A delicate balance between heart rate, blood volume, and arterial diameter maintains normal blood pressure within the human body. In the typical adult, blood pressure is considered to be normal when the systolic pressure is less than 120 mm Hg and the diastolic pressure is less than 80 mm Hg; however, there is significant variation in these numbers depending on the person’s position when blood pressure is measured. It is common for a person’s blood pressure, particularly systolic pressure, to momentarily drop 8—10 mm Hg when standing up from a sitting or lying position. This is due in part to the effects of gravity and blood pooling lower in the body. Under normal conditions, a number of events take place in our bodies to reestablish a normal functioning blood pressure. When a person’s blood pressure drops more than 20 mm Hg upon standing, this is defined as orthostatic hypotension.
Normal Control of Blood Pressure
To maintain a normal or mean arterial blood pressure, the body must create a balance between cardiac output (how much blood the heart is pumping per contraction), total peripheral resistance (how dilated the vessels are), and blood volume (how many liters of blood exist in the body). The primary structure that orchestrates all of this is called the baroreceptor (pressure sensor), which is found in the aortic arch of the vascular system. Under normal conditions, the baroreceptors fire action potentials through the autonomic nervous system to the heart and vessels, regulating heart rate and how dilated or constricted the vessels should be. If blood pressure drops, the baroreceptors will increase the number of action potentials being fired, which results in an increase in heart rate and a constriction of the great vessels of the body, both of which result in an increase in blood pressure. The converse is true if blood pressure becomes too high.
Orthostatic hypotension (OT) is defined as a drop of greater than 20 mm Hg in systolic blood pressure or a drop in diastolic blood pressure of greater than 10 mm Hg over three minutes of standing (https://www.youtube.com/watch?v=QZj0EmFV2to). There are two general categories of OT: (1) initial orthostatic hypotension, which develops within the first five minutes of standing erect, and (2) delayed orthostatic hypotension, which develops later, often 15—45 minutes after standing. There are multiple potential causes for OT. Some predisposing factors for development of OT include dehydration (decreased volume in the vascular system), deconditioning (inability of the heart to increase heart rate quickly enough to respond to a drop in pressure), nutritional factors, and aging. There are also a number of medications that can increase the incidence of OT including (1) tricyclic antidepressants, (2) antihypertensives and diuretics, (3) general vasodilators, and (4) tizanidine (Zanaflex, which is a short-acting muscle relaxant).
In addition to pharmacological agents there are a number of other pathologies/conditions that can lead to OT including (1) autonomic neuropathies, (2) Parkinson’s disease, (3) dementia, (4) dopamine β hydroxylase deficiency, (5) brainstem lesions/injuries, and (6) spinal cord injuries.
Treatment for OT is varied depending on the specific symptoms and the potential causative agent. Often, OT is a secondary symptom of some other disease process and in treating the primary disease process, the OT issue is lessened. However, there are some general behavioral changes that can be made to help individuals with benign OT. These include (1) staying well hydrated so that blood volume remains high, (2) decreasing alcohol consumption (which can cause dehydration), (3) when getting up from bed, coming to a sitting position first, and slowly allowing the body to respond to the changes in pressure, (4) wearing compression stockings, which apply pressure to the legs and prevent blood from pooling in the lower legs, (5) raising the head of the bed when sleeping, and (6) avoiding prolonged standing during the day.
Orthostatic hypotension is a significant issue within the geriatric population and often requires more treatment. Orthostatic hypotension occurs in about 20—25 percent of the elderly, and the incidence increases with the presence of coexisting disease processes. This often results in increased numbers of falls, which could lead to head injuries and fractures, particularly of the shoulder and hip. It is thought that as we age, baroreceptor sensitivity and response times become more delayed, which results in a decreased response in heart rate and peripheral resistance. Significant care must be taken to help older patients develop safe habits to prevent further injury as a result of OT.
Charles A. Ferguson
See also: Baroreceptors
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