When you think about, we shouldn’t be able to stand upright. Our ancient ancestors, floating around in the sea, used the water for everything; hydration, nutrition and support. The water buoyed them, there was no need for a spine or other elaborate bony structures. To leave the water and especially to walk upright on two legs, certain adaptations were needed. Skeletal features that evolved over time included a spine, hip bones, thigh bones and the muscles needed to keep the body upright and erect. Circulatory adaptations were needed as well. When we are upright, the heart has to pump blood against gravity to keep blood flowing to the brain. Blood pressure to the brain must be maintained otherwise when we stand up, the blood pressure would drop, the blood flow to the brain would decrease and we would pass out. How does the circulation respond to standing and why don’t we routinely pass out?
The mechanism that evolved to keep us upright is a blood pressure feedback loop that involves the heart, the brain and the blood vessels. On standing, blood pools in the veins in the legs. This causes a transient decrease in blood pressure. This drop in blood pressure is recognized by a sensor in the carotid artery (the carotid artery is the main artery supplying blood to the brain, the sensor is located in the carotid sinus, just outside of the skull). When the carotid sinus senses a lower blood pressure, it fires nerves which activate the sympathetic system. The sympathetic system constricts blood vessels, increases the heart rate and increases the amount of blood pumped by the heart ultimately restoring normal blood pressure to the brain and preventing passing out.
This mechanism is very effective at maintaining blood pressure. However, there are circumstances where the mechanism is overwhelmed causing dizziness or passing out. Instances include prolonged standing in a hot environment (think of a soldier standing for a long time in the hot sun), dehydration, or deconditioning from staying in bed for a long time. As we age, the mechanism isn’t as fast and we can have dizziness on rising quickly from a sitting or lying down position. If the mechanism isn’t working, it leads to passing out on standing, a condition called orthostatic hypotension (from the Greek ortho- straight and statikos- causing to stand; caused by standing erect; hypotension- low blood pressure). Orthostatic hypotension is defined as a drop in systolic blood pressure of 20 mmHg or more within three minutes of standing. It is a significant medical problem whose incidence rises after age 65. It occurs in 6% of elderly people in the community and up to 50% of people in nursing homes. Orthostatic hypotension puts patients at risk for falls, as well as cardiac disease. Orthostatic hypotension occurs in patients with degenerative diseases of the autonomic nervous system (for example, Parkinson’s disease). It also occurs due to an impaired autonomic nervous system (for example, diabetes). Orthostatic hypotension can be diagnosed in the office or at the hospital bedside by checking blood pressure lying down and standing up. The formal way to diagnose it is with a tilt table test (strapping a patient to an upright table and checking blood pressure frequently).
There are multiple ways to treat orthostatic hypotension. In milder forms, patients recognize that they are about to pass out and can sit or lay down to avoid falling. For more significant cases, the first step is to stop medications that can lower the blood pressure or exaggerate orthostatic symptoms. Medications to avoid include diuretics, most blood pressure meds, prostate medications (ex, Flomax or tamsulosin), antiparkinson agents and antidepressants. Other measures include increasing blood volume by keeping hydrated, ingesting salt tablets, or using a medication (fludrocortisone) which promotes salt retention. Pooling of blood in the veins of the lower legs can be decreased by using compression stockings. Lastly, medications that stimulate the sympathetic nervous system (such as midodrine or droxidopa, Northera) can be used.
POTS (postural orthostatic tachycardia syndrome) is the most common form of orthostatic intolerance in young people. It usual affects young, pre-menopausal women with as many as 500,000 cases in the US. It is defined as an increase in the heart rate of 30 beats per minute or more within 10 minutes of standing, but without a decrease in blood pressure. The typical patient is a 30-year-old woman with a heart rate increment of 40 beats per min or more. Cardiovascular deconditioning is common. Therefore, the key to treatment is exercise. The exercise regimen should avoid the upright position (swimming or recumbent bike can be prescribed) until fitness improves. Liberal intake of fluids and salt, compression stockings and sleeping with the head of the bed greater than 30 degrees are other methods. Medications can be used in severe cases and include the same agents used for orthostatic hypotension.
Whether you are young or old, if you have dizziness on rising there are measures you can take to avoid symptoms. If you have more severe symptoms or are passing out on standing, see your doctor to work through how to keep yourself upright.
