Page 299 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 265

End-tidal PCO 2 = 30 mm Hg

P CO 2 P CO 2 Arterial PCO 2
A
A
0 mm Hg 40 mm Hg 40 mm Hg
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Obstruction 46 mm Hg
0 mm Hg
Mixed Venous PCO 2

Figure 9-15 Deadspace ventilation induced by blockage of a portion of the pulmonary
blood flow (e.g., pulmonary embolism). This condition leads to a reduced PetCO 2 reading.

TRANSCUTANEOUS BLOOD GAS MONITORING

Transcutaneous blood gas monitoring involves placement of a miniature Clark
(PO ) or a Severinghaus (PCO ) electrode on the skin via a double-sided adhesive
2
2
disk. A heating coil in the electrode increases the permeability of the epidermis,
thus facilitating diffusion of gas from the underlying capillaries to the electrode.
Transcutaneous blood gas monitoring has been used more often in neonates than
in adults (Eberhard et al., 1981).

Transcutaneous PO (PtcO )
2
2
The transcutaneous PO (PtcO ) provides a noninvasive measurement of arterial
transcutaneous PO 2 (PtcO 2 ): 2 2
Measurement of PO 2 through the oxygen tension. The PtcO monitor uses a combined platinum and silver electrode
2
skin by means of a miniature Clark covered by an oxygen-permeable hydrophobic (water-repelling) membrane, with a
(PO 2 ) electrode.
built-in reservoir of phosphate buffer and potassium chloride. Since the PtcO sen-
2
sor requires an adequate blood flow to the skin, a small heating element is placed
in the silver anode to provide a constant temperature (e.g., 44ºC) to the skin.
For an adequate blood
flow to the skin, a heating Following the initial setup or site change, the PtcO should be correlated with an
2
element is used to provide arterial or capillary sample. The value from the PtcO monitor should be recorded
a constant temperature 2
(e.g., 44ºC) to the skin. immediately after obtaining the arterial or capillary sample (Klein, 2008).
In neonates, the transcutaneous PO (PtcO ) closely approximates the PaO . But
2
2
2
in adults, the PtcO measures lower than the actual PO due to thicker skin in
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2
adults. For this reason, pulse oximetry (SpO ) is the preferred method to monitor
2
the oxygenation status of adult patients.
PtcO also approximates the central organ PO (Tremper et al., 1979). It has
2
2
The clinical optimal a good correlation with the cardiac output changes in a mechanically ventilated
range of PtcO 2 for most infants
is 50 mm Hg to 70 mm Hg. patient (Shapiro et al., 1989). Since PtcO values correlate well with arterial values
2
within the whole PO range (particularly in the PO range below 100 mm Hg),
2
2
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