Beer’s law: Absorption of a given
thickness of a solution of a given concentration
is the same as twice the thickness of half the
concentration.
Lambert’s law: Each layer of equal
thickness absorbs an equal fraction of radiation
which passes through it.
The wavelengths of light chosen are (660 nm and
940 nm) since this gives a better separation of
wavelengths. An isobestic point is the point at
which two substances absorb a certain wavelength
of light to the same extent. The light is partly
absorbed by hemoglobin in an amount that differs
depending on whether or not the Hb is saturated
with oxygen. By calculating the absorption at
the two wavelengths the processor can compute
the proportion of hemoglobin which is
oxygenated. The computer within the oximeter is
capable of distinguishing pulsatile (AC
component) flow from other more static signals,
such as tissue or venous signals (DC component)
to display only the arterial flow. Diodes are
switched on in sequence with a pause where both
diodes are off. This allows the microprocessor
to compensate for ambient light. The diodes are
switched off and on in sequence hundreds of
times a second – thus the processor can detect
cyclical changes due to arterial blood flow. The
non-pulsatile component is disregarded.
These oximeters are accurate in the range of
oxygen saturation 70 to 100% (+/2%), but less
accurate under 70%. The pitch of the audible
pulse signal falls with reducing values of
saturation.
In the following
situations the pulse oximeter readings might not
be accurate:
1. Peripheral
vasoconstriction (hypovolemia, severe hypoten-
sion, cold, cardiac failure, some cardiac
arrhythmias) or peripheral vascular disease.
This is due to an inadequate signal.
2. Venous
congestion, especially when tricuspid
regurgitation produces venous pulsations.
3. A badly
positioned probe.
4. Bright
overhead lights in the theatre might cause the
oximeter to be inaccurate (Ambient light).
5. The signal
might be interrupted by surgical diathermy.
6. Shivering can
cause difficulties in picking up the signal.
7. Scattering and
refraction of light.
8. Dyes, nail
varnish, patient movement, methylene blue.
9. Tendency to
over read in the presence of carboxyhemoglobin
and under read in the presence of methemoglobin.
Uses of
pulse Oximetry:
1. Forms part of the minimum mandatory
monitoring in an operating theatre.
2. Management and weaning from ventilators in
an Intensive Care Unit.
3. Procedures done under sedation.
4. Casualty.
Pulse oximetry is
possibly the greatest advance in patient
monitoring for many years and it is hoped that
eventually its use during anesthesia and surgery
will become routine worldwide.
