Tech Note 1123 Detail

Blood Pressure Sensor Theory of Operation

Affected Products:

PS-2207 PASPORT Blood Pressure Sensor

Problem/Symptom:
Blood Pressure Sensor Theory of Operation

PASCO Solution:

Background

The PS-2207 Blood Pressure Sensor uses an oscillometric method to calculate the systolic and diastolic blood pressure of a subject. This is the most common method for automated blood pressure measurements because the method is non-invasive and simpler to automate than the traditional ascultatory method, which typically requires a carefully trained practitioner to give accurate results.

Both the asculatory and oscillometric methods of blood pressure measurement are accepted by the American Association of Critical Care Nurses; however, the oscillometric method may give slightly different results between different measurement systems of blood pressure sensors due to differences in the algorithms used to determine the systolic and diastolic pressures from the oscillometric envelope.

Theory of Operation

Maximum oscillation in the pressure of a cuff will occur at the mean arterial pressure PMA of the subject because there is maximum mechanical coupling between the cuff pressure and the arterial pressure at that point. Systolic pressure is the maximum arterial pressure during contraction of the left ventricle of the heart. The systolic and diastolic reading could be read directly from the maximum fluctuation in the cuff pressure; however, in practice the maximum oscillation from a heart seldom occurs exactly at the pressure at which the pressure in the cuff exactly matches the mean arterial pressure. Furthermore, the point at which the oscillations first begin to appear can be difficult to discriminate from the noise level; therefore, the systolic and diastolic pressures are in practice always calculated from the oscillometric envelope using algorithms that are designed to match the systolic and diastolic measurements obtained using the traditional ascultatory method:

  1. Apply a cuff to the artery in your arm and inflate so that the pressure is sufficient to cut off the flow of blood to the arm for a short time.
  2. Measure the cuff pressure as the pressure decreases at a controlled leak rate.
  3. High-pass filter the cuff pressure to obtain the oscillations in the cuff pressure as a function of time.
  4. Determine the peak amplitude Amax of the oscillometric envelope.
  5. Determine the cuff pressure corresponding to Amax. This value corresponds to the mean arterial pressure PMA of the subject.
  6. Compute oscillometric amplitudes Asystolic and Adiastolic as a function of PMA. (The exact relationship used is considered proprietary.)
  7. Determine the corresponding low-pass filtered cuff pressures corresponding to Asystolic and Adiastolic. There are the systolic and diastolic pressures.

Troubleshooting

  • Selection of a BP cuff of the appropriate size is necessary for accurate measurement of BP. (The use of a cuff that is too narrow results in an overestimation of BP, and a cuff that is too wide underestimates BP. False high pressure reading may result when the cuff is too small. False low pressure readings can result if the cuff is too large. A cuff with a bladder of an adequate size capable of going around 80% of the arm is recommended.) We have the following cuffs available:
  • Wrap the cuff snugly around upper arm with the mark on the cuff directly placed over the artery.
  • Measure baseline BP in both upper arms. For significant differences in BP, use the arm with the higher pressure.
  • Patient should be seated with back and arms supported, feet on floor, and legs uncrossed, with the upper arm at heart level. Having the arm at a non-standard position will result in a different result due to hydrostatic pressure.
  • Be sure that the patient stays still and quiet during the measurement.
  • Check that the leak rate is between 2 - 5 mmHg/min

Please see the American Association of Critical Care Nurses for more information on accepted measurement technique.

Creation Date: 11/16/2010
Last Modified:
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