Page 77 - Clinical Application of Mechanical Ventilation

Effects of Positive Pressure Ventilation 43

proportion to the increase of CO production. This is particularly important in
2
patients with impaired ventilatory reserves.

NEUROLOGIC CONSIDERATIONS

Among many other monitoring systems that influence the respiratory drive, the
central and peripheral chemoreceptors respond rapidly to the levels of CO , H ,
1
2
and O in the blood. For this reason, the degree of ventilation (CO , H ) and oxy-
1
2
2
genation (O ) can affect the normal functions of the brain.
2
Hyperventilation
Carbon dioxide acts as a vasodilator in cerebral blood vessels. During mechanical
ventilation, intentional hyperventilation is sometimes used to constrict these blood
vessels, and thus minimize intracranial pressure in patients with head trauma. Sus-
Sustained hyperventila- tained hyperventilation of less than 24 hours causes respiratory alkalosis, reducing
tion of less than 24 hours
causes respiratory alkalosis cerebral blood flow and intracranial pressure. After 24 hours, the buffer systems
and reduces cerebral blood of the body return the pH toward normal, negating the vasoconstrictor effect of
flow and intracranial pressure.
controlled hyperventilation.
If hyperventilation is prolonged, cerebral tissue hypoxia may result due to
the leftward shift of the oxyhemoglobin curve. A left shift causes higher oxygen
affinity for hemoglobin but reduced oxygen release to tissues. Sustained hyper-
ventilation also produces significant hypophosphatemia because of movement
of phosphate into the cells. Hypophosphatemia interferes with cerebral tissue
2, 3-bisphosphoglycerate metabolism by reducing ATP stores and 2,3-BPG levels, which further increases
(2, 3-BPG) is also known as the leftward shift of the oxyhemoglobin curve (Jozefowicz, 1989). Table 2-10
2,3-diphosphoglycerate
(2, 3-DPG) summarizes the neurologic changes in short-term (,24 hours) and sustained
(.24 hours) hyperventilation.

TABLE 2-10 Neurologic Changes in Hyperventilation

Condition Pathophysiologic Changes

Respiratory alkalosis (,24 hours) Decreased cerebral blood flow
Reduced intracranial pressure

Respiratory alkalosis (Prolonged .24 hours) Leftward shift of oxyhemoglobin curve
Increased O affinity for hemoglobin
2
Reduced O release to tissues
2
Cerebral tissue hypoxia
Neurologic dysfunction
Hypophosphatemia
© Cengage Learning 2014

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.