Abdominal aortic aneurysm

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Results

The population consisted of 31 individuals, 16 males and 15 females in the ages 18 to 26 years. The majority of the population had a normal BMI (Figure 5) and were between 18-20 years old (Figure 6). Table 1 and 2 demonstrates the paired sample -test for the longitudinal and transvers measurements obtained by two observers. Mean difference being 0.6 mm for the longitudinal axis and 0.05 mm for the transverse axis. The paired sample T-test showed a statistical significant difference in the longitudinal measurements between the two observers (p = 0.004). The difference between the observers´ transverse measurements did not show a statistical difference (p = 0.802) (Tabell 1-2). The interobserver variability is statistical significant between the observers’ longitudinal measurements but not between the transverse measurements. The Bland-Altman plots showed that the differences vary across the mean and that there is no systematic measurement error made by the observers, meaning that the differences are not proportional to the mean. Both plots also have over 95% of the differences within the limits of agreement and the range of their limits off agreement are all below 5 mm (Figure 7-8).

Discussion
Results

The aim of this thesis was to examine if it were any differences between the two observers when measuring the abdominal aorta with ultrasound in the longitudinal and the transverse axes. The purpose has been achieved for this thesis since the differences between the observers were determined, considering there was a statistical significant difference between the longitudinal measurements but not between the transverse measurements. According to NAAASP, it should not differ more than 5 mm between two observer’s measurements. In this study, above 95 % of the differences for longitudinal and transverse measurements were within the limits of agreement. Since the range of the limits of agreement were smaller than 5 mm, the differences has therefore no clinical relevance (5, 22). The measurements with the lowest mean difference between the two observers (0.05 mm) were the ones taken in the transverse axis. This might be explained by studies that have shown higher repeatability on measurements taken in this axis (18). The transverse axis is also the most common plane to obtain measurements in studies (5). Another possible explanation for the transverse measurements lower mean difference compared to longitudinal could be because the longitudinal measurements are more dependent of the angle placement of the caliper marker against the angle of the abdominal aorta. As seen in figure 3, the abdominal aorta in the longitudinal axis is angled down in the ultrasound image towards the patient’s head (left side of the image) and more angled up towards the patient’s lower body (right side of the image). Using the caliper tool and performing a vertical measurement, anterior to posterior in the ultrasound image of the abdominal aorta would result in a larger diameter. The caliper tool should be placed orthogonally, at a 90 degrees angle to the abdominal aortic wall (10). This angulation problem has also been discussed when ultrasound is compared to the gold standard, computed tomography (CT). When the two methods are compared, ultrasound repeatedly underestimated the diameter. But when measuring with orthogonal plane and then comparing with CT the underestimation for ultrasound decreased and resulted in a mean difference of 0.8 mm (10). The observers focus was to obtain the optimal image and a clearly visible aorta, more effort could have been addressed the angulation of the caliper tool but also the angulation of the transducer. The increased mean difference within the longitudinal measurements could therefore depend on the observer’s different angulation of the caliper tool and the angulation of the transducer. In a study comparing the reproducibility of ultrasound in nine different studies the maximum variability detected was 10 mm. All studies used different ultrasound machines, sonographers had different medical background and seven different measurements methods or combination of methods were used. Despite the variations between the studies, most of the studies did however show an inter-observer variability below the recommended limit of 5 mm and that an abdominal aortic scan can be performed by operators with varying experience (5). Like this study, the observers in eight out of nine studies performed real time ultrasound examinations and obtained the images in which the diameter would be measured. In the ninth study the interobserver variability was compared in still images. Even though the observers were inexperience in this study, the limits of agreement (Figure 7-8, -2.2 mm to + 2.4 mm for the transverse and -1.6 mm to + 2.8 mm for the longitudinal measurements, ) are closer to the lower range of the limits of agreement in the study (− 1.9 to + 1.9 mm and − 10.5 to + 10.4 mm) (5). It was also discussed that the patient’s waist circumference could affect the inter-observer variability since increased depth limits visibility (5). Since only 3 % of the study subjects in this study had a BMI above normal (Figure 5), the variations in measurements between the two observers are more likely to be explained by other factors than BMI. Despite that the observers had no previous experience of abdominal aortic ultrasound scanning the measurements were still within the recommended limit of 5 mm. However, the true diameters of the study subjects are still unknown since the measured diameters by the two observers were never controlled by experienced observers or compared to a CT examination which is the gold standard for abdominal aortic measurements (10). The population examined in this study was chosen to improve image quality and might have been a component for the low variability since they were young adults with a normal BMI, making the ultrasound examination less challenging for the observers. If the study would have been performed on men over 65 years of age, the target group for screening programmes, the data collection could have been more demanding and the outcome another. Two different studies investigated the effect of ultrasound training with novice operators. The studies concluded that education to perform abdominal aortic scans with ultrasound can make novice operators perform ultrasound examinations on the abdominal aorta. But one study also claims that despite the training they failed in more demanding cases (28). When screening for AAA, challenging cases will appear, and it is therefore important that the operators have enough experience to achieve correct results. The differences between these studies are the medical backgrounds of the novice operators and how they were trained (28, 29). Apart from the patient’s physical state, the treatment of an AAA is dependent on its diameter. A difference of 10 mm will therefore have significant clinical relevance since it affects the decision whether the AAA should be treated surgically or not. Since the diameter also conclude the existence of an AAA and if the patients should be followed over time, a difference of 10 mm is not acceptable (5). The average mean diameter in this study, both male and female, was 11.1 ± 1.2 mm which is smaller compared to another study performed on a Chinese population, 12.9 ± 2 mm (14). The smaller mean could depend on the younger population, 18-26 years compared to 45-80 years. The average mean diameter is however much smaller compared to previous studies performed on a western population, 19.1 ± 2.9 mm and 18.6 ± 2.2 (14). Since AAA is more common in countries with a Western lifestyle, most studies have been performed on a Caucasian population. Not many studies have been performed on an Asian population and the few published demonstrates various results regarding the incidence of AAA (1, 14, 16). They do however conclude that the average diameter of the abdominal aorta is smaller compared to the diameter of a western population, despite adjusting for variations in body size. Because of this, a different criterion for AAA may be needed for an Asian population since the abdominal aorta is smaller from the beginning. The criteria 30 mm used may not be suitable and the criteria of an 50% increase compared to a normal segment may therefore be preferable (14). All factors discussed highlight the importance for the training and experience needed to become a professional within ultrasound. Besides the hours of training, great knowledge regarding the anatomy and physiology of the abdominal aorta is required (28, 29).

Method

One limitation with the method was the observers´ lack of experience. The inexperience could have resulted in variations of the identification of the anatomic location of the abdominal aorta. Meaning that the measurements were obtained at different segments of the aorta. Although limited experience within ultrasound, the observers did only have minimal training connected to abdominal aortic examination and the LELE method prior to the study. The ultrasound equipment was changed during the data collection since the images from the first machine were of poor quality. A newer ultrasound machine might have improved the image quality providing more optimal conditions for the measurements. The equipment was also unfamiliar to the observers. The data was collected for one day and due to this limited time, only one measurement was obtained in each axis. Three measurements taken in each axis and a calculated mean for each observer and axis could have made the results more reliable instead of one single measurement. Since the observations were performed by inexperienced observers the results of the interobserver variability cannot be generalised to trained sonographers but could apply to other students with the same level of training. A second limitation with the method is that the observers, instead of doing a visual estimation of the maximum diameter, could have used electrocardiography (ECG) to make sure the measurements were taken at the same time in the heart cycle considering that the arterial wall fluctuates 2-4 mm during the heart cycle. The fluctuation occurs independent of the presence of an AAA, but ECG is not required in screening programmes despite that the measurements could be made more accurate (10). Even though it might not be relevant for screening in clinical practise it could have been applicable in this study to improve reliability of the measurements since without a connected ECG a difference of 2-4 mm could simply depend on the fluctuation of the arterial wall between diastole and systole if the measurements are not taken in the same cycle (10). A curved transducer with the frequency of 3,5 MHz was used since it provided the best visualisation of the abdominal aorta and therefore optimised the conditions for the measurements taken with the LELE method (10). This study uses the LELE method since it is the method used in the Swedish national screening programme but also because it measures at points that are easier to distinguish in the ultrasound image due to increased difference in acoustic impedance resulting in stronger echoes (11, 12). The three different methods, ITI, LELE and OTO, that can be used to measure the abdominal aorta with ultrasound are all well discussed in different articles but differ as to which method is preferable. In two different studies, OTO and LELE were deemed the better, however the same method that was suggested as the best was also the method that the sonographers of the studies use in everyday practise (12, 18). This also demonstrates that experienced sonographers, practising the method for obtaining the measurement, will improve the reliability of the results. However, considering that LELE was assessed as the preferable method in more than one study it became the method of choice in this study (10, 12). In contempt of the previous, an additional study suggests that LELE and OTO are two different variations of the same method since computed tomography (CT) have showed that the aortic wall is 2 mm thick. The general difference between OTO and LELE is also 2 mm and the difference between the OTO and LELE could simply depend on the aortic wall thickness. The difference between the two methods are that LELE measurements are calculated from anterior outer wall to inner posterior wall while OTO measures from anterior outer wall to posterior outer wall. Between the inner posterior wall to posterior outer wall the thickness is approximately 2 mm. The study therefore suggests that LELE and OTO are two versions of the same method (10). Further research with a larger sample, ECG and experienced observers would be needed to increase reliability and generalizability of the results. It would also be interesting to examine the average diameter of both Swedish and Vietnamese students due to the small mean aortic diameter in this study. More research is also needed regarding abdominal aortic aneurysm in Asia as well as an international standardised method for abdominal aortic measurements with ultrasound to improve interobserver variability.

Conclusion

This study showed no statistical significant difference between the observers’ transverse measurements. There was a statistical significant difference between the two observers’ longitudinal measurements, however, its clinical relevance can be considered as low since the differences were within the recommended limit of 5 mm.

Acknowledgements

The authors would like to express their gratitude towards the staff and students at the Da Nang University of Medical Technology and Pharmacy in Vietnam for their help and warm welcome. A special thanks to our supervisor in Vietnam, Dr. Phuong, whom without the collection of data would have been considerably more challenging. The authors would also like to thank the program of Minor Field Studies (MFS), The School of Health and Welfare and Jönköping University’s International Relations for the scholarship funding and opportunity to perform our thesis in Da Nang, Vietnam. A final thanks to our Swedish supervisors Emma Kramer and Ida Åström Malm.

Introduction 
Background .
Abdominal aortic aneurysm
Ultrasound
Screening worldwide
Challenges with ultrasound
Aim 
Research methodology 
Design
Equipment and measurement method
Imaging acquisition
Statistical analysis
Ethical considerations
Results 
Discussion
Results
Method
Conclusion 
Acknowledgements

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Interobserver variability when measuring the abdominal aorta with ultrasound