Cardiothoracic surgery

22 Cardiothoracic surgery






Basic considerations



Pathophysiological assessment


Careful history and appropriate examination suggest the presence of possible cardiac pathology. The initial clinical assessment is then refined and specific investigations used to confirm and quantify any disease identified (Table 22.1).


Table 22.1 Specific assessments of cardiac pathophysiological status













































Investigation Yield
ECG  
Resting Rhythm; conduction abnormalities; atrial and ventricular hypertrophy; established ischaemic changes; evidence of previous myocardial infarction
Exercise Exercise-induced ischaemic changes or arrhythmias
Chest X-ray Cardiac enlargement; valvular calcification; evidence of pulmonary oedema (Kerley B lines, pleural effusion, interstitial marking, hilar flare); absent or enlarged cardiac or great vessel structures
Thallium isotope scan Areas of low radio-uptake indicative of impaired myocardial perfusion
Echocardiography  
Precordial Ventricular contractility; valvular stenoses, regurgitation or leaflet abnormalities; intracardiac morphology, including septal defects and intracardiac masses; pericardial effusion
Transoesophageal Enhanced views of posterior cardiac structures (aortic and mitral valves, ascending aorta, great veins and posterior septae); posterior pericardial fluid collections
Cardiac catheterization  
Chamber pressures Assess left and right ventricular function via determination of left ventricular end-diastolic pressure; atrial pressures in valve disease; transvalvular gradients (Fig. 22.1)
Angiography Coronary arterial anatomy; intracardiac anatomy; trans-septal flow
O2 saturations Intracardiac shunts
Cardiac output Cardiac function and determination of secondary derived parameters, including peripheral and pulmonary vascular resistance



Specific aspects of surgical technique



Cardiopulmonary bypass (CPB)


Modern cardiac and great vessel surgery became feasible with the development of cardiopulmonary bypass. Venous blood is drained via cannulae inserted into the right atrium or venae cavae and passes to a reservoir. It is then pumped through an oxygenator, which adds O2 and removes CO2, through a heat exchanger coil so that its temperature can be varied and finally, the blood is returned to the arterial circulation via a cannula in the ascending aorta or other suitable artery (femoral, axillary) (Figs 22.1, 22.2 and 22.3). Full anticoagulation with intravenous heparin is required to prevent blood clotting in the tubing, oxygenator and pump mechanisms. Roller or centrifugal pumps are used, as these minimize red cell trauma. Semipermeable membranes, or more commonly hollow fibres, form the blood–gas interface within the oxygenator. A trained perfusion technician controls the bypass machine.





CPB stimulates a systemic inflammatory response mediated by cytokine release, complement activation and white cell activation. These changes do not generally cause clinical problems but may be implicated in post-bypass pulmonary, renal and cerebral dysfunction. Cerebral damage occurs in about 1% of cases due to intracerebral bleeding, embolization of microbubbles or arterial debris, or inadequate cerebral perfusion. Subtle deterioration in cerebral function, as detected by psychological testing, is more frequent. Coagulopathy and haemolysis are associated with prolonged bypass.



Myocardial preservation



Cardioplegia

Cardioplegic arrest achieves a still bloodless heart. A cross-clamp is applied across the ascending aorta proximal to insertion of the arterial inflow cannula. This prevents blood flow into the coronary arteries. The heart is arrested by perfusing the coronary circulation with a cardioplegic solution, delivered either antegradely via the aortic root or coronary artery ostia utilizing the native coronary arteries, or retrogradely via a catheter placed in the coronary sinus.


The essential component of a cardioplegic solution is a high potassium concentration (circa 18 mmol/l), which causes the heart to arrest in diastole. Cardioplegia is typically delivered at a temperature of 4–6°C as either a crystalloid solution or using the patient’s own blood as a vehicle. Blood-based solutions are believed to have buffering characteristics that are helpful in reducing the deleterious effects of ischaemic metabolites generated by the arrested myocardium. Cardioplegia solutions minimize myocardial energy requirements by abolishing energy expenditure on contraction and by reducing basal cellular metabolism by local tissue cooling. Reducing core temperature on bypass to 26–34°C may enhance cardiac cooling. Cardioplegia combined with mild systemic hypothermia (32°C) provides the surgeon with a safe period of cardiac arrest of up to 120 minutes permitting surgery while minimizing the risk of myocardial damage.


Coronary bypass surgery (CABG) can be performed using a technique in which an aortic clamp is intermittently applied to cut coronary flow while the heart is electrically fibrillated so as to reduce movement. The resulting brief ischaemic episodes are tolerated. This cross-clamp fibrillation technique activates mechanisms within the myocardial cells that reduce damage caused by subsequent ischaemia (preconditioning).


In some circumstances, the surgeon may elect to leave the coronary arteries perfused while on bypass and to operate on a beating heart. Recently, there has been considerable interest in performing CABG on suitable patients without the use of CPB. Proponents of ‘off-pump’ surgery claim that the risks of artificial perfusion (particularly transient cognitive impairment) are avoided and that recovery may be quicker. Many surgeons, however, feel that the bloodless, still operative field resulting from cardioplegic arrest provides the optimum conditions for high quality accurate anastomoses.



Postoperative care





Recovery time


Patients undergoing routine elective coronary or valve surgery will usually leave acute hospital care within one week. Those requiring more extensive surgery or emergency procedures may take longer to recover. Most patients will have undergone a median sternotomy (Fig. 22.4). This wound heals quickly and, as the sternal edges are approximated securely by wire or heavy sutures, chest discomfort eases rapidly. Leg vein donor sites may take longer to heal, particularly around the knee. By 2 weeks the patient should be able to walk a few hundred metres, and by 3 months should have returned to full activity, including work.




Acquired cardiac disease


Surgical intervention may be required in the management of:




Ischaemic heart disease


Ischaemic heart disease encompasses coronary artery disease and its complications, principally acute mitral regurgitation, ventricular septal defect and left ventricular aneurysm.



Coronary artery disease (CAD)


Coronary artery atheroma (Ch. 21) results in narrowing of the vessels and most patients will present for surgery because of angina or previous myocardial infarction (MI).





Coronary bypass


A coronary artery bypass graft (CABG) delivers blood to the distal coronary artery beyond a stenosis. If the distal artery is obliterated by atheroma, an endarterectomy procedure may be performed to restore the lumen. Originally, nearly all grafts comprised reversed segments of the long saphenous vein anastomosed proximally to the ascending aorta and distally to the coronary artery. Such grafts have patency rates of around 70% at 5 years and 40% at 10 years. Venous graft failure occurs as a result of intimal hyperplasia, which is thought to be, in part at least, a response to arterial pressure. The relatively high rate of vein graft failure stimulated interest in arterial grafts and led to the almost universal use of the internal thoracic artery (ITA). This is usually employed as a pedicled graft when it is left attached to the subclavian artery proximally, but can also be used as a free graft in the same manner as vein. ITA graft patency exceeds 90% at 5 years and 70% at 10 years. A common combination is to use the left ITA for the left anterior descending artery and vein grafts for the other vessels (Fig. 22.7).



The radial artery is a possible option as a free graft for use in people with poor-quality saphenous vein and critical proximal occlusion of more than 70% in the target vessel, and may be used together with ITA grafts to achieve ‘total arterial revascularization’. Occasionally, when there is a shortage of good conduit (e.g. in a ‘redo’ operation), the surgeon may consider using the right gastroepiploic artery, the short saphenous vein and the cephalic vein. Prosthetic grafts occlude early and are not used.



Results


Uncomplicated coronary surgery should carry a 2–3% risk of mortality and a 1–2% risk of stroke. Angina is relieved completely in about 70% of cases, is significantly improved in the remainder, and recurs with a frequency of about 10% per year. Successful revascularization may also improve breathlessness if it is related to myocardial ischaemia, and survival is probably enhanced in patients with left main stem and triple vessel disease. The use of arterial conduits is associated with better graft patency and improved survival. Although there is a trend in that direction for patients with multiple arterial grafts followed up beyond 10 years, the added benefit over one ITA graft placed to the left anterior descending coronary is small. This may reflect the progression of native coronary disease. Secondary prevention is mandatory in all patients with CAD and includes antiplatelet medication (aspirin) and cholesterol reduction (statin) (EBM 22.1).






Surgery for the complications of coronary artery disease



Mitral valve regurgitation (MR)







Cardiac valvular disease


Valve disease may obstruct forward flow (stenosis) or permit reverse flow (incompetence/regurgitation), or both. The aortic and/or mitral valves are primarily affected; primary tricuspid pathology is rare and pulmonary valve disease is virtually unknown. Formerly, rheumatic fever following streptococcal infection was the most common aetiological factor. This remains the case in many developing countries, but in the UK it is rare.




Surgical management


Options include valve replacement or repair. Replacement utilizes either a mechanical or a biological prosthesis. Mechanical valves have developed from the original ball-in-cage design through single disc designs to the current range of carbon bi-leaflet devices (Fig. 22.8). These should last indefinitely, but patients require lifelong warfarin to prevent thrombotic occlusion or embolism. Embolism risk is about 1–6% per year and is influenced by how accurately the INR is controlled. Mechanical valves produce audible clicks.



Biological valves are derived from:




Unstented valves and homografts offer the advantage of a larger effective orifice area minimizing the residual pressure gradient. Warfarin is not required with biological valves provided the patient remains in sinus rhythm. However, such valves deteriorate over time and after 15 to 20 years may need replacement with an increased operative risk. Unless there is a contraindication to anticoagulation, mechanical valves are commonly used in a younger age group. In young women intending to have children it is usual to advise a biological valve, with the intention of replacing it with a mechanical device when the valve fails. This avoids problems with warfarin during pregnancy (placental separation and abortion, and teratogenicity).


Repair is the preferred surgical option in regurgitation and is largely restricted to the mitral and tricuspid valves. It is superior to valve replacement, as the problems associated with prosthesis are avoided. The techniques utilized for mitral incompetence include excision of portions of redundant leaflet, repositioning of the chordae and reduction in the size of the annulus (annuloplasty). Generally, only annuloplasty is applicable to the tricuspid valve. Rarely, isolated mitral stenosis without calcification may be found, in which case division of the fused leaflets under direct vision on bypass (commisurotomy) is performed.




Aortic valve disease



Stenosis


Aortic stenosis is the commonest indication for valve surgery in the UK. Although rheumatic disease remains a common problem in underdeveloped countries, the most frequent aetiology in the Western world is calcific aortic stenosis which develops in the older population usually over 70 years. The normal aortic valve has three cusps but a congenital bicuspid valve usually calcifies from the sixth decade onwards (Fig. 22.10). Aortic stenosis causes left ventricular hypertrophy, effort angina, episodes of arrhythmia with syncope or even sudden death, and left ventricular failure.



Clinically, the patient has a slow rising pulse, a forceful apex beat and an ejection systolic murmur in the right upper parasternal area that may radiate to the root of the neck. Echocardiography will confirm a valvular gradient, which is considered severe aortic stenosis when this exceeds 60 mmHg. However, measurement of orifice area is independent of cardiac output and may be a more reliable measure. The onset of symptoms should initiate referral for surgery. Patients with cardiac failure have a low cardiac output and consequently a low gradient. In these cases, the decision to operate may be a difficult judgement, based on the absence of any other likely cause of poor left ventricular function and echocardiographic evidence of severe aortic valve disease.


In high risk patients e.g. the very elderly, those with patent ITA grafts or significant other co-morbidities, percutaneous replacement of the aortic valve may be considered (TAVI – transcatheter aortic valve insertion) where a biological valve on a holder is introduced percutaneously via the femoral artery or left ventricular apex.


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Mar 20, 2017 | Posted by in GENERAL SURGERY | Comments Off on Cardiothoracic surgery

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