It’s a question I get frequently as a nurse in an eating disorder Residential treatment program, almost unanimously among those new to treatment: “Why do I get so dizzy when I stand up.”

No doubt, it’s a frightening sensation: feeling as though you are going to pass out and fall over after standing up from a sitting position. It’s doubly unnerving when you don’t know why it’s happening.

The condition is called orthostatic hypotension (OH) and it’s the reason for the dizziness and lightheadedness experienced when one rises from a lying to sitting or sitting to standing position. More specifically, orthostatic hypotension is a drop of more than 20 mm Hg (millimeters of mercury, a unit of pressure) in systolic blood pressure (the pressure in your arteries when your heart contracts, also known as the “top” number) or more than 10 mm Hg in diastolic blood pressure (when the heart relaxes between beats or the “bottom” number).

OH is a common problem in eating disorder treatment and is often caused by acute or chronic malnutrition/dehydration. Dehydration is an overall lack of water in the body that results when a person loses more fluids than they take in and is a common occurrence in those suffering from Anorexia Nervosa (AN) or Bulimia Nervosa (BN).

There are times when dehydration can happen quickly (e.g. cases of excessive sweating due to intense exercise/extremely hot weather or in instances of excessive vomiting of diarrhea). Dehydration can also be a recurring problem (as is often the case with AN or BN) when there is chronic inadequate intake of fluids. Thus, those suffering from AN or BN who are struggling to drink enough fluids are at risk for dehydration, which heightens the risk for OH.

Before we talk more about what exactly OH is, why it happens and why it can be so dangerous, let’s discuss what blood pressure is, what the top (systolic) and bottom (diastolic) numbers mean and what are considered “good” and “bad” numbers when it comes to blood pressure.

First, the heart is a muscle and it acts like a pump, contracting to push blood to all parts of the body – also known as perfusion – to ensure they function properly. When the heart contracts (think of a squeezed fist) it pumps blood through the blood vessels to the rest of the body. The pressure in your blood vessels when the heart contracts is called the systolic blood pressure (SBP). For the most part in impatient or ED treatment, nurses and doctors prefer an SBP of at least 90 (ideal is 115 mm Hg).

When the heart relaxes between beats (unsqueeze your fist now), the pressure in your arteries is the diastolic blood pressure (DBP), which should be at least 60 mm Hg – ideal is 75 mm Hg – to adequately perfuse all parts of the body.

So now that we know what blood pressure is and what the specific numbers mean, we can better explain orthostatic hypotension. When a person is lying or sitting, the blood in their body pools in the blood vessels of the legs and feet, meaning less blood is returned to the heart. Upon rising from a lying or sitting position, your heart should beat slightly faster and the blood vessels should constrict (tighten) to keep blood pressure high enough to ensure blood is adequately being pumped to all parts of the body.

When the blood vessels do not constrict quickly enough – often because of dehydration (low fluid volume) – the BP can drop suddenly, meaning certain parts of the body are not getting the blood flow they need. This is orthostatic hypotension and symptoms can include weakness, fatigue, blurred vision, sweating, nausea, pale skin, clammy skin and feeling “shaky.” And if the brain is not getting enough blood, symptom can include dizziness, lightheadedness or fainting.

Orthostatic hypotension (OH) can have a number of causes (medications, alcohol use, postpartum complications, neurological and connective tissue disorders, diabetes) but the most common reason for OH in Residential and Inpatient eating disorder treatment is dehydration. As discussed before, dehydration occurs when the body has an inadequate amount of water. If there is not enough fluid volume in your blood vessels, there will not be enough pressure created to force the blood to all parts of the body (notably the brain), causing the some of the symptoms listed in the last paragraph. The heart will beat faster to compensate for the lack of fluid volume in the body in an attempt to raise the blood pressure but if the dehydration is too severe, the heart will be unable to do so.

To visualize it more easily, think of a water pipe. If there is a small amount of water in the pipe with no force applied to pump it through, the water will just sit still. But if a pump applies force, the water will move through the pipe. And the more water there is and the more force applied by the pump, the faster the water will move the pipe. In this analogy, the pump is the heart and the water is blood. This is why it’s so important to be sufficiently hydrated because adequate blood pressure requires adequate water and fluid intake.

As for diagnosis of OH, it’s done by taking your blood pressure both sitting and standing (or lying, sitting and standing) and seeing if there is more of a 20 mm Hg drop in SBP or a 10 mm Hg drop in DBP. In addition to changes in blood pressure, your heart rate will increase when changing positions. If your heart rate increase by more than 20 beats per minute from lying to sitting or sitting to standing, that is known as being orthostatic by pulse.

The treatment for OH is to administer fluids (usually water or Gatorade), whether it is orally, via nasogastric tube (in inpatient settings) or, in severe cases of dehydration, intravenously. Mild OH is common and can generally be treated easily and safely with oral fluids. But if you – or someone close to you – frequently experiences any of the symptoms listed in this blog or feel you may be suffering from an eating disorder, please seek help.

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Sean Jacquet joined Walden in February 2016 as a Registered Nurse in Walden’s Eating Disorder Residential Program. He received his Bachelor of Arts in Journalism from UMASS Amherst (2003) and started his career with GateHouse Media as a sports staff writer for MetroWest Daily News (Framingham, MA) and Daily News Tribune (Waltham, MA) before going back to school and receiving his Bachelor of Science in Nursing from Massachusetts College of Pharmacy and Health Sciences in Worcester, MA (2012). Prior to joining Walden, he was a staff nurse and a Nursing Supervisor at Westwood Lodge. He can be reached at sjacquet@waldenbehavioralcare.com.