33: Ischemic Heart Disease

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CHAPTER 33 Ischemic Heart Disease

Tamas Seres, MD, PhD

1 Name the known risk factors for the development of ischemic heart disease

Age, male gender, and positive family history are risk factors that cannot be modified. Smoking, hypertension, diet, hyperlipidemia, physical inactivity, obesity, and diabetes mellitus are modifiable and should be considered in all adults as risk factors for which interventions are likely to lower ischemic heart disease (IHD) risk.

2 Describe the normal coronary blood flow

The resting coronary blood flow (CBF) averages about 225 ml/min, which is 4% to 5 % of the total cardiac output in normal adults. The CBF increases threefold to fourfold to supply the extra nutrients needed by the heart at maximum exercise level. The CBF is determined by the pressure gradient between the aorta and the ventricle. There are phasic changes in CBF during systole and diastole in the left ventricle. The blood flow falls to a low value during systole, especially in the subendocardial area because of compression of the left ventricular muscle around the intramuscular vessels and the small pressure gradient between the aorta and the left ventricle (aortic systolic pressure–left ventricular systolic pressure). During diastole the cardiac muscle relaxes and no longer obstructs the blood flow through the left ventricular capillaries. The pressure gradient is high during diastole (aortic diastolic pressure–left ventricular end-diastolic pressure).

The phasic changes of the CBF are much less in the right ventricle because the force of contraction of the right ventricle is much weaker than that of the left ventricle and the pressure gradient is high between the aorta and the right ventricle during the entire cardiac cycle.

3 Describe the coronary anatomy

The right coronary artery system is dominant in 80% to 90% of people and supplies the sinoatrial node (60%), atrioventricular node (85%), and right ventricle. In 84% of the cases it terminates as the posterior descending artery (PDA). Right-sided coronary artery occlusion can result in bradycardia, heart block, and inferior or right ventricular myocardial infarction (MI).

The left main coronary artery gives rise to the circumflex artery and left anterior descending (LAD) artery, which supplies the majority of the interventricular septum (septal branches) and left ventricular wall (diagonal branches). When the circumflex gives rise to the PDA, the circulation is said to be left dominant, and the left coronary circulation supplies the entire septum and the atrioventricular node. In 40% of patients the circumflex artery supplies the sinoatrial node. Significant stenosis of the left main coronary artery (left main disease) or the proximal circumflex and left anterior descending arteries (left main equivalent) may cause severely depressed myocardial function during ischemia.

4 Explain the determinants of myocardial oxygen demand and delivery

Myocardial oxygen demand is determined by wall tension and contractility. Wall tension (T) is the product of intraventricular pressure (P), radius (R) and the wall thickness (h):

Increased ventricular volume (preload) and increased blood pressure (afterload) both increase wall tension and oxygen demand. Increase in contractility in response to sympathetic stimulation or inotropic medications increases oxygen demand also.

Increase in heart rate increases myocardial contractility but decreases the ventricular diameter and the wall tension. Thus the increase in oxygen demand caused by enhanced contractility is in large part offset by the reduction in oxygen demand that accompanies the reduced wall tension. The oxygen demand increases because more contraction is performed per minute.

Myocardial oxygen delivery is determined by oxygen content and coronary blood flow. Oxygen content can be calculated by the following equation:

Coronary blood flow is determined by coronary perfusion pressure, the time available for perfusion, and the patency of coronary arteries. Coronary perfusion pressure is altered by diastolic hypotension, left ventricular hypertrophy, and increased left ventricular end-diastolic pressure. The diastolic time for perfusion decreases with tachycardia.

5 What is the clinical manifestation of myocardial ischemia?

The clinical manifestations of myocardial ischemia are varied. Angina pectoris with or without arrhythmias or heart failure is assumed to be the classic manifestation of myocardial ischemia. However, myocardial ischemia may present as ventricular failure or arrhythmias without angina or may remain clinically silent. The dynamic nature of coronary stenosis accounts for the changes in vessel caliber that may produce pain at rest at one time and angina with varying degrees of exercise at other times.

6 How is angina graded?

The Canadian Cardiovascular Society introduced a grading system for angina:

Class I	Angina with strenuous or rapid prolonged exertion at work or recreation
Class II	Angina with walking or climbing stairs rapidly, walking uphill, or walking more than two blocks on the level and climbing more than one flight of ordinary stairs at a normal pace
Class III	Angina with walking one to two blocks on the level and climbing one flight of stairs at a normal pace
Class IV	Angina may be present at very low level of physical activity or at rest

7 Describe the pathogenesis of a perioperative myocardial infarction

MI is usually caused by platelet aggregation, vasoconstriction, and thrombus formation at the site of an atheromatous plaque in a coronary artery. Sudden increases in myocardial oxygen demand (tachycardia, hypertension) or decreases in oxygen supply (hypotension, hypoxemia, anemia, tachycardia, coronary occlusion) can precipitate MI in patients with IHD. However, recent studies have confirmed that most perioperative ischemic events are unrelated to hemodynamic perturbations, suggesting that intracoronary events may be important in the genesis of perioperative ischemia.

8 What clinical factors increase the risk of a perioperative myocardial infarction following noncardiac surgery?

There are active cardiac conditions, clinical risk factors, and minor clinical predictors based on the algorithm for risk stratification and appropriate use of diagnostic testing of cardiac patients undergoing noncardiac surgery by the American College of Cardiology/American Heart Association (ACC/AHA) Task Force on Perioperative Evaluation (2007). The guidelines integrate clinical risk factors, exercise capacity, and the surgical procedure in the decision-making process. They are discussed in more detail in Chapter 17.

9 What is the definition of recent and prior myocardial infarction?

Based on the decreased risk for perioperative reinfarction, the American College of Cardiology National Database defines recent MI as occurring >7 days but ≤30 days before the surgery. Prior MI is defined as MI >30 days before the surgery.

10 How does the type of surgery influence the risk stratification for perioperative ischemia?

High-risk surgery (risk of perioperative adverse cardiac events >5 %) includes aortic and major vascular procedures and peripheral vascular surgeries.

Intermediate-risk procedures (risk of perioperative adverse cardiac events <5 %) are: carotid endarterectomy, head and neck surgery, intraperitoneal and intrathoracic surgery, orthopedic surgery, and prostate surgery.

Low-risk procedures (risk of perioperative adverse cardiac events <1 %) are endoscopic procedures, superficial procedures, cataract surgery, and breast surgery.

11 How can cardiac function be assessed by history and physical examination?

If a patient’s exercise capacity is excellent, even in the presence of IHD, chances are good that the patient will be able to tolerate the stresses of the surgery. Poor exercise tolerance in the absence of pulmonary or other systemic disease indicates an inadequate cardiac reserve. All patients should be questioned about their ability to perform daily activities such as described in the Canadian classification of angina pectoris or the New York Heart Association (NYHA) classification of exercise tolerance. The ability to climb two-to-three flights of stairs without significant symptoms (angina, dyspnea, syncope) is usually an indication of adequate cardiac reserve as defined by >4 metabolic equivalents (METs) exercise capacity (1 MET equals 3.6 ml/kg/min oxygen uptake at rest).

Signs and symptoms of congestive heart failure include dyspnea, orthopnea, paroxysmal nocturnal dyspnea, peripheral edema, jugular venous distention, a third heart sound, rales, and hepatomegaly.

12 When would you consider noninvasive stress testing before noncardiac surgery?

The guidelines integrate clinical risk factors, exercise capacity, and the surgical procedure in the decision making process. Noninvasive stress testing should be considered if it will change the management of the patients. Examples include:

Patients with active cardiac conditions in whom noncardiac surgery is planned should be evaluated and treated per ACC/AHA guidelines before noncardiac elective surgery.

Patients with three or more clinical risk factors and poor functional capacity who require vascular surgery.

Patients with at least one or two clinical risk factors and poor functional capacity who require intermediate-risk noncardiac surgery.

Patients with at least one or two clinical risk factors and good functional capacity who are undergoing vascular surgery.

13 What tests performed by medical consultants can help further evaluate patients with known or suspected ischemic heart disease?

Exercise electrocardiogram (ECG) is a noninvasive test that attempts to produce ischemic changes on ECG (ST depression = 1 mm from baseline) or symptoms by having the patient exercise to maximum capacity. Information obtained relates to the thresholds of heart rate and blood pressure that can be tolerated. Maximal heart rates, blood pressure response, and clinical symptoms guide interpretation of the results.

Exercise thallium scintigraphy increases the sensitivity and specificity of the exercise ECG. The isotope thallium is almost completely taken up from the coronary circulation by the myocardium and can then be visualized radiographically. Poorly perfused areas that later refill with contrast delineate areas of myocardium at risk for ischemia. Fixed perfusion defects indicate infarcted myocardium.

Dipyridamole thallium imaging is useful in patients who are unable to exercise. This testing is frequently required in patients with peripheral vascular disease who are at high risk for IHD and in whom exercise test is limited by claudication. Dipyridamole is a potent coronary vasodilator that causes differential flow between normal and diseased coronary arteries detectable by thallium imaging.

Echocardiography can be used to evaluate left ventricular and valvular function and measure ejection fraction.

Stress echocardiography (dobutamine echo) can be used to evaluate new or worsened regional wall motion abnormalities in the pharmacologically stressed heart. Areas of wall motion abnormality are considered at risk for ischemia.

Coronary angiography is the gold standard for defining the coronary anatomy. Valvular and ventricular function can be evaluated, and hemodynamic indices can be measured. Because angiography is invasive, it is reserved for patients who require further evaluation based on previous tests or who have a high probability of severe coronary disease.

KEY POINTS: Ischemic Heart Disease

1. The clinical predictors, the surgical procedure, and exercise capacity should be integrated in the decision-making process to avoid adverse perioperative cardiac events.

2. Patients with active cardiac conditions (unstable or severe angina, recent MI) should be determined and treated before noncardiac elective surgery.

3. The risk of the surgical procedure should also be considered. Patients for vascular surgery represent very high risk for perioperative ischemic events.

Patients with excellent exercise capacity, even in the presence of ischemic heart disease, will be able to tolerate the stresses of noncardiac surgery. The ability to climb two-to-three flights of stairs without significant symptoms (angina, dyspnea) is usually an indication of adequate cardiac reserve.

The type of acute revascularization should be carefully planned because patients with a drug-eluting stent should be on dual-antiplatelet therapy for at least a year.

14 What are the main indications for coronary revascularization before noncardiac surgery?

Consistent with the ACC/AHA 2004 Guideline Update for coronary artery bypass graft surgery, the following patients need coronary revascularization (either coronary artery bypass grafting or percutaneous coronary intervention) before noncardiac surgery:

Patients with stable angina who have significant left main coronary artery stenosis.

Patients with stable angina who have three-vessel disease. Survival benefit is greater when left ventricular ejection fraction is less than 50%.

Patients with stable angina who have two-vessel disease with significant proximal left anterior descending stenosis and either ejection fraction less than 50% or demonstrable ischemia on noninvasive testing.

Patients with high-risk unstable angina or non–ST-segment elevation MI.

Patients with acute ST-elevation MI.

15 A patient after percutaneous coronary intervention is scheduled for surgery. What is your concern?

After percutaneous coronary intervention (PCI), patients need to be on dual-antiplatelet therapy (aspirin and clopidogrel). Discontinuation of this therapy for surgical procedure creates high risk for coronary artery thrombosis and myocardial events in the perioperative period. The appropriate timing of surgery is still under investigation, but the following guidelines are acceptable:

After balloon angioplasty nonurgent surgery can be performed with aspirin after 14 days, but ideally these patients should be on dual-antiplatelet therapy for 4 to 6 weeks.

After bare-metal or drug-eluting stent placement, nonurgent surgery can be performed with aspirin after 30 to 45 days or >365 days, respectively.

16 Why do patients with drug-eluting stents need significantly longer time than those with bare metal stents?

Drug-eluting stents in current clinical use were approved by the Food and Drug Administration after clinical trials showed they were statistically superior to bare-metal stents for treatment of coronary artery occlusions, having lower rates of major adverse cardiac events (usually defined as a composite clinical end point of death + MI + repeat intervention because of restenosis). The eluting drugs (sirolimus, paclitaxel) released from the stent inhibit endothelial surface formation inside the stent, thus requiring longer-duration dual-antiplatelet therapy.

17 Should all cardiac medications be continued throughout the perioperative period?

Patients with a history of IHD are usually taking medications intended to decrease myocardial oxygen demand by decreasing the heart rate, preload, afterload or contractile state (β-blockers, calcium channel antagonists, nitrates) and increase the oxygen supply by causing coronary vasodilation (nitrates). Generally these drugs should be administered throughout the perioperative period.

18 How would you give β-blockers to high-risk patients?

β-Blockers should be given to patients undergoing vascular surgery who are at high cardiac risk owing to the finding of ischemia on preoperative testing. They are probably indicated for patients undergoing vascular or intermediate-risk surgery in which perioperative assessment identifies coronary artery disease or more than one clinical risk factor. Finally, they should be continued in patients undergoing surgery who are already receiving these medications for the treatment of angina, symptomatic arrhythmias, or hypertension.

19 Would you give prophylactic intraoperative nitroglycerin infusion?

The usefulness of intraoperative nitroglycerin as a prophylactic agent to prevent myocardial ischemia and cardiac morbidity is unclear for high-risk patients undergoing noncardiac surgery, particularly those who have required nitrate therapy to control angina. The recommendation for prophylactic use of nitroglycerin must take into account the anesthetic plan and patient hemodynamics and must recognize that vasodilation and hypovolemia can readily occur during anesthesia and surgery.

20 What electrocardiogram findings support the diagnosis of ischemic heart disease?

The resting 12-lead ECG remains a low-cost, effective screening tool in the detection of IHD. It should be evaluated for the presence of ST-segment depression or elevation, T-wave inversion, old MI as demonstrated by Q waves, disturbances in conduction and rhythm, and left ventricular hypertrophy. Ischemic changes in leads II, III, and aVF suggest right coronary artery disease, leads I, aVL, V₅ and V₆ monitor the circumflex artery distribution, and leads V₁ to V₄ look at the distribution of the left anterior descending artery.

21 When is a resting 12-lead ECG recommended?

Patients with at least one clinical risk factor (history of ischemic heart disease, history of compensated or prior heart failure, history of cerebrovascular disease, diabetes mellitus, and renal insufficiency) who are undergoing vascular surgical procedures.

Patients with known coronary heart disease, peripheral arterial disease, or cerebrovascular disease who are undergoing intermediate-risk surgical procedures.

It is reasonable in patients with no clinical risk factors who are undergoing vascular surgical procedures.

22 How long should a patient with a recent myocardial infarction wait before undergoing elective noncardiac surgery?

The risk of reinfarction during surgery in a patient with recent MI has traditionally depended on the time interval between the MI and the procedure. The ACC/AHA task force eliminated the arbitrary 6-month time interval and suggested that elective surgery is associated with prohibitive risk only during the initial 4 to 6 weeks. The patient’s functional status following rehabilitation from an MI is probably more important than the absolute time interval. Patients with ongoing symptoms are candidates for coronary revascularization before their noncardiac procedure.

23 Outline the hemodynamic goals of induction and maintenance of general anesthesia in patients with IHD

The anesthesiologist’s goal must be to maintain the balance between myocardial oxygen demand and supply throughout the perioperative period. During induction wide swings in heart rate and blood pressure should be avoided. Ketamine should be avoided because of the resultant tachycardia and hypertension. Prolonged laryngoscopy should be avoided, and the anesthesiologist may wish to blunt the stimulation of laryngoscopy and intubation by the addition of opiates, β-blockers, or laryngotracheal or intravenous lidocaine. Maintenance drugs are chosen with knowledge of the patient’s ventricular function. In patients with good left ventricular function, the cardiac depressant and vasodilator effects of inhaled anesthetics may reduce myocardial oxygen demand. A narcotic-based technique may be chosen to avoid undue myocardial depression in patients with poor left ventricular function. Muscle relaxants with minimal cardiovascular effects are usually preferred. Blood pressure and heart rate should be maintained near baseline values. This can be accomplished by blunting sympathetic stimulation with adequate analgesia and aggressively treating hypertension (anesthetics, nitroglycerin, nitroprusside, β-blockers), hypotension (fluids, phenylephrine, inotropic drugs), and tachycardia (fluids, anesthetics, β-blockers).

24 What monitors are useful for detecting ischemia intraoperatively?

The V₅ precordial lead is the most sensitive single ECG lead for detecting ischemia and should be monitored routinely in patients at risk for IHD. Lead II can detect ischemia of the right coronary artery distribution and is the most useful lead for monitoring P waves and cardiac rhythm.

25 Would you use transesophageal echocardiography routinely in patients with high cardiac risk undergoing noncardiac surgery?

Transesophageal echocardiography (TEE) can provide continuous intraoperative monitoring of left ventricular function. In high-risk patients detection of regional wall motion abnormalities with this technique is the most sensitive monitor for myocardial ischemia. However, no study has clearly demonstrated a change in outcome from the routine use of TEE. Therefore the choice of using TEE is best left to the discretion of the anesthesia care team.

26 Is a pulmonary artery catheter reasonable to use routinely for optimization of high-risk patients? What is its potential benefit?

The use of pulmonary artery catheters in patients with IHD has not been shown to improve outcome; thus it should be restricted to a very small number of well-selected patients whose presentation is unstable and who have multiple comorbid conditions. When used, the pulmonary artery occlusion (wedge) pressure gives an estimation of the left ventricular end-diastolic pressure, which is a useful guide for optimizing intravascular fluid therapy. Sudden increases in the wedge pressure may indicate acute left ventricular dysfunction caused by ischemia. See Chapter 26.

SUGGESTED READINGS

1. Fleisher L.A., Beckman J.A., Brown K.A., et al. ACC/AHA 2007 Guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery. Circulation. 2007;116:1971-1996.

2. Newsome L.T., Kutcher M.A., Royster R.L. Coronary artery stents. Part 1. Evolution of percutaneous coronary intervention. Anesth Analg. 2008;107:552-569.

3. Newsome L.T., Weller R.S., Gerancher J.C., et al. Coronary artery stents. Part II. Perioperative considerations and management. Anesth Analg. 2008;107:570-590.

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