Aortic knob diameter in chest radiographs of healthy adults in Uganda.

The burden of cardiovascular diseases, such as aortic and degenerative diseases grows in the aging population. Chest radiograph still plays an important role in the diagnosis of cardiovascular diseases. Aortic knob diameter in chest radiographs can be used to evaluate early changes of the aortic structure and together with clinical and laboratory ndings. This study was aimed at determining the mean values of aortic knob diameter among healthy adults in Uganda. We conducted a descriptive cross-sectional study in three selected hospitals in Kampala Uganda. All participants had normal chest radiographs without radiological evidence of cardiovascular disease. Chest radiograph ndings extracted included aortic knob diameter, aortic arch diameter, transverse heart diameter and transverse thoracic diameter. All lms were independently examined by two experienced radiologists. Study participants included asymptomatic participants referred for chest radiograph examination for the purpose of pre-employment and pre-travel medical check-ups and were recruited in this study. Participants with normal chest radiograph (Posterior anterior view, erect), and age between 18-90 years, with no thoracic skeletal deformity, no known cardio-vascular disease, and normal blood pressure for age (blood pressure chart) were included in the study. Participants whose chest radiographs had signicant rotation (>0.5cm) or artifacts were excluded from the study.


Abstract Background
The burden of cardiovascular diseases, such as aortic and degenerative diseases grows in the aging population. Chest radiograph still plays an important role in the diagnosis of cardiovascular diseases.
Aortic knob diameter in chest radiographs can be used to evaluate early changes of the aortic structure and together with clinical and laboratory ndings. This study was aimed at determining the mean values of aortic knob diameter among healthy adults in Uganda.

Methods
We conducted a descriptive cross-sectional study in three selected hospitals in Kampala Uganda. All participants had normal chest radiographs without radiological evidence of cardiovascular disease.
Chest radiograph ndings extracted included aortic knob diameter, aortic arch diameter, transverse heart diameter and transverse thoracic diameter. All lms were independently examined by two experienced radiologists.

Conclusion
The aortic knob diameter was higher in males and there was a positive correlation with age, aortic arch diameter, transverse heart diameter and transverse thoracic diameter. Aortic knob diameter measurements should be done on digital screen than printed x ray lms.
Background Cardiovascular disease accounts for three quarters of all global estimated deaths in the low-and middleincome countries (1). Chest radiograph remains an important tool for investigation of cardiovascular conditions despite the advent of newer and more sophisticated imaging modalities such as echocardiography, computerized tomography (CT) and magnetic resonance imaging (MRI) in the world (2). This is mainly due to more readily available and comparatively cheaper costs of performing chest radiograph and still provides valuable clues to diagnosis of cardiovascular pathology (3).
The aorta is a major vascular supply of the human body and also plays an important role in the control of systemic vascular resistance and heart rate. This control is done via pressure-responsive receptors located in the ascending aorta and aortic arch (4). An increase and decrease in aortic pressure results in a decrease of heart rate and systemic vascular resistance, and an increase of heart rate and systemic vascular resistance respectively (5).
The Aortic knob is an important structure that refers to the hump-shaped contour of the aorta seen in a frontal chest radiograph on the left mediastinal silhouette (6). Abnormal aortic knuckle shape, enlargement and aortic nipple, may give the rst clue of cardiovascular disease (7). The aortic knob can be enlarged due to increased pressure ow in aorta, increased volume or changes in the elasticity of its wall. In adults, aortic knob enlargement can be seen in systemic hypertension, valvular insu ciency, aortic dissection, traumatic aortic injury, thoracic aortic aneurysm, and atherosclerosis (8).
Aortic knob diameter (AKD) has been shown to be a good predictor of target organ damage with a sensitivity ranging between 70-90% for different cardiovascular conditions, even in early disease (9).
Uganda is a low income country and yet is experiencing a shift in major causes of death with coronary artery disease and other forms of heart failure on the rise (13). And the normal AKD in the literature were taken from studies done among Caucasians, Americans, South Koreans, Indians and West Africans. These studies showed that variations exist amongst different populations, age groups and between sexes (14). In this study therefore, we sought to determine the normal AKD amongst healthy adults in Ugandan.

Study design and setting
This was a descriptive cross-sectional study conducted in three selected tertiary hospitals -CASE hospital, St Francis Hospital and Kiruddu Hospital -Kampala, Uganda from January 2020 to June 2020. Chest radiographs are performed as part of ongoing recruitment in government agencies like police, army and local governments. Case hospital is an urban, private, tertiary hospital in Kampala, Uganda. It is located at 67 -71 Buganda Road, on Nakasero hill. Around 100 chest radiographs are performed per a month.

Study population
Study participants included asymptomatic participants referred for chest radiograph examination for the purpose of pre-employment and pre-travel medical check-ups and were recruited in this study. Participants with normal chest radiograph (Posterior anterior view, erect), and age between 18-90 years, with no thoracic skeletal deformity, no known cardio-vascular disease, and normal blood pressure for age (blood pressure chart) were included in the study. Participants whose chest radiographs had signi cant rotation (>0.5cm) or artifacts were excluded from the study.

Sample size
A sample size of 294 participants was used using the formula proposed by Rosner B. 2015 as summarized below.

Study Procedure
Study participants who gave their consent were physically examined and enrolled into the study. A total of 294 participants' postero-anterior (PA) chest radiographs were obtained. The PA chest radiograph of all the candidates were taken in the erect position with a lm focus distance of 1.8m. The exposures were made at normal arrested inspiration. Participant's name, sex, age, height and medical history were recorded. Normal chest radiograph was con rmed by two independent radiologists. Parameters such as AKD, transverse thoracic diameter (TTD) and transverse heart diameter (THD) were evaluated for each patient ( gure 1). The aortic knob diameter was measured as the maximum transverse diameter from the lateral border of the air in the trachea to the lateral border of the aortic knob (15). The Aortic arch diameter (AAD) was measured as the most lateral extension to the right and left of the midline at the level of the aortic knob. TTD was measured as the maximum horizontal distance between the internal margins of the chest wall at the level of the right hemi diaphragm (16, 17). THD was measured as the sum of the maximum projection to the right and left heart borders from midline (16, 17).

Data analysis
A structured data collection tool was used to collect bio-demographics and relevant history (Appendix 1). Data was analyzed using STATA version 16. Descriptive statistics for continuous variables were presented as mean and standard deviation. Spear man rank correlation was used to test for the correlation between the aortic diameter measurements with continuous independent variables. In addition, Student t test was performed to test for the mean difference of AKD between groups. A p < 0.05 was considered statistically signi cant.

Results
A total of 294 participants, of whom 204(69.4%) were male and the age range between 18-89 years were recruited in the study. Overall, 196 (66.7%) participants were attending the hospital for pre-employment medical exam, 92 (31.3%) came for annual medical checkup exams, and 6 (2%) pre travel exams. The mean age of the participants was 35.3±10.5. (Table 1). Male participants had signi cantly higher mean AKD compared to their female counterparts (3.14±0.34cm versus 2.77±0.37cm, p -0.000). Those in the age group of 50-59years had signi cantly higher means than younger people in the age group of 18-29 years ( Table 2 and Table 3). The mean AKD was higher when measurement was done on a digital screen than on the chest radiograph lm (3.03±0.393cm versus 2.96±0.392cm, p -0.000). However, the males still had signi cantly higher AKD compared to female and unit increase in age would also increase the AKD (Table 5). The average AKD of the sample population was 3.03±0.39cm, the smallest was 2.2cm and the largest was 4.1 cm. The mean AAD of the study population was 5.63±0.64cm and average cardiothoracic ratio is 0.446±0.037 with highest being 0.522cm and lowest 0.366cm (Table 6). AKD was strongly and more positively correlated with age among the females than in males, the coe cient was higher among the females and the relationship was statistically signi cant at 95% con dence level r=0.7411 p-value 0.000 for females vs. r=0.5191 p-value 0.000 for males. Strong positive correlation was also observed between the AKD and AAD for both male and female. However, AKD was weakly correlated with THD, TTD and cardiothoracic ratio respectively (Table 7). Weight and height were weakly correlated though positively associated with the aortic knob diameter across both genders. However, it was all higher among the female counterpart and the relationships were not statistically signi cant at 95% con dence level.

Discussion
This study sought to determine the aortic knob diameter in chest radiographs of healthy adults in Uganda. The results of our study showed that AKD increases with age in both sexes. This can be explained by geometric and functional alterations seen with aging (18, 19). Studies in India, Nigeria, Zambia, United States and South Korea also showed similar results (10,11,(20)(21)(22).
AKD was higher in males than in females. Among males, the mean average diameter of aortic knob was 3.14±0.34cm. Population based large cohort study in Netherlands showed that men had higher mean thoracic aorta diameter than women but provided no explanation as to why (23). Mean AKD ndings in Indian population (20) and South Korean population (11) were comparable with our ndings (3.10cm±0.334 cm and 3.42±0.478 respectively). The mean aortic knob diameter among females was 2.77±0.37. This value was smaller in comparison to the value obtained in the Indian (20) study of 3.076cm±0.39 cm and the south Korean(11) study of 3.00±0.498 (Table 4). Male-3.14±0.34cm Female-2.77±0.37cm.
The mean aortic knob and arch diameters were higher in males than females which is consistent with the ndings of Ikeme and colleagues (24) and Ray and colleagues (20). On the contrary, a study done in Jamaica reported a higher aortic arch size in females than males (25). These ndings were attributed to the higher blood pressures in females than males (25). Kim and Choi observed that an aortic knob diameter (AKD) width of more than 4 cm occurred more frequently in patients with thoracic aortic disorder as compared to normal subjects (26). In this study, there was no female with an AKD > 4cm but there were 2 males whose AKD was slightly above 4cm measured on the digital screen. These were healthy males with no cardiovascular disease and normal blood pressure for age.
The AAD is another measurement for evaluation of the size of the aorta on chest radiograph (27). In this study the mean of the AAD was 5.6 ±0.64cm. This value is comparable to other values in different studies by Yousef and colleagues in Sudan (28) 5.3±0.6, and 5.3 by Umerah in Zambia (22). Anyanwu and Agwuna measured the width of the aortic shadow as the sum of the maximum extension of the aortic shadow to the right and left of the midline and found the mean of AAD varied between 4.7±0.5 cm in the Nigerian population (29).
The mean THD and mean TTD were 12.9±12.6 cm and 29.07±2.78cm respectively. This was comparable to 11.9 ± 9 cm of mean THD and 27.8± 7cm of mean TTD in the Sudanese population (28). This means, our study population had higher mean of transverse heart and thoracic diameters. Aortic knob diameter correlated positively with chest and heart diameters in the Sudanese and Nigerian studies (21,28).
In this study, we found that there was slightly higher mean AKD, when measurements done on a digital screen than on the x-ray lms and the difference were statistically signi cant. This can be explained by the fact that digital measurements on screens are correct to the nearest millimeter, which may be di cult to achieve on the plain lms due to approximation. Hence, measurements should be done on a digital screen than x-ray lms in practice. This study also demonstrated that weight and height did not signi cantly affect the size of the aortic knob.
Strengths of our study include, being a prospective study with relatively large sample size and its multicentric nature. The limitation of the study was that it was a hospital-based study which could introduce selection bias. Moreover, the number of female participants was much smaller than the male counterparts.

Conclusions And Recommendations
The aortic knob diameter was higher in males and there was a positive correlation with age, aortic arch diameter, transverse heart diameter and transverse thoracic diameter. We recommend aortic knob diameter measurements should be done on digital screen than printed x ray lms and sex-speci c aortic knob diameters should be utilized in clinical practice.  Figure 1