Page 301 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 267

CPP is the difference between the mean arterial pressure (MAP) and the intracra-
A higher CPP may be nial pressure (ICP). The relationship is shown as follows:
maintained by raising the
MAP or by lowering the ICP.
CPP 5 MAP 2 ICP
Based on the relationship of MAP and ICP, a higher CPP may be maintained
by raising the MAP or by lowering the ICP. In clinical practice ICP is usually con-
trolled within normal limits (i.e., ,20 mm Hg). However, it is unknown whether
The normal ICP is 8 to
12 mm Hg; clinical practice ICP control is necessary, providing that CPP is maintained above the critical thresh-
, 20 mm Hg. old by raising the MAP (Changaris et al., 1987; Rosner et al., 1990).
In the absence of hemorrhage, the MAP should be managed initially by maintain-
ing an adequate fluid balance. It may then be followed by using a vasopressor such
as norepinephrine or dopamine. Systemic hypotension (i.e., SBP ,90 mm Hg)
should be avoided and controlled as soon as possible because adequate systemic
Systemic hypotension perfusion is necessary to prevent cerebral ischemia (due to lack of cerebral blood
is associated with poor out- flow). For patients with severe brain injury, systemic hypotension contributes
comes in patients with severe
head injuries. to an increased morbidity and mortality. (Chesnut et al., 1993; Marmarou
et al., 1991).

SUMMARY

Monitoring in mechanical ventilation is done to provide information about the condi-
tion of the patient and the overall effectiveness of a treatment plan. The results ob-
tained from different monitoring techniques should be used and interpreted together
and should not be treated as isolated measurements. For example, a decrease in
end-tidal CO may indicate the presence of deadspace ventilation. But this assump-
2
tion must be confirmed with other supporting evidence such as a concurrent decrease
in perfusion (decrease in cardiac output or other hemodynamic values). Trending or
interpreting a series of measurements is also more meaningful since the overall condi-
tion of a patient is a dynamic process, not a set of separated events.
Finally, the condition of the patient should be assessed in conjunction with the moni-
toring results. This is because the patient may temporarily compensate for abnormal
conditions under extremely stressful settings. This erroneous “normal” measurement may
not be apparent by reviewing the laboratory results alone. Therefore, careful examina-
tion of the patient should always be a vital part of monitoring in mechanical ventilation.

Self-Assessment Questions

1. A patient suddenly develops shortness of breath and the SpO drops to 87%. The therapist should
2
anticipate a moderate increase of all of the following measurements except:
A. heart rate. C. respiratory frequency.
B. minute ventilation. D. temperature.

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