Myocardial Ischemia (Low Oxygen Supply To Heart Muscle)

Myocardial Ischemia Definition

Myocardial ischemia is injury to the heart muscle as a result of diminished or interrupted blood supply to the heart wall. The condition needs to be differentiated from myocardial infarction (heart attack). In myocardial ischemia the heart muscle undergoes injury due to an insufficient oxygen supply but the tissue does not die. This tissue injury is largely reversible in most cases but over a long period of time it can permanently damage heart function. In a myocardial infarction there is sudden death of a portion of the heart muscle tissue which can lead to a heart failure in severe cases and death. Myocardial ischemia causes a typical cardiac chest pain known as angina pectoris.

Myocardial Ischemia Incidence

Globally, heart disease is on the rise and myocardial ischemia has become more prevalent. Early diagnosis and better treatment measures and management has drastically reduced the mortality rate. Close to 10 million Americans every year experience myocardial ischemia to the extent that it presents with angina. The incidence of myocardial ischemia, however, is difficult to estimate in patients who are asymptomatic and do not undergo routine screening. With regards to angina, it is slightly more common among women than men but myocardial infarction (heart attacks) is more frequently seen in men.

Myocardial Ischemia Pathophysiology

The heart is in continuous action throughout life, receiving low-oxygen blood from the body, sending it to the lungs for re-oxygenation and then pumping it throughout the body. There are three layers of the heart wall – the innermost endocardium which includes the heart valves, the thick middle muscular layer known as the myocardium and the outermost epicardium.

The myocardium needs an adequate supply of blood laden with oxygen and nutrients to continue functioning optimally and this is known as myocardial demand. It receives its blood through the coronary arteries which originate from the aorta. The coronary arteries can widen or narrow as needed to control the blood flow for specific situation and certain times.

Coronary artery disease (CAD) is one of the common causes of myocardial ischemia and a myocardial infarction. Here the arteries narrow over time due to the build up of fatty plaques (atheromas) in the artery wall. This process is known as atherosclerosis. The narrowing may be largely asymptomatic in the early stages as the blood flow though the coronary arteries is sufficient for myocardial demand.

However, during times of increased physical activity, myocardial ischemia may arise as blood flow may not be sufficient for myocardial demand. This presents with angina pectoris in most, but not all, cases. Should the fatty plaques rupture and a blood clot form at the site, the coronary artery may be come totally occluded. Ischemia progresses to an infarct if left unattended. This is known as a heart attack.

Myocardial Ischemia Symptoms

The severity of symptoms are closely related to the degree of ischemia. It is not uncommon for many people to have no symptoms (silent ischemia), to ignore the symptoms when it is very mild or attribute it to conditions like muscular arm pain or tiredness from overexertion.

Chest Pain

The characteristic chest pain in myocardial ischemia, known as angina pectoris, is a crushing pain or tightness in the center of the chest, usually felt behind the sternum. However, it is not always typical in that sometimes it can be a burning pain, similar to heartburn, or just a feeling of pressure in the chest. The pain often radiates down to the left arm, shoulder, and to the neck and/or jaw. Sometimes the pain may occur on the right side. Although chest pain is considered to be the cardinal sign of myocardial ischemia, it can sometimes be absent even when other symptoms are present.

Other Symptoms

Many of the symptoms other than chest pain can be vague and non-specific. These symptoms include :

  • Clammy skin and rarely profuse sweating
  • Nausea and vomiting
  • Difficulty breathing marked by shortness of breath
  • Sometimes fatigue and even dizziness

Myocardial ischemia tends to occur during times of increased physical activity. The symptoms are therefore often misinterpreted as being due to overexertion and physical strain.

Myocardial Ischemia Causes

Coronary artery disease is the main cause of myocardial ischemia. Therefore myocardial ischemia, which is also referred to as ischemic heart disease, is considered to be synonymous with coronary artery disease. Certain risk factors have been associated with the condition such as :

  • High blood pressure (hypertension)
  • High blood lipids (hyperlipidemia)
  • Diabetes mellitus
  • Family history
  • Obesity
  • Cigarette smoking
  • Sedentary lifestyle

In most instance the ischemia arises with exertion when the myocardial demand due to physical activity is not met by blood supply. However, there are unstable forms where the person may experience ischemia even when at rest. Less commonly, the muscles in the coronary artery wall may go into spasm and constrict. The blood flow to the heart may be restricted. It maybe associated with pre-existing coronary artery disease or arise on its own suddenly and for no clear reason. This type of angina is known as variant angina or Prinzmetal’s angina.

The condition can also arise with severe illnesses where the blood supply is insufficient, not only to the heart but throughout the body. It may be seen with hypotension (low blood pressure) or when there is insufficient oxygen and nutrients to meet with the metabolic needs of the body. A blood clot can arise at the site of the narrowed coronary artery (thrombus) or even elsewhere in the bloodstream and travel to the coronary artery (emboli). Blood clots can cause a partial or even total occlusion of the coronary artery and usually lead to severe myocardial ischemia which can progress to an infarct (heart attack) in a short period of time.

Myocardial Ischemia Diagnosis

High risk patients should be screened for myocardial ischemia despite the lack of any signs or symptoms in some cases. Other patients who are exhibiting symptoms should immediately undergo the necessary diagnostic investigation even though some symptoms may be related to other, less severe, medical conditions. These tests include :

  • Cardiac enzymes blood tests
  • Electrocardiogram (ECG) at rest or a stress ECG for screening. A Holter monitor may also be advisable when there is no clear indication with an ECG and other tests but myocardial ischemia is suspected.
  • Coronary angioraphy (angiogram)
  • Echocardiogram
  • Gated myocardial perfusion single-photon emission computed tomography (SPECT)

Myocardial Ischemia Treatment

Myocardial ischemia needs to be managed with medication and treated with surgery where necessary. The primary aims of treatment is to restore blood flow to the heart wall and prevent a heart attack. Patients should also undertake lifestyle changes like losing weight, reducing saturated fatty acids in the diet, exercising regularly and quitting cigarette smoking to aid with the prognosis and prevent any complications.

Medication

  • Antiplatelet drugs like aspirin reduce the chance of blood clot formation within the coronary arteries.
  • Nitroglycerin to relax the muscle within the artery walls and widen the coronary artery during an angina attack.
  • Antihypertensives and other anti-angina drugs which reduce the workload on the heart and widen the blood vessels to decrease peripheral resistance.
  • Cholestrol-lowering drugs to reduce the build up of fatty plaques in the wall of the coronary artery.

Surgery

There are two main procedures that may be considered depending on the severity of myocardial ischemia.

  • Angioplasty and stenting where the coronary artery is widen with a balloon-like device at the end of a catheter and the a wire mesh is inserted to keep the artery open.
  • Coronary artery bypass surgery where a graft is inserted to re-route blood past the narrowing in the coronary artery.

References :

www.merckmanuals.com/professional/cardiovascular_disorders/coronary_artery_disease/angina_pectoris.html

www.mayoclinic.com/health/myocardial-ischemia/DS01179

emedicine.medscape.com/article/150215-overview