Patient Population

Isolated CABG operations represent about 50–60% of the cardiac surgery workload in the UK. As percutaneous coronary intervention is now often offered early in the course of ischaemic heart disease, patients presenting for CABG are now older and sicker and this trend is set to continue. In addition, people live longer due to advances in treatment of chronic diseases, including hypertension, elevated lipids, diabetes and others. Despite this, mortality from CABG is at an all-time low of between 1 and 2% due to better perioperative management. A low-risk patient with a competently executed CABG operation should have a smooth and uneventful recovery, an ITU stay shorter than 24 hours and an overall hospital stay of around 5 days. In a high-risk patient, however, with multiple risk factors and comorbidities, or after a suboptimally performed CABG operation, the procedure may be followed by complications and longer postoperative hospitalisation.

The limited functional reserve of elderly patients, the presence of chronic kidney disease, peripheral vascular disease, COPD, diabetes and impaired cardiac function are but a few of the challenges which may be faced in the management of this high-risk population.

Haemodynamic Management

In the majority of low-risk patients, there is no need for cardiovascular support and an adequate cardiac output can be obtained just with fluid management and rate control by means of temporary pacing wires.

The stroke volume, SV, is determined by preload, contractility and afterload.

If contractility and afterload are within the normal range, it is obvious that cardiac output can be optimised with fluids (preload) or by simply increasing the heart rate with temporary cardiac pacing. Determination of the optimal filling pressure is often empirical and can be estimated with the use of a fluid challenge and observing the haemodynamic response (see Chapter 14).

If adequate perfusion pressure cannot be maintained with these simple, first line measures, contractility and afterload may need to be addressed pharmacologically. Before making the diagnosis of impaired contractile cardiac function, it is important to exclude tamponade (see Chapter 3). Impaired contractility can be improved with either pharmacological or mechanical means. Dopamine and catecholamines are often the first choice but phosphodiesterase inhibitors may be beneficial in right ventricular dysfunction and elevated pulmonary vascular resistance (see Chapter 15).

The most commonly used mechanical support device is the intra-aortic balloon pump (IABP) (see Chapter 21) which, by simultaneously decreasing myocardial workload and improving coronary perfusion, can immediately alleviate myocardial ischaemia and improve poor cardiac function. The routine use of IABP in routine CABG is not necessary, but in selected high-risk patients there is a significant advantage in survival provided by this device. This was supported by several randomised trials and a recent meta-analysis.

The use of inotropic drugs, vasoconstrictors and mechanical support in the postoperative management of the CABG patient can be guided by clinical assessment. Features of low cardiac output include hypotension, poor peripheral perfusion, falling urine output despite adequate preload and metabolic evidence of impaired perfusion, such as acidosis. In a CABG patient with low blood pressure and inadequate perfusion, it can sometimes be difficult to differentiate between inadequate filling (preload), poor cardiac function (contractility) or low systemic vascular resistance (afterload). The cause may also be multifactorial. If there is doubt in interpreting clinical and simple haemodynamic findings, more invasive haemodynamic monitoring using transoesophageal echocardiography or a pulmonary artery catheter will directly provide information on the cardiac output and on the state of preload and afterload of both the pulmonary and systemic circulations, thus helping tailor the therapy to the haemodynamic needs of the patient. Routine use of a pulmonary artery catheter is controversial and is not supported by clear evidence. Nevertheless, it may play a role in high-risk patients, but the risk of adverse events and the costs associated with it suggest that its use be restricted to those patients in whom the aetiology of the haemodynamic instability cannot be identified by more simple methods and those who require multiple vasoactive agents and their fine titration.

Myocardial Infarction

Myocardial ischaemic event presenting soon after CABG surgery is a rare but troublesome finding that can be difficult to diagnose and to treat. Many factors can contribute to myocardial injury during a CABG operation, including surgical trauma due to manipulation and suturing, less than ideal myocardial protection, incomplete revascularisation, acute graft thrombosis and iatrogenic injury to the native vessels. Because of these confounding factors, the diagnosis of MI following CABG is not straightforward and uncertainties remain around the correct threshold for biomarker elevation and the need for associated criteria. According to the 2012 third universal definition of myocardial infarction after CABG, at least two of the following criteria should be present for diagnosis: cardiac biomarkers (with troponins preferred) rise >10 times 99% upper reference limit from a normal preoperative level; and new pathological Q-waves or new left bundle branch block (LBBB) or imaging or angiographic evidence of new occlusion of native vessels or grafts, new regional wall motion abnormality, or loss of viable myocardium. In daily clinical practice in many centres, cardiac biomarkers are not routinely measured after CABG surgery. Therefore, in the immediate postoperative period, when the patient is fully sedated and intubated, the first sign of possible ongoing myocardial ischaemia or infarction may be unexpected haemodynamic or rhythm instability. This finding should lead to a thorough assessment of the patient, which includes ECG for new abnormalities, echocardiogram for new regional wall motion abnormalities and cardiac biomarkers. If it is then believed that myocardial ischaemia is a genuine possibility, a low threshold for emergency angiography should be adopted. If a technical problem such as acute graft or native vessel occlusion is demonstrated, surgical revision and emergency re-grafting of the affected territory should be considered as it may save both myocardium and life. Additional treatment options include an intra-aortic balloon pump, GTN infusion, antiplatelet agents and beta-blockers.

Acute Kidney Injury

Acute kidney injury (AKI) is a common postoperative complication of cardiac surgery and is associated with an increased risk of morbidity, mortality and length of stay

Between 5% and 30% of patients undergoing CABG may suffer this complication in the postoperative period. The use of serum creatinine rather than urine output as main criterion for the diagnosis of postoperative AKI is appropriate in the cardiac surgery population, since manipulation of the urine output with diuretics makes this a less reliable indicator of AKI. A limitation of defining AKI based on serum creatinine measurements is the delay in detecting the onset of injury. Optimisation of renal perfusion, avoidance of hypovolaemia and avoidance of nephrotoxins are the only options to prevent or mitigate the effects of postoperative AKI. Pharmacological interventions have been attempted with inconsistent results, and there are no drugs that have demonstrated a definitive renal protection.

The widely used drug furosemide was found not to be protective and to be potentially harmful, as the incidence of AKI was twice that of the dopamine or placebo group in a double-blind randomised controlled trial. Similar negative results have been seen in other studies.

Other Postoperative Complications

Arrhythmia: Postoperative arrhythmia is common in cardiac surgical patients. The commonest observed arrhythmia is atrial fibrillation. The precise reasons for developing arrhythmias are not always clear, but overall the prognosis is good as frequently these recover and sinus rhythm is restored. It is important to exclude poor right ventricular perfusion as a cause of malignant ventricular arrhythmias. Occlusion of right coronary graft needs be ruled out in such cases. The treatment of postoperative arrhythmia is no different from the treatment for other patients in the ICU, and is described in Chapter 10.

Heart block: Temporary heart block is not a prominent feature of the CABG patients, but can occur. It normally resolves in the first few days, but sometimes needs implantation of a pacemaker system.

Postoperative bleeding: Patients undergoing coronary surgery often receive antiplatelet medication, which may have long lasting effects, and hence they are at a higher risk of coagulopathy. Thus postoperative bleeding can be a problem. The management of this problem is discussed in detail in Chapter 36.

Haemothorax: CABG patients are at a higher risk than other cardiac patients of bleeding in the left hemithorax when the left internal mammary artery has been used. The ICU team needs to be vigilant for this complication if haemodynamic instability is a problem or if there is a massive blood loss postoperatively.