Ischemic Heart Disease: Preoperative Management

PREOPERATIVE MANAGEMENT

The importance of ischemic heart disease—particu-larly a history of MI—as a risk factor for periopera-tive morbidity and mortality was discussed earlier. Most studies confirm that perioperative outcome is related to disease severity, ventricular function, and the type of surgery to be undertaken.

Patients with extensive (three-vessel or left main) CAD, a recent history of MI, or ventricular dysfunction are at greatest risk of cardiac com-plications. As mentioned above, current guidelines recommend revascularization when such treatment would be indicated irrespective of the patient’s need for surgery.

Chronic stable (mild to moderate) angina does not seem to increase perioperative risk substantially. Similarly, a history of prior coronary artery bypass surgery or coronary angioplasty alone does not seem to substantially increase perioperative risk. In some studies, maintenance of chronic β-receptor block-ers in the perioperative period has been shown to reduce perioperative mortality and the incidence of postoperative cardiovascular complications; how-ever, other studies have shown an increase in stroke and death following preoperative introduction of β-blockers to “at risk” patients. Consequently, as withall drugs, the risks and benefits of initiating therapy with β-blockers in at risk patients must be consid-ered. Like β-blockers, statins should be continued perioperatively in patients so routinely treated, as acute perioperative withdrawal of statins is associ-ated with adverse outcomes. ACC/AHA guidelines suggest that β-blockers are useful in patients under-going vascular surgery with evidence of ischemia on their evaluative workup (class I).

History

The history is of prime importance in patients with ischemic heart disease. Questions should encom-pass symptoms, current and past treatment, com-plications, and the results of previous evaluations. This information alone is usually enough to provide some estimate of disease severity and ventricular function.

The most important symptoms to elicit include chest pains, dyspnea, poor exercise tolerance, syn-cope, or near syncope. The relationship between symptoms and activity level should be established. Activity should be described in terms of everyday tasks, such as walking or climbing stairs. Patients may be relatively asymptomatic despite severe CAD if they have a sedentary lifestyle. Patients with dia-betes are particularly prone to silent ischemia. The patient’s description of chest pains may suggest a major role for vasospasm (variable-threshold angina). Easy fatigability or shortness of breath suggests impaired ventricular function.

A history of unstable angina or MI should include the time of its occurrence and whether it was complicated by arrhythmias, conduction dis-turbances, or heart failure. Localization of the areas of ischemia is invaluable in deciding which elec-trocardiographic leads to monitor intraoperatively. Arrhythmias and conduction abnormalities are more common in patients with previous infarction and in those with poor ventricular function. This latter group of patients will often have ICDs.

Physical Examination & Routine Laboratory Evaluation

Evaluation of patients with CAD is similar to that of patients with hypertension. Laboratory evalua-tion in patients who have a history compatible with recent unstable angina and are undergoing emer-gency procedures should include cardiac enzymes. Serum levels of cardiac-specific troponins, creatine kinase (MB isoenzyme), and lactate dehydrogenase (type 1 isoenzyme) are useful in excluding MI.

The baseline ECG is normal in 25% to 50% of patients with CAD but no prior MI. Electrocardiographic evidence of ischemia often becomes apparent only during chest pain. The most common baseline abnormalities are nonspecific ST-segment and T-wave changes. Prior infarction is often manifested by Q waves or loss of R waves in the leads closest to the infarct. First-degree AV block, bundle-branch block, or hemiblock may be present. Persistent ST-segment elevation following MI may be indicative of a left ventricular aneurysm. A long rate-corrected QT interval (QT_c> 0.44 s) may reflect the underlying ischemia, drug toxicity (usually class Ia antiarrhythmic agents, antidepressants, or phe-nothiazines), electrolyte abnormalities (hypokale-mia or hypomagnesemia), autonomic dysfunction, mitral valve prolapse, or, less commonly, a congeni-tal abnormality. Patients with a long QT interval are at risk of developing ventricular arrhythmias— particularly polymorphic VT (torsade de pointes), which can lead to ventricular fibrillation. The long QT interval reflects nonuniform prolongation of ventricular repolarization and predisposes patients to reentry phenomena. In contrast to polymorphic ventricular arrhythmias with a normal QT interval, which respond to conventional antiarrhythmics, polymorphic tachyarrhythmias with a long QT inter-val generally respond best to pacing or magnesium salts. Patients with congenital prolongation generally respond to β-adrenergic blocking agents. Left stellate ganglion blockade is also effective and suggests that autonomic imbalance plays an important role in this group of patients.

The chest film can be used to exclude cardio-megaly or pulmonary vascular congestion secondary to ventricular dysfunction. Rarely, calcification of the coronaries, aorta, or the aortic valve may be seen on the chest radiograph; such is a more common finding on CT.

Specialized Studies

When used as screening tests for the general popula-tion, noninvasive stress tests have a low predictabil-ity in asymptomatic patients, but are sufficiently reliable in symptomatic patients with suspect lesions.Holter monitoring, exercise electrocardiogra-phy, myocardial perfusion scans, and echocardiography are important in determining perioperative risk and the need for coronary angiography; how-ever, these tests are indicated only if their outcome would alter patient care.

Current ACC/AHA guidelines recommend noninvasive stress testing in patients scheduled for noncardiac surgery with active cardiac condi-tions (class I). The current guidelines also suggest that there may be benefit of such testing in patients with three or more clinical risk factors and poor functional capacity (class IIa). Likewise, they sug-gest that noninvasive testing can be of some possible benefit in patients with one or two clinical risk fac-tors undergoing intermediate risk or vascular sur-gery (class IIb). What they do not recommend is the indiscriminate use of noninvasive cardiac testing in patients with no risk factors undergoing inter-mediate-risk surgery. Consequently, indications for preoperative cardiac screening tests continue to narrow.

A. Holter Monitoring

Continuous ambulatory electrocardiographic (Holter) monitoring is useful in evaluating arrhythmias, antiarrhythmic drug therapy, and severity and frequency of ischemic episodes. Silent (asymptomatic) ischemic episodes are frequently found in patients with CAD. Frequent ischemic episodes on preoperative Holter monitoring cor-relate well with intraoperative and postopera-tive ischemia. Holter monitoring has an excellent negative predictive value for postoperative cardiac complications.

B. Exercise Electrocardiography

The usefulness of this test is limited in patients with baseline ST-segment abnormalities and those who are unable to increase their heart rate (>85% of max-imal predicted) because of fatigue, dyspnea, or drug therapy. Overall sensitivity is 65%, and specificity is 90%. The test is most sensitive (85%) in patients with three-vessel or left main CAD. Disease that is lim-ited to the left circumflex artery may also be missed because ischemia in its distribution may not be evi-dent on the standard surface ECG. A normal test does not necessarily exclude CAD, but suggests that severe disease is not likely. The degree of ST-segment depression, its severity and configuration, the time of onset in the test, and the time required for resolu-tion are important findings. A myocardial ischemic response at low levels of exercise is associated with a significantly increased risk of perioperative compli-cations and long-term cardiac events. Other signifi-cant findings include changes in blood pressure and the occurrence of arrhythmias. Exercise-induced ventricular ectopy frequently indicates severe CAD associated with ventricular dysfunction. The isch-emia presumably leads to electrical instability in myocardial cells. Given that risk seems to be asso-ciated with the degree of myocardium potentially ischemic, testing often includes perfusion scans or echocardiographic assessments; however, in ambulatory patients, exercise ECG testing is useful because it estimates functional capacity and detects myocardial ischemia.

C. Myocardial Perfusions Scans and Other Imaging Techniques

Myocardial perfusion imaging using thallium-201 or technetium-99m is used in evaluating patients who cannot exercise (eg, peripheral vascular dis-ease) or who have underlying ECG abnormalities that preclude interpretation during exercise (eg, left bundle-branch block). If the patient cannot exer-cise, images are obtained before and after injection of an intravenous coronary dilator (eg, dipyridam-ole or adenosine) to produce a hyperemic response similar to exercise. Myocardial perfusion stud-ies following exercise or injection of dipyridam-ole or adenosine have a high sensitivity, but only fairly good specificity for CAD. They are best for detecting two- or three-vessel disease. These scans can locate and quantitate areas of ischemia or scar-ring and differentiate between the two. Perfusion defects that fill in on the redistribution phase rep-resent ischemia, not previous infarction. The nega-tive predictive value of a normal perfusion scan is approximately 99%.MRI, PET, and CT scans are increasingly being used to define coronary artery anatomy and deter-mine myocardial viability.

D. Echocardiography

Th is technique provides information about both regional and global ventricular function and may be carried out at rest, following exercise, or with administration of dobutamine. Detectable regional wall motion abnormalities and the derived left ven-tricular ejection fraction correlate well with angio-graphic findings. Moreover, dobutamine stress echocardiography seems to be a reliable predictor of adverse cardiac complications in patients who cannot exercise. New or worsening wall motion abnormalities following dobutamine infusion are indicative of significant ischemia. Patients with an ejection fraction of less than 50% tend to have more severe disease and increased perioperative morbid-ity. Dobutamine stress echocardiography, however, may not be reliable in patients with left bundle-branch block because septal motion may be abnor-mal, even in the absence of left anterior descending CAD in some patients.

E. Coronary Angiography

Coronary angiography remains the definitive way to evaluate CAD and is associated with a low com-plication rate (<1%). Nonetheless, coronary angi-ography should be performed only to determine if the patient may benefit from percutaneous coro-nary angioplasty or coronary artery bypass graft-ing prior to noncardiac surgery. The location and severity of occlusions can be defined, and coronary vasospasm may also be observed on angiography. In evaluating fixed stenotic lesions, occlusions greater than 50% to 75% are generally consid-ered significant. The severity of disease is often expressed according to the number of major cor-onary vessels affected (one-, two-, or three-vesseldisease). Significant stenosis of the left main coro-nary artery is of great concern because disruption of flow in this vessel will have adverse effects on almost the entire left ventricle.

Ventriculography, measurement of the ejec-tion fraction, and measurement of intracardiac pressures, also provide important information. Indicators of significant ventricular dysfunction include an ejection fraction <50%, a left ventricular end-diastolic pressure >18 mm Hg, a cardiac index <2.2 L/min/m², and marked or multiple wall motionabnormalities.

Guidelines suggest that patients with stable angina and significant left main disease, stable angina and three-vessel disease, stable angina and two-vessel disease with an ejection fraction <50%, unstable angina, non-ST segment elevation MI, and acute ST segment elevation MI benefit from revas-cularization. This recommendation also applies to patients who are scheduled for noncardiac sur-gery (class I). Conversely, revascularization is not indicated in patients with stable angina (class III). Moreover, elective noncardiac surgery is not rec-ommended within 4–6 weeks following bare metal stent placement or within 12 months of placement of a drug-eluting stent, if the surgery requires that antiplatelet therapy be discontinued.

Premedication

Allaying fear, anxiety, and pain preoperatively are desirable goals in patients with CAD. Satisfactory premedication prevents sympathetic activa-tion, which adversely affects the myocardial oxy-gen supply–demand balance. Overmedication is equally detrimental and should be avoided because it may result in hypoxemia, respiratory acido-sis, and hypotension. A benzodiazepine, alone or in combination with an opioid, is commonly used. (The concomitant administration of oxygen via nasal cannula helps avoid hypoxemia follow-ing premedication.) Patients with poor ventricu-lar function and coexistent lung disease should receive reduced doses. Preoperative medications should generally be continued until the time of surgery. They may be given orally (with a small sip of water), intramuscularly, intravenously, sublin-gually, or transdermally.The sudden withdrawal of antianginal medication perioperatively—particularly β-blockers can precipitate a sudden, rebound increase in ischemic episodes. In the past, some clinicians pro-phylactically administered nitrates intravenously or transdermally to patients with CAD in the periopera-tive period. Although this practice may be theoreti-cally advantageous, there is no evidence of its efficacy in patients not previously on long-term nitrate therapy and without evidence of ongoing ischemia. Transdermal absorption of nitroglycerin may be erratic in the perioperative period.