Page 289 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 255

TABLE 9-7 Blood Gas Parameters and Normal Range for Adults

Parameter Monitoring Normal

PaCO 2 Ventilatory status 35 to 45 mm Hg

PaO 2 Oxygenation status 80 to 100 mm Hg
pH Acid-base status 7.35 to 7.45

© Cengage Learning 2014

values for adult patients. See Chapter 12 “Management of Mechanical Ventilation” for
examples of abnormal blood gas reports that are caused by external factors.

Assessment of Ventilatory Status

Direct measurement of arterial carbon dioxide tension (PaCO ) via arterial punc-
2
ture or indwelling catheter is the most accurate method of assessing a patient’s
ventilatory status. Hypoventilation and respiratory acidosis are present when the
PaCO is increased with a concurrent decrease in pH. This condition may be
2
corrected by increasing the frequency or tidal volume on the ventilator. On the
other hand, the frequency or tidal volume should be reduced when hyperventilation
and respiratory alkalosis occur.
When the acid-base imbalance is caused by metabolic acidosis or alkalosis, it calls
for a different ventilator management strategy. The underlying metabolic problem
must be corrected before changing the ventilator settings. Ventilator tidal volume
or frequency adjustment should not be made to “correct” metabolic acid-base
abnormalities during mechanical ventilation.
Respiratory Fatigue. The mechanically ventilated patient who develops hypercapnic
#
respiratory failure secondary to increased carbon dioxide production (VCO )
#
2
should be monitored closely. The VCO may be increased due to a hypermetabolic
2
state. This condition may lead to increased minute ventilation in an attempt to
keep up with the increasing CO production. A prolonged increase in the work
2
A prolonged increase of breathing may lead to respiratory muscle fatigue and ventilatory failure. It has
in the work of breathing may
lead to respiratory muscle been documented that excessive work of breathing (minute ventilation in excess of
fatigue and ventilatory 10 L/min.) is often associated with poor outcomes when trying to wean the patient
failure.
from mechanical ventilation (Stoller, 1991).
Patients with depressed central respiratory drive, elevated V /V , diminished com-
T
D
pliance, or respiratory muscle weakness may also develop respiratory fatigue as they are
unable to maintain an increased minute ventilation over an extended period of time.
Assessment of Oxygenation Status

Changes in the patient’s oxygen status are commonly assessed by (1) arterial oxygen
tension (PaO ), (2) alveolar-arterial oxygen tension gradient [P (A-a) O or (A-a)DO ],
2
2
2
(3) arterial to alveolar oxygen tension ratio (PaO /P O ), and (4) PaO to F O .
2
I
2
2
A
2
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.