Indirect Blood Pressure Measurements and Associated Errors
Appendix
6 - 14
Accutorr Plus™ Operating Instructions
6.4
Indirect Blood Pressure Measurements and
Associated Errors
Place the patient in a supine position to obtain true physiological pressure. If the cuff is not at
the patient’s heart level, the pressure values obtained will not reflect the true physiological
pressure. Instead, the readings will be decreased by 1.86 mmHg for every inch the cuff is
placed above the heart level and increased by 1.86 mmHg for every inch the cuff is placed
below the heart level. This effect is due to hydrostatic pressure.
Blood has weight and it is this weight that influences these blood pressure readings. The
value of the weight of blood depends on where the measurement is taken with respect to the
heart. When the patient is supine, on a flat surface, the arm is near enough to the heart level
so no adjustment of the NIBP readings will be necessary.
6.5
Precautions With Using Automatically Cycled Blood
Pressure Cuffs
Reports have been made of nerve injury occurring during use of automatically cycled blood
pressure cuffs. The authors recommend the following practices when using automatically
cycled blood pressure cuffs:
• Position and support the limb in such a way as to minimize stretching of and weight
exertion on affected nerves.
• Avoid cuff placement that applies pressure on the ulnar nerve. Cuff tubing should not exit
the cuff over the course of the ulnar nerve at the elbow.
• Select a measurement interval that provides adequate venous drainage during cuff
deflation.
• Periodically inspect the limb bearing the cuff in order to detect venostasis.
6.5.1
Cuff Size
Using a narrow cuff gives erroneously high pressure readings. If a standard cuff is applied to
an obese patient or a patient with large biceps, the extra tissue and fat will dissipate the
applied pressure, requiring an additional pressure increase to collapse the artery. On the
other hand, over-wrapping a slender arm gives erroneously low pressure readings. Too much
force per unit area is exerted. This requires less pressure to collapse the artery.
6.5.2
Other Factors
An accurate determination of blood pressure by the Accutorr Plus can be difficult if cardiac
rhythm is very irregular. Irregular cardiac rhythm changes the stroke volume from beat to
beat. This changing stroke volume may increase the time it takes the Accutorr Plus to make a
measurement. The Accutorr Plus system will take up to four successive attempts to obtain a
measurement. If a measurement cannot be made an error code will be displayed.
