Coronary Artery Disease Overview

(B) is the correct answer here. Preload is also known as ventricular volume.

The blood has to do work. In addition to he pressure work that the heart must do, it must also engage in volume work. This volume work is determined by the amount of blood that flows back into the heart at the end of diastole (end diastolic volume).

This amount of blood is known as venous return. Venous return, as you may remember, is turned into cardiac output. As more blood comes in (venous return increases), more work must be done by the heart to pump all that out, thereby requiring more oxygen. So drugs that can decrease venous return will help lower the preload and thus lower the oxygen demand of the heart. Nitrates are an example of a class of drugs that can reduce venous return.

(B) is the false statement here. There is indeed compensatory blood flow, even if a coronary artery is blocked. If an area is infarcted, nearby blood vessels can provide the necessary nutrients to the area to compensate. This is due to interconnections between arteries (anastomoses). (A) is true, as you may expect. As vessels become more obstructed due to atherosclerosis, the resistance of flow increases as well. As the resistance increases, the flow decreases. The pressure gradient between the aortic blood pressure and the right atrial pressure remains relatively unaltered. The key here is resistance. Atherosclerosis has already been discussed in the previous section (Dyslipidemia). (C) is a true statement, as mentioned before. Because the muscle contracts in systole and the compression decreases blood flow to the heart. The time of greatest blood flow is diastole. Drugs have been developed to specifically decrease the heart rate (beta-blockers) and thus allow the heart to spend more time in diastole. (D) is also true. Regular aerobic exercise will allow the heart to operate at a lower resting rate and have it be in diastole longer, thereby increasing perfusion to the heart.

(D) is the correct answer here. All of the statements are true. The following will explain why.

Statement 1 is true because the myocardial blood flow of a resting individual is approximately 250 mL/min. The heart itself pumps 5 L/min, indicating that the heart takes up approximately 5% of the total blood flow, even at rest.

Statement 2 is also true because energy must be expended to produce a cardiac output and the pressures associated with the pulmonary and systemic circulations. With this amount of work, the heart needs a good supply of oxygen and nutrients.

Statement 3 is also correct, as you may expect. Proportionate to the amount of work that is done, the blood flow to the heart increases. This is necessary to maintain adequate tissue perfusion (cardiac output) during exercise.

(A) is the correct answer. Afterload is also called ventricular systolic pressure.

The pressure that must be reached is determined by the pressure that exists outside of the heart, mainly the aorta. At the diastolic blood pressure, the aortic valve will open but only with additional pressure will the blood be pumped through. This is actually directly related to oxygen demand because if more pressure is needed to propel the blood through (high afterload) then more oxygen is needed for that purpose.

Therefore, drugs can be used to help lower the pressure. Recall from the hypetension section that most of the antihypertensives are used to lower total peripheral resistance (TPR), which will lower blood pressure over time. The same is true here. Hypetension is actually a risk factor for coronary artery disease, and you can see why that is the case.

(C) is the false statement here. Oxygen saturation at the right atrium is actually close to 40%, not 0%. As you can imagine, as it is ejected from the left ventricle, the saturation is 100%.

(A) is true. Even at rest, cardiac cells have a high metabolic rate, necessitating a high extraction rate.

(B) is also true because the resting heart extracts so much already that the reserve oxygen that can be extracted is very low.

(D) is true as well. This is expected but it is not for the most obvious reason. It is true that the skeletal muscles will receive more oxygen during exercise, but not due to a higher extraction ratio. This is due to the fact that there is greater blood flow and thus physically brings more oxygen to the tissues per unit time.

As mentioned before, metabolic autoregulation is the controller of myocardial blood flow. Recall that at the arteriole level, when cells increase their activity, they will use more oxygen, thereby reducing the amount that that the smooth muscles in the arterioles are exposed to. Smooth muscles relax to allow more blood to flow (vasodilation). So at rest, the arterioles are constricted and are dilated during exercise..

(D) is the correct answer here. Beta-blockers will decrease both the heart rate and the cardiac contractility.

For heart rate, the faster the heart is beating, the more oxygen is needed to compensate for metabolic rate. Beta-blockers, because they block the beta-1 receptors in the heart to lower the heart rate. Regular aerobic exercise will do this as well.

Cardiac contractility is directly proportional to the amount of oxygen needed to match metabolic rate. Beta-blockers, due to their blockade of the beta-1 receptors in the heart, will allow for a decrease in contractility and a decrease in the oxygen demand.

Both of these reasons are why beta-blockers are useful in the treatment of coronary artery disease.

The statement is false. Blood flow is actually greatest during DIASTOLE.

During systole, the arteries in the wall of the heart are compressed to allow blood to be ejected. In this time, flow is decreased. During diastole, however, the vessels are open to allow blood to come in and therefore has the greatest flow to the heart.

Heart rate has an impact on perfusion to the heart. When more time is spent during diastole, the heart is better perfused. When heart rate is lowered (either through regular aerobic exercise or through drugs), the vagal effect on the heart is greater, and the better the heart is perfused.

(E) if the correct answer. All of the above choices are correct. The following will explain why that is the case.

(A) is true because oxygen supply does not equal oxygen demand. The demand may be so high that the supply cannot keep up with it, leading to an ischemic attack from low oxygenation. These factors are discussed later.

(B) is actually true. Where there is atherosclerotic plaque in the coronary arteries, oxygenation following the plaque becomes a problem. The less amount of oxygen that the smooth muscles are exposed to there means that the vessels will dilate (autoregulation). While this is fine at rest, the arterioles may not be able to further dilate during exercise and metabolic byproducts can build up, activating pain receptors.

(C) is certainly true. The activation of pain receptors will yield chest pain, which is seen in exercising individuals who have coronary artery disease. At a certain heart rate, they may feel pain, so a good thing to tell them is to stay at least 20 bpm below that mark.

(B) is the false statement here. There is actually a reduction in blood flow, which may activate pain receptors. The following explanation will go into more detail.

Chronic stable angina is also called typical angina or exercise-induced angina (A) that may occur due to atherosclerosis. At rest, this isn't really a problem because the flow may be adequate. The problem occurs during some form of exercise. Beyond the blockage, there may not be enough blood flow, a phenomenon known as ischemia. A buildup of metabolites will decrease the pH and activate pain receptors. The pain is felt just below the sternum and may radiate to the left shoulder and left arm.

The ST segment of an EKG ma be elevated or depressed. It is normally isoelectric. Tissues that do now receive enough oxygen may not be synchronized with other tissues, which messes up with the electrophysiology of the heart. At the ST segment portion of the EKG, a current may be detected and hence the EKG changes (C). EKG changes will be discussed later.

There is actually another type of angina known as vasospastic angina. Smooth muscle of the coronary arteries contract in a spastic manner, yielding a rapid decrease in blood flow. There are some agents that can be used to help control the spasms.