Anaesthesia for Cardiac Surgery

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Anaesthesia for Cardiac Surgery

In the United Kingdom and much of the developed world, more than half of all cardiac surgical procedures are undertaken to revascularize ischaemic myocardium. Of the remainder, surgery for acquired valvular disease, congenital anomalies and disorders of the great vessels comprise the majority. Impaired ventricular function is not uncommon in this group of patients, the severity of which may greatly affect the conduct of anaesthesia and surgery as well as outcome. The combination of underlying cardiac pathology, comorbid conditions and concomitant medications – such as β-blockers and angiotensin converting enzyme (ACE) inhibitors – make many patients with cardiac disease susceptible to the adverse haemodynamic effects of anaesthetic agents, particularly peripheral vasodilatation. Regardless of the disease process or state, all efforts should be made to maintain haemodynamic stability and promote a positive myocardial oxygen balance during anaesthesia and throughout the postoperative period.

Undoubtedly, there is more equipment and technology on show in the cardiac surgical theatre than in other operating theatres, and the number of staff present is often large. This makes familiarity with equipment and multidisciplinary team working imperative, as well as a specialist knowledge of cardiovascular and respiratory physiology. The replacement of the functions of the heart and lungs by cardiopulmonary bypass (CPB) is often required, although some coronary surgery can be performed on the beating heart (off pump). Indeed, novel ‘minimally invasive’ methods may allow repair of various structures within the heart and even valve replacement or repair without CPB, and this is an exciting area of development.

TRENDS IN SURGICAL PRACTICE

The 6th National Adult Cardiac Surgical Database Report, published in 2009 by the Society for Cardiothoracic Surgery (SCTS) in Great Britain and Ireland, provides detailed information about trends in UK cardiac surgical practice. Although the total number of cardiac surgical procedures carried out in the UK in the period 2001–2008 increased year-on-year, coronary artery bypass graft (CABG) surgery plateaued at just under 23 000 operations per year, possibly due to advances in percutaneous intervention. Coincidentally, the number of elderly patients undergoing cardiac surgery of all types increased such that patients over the age of 75 years now make up more than 20% of the cardiac surgical population, and over 5% are over 80 years of age. Despite this, crude mortality rates decreased significantly between 2001 and 2008: 2.3% to 1.5% for isolated CABG; 5.2% to 3.5% for isolated valve surgery and 8.3% to 6.1% for combined procedures.

Ischaemic Heart Disease

The concept of aorto-coronary bypass grafting for the relief of coronary ischaemia was conceived and performed in animals in the early 1900s. It was not until the 1960s, following development of the heart-lung machine and the chance discovery of coronary angiography, that direct revascularization of the ischaemic myocardium using the autologous saphenous vein (Fig. 34.1) replaced indirect therapies such as sympathectomy, thyroidectomy and pericardial poudrage.

FIGURE 34.1 Diagrammatic representation of coronary arteries. (Adapted from JH Mackay and JE Arrowsmith (2012) Core topics in cardiac anesthesia. Cambridge University Press.)

Since being popularized in the late 1960s, coronary artery bypass grafting (CABG) has become the most commonly performed cardiac operation. The internal mammary artery is used routinely as a graft conduit, and there is good evidence that this provides good survival benefit. Complete arterial revascularization is possible using arteries such as the radial and epigastric arteries. Improved surgical techniques have increased the popularity of off-pump coronary artery surgery but its precise role remains uncertain and any advantages over surgery using CPB have not yet been proven. Technological advances in coronary stent technology, especially coated (drug eluting) stents have led to a huge expansion in the use of percutaneous coronary intervention (PCI; angioplasty, atherectomy and stenting) in the cardiac catheter laboratory. These procedures are typically performed under sedation, and length of stay in hospital and return to normal activities is undoubtedly markedly improved. However, the long-term efficacy of stenting has recently been called into question, and traditional CABG, once thought to be in terminal decline, remains a popular procedure.

Valve Disease

Stenosis or incompetence (regurgitation or insufficiency) most commonly involves the mitral and aortic valves. The most common diseases are calcific degeneration (causing aortic stenosis, with or without regurgitation), chronic rheumatic disease (affecting mitral and aortic valves) and myxomatous disease (most often causing mitral regurgitation). It should be borne in mind that valve dysfunction may occur as the result of systemic disease (e.g. carcinoid syndrome, infective endocarditis) and disruption of nearby anatomical structures (e.g. aortic regurgitation in acute dissection of the ascending aorta and mitral regurgitation following papillary muscle rupture).

Surgery usually entails repair or prosthetic replacement, guided by intraoperative transoesophageal echocardiography (TOE). The use of bioprosthetic or ‘tissue’ (porcine, bovine, cadaveric homograft) valves obviates the necessity for, and risks associated with, life-long anticoagulation but exposes the patient to the prospect of reoperation within 15–20 years. In contrast, mechanical (tilting disc) valves tend to last longer than bioprostheses and are therefore better suited to younger patients and those already anticoagulated for other reasons (e.g. chronic atrial fibrillation). Improvements in technology have led to some prostheses lasting more than 20 years, especially in patients aged > 70 years at the time of surgery. Minimally invasive transcatheter aortic valve replacement (TAVR) is now possible, making ‘redo’ sternotomy unnecessary in case of valve failure, by inserting a new tissue valve within the old prosthesis.

Congenital Heart Disease

Congenital heart disease has an incidence of 6–8 per 1000 live births. The majority of lesions requiring surgery are repaired during childhood in specialist paediatric cardiac surgical centres. Conditions such as a small atrial septal defect, partial anomalous pulmonary venous drainage or a bicuspid aortic valve may not present until adulthood. As a result of improvements in paediatric surgical and medical care, many patients now survive well into adulthood, and may require repeat surgery or other cardiac procedures. Specialist grown-up congenital heart (GUCH) disease centres have been created to cater for the often complex needs of this group of patients. A further description of these procedures is beyond the scope of this chapter.

Cardiopulmonary Bypass

The essential components of a cardiopulmonary bypasshe essential components of a cardiopulmonary bypass (CPB) circuit (Fig. 34.2) are:

FIGURE 34.2 Components of a cardiopulmonary bypass circuit. (Adapted from JH Mackay and JE Arrowsmith (2012) Core Topics in Cardiac Anesthesia. Cambridge University Press.)

venous reservoir

heat exchange unit

cardioplegia delivery system

connecting tubes and filters.

Full anticoagulation of the patient, typically with unfractionated heparin, is required to prevent coagulation in the CPB circuit caused by contact between the blood and the plastic components, which would otherwise lead to potentially lethal CPB/oxygenator blockage and failure. Despite anticoagulation, blood/plastic contact leads to the release of a number of active substances which cause vasodilatation, consumption of clotting factors and fibrinolysis. These include cytokines, thromboxane A2 and leukotrienes, and they are responsible for the hypotension and increased bleeding associated with CPB.

Blood from the venous side of the circulation, the venae cavae or right atrium, is drained by gravity to a venous reservoir, from where it is pumped into a gas exchange unit (oxygenator) where oxygen is delivered to, and carbon dioxide removed from, the blood. The blood can also be cooled or warmed efficiently at this point, using water pumped through a countercurrent heat exchanger located within the oxygenator. Oxygenated or ‘arterialized’ blood is then delivered into the systemic circulation, usually via a cannula in the ascending aorta. The heart and lungs are thus ‘bypassed’ or isolated and their function maintained temporarily by mechanical equipment remote from the body. Any blood in or around the bypassed heart (whether spilt or drained) may be drained and returned to the venous (cardiotomy) reservoir for filtration, oxygenation and subsequent return to the circulation.

Venous Reservoir

A 500–2000 mL reserve of circulating volume permits the delivery of a constant system flow during times when venous drainage is inadvertently reduced or deliberately impeded. Clinical systems are described as being ‘open’ (blood in contact with air) or ‘closed’ (blood in a soft, flexible container not in contact with air). In order to prevent air entrainment, most systems incorporate a critical level alarm which automatically stops the CPB pump if the reservoir becomes empty.

Pumps

Roller pumps displace blood around the circuit by intermittent, semi-occlusive compression of the circuit tubing during each rotation. Intermittent acceleration of the roller head can be used to produce a ‘pulsatile’ pressure waveform although there is little evidence that a more physiological flow pattern improves outcome. Alternatively, a centrifugal pump may be used. Movement of a disc at very rapid speeds (> 3000 revolutions per minute) leads to exertion of gravitational force on blood and results in propulsion at a flow which is dependent on the resistance (afterload) offered by the arterial tubing and the patient’s systemic vascular resistance. There is some evidence that centrifugal pumps cause less blood component damage and activation, but this has not translated into improved outcome, and their use is usually confined to prolonged or complex surgery. Unlike roller pumps, which impede all flow when stopped, centrifugal pumps permit passive retrograde blood flow when switched off.

Oxygenator

Membrane oxygenators comprise a semi-permeable membrane which separates gas and blood phases and through which gas exchange occurs. Commercially available devices have an effective exchange area of around 7 m² – one tenth of the alveolar surface area of an adult.

Connecting Tubes, Filters, Manometer, Suction

These must be sterile and non-toxic and should damage blood as little as possible. A filter should also be incorporated in the arterial line to remove particulate and gaseous emboli which would otherwise pass directly to the aorta and cause blood vessel occlusion. Low-pressure suction pumps are supplied to vent blood collecting in the pulmonary circulation or left ventricle during bypass and also to remove shed blood from the surgical field. The blood is collected in the ‘cardiotomy’ reservoir, filtered and returned to the main circuit. Cardiotomy suction causes damage to blood components.

Fluid Prime

The CPB circuit must be primed with fluid (de-aired) prior to use. When CPB is commenced and the patient’s blood is mixed with the clear fluids which prime the bypass circuit, the haematocrit decreases by approximately 20–25%. Although oxygen content is reduced, oxygen availability may be increased by improved organ blood flow resulting from reduced blood viscosity. In some patients (low body weight, children or preoperative anaemia, when dilution would reduce the haematocrit to below 20%), blood may be added to the prime. In the normal adult, ‘clear’ primes are used almost exclusively (usually a crystalloid/colloid mixture). Most units have individual recipes for addition to the prime (e.g. mannitol, sodium bicarbonate and potassium) to achieve an isosmolar solution at physiological pH.

PREOPERATIVE ASSESSMENT

In recent years, there has been a trend towards the assessment of elective patients in pre-admission clinics, typically one to two weeks before surgery. This allows routine paperwork, laboratory tests and radiological imaging to be completed before admission, which may not be until the day of surgery. Despite undergoing an extensive array of specialized investigations to diagnose and quantify cardiac disease, there is evidence that a significant number of cardiac surgical patients have additional and hitherto undocumented pathology. Thorough preoperative evaluation by the anaesthetist remains an essential component of perioperative care. This should, at the very least, include confirmation of the documented history and symptoms, documentation of current drug therapy, a review of the results of diagnostic investigations and a physical examination focused on the cardiovascular and respiratory systems.

Exercise Electrocardiography

Exercise (treadmill) testing is frequently used as a screening test before coronary angiography. Various stress protocols are used, in which a standard exercise test provokes ischaemic changes and symptoms. Changes in rhythm, rate, arterial pressure and conduction are recorded. Although it has relatively low sensitivity and specificity (60–70%) for coronary artery disease, it does provide some indication of effort tolerance.

Cardiac Catheterization

Left heart catheterization typically comprises coronary angiography, aortography, left ventriculography and manometry. This provides the following information (Table 34.1):

TABLE 34.1

Cardiac Catheterization

Technique	Procedure	Parameter
Manometry	Pressure measurement with catheter in aortic root and LV	Aortic valve gradient LV end-diastolic pressure
Angiography	Coronary arteries selectively cannulated, contrast injected	Coronary anatomy
Ventriculogram	Catheter in LV, contrast injected very rapidly	LV size and function Ejection fraction Severity of mitral regurgitation
Aortogram	Catheter in aortic root, contrast injected	Severity of aortic regurgitation

LV, left ventricle.

site and severity of coronary artery disease

mitral and aortic valve function

left ventricular (LV) morphology and function

The efficiency of ventricular contraction (ejection fraction) can be estimated using the formula:

Right heart catheterization allows measurement of right heart and pulmonary artery pressures. When combined with measurements of cardiac output, these can be used to determine the pulmonary and systemic vascular resistances (Table 34.2).

TABLE 34.2

Measurements Obtained During Cardiac Catheterization

	Parameter	Normal Values
Left heart	Systemic arterial/aortic pressure	< 140/90 (mean 105) mmHg
	LV pressure	< 140/12 mmHg
Right heart	RA pressure	< 6 (mean) mmHg
	RV pressure	< 25/5 mmHg
	PA pressure	25/12 (mean 22) mmHg
	PAWP	12 mmHg
	Cardiac index	2.5–4.2 l min^– 1 m^– 2
	PVR	100 dyne s cm^– 5
	SVR	800–1200 dyne s cm^– 5

LV, left ventricle; RA, right atrium; RV, right ventricle; PA, pulmonary artery; PAWP, pulmonary artery wedge pressure; PVR, pulmonary vascular resistance; SVR, systemic vascular resistance.

Echocardiography

Transthoracic echocardiography (TTE) is used frequently to define cardiac anatomy and assess ventricular and valvular function. TTE is non-invasive, and can be performed at intervals to monitor disease progression and to optimize the timing of surgical intervention before irreversible ventricular damage has occurred. It may also assist planning of the type of intervention required. Doppler techniques allow recognition of the direction and velocity of blood flow and are valuable in the diagnosis of valvular disease.

Unfortunately, TTE is of limited use in obese patients and patients with chronic lung disease (because of poor ultrasound windows caused by tissue or air). In addition, certain parts of the heart may not be visualized adequately because of their distance from the probe (such as the left atrium and interatrial septum). Therefore, transoesophageal echocardiography (TOE) may be required preoperatively (usually performed under sedation). TOE may also be indicated in mitral valve pathology to aid surgical decision-making between valve replacement and repair.

Radionuclide Imaging

By imaging the activity of an appropriate radioisotope as it passes through the heart or into the myocardium, ventricular function and myocardial perfusion can be assessed. Technetium images blood volume and can be used to demonstrate abnormal wall motion and EF. Thallium, which is taken up by the myocardium, may be used to assess regional blood flow. These techniques can be used before and after exercise or pharmacologically-induced stress, e.g. dobutamine infusion.

Computed Tomography/Magnetic Resonance Imaging

ECG-gated, multi-slice scanning, real-time motion and 3D reconstruction has led to the incorporation of CT and MRI in the preoperative assessment of many cardiac surgical patients. CT can demonstrate coronary anatomy and disease less invasively than traditional angiography, and MRI can be used to assess valvular lesions, especially in complex cases or when previous surgery has taken place. Anatomy of the aorta and pulmonary arterial system can be delineated and parameters measured accurately, facilitating surgical decision-making.

Additional Investigations

Respiratory function tests, arterial blood gas analysis, carotid ultrasonography, creatinine clearance and evaluation of a permanent pacemaker or cardio-defibrillator should be conducted as appropriate.

Preoperative Drug Therapy

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