Chapter 1 Thorax – part 2

Intro

 Myocardial Infarction

With sudden occlusion of a major artery by an embolus (G. embolos, plug), the region of myocardium supplied by the occluded vessel becomes infarcted (rendered virtually bloodless) and undergoes necrosis (pathological tissue death). The three most common sites of coronary artery occlusion and the percentage of occlusions involving each artery are the:

  1. Anterior IV (LAD) branch of the LCA (40–50%).
  2. RCA (30–40%).
  3. Circumflex branch of the LCA (15–20%)

Sites 1–3 account for at least 85% of all occlusions.

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An area of myocardium that has undergone necrosis constitutes a myocardial infarction (MI). The most common cause of ischemic heart disease is coronary artery insufficiency resulting from atherosclerosis.

Coronary Atherosclerosis

The atherosclerotic process, characterized by lipid deposits in the intima (lining layer) of the coronary arteries, begins during early adulthood and slowly results in stenosis of the lumina of the arteries. As coronary atherosclerosis progresses, the collateral channels connecting one coronary artery with the other expand, which may initially permit adequate perfusion of the heart during relative inactivity. Despite this compensatory mechanism, the myocardium may not receive enough oxygen when the heart needs to perform increased amounts of work. Strenuous exercise, for example, increases the heart’s activity and its need for oxygen. Insufficiency of blood supply to the heart (myocardial ischemia) may result in MI.

Atherosclerosis: stages of development in a coronary artery.

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Slowly Progressive Coronary Artery Disease

In slow occlusion of a coronary artery, the collateral circulation has time to increase so that adequate perfusion of the myocardium can occur when a potentially ischemic event occurs. Consequently, MI may not result. On sudden blockage of a large coronary branch, some infarction is probably inevitable, but the extent of the area damaged depends on the degree of development of collateral anastomotic channels. If large branches of both coronary arteries are partially obstructed, an extracardiac collateral circulation may be used to supply blood to the heart. These collaterals connect the coronary arteries with the vasa vasorum (small arteries) in the tunica adventitia of the aorta and pulmonary arteries and with branches of the internal thoracic, bronchial, and phrenic arteries. Clinical studies show that anastomoses cannot provide collateral routes quickly enough to prevent the effects of sudden coronary artery occlusion. The functional value of these anastomoses thus appears to be more effective in slowly progressive CAD in individuals that are physically active.

Angina Pectoris

Pain that originates in the heart is called angina or angina pectoris (L. angina, strangling pain + L. pectoris, of the chest). Individuals with angina commonly describe the transient (15 sec to 15 min) but moderately severe constricting pain as tightness in the thorax, deep to the sternum. The pain is the result of ischemia of the myocardium that falls short of inducing the cellular necrosis that defines infarction.

Most often, angina results from narrowed coronary arteries. The reduced blood flow results in less oxygen being delivered to the cardiac striated muscle cells. As a result of the limited anaerobic metabolism of the myocytes, lactic acid accumulates and the pH is reduced in affected areas of the heart. Pain receptors in muscle are stimulated by lactic acid. Strenuous exercise (especially after a heavy meal), sudden exposure to cold, and stress all require increased activity on the part of the heart, but the occluded vessels cannot provide it. When food enters the stomach, blood flow to it and other parts of the digestive tract is increased. As a result, some blood is diverted from other organs, including the heart.

Anginal pain is relieved by a period of rest (1–2 min are often adequate). Sublingual nitroglycerin (medication placed or sprayed under the tongue for absorption through the oral mucosa) may be administered because it dilates the coronary (and other) arteries. This increases blood flow to the heart, while decreasing the workload and the heart’s need for oxygen because the heart is pumping against less resistance. Furthermore, the dilated vessels accommodate more of the blood volume, so less blood arrives in the heart, relieving heart congestion. Thus the angina is usually relieved. Such angina provides a warning that the coronary arteries are compromised and that there is a need for a change of lifestyle, a healthcare intervention, or both.

The pain resulting from MI is usually more severe than with angina pectoris, and the pain resulting from the infarction does not disappear after 1–2 min of rest.

Coronary Bypass Graft

Patients with obstruction of their coronary circulation and severe angina may undergo a coronary bypass graft operation. A segment of an artery or vein is connected to the ascending aorta or to the proximal part of a coronary artery and then to the coronary artery distal to the stenosis. The great saphenous vein is commonly harvested for coronary bypass surgery because it (1) has a diameter equal to or greater than that of the coronary arteries, (2) can be easily dissected from the lower limb, (3) and offers relatively lengthy portions with a minimum occurrence of valves or branching. Reversal of the implanted segment of vein can negate the effect of a valve if a valved segment must be used. Use of the radial artery in bypass surgery has become increasingly more common. A coronary bypass graft shunts blood from the aorta to a stenotic coronary artery to increase the flow distal to the obstruction. Simply stated, it provides a detour around the stenotic area (arterial stenosis) or blockage (arterial atresia). Revascularization of the myocardium may also be achieved by surgically anastomosing an internal thoracic artery with a coronary artery.

Triple coronary artery bypass.

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Coronary Angioplasty

In selected patients, surgeons use percutaneous transluminal coronary angioplasty in which they pass a catheter with a small inflatable balloon attached to its tip into the obstructed coronary artery. When the catheter reaches the obstruction, the balloon is inflated, flattening the atherosclerotic plaque against the vessel’s wall. The vessel is stretched to increase the size of the lumen, thus improving blood flow. In other cases, thrombokinase is injected through the catheter; this enzyme dissolves the blood clot. Intraluminal instruments with rotating blades and lasers have also been employed. After dilation of the vessel, an intravascular stent may be introduced to maintain the dilation. Intravascular stents are composed of rigid or semirigid tubular meshes, collapsed during introduction. Once in place, they expand or are expanded with a balloon catheter, to maintain luminal patency.

Percutaneous transluminal angioplasty.

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Collateral Circulation via the Smallest Cardiac Veins

Reversal of flow in the anterior and smallest cardiac veins may bring luminal blood (blood from the heart chambers) to the capillary beds of the myocardium in some regions, providing some additional collateral circulation. However, unless these collaterals have dilated in response to pre-existing ischemic heart disease, especially in conjunction with physical conditioning, they are unlikely to be able to supply sufficient blood to the heart during an acute event and thus prevent MI.

Electrocardiography

The passage of impulses over the heart from the SA node can be amplified and recorded as an electrocardiogram (ECG or EKG). Functional testing of the heart includes exercise tolerance tests (treadmill stress tests), primarily to check the consequences of possible coronary artery disease. Exercise tolerance tests are of considerable importance in detecting the cause of heartbeat irregularities. Heart rate, ECG, and blood pressure readings are monitored as the patient does increasingly demanding exercise on a treadmill. The results show the maximum effort a patient’s heart can safely tolerate.

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A.Electrocardiography (ECG).

B.Relationship of electrocardiogram to conducting system of heart.

Coronary Occlusion and Conducting System of Heart

Damage to the conducting system of the heart, often resulting from ischemia caused by coronary artery disease, produces disturbances of cardiac muscle contraction. Since the anterior IV branch (LAD) gives rise to the septal branches supplying the AV bundle in most people, and branches of the RCA supply both the SA and AV nodes, parts of the conducting system of the heart are likely to be affected by their occlusion, and a heart block may occur. In this case (if the patient survives the initial stages), the ventricles will begin to contract independently at their own rate: 25–30 times per minute (much slower than the slowest normal rate (40–45 times per minute). The atria continue to contract at the normal rate if the SA node has been spared, but the impulse generated by the SA node no longer reaches the ventricles.

Blood supply of conducting system of heart. AV, atrioventricular; SA, sinu-atrial.

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Damage to one of the bundle branches results in a bundle branch block, in which excitation passes along the unaffected branch and causes a normally timed systole of that ventricle only. The impulse then spreads to the other ventricle via myogenic (muscle propagated) conduction, producing a late asynchronous contraction. In these cases, a cardiac pacemaker (artificial heart regulator) may be implanted to increase the ventricular rate of contraction to 70–80 per minute.

With a VSD, the AV bundle usually lies in the margin of the VSD. Obviously, this vital part of the conducting system must be preserved during surgical repair of the defect. Destruction of the AV bundle would cut the only physiological link between the atrial and ventricular musculature, also producing a heart block as described above.

Artificial Cardiac Pacemaker

In some people with a heart block, an artificial cardiac pacemaker (approximately the size of a pocket watch) is inserted subcutaneously. The pacemaker consists of a pulse generator or battery pack, a wire (lead), and an electrode. Pacemakers produce electrical impulses that initiate ventricular contractions at a predetermined rate. An electrode with a catheter connected to it is inserted into a vein and its progression through the venous pathway is followed with a fluoroscope, a device for examining deep structures in real time (as motion occurs) by means of radiographs. The terminal of the electrode is passed through the SVC to the right atrium and through the tricuspid valve into the right ventricle. Here the electrode is firmly fixed to the trabeculae carneae in the ventricular wall and placed in contact with the endocardium.

Restarting Heart

In most cases of cardiac arrest, first-aid workers perform cardiopulmonary resuscitation (CPR) to restore cardiac output and pulmonary ventilation. By applying firm pressure to the thorax over the inferior part of the sternal body (external or closed chest massage), the sternum moves posteriorly 4–5 cm. The increased intrathoracic pressure forces blood out of the heart into the great arteries. When the external pressure is released and the intrathoracic pressure falls, the heart again fills with blood. If the heart stops beating (cardiac arrest) during heart surgery, the surgeon attempts to restart it using internal or open chest heart massage.

Fibrillation of Heart

Fibrillation is multiple, rapid, circuitous contractions or twitchings of muscular fibers, including cardiac muscle. In atrial fibrillation, the normal regular rhythmical contractions of the atria are replaced by rapid irregular and uncoordinated twitchings of different parts of the atrial walls. The ventricles respond at irregular intervals to the dysrhythmic impulses received from the atria, but usually circulation remains satisfactory. In ventricular fibrillation, the normal ventricular contractions are replaced by rapid, irregular twitching movements that do not pump (i.e., they do not maintain the systemic circulation, including the coronary circulation). The damaged conducting system of the heart does not function normally. As a result, an irregular pattern of uncoordinated contractions occurs in the ventricles, except in those areas that are infarcted. Ventricular fibrillation is the most disorganized of all dysrhythmias, and in its presence no effective cardiac output occurs. The condition is fatal if allowed to persist.

Defibrillation of Heart

A defibrillating electric shock may be given to the heart through the thoracic wall via large electrodes (paddles). This shock causes cessation of all cardiac movements and a few seconds later the heart may begin to beat more normally. As coordinated contractions and hence pumping of the heart is re-established, some degree of systemic (including coronary) circulation results.

Cardiac Referred Pain

The heart is insensitive to touch, cutting, cold, and heat; however, ischemia and the accumulation of metabolic products stimulate pain endings in the myocardium. The afferent pain fibers run centrally in the middle and inferior cervical branches and especially in the thoracic cardiac branches of the sympathetic trunk. The axons of these primary sensory neurons enter spinal cord segments T1 through T4 or T5, especially on the left side.

Cardiac referred pain is a phenomenon whereby noxious stimuli originating in the heart are perceived by a person as pain arising from a superficial part of the body—the skin on the left upper limb, for example. Visceral referred pain is transmitted by visceral afferent fibers accompanying sympathetic fibers and is typically referred to somatic structures or areas such as a limb having afferent fibers with cell bodies in the same spinal ganglion, and central processes that enter the spinal cord through the same posterior roots (Hardy and Naftel, 2005).

Anginal pain is commonly felt as radiating from the substernal and left pectoral regions to the left shoulder and the medial aspect of the left upper limb. This part of the limb is supplied by the medial cutaneous nerve of the arm. Often the lateral cutaneous branches of the 2nd and 3rd intercostal nerves (the intercostobrachial nerves) join or overlap in their distribution with the medial cutaneous nerve of the arm. Consequently, cardiac pain is referred to the upper limb because the spinal cord segments of these cutaneous nerves (T1–T3) are also common to the visceral afferent terminations for the coronary arteries. Synaptic contacts may also be made with commissural (connector) neurons, which conduct impulses to neurons on the right side of comparable areas of the spinal cord. This occurrence explains why pain of cardiac origin, although usually referred to the left side, may be referred to the right side, both sides, or the back.

Areas of cardiac referred pain (red).

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The Bottom Line

Heart

Heart: The heart is a dual suction and pressure pump that propels blood through the infinite double loop formed by the pulmonary and systemic circuits. ? The right heart serves the former and the left heart the latter. ? The heart is shaped like a tipped-over pyramid, with the apex directed anteroinferiorly and to the left and the base opposite the apex (posterior). ? Each side of the heart includes a receiving chamber (atrium) and a suction-compression-expulsion chamber (ventricle). ? The bilateral chambers (and thus the high-pressure systemic and lower-pressure pulmonary circuits) are separated by a cardiac septum that is largely muscular but partly membranous. ? AV valves are placed between unilateral chambers to facilitate two-stage (accumulate and then eject) pumping. ? Oneway semilunar valves (pulmonic and aortic) placed at the exit on each side prevent backflow (except that which fills the coronary arteries) and maintains the diastolic pressure of the arteries. ? The chambers have a glistening endothelial lining, the endocardium; a muscular wall or myocardium, the thickness of which is proportional to the internal pressures occurring within the specific chamber; and a glistening outer covering (the visceral layer of serous pericardium, or epicardium). ? The myocardium of the atria and ventricles (and the myogenic propagation of contracting stimuli through it) is attached to and separated by the connective tissue of the fibrous skeleton of the heart. ? The fibrous skeleton consists of four fibrous rings, two trigones, and the membranous parts of the cardiac septa. ? Only specialized muscle conducting contractile impulses from the atria to the ventricles penetrates the fibrous skeleton at defined sites. ? The fibrous skeleton provides attachment for the myocardium and cusps of valves and maintains the integrity of the orifices.

Coronary circulation: The circulatory system of the myocardium is unique in that the coronary arteries fill during ventricular diastole as a result of aortic recoil. They are typically (but not necessarily) functional end arteries. ? The right coronary artery (RCA) and circumflex branch of the left coronary artery (LCA) supply the walls of the atria via small branches. ? The RCA typically supplies the SA and AV nodes, the myocardium of the external wall of the right ventricle (except its anterior surface), the diaphragmatic surface of the left ventricle, and the posterior third of the IVS. ? The LCA typically supplies the anterior two thirds of the IVS (including the AV bundle of conductive tissue), the anterior wall of the right ventricle, and the external wall of the left ventricle (except the diaphragmatic surface). ? The capillary beds of the myocardium drain primarily into the right atrium via veins emptying into the coronary sinus. However, the vein also may enter directly into the chambers via the smallest cardiac veins. Both pathways lack valves.

Conducting, stimulating, and regulating system of heart: The conducting system of the heart consists of specialized intrinsic nodes that rhythmically generate stimuli and bundles of modified cardiac muscle that conduct the impulses. The result is the coordinated contraction of the atria and ventricles. ? The rate of generation and speed of conductivity are increased by the sympathetic division and inhibited by the parasympathetic division of the ANS to meet demands or conserve energy. ? The impulse-generating sinu-atrial (SA) node and the relaying atrioventricular (AV) node are typically supplied by nodal branches of the RCA. The atrioventricular bundle and its branches are primarily supplied by septal branches of the LCA. ? Occlusion of either coronary artery with subsequent infarction of nodal or conductive tissue may require placement of an artificial cardiac pacemaker. ? The effect of the ANS on the coronary arteries is paradoxical. Sympathetic stimulation produces vasodilation and parasympathetic stimulation produces vasoconstriction.