Page 300 - Clinical Application of Mechanical Ventilation

266 Chapter 9

the PtcO can be used as an indicator of hypoxemia (Huch et al., 1974). The clini-
2
cal optimal range of PtcO for most infants is 50 mm Hg to 70 mm Hg (Klein,
2
2008).
Limitations. Accuracy of the PtcO electrode is affected by skin edema, hypothermia,
Accuracy of the PtcO 2 2
electrode is affected by skin and capillary perfusion status. PtcO becomes less accurate when the measuring range
2
edema, hypothermia, and is greater than 80 mm Hg (Palmisano et al., 1990). When cardiac output decreases, as
capillary perfusion status.
with the patient in shock, a disproportionate fall in PtcO occurs.
2
Two other disadvantages of transcutaneous monitors are the need for frequent site
changes (every 4 hours) to avoid erythemia and burns to the infant’s skin, and a long
equilibration time after each site change.

Transcutaneous PCO (PtcCO )
2
2
Transcutaneous PCO (PtCO ) monitoring is done to provide a means of con-
transcutaneous PCO 2 (PtcCO 2 ): 2 2
Measurement of PCO 2 through tinuous ventilatory assessment. The PtcCO is measured by heating the underlying
2
the skin by means of a miniature skin to 44°C (40°C to 42°C in neonates, maximum 45°C), which facilitates CO
Severinghaus (PCO 2 ) electrode. 2
diffusion across the skin to the CO electrode.
2
The correlation between PtcCO and PaCO is good in neonates as long as
2
2
perfusion is normal. This correlation in adults shows mixed results, but in gen-
eral the PtcCO may be useful as a monitoring tool once the trend has been
2
established.
Limitations. It should be noted that PtcCO values are usually higher than PaCO
2
2
values. This is due to increased CO production as underlying tissues are heated
2
(Marini, 1988). In addition, during shock or low perfusion states, the PtcCO mea-
2
sures higher than the actual PaCO due to increased accumulation of tissue CO
2
2
(Tremper et al., 1981).
CEREBRAL PERFUSION PRESSURE

Cerebral perfusion pressure (CPP) is the pressure required to provide blood
Maintenance of adequate flow, oxygen, and metabolites to the brain. Under normal conditions, the brain
CPP reduces mortality.
regulates its own blood flow regardless of the systemic blood pressure and cere-
bral vascular resistance. This autoregulation may be lost following head trauma,
cerebral perfusion pressure where the cerebral vascular resistance is often greatly elevated. The brain also
(CPP): Pressure required to
provide blood flow, oxygen, and becomes vulnerable to changing blood pressures. Depending on the degree of
metabolite to the brain. CPP 5 decrease in cerebral perfusion, effects on the brain may range from cerebral isch-
MAP 2 ICP. Normal range 5 70 to
80 mm Hg. emia to brain death (Bouma et al., 1990; Marion et al., 1991).
The optimum level of CPP is not defined, but the critical threshold is believed
to be from 70 to 80 mm Hg. Mortality increases about 20% for each 10 mm Hg
CPP should range
between 70 and 80 mm Hg. drop in CPP. In studies involving severe head injuries, 35% reduction in mortality
was achieved when the CPP was maintained above 70 mm Hg (Bouma et al., 1992;
Rosner et al., 1990).

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.