7. Pulse oximetry instead of PaO2 and SaO2
Summary: The pulse oximeter measures SpO2, which in
most situations closely correlates with SaO2 as measured by the
co-oximeter. However, in several situations the pulse oximeter can be dangerously
misleading, and should not be used without blood gas confirmation.
Pulse oximetry has been heralded as "arguably the most
significant technological advance ever made in the monitoring of the
well being and safety of patients during anaesthesia, recovery and
critical care" (Severinghaus 1986) and as "the greatest advance in
patient monitoring since electrocardiography" (Hanning 1995).
Clearly, this is a device with which all care givers should be familiar.
So much has been written about pulse oximetry that reviews of the
subject appear frequently (Severinghaus 1992; JAMA 1993; Wahr
Despite its acknowledged importance and simplicity of use, the
pulse oximeter is often mis-used and its measurement misunderstood.
The following points bear emphasis for anyone using a pulse oximeter
Unlike carbon monoxide,
metHb does depress the SpO2
reading, but not linearly (Eisenkraft 1988; Barker 1989; Watcha
1989; Ralston 1991). MetHb decreases SpO2, but the fall in SpO2
is only by about one-half of the metHb concentration, until a
reading of 85% is reached; at this point, further increases in
%metHb do not lower the SpO2 any further. Thus a pulse
oximetry reading of 90% could represent:
- Pulse oximetry does not differentiate carboxyhemoglobin from
oxyhemoglobin (Barker 1987; Raemer 1989). Pulse oximetry
emits two wavelengths of light, 660 nm and 940 nm. Oxygenated
(HbO2) and deoxygenated hemoglobin (Hb) reflect these two
wavelengths differently, allowing the oximeter to distinguish
between them. Light transmission at 660 nm is mainly from
HbO2, and at 940 nm is mainly from Hb. It turns out that COHb
reflects just as much 660 nm wavelength light as HbO2, and thus
COHb is read as HbO2 by the pulse oximeter. Thus, for example,
a patient with a true SaO2 of 85%, plus 10% COHb, will have a
pulse oximetry SpO2 reading of about 95%. For this reason pulse
oximeters should never be used to assess oxygenation in anyone
who might have CO poisoning.
- Pulse oximetry does not reliably distinguish between oxygen
desaturation from a low PaO2 and from excess methemoglobin
a) a low PaO2 causing oxygen desaturation of 10% (i.e., a
true SaO2 of 90%); or
b) normal PaO2 with methemoglobin in excess of 10%; or
c) some combination of a low PaO2 and excess metHb
As with CO, pulse oximeters should never be used to assess
oxygenation in anyone who might have excess methemoglobin.
Methylene blue is used to treat severe cases of excess
methemoglobin; like many intravenous pigments, methylene blue
causes a major drop in SpO2, and is another reason to avoid the
pulse oximeter altogether when managing this problem (Wahr 1995).
Clinically acceptable precision for SpO2 is within ±3% of the
SaO2, but the degree of precision varies among oximeter models
(Leasa 1992). Numerous studies have appeared correlating the
precision or accuracy of different oximeters, and the results are
variable. Knowing whether a particular model at a particular time
is over- or under-estimating oxygen saturation would require
measuring SaO2 (in a co-oximeter) at the time SpO2 is also
measured. This is obviously not practical nor desirable. Instead,
it seems prudent to assume that SpO2 is over-estimating SaO2,
and to take some action whenever SpO2 falls below 93%. Such action
would depend on the clinical situation, of course (e.g., close
monitoring of vital signs and cardiac rhythm; adding or increasing
supplemental O2; beginning another treatment; checking an arterial
Pulse oximetry may give a false sense of security if the patient has
adequate oxygen saturation but a declining PaO2.
Because of the relatively flat portion of the O2 dissociation
curve above a PaO2 of 60 mm Hg, and especially above 100 mm Hg,
PaO2 could drop significantly without an appreciable change in
SpO2 (Figure 6-5).
Pulse oximetry may give a false sense of security if the patient has
adequate oxygen saturation but a rising PaCO2.
A sedated or anesthetized patient receiving supplemental oxygen can maintain
adequate SaO2 without adequate ventilation. In the most extreme
cases this is called apneic oxygenation: diffusion of a high FIO2
into the lungs maintains oxygenation, while the PaCO2 rises to
life-threatening levels (Davidson 1993; Hutton 1993, Ayas 1998).
Even with extreme acidosis SaO2 may stay in the normal range,
especially if PaO2 is maintained above normal (Figure 6-6).
(Oxygenation by diffusion is also used in the "apnea test" for brain
death. Patients without spontaneous breathing, in whom neuro-logic exam
points to brain death, are oxygenated by diffusion
though an endotracheal tube without any mechanical breathing.
Total absence of respirations when PaCO2 reaches 60 mm Hg or
higher, without confounding factors such as hypothermia or drug
overdose, confirms brain death.)
Pulse oximetry may be unreliable if there is poor tissue
perfusion, vasoconstriction or hypothermia. This problem is most
often seen in patients with decreased vascular flow to the
extremities. The machines only work if there is a strong pulse.
With a weak pulse, the SpO2 reading may "stick" on a falsely low
or high value.
Pulse oximetry can be mis-used by people unfamiliar with how it
works and what is measured. A 1994 study showed that doctors
and nurses were surprisingly ignorant about some basic oximetry
principles, and made serious errors in interpretation of readings
(Stoneham 1994). For example, 30% of doctors and 93% of
nurses thought the oximeter measured PaO2 or oxygen content.
In the same study only one doctor and one nurse (3% of each sample)
knew that an oximeter requires pulsatile flow of blood under the
Of course the pulse oximeter is so simple to use that anyone can record
an SpO2, whereas only trained laboratory personnel can run a
blood sample through a co-oximeter.
This ease of use invites mis-application. I have seen health care personnel
at all levels -- physicians, nurses, therapists -- unintentionally chart a false reading
because they were unfamiliar with the device and the pitfalls
mentioned above. This problem is compounded when a charted value
is taken as reliable by other health care workers, when in fact it may
be totally erroneous.
A patient in the ER has a blood gas drawn at the same time as
a pulse oximetry measurement. The arterial PaO2 is 77 mm
Hg (breathing room air). Below are three values for arterial
oxygen saturation, with information about how each was
This question illustrates the three ways oxygen saturation is usually obtained.
By far the most reliable method is direct
measurement of an arterial sample in the co-oximeter. Using four
wavelengths of light, the co-oximeter makes direct measurements of
oxyhemoglobin, carboxyhemoglobin and methemoglobin, and so gives
the only true measurement of SaO2. Pulse oximetry, on the other
hand, includes carboxyhemoglobin and a variable amount of any
excess met-hemoglobin in its reading of oxygen saturation, and so will
overestimate SaO2 in the presence of excess dyshemoglobins.
As for the calculated SaO2, it is only reliable if nothing else but
oxygen is binding to hemoglobin, which you can't know from just the PaO2.
In this example the patient actually had 10% carboxy-hemoglobin and 2% methemoglobin,
so the calculated SaO2 of 95% was misleading.
In summary, it is always important to know where
your oxygen saturation value is coming from, and to interpret it
Return to Introduction of "Non-invasive blood gas interpretation"
Return to Table of Contents for All You Really Need to Know to
Interpret Arterial Blood Gases