Chapter 154

INTRODUCTION

The right and left coronary arteries are responsible for the delivery of oxygenated blood to the myocardium. The right coronary artery is responsible for supplying oxygen to the right atrium, the right ventricle, and the inferior portion of the left ventricle, sinoatrial and atrioventricular nodes. The distribution of the left coronary artery (LCA) includes the left atrium, the interventricular septum, and the anterolateral walls of the left ventricle. Branches of the LCA include the circumflex artery, which supplies the lateral wall of the left ventricle, and the left anterior descending artery (LAD), which supplies the anterior wall of the left ventricle as well as the interventricular septum. To supply the myocardium with oxygen, the blood flows from the epicardial vessels to the endocardial vessels, and then returns to the right atrium via the coronary sinus.

CORONARY CIRCULATION PHYSIOLOGY

The coronary perfusion pressure (CPP) is dependent on the difference between the aortic diastolic pressure (ADP) and the left ventricular end-diastolic pressure (LVEDP), according to the following equation: CPP = ADPLVEDP. The LVEDP is an approximation of the resistance to coronary blood flow during diastole and is used because it can be inferred with standard invasive monitors such as pulmonary artery catheters. It should be recognized that there are other factors that can contribute to resistance to coronary artery blood flow besides LVEDP, such as intrinsic intramyocardial tissue pressures, that are not easily quantifiable with clinical monitors. As the left ventricle contracts during systole, it occludes the intramyocardial portion of the coronary arteries and results in intermittent perfusion of the left ventricle during diastole only, and actually some degree of retrograde coronary blood flow during systole (Figure 154-1). However, the right ventricle receives continuous perfusion during both systole and diastole.

FIGURE 154-1

Coronary blood flow during the cardiac cycle.

(Reproduced with permission from Butterworth JF, Mackey DC, Wasnick JD, Morgan and Mikhail's Clinical Anesthesiology, 5th ed. McGraw-Hill; 2013.)

To summarize, it is important to note that the effective CPP is directly proportional to ADP, but inversely proportional to LVEDP as well as heart rate (HR). The effective CPP increases with: (1) increases in ADP; (2) decreases in LVEDP; and (3) decreases in HR as the diastolic time extends, leading to prolongation of the time interval for left ventricular coronary blood. In contrast, the CPP decreases with: (1) decreases in ADP; (2) increases in LVEDP; and (3) increases in HR.

MYOCARDIAL OXYGEN BALANCE

Effective CPP ranging from 50 to 120 mm Hg, produces coronary blood flows of 60-80 mL/100 g tissue/min in the average adult at rest. Myocardial oxygen consumption at rest is between 7 and 10 mL/100 g tissue/min; with exercise this can increase five- to sixfold. At the same time, coronary blood flow increases ...

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