Description of Pathology
Aortic stenosis is the narrowing of the aortic valve within the left ventricle of the heart. The aortic valve is virtually the one-way “door” between the heart’s left ventricle and the rest of the body. Aortic stenosis can be compared to that door not being able to open or close all the way. It is among the most common forms of heart diseases in which the valves are affected. (Manning, 2013) Patients with symptomatic aortic stenosis have less than a three-year survival rate if they do not seek treatment. (Spaccarotella, Mongiardo, & Indolfi, 2011) With a survival rate this low, aortic stenosis has the potential to be highly fatal if left untreated.
Body Systems Involved
A valvular heart disease means that one or more valves of the heart are affected. Aortic stenosis targets only one valve: the aortic valve of the left ventricle. The entire cardiovascular system is affected because of this. Aortic stenosis affects the muscular abilities of the heart, the pressure in the blood vessels, and the amount of blood leaving the heart. The rest of the body can also face repercussions due to a lack of adequate blood flow to meet the demands of other organ systems.
Normal Anatomy of the Cardiovascular System
The cardiovascular system consists of blood, the heart, and blood vessels. In an easy to understand analogy, the cardiovascular system is like the cooling system of a car. The car circulates coolant, while the cardiovascular system circulates blood. Both systems are driven by a pump (water pump versus heart). Finally, the fluid is carried in tubing which would be radiator hoses in the car and blood vessels in the cardiovascular system. (Martini & Bartholomew, 2017) Aortic stenosis focuses mainly on the heart portion of the cardiovascular system. The heart is located within the thoracic cavity, deep to the sternum and between the two lungs. There are three distinct layers of the heart: the epicardium (the outermost layer), the myocardium (the muscle layer), and the endocardium (the innermost layer). The heart consists of four inner chambers: the right atrium, right ventricle, left atrium, and left ventricle. The left ventricle is unique because there is a thicker layer of myocardium in this region. This allows for enough force to be exerted to expel blood from the heart to the rest of the body. The aortic valve is between the left ventricle and the aorta, which is the major artery closes to the heart that allows blood to flow to the rest of the body. Normally, the aortic valve has three cusps, or flaps, which open to allow blood to expel from the heart and close to prevent blood from pouring back into the heart.
Normal Physiology of the Cardiovascular System: The Heart
Deoxygenated blood travels through the superior and inferior vena cava and enters the heart in the right atrium. The tricuspid valve separates the right atrium from the right ventricle. Once this valve opens, blood enters the right atrium. Unlike the left ventricle, the myocardium of the right ventricle is thinner since the blood only needs to travel a short distance to the pulmonary circuit. Once the right atrium contracts, blood is pumped through the pulmonary valve, which is very similar to the aortic valve. Blood then travels through the pulmonary circuit and returns to the heart from the left and right pulmonary veins and enters the left atrium. While the blood was in the pulmonary circuit, it became oxygenated by the lungs. Separating the left atrium from the left ventricle is the bicuspid valve. Blood now enters the left ventricle and prepares to begin its journey to the rest of the body. First, the blood must be pumped from the left ventricle through the aortic valve and to the aorta. The aortic valve, or aortic semilunar valve, has three symmetrical cusps that support each other, which prevents the backflow of blood into the left ventricle. (Martini & Bartholomew, 2017)
Mechanism of Pathophysiology
Aortic stenosis develops over an extensive period, sometimes decades. There are a few main stages in the development of aortic stenosis. The first stage is only inflammation of the valve. After inflammation, a period of fibrosis and thickening of the valve occurs. (Czarny & Resar, 2014) Eventually, the valve becomes completely calcified. Each stage causes either a slight or significant obstruction to the normal opening and closing procedures of the valve. When this obstruction occurs, there are three main consequences: angina, syncope, and heart failure. Angina is caused by a disturbance in myocardial oxygen supply and demand. Decreased myocardial oxygen supply is a result of two things: reduced mean arterial pressure (MAP) and decreased coronary blood flow. MAP declines due to the obstruction of left ventricular outflow, resulting in decreased coronary blood flow. This obstruction also lengthens the systolic ejection period, which causes a decrease the time that the heart spends in diastole. Coronary perfusion, the passage of blood through the arteries of the heart, happens while the heart is in diastole. Reducing this period decreases coronary blood flow, leading to increased myocardial oxygen demand and decreased myocardial oxygen supply, or ischemia. (Czarny & Resar, 2014) Ischemia of the heart results in angina, which is characterized by constricting pain in the chest. Syncope is a direct repercussion of the heart not being able to meet the demands of the body. In aortic stenosis, cardiac output is limited by valvular obstruction. This limitation can cause a lack of adequate blood flow to the brain. When the brain is hypoxic, it is unable to function in the way it needs to, leading to a temporary, or sometimes permanent, loss of consciousness. Heart failure is more common in the later stages of aortic stenosis. The myocardium of the left ventricle is already thick to compensate for the pressure required for blood to reach the entire body. With aortic stenosis, the aortic valve is unable to open all the way, therefore requiring the left ventricle to exert even more force to overcome this obstruction. Muscles normally grow or shrink due to repeated forces. This is how bodybuilders “build” their muscles from repeated weight lifting exercises. The heart is not exempt from this phenomenon. Over time, the left ventricle muscles grow even larger to compensate for the added obstruction of the aortic valve. The coronary blood vessels do not grow because of this added stress. Therefore, the same blood vessels still supply the same amount of blood to a larger muscle. The result is a muscle that is threatened with cell death due to inadequate blood supply. Heart failures are usually left-sided when associated with aortic stenosis. (Czarny & Resar, 2014)
Prevention
The direct mechanisms that lead to the development of aortic stenosis remain poorly understood even with today’s advanced medicine. The calcification of the aortic valves usually takes decades to occur, indicating that age is an important factor in the development of aortic stenosis. There have been recent studies showing that there is an association between this calcification and usual risk factors for cardiovascular disease. Age, male gender, smoking, and hypertension should be noted as important risk factors. (Czarny & Resar, 2014) Rheumatic fever has also been associated with inflammation of heart valves. Rheumatic fever is a rare complication after infection from the streptococcus bacteria. Normally, the body would make antibodies to the streptococcus bacteria, however, in some people the antibodies may attack cells of the body itself. If the antibodies end up targeting the heart valves, inflammation occurs. (Tidy, 2015) An effective prevention strategy would be to consume all prescribed antibiotics when dealing with a streptococcus infection. This is one of the reasons why doctors persistently advise patients to not stop taking their medication when they feel better. Other causes of aortic stenosis tend to be genetic deformities and inborn conditions affecting the heart. (Tidy, 2015) When dealing with genetic conditions, prevention is something that is hard to pinpoint. Once a genetic deformity is confirmed, the focus tends to switch to management and treatment.
Treatment
There are several different approaches to treating aortic stenosis. In some cases, where the obstruction is mild and the patient is asymptomatic, there is usually no need for treatment. Medications such as ACE inhibitors or diuretics may be prescribed if the obstruction begins to increase and symptoms are present. (Tidy, 2015) ACE inhibitors, or angiotensin-converting enzymes, work by making the heart’s job easier by reducing the amount of work it must do. They also ease symptoms of heart failure. Diuretics are commonly referred to as “water pills.” They are an effective form of medication for the treatment of heart failure induced by aortic stenosis because they trigger increased urine output from the kidneys. Thus, excess blood and fluid that may have built up due to heart failure are eliminated. In most symptomatic cases, medication is usually skipped and surgical procedures are the go-to for treating aortic stenosis. The surgical procedure performed is usually one of the following: valvotomy, valve replacement, or balloon valvuloplasty. (Tidy, 2015) Valvotomies widen the valve and require open heart surgery. Valve replacements also require open heart surgery and involve either a mechanical or tissue replacement valve. Finally, a valvuloplasty involves stretching the valve with a balloon inserted into valve and inflating it with a catheder. This is achieved without performing open heart surgery. This procedure is typically done for patients that are not good candidates for open heart surgery. The results from a valvuloplasty are not as long-lasting as results from a valvotomy or a valve replacement surgery.