Philips M3000A, Service Manual

Page: 48 / 293

Non-invasive Blood Pressure (NBP) Measurement
32 Introduction to the Instrument
measurement.
Pressure Pump—Inflates the cuff to preset limits, once or repeatedly, depending on the
measurement method used.
Pressure Sensor—Measures cuff pressure using solid-state technology.
Overpressure Safety System—Triggers alerts at given pressures and time limits, and
deflates the cuff.
Bandpass Filter—Extracts arterial pressure oscillations from the cuff pressure.
Deflation System—Automatically deflates the cuff at steps of a given magnitude.
NBP Measurement Characteristic
See “Selecting NBP Measurement Characteristic” on page 87.
Theory of Operation
As NBP signals pass from the patient to the Monitor, they progress through stages
corresponding to logical sections of the circuit, as shown in the block diagram. Circuit-
related faults can generally be isolated to one of the stages.
1 Acquisition—Signals from the patient are received by the pressure sensor through the
cuff, which is connected to the circuit by a single tube. The cuff is inflated, deflated, and
monitored by a pump, deflation system, and safety system controlled by a microproces-
sor.
a. Cuff Inflation—During the initial cuff inflation, the cuff is inflated by the
pressure pump to a set pressure which is determined by the patient size.
Thereafter the cuff is inflated by the pressure pump to a cuff pressure above the
patient’s systolic pressure. Depending on the measurement method used, inflation
occurs once or repeatedly. When the cuff pressure is greater than the systolic
pressure, the artery is occluded; the pressure sensor then detects only the cuff
pressure.
b. Cuff Deflation—Cuff pressure is automatically released by the deflation system
in steps until the artery is only partially occluded. At that point, measurement and
processing of arterial pressure oscillations begin and continue as the cuff pressure
is progressively released.
2 Detection—The arterial pressure oscillations are superimposed on the cuff pressure.
They are extracted from the cuff pressure by a digital bandpass filter in the microcontrol-
ler.
3 Measurement—As the cuff is deflated, the magnitude of the oscillations as a function of
cuff pressure increases until the mean arterial pressure is reached. When cuff pressure
falls below the mean arterial pressure, oscillation magnitude begins to decrease.
The systolic and diastolic blood pressure values are deduced from the oscillometric sig-
nal by extrapolation. Differences in the results with the standard stethoscope method can
be expected. The NBP accuracy complies with AAMI SP-10.