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OEM MAXNIBP® Factors That Affect Blood Pressure Blood pressure is dynamic and undergoes minor adjustments constantly. The following systems or variables may affect blood pressure: Autonomic nervous system Provides continuous feedback to the brain, adjusts the cardiac rate and determines the strength of heart contractions required by the body. Endocrine system Contains various hormones that affect blood pressure, particularly norepinephrine (noradrenalin), epinephrine (adrenalin), and cortisol . Norepinephrine generally results in increased vasoconstriction while epinephrine results in the dilation of vessels adjacent to muscle cells. Both epinephrine and norepinephrine increase heart rate. Elasticity of the blood vessels Ability of arteries and capillaries to contract and expand as needed affects blood pressure. When healthy, vessel elasticity is high and blood pressure is not affected. The oscillometric blood pressure reading of elderly patients or other patients whose arterial elasticity is compromised may differ from an intra-arterial reading due to the stiffness of the arterial wall. Cardiac output and blood volume Amount and frequency of blood pumped from the heart per minute, as well as the amount of blood in the circulatory system, play important roles in measuring blood pressure. Position of body Standing tends to elevate readings. Raising the arm also affects readings. Medications Some medications can alter blood pressure. BP Cuff location Expect to see a difference between measurements taken on the right and the left arm, and at other locations at which blood pressure may be taken. BP Cuff sizing A cuff that is not the appropriate size for a patient can result in inaccurate readings. This is especially true of a cuff that is too narrow – readings will be falsely high. BP Cuff positioning A partially encircling bladder must be properly centered over the brachial artery to ensure proper artery compression. Quality of equipment Manual or automatic equipment that is not properly calibrated may result in inaccurate readings. Operator subjectivity The clinician may have a preconceived notion about what the blood pressure will be “I know that systolic is around 130mmHg, so I don’t start listening until I am close”. Difficulty hearing An auscultatory device relies on the clinician’s ability to hear arterial sounds. If there is background noise or the clinician’s hearing acuity is impaired, he will not be able to correctly sense the systolic or diastolic points. Even number bias Clinicians will tend to report only even numbers when using some mercury columns or gauges. Viewing angle The viewing angle of a mercury column or aneroid gauge will affect what number is read. It must be viewed straight on. Time of day Blood pressure is usually lower in the morning and when when sleeping. Eating/Drinking Certain foods tend to increase blood pressure. Weight Blood pressure can increase with weight gain. The body is asking the heart to do more work. Age Children tend to have lower readings, with systolic pressures typically under 100mmHg. Emotional status Stress may elevate readings, while relaxation may lower them. References: 1. The Role of the Endocrine System in Blood Pressure. [Online] Sited 5-18-10. http://www.severehypertension.net/hbp/more/the-role-of-the-endocrine-system-in-blood-pressure/ 2. Geddes LA, ME, PhD, Handbook of Blood Pressure Measurement, Humana Press, Clifton NJ, 1991. 3. Forster FK and Turney D, Journal of Biomechanical Engineering, Vol. 108, pp 359-364, Nov 1986. For more information: Toll Free: 800.227.4414 Local: 203.488.6056 Ask for OEM Product Manager CAS Medical Systems Inc. 44 E Industrial Rd, Branford, CT 06405 USA www. CASMED.com MAXNIBP Factors Affect BP Rev 0.2 5102010