A 72-year-old woman with a past medical history significant for hypertension and a 55-pack-year history of tobacco use is scheduled for a right femoral-popliteal bypass. The patient’s symptoms include claudication of her right leg and mild shortness of breath. She reports being able to walk up two flights of stairs, with only minimal shortness of breath. She takes Lisinopril for hypertension. Her preoperative laboratory test reports include a hemoglobin (Hgb) of 11.9 g per dL, creatinine 1.1 mg per L, glucose 92 mg per dL, Na+ 140 mEq per L, and K+ 4.3 mEq per L. Preoperative electrocardiogram shows a normal sinus rhythm at a rate of 82, with no Q-waves or ST-segment changes. The patient undergoes a general anesthetic with endotracheal intubation and mechanical ventilation. After 1 hour of surgery, the electrocardiogram reveals a new 1.5 mm ST-segment depression in leads II, III, and aVF. Her vital signs are blood pressure 95/43 mm Hg, heart rate 95 bpm, Spo2 98%, and temperature 36.8°C, with an estimated blood loss of 600 mL. A bolus of intravenous crystalloid solution is administered, with a consequent increase in blood pressure. Subsequently, the patient is treated with intravenous metoprolol to reduce the heart rate to 60 bpm. Intraoperative Hgb measurement is 9.6 g per dL, and a unit of packed red blood cells is transfused. Following these interventions, the ST segments return to baseline levels. The surgical procedure is completed. The patient is extubated uneventfully in the operating room and taken to the intensive care unit for observation.
How Is Epidemiology Defined?
Epidemiology is defined as (i) a branch of medical science that deals with the incidence, distribution, and control of disease in a population; and (ii) the sum of the factors controlling the presence or absence of a disease or pathogen.1 For the anesthesiologist, epidemiology involves the use of known studies and guidelines to predict who may be at risk for developing specific complications in the perioperative setting. An anesthesiologist considers the factors that determine the safety of an anesthetic and formulates an anesthetic plan utilizing monitors, medications, and perhaps even plans further diagnostic testing to ensure the safety of the patient. The anesthesiologist also provides this information to the patient to obtain informed consent for the planned anesthetic. In this chapter, the focus will be on the ability to predict which patients may be at risk for developing cardiovascular complications in the preoperative, intraoperative, and perhaps most important, the postoperative period.
Cardiovascular complications most frequently imply perioperative myocardial infarction (MI); however, sudden cardiac arrest, stroke, and myocardial ischemia, as evidenced by the electrocardiographic changes with or without hemodynamic perturbations, are all forms of cardiovascular complications.
What Measures Have Been Taken to Assess Cardiac Risk for Noncardiac Surgery?
More than 28 million patients undergo anesthesia for surgical procedures each year in the United States. With the aging population, that number is predicted to reach 40 million in just a few years. Approximately 8 million of the patients undergoing anesthesia each year have known coronary artery disease or coronary risk factors.2 More than 50,000 patients suffer a perioperative MI, and approximately 1 million incur perioperative cardiac complications.
During the second half of the 20th century, many attempts at developing indices to assess perioperative risk were put forward. The development of the now ubiquitous American Society of Anesthesiologists (ASA) Physical Status Classification of surgical mortality by Dr. Robert D. Dripps emerged from this period as one of the early attempts. Several years later in 1977, Goldman et al. developed the Cardiac Risk Index. Since then, there have been many indices offered to stratify the risk of cardiovascular complications in noncardiac surgery.3
As the formal practice of evidence-based medicine emerged over the last few decades, numerous studies that have examined the predilection to cardiovascular complications provide a structure from which practicing physicians can make reasonable predictions about which patients may be at high risk for cardiovascular complications. In fact, a task force involving experts throughout the medical community set out to review the body of literature pertaining to cardiovascular risk and risk factors in the perioperative setting to develop guidelines for managing patients at risk for cardiovascular disease. In 1996, the American College of Cardiology (ACC) and the American Heart Association (AHA) released 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 Perioperative Cardiovascular Evaluation for Noncardiac Surgery) which was subsequently updated in 2002, incorporating new information and studies.4
In this chapter, we will review the current guidelines of risk stratification for cardiovascular risk, including the ACC/AHA guidelines. We will also examine the most current evidence concerning the effectiveness of diagnostic studies in predicting cardiovascular complications, as well as potential interventions aimed at reducing the risk of cardiovascular complications in the perioperative period. The bulk of this chapter will be dedicated to the examination of cardiovascular risk for the patient undergoing noncardiac surgery. At the end of this chapter, we will address specific risk factors for cardiovascular complications in patients undergoing cardiac surgery. This chapter is designed to assist the clinical anesthesiologist in making informed decisions about the risk of cardiovascular complications and to guide the use of preoperative testing in evaluating such risk.
What Are the Methods Used to Establish Risk?
▪ PATIENT HISTORY
A carefully addressed patient history may quickly identify high-risk factors, as well as the necessity and appropriateness of preoperative testing. Patients who provide a history that includes a previous MI, current or recent chest pain, complaints of worsening shortness of breath, worsening edema, and pacemaker and/or defibrillator placement should all be considered for further cardiac workup. Each of the above mentioned features or symptoms is associated with a higher cardiovascular risk. Patients who have pain while walking or pain of the extremities at rest may have peripheral vascular disease, which may suggest occult coronary artery disease in the absence of cardiac symptoms.
Several comorbidities have been suggested as factors that increase the risk of cardiovascular complications in the perioperative setting. A history of diabetes mellitus, renal impairment, pulmonary disease, and hematologic perturbations such as anemia are the most commonly described. Diabetes mellitus not only has an increased association with coronary artery disease, but the pathophysiologic effects on the visceral nervous system may lead to “silent” ischemia that does not manifest as chest pain. Furthermore, congestive heart failure is more common among elderly patients with diabetes mellitus than without, even when angiotensin-converting enzyme inhibitors are used appropriately. Renal impairment is also associated with increased cardiovascular risk. Azotemia alone has an association with cardiovascular disease and an increase in cardiovascular events.4 Lee et al. demonstrated that a creatinine level >2.0 mg per dL was an independent predictor of cardiovascular complications.5
Both obstructive and restrictive pulmonary disease can lead to perioperative respiratory complications. In addition, hypoxia, hypercapnia, and acidosis can lead to worsening cardiovascular performance, and ultimately contribute to cardiovascular complications. Most significantly, the presence of pulmonary disease may limit the use of β-blockers, one of the most commonly used medical therapies for decreasing cardiovascular events. Anemia can further worsen myocardial supply, leading to potential worsening of ischemia, as well as heart failure. A hematocrit <28% is associated with an increased risk of myocardial ischemia and cardiovascular complications in prostate and vascular surgical patients.4
The Revised Cardiac Risk Index identifies six factors of a patient’s history that can be used to determine the risk of major cardiac complications in the perioperative setting.5 The factors are of approximately equal prognostic value and include the following:
High-risk surgery
Ischemic heart disease
History of congestive heart failure
History of cerebrovascular disease
Insulin therapy for diabetes
Preoperative serum creatinine >2.0 mg per dL (see Table 14.1).
In a prospective evaluation of 1,422 patients, the presence of ≥2 of these factors identified patients with moderate (7%) and severe (11%) cardiovascular complication rates (see Table 14.2). However, according to the ACC/AHA guidelines, only four clinical features serve as major predictors (see Table 14.3) of cardiac complications:
TABLE 14.1 Factors that Increase the Risk of Perioperative Cardiac Complications in Patients Undergoing Noncardiac Surgery and Indications for the Use of Perioperative β-Blocker Therapy
Yes, if poor status is thought to be due to coronary artery disease or heart failure
aData are from Lee TH, Marcantonio ER, Mangione CM, et al. Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation. 1999;100:1043; Reilly DF, McNeely MJ, Doerner D, et al. Self-reported exercise tolerance and the risk of serious perioperative complications. Arch Intern Med. 1999;159:2185.
bIschemic heart disease includes angina and prior myocardial infarction.
cHigh-risk surgery includes intraperitoneal, intrathoracic, and suprainguinal vascular procedures.
dPoor functional status is defined as the inability to walk four blocks or climb two flights of stairs. CI, confidence interval.
Unstable coronary syndromes include acute or recent MI, as well as unstable and severe stable angina. Significant arrhythmias include second-degree or higher atrioventricular block, uncontrolled supraventricular tachycardia, or symptomatic ventricular arrhythmia. Major predictors are those risk factors that have consistently been associated with a high perioperative risk, as well as an increased perioperative risk for MI, heart failure, and death.
TABLE 14.2 Major Cardiac Complication Rates and 95% CIs in Derivation and Validation Cohorts Stratified by Risk Classification System
bRCRI vs OCRI (p = 0.02); RCIR vs MCRI (p < 0.0001); RCRI vs ASA (p = −0.018)
Revised cardiac risk factors include high-risk surgery, ischemic heart disease, congestive heart failure, history of cerebrovascular disease, insulin therapy for diabetes, and preoperative serum creatinine. CI, confidence interval; ASA, American Society of Anesthesiologists; Pop, population; ROC, receiver opening characteristic; SE, standard error; OCRI, original cardiac risk index; MCRI, modified cardiac risk index; RCRI, revised cardiac risk index
Reprinted and adapted from Lee TH, Marcantonio ER, Mangione CM, et al. Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation. 1999;100:1043-1047.
Equally important in the patient history is the assessment of functional capacity and/or exercise tolerance. Despite the presence of cardiovascular risk factors, patients who are asymptomatic and involved in routine, vigorous activity may not warrant further cardiac workup. In an effort to simplify the determination of a patient’s functional capacity, the ACC/AHA guidelines (see Table 14.4) incorporate a method of stratifying functional capacity based on a calculated metabolic equivalent (MET). Patients exhibiting <4 METs, coupled with either the presence of intermediate-risk factors (Table 14.3) or undergoing high-risk surgical procedures, should be referred for noninvasive testing.
TABLE 14.3 ACC/AHA Guidelines: Clinical Predictors of Increased Perioperative Cardiovascular Risk (Myocardial Infarction, Heart Failure, and Death)
Major
Unstable coronary syndromes
▪
Recent myocardial infarctiona with evidence of important ischemic risk by clinical symptoms or noninvasive study
▪
Unstable or severeb angina (Canadian class III or IV)c
Decompensated congestive heart failure
Significant arrhythmias
▪
High-grade atrioventricular block
▪
Symptomatic ventricular arrhythmias in the presence of underlying heart disease
▪
Supraventricular arrhythmias with uncontrolled ventricular rate
Severe valvular disease
Intermediate
Mild angina pectoris (Canadian class I or II)
Prior myocardial infarction by history or pathologic Q-waves
Rhythm other than sinus (e.g., atrial fibrillation)
Low functional capacity (e.g., inability to climb one flight of stairs with a bag of groceries)
History of stroke
Uncontrolled systemic hypertension
aThe American College of Cardiology National Database Library defines recent MI as >7 d but ≤1 mo (30 d).
bMay include “stable” angina in patients who are unusually sedentary.
cCampeau L. Grading of angina pectoris. Circulation. 1976;54:522-523.
ECG, electrocardiogram.
Reprinted with permission from Eagle KA, Berger PB, Calkins H, et al. ACC/AHA guideline update for perioperative cardiovascular evaluation for noncardiac surgery—executive summary a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1996 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery). Circulation. 2002;105:1257.
TABLE 14.4 Estimated Energy Requirements of Daily Activitiesa
METs
Criteria
1
Can you take care of yourself?
Eat, dress, or use the toilet?
Walk indoors around the house?
Walk a block or two on level ground at 2-3 mph or 3.2-4.8 km/h?
Do light work around the house like dusting or washing dishes?
4
Climb a flight of stairs or walk up a hill?
Walk on level ground at 4 mph or 6.4 km/h?
Run a short distance?
Do heavy work around the house like scrubbing floors or lifting or moving heavy furniture?
Participate in moderate recreational activities like golf, bowling, dancing, doubles tennis, or throwing a baseball or football?
10
Participate in strenuous sports like swimming, singles tennis, football, basketball, or skiing?
aAdapted from the Duke Activity Status Index and AHA Exercise Standards MET,metabolic equivalent.
Reprinted with permission from Eagle KA, Berger PB, Calkins H, et al. ACC/AHA guideline update for perioperative cardiovascular evaluation for noncardiac surgery—executive summary a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1996 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery). Circulation. 2002;105:1257.
▪ PHYSICAL EXAMINATION
In addition to a detailed medical history, a complete physical examination should be performed, with particular attention to a comprehensive cardiovascular examination. Vital signs should be evaluated. Previously unrecognized, untreated, or poorly treated hypertension may be of concern. Patients exhibiting stage 3 hypertension (systolic blood pressure > 180 mm Hg and diastolic blood pressure >110 mmHg) should be evaluated and treated before surgery. The data on which this recommendation is based is limited; furthermore, the impact that severe, poorly controlled hypertension (stage 3) has on perioperative mortality is unclear.6 Hypertension alone is considered a minor risk factor; however, it must be considered in conjunction with additional clinical risk factors, the patient’s exercise capacity, and the surgical risk as assigned by the ACC/AHA guidelines.
A new or worsening heart murmur may also be of concern, particularly if it is a harsh, crescendodecrescendo murmur heard at the left upper sternal border. This murmur is characteristic of aortic stenosis and may prompt further workup. As indicated in the AHA guidelines, severe or critical aortic stenosis has a high risk of cardiovascular complications, including acute MI and asystole.7 Similarly, both stenotic and regurgitant murmurs of the mitral valve are also associated with heart failure. Signs of dependent edema, especially in the lower extremities, with or without accompanying dyspnea, may be signs of heart failure as well, regardless of valvular disease. Pulmonary rales, elevated jugular venous pressure, hepatojugular reflux, and a third heart sound may also be hallmarks of heart failure.4
▪ ELECTROCARDIOGRAM
A routine electrocardiogram is often one of the first diagnostic studies to be performed—and arguably one of the simplest—in assessing cardiovascular risk. Any abnormal finding (e.g., arrhythmia, Q-waves, ST-segment changes, etc.) in high-risk patients confers a notable increase in perioperative risk, as much as 300%.8 However, a normal electrocardiogram in a low-risk patient has very low sensitivity and often does not discriminate any further stratification of risk. Owing to the swift and noninvasive nature of an electrocardiogram, it is frequently performed as an early test in assessing perioperative cardiac risk, but are often most valuable when used in conjunction with additional studies and in patients with clinically determined intermediate or high risk.
▪ EXERCISE TREADMILL TESTING
The “stress response” is the natural reaction of the body to surgery. A well designed anesthetic, including the use of narcotics, volatile or intravenous agents, antibiotics, and regional anesthesia and so on, can reduce the level of psychologic stress experienced by the patient. In spite of the anesthesiologist’s best efforts, the conditions brought about by surgery elicit a stress response to some degree; for the cardiovascular system, this response often includes tachycardia, hypertension, and increased tissue oxygen demand. One of the noninvasive studies, termed a stress test, is an exercise treadmill test. This is a diagnostic study that induces “stressful” conditions on the heart, pulmonary system, and peripheral vasculature to ascertain the patient’s tolerance of increased heart rate and subsequent cardiac oxygen demand. The exercise test typically involves the use of either a treadmill or a stationary bicycle, an electrocardiogram, and blood pressure monitoring. In general, exercise testing is a safe procedure; however, MI and even death have been reported during routine exercise testing, and can be expected to occur at a rate of up to 1 per 2,500 tests.9 As a result, the AHA has developed recommendations for both absolute and relative contraindications for proceeding with exercise treadmill testing (see Table 14.5). In addition, patients unable to walk or run on a treadmill (i.e., patients with severe arthritis, paralysis, or other neurologic conditions, amputees, etc.) require an alternative means of inducing an increased heart rate, which will be discussed in the subsequent text.
Patients with suspected or known coronary artery disease and new or changing symptoms that suggest ischemia should generally undergo exercise testing to assess the risk of future cardiac events. In fact, the exercise treadmill test is designed to produce ischemia in those with significant risk factors for coronary artery disease. Several studies have demonstrated that a positive ischemic response and a low exercise capacity can predict the outcome following noncardiac surgery.2 An early study by Mangano et al. demonstrated an incidence of postoperative MI in 37% of patients who underwent vascular surgery and had a positive ischemic response by exercise treadmill testing, as opposed to a 1.5% incidence of perioperative MI in those who did not. Other studies, however, have not demonstrated such definitive results. It has been found that a 12-lead resting electrocardiogram and exercise capacity were independent predictors of perioperative cardiac complications, and not necessarily the variables related directly to ischemia.
TABLE 14.5 Absolute and Relative Contraindications to Exercise Treadmill Testing
Absolute Contraindications
Acute myocardial infarction (within 2 days)
Unstable angina not previously stabilized by medical therapy
Uncontrolled cardiac arrhythmias causing symptoms or hemodynamic compromise
Symptomatic severe aortic stenosis
Uncontrolled symptomatic heart failure
Acute pulmonary embolus or pulmonary infarction
Acute myocarditis or pericarditis
Acute aortic dissection
Relative Contraindications
Left main coronary stenosis
Moderate stenotic valvular heart disease
Electrolyte abnormalities
Severe arterial hypertension
Tachyarrhythmias or bradyarrhythmias
Hypertrophic cardiomyopathy and other forms of outflow tract obstruction
Mental or physical impairment leading to inability to exercise adequately
High-degree atrioventricular block
Reprinted with permission from Gibbons RJ, Balady GJ, Bricker JT, et al. ACC/AHA 2002 guideline update for exercise testing: Summary article: A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1997 Exercise Testing Guidelines). Circulation. 2002;106:1883.
▪ MYOCARDIAL PERFUSION STUDIES
In patients for whom further preoperative testing is desired (i.e., high-risk patients) yet who are unable to perform an exercise treadmill test, a myocardial perfusion scan (scintigraphy) may be considered. Patients with exercise limitations are often at highest risk, including those with peripheral vascular, neurologic, or orthopedic disease. A dipyridamole/thallium or adenosine/thallium perfusion scan induces coronary vasodilation and assists in elucidating regions of redistribution defects. Dipyridamole blocks adenosine reuptake, thereby increasing adenosine concentration in the coronary vessels. Adenosine acts as a direct coronary vasodilator. By infusing these vasodilators, flow is preferentially distributed to areas distal to normal coronary arteries, and minimizes flow to areas distal to coronary stenoses. A radioisotope, thallium (99m-Technetium sestamibi is also used), is injected. Regions of normal myocardium appear on initial imaging, whereas areas of myocardial necrosis or areas distal to significant coronary lesions remain dark. Several hours later, a second infusion of radioisotope is injected. Areas that remain as defects represent regions of old scar, whereas those that reappear as normal suggest areas at risk for myocardial ischemia. Redistribution defects can be quantified; large areas of defect are associated with increased cardiac risk.
Numerous studies have examined the utility of perfusion scans for risk stratification, and much controversy has arisen over the findings. In the mid-1980s, positive perfusion scans were found to correlate with adverse perioperative events. On the other hand, very few adverse events occurred in patients who had no redistribution defects (e.g., a negative scan). As such, dipyridamole thallium scans found widespread use in the ensuing years. In the early 1990s, however, the former findings were challenged by a prospective, triple-blinded study which, contrary to previously reported findings, revealed no association between redistribution defects and adverse outcomes.2,10 Later studies corroborated the fact that thallium redistribution was not significantly associated with adverse perioperative cardiac events,10,11 and as a result, the routine use of perfusion scans decreased. More recent studies have demonstrated a greater predictive value of perfusion scans if used in conjunction with more complicated clinical markers and technical examinations. However, the most significant study may have been that by Baron et al., in which it was reported that myocardial perfusion imaging did not provide independent prognostic value beyond that of clinical risk stratification.11
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