GE Medical Systems marquette Dash 2000, Operator'S Manual, Page: 217 / 272

Revision C Dash 2000 Patient Monitor 11-15
2005873-001
The following clinical questions are frequently asked about SPO2 monitoring. These may help in troubleshooting
when monitoring SPO2.
Why does the monitor
(pulse oximeter)
sometimes read differently
than an ABG?
How does a pulse oximeter
”read” the various types of
hemoglobins?
What effect can ambient
light have on pulse
oximetry monitoring?
Arterial Blood Gas analyzers calculate the O2 saturation based on normal
values for pH, PaCO2, Hb, temperature, etc. (i.e., a normal oxyhemoglobin
dissociation curve). Depending on the patient's physiologic and metabolic
status, his curve and all his values may be shifted away from ”normal.” Thus
the oximeter, which
measures O2 saturation, may not agree with the ABG.
A CO-Oximeter is an analyzer which also measures O2 saturation. The
saturation of hemoglobin may be much closer to a pulse oximeter in these
cases.
All pulse oximeters utilize two wavelength absorption. This is because reduced
hemoglobin (RHb) and oxyhemoglobin (HbO2) absorb these two wavelengths
differently. The hemoglobin saturation is then figured from the measured
amounts of the hemoglobins: (SpO2–HbO2)/(HbO2 + RHb).
Carboxyhemoglobin (COHb) absorbs similarly to HbO2 and thus can raise the
SpO2. Normal levels of COHb are 1-2%. Methemoglobin (MetHb) usually
represents less than 1% total Hb, but in cases such as some IV dyes,
antibiotics, etc. this level may go up sharply. MetHb absorbs similarly to RHb,
and thus could lower the SpO2 reading. Fetal Hb absorbs like adult Hb, thus
the oximeter has the same accuracy with neonates and adults.
Ambient light can have numerous effects. In the newborn, ICU bili lights can
affect the readings of the oximeters. Outside light striking the probe detector
can give poor waveform and inaccurate readings. Sunlight or bright indoor
lights can have the same effect in other areas of the hospital using oximetry.
Error messages of PROBE IS OFF THE PATIENT or PULSE SEARCH, etc.
are possible. Shielding the probe with opaque tape, the posey wrap or other
material can thus increase oximetry accuracy—verified by good waveform and
signal strength.