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):

image

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:

image

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?

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

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