compartment. Peripheral resistance is determined by functional and anatomic changes in small arteries and arterioles. The pathophysiology of essential hypertension is an area of research, and until now remains not well understood, but many theories have been proposed to explain this. What is known is that cardiac output is raised early in the disease course, with total peripheral resistance (TPR) normal; over time cardiac output drops to normal levels but TPR is increased. Three theories have been proposed to explain this: An overactive Renin–angiotensin system leads to vasoconstriction and retention of sodium and water. The increase in blood volume leads to hypertension. An overactive sympathetic nervous system, leading to increased stress responses. It is also known that hypertension is highly heritable and polygenic (caused by more than one gene) and a few candidate genes have been postulated in the etiology of this condition. Essential hypertension can lead to impaired white matter of the brain, which is accompanied by specific cognitive impairment. Diagnosis For most patients, health care providers diagnose high blood pressure when blood pressure readings are consistently 140/90 mmHg or above. A blood pressure test can be done in a health care provider's office or clinic. To track blood pressure readings over a period of time, the health care provider may ask the patient to come into the office on different days and at different times. The health care provider also may ask the patient to check readings at home or at other locations that have blood pressure equipment and to keep a written log of results. The health care provider usually takes 2–3 readings at several medical appointments to diagnose high blood pressure. Using the results of the blood pressure test, the health care provider will diagnose prehypertension or high blood pressure if: For an adult, systolic or diastolic readings are consistently higher than 120/80 mmHg. A child's blood pressure numbers are outside average numbers for children of the same age, gender, and height. Once the health care provider determines the severity, he or she can order additional tests to determine if the blood pressure is due to other conditions or medicines or if there is primary high blood pressure. Health care providers can use this information to develop a treatment plan. History Prior to the work of Australian cardiovascular physiologist Paul Korner, in the 1940s, little was known about essential hypertension. See also DASH diet References Hypertension