This document explains how the BP-2000 Blood Pressure Analysis System measures the heart rate, and how it uses the pulse signal to determine blood pressure. Two levels of explanation are provided for how the instrument determines systolic and diastolic blood pressure.
The BP-2000 uses transmission photoplethysmography, in which variations in the amount of light transmitted through the tail is the basic signal that is analyzed to determine the blood pressure. Every time the heart beats, a pressure wave travels through the vascular system. When it reaches the tail, the vessels dilate slightly, and thus scatter more red light. (This corresponds to systole.) As the pressure wave passes, the vessels in the tail get smaller as the pressure decreases, and less light is scattered. (This corresponds to diastole.) The variation in light transmitted through the tail is what the BP-2000 displays on the computer screen.
The diastolic and systolic pressure can be determined by monitoring the vessel dilation as the occlusion cuffs inflates. Here is a plot of actual data showing where diastolic and systolic are measured:
Here are two explanations of how the systolic and diastolic pressures are determined:
Where the waveform amplitude starts to decrease is the diastolic pressure.
Where the waveform decreases to and remains at a steady value is the systolic pressure.
More Complicated Explanation
When the BP-2000 software has determined the pulse rate, it starts inflating the occlusion cuff. When the occlusion pressure reaches the diastolic pressure, the amplitude of the waveform starts to decrease, since the vessels cannot dilate as much in response to the heartbeat. When the occlusion pressure equals the systolic pressure, the waveform remains at a steady value and thereafter does not decrease, since the vessels cannot dilate at all.