Cardiologists and internists are frequently asked to “clear” patients for surgery. This term is a misnomer. The role of the perioperative consultant is to summarize accurately the cardiac risk of surgery while identifying strategies to maximize the odds of a positive surgical outcome. Such assessment is a prerequisite for both the patient and the surgeon when faced with decision making regarding surgery, as this information can facilitate the timing and appropriateness of interventions.

Several studies have established that perioperative cardiac risk is best assessed by clinical evaluation and this is true for patients with PVD. A good history and physical examination can provide a very good estimate of operative risk. This estimate can then be modified and/or improved by selective testing.

The testing of patients prior to noncardiac surgery, whether by treadmill stress testing, treadmill testing with nuclear imaging, or dobutamine stress echocardiography or with MRI stress testing, is most appropriate for patients for whom: (a) stress-testing can effect a change in management and/or alter overall outcome, or (b) cardiac risk for surgery is unclear. In contrast, patients deemed to be at low or high clinical risk of perioperative cardiac events do not benefit from further testing. Such tests are quite effective when applied selectively based on clinical assessment. The use of Bayes theorem (using pre-test probability to assess the predictive value of
a test), is thus very much applicable to pre-operative assessment as it is in other medical interventions.

Prior to the 1970s, the only clinical risk classification system widely available for preoperative assessment was that created by the American Society of Anesthesiology; this is still used today (Table 28.1). According to this classification, patients with imminently life-threatening disease are placed into category 4 or 5, whereas those with activity-limiting disease are classified as 3. The overwhelming majority of patients fall into category 2, that is, those with mild systemic disease that is well controlled. The biggest drawback of this instrument is that it poorly defines the perioperative risk faced by a patient, (especially those in categories 1 and 2), and provides little guidance with respect to further management. Furthermore, its lack of specificity meant that wide latitude was given in determining what constitutes a systemic disease, a severe disease, or an incapacitating disease. The ASA classification system thus did little to improve the understanding of perioperative risk. Faced with this dearth of guidance, Goldman et al. identified specific clinical factors that were associated with increased perioperative risk. Based on 1,000 consecutive patients referred for evaluation prior to noncardiac surgery, Goldman et al. identified clinical variables most associated with life-threatening or fatal cardiac complications of surgery. A score was assigned to each of these factors based on the strength of association, allowing for the creation of a risk index. This study was the first comprehensive attempt to identify clinical factors associated with perioperative risk. The authors demonstrated that (within their derivation cohort) calculated risk scores correlated with perioperative complications and death. High-risk clinical features included severe valvular heart disease, active heart failure, recent myocardial infarction, older age, arrhythmia, and/or a serious medical comorbidity. Detsky et al. performed a similar analysis on 455 patients prospectively assessed preoperatively. These investigators also demonstrated that a risk-index score could predict the likelihood of a perioperative cardiac event. Interestingly, they included descriptive and remote factors such as severity of angina, a history of pulmonary edema, or a history of prior myocardial infarction in their risk model.

More recently, Lee et al. derived and then validated a risk-assessment index for perioperative cardiac complications of noncardiac surgery among 4,315 patients undergoing elective noncardiac surgery at the Brigham and
Womens’ Hospital. Clinical correlates of cardiac complications were modeled using stepwise logistic-regression analysis. The authors identified six clinical markers of risk and then tested these variables in a separate validation cohort of patients (Table 28.2). By assigning one point to each of these six cardiac risk factors, a risk index was derived and found to be highly predictive of perioperative cardiac complications.

L’Italien et al. developed a risk-prediction model specifically for patients undergoing vascular surgery (Table 28.3). The investigators focused on 1,081 consecutive patients with PVD undergoing major vascular surgery, a cohort at increased risk of perioperative cardiac complications. Multivariate analysis identified five clinical variables; advanced age, angina, history of myocardial infarction, diabetes mellitus, and history of heart failure, associated with increased perioperative risk. This clinical model stratified patients correctly into low, moderate, and high-risk patients with observed cardiovascular events rate of 3, 8, and 18%, respectively. More importantly, these investigators found that the predictive power of preoperative risk assessment was improved with the combined use of both clinical risk- assessment and selective dipyridamole-thallium testing. They noted no additional benefit from thallium testing in clinically low or high-risk groups.

Based on these and other studies, the American Heart Association/American College of Cardiology Foundation (AHA/ACCF) preoperative guidelines have identified clinical markers of risk for perioperative cardiac events. It is informative to note that these risk factors do not include many well-known markers of risk for atherosclerotic disease. For instance, no study has identified elevated cholesterol, high blood pressure, or tobacco use as being associated with increased risk of perioperative events. Although these factors are contributors to the development of atherosclerosis (thus increasing the long-term risk for cardiac events), they do not identify patients at immediate risk for cardiac events (i.e., those at risk for perioperative events). Thus, the clinical factors associated with increased risk of perioperative events appear to be markers associated with increased biological activity of coronary lesions rather than those associated with the initiation and propagation of these entities (Table 28.4).

An impressive number of clinical studies in a wide range of patient populations have evaluated the effectiveness of thallium stress testing to predict the risk of perioperative cardiac events. In spite of the varied patient cohorts examined and the time elapsed from the first study to the last, there is a remarkable uniformity of findings in terms of the predictive power of this test in identifying those at risk of perioperative cardiac events. In patients with a history of PVD requiring vascular surgery, numerous studies have shown that the negative predictive value of thallium stress testing remains close to 100%. That is to say, those with a normal thallium stress test have an almost 100% likelihood of being free from perioperative cardiac events. Conversely, the positive predictive value was remarkably and consistently poor at about 12%. Thus, patients with an abnormal thallium stress test only had a 12% chance of having a perioperative cardiac event. This makes thallium testing of questionable utility as a screening test, as many patients facing vascular surgery are likely to have an abnormal thallium test sans perioperative cardiac event.

A review of published papers evaluating dobutamine echocardiogram testing reveals strikingly similar results. Throughout the decade of the 1990s, numerous studies evaluating the ability of dobutamine echocardiography to predict perioperative cardiac events found a near-perfect 99% negative predictive value, but a disappointing 15% positive predictive value. Thus, both modalities seem quite powerful in their ability to identify patients who will be free of perioperative cardiac events, but imprecise in identifying patients at increased risk.