Chapter 4 – Conduct of Anaesthesia

Abstract

The aims of anaesthesia for cardiac surgery are: prevention of perioperative cardiac ischaemia and arrhythmias, tight haemodynamic control, avoidance of non-cardiac complications and early tracheal extubation. This chapter deals with the management of low-risk patients undergoing elective CABG surgery.

Chapter 4 Conduct of Anaesthesia

Andrew I. Gardner and Paul H. M. Sadleir

Preoperative Assessment

For the majority of elective patients, preoperative assessment should take place several days before surgery in a pre-admission clinic. This allows an assessment of the patient’s ability to withstand the intended surgical procedure and provides an opportunity to explain anaesthetic procedures, obtain consent for specific interventions and discuss appropriate cessation of medication prior to surgery. Having an interval between assessment and surgery permits the early identification of potential problems, allows additional investigations to be undertaken, and alerts support services, such as transfusion, pacing and critical care, of likely demand. This approach significantly reduces the likelihood of delays or cancellation on the day of surgery. This is particularly important for patients considered suitable for admission to hospital on the day of surgery.

The presence of previously documented symptoms and signs (see Chapter 2) should be verified, the results of preoperative investigations (in particular coronary angiography and echocardiography) reviewed and any new or undiagnosed problems excluded.

In addition to a routine systematic preoperative history and examination, specific areas of interest include:

Intended conduit harvest sites: may restrict placement of monitors and cannulae
Recent history of anticoagulant therapy (see below)
Permanent pacemaker/implantable defibrillator: may need reprogramming before induction of anaesthesia
Oesophageal pathology: may be a relative or absolute contraindication for TOE
Religious or cultural beliefs: (e.g. Jehovah’s Witness and blood product transfusion)

All regular anti-anginal, antihypertensive and anti-cardiac failure medications should be continued in the preoperative period (Table 4.1).

Table 4.1 A guide to which regular medications should be continued until the day of surgery

Continue	Controversial	Discontinue
Statins (decreases in-hospital mortality and need for RRT) β-blockers (reduced risk of post-CABG AF) Nitrates Calcium antagonists Potassium channel openers Corticosteroids Antidysrhythmics Bronchodilators	Aspirin ACE inhibitors Angiotensin receptor blockers	Thienopyridines (e.g. clopidogrel, prasugrel) GP IIb/IIIa inhibitors (e.g. tirofiban) DOACs (e.g. rivaroxaban) Diuretics NSAIDs MAO inhibitors Biguanides (metformin)

RRT, renal replacement therapy; GP, glycoprotein; DOAC, direct-acting oral anticoagulant; MAO, monoamine oxidase.

Oral hypoglycaemic agents should be managed according to institutional protocols to maintain normoglycaemia during the fasting periods. It is recommended that sodium-glucose co-transporter-2 inhibitors be stopped two days before surgery to reduce the risk of perioperative euglycaemic diabetic ketoacidosis. This may require the introduction of other diabetic therapies to ensure normoglycaemia between preoperative assessment and surgery.

The optimal timing of cessation of antiplatelet and anticoagulant therapy is determined by balancing the risk of perioperative bleeding against the risk of thrombotic complications.

Thienopyridines should be discontinued at least 5 days before surgery.
Aspirin withdrawal remains controversial: continuation of therapy during the 5 days before surgery may reduce early (in-hospital) mortality and improve graft patency without increasing the risks of reoperation for bleeding or transfusion. However, published evidence is inconclusive; in many centres concern about the risk of haemorrhagic complications still prompts aspirin cessation 7-10 days before surgery, while in other centres aspirin therapy is continued up until the day of surgery, particularly in patients with critical coronary artery stenoses.
Direct-acting oral anticoagulants (e.g. dabigatran, rivaroxaban, apixaban) should be discontinued 72 hours before surgery .
Other antithrombotic agents used in secondary prevention, such as tirofiban or unfractionated heparin by infusion, are typically withdrawn 2-4 hours before surgery.

Premedication

Despite trends in other anaesthetic subspecialties, sedative premedication remains common in cardiac anaesthetic practice. The stated goals are: minimization of the risk of cardiac ischaemia secondary to anxiety, hypertension and tachycardia while at the same time avoiding respiratory depression. Although opioids, benzodiazepines and antihistamines are commonly prescribed, the final choice of drugs is subject to institutional variability (Box 4.1). The use of gabapentanoids as premedication is increasing, for both attenuation of sympathetic response during general anaesthesia and reduction of acute and chronic pain after cardiac surgery. Longer acting amnestic drugs have the advantage of covering the early ICU period. As respiratory depression is a known sequel of sedative premedication, supplemental oxygen should be prescribed and administered until induction of anaesthesia.

Box 4.1 Examples of premedicant drugs in cardiac anaesthesia

Oral (90–120 minutes prior to induction of anaesthesia):

Lorazepam 2–4 mg
Temazepam 10–20 mg
Clonidine 100–150 μg
Pregabalin 75–150 mg
Methadone 0.1–0.2 mg kg^–1

Intramuscular (45–60 minutes prior to induction of anaesthesia):

Morphine sulphate 0.2–0.3 mg kg^–1 + hyoscine hydrobromide 200–400 μg

Patients are often given supplemental oxygen after the administration of sedative premedicants

Preparation

As for all anaesthetic procedures, the availability of drugs, equipment and staff (surgeon, nursing staff, perfusionist) should be checked prior to the patient’s arrival in the operating suite. Drugs that should be immediately available include: inotropes, antidysrhythmics, calcium, magnesium, heparin and protamine (Box 4.2).

Box 4.2 Preparation for cardiac anaesthesia

Equipment
- Anaesthetic machine, laryngoscopes and intubation aids, suction apparatus
- Monitoring (standard anaesthetic monitoring plus pressure transducers, depth-of-anaesthesia monitoring, TOE, ABG, ACT)
- Infusion pumps and transfusion apparatus
- Arterial and venous cannulae
- Defibrillator and external pacemaker box
- Ultrasound for venous access
Drugs to be drawn up
- Anaesthetic
- Local anaesthetic (lidocaine)
- Analgesic (e.g. fentanyl, sufentanil, remifentanil, alfentanil)
- Muscle relaxant (e.g. pancuronium, rocuronium)
- Induction agent (e.g. etomidate, propofol, midazolam)
- Cardiovascular
- Vagolytic (atropine, glycopyrrolate)
- Vasopressor (metaraminol, phenylephrine)
- β-blockers
- Other
- IV fluids
- Prophylactic antibiotics
- Anticoagulant (heparin)
- Antifibrinolytic agents (tranexamic acid, ε-aminocaproic acid)

Anaesthetic/Operating Room

On arrival, the identity of the patient should be verified and consent for surgery confirmed in accordance with the WHO ‘Safe Surgery’ checklist. The availability of cross-matched blood should also be checked. The operative site should be clearly marked if appropriate (e.g. thoracotomy, radial artery harvest site). Non-invasive monitoring (ECG, NIBP and pulse oximetry) is then instituted prior to any anaesthetic procedures.

Vascular Access and Invasive Monitoring

Cannulae are sited under local anaesthesia in a forearm vein (14 G) and the non-dominant radial artery (20 G). In the case of non-dominant radial artery harvest, either the dominant radial artery or femoral artery may be cannulated. An IV infusion is then commenced, and arterial pressure transduced and monitored. Femoral arterial pressure monitoring is used when both radial arteries are required for conduits.

Insertion of central (i.e. internal jugular or subclavian) venous cannulae may then be undertaken; however, in many centres this is deferred until after induction of anaesthesia. With appropriate training, the use of ultrasound to guide internal jugular vein cannulation has been shown to reduce complications and the number of attempts required.

Inserting a PAFC in low-risk CABG surgery patients has little impact on clinical management or outcome, and routine use of the device is declining. A PAFC sheath may, however, be inserted for central vascular access, and has the advantage of facilitating subsequent PAFC insertion if indicated after separation from CPB. Many cardiac anaesthetists use a PAFC sheath in combined or complex procedures, and in patients with poor LV function.

Depth-of-anaesthesia monitoring by processed EEG devices may reduce the incidence of awareness in this group of elective CABG patients long considered to be at high risk. An additional advantage of using these devices is that it is possible to reduce the dose of anaesthetic agents and consequently reduce drug-induced cardiovascular depression and cost.

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