In the current issue of Circulation, Poldermans et al1 report on the long-term prognostic value of dobutamine stress echocardiography in patients undergoing major vascular surgery. Their findings add to the growing literature on the use of exercise and dobutamine stress echocardiography as adjuncts in the assessment of prognosis among patients with known or suspected coronary artery disease. To date, the published experience with dobutamine stress echocardiography for assessment of prognosis and perioperative risk is relatively small compared with that using nuclear perfusion imaging techniques. Stress echocardiography is a more recently developed technique to detect coronary artery disease and myocardial ischemia, and all studies related to prognosis have been published since 1991. However, stress echocardiography is of increasing importance because of the increasing availability these techniques and because ofseveral advantages it offers over nuclear perfusion imaging. In addition to providing apparently equivalent data with respect to the presence and extent of coronary artery disease and myocardium at risk, dobutamine stress echocardiography allows assessment of valvular anatomy and function as well as resting and stress ventricular systolic function. This allows a more complete assessment of overall cardiac function, pertinent especially among patients with a history of congestive heart failure or cardiac murmur. Finally, stress echocardiographic techniques appear to have lower associated costs than the equivalent nuclear perfusion imaging counterparts, which may become increasingly important as the healthcare environment requires the delivery of cost-effective medical care.
The Present Report
Poldermans et al2 and others3 4 have previously published reports on the utility of dobutamine stress echocardiography in the assessment of prognosis in a general population4 and for the identification of patients at increased perioperative risk during major vascular surgery.2 3 The report in the current issue of Circulation is important in that it describes the long-term prognostic data afforded by preoperative functional testing with dobutamine stress echocardiography in an unselected population undergoing major vascular surgery. The authors found a 10.1% incidence of “hard” adverse events (cardiac death, nonfatal myocardial infarction, or coronary revascularization) during the follow-up period of 19±11 months. On the basis of a combination of clinical and stress echocardiographic data, the investigators were able to divide patients into low-, medium-, and high-risk groups. Low-risk patients had no clinical history of prior myocardial infarction and a normal stress test or either (but not both) a history of prior infarction or limited ischemia on stress testing. There were 12 events among 247 such patients (4.9% incidence; hazards ratios, 1.0 to 3.1). Moderate-risk patients had a history of prior myocardial infarction and evidence of limited ischemia on dobutamine stress echocardiography or evidence of more extensive ischemia with no history of prior infarction. There were 9 events among 46 such patients (20% incidence; hazards ratios, 8.8 to 10.3). High-risk patients had both a clinical history of prior myocardial infarction and evidence of extensive ischemia on dobutamine stress echocardiography. There were 11 events among 21 such patients (52% incidence; hazards ratio, 31.5). Prediction of prognosis was therefore based on a combination of data derived from clinical information and functional testing. Like the findings in previous studies using nuclear perfusion imaging,5 6 semiquantification of both extent of prior infarction and extent of inducible ischemia on echocardiographic imaging were important predictors of prognosis.
On the basis of their findings, the authors recommend that all patients with at least one risk factor for an adverse event (age >70 years, Q waves on ECG, angina pectoris, diabetes mellitus, or history of ventricular arrhythmia7 ) undergo preoperative testing with dobutamine stress echocardiography. In considering which patients are likely to benefit from noninvasive testing as part of preoperative assessment, it would be useful to review the specific goals of preoperative assessment and the role that noninvasive testing can play.
Preoperative Assessment of Risk
The preoperative evaluation of patients with known or suspected coronary artery disease undergoing noncardiac surgery should be designed with two specific goals. The first is to ensure the safe performance of surgery and a perioperative period free of adverse cardiac events. The second is the identification of patients with a poor long-term prognosis. The latter helps ensure that patients undergoing a major surgical procedure have a long-term prognosis that justifies the procedure, with a likelihood that the patient can enjoy the results of surgical intervention. In addition, it provides an opportunity to recognize individuals at risk of subsequent morbid cardiac events who come to medical attention only because of a need for noncardiac surgery and for whom long-term prognosis can be improved with institution of appropriate therapy. This may include antihypertensive medications, anti-ischemic drugs, antiplatelet or anticoagulant therapy, lipid-lowering agents, and lifestyle changes in diet and exercise. Also, for selected patients, coronary revascularization may be appropriate.
The spectrum of noncardiac surgery is such that various clinical variables can be used to identify patients at increased risk of perioperative cardiac events.7 8 Some of these variables are surgery-specific, and patients undergoing aortic and major vascular procedures are among those at greatest risk. This associated risk is most likely related to two factors, the first of which is cardiovascular stress inherent to the surgical procedure. Major arterial operations are often time-consuming and may be associated with significant fluctuations in cardiac preload and afterload. The resultant increase in myocardial oxygen demand creates the potential for an induced mismatch between oxygen consumption and delivery, such that myocardial ischemia is induced in patients with otherwise clinically silent coronary stenoses.
Importantly, the second factor associated with increased cardiac risk during vascular surgery relates to the surgical population itself. Because risk factors that contribute to peripheral vascular occlusive disease also contribute to the development of coronary atherosclerosis, the prevalence of coronary artery disease among these patients is higher than that among a general surgical population. Furthermore, because of physical limitations inherent to patients with advanced peripheral vascular disease, a sedentary lifestyle in the months or years before surgery may limit the development of typical symptoms of exertional angina pectoris. As a result, among patients with advanced peripheral vascular disease, there is a high prevalence of significant coronary artery disease that remains occult before surgery.
Noninvasive Testing in Aortic and Vascular Surgery
Preoperative noninvasive functional testing before aortic or major vascular surgery has been shown to help identify patients at increased risk of suffering an adverse cardiac event in the perioperative period.2 3 6 9 In this high-risk population, it has become commonplace to include a form of cardiac stress testing to define the extent of ischemia in patients with known coronary artery disease and to screen for occult disease among others, particularly among diabetics or those with prior congestive heart failure.7 In general, exercise capacity has been shown to be a good predictor of perioperative prognosis. However, because of limited exercise tolerance among patients with surgical vascular occlusive disease, most testing in this population uses a form of pharmacological stress in conjunction with cardiac imaging. Early studies demonstrated that dipyridamole thallium scintigraphy6 9 is useful in defining risk of a cardiac event during noncardiac surgery. Subsequently, Poldermans et al2 and others3 demonstrated that dobutamine stress echocardiography provides prognostic information predictive of perioperative cardiac events equivalent to that afforded by nuclear imaging techniques.
Selection of Patients for Noninvasive Testing
The Bayes theorem states that the accuracy of a test is affected by the prevalence of disease in the population tested.10 It is a function of Bayes' theorem that noninvasive testing for the detection of coronary artery disease will have its greatest impact among a population with an intermediate pretest likelihood of disease. Among a low-risk population, an abnormal test result may be more likely to represent a false-positive than a true-positive finding. Similarly, among a population with a high pretest likelihood of disease, a normal test result may be more likely to represent a false-negative than a true-negative finding.
Because there is a high prevalence of coronary artery disease and risk factors for it among the population undergoing vascular surgery, noninvasive testing has become a common accompaniment to preoperative assessment of risk. However, the specific goal of preoperative assessment for noncardiac surgery is the identification of patients at risk of a perioperative or subsequent adverse cardiac event, not simply the detection of occlusive coronary artery disease. Applying Bayes' theorem to noninvasive testing before major vascular surgery, the greatest impact of testing is among patients whom clinical data indicate to be at intermediate risk of an adverse event.11 It has become apparent that subgroups of patients at low risk and at high risk of an event and those for whom noninvasive testing will not further clarify prognosis can be identified from clinical data alone. It is believed that limiting testing to only those patients at intermediate risk of a cardiac event will have substantial impact on the frequency with which testing is required and the subsequent cost of preoperative assessment without having an adverse effect on patient outcomes.11
Guidelines for Evaluation
In a recent collaborative effort, the American College of Cardiology and the American Heart Association issued a task force report on guidelines for perioperative cardiovascular evaluation for noncardiac surgery.12 In this report, patients undergoing aortic and vascular surgical procedures are recognized as being at increased risk of a perioperative event on the basis of surgery-specific risks of these procedures and the specific patient populations that require them. However, it was recognized that the known presence of coronary artery disease did not in itself predict a high risk for a perioperative cardiac event. Specifically, among patients who had known disease and either recent coronary revascularization or a recent functional study with favorable results and no recent change in symptoms, further noninvasive testing would not affect perioperative prognosis.
Identification of Patients at Risk
The American College of Cardiology/American Heart Association task force report12 recommends initial preoperative assessment of risk for noncardiac surgery based on clinical variables specific to both the patient and the anticipated surgery. For patients at intermediate risk of an event, noninvasive testing may be useful as an adjunct to available clinical information. Because subgroups at low risk and at high risk can be identified on the basis of clinical data, similar recommendations are made for patients undergoing vascular surgery.
In a recent report, L'Italien and colleagues13 derived an algorithm based on clinical data for the estimation of long-term risk among patients undergoing major vascular surgery. On the basis of a model incorporating incremental surgical, clinical, and noninvasive data, populations at low, moderate, and high risk of a late cardiac event were identified. The analysis found that incorporation of noninvasive testing results did not substantially affect estimation of risk based on clinical data. Specifically, the 2-year event-free survival among patients clinically at low risk was 95±1% based on clinical variables alone and 96±1% after results of noninvasive testing were incorporated. The 2-year event-free survival among patients clinically at high risk was 61±9% based on clinical variables alone and 66±8% after results of noninvasive testing were incorporated. Therefore, clinical markers alone are often sufficient to identify patients at low enough risk that noninvasive testing is unlikely to alter management. Similarly, clinical markers alone may be sufficient to identify patients with a poor enough prognosis that management may be dictated by clinical and invasive markers rather than by noninvasive testing. It is the group of patients at intermediate risk who may be stratified into higher- and lower-risk patients on the basis of results of noninvasive testing and for whom noninvasive testing should be performed.
Poldermans et al have added important data to the body of literature supporting the use of dobutamine stress echocardiography to define long-term cardiac risk for patients undergoing major vascular surgery. In the application of this or any other noninvasive study, it will remain important to select the appropriate population for testing. In the case of noninvasive testing before vascular surgery, the population should consist of patients at intermediate risk of an adverse cardiac event rather than the whole population with known or suspected coronary artery disease. If this is done, testing can be restricted to a group for whom results will affect clinical decision making. Clinical assessment should remain the first method of patient screening. Among patients identified by clinical markers as at intermediate risk for a perioperative or late cardiac event, noninvasive testing such as dobutamine stress echocardiography may be used to better stratify risk and to help guide perioperative and subsequent cardiac management.
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
- Copyright © 1997 by American Heart Association
Poldermans D, Arnese M, Fioretti PM, Boersma E, Thomson IR, Rambaldi R, van Urk H. Sustained prognostic value of dobutamine stress echocardiography for late cardiac events after major noncardiac vascular surgery. Circulation.. 1997;95:53-58.
Poldermans D, Fioretti PM, Forster T, Thomson IR, Boersma E, El-Said EM, du Bois NAJJ, Roelandt JRTC, van Urk H. Dobutamine stress echocardiography for assessment of perioperative cardiac risk in patients undergoing major vascular surgery. Circulation.. 1993;87:1506-1512.
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L'Italien G, Paul SD, Hendel RC, Leppo JA, Cohen MC, Fleischer LA, Brown KA, Zarich SW, Cambria RP, Cutler BS, Eagle KA. Development and validation of a Bayesian model for perioperative cardiac risk assessment in a cohort of 1081 vascular surgical patients. J Am Coll Cardiol.. 1996;27:779-786.
Eagle KA, Brundage BH, Chaitman BR, Ewy GA, Fleisher LA, Hertzer NR, Leppo JA, Ryan T, Schlant RC, Spencer WH III, Spittell JA, Twiss RD. Guidelines for perioperative cardiovascular evaluation for noncardiac surgery. Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Peri-operative Cardiovascular Evaluation for Noncardiac Surgery). Circulation.. 1996;93:1278-1317.
L'Italien GJ, Paul SD, Hendel RC, Cohen MC, Fleischer LA, Brown KA, Zarich SW, Leppo JA, Eagle KA. Cardiac risk assessment following vascular surgery: independent validation of a Bayesian prediction model. J Am Coll Cardiol.. 1996;27:317A. Abstract.