Blood Pressure Monitor
5.5 Motor and Valve Control
After the pressure reaches the maximal value of 180 mm Hg the program starts to stabilize the system. It establishes a shorter
duty cycle for motor control and a longer duty cycle for valve control and starts to measure the blood pressure. The flowchart
below shows how to implement the motor control.
Figure 10. Motor control flowchart
The flowchart shows both pressures, the systolic which represents the maximum pressure exerted (cuff inflates) when the heart
contracts and diastolic which represents the pressure in the arteries when the heart is at rest (cuff deflates).
5.6 Obtaining Heart Beats at BloodPressure.c
Blood pressure is one of the principal vital signs. Blood pressure is a force on the walls of blood vessels exerted by the blood
circulating. During each heartbeat, blood pressure varies between systolic pressure and diastolic pressure. For a better diagnose
and control of hypertension it is important to obtain the heart rate.
5.6.1 Obtaining the Heart Rate
The pressure sensor has one output split in two. The two outputs are taken from the pressure sensor to remove noise to get a
better measurement. One output is filtered and obtains the specific frequency of the heart. The other output is for sensing the
pressure receiver. When the cuff is attached to a person’s arm and it is deflating you can see slight variations in the overall
pressure on the cuff. This variation in the pressure from the cuff is actually due to the pressure change from blood circulating.
This variation is amplified through a high-pass filter designed at 1 Hz and set to an offset. The heart rate is calculated with the
resulting signal.
5.6.2 Oscillometric Method
This method consists in taking samples from the oscillations caused by the blood flow. While the cuff is deflated from the
systolic level, the blood starts to circulate through the obstructed artery producing vibrations. When the pressure cuff is decreasing,
the oscillations increase up to a maximal amplitude; after decreasing, the blood flow is normal. The pressure in the cuff in the
maximal oscillation is known as mean arterial pressure (MAP). When oscillation starts to increase quickly, this is the systolic
pressure (SBP)(top number). When oscillations decrease quickly, this is the diastolic pressure (DBP). Figure 11 shows the
pressure and the heart rate.
Implementing a Glucometer and Blood Pressure Monitor Medical Devices, Rev. 1, 4/2010
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