Page 293 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 259

Accuracy and Clinical Use of Pulse Oximetry

Pulse oximetry has been used as a reliable noninvasive means of monitoring oxy-
genation in mechanically ventilated patient. SpO of .95% has a strong correlation
2
with PaO of .70 mm Hg with a sensitivity of 100% (Niehoff et al., 1988).
2
SpO 2 above 92% SpO can be used to facilitate F O weaning. The F O may be reduced to an
I
2
2
2
I
correlates with a PaO 2 above appropriate level by use of a single arterial blood gas measurement followed by
60 mm Hg.
multiple pulse oximetry measurements (Rotello et al., 1992). Oxygenation of the
ventilator-dependent patient can be assured when the SpO is kept above 92% as
2
this level correlates with a PaO above 60 mm Hg (Jubran et al., 1990). Table 9-9
2
outlines other clinical application of pulse oximetry.
Limitations of Pulse Oximetry
SpO has good correlation with arterial oxygen saturation (SaO ) when the SaO is
2
2
2
SpO 2 has good cor- 95% or greater (Niehoff et al., 1988). SpO becomes less accurate as SaO decreases,
2
2
relation with arterial oxygen and over estimation of a patient’s oxygenation status may result.
saturation (SaO 2 ) when the
SaO 2 is 95% or greater. The accuracy of pulse oximetry can be affected by factors such as artifact and
underlying patient conditions. Artifact due to motion remains a cause of inaccurate
measurement despite corrective efforts (Pologe, 1987). Sunlight has been reported
to give a falsely low SpO measurement (Abbott, 1986). Nail polish (primarily blue,
2
green, and black), and intravascular dyes can also give a falsely low SpO reading
2
(Welch, DeCesare & Hess, 1990). Improper placement of the oximeter probe can
give a faulty SpO reading as well. If a patient is wearing nail polish, the probe may
dyshemoglobins: Hemoglobins 2
that do not carry oxygen (e.g., be placed sideways (White et al., 1989).
carboxyhemoglobin, methe- Pathologic factors such as low perfusion states and presence of dyshemoglobins may
moglobin). In the presence of
dyshemoglobins, pulse oximeter lead to SpO measurements that are higher than the actual SaO (Schnapp & Cohen,
2
2
reads higher than actual SaO 2 .
1990). Table 9-10 shows the factors that affect the accuracy of pulse oximetry.
Integrated Pulse CO-Oximetry
Low perfusion and pres-
ence of dyshemoglobins may
lead to SpO 2 that are higher In addition to the SpO and PR (pulse rate), an integrated pulse CO-oximetry (Masimo
2
than the actual SaO 2 .
Rainbow SET®) uses signal extraction technology to measure a patient’s hemoglobin
TABLE 9-9 Clinical Application of Pulse Oximetry

Clinical Application Examples

Monitor oxygenation status Mechanical ventilation Intubation
Surgery
Titrate F O 2 Increase F O in hypoxemia
2
I
I
Decrease F O in weaning
I
2
Verify ABG accuracy Compare O saturation readings to rule
2
out venous sample.
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