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Anaesthesia for cardiac surgery


In the UK and much of the developed world, more than half of all cardiac surgical procedures are undertaken to revascularise 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 pathological conditions, comorbid conditions and concomitant medications 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 teamwork 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.

The number of older patients undergoing cardiac surgery of all types has increased over recent years such that patients older than age 75 now make up more than 20% of the cardiac surgical population, and more than 5% are older than 80.

Ischaemic heart disease

Since being popularised in the late 1960s, revascularisation by coronary artery bypass grafts (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. Improved surgical techniques have increased the popularity of off-pump coronary revascularisation, 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), such as angioplasty, atherectomy and stenting of coronary arteries ( Fig. 42.1 ), in the cardiac catheter laboratory. 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.

Fig. 42.1, Diagrammatic representation of coronary arteries.

Valvular 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 after 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, lifelong 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 older than 70 years at the time of surgery.

Transcatheter aortic valve replacement (TAVR; also known as transcatheter aortic valve implantation, TAVI) has become much more common in the last few years. It allows replacement of the aortic valve without sternotomy or CPB, and the prosthetic tissue valve is rolled up and crimped into a tube-like shape before deployment. The most common approach is transfemoral, during which the femoral artery is accessed and a wire passed up into the ascending aorta under fluoroscopic control. The aortic valve is crossed, and the prosthetic valve is then inserted before being deployed; exact positioning is crucial to minimise the risk of valve embolism. The most common complication of the procedure is damage to, or dissection of, the femoral or iliac artery. If the femoral/iliac artery is not suitable because of small size or excessive calcification, alternative approaches, including transapical (mini-thoracotomy) or transaortic (mini-sternotomy), are possible. Anaesthetic management for transfemoral TAVR commonly involves conscious sedation and regional blockade, because this is associated with greater haemodynamic stability and reduced procedure and recovery time.

Arrhythmias

Most ablation procedures undertaken to treat arrhythmias are minimally invasive and undertaken by cardiologists in the cardiac catheter laboratory. Until recent times, the majority of these were facilitated by sedation administered by the cardiologist and/or cardiac catheter nurses. However, recent studies have demonstrated improved ‘cure’ rates and longevity when facilitated by general anaesthesia, leading to an increase in the demand for anaesthetic services in cardiac catheter labs. The majority of procedures are for recurrent atrial fibrillation or flutter or supraventricular tachycardias. Mapping the heart for electrical activity and subsequent ablation takes 2–3 hours on average but can last up to 4 or 5 hours. Haemodynamic instability is more of a feature during ablation for ventricular tachycardia, during which invasive arterial monitoring is recommended. Processed EEG monitoring is recommended for all ablation treatments, as excessively deep anaesthesia (e.g. bispectral index <25) is associated with failure to map electrical currents.

Implantable rhythm management devices

Sedation or general anaesthesia may be required for pacemaker or implantable cardioverter defibrillator (ICD) insertion, especially if tunnelled deep under the pectoral muscles or if ventricular arrhythmias are to be induced to check function. In patients with cardiomyopathy or severely impaired left ventricular (LV) function, ICDs may be implanted prophylactically to prevent sudden death. Implantable cardioverter defibrillators are now the definitive therapy for patients at high risk for malignant ventricular arrhythmias (primary prophylaxis) and for patients who have survived a malignant arrhythmia (secondary prophylaxis). Consideration should be given to suspending the defibrillator function of ICDs during surgery, as diathermy may prompt unnecessary defibrillation ( Box 42.1 ).

Box 42.1
Potential adverse events and outcomes in patients with implantable cardiac defibrillators (ICDs)

  • Damage to the device, the leads or site of lead implantation

  • Failure to deliver pacing, defibrillation or both

  • Changes in pacing behaviour

  • Inappropriate delivery of a defibrillatory shock

  • Inadvertent electrical reset to backup pacing modes

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 adult congenital heart disease (ACHD) centres have been created to cater to 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 CPB circuit ( Fig. 42.2 ) are:

  • venous reservoir;

  • pump;

  • oxygenator;

  • heat exchange unit;

  • cardioplegia delivery system; and

  • connecting tubes and filters.

Fig. 42.2, Components of a cardiopulmonary bypass circuit.

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 or 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-A 2 and leukotrienes, and they are responsible for the hypotension and increased bleeding associated with CPB.

Blood from 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 ‘arterialised’ 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 aspirated 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 systemic 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). 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, semiocclusive 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 min) 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 semipermeable membrane which separates gas and blood phases and through which gas exchange occurs. Commercially available devices have an effective exchange area of around 7 m 2 – one-tenth of the alveolar surface area of an adult.

Connecting tubes, filters, manometer, suction

Tubes, filters, manometer and suction 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) before 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 (those with low body weight, children or patients with preoperative anaemia, when dilution would reduce the haematocrit to <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 1–2 weeks before surgery. Despite undergoing an extensive array of specialised investigations to diagnose and quantify cardiac disease, there is evidence that a significant number of cardiac surgical patients have additional and hitherto undocumented pathological conditions. Thorough preoperative evaluation by the anaesthetist remains an essential component of perioperative care (see Chapter 19 ).

Exercise electrocardiography

Exercise (treadmill) testing is often 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 catheterisation

Left-sided heart catheterisation typically comprises coronary angiography, aortography, left ventriculography and manometry. This provides the following information:

  • Site and severity of coronary artery disease

  • Mitral and aortic valve function

  • Left ventricular morphology and function

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


( end-diastolic volume − end-systolic volume ) / end-diastolic volume

Right-sided heart catheterisation allows measurement of right-sided heart and pulmonary artery pressures. When combined with measurements of cardiac output, these can be used to determine pulmonary and systemic vascular resistances ( Table 42.1 ).

Table 42.1
Measurements obtained during cardiac catheterisation
Parameter Normal values
Left side of the heart Systemic arterial/aortic pressure <140/90 (mean 105) mmHg
LV pressure <140/12 mmHg
Right side of the 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; PA, pulmonary artery; PAWP, pulmonary artery wedge pressure; PVR, pulmonary vascular resistance; RA, right atrium; RV, right ventricle; SVR, systemic vascular resistance.

Echocardiography

Transthoracic echocardiography (TTE) is often used to define cardiac anatomy and assess ventricular and valvular function. It is non-invasive and can be performed at intervals to monitor disease progression and to optimise 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 visualised adequately because of their distance from the probe (such as the left atrium and interatrial septum). Therefore TOE may be required preoperatively (usually performed under sedation); TOE may also be indicated in mitral valve pathological conditions 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).

CT/MRI

Electrocardiogram-gated, multislice scanning, real-time motion and 3-dimensional reconstruction have led to the incorporation of CT and MRI in the preoperative assessment of many cardiac surgical patients. Computed tomography 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.

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

Care is required to balance the risks of discontinuation of medication in the perioperative period against the risk of major adverse cardiovascular events (e.g. withholding antiplatelet agents such as aspirin and clopidogrel).

  • Beta-blockers. Continued administration of these drugs up to the time of surgery is desirable because discontinuation may increase the risk of perioperative myocardial infarction.

  • Calcium channel antagonists. Have a negative inotropic effect, but it is preferable to continue therapy.

  • Nitrates. Should be continued to prevent rebound angina.

  • Digoxin. Discontinued 24–48 h before surgery to diminish digoxin-associated arrhythmias after surgery.

  • Diuretics. Should be continued until the day before surgery.

  • Antiplatelet agents. Aspirin and clopidogrel are usually stopped up to 1 week before surgery to permit platelet function to return towards normal. However, there is recent evidence that stopping aspirin is associated with increased morbidity, and it is continued throughout the perioperative period in many centres.

  • Anticoagulants. Management is discussed in Chapter 19 . The international normalised ratio (INR) should be 1.5 or less in patients taking warfarin, with a target INR of 2.5.

  • Angiotensin-converting enzyme (ACE) inhibitors/angiotensin receptor blockers (ARBs). May produce significant vasodilatation and hypotension intra- and postoperatively. Perioperative use varies from unit to unit; they may be stopped up to 1 week before surgery or continued until the day of operation.

  • Potassium channel activators. May be continued up to the day of operation.

Investigations

Investigations are generally identical to non-cardiac surgery (see Chapter 19 ). Specific differences include the following:

  • Chronic anaemia (Hb <130 g L −1 ) should be investigated and treated because it is associated with blood transfusion and worsened outcomes, including complications, prolonged ICU and hospital stay and increased mortality.

  • Quantitative platelet or leucocyte abnormalities should also be excluded.

  • Clotting studies should be performed before surgery. In the absence of anticoagulant administration, the finding of a seemingly trivial prolongation of the activated partial thromboplastin time (APTT) should prompt further investigation because it may indicate the presence of a factor deficiency.

Risk assessment

Despite advances in surgical techniques, anaesthesia and critical care, cardiac surgery still carries a finite risk of death and serious complications. Although this risk has decreased steadily over the last 10 years (<1% for isolated CABG or aortic valve replacement in young male patients), outcomes vary from centre to centre and from surgeon to surgeon. Although helping the patient to understand the benefits (symptomatic and prognostic) and risks of surgery during the consent process is the responsibility of the surgeon, it is essential that the anaesthetist understands how risk is assessed so that the patient is not given contradictory information.

The European System for Cardiac Operative Risk Evaluation (EuroSCORE) provides a robust risk assessment and can be calculated easily at the bedside ( Table 42.2 ). The EuroSCORE has been validated in the UK, Europe and North America and has been found to be predictive of major complications, duration of ICU stay and resource utilisation.

Table 42.2
The European System for Cardiac Operative Risk Evaluation (logistic version II) risk stratification model (EuroSCORE)
(From Nashef, S. A. M., Rogues, F., & Sharples, L. D., et al. (2012) EuroSCORE II. European Journal of Cardio-Thoracic Surgery. 41, 734–745.)
Factor Description Coefficient
Age ≤60 years
>60 years		 0.0285181
(Age –60) × 0.0285181
Sex Female 0.2196434
Renal impairment CC 50–85 ml min –1
CC <50 ml min –1
On dialysis		 0.303553
0.8592256
0.6421508
Chronic lung disease Bronchodilators or steroids 0.1886564
Extracardiac arteriopathy Claudication, carotid stenosis >50%, abdominal aortic, limb artery or carotid surgery planned or undertaken 0.5360268
Diabetes mellitus On insulin therapy 0.3542749
Poor mobility Severe impairment of mobility secondary to musculoskeletal or neurological dysfunction 0.2407181
Previous cardiac surgery Pericardium opened 1.118599
Active endocarditis On antibiotics 0.6194522
Critical preoperative state VT, VF, cardiac massage, invasive ventilation, inotropic support, IABP, acute renal failure 1.086517
Unstable angina Angina at rest requiring i.v. nitrates (CCS class IV) 0.2226147
Functional status NYHA class II
NYHA class III
NYHA class IV		 0.1070545
0.2958358
0.5597929
LV function Moderate (LV EF 31%–50%)
Poor (LV EF 21%–30%)
Very poor (LV EF ≤20%)		 0.3150652
0.8084096
0.9346919
Recent myocardial infarction Within 90 days 0.1528943
Pulmonary hypertension Moderate (PAP systolic 31–55 mmHg)
Severe (PAP systolic >55 mmHg)		 0.1788899
0.3491475
Urgency Urgent (intervention or surgery on the current admission for medical reasons)
Emergency (operation before the beginning of the next working day)
Salvage (external cardiac massage) en route to the operating theatre or before induction of anaesthesia		 0.3174673
0.7039121
1.362947
Other than isolated CABG Single non-CABG procedure
Two procedures
Three procedures		 0.0062118
0.5521478
0.9724533
Surgery on thoracic aorta 0.6527205
Mathematical constant −5.324537
CABG, Coronary artery bypass graft; CC, creatinine clearance; CCS, Canadian Cardiovascular Society; EF, ejection fraction; IABP, intra-aortic balloon pump; LV, left ventricle; NYHA, New York Heart Association; PAP, pulmonary artery pressure; VF, ventricular fibrillation; VT, ventricular tachycardia.
Predicted mortality = e (sum of coefficients) / 1 + e (sum of coefficients) .

Monitoring

Extensive and accurate physiological monitoring is essential throughout the perioperative period for the safe practice of cardiac surgery. Instrumental monitoring should be considered an adjunct to, rather than a replacement for, routine clinical observation of the patient.

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