Off-pump cardiopulmonary bypass

Background

This case aims to elicit knowledge and understanding of off-pump coronary artery bypass graft (OPCABG) surgery.

Learning outcomes

1 Outline differing degree of coronary artery disease and treatment options for each

2 Discuss advantages and disadvantages of OPCABG

3 Describe an anaesthetic plan for such a case.

CPD matrix matches

2A03; 2A07

Case history

You are asked to anaesthetize a 64-year-old lady for coronary bypass graft surgery on your elective list tomorrow. She has presented to the cardiothoracic ward the evening prior to surgery, having already been assessed at a surgical pre-admission clinic.

Following discussion with the surgical team, she has opted to have her coronary bypass grafting done ‘off pump’. The surgeon has indicated that she will require three grafts to be performed.

Her past medical history includes symptomatic ischaemic heart disease, hypertension, hyperlipidaemia, and type II non-insulin-dependent diabetes mellitus. She has a raised BMI of approximately 32 and is a lifelong smoker of 20 cigarettes per day.

What are the indications for coronary artery bypass grafting?

◆ The aim of CABG is to improve survival and to relieve the symptoms of myocardial ischaemia; these occur when an area of the myocardium receives an inadequate blood supply due to the occlusion (partial or full) of a coronary artery by atheromatous plaque or thrombus

◆ Emergency CABG is recommended in patients with acute MI, in whom a primary PCI has failed or cannot be performed

◆ CABG is recommended for patients with >50% stenosis of the left main coronary artery to improve survival

◆ CABG is advisable in patients who have a >70% stenosis in three or more coronary arteries or if the proximal left anterior descending artery is stenosed plus one other coronary artery

◆ CABG is advised in patients who are unable to tolerate antiplatelet therapy and are thus precluded from PCI.

What do you understand by the term ‘off-pump’?

◆ ‘Off-pump’ indicates that the patient will not require cardiopulmonary bypass (CPB) in order to have her CABG carried out

◆ This means that the patient’s surgery will be performed on a beating heart.

What are the perceived potential advantages of off-pump coronary artery bypass grafting?

◆ The intended benefits of OPCABG relate to the avoidance of CPB and not needing to cross-clamp the ascending aorta

◆ Reduced systemic inflammatory response

◆ Reduced incidence of consumptive coagulopathy

◆ Reduced incidence of neurological dysfunction

◆ Reduced incidence of renal dysfunction

◆ Reduced blood loss and requirement for transfusion of red blood cells and coagulation factors.

What makes off-pump coronary artery bypass grafting technically possible?

◆ Cardiac stabilization devices are used to stabilize a small area of the myocardium around the area where the anastomosis between the coronary artery and the graft is being carried out

◆ Myocardial protection from ischaemia is achieved through the use of a surgical shunt that is placed in the coronary artery to facilitate distal perfusion, whilst the anastomosis is being carried out. The shunt is then removed prior to completion of the anastomosis

◆ Myocardial protection is also facilitated by anaesthetic technique by reducing the myocardial oxygen demand by preventing tachycardia and also by maintaining the coronary perfusion pressure through the maintenance of an adequate MAP.

What complications should you be aware of when anaesthetizing patients for off-pump coronary artery bypass grafting?

◆ The most significant complications of OPCABG relate to the movement of the heart in order to access the distal, lateral, and posterior coronary arteries. This can result in haemodynamic instability via three mechanisms:

1 Moving the heart into a vertical position causes a dramatic fall in cardiac output due to changes in right heart pressures and ventricular filling having to occur through flow against gravity, and therefore an increased reliance on atrial contraction for ventricular filling. There is also a concomitant reduction in venous return

2 Cardiac wall stabilization devices apply pressure to the ventricular wall, which results in localized areas of ventricular wall motion abnormality

3 The vertical heart distorts the tricuspid and mitral valves, which result in a significant increase in valvular regurgitation

◆ Patients are also at risk of intraoperative ischaemia, as there will be a period where the already threatened myocardium will have its blood supply reduced or even stopped

◆ Arrhythmias: these can be either atrial or ventricular, although there is a higher incidence of tachyarrhythmias than of bradyarrhythmias

◆ Sustained haemodynamic instability or persistent ventricular fibrillation may require the patient to be stabilized on CPB. Facilities for urgent conversion from off-pump to on-pump should be available in theatre.

What information would you like to obtain during your preoperative visit?

History

◆ Presence of symptoms relating to their ischaemic heart disease and the severity of these symptoms, i.e. whether they have stable or unstable angina or nocturnal symptoms

◆ Functional capacity or exercise tolerance

◆ Frequency with which they have to use sublingual GTN

◆ Concurrent comorbid conditions

◆ Previous anaesthetic history

◆ Current medication history, including potential allergies and if still taking any anticoagulant or antiplatelet therapy

◆ History of reflux symptoms

◆ Smoking history.

Examination

◆ Airway assessment, including assessment of dentition

◆ Cardiorespiratory examination, with particular emphasis on identifying signs of congestive cardiac failure or concurrent respiratory disease

◆ Baseline vital observations: NIBP, HR, temperature, RR

◆ Height, weight, BMI

◆ Ease of vascular access, given her raised BMI

◆ Ease of palpation of radial arteries (preoperative coronary angiography is often done via the right radial artery that can be difficult to palpate for a period afterwards).

Case update

This lady describes angina symptoms of chest tightness on exertion, usually precipitated when she is in a hurry to arrive somewhere on time and has to walk briskly for >200 yards. Her angina does not come on at rest, and it is relieved by using her GTN spray. She has previously had a general anaesthetic for an emergency Caesarean section and reports that, other than discomfort, she had no adverse reaction to her general anaesthetic. She normally takes aspirin but has stopped this 3 days ago. She is also taking atenolol, lisinopril, bendroflumethiazide, metformin, and rosiglitazone.

Her BP is 162/88, HR 58 bpm, temperature 36.2°C, and RR 18 breaths/min.

What investigations would you wish to have been performed?

Many investigations will have been done prior to referral for CABG, and those results should be available to you.

◆ Coronary angiography: used to identify and quantify coronary artery lesions

◆ Exercise tolerance test: also known as the treadmill test, which uses the Bruce protocol of increasing physical activity to determine if ischaemia is reproducible. Not always performed, as it is used more often as a diagnostic tool for individuals with less severe disease

◆ Echocardiography: will allow the assessment of LV function and identify any areas of regional wall motion abnormality. It will also allow the identification and assessment of any valvular lesions. Stress echocardiography may be of use to identify areas of inducible ischaemia.

Other investigations that you will need with up-to-date results are:

◆ ECG: indicates the presence of LV hypertrophy, conduction abnormalities, the presence of a sinus rhythm or an underlying arrhythmia, and ischaemic changes. An ECG should be done on admission to the cardiothoracic unit and should serve both as a diagnostic aid and as a baseline for comparison with subsequent ECGs

◆ Blood tests: FBC, coagulation studies, U&E, including calcium and magnesium, and blood glucose, as a minimum, and any others that are deemed necessary, based on the patient’s concurrent comorbid conditions or current medications. A blood sample should be sent to blood bank to ensure that cross-matched blood will be available

◆ CXR: may demonstrate cardiomegaly or signs of congestive cardiac failure. Also any lung lesions or lung disease should be looked for

◆ Pulmonary function tests: a useful baseline, especially in smokers.

What information would you like to provide to the patient during your preoperative visit?

◆ Planned anaesthetic technique, including risks and benefits, in order to gain informed consent

◆ Which of their usual medications to take or withhold. It is important to give explicit instructions to both patient and nursing staff and to document clearly in the prescription record. In this patient, it is important to continue their β-blocker, and this should be administered on the morning of surgery. They should also have in place a perioperative plan for the control of their blood glucose; this is usually achieved through the administration of an insulin infusion

◆ Antiplatelet therapy is usually stopped 7 days prior to surgery; however, this practice varies, according to surgical preference. In high-risk patients, aspirin may be continued purposefully, with the knowledge that this may increase the risk of intra- and post-operative blood loss

◆ Information as to invasive monitoring

◆ Information regarding the use of TOE intraoperatively

◆ Post-operative ventilation and critical care, although one of the advantages of OPCABG is the possibility of extubation at the end of the procedure

◆ Post-operative analgesia

◆ Premedication

◆ Fasting guidelines

◆ Answers to any questions that the patient may have.

How would you conduct anaesthesia for this patient?

◆ It is common to prescribe premedication to patients for cardiac surgery, and this may take the form of: anxiolysis, e.g. benzodiazepines; anti-reflux, e.g. a proton pump inhibitor (PPI) or H2 receptor antagonist; or analgesic medication, e.g. paracetamol or opioid medications. In this instance, the premedication should be with short-acting agents, as one of the goals of ‘off-pump’ CABG is extubation at the end of the case

◆ Inform the anaesthetic team of your management plan, with emphasis on any particular risks or concerns regarding the patient’s care

◆ Draw up and clearly label all drugs and infusions prior to the patient’s arrival

◆ Check the anaesthetic machine and equipment are functioning appropriately and that the monitoring equipment is present and functional

◆ On the patient’s arrival in the anaesthetic room, check the patient’s identification, and comply with the WHO and local checklist protocols

◆ Site a large-bore peripheral cannula, and insert an arterial line under local anaesthetic

◆ Central venous cannulation (for CVP monitoring and the administration of vasoactive infusions) ± pulmonary artery catheter placement (for monitoring pulmonary artery pressures and measuring the cardiac output) are usually carried out after the induction of anaesthesia but can be carried out under local anaesthetic in the more unwell patient

◆ Any of the common induction agents would be suitable, but it is the dose and rate of administration that are important. Opioids and benzodiazepines are commonly used co-induction agents

◆ A neuromuscular-blocking agent is used to facilitate endotracheal intubation

◆ Either TIVA or a volatile-based anaesthetic for maintenance would be equally acceptable

◆ The core temperature should be monitored, and an aggressive approach to maintaining normothermia undertaken, including the use of warmed IV fluid, forced air warmers, and raised theatre ambient temperature

◆ A urinary catheter is inserted to allow accurate monitoring of the urine output

◆ TOE is considered by many to be routine for patients undergoing cardiac surgery (except if there is a specific contraindication, e.g. oesophageal disease)

◆ ABGs are performed, as indicated or as per unit protocols. Parameters that are specifically looked for are K⁺ levels (≥4.5 mmol/L), Hb, acid–base status, and glucose

◆ Heparin 150 IU/kg (half of the dose given to patients requiring CPB) is given on instruction of the surgeon in the aim of achieving an activated clotting time (ACT) of >300 s.

When compared to on-pump coronary artery bypass grafting, does off-pump coronary artery bypass grafting result in reduced incidence of morbidity and mortality?

◆ A recently published systematic review and meta-analysis by the Cochrane trials group has indicated:

1 OPCABG showed an increase in all-cause mortality

2 OPCABG showed a reduction in the incidence of AF

3 There was no statistical difference in the incidence of MI, stroke, or renal insufficiency

◆ The authors concluded that CABG should be carried out on-pump, using sternotomy, CPB, and cardiac arrest, unless there are contraindications to aortic cannulation and CPB

◆ The majority of studies into OPCABG indicated a reduction in the number of transfusion products required.

Summary

Cardiac anaesthesia for OPCABG should only be performed by cardiac anaesthetists who are familiar with the indications for the procedure and with the complications associated with the manipulation of the beating heart and the resultant cardiovascular instability. Expertise in intraoperative TOE is helpful and can be used to both monitor the patient and make diagnoses that may assist the surgical team. OPCABG is indicated in those patients in whom there are contraindications to aortic cannulation and/or CPB.