Page 415 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 381

The following two-step procedure may be used to estimate the needed F O for a
2
I
desired PaO assuming that there is no significant deadspace or shunt abnormalities
2
(Chang, 2012)
PaO desired
2
Step 1: P O needed = (a/A ratio)
A
2
(P O needed + 50)
Step 2: F O = A 2
I
2
713
P O needed: Alveolar oxygen tension needed for a desired PaO 2
A
2
PaO desired: Arterial oxygen tension desired
2
a/A ratio: Arterial/alveolar oxygen tension ratio; (PaO /P O before
2
2
A
changes)
F O : Inspired oxygen concentration needed for a desired PaO 2
2
I
50: normal PaCO /Respiratory Quotient 5 (40/0.8) mm Hg
2
713: P 2 P H 2 O 5 (760 2 47) mm Hg
B
Oxygen and Ventilation. Most patients with respiratory acidosis or ventilatory failure
Hypoxemia related
to hypoventilation may be are also hypoxemic. Hypoxemia related to hypoventilation may be partially cor-
partially corrected by improv- rected by improving ventilation. In most cases, supplemental oxygen is also needed
ing ventilation. In most cases,
supplemental oxygen is also for the treatment of hypoxemia. In a clinical setting, an elevated PaCO along with
2
needed to treat hypoxemia. hypoxemia should be managed with ventilation and oxygen.
Oxygen and PEEP. Oxygen therapy alone may not be sufficient if the hypoxemia is
caused by intrapulmonary shunting. This type of refractory hypoxemia requires
refractory hypoxemia: Hypox-
emia that is commonly caused oxygen and continuous positive airway pressure (CPAP) or positive end-expiratory
by intrapulmonary shunting and pressure (PEEP). CPAP is used for patients with adequate spontaneous ventilation
does not respond well to high or
increasing F I O 2 . for a sustainable normal PaCO . PEEP is used for patients requiring mechanical
2
ventilation.
Oxygen Toxicity. Sufficient oxygen should be given to the patient to maintain a PaO 2
of around 80 mm Hg (lower for COPD patients). Excessive oxygen must be avoided
Refractory hypoxemia
responds well to supplemental because of the increased likelihood of developing oxygen toxicity, ciliary impair-
oxygen when used with
CPAP or PEEP. CPAP is used for ment, lung damage, respiratory distress syndrome, and pulmonary fibrosis (Otto,
patients with adequate spon- 1986). Since these complications may occur within 12 to 24 hours of exposure to
taneous ventilation for a sus-
tainable normal PaCO 2 . PEEP 100% oxygen, the general guideline is to use an F O lower than 60% and limit use
I
2
is used for patients requiring of high levels of F O for less than 24 hours (Winter et al., 1972).
mechanical ventilation. I 2
Improve Ventilation and Reduce
Mechanical Deadspace

Adequate ventilation is a prerequisite to oxygenation. Hypoxemia caused by hy-
poventilation is usually supported by supplemental oxygen during mechanical

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