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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
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
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.
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
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.
Certain foods tend to increase blood pressure.
Blood pressure can increase with weight gain. The body is asking the heart to do more work.
Children tend to have lower readings, with systolic pressures typically under 100mmHg.
Emotional status
Stress may elevate readings, while relaxation may lower them.
1. The Role of the Endocrine System in Blood Pressure. [Online] Sited 5-18-10.
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.
MAXNIBP Factors Affect BP Rev 0.2 5102010