by Avril Lowry
Cardiovascular disease and its contributory factors is the most significant public health problem that we have faced in recent times.
The WIN Continuing Education section is continuing to focus on this important clinical area throughout 2007, given that it impacts on all areas of the Irish health service.
Diabetes mellitus is one of the primary risk factors for the development of coronary heart disease, and the focus on diabetes management will also continue this year. Guidelines for the management of both CVD and diabetes include: risk factor management, lifestyle modification, pharmacological intervention and patient education.
This month the Cardiology Module focuses on the benefits of coronary artery bypass surgery (CABG), the principle of which is to provide blood supply to sections of the heart muscle restricted by a blocked artery.
The management of coronary heart disease has evolved significantly in recent years due to developments in both surgical and percutaneous revascularisation techniques. The majority of patients with chronic stable angina are still treated with medical therapy, however revascularisation with either coronary artery bypass grafting or percutaneous coronary intervention is necessary in several subgroups.
Although percutaneous coronary intervention (PCI) has relieved many patients of the symptoms of ischaemic heart disease and reduced the number of people currently awaiting cardiac surgery, there are still numerous patients who will benefit from coronary artery bypass grafting (CABG).1 The principle of CABG is to provide blood supply to sections of the heart muscle currently restricted by a blocked artery. The benefits of surgery are magnified in patients with more severe disease, certain anatomical patterns of disease and those with impaired left-ventricular disease.2
Survival for patients with poor left ventricular function in association with vessel disease was significantly improved with CABG – 88% versus 66%.3 Improved ventricular function may reflect hibernating myocardium, which refers to an ischaemia-induced decline in cardiac contractility that can be reversed over several days or weeks following revascularisation.
The conduit that supplies the new blood supply can be an internal mammary artery, radial artery or a section of saphenous vein removed from the leg. Studies show that 90% of internal mammary artery grafts are patent after 10 years in comparison to only 60% of grafts using saphenous vein (SVG). Graft patency is dependent on a number of factors including the type of graft used, the size of the coronary artery that the graft is anastomosing with, and the skill of the operator performing the procedure. Furthermore enhanced perioperative mortality can be associated with coronary artery diameter. This fact may explain the increased risk that has been observed in women, particularly those less than 50 years of age, as the diameter of women’s vessels is known to be smaller than those of men.
Ideally a multidisciplinary team should evaluate patients at a pre-admission clinic four to six weeks prior to surgery. This assists in streamlining services and provides education to patients and families regarding their cardiac condition and proposed surgery. This is an ideal time to refer patients to other disciplines for further workup or management of existing conditions, which may complicate post-operative recovery.
Patients’ risk factors are evaluated using the Euroscore algorithim a prediction for cardiac surgical mortality. Other factors which must be taken into consideration pre-operatively include:
Improvements in surgical techniques, cardiopulmonary bypass and anaesthesia have led to a significant reduction in morbidity after CABG. Despite these advancements patients undergoing CABG currently have higher risk factors8 than those in the past. The procedure takes approximately four to five hours.
Following line insertion and intubation by the anaesthetist the patient undergoes a procedure in four stages:
An optimal surgical result depends on protecting the heart from damage that might ensue during a corrective operation. Protective methods include hypothermia which lowers cellular energy requirements during ischaemia and the administration of cardioplegia. Cardioplegia is a high concentration of potassium delivered to the heart which arrests the heart during diastole.
The major complications following CABG include low cardiac output, cardiac complications, bleeding, pericardial effusions, pleural effusion, stroke and wound infection.
Low cardiac output
Low cardiac output is a frequent complication post-operatively, primarily due to left ventricular dysfunction, which can result from:
This complication is often transient and responds to fluid replacement and or INOtropic support.
Perioperative MI diagnosis is difficult to make following CABG as cardiac enzymes will automatically be elevated and ECG changes may reflect post-operative pericardial inflammation. A new Q wave on the post-operative ECG is strongly suggestive of a new MI with a poor outcome.10
Arrhythmias, most often tachyarrhythmias are common after CABG. Atrial fibrillation (AF) and atrial flutter occur frequently after most types of cardiac surgery. AF has been reported in up to 15%-40% of patients in the early post-operative period following CABG, and 60% undergoing valve replacement plus CABG. The initial treatment of AF is beta-blockers and/or amiodarone to control rate. AF that develops following CABG is usually self limiting in patients without a prior history, and reverts spontaneously to sinus rhythm within 24 hours in 80% and six to eight weeks in 90% of patients.7
Some blood loss is anticipated following CABG. Excessive bleeding requiring reoperation can occur in 4% of the population.13 Anti-platelet agents such as clopidogrel have a variable effect on bleeding risk. Clopidogrel therapy administered within five days of the procedure appears to be associated with an increased bleeding risk.1,6
Post-operative pericardial effusion is more common although many patients are asymptomatic. The effusion is usually present by the second post-operative day, but may not occur until day 10. In most cases, the effusion is small and clinically insignificant, however if large it may result in tamponade and haemodynamic instability, requiring urgent reoperation.
Pleural effusions are common post-operatively, occurring in up to 90% of patients who have undergone CABG. Most effusions develop as a consequence of the surgical procedure itself and follow a benign course. The effusions are usually small, left sided and don’t require treatment.
Neurological complications are an important cause of morbidity and mortality following CABG. The major neurological complications include stroke, neuropsychiatric abnormalities such as cognitive dysfunction, and peripheral neuropathy. This is a risk factor which increases with the patient’s age. 9
Sternal wound infection
Sternal wound infection (mediastinitis) following CABG has been reported in 0.9%-1.3% of patients.11 It is usually detected within the first two weeks, but the onset can be delayed for more than one month. Streptococcus and staphylococcus are the most frequent organisms cultured. A number of risk factors have been identified for the development of mediastinitis after CABG, although the same risk factors were not noted in all studies. These include:
Leg wound complications
The reported incidence of leg wound complications after SVG harvesting varies widely, ranging from 3%-24%. The most common manifestations are usually minor and do not require surgery.
Cardiac surgery is not without its risks but improvements in surgical technique have led to a steady reduction in morbidity following CABG, despite the fact that patients currently undergoing the procedure have a higher risk profile due to associated comorbidities. The perioperative and in-hospital mortality rate following CABG averages about 1% for the lowest risk elective patients and 2%-5% for all patients.3 The majority of patients who undergo uncomplicated CABG are discharged within five to seven days post-operatively and recover well at home with family and public health nurse support.
Avril Lowry is a cardiothoracic advanced nurse practitioner at St James’s Hospital, Dublin
Next month: A focus on the role of the cardiothoracic advanced nurse practitioner
References on request from email@example.com (quote: Lowry A. WIN 2007; 15(2): 41-42)