Background and Indications for Examination
Abdominal aortic aneurysm (AAA) has an adult incidence of 2%–4%, with approximately 10% of these cases occurring in people over the age of 65. The prevalence increases with risk factors for vascular disease, particularly smoking and hypertension. AAA has a male to female preponderance of 8:1 and may be present in as many as 10% of elderly male patients with at least one risk factor.
The rapid and accurate diagnosis of AAA at the bedside is important in decreasing the morbidity and mortality that may be associated with this disease entity. Bedside ultrasound has been shown to be a quick and reliable method to diagnose AAA in the critical care setting. Sensitivity and specificity of ultrasound are high when the aorta is visualized completely.
AAA rupture is among the top leading causes of death in the United States. When aneurysms rupture, they are highly lethal, with a mortality rate exceeding 50%. Ultrasound is not sensitive for rupture, as most ruptures occur into the retroperitoneum, which is a difficult area to visualize with ultrasound. In a stable patient, computed tomography (CT) still represents the best method for defining the extent of the aneurysm and the presence of leak or rupture. Although ultrasound does not detect rupture well, it has been shown to expedite care in patients who are hemodynamically unstable. In hemodynamically unstable symptomatic patients (abdominal pain, flank pain, back pain), the presence of an aneurysm is likely to mean leak or rupture and should prompt immediate preparation for operative intervention.
Ultrasound may also visualize aortic dissection. While ultrasound should not be used to rule out dissection (sensitivity is not good), specificity is high when a flap or false lumen is visualized. In particular, thoracic aneurysm and dissection are emergent, deadly diseases that may also be seen using bedside cardiac ultrasound, and may be a cause of pericardial effusion or tamponade. Contrast-enhanced CT, magnetic resonance imaging (MRI), and transesophageal echocardiography (TEE) remain the modalities of choice for thoracic aortic pathology.
Probe Selection and Technical Considerations
Lower frequencies, which provide better penetration, may be helpful in obese patients or when significant bowel gas is present.
Tissue harmonics may help to sharpen the image and improve visualization of the aorta and associated deep vessels.
The focal zone should be adjusted and placed at the depth of the aorta. This improves the lateral resolution of the image.
Color-flow Doppler detects blood flow and therefore is helpful in identifying a blood vessel and distinguishing it from other artifacts that may be present in the abdomen. It is important to angle the probe in order to avoid having the beam perpendicular to the blood flow, which will erroneously appear on the screen as a lack of color. In addition, by decreasing the pulse repetition frequency (PRF, also known as scale) or increasing the color gain may help to detect color flow.
Time-gain compensation (TGC) can be adjusted in order to increase the far gain of the image. This produces an enhanced brighter signal returning from the far field and a resultant clearer image. Adjustment of this control may be useful in obese patients with difficult anatomy.
It is important to start with adequate depth, but it should be decreased appropriately once the landmarks are identified so that the structures of interest make up approximately three-fourths of the screen. An image that is either too deep or too shallow can be disorienting.
Normal Ultrasound Anatomy
The aorta originates at the left ventricular outflow tract (LVOT), distal to the aortic valve. The aortic root and descending thoracic aorta are usually best seen and measured with transthoracic echocardiography on a parasternal long-axis view of the heart. The descending aorta can be seen behind the heart on this view, which is illustrated in the echo chapter of this book. In some patients, the aortic arch can be seen using a suprasternal notch view. The sonographer can follow the carotid arteries down to their union with the brachiocephalic artery and the aortic arch. This view is challenging due to the presence of venous structures, as well as the lungs and trachea.
The aorta enters the diaphragm through a hiatus at T12 to become the abdominal aorta. The vessel lies just anterior to the spine and follows its curvature anteriorly as it travels distally.
In a transverse view, the probe indicator should be pointed to the patient’s right side. In this orientation, the vertebral body of the spine should be seen as a bright, curving hyperechoic structure that shadows distally. This is called the spine sign and serves as one of the landmarks for identifying the abdominal aorta. The normal aorta is circular in cross section and is seen anterior to the spine, on the right side of the screen (patient’s left). The second important landmark is the inferior vena cava (IVC), which can also be visualized in this view, on the left side of the screen adjacent to the aorta, in a circular or teardrop conformation. The spine and IVC are thus the primary landmarks used to correctly identify the abdominal aorta, and should be routinely identified in order to avoid common pitfalls (Fig. 5-1).
The first visible branch of the abdominal aorta is the celiac trunk. It exits anteriorly and branches into the splenic artery (coursing toward the right side of the screen), the common hepatic artery (coursing toward the left side of the screen), and the left gastric artery (not usually visualized on ultrasound). In a transverse orientation, the celiac and its two branches form the “seagull sign,” with the splenic and common hepatic artery forming the wings (Fig. 5-2).
The next branch seen is the superior mesenteric artery (SMA), which branches anteriorly off the proximal aorta distal to the celiac trunk. In a transverse view, the SMA is seen as a circle with some surrounding hyperechoic fatty tissue with an appearance similar to a “mantle clock” (Fig. 5-3). The celiac trunk and SMA can also be seen in a longitudinal or sagittal view when the probe is rotated clockwise 90° with the indicator pointing cephalad (Fig. 5-4).