Upon standing, approximately 500mL of blood becomes trapped in the distensible veins that are located below the heart. Additionally, plasma volume is lost to interstitial fluid, decreasing the amount of blood that flows back to the heart (venous return). Normally, these changes are detected by receptors in the blood vessels (stretch and barorecptors) and this results in a decrease in parasympathetic (including vagal) and increase in sympathetic tone. The purpose of this test is to monitor the patient’s heart rate, blood pressure, and plasma concentrations of substances (e.g. norepinephrine and epinephrine) that regulate heart rate and blood pressure when the patient is both lying down and standing. The results of a posture study allow the assessment of the body’s hemodynamic response to standing.
Prior to the test, the patient will have an IV catheter inserted into his/her arm. This will be used to draw blood during the test. The patient will lie down and remain still while baseline heart rate, blood pressure, and serum concentrations (obtained by blood draw from the IV line) are measured. Following these measurements, the patient will stand for up to 30 minutes while heart rate, blood pressure, and plasma concentrations are measured.