Medical brain notes 30

  DISORDERS OF THE CARDIOVASCULAR SYSTEM INTRODUCTION The cardiovascular system comprises the heart, with its covering pericardium, and blood vessels, that is, arteries, arterioles, capillaries, venules and veins, which enclose and distribute blood to the tissues. The heart is a muscular pump weighing about 500 g in an adult and is the strongest muscle in the body. It can contract for over a hundred years nonstop, beating about 100 000 times a day at a rate of about 70 beats min –1 , to pump blood ) around the body. This supplies, for example, oxygen and nutrients to the tissues and removes their waste products. However, in such a complicated system many things can go wrong. In the developed countries, heart disease is responsible for about half of the annual deaths. Much of this is due to lifestyle, for example smoking, poor diets and excess weight. In developing countries rheumatic heart disease is more of a problem but this has decreased in the developed countries.

  THE HEART AND CIRCULATORY SYSTEMS The heart ( Figure 14.1 (A)  and  (B) ) is a hollow, four-chambered, muscular organ situated approximately in the center of the chest. A smooth layer called the  endocardium  lines the inside of the chambers. The wall of the heart or  myocardium  is rich in cardiac muscle tissue arranged into three layers. Theinner layer is circular and thicker in the wall of the left ventricle than in the right. The outer layers of muscle spiral around the ventricles and extend to the fibrous attachments of the four valve rings. On contraction they tend to pull the chamber of the ventricle towards the valve rings. The exterior of the heart is a tough, fibrous layer that is partly covered by fat. The heart is enclosed by a double membrane system called the pericardium. Internally, the heart is centrally divided by a septum that prevents oxygenated and deoxygenated blood from mixing. Each side is subdivided into an upper chamber or ...

  BLOOD VESSELS The blood vessels comprise arteries, arterioles, capillaries, venules and veins ( Figures 14.5 and 14.6 ). Arteries are strong and flexible and carry blood away from the heart. They are subjected to the highest pressure, and their resilience helps to maintain blood pressure while the heart is in between beats. The smaller arteries and arterioles have muscular walls and their diameters can be adjusted to regulate blood flow to a particular region of the body. Capillaries are thin-walled vessels that allow oxygen, nutrients and other materials, for example hormones, to diffuse from the blood to the tissues and waste products to pass from the tissues into the blood. The capillaries are links between the arteries and arterioles and the venules and veins. Veins are, in general, larger in diameter than arteries but have much thinner walls. They transport blood back to the heart but at much lower pressures and speeds than is found in arteries. ARTERIES Arteries are elastic...

  BLOOD PRESSURE The blood pressure is the hydrostatic force that the blood exerts against the wall of a blood vessel and that propels blood around the body. It is determined partly by cardiac output and partly by the peripheral resistance. As already stated, it is greater in arteries than in veins and is highest in the arteries when the heart contracts (systole). This is called the systolic pressure. When the heart contracts blood enters the arteries faster than it can leave through the capillaries so the vessels stretch under the pressure. This bulging of the arteries is the  pulse  that can be felt at a number of sites in the body. During diastole, the artery walls snap back, but the heart contracts again before enough blood has flowed into the capillary bed to relieve the pressure. This is the peripheral resistance. As a result, there is still a substantial blood pressure even during diastole, the diastolic pressure, and so blood flows into the arterioles more or less...

  INVESTIGATING CARDIAC FUNCTION IN HEALTH AND DISEASE Clinicians need to be able to diagnose cardiovascular problems and to monitor their treatment. Obviously, noninvasive methods are to be preferred as far as possible for the comfort of the patient. THE ELECTROCARDIOGRAM (ECG) The spread of depolarization and repolarization through the muscle mass of the heart is accompanied by measurable electrical potentials, which may be recorded through electrodes placed on the skin. The record of these potentials is called an  electrocardiogram  ( ECG ) and represents the aggregate or resultant electrical activity associated with the action potentials of millions of individual cells, each of which has an amplitude and direction. Recording a patient’s ECG is a way of investigating the electrical activity of the heart and when the pattern is abnormal it is invaluable in diagnosing a variety of heart complaints. The test is quick, simple and painless, and provides information on the h...

  INVESTIGATING CARDIAC FUNCTION IN HEALTH AND DISEASE Clinicians need to be able to diagnose cardiovascular problems and to monitor their treatment. Obviously, noninvasive methods are to be preferred as far as possible for the comfort of the patient. THE ELECTROCARDIOGRAM (ECG) The spread of depolarization and repolarization through the muscle mass of the heart is accompanied by measurable electrical potentials, which may be recorded through electrodes placed on the skin. The record of these potentials is called an  electrocardiogram  ( ECG ) and represents the aggregate or resultant electrical activity associated with the action potentials of millions of individual cells, each of which has an amplitude and direction. Recording a patient’s ECG is a way of investigating the electrical activity of the heart and when the pattern is abnormal it is invaluable in diagnosing a variety of heart complaints. The test is quick, simple and painless, and provides information on the h...

  ENDOCARDITIS Endocarditis is an inflammation of the endocardium, the interior lining of the heart and its valves. It most often results from bacterial infection that may originate from bacteria in the blood or as a result of heart surgery. The bacteria in the blood may enter from a skin wound or even from small injuries occurring when chewing food or brushing the teeth. Injecting drug users and patients with prolonged catheter use are also at risk. Abnormal or damaged valves are more susceptible than normal ones, and people with artificial valves are at risk. Bacteria and blood clots can accumulate on the valves (called  vegetation ) and can then break loose and block vessels elsewhere in the body causing strokes, heart attacks pulmonary embolisms or infecting the area where they lodge. Acute infective endocarditis has a rapid onset and can be life threatening, unlike subacute infective endocarditis which develops slowly over weeks and months. In the acute form, the symptoms...

  ABNORMAL HEART RHYTHMS The normal heart rate is between 60 and 100 beats per min but much lower rates may be encountered and are quite normal in young adults who are physically fit. As has been mentioned, the rate also responds to exercise or inactivity and also to pain and anger. An inappropriately fast heartbeat is called  tachycardia  and an abnormally low rate  bradycardia . Abnormal rhythms arefrequently encountered and can be regular or irregular. The contractions of the heart muscle fibers and therefore heartbeat is controlled by electrical discharges that flow through the heart along distinct pathways at controlled speeds. If disturbances occur with the flow of the electrical discharge then  arrhythmias  in these contractions can occur. These range from the harmless to the life threatening. For example, minor arrhythmias can be caused by excessive alcohol consumption, smoking, stress or exercise. Thyroid hormones also affect the heart rate and an ...

  CARDIAC FAILURE Cardiac failure is the inability of the heart to maintain an adequate cardiac output, that is pump a volume of blood per minute sufficient to meet the demands of the body. The heart does not stop beating as is often thought, but its diminished ability imposes severe demands. It is a serious condition but commoner in older people. The incidence is about one in a hundred for individuals over 65 years, and irrespective of the cause, the prognosis is poor. Approximately 50% of patients will die within two years, although new drug treatments are improving mortality and morbidity. There are many possible causes of heart failure and, indeed, any disease that affects the heart and circulatory system can lead to heart failure. The commonest of these is coronary artery disease that limits the flow of blood, and hence oxygen and nutrients to the heart muscle and can lead to heart attack. Bacterial and viral infections can also damage the heart muscle, as can diabetes, an ove...

  ISCHEMIC HEART DISEASE Myocardial ischemia, meaning a lack of oxygen to the myocardium, is the result of an imbalance between the demand of the myocardium for oxygen and the amount supplied. There are several possible causes of this condition. Firstly, the blood flow through the coronary arteries may be reduced because of mechanical blockages, such as atheroma (plaque), thrombosis (a clot), spasm, ostial stenosis, arteritis or any sort of blockage (an embolism) due to, for example, tumor cells or an air bubble. Secondly, a decreased flow of oxygenated blood to the myocardium because of anemia ,  hypotension  (low blood pressure) or carbon monoxide poisoning. Thirdly, an increased demand for oxygen caused by exercise or myocardialhypertrophy that requires an increase in cardiac output. Note that in the last two scenarios, the coronary arteries may be healthy. The commonest cause of ischemic heart disease is coronary atheroma, which obstructs the flow of blood through the...

  CARDIOMYOPATHIES Cardiomyopathy is a progressive disorder that impairs the function of the ventricular muscle walls. It may come about as a result of a number of diseases or may have no identifiable cause.   DILATED CONGESTIVE CARDIOMYOPATHY Dilated congestive cardiomyopathy is not a single condition but a group of heart disorders in which the ventricles have enlarged but are still not able to pump enough blood to meet the needs of the body. Heart failure may result. The commonest cause of the defect in developed countries is widespread coronary artery disease, which leads to an inadequate blood supply to the heart muscle. This causes damage and the undamaged muscle then stretches in compensation. If this is inadequate to meet body needs, then dilated congestive cardiomyopathy develops. Its symptoms are shortness of breath on exertion and a rapid onset of tiredness due to the weakening of the heart’s pumping action. The heart rate speeds up so blood pressure is normal or low...