Mallinckrodt NELLCOR NPB-4000 Service Manual page 110

Section 12: NIBP - Theory of Operation
12-2
Figure 12-2: NIBP System Block Diagram
Prior to making blood pressure measurements, the cuff is placed around a limb,
typically an upper arm (left or right). The NIBP measurement system is initiated
by commands responding to pressing the NIBP Start/Stop switch on the front
panel. The cuff is inflated at a rate controlled by the pump excitation current to
a pressure above systolic, at which level the artery is effectively occluded. Then,
as the cuff pressure is reduced at constant rate, the pressure transducers provide
analog signals to the digital hardware where they are digitized and processed to
obtain systolic, diastolic, and mean blood pressure values, as well as the beat
rate.
The technique for translating the pressure signals into the required measurements
makes use of the action of the oscillatory characteristic of the returning arterial
pressure. The phenomenon is illustrated in the diagram below.
See Figure 12-3. Systolic pressure is defined as the point at which oscillation
increases sharply. As the cuff deflates, oscillation amplitude increases to a
maximum, and then decreases. The point of peak oscillation amplitude is the
mean arterial pressure. The point at which oscillations cease is defined as the
diastolic pressure
Figure 12-3: Oscillatory Characteristics Diagram