AORTA By Dr. Robert Hyde University of Pittsburgh Medical Center INDICATIONS: • • • •
Pain in the back, abdomen, flank or groin Syncope or dizziness Unexplained hypotension Palpable abdominal mass KEY QUESTION:
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Is the abdominal aorta enlarged? TECHNIQUE:
A low frequency transducer (2-5 MHz) with large footprint (typically a curvilinear probe) is ideal for the assessment of the aorta. • Begin in the midline in the transverse plane, with probe indicator aimed toward the patient’s right. Identify the proximal aorta, spine, IVC, and celiac axis. o The spine is an important landmark and without visualization it is possible to mistake a more superficial structure for the aorta • While maintaining gentle downward pressure on the transducer, slide the transducer caudally towards the umbilicus. The celiac axis appears first and due to its appearance, it is sometimes referred to as having a seagull appearance (Figure 1). Measure the proximal aorta at or above the celiac axis, in the anterior-posterior plane, from outer wall to outer wall. • Continue to slide the transducer caudally and identify the superior mesenteric artery (SMA). The SMA begins about 1 cm caudad to the celiac axis and is surrounded by hyperechoic tissue. It is sometimes referred to as a having a “mantle clock” appearance (Figure 2). Measure the mid aorta just distal to the SMA, in the anterior-posterior plane, from outer wall to outer wall. • Continue to slide the transducer caudally until you reach the aortic bifurcation, usually at the level of the umbilicus. The distal aorta is measured just proximal to the bifurcation, in the anterior-posterior plane, from outer wall to outer wall. • As one progresses distally towards the aortic bifurcation, the aorta becomes more superficial. Adjust depth as needed. • Scan past the bifurcation and measure each common iliac artery. • Return to the subcostal space and scan the aorta in the sagittal plane, with the probe indicator aimed towards the patient’s head. Identify the aorta with spinal stripe, celiac axis and SMA (Figure 3). •
AORTA By Dr. Robert Hyde University of Pittsburgh Medical Center
Figure 1. Proximal Aorta-Celiac Axis
Figure 2. Mid Aorta-SMA
AORTA By Dr. Robert Hyde University of Pittsburgh Medical Center
Figure 3. Aorta Sagittal
MEASUREMENTS: • •
The diameter of the aorta should decrease as it courses inferiorly. An abdominal aortic aneurysm (AAA) is defined as having a diameter of > 3.0 cm (Figure 4) or an increase in diameter of at least 1.5 times that of the proximal segment. An iliac artery aneurysm is defined as having a diameter > 1.5 cm.
Figure 4. AAA
AORTA By Dr. Robert Hyde University of Pittsburgh Medical Center
PEARLS AND PITFALLS: •
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Classic presentation of abdominal/back/flank pain, palpable abdominal mass and hypotension is present in only approximately 25% of cases. AAA may masquerade as renal colic and urologic symptoms are estimated to be present in 10% of cases. The elderly population is especially at risk. Obesity and bowel gas can be challenging factors. Apply constant, gentle downward pressure with the transducer and use a large footprint probe if available. Measure the aorta in the transverse plane, perpendicular to the long axis of the aorta. This is especially important in ectatic aortas, where an oblique measurement may misrepresent the true transverse diameter. The most reliable measurement is anterior-posterior. Measurements taken in the sagittal plane are more likely to underestimate the true aortic diameter. Carefully measure the aorta from outer wall to outer wall. Atherosclerotic plaque and intraluminal clot may have variable echogenic appearance and should be included in the measurement of aortic diameter (Figure 5). Confirm the presence of any abnormality by imaging in two orthogonal planes. Do not mistake the IVC for the aorta. The key landmark to identify is the spine, with the IVC located towards the patient’s right and the aorta towards the patient’s left. Bedside sonography is a very useful test for detecting the presence of an abdominal aortic aneurysm. However, it is not the definitive test for aortic dissection. Keep in mind that the abdominal aorta is a retroperitoneal structure and the presence of retroperitoneal hemorrhage cannot be reliably identified by ultrasound; thus, additional imaging may be warranted.
AORTA By Dr. Robert Hyde University of Pittsburgh Medical Center
Figure 5. AAA with intraluminal clot