The Harvard Medical School Family Health Guide defines normal blood pressure as a systolic pressure of less than 120 and a diastolic pressure of less than 80. Systolic pressure represents the force of the blood on the artery walls as the heart contracts. Diastolic blood pressure represents the force of the blood as the heart rests between beats. Body position sometimes affects blood pressure, making it important to measure blood pressure in several positions for accuracy.
Medical professionals use three common positions when taking a patient’s blood pressure. In a standard medical office, the patient usually sits in a chair with his arm extended. Patients in the hospital usually are lying in bed when a nurse or medical assistant takes their vital signs. Some nurses also measure blood pressure while a patient stands.
The sitting position produces different blood pressure measurements than the lying-down position. The National Guideline Clearinghouse explains that diastolic blood pressure is approximately 5 mm/Hg higher when sitting than when lying down. Reports indicate that systolic pressure is 8 mm/Hg higher in the lying-down position than in the sitting position, but only when the person taking the blood pressure measurement positions the patient’s arm so that it is at the same level as the right atrium of the heart. Back support and leg position also play a role in blood pressure measurements. Crossing the legs increases systolic pressure by as much as 8 mm Hg.
MayoClinic.com defines orthostatic hypotension as a type of low blood pressure that occurs when you stand up after lying down or sitting. This results in dizziness and lightheadedness. In some cases, orthostatic hypotension causes fainting. Doctors refer to this condition as postural hypotension. Some people have a higher risk of developing this condition. Postural hypotension usually occurs in older adults because of the decreased ability to regulate blood pressure. Some heart conditions increase the risk of low blood pressure. Other risk factors for orthostatic hypotension include crossing the legs, heat exposure and the use of diuretics, heart medications, tricyclic antidepressants, Viagra and high blood pressure medications.
Arm position also plays a role in accurate blood pressure measurements. If the upper arm is below the right atrium of the heart, it results in elevated readings. If the patient raises her arm above heart level, it results in low readings. Arm position may alter a blood pressure measurement as much as 10 mm/Hg, according to the National Guideline Clearinghouse.
Accurate blood pressure measurements help medical care providers monitor the progression of some conditions, so it is important to understand the effects of body position on blood pressure. If you have kidney disease, heart disease, hypertension (high blood pressure) or any other condition that requires regular blood pressure measurement, discuss body position with your doctor to learn the proper technique for taking accurate readings.