m diameter p e r p e n d i c u l a r to the longitudinal axis defined. The echoes f r o m the walls w e r e o p t i m i s e d a n d after locking the echo-tracker to the inner surface of the a n e u r y s m walls the d i a m e t e r w a s calculated as the m e a n of three consecutive registrations. The height a n d b o d y w e i g h t w e r e m e a s u r e d in each patient a n d b o d y surface area calculated according to d u Bois' f o r m u l a (body surface area, c m 2 = w e i g h t °425 kg x height °72~, c m x 71.84). The a b d o m i n a l aortic size is d e p e n d e n t on age, b o d y surface area a n d sex. By using these variables the n o r m a l aortic d i a m e t e r could be calculated according to the n o m o g r a m s of Sonesson et al. (1994). 3 The n u m b e r of times b y w h i c h the actual a n e u r y s m diameter exceeds the predicted n o r m a l aortic d i a m e t e r w a s defined. M e d i a n (range) values are given, a n d differences b e t w e e n g r o u p s w e r e assessed
50
4O
30 ~9
E
10
M
F
(a)
~
M
F
(b)
Fig. 2a. The aneurysm diameter (mm) in males (m) and females (f). Fig. 2b. The aneurysm size calculated as the number of times by which the actual aneurysm diameter exceeds the predicted normal aortic diameter. Note the larger size in females. Eur J Vasc Endovasc Surg Vol 15, April 1998
T. L~nne e t a l .
310
Table 1. Size evaluation of abdominal aortic aneurysms.
Sex
n
Age (years)
AAA BSA diameter (ram) (m 2)
Predicted Predicted AO diameter3 AAA size
Males and females
147
Males
113
72 49-92 72 53-92 74 49-85 NS
48 40-55 48 40-55 46.5 40-55 NS
19.4 14.3-21.6 19.7 18-21.6 16.4 14.3-17.8 p<0.0001
Females
34
Significance males/females
1.85 1.42-2.37 1.89 1.47-2.37 1.63 1.42-1.95 p<0.0001
2.51 1.90-3.53 2.46 1.90-2.94 2.93 2.25-3.53 p<0.0001
Patients with abdominal aortic aneurysms, 40-55 mm (median)/range. using the M a n n - W h i t n e y U-Test at the level of significance p<0.05.
Results
The AAA diameter was 48 m m (40-55). The BSA was 1.85 m 2 ( 1 . 4 2 - 2 . 3 7 ) and the predicted AO size 19.4 m m (14.3-21.6). The calculated increase of size from the predicted AO diameter in the individual aneurysms was 2.51 (1.9-3.53), that is the spread of data doubled as c o m p a r e d to conventional diameter measurements. W h e n females and males were studied separately the AAA diameter was 46.5 and 48 mm, respectively (NS). Since the BSA was significantly lower in w o m e n than in men, 1.63 (1.42-1.95) and 1.89 (1.47-2.37), respectively (p<0.0001), also the predicted normal aortic size was lower, 16.4 (14.3-17.8) vs. 19.7 (18-21.6) (p<0.0001). Thus, the AAA diameter increase from the predicted size was larger in w o m e n than in men; 2.93 (range 2.25-3.53) vs. 2.46 (range 1.90-2.94), respectively (p<0.0001) (Table 1). Figure 1 shows the a n e u r y s m diameter normalised to 100% and the differences in spread of data (ranges) w h e n calculations were based either on the diameter in m m or the n u m b e r of times b y which the actual a n e u r y s m diameter exceeded the predicted normal aortic diameter. It is seen that the spread of data increases from 30 to 65%, i.e. b y a factor of 2 using the latter definition. Figure 2a shows the a n e u r y s m diameter in m m in the males and females respectively. No differences are seen. In Fig. 2b the a n e u r y s m size is calculated as the n u m b e r of times b y which the actual a n e u r y s m diameter exceeds the predicted normal aortic diameter. Note the significantly larger size in females as c o m p a r e d to males (p<0.0001).
Discussion
This study shows clear differences in the size of abdominal aortic aneurysms (AAA) w h e n evaluated in Eur J Vasc Endovasc Surg Vol 15, April 1998
m m as c o m p a r e d to the increase in size from the predicted value of the normal aorta in each individual. The latter m e t h o d m a y be preferred from a pathophysiological point of view and might add information concerning the growth and rupture patterns of AAAs. The prevalence of AAA has s h o w n an increasing trend within the last decades. This does not seem to be related to more reliable diagnostic methods alone. 7'8 Recent screening studies indicate a prevalence of AAA in m e n over 65 years of about 5%. 9'1° Further, there is a genetic influence with about one-quarter to one-third of the male near relatives to patients with AAA, also having AAA. 1~'12Several studies have s h o w n that aneurysms with a diameter >55-60 m m tend to increase in diameter and the n u m b e r of ruptures also increase in an exponential fashion. This varies, of course, between individual aneurysms ~3 and is w h y vascular surgeons r e c o m m e n d surgical treatment in AAAs >55-60 mm. In the same manner, AAAs less than 40 m m are treated conservatively. However, between 40 and 55 m m there is no consensus in favour of surgical treatment or not. Two large prospective r a n d o m i s e d studies are addressing this issue at the m o m e n t and will hopefully show the preferable treatment. ~4 However, several definitions of AAA are currently used. The Society for Vascular Surgery and the International Society of Cardiovascular Surgery (SVS/ISCVS) define an ane u r y s m as a 50% dilatation of the normal artery adjusted for gender and radiological modality. 2 Sterpetti et al. 1~ suggest a ratio of infrarenal to suprarenal diameters of > 1.5. Collin ~6defines an abdominal aortic a n e u r y s m as a maximal infrarenal diameter of 40 m m or more, or exceeding the maximal diameter of the aorta between the origin of the superior, mesenteric and left renal arteries b y at least 5 mm. W h e n comparing these definitions on p o p u l a t i o n data concerning frequency and prevalence, large differences are seen. Therefore, until a clear uniform definition is used, the true epidemiology of aortic a n e u r y s m s will not be elucidated. The SVS/ISCVS definition seems the most reasonable from a pathophysiological point of view. H o w ever, until recently there has been no knowledge
A Dynamic View on the Diameter of Abdominal Aortic Aneurysms
concerning the normal infrarenal aortic size. 1 In 1994 Sonesson et al. 3 published a paper that defined the normal aortic diameter in relation to sex, age and body surface area with nomograms to be used in the prediction of an individual aortic size. For example, if the maximum diameter of an abdominal aortic aneurysm is 40 mm in a 75-year-old male with a BSA of 2.3 m 2, the expected aortic diameter obtained is 21.9 mm. 3 By dividing 40 m m by 21.9 it is shown that the aneurysm in this patient has increased 1.83 times the expected diameter at this age and body size. On the other hand, if an abdominal aortic aneurysm of 40 mm is encountered in a 50-year-old female with a BSA of 1.4 m 2, the normal aortic diameter in this patient is defined to be 13.9 mm 3 and the aneurysm of this patient has enlarged by 2.88 times. Thus, a discrepancy of almost 100% in enlargement is seen. Stringfellow et a]. 17 used finite element analyses to determine the wall stress distribution in AAA. They found that an aneurysm of similar size arising from a small aorta had much greater wall stress than had an aneurysm arising from a larger aorta. Further, from a biological point of view it seems reasonable to assume that a 40 mm large aneurysm that has expanded 2.9 times from the original size has a more diseased arterial wall than the 40 mm large aneurysm that has expanded only 1.9 times. Similar reasoning was used by Ouriel et al. I8, who standardised the aneurysm diameter to the transverse diameter of the third lumbar vertebrae body as an index of body size. They found this to be a predictor of rupture when the threshold ratio of 1.0 was used. The study by Limet et aI. 19 shows that the risk of rupture is closely related to a relative change in size of an aneurysm more than the change in mm. However, they related the diameter change to the initial aneurysm size at diagnosis and not the predicted normal aortic size. It is possible that their figures would improve, if this had been performed. Another point of concern is the sex-specific frequency of AAA. Several studies have shown a 2-4 times higher AAA frequency in men than in women and that aneurysms start to appear later in women. However, these data are based on a standard diameter definition without any differences between sexes taken into account. With the differences in normal aortic diameter between males and females born in mind, this might indicate an underestimation of the number of aneurysms in women. Further, this might also be the case in the gender-specific inheritance patterns with male near relatives more prone to aneurysm formation than corresponding females. 11'2° To our knowledge no study so far has addressed the prognosis of small aortic aneurysms in relation to sex. However,
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several large studies, such as The UK Small Aneurysm Trial, The Veteran Aneurysm Detection and Management Study and The Canadian Small Aneurysm Treatment Trial, may elucidate this factor.21The evaluation of our AAAs showed that women had significantly larger AAAs than males, when the size of the aneurysm was related to the predicted normal aortic diameter in the individual patients (Fig. 1). Whether this fact may be of clinical importance cannot be elucidated by this study, since the number of AAAs were relatively small and therefore no outcome was studied. However, Katz et al. (1997)22 showed, in a study on 41 358 patients with AAA, that women were less likely to undergo aortic reconstruction and when they did were less likely than men to survive to discharge. 22 The reliance on AAA diameter as the primary indication for surgery might have resulted in referral for surgery at a later stage of disease among women. This indicates a need for individualisation in the evaluation of an aneurysmal size. In summary, no consensus so far has been reached concerning diagnostic criteria for AAA. A reasonable point of view is to define an aneurysm as a permanent, localised dilatation of an artery exceeding 50% of the expected normal diameter, this now being possible due to knowledge of the normal aortic size. This may facilitate the question of how to treat aneurysm of smaller size (i.e. 40-55 mm) that in fact may be large in relation to the individual's normal predicted aortic diameter.
Acknowledgements This study was supported by grants from the Medical Faculty, Lund University, the Medical Research Council (no. 12661), the Funds of Malta6 University Hospital, and the Swedish Society of MedicIne. We thank the staff of the Department of Clinical Physiology for skilled assistance.
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Accepted 15 October 1997