Clinical Cases in Anesthesia : Recent Myocardial Infarction

Clinical Cases in Anesthesia : Recent Myocardial Infarction

RECENT MYOCARDIAL INFARCTION

A 68-year-old woman with multiple cardiac risk factors had sudden onset of crushing substernal chest pain. Despite aggressive thrombolytic therapy, the patient had electrocardiogram (ECG) evidence of a transmural antero-lateral myocardial infarction (MI). Three weeks following the MI, the patient develops acute cholecystitis, and presents for a cholecystectomy.

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.

What is the cardiac risk in this patient? What addi-tional investigations should be performed?

This patient is an elderly woman with known CAD, and a recent MI who is going for emergency surgery. There are several important factors that require consideration. The first of these is the post-MI course. If she has recurrent pain, CHF, or late ventricular dysrhythmias (>48 hours post-MI) she has a 15–30% risk of death or re-infarction in her first post-infarct year even without surgery.

Another issue is whether there was evidence of reperfu-sion following thrombolytic therapy. This would include pain relief, reperfusion dysrhythmias, large increases in creatine phosphokinase (CPK) enzyme levels, and an improvement in the ECG without evidence of MI. Anticoagulant therapy is of importance. Heparin therapy used for patients with recurrent chest pain would have to be stopped prior to surgery. Recent studies suggest that the timing may be very important. Patients whose heparin was stopped for more than 9.5 hours were more likely to develop recurrent ischemia requiring urgent intervention.

The majority of patients who have received throm-bolytic therapy have significant residual stenosis in vessels that have been reperfused, and they are often investigated with early cardiac catheterization, especially if they had a complicated infarction. Some centers treat patients who are doing well as they do any patient with a recent uncomplicated infarct, that is they perform a modified symptom-limited stress test prior to discharge (on post-MI day 5–7), and a symptom-limited stress test 6 weeks later.

The presence of sepsis is an important issue. The hemo-dynamic changes associated with sepsis may significantly stress the myocardium. These include an increased cardiac output because of endotoxin-induced vasodilation, and myocardial depression from myocardial depressant factor.

If the patient must have an urgent surgical procedure and no additional cardiac studies have been performed (e.g., stress test or angiogram), one should assume the patient has significant CAD. If time permits, a transthoracic echocardiogram (TEE), specifically assessing wall motion, LV ejection fraction, and mitral valve function would provide useful information.

What are the implications for anesthetic management when coronary revascularization is performed before noncardiac surgery?

The ACC/AHA Guidelines for Perioperative Cardio-vascular Evaluation for Noncardiac Surgery provide a stepwise algorithm for the preoperative assessment of the patient with an increased risk for PCE. According to these recommendations, patients with coronary revasculariza-tion within the last 5 years without significant change in symptoms or a favorable cardiac evaluation within the last 2 years may proceed for surgery without further testing. An increasing number of patients are presenting for non-cardiac surgery with prior percutaneous coronary artery stenting (new drug-eluting stents have recently been intro-duced), and more patients are taking a combination of anticoagulant and antiplatelet medications, all of which may influence anesthetic management.

Recent data on coronary artery interventional therapy shows an increased incidence of PCE in patients with prior percutaneous coronary myocardial revascularization. A retrospective study by Posner et al. (1999) looked for adverse cardiac outcomes after noncardiac surgery among 686 patients with prior percutaneous transluminal coronary angioplasty (PTCA). Patients with prior PTCA had twice the rate of adverse cardiac outcomes compared with normal subjects, 7 times the rate of angina, almost 4 times the rate of MI, and twice the rate of CHF. Patients who underwent PTCA within 90 days of noncardiac surgery had twice the rate of perioperative MI compared with patients with uncorrected CAD. Kaluza et al. (2000) found a high number of MIs, major bleeding episodes, and fatal events in patients who underwent coronary stent placement less than 2 weeks before noncardiac surgery. Wilson et al. (2003) reviewed a larger cohort at the Mayo Clinic and found that the period of increased risk extended to 6 weeks following stent placement. It is unclear at present, but drug-eluting stents may extend the period of risk even longer by virtue of their inhibition of neointimal formation.

Antiplatelet drugs that prevent thrombosis of the newly stented coronary arteries, such as GPIIb/IIIa receptor antagonists and ADP inhibitors, have profound anticoagu-lative properties, and recommendations about when these drugs should be discontinued prior to neuraxial anesthesia have been created and periodically updated by the American Society of Regional Anesthesia and Pain Medicine (http://www.asra.com). In emergency procedures, these patients demonstrate increased risk of perioperative bleeding and platelet transfusions may be necessary to achieve hemo-stasis. When these antiplatelet regimens are discontinued for elective surgery shortly after coronary interventions, the risk of stent thrombosis is probably increased, especially in the setting of the hypercoagulable state that frequently is present in the postoperative period.

In summary, recently published data suggest an increased risk for patients presenting for noncardiac surgery who have undergone percutaneous coronary interventions with stent placement within the 2 months prior to surgery. While the data are preliminary, elective surgery should be undertaken with caution and attention should be paid to the management of anticoagulation in the perioperative period.

What intraoperative monitors would you use?

A general goal in these patients is to maintain intraoperative hemodynamics within 20% of preoperative values. Therefore, in addition to the standard intraoperative monitors, other monitors that should be considered include an intra-arterial line, a pulmonary artery catheter (PAC), and a TEE. An intra-arterial line would be the optimum way of monitoring blood pressure (BP) beat-to-beat. Although 40% of intraoperative ischemic episodes are not related to aberrations in hemodynamics, there are studies demonstrating that inadequate management of hemody-namic abnormalities may increase risk. Hypotension (BP <30% baseline for greater than 10 minutes) has been shown to be a strong predictor of PCE in one study. On the other hand, there are no studies demonstrating conclusively that hypertension is associated with adverse outcome. Tachycardia has not been definitely shown to be associated with PCE, although studies suggest a relationship.

The easiest technique for myocardial ischemia monitoring in the anesthetized patient is with a multiple-lead ECG. Monitoring precordial chest leads V4 and V5 detects greater than 90% of ischemic events that would be seen on a 12-lead ECG, but it has been reported to have as low as a 9% sensitivity compared with the gold standard (myo-cardial lactate extraction). ST-segment depressions and T-wave morphology changes are most commonly seen. However, there are patients in whom the ECG is not an effective intraoperative monitor of myocardial ischemia, such as those with LV hypertrophy, conduction abnormal-ities, and ventricular pacemaker dependence.

The development of V waves on the pulmonary artery wedge pressure waveform may be an indication of myocar-dial ischemia, but it is not sensitive or specific enough to be regarded as a reliable monitor for this purpose (Figure 3.1).

The utility of the PAC, however, extends beyond its ques-tionable ability to detect ischemia. It provides information about the patient’s intravascular volume status, a quantitative estimate of myocardial compliance, and allows for calcula-tion of cardiac output and other hemodynamic measure-ments, such as systemic vascular resistance and stroke volume. A PAC would be mandatory if this patient showed signs of CHF preoperatively.

The TEE is the most sensitive detector of intraoperative ischemia, and it is capable of detecting ischemia earlier than any other modality. However, studies have questioned its specificity. Specifically, it is not clear what TEE changes are predictive of ischemia and PCE. In the largest study of patients with or at-risk for CAD who were scheduled for noncardiac surgery, Mangano and Goldman (1995) did not find that LV wall motion abnormalities were predictive of ischemia or PCE. The TEE also provides physiologic information, such as estimates of LV ejection fraction and intravascular volume status, which may help with intraoperative management in patients with ventricular dysfunction.

What additional drugs would you have prepared?

Intravenous nitroglycerin, esmolol, and vasopressors should be immediately available to treat ischemia and hemodynamic aberrations. Phenylephrine is particularly useful in restoring myocardial blood flow in hypotensive patients without causing major increases in myocardial oxygen consumption due to tachycardia

What anesthetic technique would you use?

The anesthetic technique has not been shown to be a predictor of PCE. Thus, the anesthetic technique used should be based on the patient assessment and the best tech-nique for maintaining stable intraoperative hemodynamics and adequate postoperative analgesia. There is no defini-tive evidence that one anesthetic technique is safer than another. Tachycardia should be avoided in patients with CAD, thus, agents such as ketamine and pancuronium are probably best avoided. There is some preliminary evidence that epidural analgesia in the postoperative period is asso-ciated with a lower incidence of PCE.

How would you manage this patient  postoperatively?

 Ideally, the patient should be monitored in an intensive care setting postoperatively. Furthermore, the results of the study by Rao et al. (1983) suggest that patients may benefit from a more prolonged stay (at least 3 days) in the intensive care unit with intensive hemodynamic monitoring.