Page 537 - Clinical Application of Mechanical Ventilation

Critical Care Issues in Mechanical Ventilation 503

The most common cause of energy depletion and failure is a drop in cerebral per-
fusion (global ischemia). Cerebral perfusion is governed by the cerebral perfusion
pressure (CPP). In turn, the CPP is the difference between the mean arterial pressure
(MAP) and intracranial pressure (ICP). CPP 5 MAP 2 ICP. Perfusion-related
CPP 5 MAP 2 ICP conditions (e.g., cardiac arrest, hypotension) are conditions that cause a decrease in
MAP and a direct reduction in CPP and cerebral perfusion. Non-perfusion-related
conditions (e.g., traumatic brain injury, increase in ICP) cause an indirect reduction
in CPP and cerebral perfusion.

Symptoms. In mild cases of HIE, symptoms range from difficulty concentrating or
paying attention, poor judgment or coordination, euphoria, and extreme lethargy.
In more advanced or severe cases of cerebral oxygen deprivation, seizures and coma
may result (Kohnle, 2011). In respiratory care, most patients with lung diseases and
hypoxia may exhibit signs of mild HIE. They usually respond to oxygen therapy
and other respiratory therapy very well. The following sections discuss the topics
related to severe cerebral hypoxia.

Cerebral Perfusion Pressure

Cerebral perfusion pressure (CPP) is the pressure required to provide blood flow,
Inadequate cerebral oxygen, and metabolites to the brain. Under normal conditions, the brain regulates
perfusion can cause a wide
range of neurological prob- its own blood flow regardless of the systemic blood pressure and cerebral vascular
lems ranging from cerebral resistance. However, the brain becomes vulnerable in conditions of severe hypoten-
ischemia to brain death.
sion (e.g., cardiac arrest). The autoregulation ability of the brain may also be lost
following head trauma, whereas the cerebral vascular resistance is often greatly
elevated. Depending on the severity of reduction in cerebral perfusion, neurological
effects on the brain may range from cerebral ischemia to brain death (Bouma &
Muizelaar, 1990; Marion et al., 1991).
The normal cerebral per- There is no class I evidence for the optimum level of CPP, but the critical threshold
fusion pressure (CPP) should
range between 70 and 80 mm is believed to be from 70 to 80 mm Hg. The mortality rate increases about 20%
Hg. Mortality rate increases for each 10 mm Hg drop in CPP. In studies involving severe head injuries, a 35%
about 20% for each 10 mm Hg
drop in CPP. reduction in mortality was achieved when the CPP was maintained above 70 mm Hg
(Bouma et al., 1992; Rosner et al., 1990).

Decrease in CPP Due to Cardiac Arrest

Cardiac arrest and shock are two common causes of severe hypotension. Based
on a 2002 World Health Organization report, the incidence of cardiac arrest
is estimated between 36 and 128 per 100,000 persons per year. CPR was per-
formed in 86% of these cases with a successful return of spontaneous circulation
in 17 to 49% of these resuscitated individuals. However, about 80% of patients
who initially survive a cardiac arrest remain in a coma for different durations,
about 40% remain in a persistent vegetative state, and 80% are dead at one year.
(Madl et al., 2004).
Hypotension as a result of cardiac arrest is essentially a mechanical failure,
wherein the pump (heart) is not generating adequate pressure to provide perfusion.

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