• Activation of the clotting cascade could occur, causing intravascular coagulation. This is prevented by anticoagulating with high-dose heparin (300 IU/kg) which is later reversed with protamine sulphate on coming off bypass

• Clotting factors and platelets are consumed in the extracorporeal circuit (mainly within the oxygenator) leading to a bleeding diathesis. Blood products may be needed to reverse this

• Activation of the complement cascade and other inflammatory mediators may result in systemic inflammatory response syndrome (SIRS) after bypass

Cyanotic heart disease

Cyanotic heart disease exists when there is mixing of systemic arterial and venous blood through a predominantly right-to-left cardiac shunt. The most common examples are:

Acyanotic heart disease

Acyanotic congenital heart disease may involve:

Palliating congenital cardiac disorders

When pulmonary blood flow is reduced (as in tricuspid atresia, tetralogy of Fallot or pulmonary artery stenosis), palliation aims to increase pulmonary flow by creating a shunt between the systemic arterial circulation and the pulmonary artery (see Fig. 44.2).

When pulmonary blood flow is too great (e.g. VSD or truncus arteriosus), the aim is to reduce the pulmonary flow by artificially narrowing the main pulmonary artery by external banding.

Correcting congenital cardiac disorders

Surgical correction of congenital heart disease is based on accurately identifying the lesion or lesions, then performing procedures which restore the normal flow and functioning of the heart. Correction may be straightforward (e.g. PDA closure; resection of coarctation: VSD repair) or highly complex (e.g. total correction of Fallot’s tetralogy; correction of anomalous pulmonary venous drainage). These should only be performed in specialist paediatric cardiac surgical units.

Pathophysiology

Coronary heart disease (ischaemic heart disease) is nearly always atherosclerotic, with subintimal thickening caused by deposition of cholesterol-containing lipids, plus hyperplasia of media smooth muscle cells which migrate into the subintimal area. Together, these changes reduce the luminal diameter. Since resistance to flow is proportional to the fourth power of the radius (Poiseuille’s Law), a small change in cross-sectional area causes a dramatic reduction in coronary blood flow. When assessed on angiography, a 50% reduction in coronary artery diameter shown in two planes is regarded as significant.

Acute coronary ischaemia is usually brought on by thrombosis of already narrowed coronary arteries, in some cases precipitated by haemorrhage into atherosclerotic plaques.

The worldwide distribution of coronary atherosclerosis is patchy. It is predominantly a disease of developed countries. Men are at greater risk than women, with 45% of men over 65 having some manifestation of it. Areas of historically high incidence include Scotland and Finland. The rates in some countries, notably the USA and much of Europe, have been falling in recent years, largely as a result of reduced cigarette smoking, control of hypertension and perhaps changes in diet and exercise.

Risk factors for coronary atheroma (and atherosclerosis elsewhere) include unfavourable cholesterol and lipid profiles (often hereditary), cigarette smoking, diabetes, hypertension, obesity and a sedentary lifestyle (and perhaps a life of severe unrelieved stress). The presentations of coronary heart disease are outlined in Table 44.3.

Control of predisposing factors

The first step in treatment is usually to try to persuade the patient to modify risk factors known to contribute to disease progression. There are two purposes: if progression can be arrested, physiological development of collateral blood supply can proceed, which may make intervention unnecessary. Secondly, occlusion of any form of revascularisation is more likely if risk factors, particularly cigarette smoking, continue. On average, patients who continue to smoke gain no benefit from coronary artery bypass grafting (CABG). Smokers who fail to improve thus suffer the risk and discomfort of surgery having squandered limited hospital resources. Nothing can yet be done about hereditary factors, but smoking, obesity and inactivity can be tackled. The benefits of lowering blood lipid levels are well established. Statins are widely used for this but have other beneficial effects including reducing arterial wall inflammation.

Management of coronary artery disease

Coronary artery disease can be managed conservatively (‘medical management’) or by interventions employing percutaneous techniques or surgical bypass grafting. The anatomy of the coronary arteries is shown in Figure 44.3.

Aortic valve disease

Aortic stenosis is caused most commonly by senile degeneration and calcification of a normal tricuspid aortic valve, affecting 2–4% of patients in their 6th to 8th decades. Bicuspid aortic valves are the most common valvular anomaly, affecting 1–2% of the general population, and may degenerate in the 4th to 5th decade of life. Aortic valve regurgitation may result from aortic leaflet abnormality (e.g. leaflet prolapse or perforation) and/or abnormal dilatation of the aortic valve annulus, a condition called annulo-aortic ectasia.

Severely stenotic aortic valves are usually treated by resection and replacement with a prosthetic valve. Some regurgitant valves are repaired but long-term durability is uncertain. A recent treatment for patients with aortic valve stenosis unfit for valve replacement (AVR) is a catheter technique via the left ventricular apex or femoral artery. It involves balloon dilatation of the stenotic valve and insertion of a stent-mounted prosthetic valve into the space. This transcatheter aortic valve insertion (TAVI) technique is less invasive than surgical AVR and is gaining acceptance across the Western world.

Mitral valve disease

Mitral stenosis following rheumatic fever can be successfully treated by valvotomy (separation of fused valve leaflets), if performed before calcification makes the leaflets immobile. In the early days of cardiac surgery, Cutler and Levine (1925) introduced semi-closed, blind mitral valvotomy, with mechanical dilatation of the valve via the left auricular appendage. With the heart beating, the auricle was opened, a finger or mechanical device inserted and the valve rapidly dilated. Symptomatic relief was usually reasonable, but the valvotomy was often incomplete, short-lived or caused splits in the leaflets rather than between them. Later, direct open valvotomy under cardiopulmonary bypass became popular. More precise and long-lasting results could be achieved by this method. More recently, there has been a return to closed valvotomy, with percutaneous trans-septal balloon dilatation of the stenosed valve.

In regurgitant valvular disease, surgical valve repair is possible in most patients but has been most successful for myxomatous or degenerate regurgitation. This conservative technique has a lower perioperative risk than valve replacement and better preserves left ventricular function. Thus, the overall functional result may be better than valve replacement. Repair techniques may also be employed for diseased tricuspid or aortic valves. Only where valvotomy or repair is inappropriate or has failed are valves replaced.

Valve prostheses (Fig. 44.5)

The leaflets of prosthetic heart valves are constructed from artificial material or biological tissue. Various types of prosthetic valves are available (see Fig. 44.5). The mechanical demands on replacement heart valves are extreme. They must cause minimal restriction to blood flow when open, yet prevent reflux when closed; they must be biocompatible, non-thrombogenic, resistant to infection and, most demanding of all, capable of opening and closing 70 times a minute for many years without mechanical failure. Modern mechanical valves are durable but thrombogenic and generally require lifelong anticoagulation. Anticoagulation carries its own risk with a mortality of about 2% over 5 years. Even with effective anticoagulation, there is still a small risk of arterial embolism.

Replacement tissue valves may be homografts (human) or xenografts (animal origin). Xenografts are most commonly made from bovine pericardium or porcine heart valves. In general, tissue valves are less thrombogenic (anticoagulation not necessary) but less durable through degeneration as time goes by. However, in patients over 60 years given an aortic bioprosthesis, 90% or more are free from structural valve degeneration at 10 years. Homograft (human) valves may be more resistant to degeneration and are preferred for young patients.

Infection of valve prostheses is devastating but fortunately rare. The risk is least for homograft valves. Prosthetic valve endocarditis carries a very high mortality and needs protracted antibiotic treatment, often requiring explantation (removal) of the infected prosthesis.

Indications for valve surgery

Valvular heart surgery is most successful when carried out at the right time, namely when risks of surgery are least and potential benefits greatest and before irreversible ventricular dysfunction has occurred.

In aortic or mitral valve stenosis, surgery is indicated when patients become symptomatic. In aortic or mitral regurgitation, surgical intervention is best when there is left ventricular dilatation in an asymptomatic patient.

Aortic dissection

In aortic dissection, a breach in the luminal surface allows blood to leak out and split the media then flow along a false lumen for variable distances proximally and/or distally. The dissection flap may occlude any aortic branch and disrupt the aortic valve. The likely aetiology is degeneration of elastin and collagen in the media. Dissection occurs most often in the ascending aorta (65%), but also in the aortic arch (10%), or the descending thoracic aorta just distal to the ligamentum arteriosum (20%).

Dissection is classified according to the part affected; the most widely used method is the Stanford system. Type A indicates that only the ascending aorta is involved while type B is when any other part of the thoracic aorta is affected. An alternative classification method is that of DeBakey (see Fig. 44.6).

Without surgery, Stanford type A dissection carries an 80% mortality in the first month; with surgical management this falls to less than 20%. The operation involves resecting the ascending aorta and replacing it with a synthetic vascular graft. If the dissection reaches the aortic valve, it is likely to be disrupted causing acute severe regurgitation and perhaps occluding the coronary artery origins. In these cases, the valve will need resuspending or replacing.

Uncomplicated type B dissections have a 20% mortality at 30 days, and the outcome is similar whether surgically or medically managed. Medical management is by control of hypertension. Surgery is required in type B dissections complicated by aortic rupture, occlusion of vital branches, progressive dissection or, later, by aneurysm formation. Note that abdominal aortic aneurysms occur years later in about 50% after thoracic dissection.

Thoracic aneurysms

Aneurysmal dilatation of the thoracic aorta may occur in the ascending part (Fig. 44.7), the arch or the descending part. There is a risk of rupture similar to abdominal aortic aneurysms and surgical intervention is usually recommended when the aneurysm reaches 6 cm diameter. Replacing the descending aorta, particularly when there is thoraco-abdominal disease, threatens the main blood supply of the spinal cord (the artery of Adamkiewicz at about T10) so that paraplegia complicates 10–30% of these operations. There is a growing trend towards endoluminal stent-grafting for appropriate thoracic aneurysms with the promise of lower morbidity and mortality.

Trauma to the thoracic aorta

This may result from blunt or sharp injury. Blunt trauma is usually associated with severe deceleration as occurs in head-on impact in road traffic collisions. The common injury is a partial or complete transsection of the aorta at the junction between the arch and the descending aorta close to the ligamentum arteriosum. Complete transsection with free rupture is rapidly fatal; partial transsection with an intact adventitia and a contained haematoma is liable to rupture at any time and must be diagnosed to enable early emergency treatment. There is a trend towards treating these with intraluminal stent grafts via the femoral artery. Late aneurysm formation may also occur in those patients who survive an untreated occult aortic transsection.