How do you evaluate the cardiac risk in a patient scheduled for noncardiac surgery?
The preoperative cardiac evaluation and assessment of any patient includes a review of the history, physical examination, and laboratory results, and knowledge of the planned surgical procedure. The history should assess the presence, severity, and reversibility of coronary artery disease (CAD) (risks factors, anginal patterns, his-tory of myocardial infarction), the clinical assessment of left and right ventricular function (exercise capacity, pul-monary edema, pulmonary hypertension), and the presence of symptomatic dysrhythmias (palpitations, syncopal or pre-syncopal episodes). Patients with valvular heart disease should also be asked about the presence of embolic events.
On physical examination, particular attention should be paid to the vital signs, specifically the heart rate, blood pressure, and pulse pressure (determinants of myocardial oxygen consumption and delivery), the presence of left- or right-sided failure (jugular venous distention, peripheral edema, pulmonary edema, or an S3), and the presence of murmurs. Baseline laboratory tests include a chest radio-graph to assess heart size, and an ECG. Further evaluation depends on the results of the above preliminary investiga-tions, as well as the planned surgical procedure.
The significance of historical and laboratory data is the subject of much controversy. It is not really known how pre-dictive these variables are of patient outcome. For example, a review of the literature suggests a variable contribution from patient age. In general, it is believed that age has no effect on resting parameters of cardiac function, such as ejection fraction, left ventricular dimensions, and wall motion. It is believed that older patients have decreased reserve and decreased response to stress. However, not all studies show a relationship between age and perioperative cardiac events (PCE). A PCE is generally defined as post-operative unstable angina, myocardial infarction (MI), congestive heart failure (CHF), or death from cardiac causes.
The current standard of care is defined by the most recent updated American College of Cardiology/American Heart Association (ACC/AHA) Guidelines for Perioperative Cardiovascular Evaluation for Noncardiac Surgery. The general paradigm is that patients are risk-stratified based upon patient-related clinical predictors of PCE, the risk imparted by the surgical procedure, and the appropriate use of noninvasive testing. In elective procedures, this algo-rithmic approach should be used by internists, surgeons, and anesthesiologists for the appropriate management of the cardiovascular evaluation strategy.
Unstable angina is a major clinical predictor of PCE in the ACC/AHA Guidelines, and chronic stable angina is an intermediate clinical predictor of PCE. Fleisher and Barash (1992) suggested that patients should be classified in a more functional way. They contended that not all patients with stable angina have the same disease process (i.e., coro-nary anatomy, frequency of ischemia, and left ventricular (LV) function). The number of ischemic episodes is espe-cially difficult to quantitate without some sort of continu-ous monitoring (ambulatory ECG). This information is probably important since more than 75% of ischemic episodes are silent and more than 50% of patients with CAD (not just diabetics) have silent ischemia. It is not clear what the role of silent ischemia is in myocardial injury, although it seems to portend a worse prognosis if present in patients with unstable angina or post-MI patients.
Noninvasive studies are designed to determine the risk of ongoing ischemia (and the quality of LV function in some instances), and include ambulatory ECG (Holter monitoring), exercise stress tests, nuclear perfusion scans and function studies, and echocardiography. Exercise, steal-inducing drugs (dipyridamole or adenosine), or dobutamine are commonly used to induce reversible ischemia for noninvasive studies. Angiography may be per-formed if the noninvasive studies are highly suggestive of CAD and coronary intervention is logical for the patient from a global cardiovascular disease standpoint. A patient suspected of having mild disease may benefit from an aggressive investigation if the surgical procedure is associated with a high incidence of PCE. The same patient scheduled for a procedure with minimal cardiac risk probably does not warrant further testing.
The relationship between history of infarction and PCE varies significantly based upon the age of the infarction. Recent infarctions are defined by cardiologists as those within the last 7–30 days, and are acknowledged as a major clinical predictor of PCE. Prior MI by history or pathologic Q waves on the ECG is an intermediate clinical predictor. This is somewhat complicated to interpret in anesthesia practice because anesthesiologists traditionally refer to recent infarctions as those occurring within the preceding 6 weeks to 6 months. The classic “re-infarction” studies from data collected 20–40 years ago, found that patients with an infarct within 3 months had a 5.7–30% incidence of re-infarction. Between 3 and 6 months the risks vary from 2.3% to 15%, and an infarct more than 6 months prior to surgery is associated with a 1.9–6% incidence. The mortality of myocardial re-infarction was about 50%, and this figure varies very little among the various studies. The lower numbers in each group are from the study of Rao et al. (1983), in which aggressive hemodynamic monitoring was used and patients recovered in the intensive care unit post-operatively. The problem with applying these data to modern care is that they precede the widespread use of β-blockers, coronary interventions, and enzyme-based diagnosis of infarctions. Nevertheless, there is no doubt the more recent MIs represent a significant risk factor for PCE. The severity of the infarction must also be considered.
Medical literature distinguishes mortality in Q wave versus non-Q wave MIs, involving the right versus the left coronary artery distribution, uncomplicated versus com-plicated infarcts (recurrent pain, CHF, or dysrhythmias) and negative versus positive post-MI exercise stress test results. It seems reasonable to assume that mortality rates from (recent) MIs should not all be classified together based solely on the time since the infarction.
CHF in the general population has a poor prognosis. There is only an approximately 50% 5-year survival, although this may be improving with modern afterload-reduction and antidysrhythmic therapies. Patients with LV ejection fractions less than 30% have approximately 30% 1-year mortality. The ACC/AHA Guidelines include uncompensated CHF as a major clinical predictor and com-pensated or prior CHF as an intermediate clinical predictor.
Dysrhythmias are not an uncommon problem. They are usually benign, except in patients with underlying heart disease, in whom they serve as markers for increased mor-bidity and mortality. For example, many patients with LV dysfunction and dysrhythmias die from LV failure and not from a dysrhythmia. Acknowledged major clinical predictors include high-degree atrioventricular block, symptomatic ventricular dysrhythmias in the presence of underlying heart disease, and supraventricular dysrhythmias with uncon-trolled ventricular rate. Minor predictors include abnormal ECG (i.e., LV hypertrophy, left bundle branch block, and ST-T wave abnormalities). Rhythm other than sinus (e.g., atrial fibrillation) is also a minor clinical predictor.
Patients with valvular heart disease are difficult to evalu-ate because the lesions cause changes which are independ-ently associated with increased risk (i.e., CHF, rhythm changes). Severe valvular disease, however, is considered a major clinical predictor.
Routine laboratory tests, such as ECG, chest radiography, electrolytes, BUN and creatinine, and complete blood counts may also have some predictive value. However, normal ECGs may be present in up to 50% of patients with CAD. The most common ECG findings in patients with CAD are ST-T wave abnormalities (65–90%), LV hypertro-phy (10–20%), and pathologic Q waves (0.5–8%).
It is generally agreed that patients with a “combined” risk of PCE (based upon patient and surgical factors) of greater than 10% warrant further study. The noncardiac surgical procedures associated with the highest PCE rate are mostly vascular surgical procedures. Peripheral vascular and aortic surgeries have high PCE rates, while carotid artery surgery has PCE rates of about 5%. While the data are still emerging, it appears that endovascular repairs have low associated risk. The high PCE rate is usually attributed to the high incidence of CAD in vascular patients (estimated to be as high as 90%), and to the stress imposed on the myocardium by hemodynamic changes.
The metabolic changes induced by surgery, such as increased levels of stress hormones, and increases in platelet adhesiveness, are also implicated as factors that increase PCE. Nonvascular surgical procedures associated with higher morbidity and mortality include intrathoracic and intra-abdominal surgery. Presumably, the increased risks are because of the greater hemodynamic changes associated with large fluid shifts, and compression of the great veins, as well as aberrations in cardiopulmonary function during thoracic surgery. Emergency surgery is also associated with increased risk. Procedures associated with a lower risk of PCE include extremity surgery, transurethral prostate resections, and cataract surgery. Therefore, the risk of surgery must always be included in the estimation of patient risk, and this is constantly changing due to the emergence of less invasive techniques that cause less physiologic disturbance.
Thus, the assignment of “cardiac risk” to a particular patient for a particular surgical procedure is difficult, but there are guidelines that should be followed. Further evalu-ation should depend on whether the information gained would change the planned surgical or anesthetic manage-ment. These changes in management might include altering the surgical procedure to one associated with lower risk, medical or surgical treatment of CAD, perioperative anti-coagulation, or perhaps more aggressive intraoperative and postoperative monitoring. Although many of these strat-egies sound logical, there is relatively weak evidence of outcome improvements with interventions. Interventions that are probably effective in reducing PCE include β-adrenergic blockade and prevention of hypothermia.
The use of myocardial revascularization by percuta-neous coronary angioplasty/stent placement or coronary artery bypass grafting prior to elective noncardiac surgery for PCE risk reduction is a very controversial subject. If myocardial revascularization is considered appropriate from a cardiovascular disease management perspective then it may be beneficial, but the risks associated with the “preoperative” myocardial revascularization must be added to those associated with the planned noncardiac surgery. In many cases, the combined risk may be prohibitive. There is also emerging evidence that surgery in the early period following coronary artery stent placement is extremely risky (see below).