Page 294 - Clinical Application of Mechanical Ventilation

260 Chapter 9

TABLE 9-10 Factors That Affect the Accuracy of Pulse Oximetry

Factors Type of Inaccuracy

Sunlight SpO measures lower than actual
2
SaO .
Nail polish 2
Fluorescent light

Intravenous dyes

Dyshemoglobins SpO measures higher than
2
actual SaO .
2
(methemoglobin, sulfahemoglobin,
carboxyhemoglobin)

Low perfusion states
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(SpHb™), oxygen content (SpOC™), carboxyhemoglobin (SpCO®), methemoglobin
Integrated pulse (SpMet®), pleth variability index (PVI®), and perfusion index (PI).
CO-oximetry is capable of
measuring the hemoglobin Pleth variability index (PVI®)(Masimo Corp., Irvine, CA, USA) is an algorithm
(SpHb™), oxygen content
(SpOC™),carboxyhemoglobin allowing for automated and continuous calculation of the respiratory variations in
(SpCO®), methemoglobin the pulse oximeter plethysmographic (∆POP) waveform amplitude. PVI has been
(SpMet®), pleth variability
index (PVI®), and perfusion used to predict fluid responsiveness in mechanically ventilated patients during gen-
index (PI). eral anesthesia (Cannesson et al., 2008).
Perfusion index (PI) is a relative assessment of the pulse strength at the monitor-
ing site. The PI display ranges from 0.02% (very weak pulse strength) to 20% (very
strong pulse strength). During sensor placement, use a site with the highest PI num-
ber (strongest pulse amplitude). The PI is influenced primarily by the amount of
blood at the monitoring site, not by the level of oxygenation in the blood. Prelimi-
nary data from one study show that low PI values correspond with illness on neo-
nates (DeFelice et al., 2002). Other applications of PI include assessment of pain
in the anesthetized state (Hagar et al., 2004) and as an early indicator of successful
epidural block in laboring women (Kakazu et al., 2005).

END-TIDAL CARBON DIOXIDE MONITORING

End-tidal carbon dioxide monitoring is done to monitor a patient’s ventilatory
end-tidal carbon dioxide
monitoring: The CO 2 level status. Once a good correlation is established between PaCO and end-tidal PCO
2
2
measured at the end of exhalation; (PetCO ), the number of routine blood gases may be reduced. In addition, changes
measured in mm Hg. 2
of the PetCO values and waveforms may also be obtained and interpreted for ad-
2
ditional information about the patient/ventilator system.

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