From the Guidelines
The aortic knob on a chest X-ray is the visible outline of the distal arch of the aorta as it curves from the ascending to the descending portion, appearing as a rounded prominence along the upper left border of the mediastinum, just below the clavicle, and its normal contour is crucial for evaluating signs of aortic dissection, such as disruption of the normally distinct contour of the aortic knob 1. When evaluating the aortic knob on a chest X-ray, it is essential to consider its size, shape, and position, as changes can indicate various cardiovascular conditions, including aortic aneurysm, hypertension, or atherosclerotic disease.
- Key features of the aortic knob include:
- A smooth, well-defined border without significant enlargement
- A rounded prominence along the upper left border of the mediastinum, just below the clavicle
- The presence of calcium deposits in the aortic wall, making it more radiopaque and visible against the darker lung fields
- Signs of aortic dissection on chest X-ray findings, as outlined in the 2022 ACC/AHA guideline for the diagnosis and management of aortic disease 1, include:
- Mediastinal widening
- Disruption of the normally distinct contour of the aortic knob
- "Calcium sign," which appears as a separation of the intimal calcification from the aortic wall of >5 mm
- Double density appearance within the aorta
- Tracheal deviation to the right
- Deviation of the nasogastric tube to the right The evaluation of the aortic knob is a critical component of chest X-ray interpretation, and any abnormalities should be thoroughly investigated to ensure prompt diagnosis and treatment of potential cardiovascular conditions 1.
From the Research
Definition and Location of the Aortic Knob
- The aortic knob is a radiographic landmark seen on chest X-rays, representing the distal aortic arch 2.
- It is formed by the foreshortened aortic arch and a portion of the descending aorta 3.
Clinical Significance of the Aortic Knob
- The aortic knob width is associated with subclinical left ventricular dysfunction in hypertensive patients, with a cutoff value of 37 mm having 85.9% sensitivity and 86.4% specificity 3.
- A larger aortic knob width is independently related to higher central systolic blood pressure in patients with known or suspected coronary artery disease 4.
- The aortic knob width reflects left ventricular diastolic function, with an inverse correlation between aortic knob width and peak filling rate and one-third mean filling rate 5.
- A greater aortic knob width is independently predictive of increased aortic pulse pressure in patients undergoing elective invasive coronary angiography 6.
Measurement and Positioning of the Aortic Knob
- The distance from the top of the distal aortic arch (aortic knob) to the left subclavian artery can be measured using three-dimensional computed tomography angiography 2.
- The carina may be a more reliable landmark for positioning the intraaortic balloon pump than the aortic knob, with a distance of 2 cm above the carina providing an adequate position for the IABP tip in 95.3% of patients 2.