The first and oldest method is Photoplethysmography (PPG),
a light-based technology. It records the first appearance of the
pulse while deflating the occlusion cuff, and the disappearance
of pulses upon inflation of the occlusion cuff. PPG uses an
incandescent or LED light source to record the pulse signal wave.
The light source illuminates a small spot on the tail and attempts
to record the pulse.
PPG is relatively inaccurate since the readings are based solely
on the amplitude of a single pulse and cannot precisely measure
the systolic blood pressure or the heart beat. There are many
limitations to a light-based technology, such as over-saturation of
the blood pressure signal by ambient light, extreme sensitivity to
the rodent’s movement (motion artifact) and difficulty obtaining
adequate blood pressure signals in dark-skinned rodents
(Pigmentation Differentiation). Light-based sensors also cause tail
burns from close contact and prolonged exposure.
Diastolic blood pressure cannot be measured by PPG since the
technology records only the first appearance of the pulse. If the
diastolic blood pressure is displayed on the PPG instrumentation,
it is an estimation only, calculated by a software algorithm, and
not a true measurement.