Disorders of the arterial system

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Contents

Introduction

The clinical approach to peripheral vascular disease should be based on knowledge of the natural history of the particular disorder, options for non-operative management, particularly risk factor modification and the likely outcome of any proposed intervention.

Epidemiology

Patients with peripheral arterial disease usually die from co-existent coronary or cerebrovascular disease. Heart, stroke and vascular disease combined caused 37.6% of all deaths in Australia in 2002, more than any other disease group. The prevalence of diabetes has doubled over the last 20 years and now affects 8% of the adult population.

Atherosclerosis

In 90% of adults with peripheral arterial disease, the cause is atherosclerosis. The other 10% of less common causes include peripheral thromboembolism either from the heart or an aneurysm, trauma, fibromuscular disease and rare inflammatory conditions.

The pathogenesis of atherosclerosis is complex and can begin in the second decade of life, with implications for the timing of preventative measures. It may take many years for atherosclerotic plaque to enlarge enough to narrow an artery. Peripheral vascular disease may therefore remain asymptomatic for many years.

The earliest detectable pathological feature of atherosclerosis is fatty streaking, which later evolves into the fibrous plaque. More complex lesions occur with intra-plaque haemorrhage, calcification or disruption of the overlying fibrous cap.

Once the diameter of an artery is reduced beyond 60%, the stenosis becomes critical, reducing blood flow. To compensate, a collateral circulation forms. The clinical outcome of arterial occlusion depends on the adequacy of the arterial collateral. If there is little collateral present then ischaemia can be extreme. With well-developed collateral, arterial occlusion may even be asymptomatic.

Aneurysms

An aneurysm is defined as a permanent, localised dilation of an artery that has at least a 50% increase (1.5 times) in diameter as compared with the expected normal diameter of that artery. When an artery dilates it may rupture causing internal bleeding (typical of large abdominal aortic aneurysms) or form laminated thrombus in the aneurysm sac that can embolise or thrombose (typical of popliteal aneurysms) resulting in limb loss.

Patterns of arterial aneurysmal disease

The aetiology of abdominal aortic aneurysms is multifactorial. There is a 10% familial association in firstorder siblings and an association with genetic disorders such as Marfan's or Ehlers-Danlos syndrome.

A patient with an abdominal aortic aneurysms is likely to have other aneurysms involving the iliac arteries in 40% and popliteal in 15%. Conversely, aortic aneurysms are present in about 30% of patients with bilateral popliteal aneurysms. An aneurysm in one anatomic location should therefore prompt a search for aneurysms elsewhere.

Epidemiology

The prevalence of abdominal aortic aneurysm increases with age from 4.8% in men aged 65–69 years to 10.8% for men aged 80–83 years and is higher in men, those with hypertension, smokers and in Caucasians. The mortality from a ruptured abdominal aortic aneurysm is over 75%, with most patients dying before they can reach hospital. Even with surgery there is a 50% mortality rate. Every effort therefore must be made to identify and treat abdominal aortic aneurysm before rupture occurs.

Once an aneurysm reaches a diameter greater than 5 cm, the risk of rupture increases exponentially with further expansion. At a diameter of 5–6 cm, the 5-year risk of rupture is 25%, from 6–7 cm35%and over 7 cm it is 75%.

Clinical presentation

Most abdominal aortic aneurysms are asymptomatic unless rupture is impending, in which case, the presentation can be dramatic, with hypotension and abdominal pain radiating through to the back. More usually, an abdominal aortic aneurysm is an unexpected finding, either on astute routine physical examination or on incidental ultrasound or CT scanning.

Evaluation

The diagnosis of a suspected abdominal aortic aneurysm is best initially confirmed by ultrasound examination, although calcification of the aortic wall can be seen in plain X-rays. Three-dimensional computed tomographic (CT) reconstruction can show the morphology of an aortic aneurysm in exquisite detail. Ultrasound examination is also the best modality for population screening or to follow patients with small abdominal aortic aneurysms to check for expansion and the easiest way to confirm the diagnosis of a suspected popliteal aneurysm.

Treatment

Small abdominal aortic aneurysms can be observed for expansion by regular ultrasound examination. There is general agreement that once the diameter of an abdominal aortic aneurysm exceeds 5 cm, the risk of rupture is sufficient to justify intervention.

Endoluminal repair of aneurysms

The endovascular method of treating an abdominal aortic aneurysm involves the insertion of an aortic graft through the common femoral artery via a catheter, avoiding the morbid open abdominal incision.

Not all patients are suitable to treat by the endoluminal method. In order to anchor the aortic endograft, enough normal aorta (>2.5 cm) is needed below the renal arteries to securely fix the device in place. Secure fixation may be impossible if this segment of aorta is too dilated. Narrow, calcified and tortuous iliac and femoral arteries also pose technical challenges. Many of these anatomic limitations may be overcome with further technical advances in endograft design and delivery systems.

The same concept can be extended to treat other peripheral aneurysms, for example in the thoracic aorta or popliteal artery.

Open surgical repair of aneurysms

The open method of treating an abdominal aortic aneurysm has been used for more than 50 years, and late graft failure and rupture is rare. Conventional repair of an abdominal aortic aneurysm involves a laparotomy, clamping of the aorta and sewing in a prosthetic arterial graft to replace the aneurysmal aorta. This can be done in a straight or bifurcated configuration.

In other settings, the aneurysm can be ligated and circulation maintained by a bypass procedure. This is the commonest form of treatment for popliteal aneurysms.

Complications

Randomised trials have confirmed a short-term advantage with endovascular repair with a lower perioperative mortality of 1.2% compared to 4.6% for open repair. General complications, particularly cardio-respiratory, are more common with open repair, but there is a higher incidence of local vascular or implant related complications after endovascular repair. It is not yet known if the immediate advantage of endovascular repair will be sustained in the longer term because of late problems that can develop with endografts, including device failure or migration causing endoleak (blood flow communicating with the aneurysm sac), leading to ongoing aneurysm expansion and possible late rupture.

Ruptured aneurysms or more extensive aneurysms involving the thoracoabdominal aorta are generally not suitable to treat by the endoluminal method, although this is now being investigated, and the complication rate is higher. Serious complications include renal dysfunction, paraplegia and ischemic colitis. Late prosthetic graft infection is rare but can occur after open or endovascular repair and lead to graft-enteric fistula years after surgery.

Chronic limb ischaemia

Chronic limb ischaemia is the commonest clinical problem occurring in patients with peripheral vascular disease (Patterns of disease affecting the lower limb).

Clinical presentation

Patients may present with intermittent claudication, a characteristic muscle pain induced by exercise, relieved by rest and recurring on walking the same distance again. Similar pain occurs less commonly on a neurogenic basis from spinal cord or nerve root compression but can be distinguished from arterial claudication by physical examination and exercise testing.

The risk of limb loss is low, with less than 10% chance of amputation in 10 years. A mortality of 5% per year can be expected from associated medical comorbidities, with coronary events the usual cause of death. The outlook is worse in claudicants with diabetes who continue to smoke.

With more severe ischaemia, pain can affect the limb at rest and is usually worse at night. The next stage is the onset of ischaemic ulceration or gangrene. In either situation, the risk of limb loss is high.Without restoration of adequate blood supply, the chance of amputation is about 30% within 3 months. A mortality rate of 10% per year can be expected, with the higher mortality reflecting the more severe, generalised disease in such patients.

Clinical evaluation

The aorta and femoral pulses should be palpated in every routine physical examination. The femoral pulse is generally easy to feel and therefore a good site to check cardiac rate and rhythm. The degree of arterial calcification can be estimated by the rigidity of the pulse.

Palpation of the popliteal artery is more difficult. Whenever the popliteal pulse is prominent and easy to feel, a popliteal aneurysm should be suspected.

Investigations

As atherosclerosis is a generalised disease, investigations should be carried out to assess risk factors and the involvement of other arterial beds, thinking particularly of the coronary and carotid arteries.

Measurement of ankle pressure

The Ankle Brachial Index (ABI) can be derived by dividing the highest ankle systolic pressure by the arm systolic blood pressure. This measurement adds a degree of objectivity to the detection and grading of any arterial occlusive disease present.

  • Normal range >0.95
  • Intermittent claudication 0.9–0.4
  • Rest pain 0.4–0.15
  • Gangrene <0.15

In patients with diabetes or chronic renal failure their arterial walls may be calcified and not compressible by a blood pressure cuff, resulting in falsely elevated ABI determination.

Duplex ultrasound imaging

Ultrasound can also be used to image blood vessels, with the anatomic display of the artery complementing the physiological information derived from ankle systolic pressure measurement. This is the simplest and most cost-effective way to confirm the diagnosis and anatomic location of suspected peripheral arterial disease.

Angiography

Angiography is regarded as the diagnostic ‘goldstandard’. The complication rate is low, related to arterial injury or adverse reaction to the contrast agent used. Ultrasound has replaced angiography for many applications because these risks are avoided. Angiography is now more selectively used on an ‘intention to treat’ basis after duplex imaging. Management

Management

Non-operative management (Suggested management algorithm for intermittent claudication. Modified from ACS Working group. Management of peripheral arterial disease (PAD). J Vascular Surgery 31, Part 2, January 2000.)

Suggested management algorithm for intermittent claudication. Modified from ACS Working group. Management of peripheral arterial disease (PAD). J Vascular Surgery 31, Part 2, January 2000.

Conservative treatment should be offered as the initial treatment for patients with intermittent claudication, summarised in the advice ‘Stop smoking, lose weight and exercise.’

Intervention should be considered if intermittent claudication affects essential daily activity or threatens employment.

Endovascular procedures

Arterial stenoses or occlusion may be dilated or stented by minimally invasive catheter technology. Balloon angioplasty enlarges the lumen by a controlled dissection. The risk of serious complications is about 2%. In the iliac arteries an initial success rate of about 90% is well maintained over a period of 5 years. In the superficial femoral artery the results are less satisfactory. Metallic stents may be deployed to maintain the lumen after angioplasty, improving durability of arterial dilatation but adding to the cost of intervention. Many of the local complications of balloon angioplasty can be controlled by stent placement.

Operative management

Surgical bypass is done by anastomosing a conduit to carry blood around an occluded arterial segment, even down to the tibial arteries in the foot. A prosthetic graft may be needed if autogenous vein, the preferred graft material, is unavailable.

Femoropopliteal bypass has an operative mortality of 1–2% and a 5-year patency of 50–70%, with the best results achieved when autogenous vein can be used as graft material. Aortofemoral bypass has an operative mortality of 2–5% and a 5-year patency of 80%. This operation is now infrequently performed because of the comparable results and lower morbidity of endovascular methods such as iliac arterial dilatation and stenting.

Surveillance

Whether the patient has been treated by endovascular means or by bypass surgery, regular surveillance is required to detect and correct late structural problems. Re-stenosis is usually due to neointimal hyperplasia, which occurs most frequently in the first 18 months after intervention and can be detected by ABI measurement and ultrasound scanning.

The diabetic foot

Patients with long-standing diabetes mellitus are predisposed to foot complications on the basis of arterial occlusive disease, neuropathy and infection. Coexistent arterial disease, with calcification of the tibial arteries, is most likely in diabetics who smoke.

Diabetic neuropathy may affect motor, sensory and autonomic nerves. Sensory neuropathy results in loss of pain sensation. Motor neuropathy results in paralysis and atrophy of the small muscles of the foot. This produces clawing of the toes, with neuropathic ulceration forming under the metatarsal heads, maximum area of load bearing. Diabetics are also susceptible to infection.

Clinical presentation

The foot lesion may be the presenting feature of maturity-onset non-insulin-dependent diabetes mellitus (NIDDM). The patient may present with a ‘punched out’ ulcer or area of localised gangrene, or acutely with a major infection in the foot.

Assessment is directed at determining the extent of local tissue damage and the adequacy of the blood supply. Plain X-rays of the foot will reveal any underlying orthopaedic abnormality such as metatarsal phalangeal dislocation or show evidence of osteomyelitis.

Management

Prevention is a major goal by careful control of the diabetes and foot care, to removing callus that can precede ulceration. Surgery has an important role, requiring inter-disciplinary co-operation. Vascular surgeons are involved in improving lower extremity blood supply and orthopaedic surgeons in correcting local bone or soft tissue complications of diabetes. The outlook is worst for patients with diabetic foot ulceration due to peripheral arterial disease who continue to smoke.

Acute limb ischaemia

Acute arterial ischaemia can be caused by trauma (see the section “Vascular Trauma”) or by non-traumatic conditions, notably arterial embolism or thrombosis of a pre-existing diseased arterial segment. Unless blood flow is restored within hours, irreversible tissue damage will occur, leading to possible amputation.

Pathophysiology

The common causes of non-traumatic acute limb ischaemia are embolus, thrombosis or graft occlusion following prior surgery. The most common site of origin of an embolus is the heart, due to atrial fibrillation or after myocardial infarction.

Arterial thrombosis can be precipitated by preexisting arterial disease, resulting in sudden occlusion at the site of an atheromatous arterial stenosis.

Evaluation

Features such as rapid onset of ischaemia, presence of atrial fibrillation or recent myocardial infarction, or absence of a history of claudication are more common in patients with embolus. The ABI will indicate the severity of the ischaemia. In almost all cases angiography is necessary to define the site and extent of obstruction.

Treatment

The survival of the limb is threatened if there is loss of sensation and muscle tenderness or weakness. The aim is to restore arterial inflow, which may done by thrombolytic therapy to dissolve the occluding thrombus, which is successful in 30% of cases, decreasing the need for surgical thrombectomy or bypass.

The mortality remains on the order of 10% and the rate of amputation about 20%.

Upper extremity

Upper extremity ischaemia occurs far less commonly than lower extremity ischaemia and is usually due to non-atherosclerotic causes.

Thoracic outlet syndrome

The subclavian artery, vein and T1 nerve root pass over the first rib to enter the arm. These structures can be compressed in the presence of cervical ribs or with the hypertrophic musculature that follows hard work or in athletes.

The clinical presentation will vary, depending on whether the artery, vein or nerve is predominantly compressed. In selected patients, removal of the first rib, usually by a transaxillary approach, will relieve arterial, venous or neural compression.

Vasospastic disorders

Raynaud's syndrome describes the changes which result from intermittent vasospasm of the arterioles in the hands or feet which occurs after exposure to cold. There is a classic sequence of colour change from pallor to cyanosis to redness as the arterioles first spasm, then slowly recover.

This can be secondary to an underlying connectivetissue disorder such as scleroderma or rheumatoid arthritis.

Management is directed towards treating the underlying condition but with severe digital ischaemia causing tissue loss, vasodilator therapy or occasionally endoscopic surgical sympathectomy is indicated to dilate the digital arteries and improve cutaneous circulation.

Vascular trauma

Vascular injury may be blunt or penetrating. Most arterial injury in Australia is due to blunt trauma sustained in motor vehicle accidents, but there is an increasing incidence of penetrating trauma related to urban violence.

Patterns of arterial injury

Arterial transection

This will present with haemorrhage and evidence of distal ischaemia. The severity of the ischaemia depends on the collateral circulation. With complete transection, arterial spasm and contraction occurs so that bleeding may spontaneously stop as the distal end of the artery thromboses. With partial transection, arterial contraction cannot occur so bleeding can be profuse while the distal pulses remain present.

Arteriovenous fistula

If a penetrating injury involves the adjacent artery and vein, a fistula between the two may develop, with shunting of blood from artery to vein. External bleeding may therefore be minimal.

Closed injury/intimal dissection

This type of injury is most commonly encountered with blunt injury and is dangerous because there may be no external haemorrhage and therefore the possibility of arterial injury overlooked. The artery wall usually remains intact but may weaken, producing a false aneurysm classically seen with traumatic dissection of the thoracic aorta.

Certain orthopaedic injuries such as supracondylar fracture of the humerus or dislocation of the knee are associated with concomitant arterial injury.

Clinical presentation

Vascular injury should be suspected when there is pulsatile bleeding, signs of distal ischemia, an expanding haematoma or a thrill or bruit overlying a site of suspected arterial injury.

Diagnosis

Regular clinical review with a high index of suspicion is essential in detecting arterial injury. Prompt assessment in an operating theatre, ideally equipped for intra-operative angiography, is required for arterial injury associated with ongoing haemorrhage or acute ischaemia. Less obvious arterial injury will usually be detected with measurement of the ABI, duplex scanning and diagnostic angiography. Arterial spasm should not be assumed as an explanation for limb ischaemia after injury unless intimal disruption has been excluded by angiography or surgical exploration.

Treatment

Arterial inflow must be restored within 4-6 hours of acute injury to prevent permanent muscle damage and limb loss.

The technique of arterial repair depends on the pathology of the injury. In some cases of lacerated wounds direct repair can be performed. If a segment of artery has been damaged an interposition graft may be necessary to bridge the defect. Covered stents are also used in selected cases.

Reperfusion syndrome can occur when blood supply is restored to muscle damaged by ischaemia, so that the breakdown products from ischaemic muscle necrosis are washed into the general circulation. Systemic features of reperfusion syndrome include hyperkalaemia and myoglobinuria that can cause sudden death, adult respiratory distress syndrome and cardiac or renal failure.

If muscle swelling is anticipated following restoration of blood flow, a fasciotomy will relieve high intracompartmental pressure.
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