Medical Applications User Guide (pdf) - Freescale Semiconductor

4.2
Heartbeat Detection
The heartbeat rate is considered one of the
vital patient measurements. The following
procedure is used to obtain this measurement.
While deflating a cuff that is attached to a
person’s arm, you can see slight variations
in the overall cuff pressure (Figure 4-2). This
variation in the cuff’s pressure is due to the
pressure change from blood circulation. This
variation is amplified through a filter designed
at 1 Hz, and set to an offset. This new signal
is the heartbeat signal.
The signal in Figure 4-3 shows variations in the
pressure signal and is a graphical representation
of a patient’s heartbeat over time.
4.3
Systolic and Diastolic
Measurements
Heartbeat detection is a simple oscillometric
method used to determine systolic blood
pressure (SBP) and diastolic blood pressure
(DBP). The simplified measurement is
based on the idea that the amplitude of the
heartbeat signal changes as the cuff is inflated
over the SBP. While the cuff is deflated, the
amplitude of the heartbeat signal grows as
the cuff pressure passes the systolic pressure
of the patient. As the cuff pressure is further
reduced, the pulsations increase in amplitude
until the pulsations reach a maximum pulse
known as the mean arterial pressure (MAP),
and then reduce rapidly until the diastolic
pressure is reached (Figure 4-4).
4.4
Invasive Blood Pressure
Monitors
The most accurate way to measure blood
pressure is to take the measurement directly
from an arterial line. The advantage of this
method is continuous measurement, versus
a discrete measurement in the non-invasive
method.
Freescale has long been a provider of sensors
for the invasive blood pressure monitoring
segment. Figure 4-6 shows different types of
packaging for Freescale pressure sensors.
Home Portable Medical
Figure 4-1: Blood Pressure Monitor (BPM) General Block Diagram
Blood Pressure Monitor (BPM)
Power
Management
Pressure Sensor
Inertial
Sensor
Amplifier
DC Brush
Motor Control
Pump Motor
Bleed Valve
Freescale Technology Optional
Figure 4-2: Heartbeat Signal
Figure 4-2: Heartbeat Signal
1800
1600
1400
1200
1000
800
600
400
200
Wireless
Comm
Keypad
freescale .com/medical 25
SPI/I 2 C
SPI/I 2 C
Main System
Pressure
MCU
USB
To PC
Display
Non-Volatile
Memory
Sensor System
(Intergrated with main system
for wrist applications or with cuff
for all other applications)
0
1 449 897 1345 1793 2241 2689 3137 3585 4033 4481 4929 5377 5825 6273 6721
Figure 4-3: Heartbeat over Time
Figure 4-3: Heartbeat over Time
2500
2000
1500
1000
500
0
Heartbeat
1 458 915 1372 1829 2286 2743 3200 3657 4114 4571 5028 5485 5942 6399 6856
Pressure
Heartbeat