Ischemia

Ischemia is one of the most studied medical conditions. It is associated with various infectious and non-infectious medical conditions. The term ischemia is derived from the Greek word Icheim (restrain) referring to the lack of blood supply to a respective tissue. The occurrence of ischemia in the body triggers many biochemical abnormalities in the body therefore creating insufficiency of metabolites to the tissue.

Studies indicate that almost fifty percent of deaths around the world happen because of ischemia. The most prevalent forms of ischemia are associated with the cardiovascular system and brain. Ischemia progresses itself in to metabolic disorders such as hypoxia. Hypoxia is a fatal condition in which there is oxygen deficiency to the cells causing cellular damage and death. The predisposing factors for hypoxia may include blockage of arteries in a respective circulatory region preventing the blood supply. Ischemia gradually results in tissue necrosis and damage to blood vessels restricting the flow of blood into the tissues.

Biochemical changes such as accumulation of metabolized waste products occur as a result of ischemia in relation to conditions such as embolism, atherosclerosis, thrombosis and compression. This metabolic waste accumulation eventually leads to increase in toxin levels in the body. Cell death is one of the main outcomes of ischemia. The complications of ischemia range from reversible forms to irreversible forms depending on the type of organ damage.

Physiological studies indicate the significance of parenchyma cells in the occurrence of ischemia. Parenchyma cells located in each organ have a specific threshold. In the cardiac muscle, the threshold levels are 20-30 min whereas in the neural cells it is only 3-4 min. Parenchyma cells are more prone to damage than the cells of the stoma and hence the difference in these threshold values indicate the exact etiology of ischemia.

Ischemia occurring in the cardiac region is predominantly because of the hypoxia present in the sub endocardial region. Simultaneously in the central nervous system, it is because of the watershed infarcts. Hence the effect of ischemia is directly proportional to the micro vascular anatomy of the organ systems. Another form of ischemia is silent ischemia. It is an asymptomatic condition often found in people who are old, women and have a history of diabetes. Silent ischemia does not present any visible clinical signs of illness, however through diagnostic examinations such as electrocardiogram and stress testing; the insufficient blood supply to the cardiac muscle can be identified.

Mesenteric Ischemia

Acute mesenteric ischemia is caused due to inadequate blood flow the the mesentery artery and results in tissue hypoxemia. Symptoms such as acute abdominal pain, nausea, diarrhea and vomiting are noticed. Some feel acute pain on eating - 'abdominal angina'. The narrowed mesenteric artery is usually a result of chronic atherosclerosis. Anticoagulation and vasodilation are resorted to. Often surgery is done to treat mesenteric ischemia due to embolus or thrombosis.

Treatment

Ischemia is treated by administering blood thinning drugs such as aspirin. This is done to prevent the clot formation in the blood vessels. Temporary opening of the arterial walls is done by using nitro-glycerine medication. Since ischemia is the precursor for the occurrence of tachycardia, beta blockers are used to relax the heart muscle. Other medications include calcium channel blockers and cholesterol lowering drugs.

Cardiac Arrhythmia

Cardiac arrhythmia or Cardiac dysrhythmia refers to a disturbance of the heart rhythm. When the regular heart rhythm is disturbed, it can lead to symptoms that can range from mild to life-threatening. The heart may have slower beats or there may be a blockage of the electrical pathway of the heart. One of the most common form of cardiac arrhythmia is atrial fibrillation. This occurs in older persons when the upper atrial chambers of the heart do not pump correctly. This can lead to blood clots. Heart failure or electrolyte imbalance can bring on cardiac arrhythmia.

Tachycardia or Tachydysrhythmia is a condition where there is rapid heartbeat due to inefficiency in the blood circulation. Tachycardia is a condition where the heart rhythm is more than 100 beats/minute. This can happen due to stress, hyperthyroidism or alcohol. On the other hand, Bradycardia or bradyarrhythmia is a condition where the heart rhythm is less than 60 beats/min. A ventricular arrhythmia can be life-threatening. This happens when there is ventricular fibrillation. It is essential to treat this condition and restore the rhythm within minutes to prevent heart damage and death. Allergic reactions can trigger arrhythmia.

Some persons suffering from cardiac arrhythmia notice symptoms such as dizziness, fainting and lightheadedness. There may be a fluttering or pounding sensation in the chest. Anti arrhythmic agents such as amiodarone and sotalol are prescribed to maintain the normal rhythm of the heart. Amiodarone is effective atrial flutter and to establish heart's normal rhythm.

In cases of atrial fibrillation, Warfarin is used to prevent blood clots. Medication for cardiac dysrhythmia includes beta blockers such as metoprolol and atenolol to reduce the heart rhythm. An electronic cardiac pacemaker may be implanted to regulate the heart beat.

Adenosine Stress Test

Coronary artery disease is diagnosed by many stress tests such as echocardiography and treadmill. They determine cardiac activity with details of blood circulation and blood pressure. The stress tests are measured based on activity of the heart when subjected to exertion. The coronary arteries are examined for accumulation of plaque through triglyceride estimation and various other tests. In many scenarios the results of stress test are correlated with the laboratory analysis to identify the underlying conditions such as atherosclerosis.

Stress examinations of the heart also help in the detection of ischemia and arrhythmia. The adenosine stress test is done to identify underlying coronary artery disease. Adenosine helps in inducing the vasodilation of the coronary artery directly through the activation of the A2A receptor. The myocardial blood volume increases to a greater extent through this activity. The stenotic coronary arteries display an attenuated hyperemic response in the myocardial region of the heart. The radio tracer for the identification of coronary artery disease undergoes heterogeneous distribution. Adenosine is an endogenous nucleoside predominantly produced in the arterial vascular region. Its action is mediated through the G protein coupled adenosine receptors.

Methodology of adenosine infusion

The adenosine infusion is administered at a rate of 140 mcg/kg/min. The echocardiogram is taken by attaching twelve leads. The values of ECG are taken every minute. Along with these procedures, the blood pressure is consistently monitored for stability during the entire procedure of adenosine stress testing.

A2A receptor protein plays a significant role in the regulation of inflammatory response and immune response. A2A agonists act as effective catalysts in the identification of various diseases such as myocardial infarction and infectious diseases. These receptors help in the reduction of tissue damage.

Patient information and risk factors

The patient is advised not to consume any solid food. Water and juices are allowed prior to the procedure. The nuclear technologist injects the cardiolite and adenosine into the blood stream through an intravenous line. The cardiolite is administered to capture the cardiac images.

Water is given to patients before the start of the imaging procedure through the induced cardiolite. Adenosine is introduced through the IV line after acquiring the resting images. Anticipated side effects of adenosine are generally nausea, angina, shortness of breath and flushing. Most of these side effects are monitored by the technicians. The side effects are usually short lived. A second round of cardiolite is administered after the adenosine to acquire the second set of images. In order to reduce the side effects, patients are advised to walk on the treadmill. The entire procedure lasts for a period of two and a half hours. A comparative account is made of both sets of images at resting stage and also during stress (exercise induced). Patients are advised to take fluids preferably juices after the procedure.

The adverse effects of adenosine stress test include wheezing, hypotension with a systemic hypotension of less than 80 mm of hg, second degree heart block and cyanosis. Pregnant and lactating women are not advised to undergo adenosine stress test because of radio tracers. Incidences of hypersensitivity to adenosine have also been reported. Although adenosine stress is pharmacologically recommended for myocardial stress, one of the predominant side effects during the procedure is myocardial infarction.