Abdominal aortic aneurysm
Abdominal aortic aneurysm is the commonest type of aortic aneurysm and is found in 2 per cent of the population at autopsy; 95 per cent are due to atherosclerosis and 95 per cent occur below the renal arteries.
Symptomatic aneurysms cause either minor symptoms, such as back pain and abdominal pain, or sudden, severe symptoms when they expand and rupture.
Asymptomatic aneurysms are found incidentally on physical examination, radiography or ultrasound investigation.
Ruptured abdominal aneurysm
Abdominal aortic aneurysms can rupture anteriorly into the peritoneal cavity (20 per cent) or posteriorly into the retroperitoneal space (80 per cent). Less than 50 per cent of patients with rupture survive to reach hospital.
Anterior rupture results in free bleeding into the peritoneal cavity. Very few of these patients reach hospital alive. Those who do have had a prolonged period of hypotension and shock, and consequently the results of surgery are poor.
Posterior rupture produces a retroperitoneal haematoma. There is a brief period in many of these patients when a combination of moderate hypotension and the resistance of the retroperitoneal tissues stops the haemorrhage. The patient remains conscious, but in severe pain. If no operation is performed, the mortality is 100 per cent. Operation results in a better than 50 per cent survival.
To achieve the best results, the diagnosis must be made early. The clinical features include sudden, severe back pain, accompanied in some cases by a brief loss of consciousness. The femoral pulses in one or both groins may be diminished or absent. A pulsatile mass is palpable in the abdomen and there are signs of shock. The procedure is as follows.
• Two good intravenous infusion lines and a central venous pressure line must be inserted as soon as the patient arrives in hospital or the diagnosis made once admitted.
• Blood is sent for immediate cross-match of 8 units.
• Infusion of saline, or volume-expanding fluids , is given to raise the systolic blood pressure to approx. 100 mmHg. On no account should the blood pressure be allowed to rise unduly as further uncontrolled haemorrhage may be stimulated.
• A urinary catheter is passed.
• If the patient appears to be stable, although in pain, the operation may be delayed until cross-matched blood is ready but the patient should still be transferred immediately to the operating room so that surgery may be commenced immediately if haemodynamic problems develop.
It should always be remembered that the definitive treatment of burst aneurysm is operation, not monitoring and resuscitation.
Abdominal aneurysm without rupture
Patients most commonly present without symptoms although they may have pain, usually felt in the back in the lumbar region and in the upper abdomen. In addition, pain can occur in the thigh and groin due to nerve compression. Gastrointestinal, urinary and venous symptoms can also be caused by abdominal aneurysm. As a general rule, in the presence of a pulsatile mass if symptoms cannot be reasonably explained by another lesion, they must be assumed to be due to the aneurysm (until proved otherwise) and the aneurysm placed in the symptomatic group.
Indication for surgery
Without surgery, 80 per cent with a symptomatic aneurysm will be dead in a year. With surgery, 80 per cent will be alive in a year. Surgery is indicated, therefore, in patients who are otherwise medically fit. The risk of operation is increased particularly in the presence of hypertension, chronic airway disease, recent myocardial infarction and impaired renal function. Chronological age is not a bar to surgery, but few patients are fit enough for this type of procedure once over the age of 80.
Aneurysm found incidentally on examination, radiography or ultrasound in an otherwise fit patient needs repair if over 5.5 cm in diameter on ultrasound. The annual incidence of rupture rises from 1 per cent in aneurysms that are 5.5 cm in diameter to over 20 per cent in those that are 7 cm in diameter. As elective surgery carries a 2—5 per cent mortality, the balance is in favour of surgery once the diameter is above 5.5 cm, provided there are no medical contraindications to surgery.
After taking a careful history and examining the patient, the following investigations are performed: urine analysis to exclude diabetes, in particular, haemoglobin estimation, full blood count, erythrocyte sedimentation rate (ESR), blood group and cross-match if surgery is contemplated within a few days; electrocardiogram (ECG); liver function tests; blood lipids; electrolytes and urea; chest radiography and ultrasonography of the abdomen to assess aneurysm diameter.
An aortogram may be useful in delineating the proximal and distal extent of the aneurysm before surgery; it does not permit assessment of diameter because the sac is usually filled with circumferential clot leading to a falsely narrow angiographic appearance. Involvement of the renal arteries by the aneurysm should be suspected if it is not possible to palpate the upper limit of the aneurysm below the xiphisternum with the patient lying flat (only 5 per cent of cases). In such circumstances aortography, computerised tomography (CT) and/or magnetic resonance imaging are essential.
Open surgical procedure
Under general anaesthesia, with the patient lying supine with a urinary catheter and central venous line in situ, a full-length midline or upper transverse incision is made. The small bowel is lifted to the patient’s right and the aorta identified. The posterior peritoneum overlying the aorta is opened and the upper limit of the aneurysm is identified. A plane is sought between the aorta and vena cava below the left renal vein. The iliac arteries are then dissected free from surrounding structures, heparin is given, and clamps are applied above and below the aneurysm. The aneurysm is opened longitudinally to the right of the inferior mesenteric artery, and back-bleeding from lumbar and mesenteric vessels controlled by sutures placed from within the aneurysm sac. The graft is then sutured end to end inside the aneurysm sac (2/0 or 3/0 Prolene). The upper clamp is released and haemostasis achieved. The lower end is then sutured to the aortic bifurcation in a similar manner. Clamps are released carefully, to one leg at a time, because hypotension and arrhythmias can occur if release is too rapid. The aneurysm sac is then closed round the graft and the posterior peritoneum closed to exclude the graft and suture lines from the intestine (to reduce the risk of fistula formation). The abdomen is then closed in layers. Occasionally, when the iliac vessels are also involved with dilatation or severe atheroma, it is necessary to construct an aorto-bi-iliac or aorto-bifemoral bypass, rather than use a simple aorto-aortic tube graft.
Endoluminal stent-graft procedure
Many major vascular surgical centres are now able to offer this minimally invasive treatment for certain aortic aneurysms, generally on an elective or a semi-elective basis. The aorta is accessed via the common femoral arteries, which are exposed surgically. Under radiological control, a delivery system is guided up into the aorta and a stent-graft placed within the aortic sac; this usually comprises aortic body and one iliac, the opposite iliac being replaced by a separate single iliac stent-graft introduced from the opposite common femoral artery. The stent system must be able to produce a blood-tight seal at the uppermost (infrarenal aortic) level of the graft, at both iliac levels distally, and at the junction between the aorto-uni-iliac stent-graft and its contralateral iliac partner. The technique has been carried out successfully so far in a few thousand patients world-wide but concerns remain about stent-graft fragmentation with the passage of time, and leakage (endoleak) at the interface of vessel and stent-graft or from patent lumbar arteries.
The commonest complications following open repair of abdominal aortic aneurysms are respiratory (lower lobe consolidation, atelectasis and ‘shock lung’). Haemorrhage occurs in relatively few cases provided anticoagulation is not continued beyond the immediate operative period and haemostasis is satisfactory at the end of the procedure. A degree of colonic ischaemia due to lack of collateral blood supply occurs in 10 per cent of all cases; fortunately this severe complication usually resolves spontaneously. Renal failure and infection of the graft are rarely seen following nonurgent procedures, but can complicate procedures for repair of ruptured aneurysm. Other complications are sexual dysfunction, flstula formation and spinal cord ischaemia.
Aortoduodenal fistula is an uncommon but treatable complication of abdominal aortic replacement surgery. It should be suspected whenever haematemesis or melaena occurs in the months or years after operation. A successful outcome may be achieved by prompt operation, separating aorta from duodenum, closing the holes and interposing some omentum.
Popliteal aneurysm accounts for 70 per cent of all peripheral aneurysms. Two-thirds of them are bilateral. Three-quarters develop complications within 5years if treated conservatively. Careful examination of the abdominal aorta is indicated if a popliteal aneurysm is found because one-third are accompanied by aortic aneurysms. Popliteal aneurysms present as a swelling behind the knee, or with symptoms due to complications, such as severe ischaemia of sudden onset following thrombosis, or ischaemic ulceration of the toes due to emboli. Surgery, possibly preceded by intra-arterial thrombolysis, is indicated urgently in the presence of complications to prevent amputation, and in asymptomatic cases, to prevent complications. Diagnosis can be difficult and relies on palpation of a pulsatile mass behind the knee. As a general guide, if the popliteal pulse is easily felt in a patient who is not thin, the presence of an aneurysm should be considered. Ultrasound and CT scan can be helpful in confirming the diagnosis. Treatment is either a bypass~ graft with ligation of the aneurysm or an inlay graft.
Femoral aneurysm. True aneurysm of the femoral artery is uncommon. Complications occur in less than 3 per cent and so conservative treatment is indicated initially. Look for aneurysms elsewhere: more than half are associated with abdominal or popliteal aneurysms.
False aneurysm of the femoral artery occurs in 2 per cent of patients after arterial surgery at this site. Many are infective in origin and rupture is possible, making surgical treatment indicated. Local repair with reanastomosis at the groin under suitable antibiotic cover may be successful, but bypass, clear of the infected area, with subsequent excision of the infected graft is often the only way of preventing further problems.
Iliac aneurysm usually occurs in conjunction with aortic aneurysm and rarely occurs on its own. On its own, it is difficult to diagnose clinically and so 50 per cent present already ruptured. Surgical treatment is indicated, with bypass, and exclusion of the aneurysm by ligation above and below the dilatation.
Aneurysms of the ascending aorta and arch require cardiopulmonary bypass for reconstruction to be undertaken. These, together with dissecting aneurysms of the thoracic aorta and traumatic false aneurysms in that situation.
Arteriovenous fistula (AVF)
Communication between an artery and a vein (or veins) may be either a congenital malformation, or acquired by the trauma of a penetrating wound or a sharp blow. Arteriovenous fistulas are also created surgically in the arms or legs of patients undergoing renal dialysis. All arteriovenous communications have a structural and a physiological effect.
The structural effect of arterial blood flow on the veins is characteristic as they become dilated, tortuous and thick walled (arterialised). It also makes the lesions diffuse and so renders surgical procedures difficult.
The combination of an uncontrolled leak from the high-pressure arterial system and an enhanced venous return and venous pressure results in an increase in pulse rate and cardiac output. The pulse pressure is high if there is a large and persistent shunt. Left ventricular enlargement and, later, cardiac failure occur. A congenital fistula in the young may cause overgrowth of a limb. In the leg, indolent ulcers may result from relative ischaemia below the short circuit.
Clinically, a pulsatile swelling may be present if the lesion is relatively superficial. On palpation, a thrill is detected and auscultation reveals a buzzing continuous bruit. Dilated veins may be seen, in which there is a rapid blood flow. Pressure on the artery proximal to the fistula causes the swelling to diminish in size, the thrill and bruit to cease, the pulse rate to fall [known variously as Nicoladoni’s (1875) or Branham’s (1890) sign] and the pulse pressure to return to normal.
Arteriography confirms the lesion, which is noteworthy for the speed with which venous filling occurs. It is often difficult to pinpoint the actual site of the fistula.
Embolisation by the radiologist or excision is advocated only for severe deformity or recurrent haemorrhage. It is often wise to enlist the aid of a plastic surgeon so that proper ablation and reconstruction can be effected. Ligation of a ‘feeding’ artery is of no lasting value and is likely to be detrimental as it may preclude treatment by embolisation.
The acquired lesions especially tend to be progressive and embolisation or operation is indicated if feasible and safe. At operation the vessels are separated and, if possible, repaired by suture, any intervening sac being excised. Failing this, ligation of the involved artery and vein is required both above and below the lesion (quadruple ligation). Bypass grafts may be required.