Page 1611 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

1130 PART 10: The Surgical Patient

hypotension and should not be used as a substitute for monitoring
112
MAP and ICP. 85
The state of autoregulation is an important determinant of the
response to CPP manipulation. Patients with intact autoregulation will
tolerate higher CPP, but after acute TBI, autoregulation may be region-
Intracranial pressure CBF is regulated by metabolism and does not show a linear relationship
ally or globally impaired. The more viable the brain tissue, the greater the
to CPP; the greater the brain injury, the more CBF appears to be influ-
enced by CPP. Maintaining CPP >70 mm Hg may require fluid and
109
113
vasopressor therapy and has been associated with the development of
acute respiratory distress syndrome (ARDS), cerebral edema, and myo-
cardial complications that can compromise cerebral oxygen delivery. In
C D a randomized controlled trial of a cerebral blood flow-targeted protocol
using CPP >70 mm Hg versus an ICP-targeted protocol with ICP main-
A B tained at <20 mm Hg after TBI, there was no significant difference in
Intracranial volume outcome between the groups. However, the risk of developing ARDS
was 5 times greater in the CPP-targeted group that received more ino-
FIGURE 118-12. Intracranial compliance curve. The Monro-Kellie doctrine states that pressors and intravenous fluid. 114,115 The ARDS patients were 2.5 times
243
the skull is rigid and brain, CSF, and blood are incompressible structures; therefore, an increase more likely to develop refractory intracranial hypertension and be veg-
in any intracranial component must be accompanied by displacement of brain, CSF, or blood, or etative or dead at 6-month follow-up.
an increase in ICP. Once the ICP increases and compliance is reduced, smaller changes in volume Both low CBF and CPP are associated with poor outcome after TBI;
can cause relatively larger changes in ICP (A-B vs C-D). Likewise only small amounts of CSF however, the determination of clinically beneficial thresholds for CPP
drainage can lead to large decreases in ICP. remains under investigation. 85,116,117 Correlations over time between the
MAP and the ICP, called the pressure reactivity index (PRx), can be
cerebral perfusion pressure (CPP), where MAP-ICP = CPP; however, the used to determine if autoregulation is intact after TBI and lack of auto-
degree of ICP elevation that may result in herniation is variable. regulation is also associated with poor prognosis ; however, there is no
118
ICP monitoring may be achieved via ventriculostomy or brain confirmation that PRx determined “optimal” CPP improves outcome. 119
parenchymal probes. Subarachnoid bolts, subdural, and epidural ICP Currently there is no evidence from controlled clinical trials to indi-
monitors are inaccurate and no longer in common use. A ventricu- cate an optimal CPP goal in terms of reducing secondary ischemic injury
87
lostomy (or external ventricular drainage [EVD] device) placed by or improving the neurological outcome; however, published guidelines
direct catheterization of a lateral ventricle and utilizing a fluid-coupled state as a level III recommendation that the treatment range for CPP
87
microstrain gauge transducer system is the most precise and reliable should be 50 to 70 mm Hg. It is important to note that CPP includes
85
form of ICP monitoring. It is also the most invasive, carrying the great- the mean arterial pressure at the level of the internal carotid artery. The
est risk of infection and hemorrhage. EVDs are more often placed in gradient between ICP and MAP measured at cardiac level should be
the nondominant hemisphere (usually right side) but may be placed in taken into account, especially when the head of bed is elevated.
the dominant hemisphere if indicated. While efforts should be made to
remove ventriculostomy catheters as early as possible, monitoring dura-
tion after 10 days is not associated with an increased infection rate. CSF CEREBRAL BLOOD FLOW
26
may be sent for cell count, Gram stain, and culture as indicated. The normal average cerebral blood flow (CBF) is 50 mL/100 g brain
EVD is potentially therapeutic, allowing the withdrawal of cerebrospi- tissue per minute. It is lower in the less metabolically active white
120
nal fluid (CSF) and blood (in the case of intraventricular hemorrhage) matter (average 30 mL/100 g/min) and higher in the gray matter (average
which may be used to reduce ICP (Fig. 118-12) or prevent obstructive 70 mL/100 g/min). Autoregulation is the process whereby the mean
121
hydrocephalus. Fiberoptic (Integra Life Sciences Corp., Plainsboro, NJ) CBF is maintained at 50 mL/100 g/min despite fluctuations in MAP, as
or microprocessor (Codman and Shurtleff, Inc., Raynham, MA) probes demonstrated by the autoregulation curve (Fig. 118-13). Under normal
may be placed directly into the brain parenchyma via a burr hole or at conditions, the CBF is maintained over a MAP range of 50 to 150 mm Hg
the time of surgery, or placed within ventriculostomy catheter systems, and is tightly linked to cerebral metabolic rate. After TBI, autoregula-
109
but they cannot be recalibrated. Intraparenchymal fiberoptic monitoring tion is lost; however, this occurs in a heterogeneous pattern—greater in
of ICP provides equivalent, statistically similar pressure measurements the areas of injury and less or intact in undamaged areas.
when compared to intraventricular monitors and is valuable when con-
tinuous cerebrospinal fluid drainage is needed since ICP measurement
via ventriculostomy during drainage of CSF may be less reliable. A
107
recent retrospective study comparing EVD versus fiberoptic parenchy- 100
mal ICP monitoring in adult TBI patients found that EVDs were associ-
ated with prolonged ICP monitoring, increased ICU length of stay, and
more frequent device-related complications ; however, a prospectively
108
designed study is needed to confirm these observations.
CEREBRAL PERFUSION PRESSURE Cerebral blood flow (mL/100 g/min) 50
Monitoring CPP or ICP gives only limited information regarding cere-
bral blood flow. ICP and its influence on CPP is often used implicitly or
explicitly as a surrogate for cerebral perfusion; however, perfusion also
depends on cerebrovascular resistance (CVR) with CBF = CPP/CVR 0
and is tightly regulated by cerebral metabolism. 109 0 50 100 150 200
When autoregulation is relatively intact, low CPP is associated Cerebral perfusion pressure (mm Hg)
with increased ICP through compensatory vasodilation in response FIGURE 118-13. Cerebral autoregulation. CBF is tightly linked to the cerebral metabolic
to decreased perfusion pressure. 110,111 CPP is a marker for systemic rate in normal brain tissue over a wide range of MAP and CPP.

section10.indd 1130 1/20/2015 9:20:19 AM