8.11. Applying Anatomy
Myocardial Infarction
The medical event that is commonly referred to as a heart attack. Atherosclerosis is well-developed in the coronary arteries and the fatty plaques bulge into the lumen of the arteries. A fatty plaque then ruptures due to the turbulent blood flow. An infarct (blood clot) develops at the site of the ruptured plaque. The infarct can block a large proportion of the lumen of the artery, which means the myocardium does not receive blood (and thus oxygen and glucose) for respiration. Cardiac myocytes quickly die. These pathological processes are demonstrated in the accompanying diagram.
The location of the infarct will determine the effects on the heart. For example, a block in the right coronary artery, proximal to the atrial branches that supply the SA node, could cause serious electrical conduction problems. A thrombus in the anterior interventricular artery, also called the left anterior descending (LAD) artery, will affect contraction of the anterior ventricular wall and the stability of the interventricular septum. A proximal LAD thrombus will result in more diseased myocardium than a distal LAD thrombus.
Other Clinical Connections
Angina pectoris
The main symptom of angina is chest pain during exertion, but not rest. During exercise the heart rate and force of contraction increases. However, in the patient with coronary artery atherosclerosis (narrowed arteries), an insufficient volume of blood reaches the respiring myocardium and the death of some cardiac myocytes occurs, leading to pain. After rest, the pain reduces and disappears.
Transposition of the great arteries
This is a congenital defect where the pulmonary trunk and aorta are connected to the wrong ventricles of the heart (i.e. the aorta leaves from the right ventricle and the pulmonary trunk leaves from the left ventricle). Surgery is performed to correct the situation and thereby prevent excess strain on the heart.
Cardiac tamponade
This can present in some patients secondary to:
A stab wound to the heart; blood leaks from the heart's chambers into the pericardial cavity in this 'traumatic' tamponade.
Pericarditis, which can be infective or associated with autoimmune conditions. Pus-contained fluid from the serous pericardial layers builds up in the pericardial cavity.
The build-up of blood limits the expansion of the ventricles, which do not fill fully. This produces a clinical triad caused Beck's triad, which features:
Muffled heart sounds (due to the blood/pericardial fluid distorting the ausuclated heart valves closing.
Distended neck veins (i.e. jugular veins, due to venous back pressure due to low cardiac output).
Hypotension (due to blood in the pericardium preventing full ventricular contraction and relaxation).