Abdominal Aortic Aneurysm (AAA) is an outpouching at the area of the weakness in the abdominal aorta involving all three layers of the artery wall. An aneurysm is defined as an increase of greater than 50% from the vessel’s original size – for an abdominal aortic aneurysm, this equates to a diameter of approximately 3 cm. The larger the aneurysm, the greater the risk of rupture. AAAs are commonly diagnosed either incidentally or when they become symptomatic or rupture. Whereas morbidity and mortality are low for elective treatment of an AAA, outcome for a ruptured AAA (rAAA) remains poor even when prompt treatment is provided.
Case Study: A 67-year-old man who is a long-term smoker arrives in the emergency department (ED) via ambulance with complaints of severe back and abdominal pain. He reports that the pain is tearing in nature and constant. Upon arrival in the ED, the patient’s heart rate is 130, blood pressure is 70/40, and respiratory rate is 20. Glasgow Coma Scale (GCS) score is 15, and he is able to hold a conversation. Electrocardiography (ECG) shows sinus tachycardia with no ST-segment changes. Examination of the abdomen reveals a tender pulsatile mass. There is a high clinical index of suspicion for a symptomatic or rAAA.
In this particular case study, the appropriate diagnostic approach is ultrasound (US) which is an easy imaging study that most emergency physicians can perform effectively. In an emergency, aortic evaluation is often done as an extension of FAST (focused assessment with sonography in trauma), whereby the diameter of the abdominal aorta can be assessed for the presence of an aneurysm. The presence of free fluid, the visualization of an AAA, and the clinical correlation usually are strongly indicative of a symptomatic or rAAA. In an elective or clinical situation, US provides an inexpensive and effective method of screening for an AAA that may require treatment or ongoing surveillance. Studies show that aneurysm-related mortality can be reduced by employing ultrasound-based screening of at-risk population groups, capturing patients with previously undetected AAAs, and proceeding to appropriate treatment on the basis of the rupture risk.
In this case study, computed tomography angiography (CTA) is not appropriate, because of the patient’s hemodynamic instability, especially if abdominal US visualizes an AAA. Attempts to perform CTA will only delay the necessary surgical treatment. Nevertheless, CTA remains the gold standard for diagnosing rAAAs and may still be indicated if the patient is hemodynamically stable enough and if endovascular aneurysm repair (EVAR) for an rAAA (rEVAR) is being considered as an option. It may also reveal alternative pathologic conditions, such as ruptured viscera or aneurysms of arteries other than the abdominal aorta.
RISK OF RUPTURE
The presence of an aneurysm that is at least 3 cm in its largest dimension implies some degree of rupture risk. As might be expected, a larger aneurysm carries a higher risk of rupture and ensuing morbidity and mortality even when treated promptly. A smaller aneurysm still carries a risk of rupture, but the risk is so small that elective repair is not indicated, despite the low incidence of complications from such treatment.
Clinical trials support aneurysms smaller than 5.5 cm do not benefit from early intervention as compared with those larger than 5.5 cm. It has been suggested, however, that this threshold should be lowered to 5 cm for female patients, who are at greater risk for rupture with AAAs larger than 5 cm.
Ongoing surveillance is recommended for patients with aneurysms smaller than 5.5 cm; as a rule, surveillance should be more frequent in those with aneurysms larger than 4 cm. These patients should be on best medical therapy for optimization of cardiovascular risk status. Antiplatelet agents, statins, and smoking cessation have all been shown to decrease cardiovascular risks.
Large-bore access in the cubital fossa is mandated, but fluid resuscitation is not required in this particular case study because BP is sufficient to maintain cerebral and cardiac perfusion. Permissive hypotension prevents further blood loss from the rupture and improves the outcome of an rAAA. Attempting to elevate BP to the normal range might precipitate further intra-abdominal blood loss. Obviously, rAAA patients may present with a wide spectrum of shock, from the patient with a stable contained rupture to the patient who is essentially moribund.
CLINICAL AND SURGICAL INDICATIONS
Studies predicting outcomes in rAAA patients are derived from retrospective analysis of characteristics predictive of poor outcome. A Glasgow Aneurysm Score (GAS) higher than 85 is predictive of mortality. Similarly, a Hardman index score of 2 predicts a mortality of 80%. Nevertheless, such scores should not be used as the sole basis of the clinician’s decision whether to opt for operative management.
Clinical indications for surgical repair of noninfective aneurysms are: An aneurysm size in excess of 5.5 cm; rAAA; A tender or symptomatic aneurysm or an aneurysm that is growing rapidly (>10% annually). A tender aneurysm is an indicator of impending rupture; the rapid growth in size stretches the retroperitoneal tissue and causes pain. Because larger aneurysms can cause discomfort during examination, the examiner may be uncertain whether the pain a patient feels is due to the large size of the aneurysm or to true tenderness signaling impending rupture. In some cases, stranding in the periaortic tissue may be observed on CTA in a tender but nonruptured aneurysm. Similarly, rapidly growing aneurysms are thought to be associated with a higher risk of rupture because of their potential to grow substantially between surveillance scans. In these scenarios, it may be safer for the patient to undergo elective repair even if the aneurysm is smaller than 5.5 cm.
SURGICAL OPTIONS & TECHNIQUES
In a situation where the AAA has not ruptured, once the decision has been made to intervene, options are as follows: Open AAA repair and EVAR. Currently, with the advent of EVAR, elective open AAA repair is being performed less frequently; it is more commonly used in younger patients or in patients whose anatomy is not suitable for EVAR. As a general rule, the patient must have adequate cardiovascular and respiratory fitness and a life expectancy of at least 2 years. Open AAA repair can be carried out via either a transperitoneal or a left retroperitoneal approach, each of which has advantages and disadvantages. The transperitoneal approach affords good access to all vessels, including the common and external iliac vessels on both sides, and allows inspection of abdominal organs; however, it can be cumbersome and increases the risk of bowel injury in the setting of a previous laparotomy. The left retroperitoneal approach avoids bowel adhesions while accessing the aorta, especially the juxtarenal and suprarenal aorta; however, it affords only limited access to the right iliac vessels, making bifurcated repair difficult. Systemic heparin is frequently given in the elective setting but is generally avoided in cases of rAAA.
An aortic crossclamp is usually placed in the infrarenal position, but suprarenal clamps may sometimes be required until the sac is opened and depressurized and an infrarenal clamp can be placed. In some rAAA cases, supraceliac clamps may be required to gain control. A bifurcated graft may be required if the aneurysm involves the iliac vessels. Usually, the graft is sewn in as an inlay, but occasionally, the aorta may be transected and the graft sewn on in an end-to-end fashion. The inferior mesenteric artery typically is not replanted unless it is of a large caliber. The aneurysm sac is usually closed over the graft, with special attention to covering the upper anastomosis. It has been suggested that this may decrease the incidence of aortoduodenal fistulas.
For EVAR, various off-the-shelf devices are commonly available. However, there are some conditions that must be met to ensure success. The anatomy of the aneurysm is by far the most important factor dictating whether an endograft is a suitable choice for a given patient. – – – First, to ensure good proximal sealing of the endograft, an adequate “neck” is required; this is usually defined as 15 mm between the lowest renal artery to be preserved and the start of the aneurysm, though some devices only require 10 mm. This boundary is being challenged by the ongoing development of newer devices. For juxtarenal or pararenal aneurysms without an adequate neck, an alternative endovascular technique would be required, such as the use of an endograft plus chimney stents or the use of a custom-made fenestrated or branched device. – – – Second, because access to the aorta is commonly obtained via a transfemoral approach, adequate-sized access vessels must be available. Severely diseased, calcified, or tortuous iliac vessels or a stenosed aortic bifurcation may prevent passage of the endograft to the desired destination. Adjunctive procedures (eg, angioplasty, stenting, or placement of surgical conduits) may be performed to facilitate delivery of the endograft. Other anatomic factors (eg, neck angulation, thrombus in the neck, or tortuous anatomy) may persuade the surgeon to use one device in preference to others. Patient factors (eg, renal function) may also dictate whether EVAR is the best option.
The perioperative mortality associated with elective EVAR is approximately 1.5% in most major studies, which is significantly better than that associated with open aneurysm repair (~ 4.5%) in other trials. Long-term outcomes of EVAR have been widely studied; the annual rate of reintervention for stent-graft–related problems is approximately 5%, and the annual risk of rupture after implantation is approximately 1%.
Aneurysm- and graft-related complications include the following:
- Graft occlusion
- Renal artery occlusion
Endoleaks are divided into four types:
- Type I – Lack of seal at proximal or distal sealing zones, resulting in arterial pressurization of the aneurysm sac
- Type II – Backbleeding from patent lumbar vessels or the inferior mesenteric artery
- Type III – Graft dissociation or tear through the graft material
- Type IV – Graft porosity
Type I and III endoleaks require urgent treatment because the aneurysm sac remains pressurized and continues to be at risk for rupture. Type II endoleaks generally do not require treatment unless there is ongoing sac expansion.
SURGICAL OUTCOMES & SURVIVAL
- Perioperative mortality for EVAR ranged from 0.5% to 1.7%, whereas open repair was in the range of 3-5%.
- The combined rate of operative mortality and severe complications was 4.7% and 9.8% for open repair.
EVAR is undoubtedly associated with lower perioperative morbidity and mortality, but the survival benefit seems to be lost over the longer term.
At 2 years after intervention, the overall survival benefit of EVAR is lost. The similarity in overall mortality was due to an increased proportion of cardiovascular-related deaths in EVAR patients. At 6-year follow-up, EVAR again conferred no survival advantage in clinical trials, and the rates of aneurysm-related deaths were similar in studies and this finding was repeated at 15-year follow-up.
Studies have suggested that more repeat interventions are required for patients treated with EVAR:
- The 2-year reintervention rate was 30% for EVAR and 19.1% for open repair (a statistically significant difference).
- The annual risk of reintervention in EVAR patients was 5%.
In the case study (above), more vascular surgeons would probably opt for open repair. Given the long history of open AAA repair, most vascular surgeons and the institutions where they work should be comfortable dealing with AAAs both in the elective setting and in the context of rupture. Generally, surgeons, anesthesia personnel, and nursing staff members will all be well aware of the steps and equipment required for open rAAA repair.
In contrast, treatment of an anatomically suitable rAAA by means of EVAR (ie, rEVAR) is a relatively new concept that often cannot be implemented, whether because the surgeons lack the necessary experience, because the available infrastructure is inadequate, or both. To perform rEVAR, the surgeon must have rapid access to the angiography suite or an angiography-capable hybrid operating room, must have a wide range of stent grafts readily available, and must have the assistance of nurses skilled in the smooth handling of angiography wires and devices. To date, only specialized medical centers have been set up to perform rEVAR.
As of April 2017, trials supported an EVAR-first approach to rAAA management offered no significant survival benefit at 1 year but was associated with shorter hospital stays, afforded patients better quality of life, and was cost-effective.
MEDICAL MALPRACTICE – AAA MISDIAGNOSIS
Only one-third of patients with AAA clinically present with the classic triad of abdominal pain, shock, and a pulsatile abdominal mass. Because these typical features are frequently absent, misdiagnosis is a common problem. As a result, emergency physicians must be aware that leaking or ruptured AAA may present with atypical signs and symptoms frequently leading to such erroneous diagnosis as renal colic, diverticulitis, or gastrointestinal bleeding. Any patient with or without hypotension, who presents with abdominal pain, flank pain, or back pain must be evaluated for a symptomatic AAA.
The signs and symptoms of an AAA are easily confused with such disease entities as renal colic diverticulitis, GI bleed, musculoskeletal pain, etc. Failure to diagnosis an AAA may result in a disaster both for the patient and the ED physician. The physician’s duty to the patient mandates practicing with a high clinical index of suspicion for AAA when evaluating any patient with risk factors who presents with abdominal, back, or flank pain, with or without associated hypotension. Patients may present with the additional confounding findings of an unexplained drop in hemoglobin, hematuria, a left lower quadrant tender mass, or syncope. The evaluation for an AAA must be aggressively pursued in such patients with ultrasound (standard of care is CTA if the patient is hemodynamically stable) in order to avoid the failure to diagnosis this condition. If the diagnosis is made and surgery undertaken while the patient’s blood pressure is stable, the mortality rate is low. However, if the diagnosis and surgical therapy is delayed, and the hematoma no longer is contained, the blood pressure will fall, and with it the chance for a successful outcome. The reasonable and prudent ED physician must include AAAs in the differential diagnosis whenever evaluating patients with these clinical presentations. Listening to the patient is paramount as the history alone in many cases of AAAs drives the workup and makes the diagnosis.
Kathleen A. Mary, RNC, Legal Nurse Consultant Certified is an honored medical expert and lifetime clinical scholar valued immeasurably by her plaintiff and defense attorney-clients as a time/cost-efficient asset to medical malpractice, personal injury and product liability claims. Kathleen provides flawless investigative navigation of meritorious complexities, meticulous comprehensive medical record reviews, locates trusted preeminent experts, is a recognized medical researcher and lifetime standard of care clinical consultant. For over 25 years, 100% of Kathleen’s cases (hundreds) have been positively settled without trial. Please contact Kathleen for your next medical-legal case.