Fig. 17.1
The anatomy of the coronary arteries, showing their location in relation to the chambers of the heart and main branches
The right coronary artery (RCA) arises from the right coronary sinus of Valsalva. Its branches include the SA nodal artery, the right marginal artery, the AV nodal artery and the posterior interventricular artery (usually, although anatomy can vary between individuals). The RCA usually supplies the:
Right atrium.
Right ventricle (most of it).
Diaphragmatic surface of the left ventricle.
Interventricular septum (part of it).
SAN.
AVN.
The left coronary artery (LCA) arises from the left aortic sinus. Its branches are the left anterior descending artery and the circumflex artery. The LCA usually supplies the:
Left atrium.
Left ventricle (most of it).
Right ventricle (part of it).
Interventricular septum (part of it).
Coronary Dominance
Coronary dominance is determined by the artery that gives rise to the posterior interventricular artery.
In approximately 70 % of people this is the RCA.
In approximately 15 % of people it arises from the circumflex artery.
In the remaining 15 % of the population, both arteries contribute to the formation of the posterior interventricular artery – co-dominance.
The Heart Valves
There are two sets of heart valves:
Atrio-ventricular valves (also known as the mitral and tricuspid valves) which are found between the atria and the ventricles.
Ventriculo-arterial valves (also known as the aortic and pulmonary valves or the semilunar valves) which are found in the arteries leaving the heart.
The mitral and aortic valves are found in the left side of the heart and the tricuspid and pulmonary valves are found in the right side of the heart. The tricuspid valve has three cusps (anterior, posterior and septal), and prevents backflow of blood from the right ventricle into the right atrium during ventricular systole. The mitral valve, so named because of its resemblance to a bishop’s mitre, has two cusps, the large anterior cusp and the smaller posterior cusp. The mitral valve prevents backflow of blood from the left ventricle into the left atrium during ventricular systole. The semilunar valves each have three cusps. They close in order to prevent backflow of blood from the aorta and pulmonary trunk back into the ventricles. The aortic cusps insert into the aortic annulus. Above each of these, the vessel walls dilate, forming a sinus. The left and right coronary arteries arise from the left and right coronary sinuses respectively (sinuses of Valsalva).
Types of Heart Valve Replacement
Aortic and mitral valve replacements are currently more common that valve repairs. Valve replacements are broadly divided into two categories: mechanical and tissue.
Mechanical Valve Replacements
Mechanical valves are usually made of carbon.
Advantages:
Extremely durable.
Should last over 30 years in most patients.
Usually a lower risk of failure than tissue valve replacements.
Disadvantages:
Risk of thrombosis means that lifelong anticoagulation is necessary, usually in the form of warfarin.
Tissue Valve Replacements
Allografts:
From a person of the same species e.g. cadaveric homograft.
Autografts:
A different valve from the same patient e.g. the Ross Procedure (the use of a pulmonary valve as an aortic valve replacement – an allograft replaces the pulmonary valve).
Xenografts:
From a different species e.g. porcine.
Historically, tissue valve replacements have shown a significant risk of valve failure over time due to structural valvular deterioration. However, the latest data suggests the deterioration curve is no longer as steep. One reason for this may be due to living valve substitutes, as in the case of the Ross Procedure. Unlike mechanical valve replacements, lifelong anticoagulation is not required in patients who undergo a tissue valve replacement.
Core Knowledge
Cardiothoracic surgery is divided into two broad subspecialties: congenital (i.e. paediatric cardiothoracic surgery) and adult. Adult cardiothoracic surgery is further subspecialised into cardiac and thoracic subspecialties. Cardiothoracic surgeons also perform heart and lung transplantations.
Congenital
Congenital heart defects can be classified as either cyanotic or acyanotic.
Cyanotic cardiac lesions can be remembered using the 1–5 mnemonic:
1 persistent vessel: Persistent ductus arteriosus (PDA).
2 vessels transposed: Transposition of the great arteries (TGA): a group of congenital defects involving the abnormal spatial arrangement of the aorta, venae cavae, pulmonary arteries and/or pulmonary veins.
3 – tricuspid: Tricuspid atresia: complete absence of the tricuspid valve.
4 – tetralogy: Tetralogy of Fallot: pulmonary stenosis, overriding aorta, ventricular septal defect and right ventricular hypertrophy.
5 letters – Total anomalous pulmonary venous return (TAPVR): the four pulmonary veins do not form normal attachments with the left atrium.
Cyanotic heart lesions are associated with deoxygenated blood entering the systemic circulation, therefore bypassing the lungs. Acyanotic heart lesions occur when there is “shunting” of blood from the left to the right side of the heart. The most common cause of an acyanotic heart lesion is a ventricular septal defect (VSD) or an atrial septal defect (ASD), although there are others. Often, but not always, patients benefit from early anatomical correction. It is important to consider both the anatomical and physiological consequences of repair.
Adult
Coronary Artery Bypass Grafting (CABG) is the most common adult cardiac procedure, followed by replacement of the aortic valve. Other common procedures include repair or replacement of the mitral valve, surgery to the thoracic aorta and heart failure surgery (including ventricular assist device implantation and transplantation).
Much of adult thoracic surgery is concerned with cancer.
The Heart Team
The emergence of new technologies is blurring the boundaries of expertise in cardiology, interventional radiology and surgery. The presence of competing treatment options, which may require the input of multiple specialists, has resulted in the development of the Heart Team. This team should be involved in the decision making process regarding diagnosis and management of the patient, with the aim of providing patients with a range of different options across specialties. This avoids the bias of any single specialist. A good example of this is coronary artery disease (CAD), where current guidelines recommend discussion with the heart team. The team should present all appropriate treatment options (e.g. surveillance, percutaneous coronary intervention and CABG) to the patient.
Investigations
Investigations are useful in that they can occasionally explain symptomatology (e.g. anaemia precipitating angina in CAD), direct therapy appropriately by eluding to the appropriateness of various interventions, allow assessment of the severity of disease and estimate prognosis in certain circumstances.
Blood Tests
Full Blood Count (FBC):
This measures haemoglobin and therefore checks for anaemia, which can be a cause of chest pain or exacerbate heart failure.
Haemolytic anaemia may occur due to prosthetic heart valves – red blood cells are damaged as they pass through the valves.
Lipids:
Measured in the risk factor assessment for CAD.
Urea, Electrolytes and Creatinine:
Electrolyte disturbances can promote arrhythmia (especially potassium and magnesium).
To monitor renal function which can be affected by cardiopulmonary bypass.
Troponin:
The first cardiac biomarker to rise after MI.
Should be tested on admission and re-tested 12h after the onset of maximum chest pain.
Important in detecting myocardial cell death.
Always high when myocardial tissue is damaged.
WARNING: Can be raised in many other conditions.
Other Investigations
Electrocardiogram (ECG):
Changes to the ECG can indicate cardiac ischaemia or arrhythmias.
Chest X-ray:
Can show results of previous surgery e.g. sternal wires.
Can also show signs of heart failure which can be remembered by ABCDE:
Alveolar oedema.
Kerley B lines (lines that run perpendicular to the pleura).
Cardiomegaly (enlarged heart).
Upper lobe diversion (prominent vessels in the upper lobe).
Pleural effusion.
Echocardiogram:
Can be used to assess the structure of the heart e.g. the valves.
Can be used assess contractile function.
Can be used to assess valve regurgitation (classified as none, mild, moderate or severe).
Can measure ejection fraction:
A measurement of how much blood is pumped out of the left ventricle with each contraction (as a percentage of the blood in the ventricle).
Normal is 55–70 %.
Below 40 % should be investigated in light of the whole scan.
MRI scan:
To visualise complex anatomy.
Stress testing:
Tests heart function during physical activity;
Angiogram:
Visualises blood vessels.
Can be diagnostic in CAD and can evaluate the pattern and extent of disease.
Angiography:
Visualises blood vessels.
For thoracic procedures, the following investigations should be considered:
CT scan.
MRI scan.
Bone scan: May detect metastatic bone disease.
PET scan: Can check for metastatic cancers.
Bronchoscopy.
Thoracotomy: Occasionally used to stage lung cancer.
Lung function tests:
Spirometry:
Forced expiratory volume in 1 sec (FEV1).
Forced vital capacity (FVC).
Peak expiratory flow (PEF).
Total lung capacity (TLC).
Tidal volume (TV).
Transfer factor.
General Indications for Surgery
The New York Heart Association (NYHA) functional classification places patients into categories depending on their tolerance for physical activity. It is used to assess the severity of a range of symptoms of heart failure. A certain NYHA class is often used as an indication for a surgical procedure. The four classes are described below:
NYHA Class I:
Cardiac disease, but no limitation of physical activity.
NYHA Class II:
Slight limitation of physical activity. Ordinary physical activity causes symptoms such as fatigue, palpitations, angina and dyspnoea.
NYHA Class III:
Significant limitation of physical activity. Less than ordinary activity e.g. getting dressed, causes symptoms such as fatigue, palpitations, angina and dyspnoea.
NYHA Class IV:
Symptoms present at rest.
General Contraindications for Surgery
Some conditions or co-morbidities make patients unsuitable for surgery. General contraindications for surgery include:
Severe renal failure.
Extremely poor cardiac function.
Severe sepsis (defined in Chap. 8: Post-Operative Care).
Uncontrollable bleeding or coagulopathy.
Cardiopulmonary Bypass
The first successful open heart operation utilizing cardiopulmonary bypass (CPB) was performed by John Gibbon in 1953. CPB allowed the acceleration of research into cardiothoracic surgery. CPB takes over the function of the heart and lungs during surgery allowing the heart to be arrested and emptied of blood creating a stable field for safe surgery. CPB involves diverting venous blood to a machine, which oxygenates, filters and returns blood to the arterial circulation. Cannulas therefore need to be placed in the superior vena cava (SVC) and inferior vena cava (IVC), diverting blood away from the heart and lungs. Blood is then returned to the ascending aorta. The temperature of the blood is regulated using a heat exchanger. Whether the blood is warmed or cooled depends on the procedure being performed – during CABG, blood is often cooled to protect the brain.
Cardiopulmonary Bypass: Step by Step Summary
- 1.
Prior to CPB full anticoagulation must be achieved, normally using heparin. This prevents clotting of blood during CPB.
- 2.
The bypass circuit is primed with crystalloid and heparin.
- 3.
The aorto-pulmonary window is dissected to allow access via a cannula.
- 4.
Deoxygenated blood is drained from either the venae cava or the right atrium into a reservoir using a venous cannula.
- 5.
Blood is filtered and oxygenated by an artificial lung.
- 6.
Oxygenated blood is pumped back into the aorta via an arterial cannula.
- 7.
The ascending aorta is cross-clamped below the arterial cannula, preventing back flow of blood into the left ventricle.
- 8.
A venting cannula is commonly placed in the right superior pulmonary vein to assist in maintaining a bloodless field.
- 9.
Anticoagulation is reversed by the administration of protamine (a heparin binding peptide) at the end of the procedure.
- 10.
After the procedure, the patient is rewarmed.
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