Page 252 - Clinical Application of Mechanical Ventilation

218 Chapter 8

ARDS, pulmonary edema, and carbon monoxide poisoning are examples that
often require ventilatory support for the primary purpose of oxygenation.
Hypoxemia can be assessed by measuring the PaO , or the alveolar-arterial oxy-
alveolar-arterial oxygen 2
pressure gradient [P (A-a) O 2 ]: gen pressure gradient [P (A-a) O ]. Severe hypoxemia is present when the PaO is less
2
2
The difference of P A O 2 and PaO 2 . A than 60 mm Hg on 50% or more of oxygen or less than 40 mm Hg at any F O .
gradient over 450 mm Hg while I 2
on 100% oxygen indicates severe P (A-a) O is the difference of P O and PaO . The normal P (A-a) O at 21% F O should
2
2
2
2
I
A
2
hypoxemia or intrapulmonary be less than 4 mm Hg for every 10 years of age. On 100% oxygen, every 50-mm Hg
shunting.
difference in P (A-a) O approximates 2% shunt (Shapiro et al., 1994).
2
P (A-a) O 5 P O 2 PaO
A
2
2
2
See Appendix 1 for
example.
PaO is obtained from arterial blood gas analysis and P O can be calculated as
A
2
2
follows:
A simplified alveolar air equation:
P O 5 (P 2 PH O) 3 F O 2 (PaCO /R)
2
2
2
2
A
I
B
where P 5 barometric pressure, PH O 5 water vapor pressure (47 mm Hg at
2
B
PaCO 2 /0.8 may be 37°C), and R 5 respiratory quotient (estimated to be 0.8). P O is mainly affected
2
A
changed to PaCO 2 3 1.25.
by changes of F O , PaCO , and P .
I
2
2
B
Patients with ALI and ARDS share three common clinical manifestations: acute
onset, bilateral infiltrates on frontal chest radiograph, and normal pulmonary
capillary wedge pressure (PCWP) of ≤18 mm Hg. The only distinguishing feature
separating ALI and ARDS is the degree of hypoxemia or PaO /F O (P/F) ratio.
PCWP of ≤18 mm Hg is 2 I 2
used to rule out pulmonary The threshold for ALI is a P/F value ≤300 mm Hg. For ARDS, the P/F threshold is
edema or bilateral infiltrates ≤200 mm Hg (Bernard et al., 1994). Since severe hypoxemia is the hallmark of ALI
caused by cardiogenic
pulmonary edema. and ARDS, the P/F ratio can be used to assess the degree of hypoxemia in critically
ill patients. The P/F ratio is calculated by:
P/F 5 (PaO / F O ) mm Hg
2
2
I
Prophylactic ventilatory
support is provided in clinical
conditions in which the risk Prophylactic Ventilatory Support
of pulmonary complications,
ventilatory failure, or oxygen-
ation failure is high. Prophylactic ventilatory support is provided in clinical conditions in which the
risk of pulmonary complications, ventilatory failure, or oxygenation failure is high.
In addition, prophylactic or early commitment of the patient to the ventilator can
Untreated tension pneu- minimize hypoxia of the major body organs. It can also reduce the work of breath-
mothoraxis is an absolute con- ing and oxygen consumption and thus preserve and rest the cardiopulmonary
traindication for mechanical
ventilation. system, and promote patient recovery (Otto, 1986). Indications for prophylactic
ventilatory support are outlined in Table 8-5.
CONTRAINDICATIONS
Since positive pressure ventilation is contraindicated in untreated tension pneumo-
thorax, mechanical ventilation at any positive pressure level must not be done with-
out a functional chest tube to relieve the pleural pressure. Other contraindications
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