According to the American Heart Association, more than 73.6 million Americans over the age of 20 have high blood pressure (hypertension)--about one in every three adults. When most of us refer to high blood pressure, we're actually referring to a specific type, called primary or essential hypertension. Essential hypertension affects between 90 to 95 percent of those diagnosed with high blood pressure and has no known cure. There is also another kind--secondary hypertension--that accounts for a much smaller segment of the population and is usually associated with other diseases or complications like renal disease, stress, sleep apnea, pheochromocytoma or aortic coarction.
The Natural Progression
While most experts agree that there is no known cause of primary hypertension, they do agree on a natural progression of events that occurs leading up to its diagnosis: early elevations in blood volume and cardiac output that result in increased vascular resistance.
Blood Volume
Blood volume is affected by ingesting and excreting water and sodium through the kidneys. The key mechanism for this is the renin-angiotensin-aldosterone system that leads to sodium retention and water loss. One popular drug used for lowering blood pressure is an ACE inhibitor (short for angiotensin converting enzyme inhibitor) that produces sodium and water loss, reducing the blood volume. Another popular drug called spironolactone blocks aldosterone receptors to increase the loss of sodium and water loss, resulting in the loss of blood volume.
Cardiac Output
As complex as it seems, the heart is a simple pump. When the left ventricle squeezes blood out of the heart, the result is the cardiac output. Each time the heart beats, it ejects a quantity of blood called the stroke volume. To calculate the cardiac output of the heart, multiply the stroke volume by the heart rate: CO = SV x HR
Systemic Vascular Resistance
In addition to the physical effects of the blood volume and cardiac output, many feel that early increases in the two initiate systemic vascular resistance, which is the total resistance of the blood flow by all of the circulation system in the body, excluding the pulmonary vasculature--often called the total peripheral resistance, or TPR. Certain conditions in the body can cause increases in vascular resistance (called vascoconstriction) and can also be affected by the diameter of the blood vessels and the viscosity (or thickness) of the blood. Systemic vascular resistance can be calculated by multiplying the mean arterial pressure, minus the central venous pressure, divided by the cardiac output: SVR = (MAP - CVP) / CO
Vascular Tone and Sympathetic Activity
Two other major factors that affect primary hypertension are vascular tone and sympathetic activity. Vascular tone refers to the amount of constriction of the blood vessels, relative to its normal, dilated state. Sympathetic activity constricts arteries and arterioles and increases the resistance against blood flow in the arteries. An example of sympathetic activity in the heart is when the left ventricle squeezes blood out of the heart.
Treatment of Primary Hypertension
There are a number of inherent mechanisms that contribute to primary hypertension--none of them curable. However, through the use of of antihypertensive drugs like diuretics, vasodilators, cardioinhibitory drugs and centrally-acting sympatholytics, the underlying effects of the disease can be safely treated with a minimum of side effects. It also helps to lose weight, reduce your sodium intake, stop cigarette smoking and get exercise if you're at risk for hypertension.
