Page 50 - Clinical Application of Mechanical Ventilation

16 Chapter 1

TABLE 1-7 Causes of Decreased Diffusion Rate

Type of Diffusion Problem Clinical Conditions

Decrease in P(A-a)O gradient High altitude
2
Fire combustion
Thickening of A-C membrane Pulmonary edema
Retained secretions
Decreased surface area of A-C Emphysema
membrane Pulmonary fibrosis

Insufficient time for diffusion Tachycardia
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dioxide diffuses from the pulmonary arterial capillaries (PvCO = 46 mm Hg) to
2
the alveoli (P CO 5 40 mm Hg) with a net pressure gradient of only 6 mm Hg.
2
A
This is possible because the gas diffusion coefficient for carbon dioxide is 19 times
greater than that for oxygen.
Diffusion of oxygen is greatly impaired when the inspired oxygen tension (P O )
2
I
is reduced. The inspired oxygen tension is directly related to the barometric pres-
sure. At high altitude where the barometric pressure is low, the inspired oxygen ten-
sion is also low. This leads to a condition known as hypoxic hypoxia.
hypoxic hypoxia: Lack of oxygen
in the organs and tissues due to The P O is also reduced in a burning enclosure as combustion consumes oxygen
2
I
a reduction in inspired oxygen in the air. Patients who suffer from smoke inhalation are at risk for developing hy-
tension.
poxic hypoxia. In addition to the reduced oxygen concentration and tension, the
lung functions are impaired by the presence of carbon monoxide, toxic gases, and
inert particles found in a burning environment (Wilkins, 1998).
In addition to the pressure gradient and diffusion coefficient, the gas diffusion
rate is also affected by the thickness of the A-C membrane, the surface area of the
A-C membrane, and the time available for diffusion to take place. Factors that can
decrease the diffusion rate are shown in Table 1-7.
Conditions in Table 1-7 induce poor or inadequate gas diffusion and can severely
hinder the oxygenation process. Hypoxemia and hypoxia are usually the end results.
Severe hypoxemia and hypoxia may lead to hypoxic pulmonary vasoconstriction,
pulmonary hypertension, and cor pulmonale.

OXYGENATION FAILURE

Oxygenation failure is defined as severe hypoxemia that does not respond to moderate
to high levels of supplemental oxygen. It may be caused by hypoventilation, ventilation/
perfusion mismatch, or intrapulmonary shunting. Regardless of the etiology of oxygen-
ation failure, mechanical ventilation may be needed to minimize the work of breathing
and provide oxygenation support.

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