Case

A 71-year-old female with history of hypertension presents to the ED worsening lower abdominal pain and constipation for one week. She describes the pain as intermittent, crampy, and "like gas but worse." There is no temporal relationship to food, associated fever, chills, nausea, vomiting, diarrhea, urinary symptoms, or bleeding. The patient is compliant with her blood pressure medications but admits to a 50-pack year smoking history.

Initial vital signs are; Temp- 98.1F, HR 70, RR 20, BP 125/72. The patient is in mild distress due to pain. Lung and heart sounds are normal. Abdominal exam is significant for an obese, mildly distended but non-tender abdomen with normal bowel sounds and a mass just left of midline.

Discussion

Abdominal Aortic Aneurysms (AAA) are elusive time bombs that kill 15,000 Americans each year. 75% of patients are not aware of the AAA when complications first occur contributing to a 30% rate of initial misdiagnosis. The classical triad of hypotension, back pain, and pulsatile abdominal mass occurs in only 50% of ruptured abdominal aortic aneurysms. AAA’s commonly masquerade as a variety of other diagnosis including renal colic, diverticular disease and gastrointestinal bleeding. Furthermore, the sensitivity of detecting AAA by physical exam is only 68%. 90% of patients with ruptured AAA will die before ever reaching the hospital (1).

Limited ED US demonstrating an anterior to posterior diameter of greater than 3cm from outside wall to outside wall in the transverse plane supports the diagnosis of AAA. The entire abdominal aorta to the proximal illiacs must be visualized. Most AAA's are infrarenal and smaller often lethal sacular aneurysms can be missed without careful inspection. Bowel gas especially in the transverse colon must be displaced with gentle yet consistent pressure or adequate visualization from above and below. The probe is rocked downwards from above and upwards from below to bring the entire aorta into the scanning plane. Repositioning the patient in the left lateral decubitus position and placing the probe in the right anterior axillary line utilizes the liver as an acoustic window providing an alternative view in difficult patients (1).

The outside walls of the aorta must be carefully identified since intramural thrombus creates a false lumen much smaller than the actual aneurysm. When obesity, persistent bowel gas, or a tortuous aorta prevents complete visualization additional diagnostic test must be used to definitely exclude the diagnosis. Although US is extremely reliable in determining whether an AAA is present, CT scan is often next performed to detect the intra or retroperitoneal bleeding in stable patients.

Limited ED US for AAA improves patient care by accurately and dramatically decreasing the time to diagnosis and transfer to the operating room. Multiple studies have demonstrated the sensitivity of Limited ED US for the detection of AAA to be 100% even in the hands of novice physicians with limited training (1). The use of US reduces the time needed to diagnosis from the average 83 minutes needed to obtain and read a CT scan to an average of 5.4 minutes. Other modalities such as MRI and angiography require prolonged and dangerous trips outside the friendly confines of the ED (2).

In our institution, limited ED US to Rule out AAA is performed as part of the physical exam in both stable and unstable patients. In the stable middle age to elderly patient with atraumatic back pain, US is performed as a diagnostic test to rule out the presence of an AAA without the need for additional CT scan. In the high-risk patient, US is used to evaluate for the presence of an AAA and the need for additional testing or immediate transfer to the operating room. In the stable patient, the presence of an AAA on US prompts an immediate consult to vascular surgery, type and cross match and emergent CT scan. In the unstable patient, the presence of an AAA on US prompts immediate consultation with vascular surgery and transfer to the OR without further diagnostic testing.

Conclusion

An emergent CT scan confirmed the presence of the AAA with extension to the right iliac artery. The patient was immediately admitted to the SICU for urgent repair by endoscopic grafting. In summary, limited ED US provides a readily learned, extremely sensitive, rapid, and life saving tool for the diagnosis of AAA.

References

1. Barkin AZ, Rosen CL. Ultrasound detection of abdominal aortic aneurysm. Emerg Med Clin North Am 2004; 22(3):675-682.
2. Plummer D. Abdominal Aortic Aneurysm. Emergency Ultrasound. McGraw Hill, 2007: 129-141.