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 (Fig.
15.47). 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 (Chapter 4), is given to
raise the systolic blood pressure to approx.
•
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.
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
Symptomatic
aneurysm
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.
Asymptomatic aneurysm
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.
Investigations
After taking a careful history and examining
the patient, the following investigations are performed: urine analysis to
exclude diabetes, in particular, haemoglobin estimation,
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 (Figs 15.49,15.50
and 15.51).
open
surgical procedure (Fig 15.52 and Fig 15.53)
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 (Fig.
15.54) 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.
Postoperative complications
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
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.
Peripheral
aneurysm
Popliteal
aneurysm accounts for 70 per cent of all peripheral aneurysms.
Two-thirds of them are bilateral. Three-quarters develop complications within 5
years 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 (Fig. 15.55) 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, are
considered in Chapter 48.
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
Structural effect
The
structural effect of arterial blood flow on the veins is characteristic as they
become dilated, tortuous and thick walled (arterialised) (Fig.
15.56). It also
makes the lesions diffuse and so renders surgical procedures difficult.
Physiological effect
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.
Clinical signs
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
Arteriography
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.
Treatment
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.