CORONARY ARTERY DISEASE [Myocardial oxygen supply / Angina pectoris / Mechanism of Cardiac pain / Treatment of Angina] 


          cardiovascular diseases are the number 1 cause of death globally. An estimated 17.9 million people died from CVDs in 2016, representing 31% of all global deaths. Of these deaths, 85% are due to heart attack and stroke. In India, studies have reported increasing prevalence of coronary artery disease over the last 60 years, from 1% to 9-10% in urban populations and <1% to 4-6% in rural populations.


Myocardial oxygen supply

     The myocardial oxygen supply depends on; (1) Oxygen content of the arterial blood, and  (2) The rate of coronary blood flow. The oxygen content of arterial blood may be decreased because of decreased hemoglobin concentration or because of poor systemic blood oxygenation (hypoxic hypoxia). Thus, angina may be a presenting feature of a patient with severe anemia or lung disease. In the absence of anemia or lung disease, oxygen supply to the heart is determined by rate of coronary blood flow.

     In most other organs, because of greater pressure head, blood flow is greater during systole than in diastole of the heart . However, in case of myocardium, the reverse is true. The coronary arteries. Branches of epicardial arteries run into and supply blood to the myocardium are called subendocardial coronary vessels. During systole, myocardial contraction has a strangulating effect on the blood vessels passing through the cardiac muscle fibers. Because of this, blood flow in the subendocardial vessels stops. As a result, most myocardial perfusion occurs during diastole when the subendocardial coronary vessels are patent because of absence of extramural pressure.




         Although coronary vessels are supplied with sympathetic and parasympathetic nerve fibers, the coronary vascular resistance is chiefly determined by intrinsic metabolic factors rather then neutral control. Local vasodilator metabolites such as adenosine (chiefly) and other products of anoxic metabolism (lactate, H+, certain prostaglandins) regulate coronary blood flow by a direct action on vascular smooth muscle. During exercise, greater release of local vasodilator metabolites assures greater blood flow in the coronary arteries.

        Atherosclerosis narrowing of coronary arteries produces its effects mainly by hypo-perfusion (decreased blood supply) of the myocardium. The effect may range from angina to myocardial infarction.


Angina Pectoris

        In normal individuals during exercise, by the local metabolite control, coronary arteriolar resistance decreases in proportion to the increase in O2 demand of the myocardium. Thus coronary blood flow increases in proportion to the oxygen demand of myocardium. When atherosclerotic narrowing is greater than 60-70%, coronary blood flow cannot increase during exercise in spite of presence of vasodilator metabolites. Therefore, myocardial ischemia results; Which is commonly intermittent (only during exertion). Angina pain is characterized by the fact that it occurs only at times of increased myocardial oxygen demand such as exertion or emotional excitement, but subsides by rest. Such a condition is known ass angina. In angina, pain may be localized to substernum or referred to left arm, neck, jaw.


Symptoms

     *Pain or discomfort that can spread to the chest, jaw, shoulders, arms(mostly the left arm) and back.

     *Chest tightness, burning, heaviness, feeling of squeezing or not being able to breathe.

     *Angina will sometimes cause dizziness, paleness, weakness. 


THE MECHANISMS OF CARDIAC PAIN

     It is presumed that pain of angina pectoris results from the release of anoxic metabolites (adenosine, bradykinin) by the myocardium excite the sensory ends of the sympathetic and vagal afferent fibers supplying the heart. Within the spinal cord, cardiac sympathetic afferent impulses may converge with impulses from somatic thoracic structure, Which may be the basis for referred cardiac pain, for (example, to the left arm).


Pathophysiologic Basis of Treatment Of Angina

    For the immediate relief of angina pain, the patient is advised to take a sublingual tablet of nitroglycerin.

    Nitroglycerin is converted to a powerful vasodilator nitric oxide (NO) in the body. It may produce some dilation of coronary arteries, but the major action is venous vasodilation. Venodilation cause pooling of blood within the venous system, reducing preload to the heart. This cause a decrease in cardiac work, and cardiac oxygen demand and hence relieves angina pain.