Page 80 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Diffusion Abnormalities
O 2 has to diffuse from the alveoli to hemoglo- brief. In effect, diminution of the diffusion
bin in the erythrocytes, and CO 2 from the area (e.g., after unilateral lung resection) also
erythrocytes into the alveoli. The amount of means a shorter contact time in the remaining
gas (M ˙ ) that diffuses across the diffusion barri- lung tissue, because the same amount of blood
er between alveoli and blood per unit time is will now pass through a reduced amount of
proportional to the diffusion area (F) and the lung tissue per unit of time. An increased O 2
difference in partial pressure between alveolar demand during physical exercise forces an in-
gas (PA) and blood (Pblood), and inversely pro- crease in cardiac output and can thus reveal a
Acid–Base Balance way (d): –1 × transport. In order for the same amount of gas
diffusion abnormality.
portional to the length of the diffusion path-
Abnormal diffusion primarily affects O 2
M ˙ = K × F (PA – Pblood)/d.
to diffuse per time, the O 2 gradient must be
twenty times greater than the CO 2 gradient.
Krogh’s diffusion coefficient K is about 20
Should the diffusion capacity in an alveolus
times greater for CO 2 than for O 2 . The diffusion
be diminished while ventilation remains con-
capacity D (= K × F/d) is about 230 mL × min
–1
(1.75 L × min
–1
–1
× mmHg ) in a healthy
Respiration, kPa A diffusion abnormality exists when the ra- stant, O 2 partial pressure will fall in the blood
person.
leaving the alveolus. If all alveoli are similarly
affected, O 2 partial pressure will fall in the pul-
monary venous (and thus systemic arterial)
tio of diffusion capacity to lung perfusion (or
The diffusion capacity may be reduced by
partial pressure will necessarily be lower also
4 cardiac output) is reduced. blood. If O 2 consumption remains constant, O 2
increased distance (→ A). When a pulmonary in deoxygenated (systemic venous) blood
edema occurs (→ p. 80), raised intravascular (→ B2). For this reason patients with a diffu-
pressure means plasma water is exuded into sion abnormality get blue lips on physical ex-
the interstitial pulmonary tissue or into the al- ertion (central cyanosis; → p. 84). The primary
veoli, and thus increases the diffusion dis- effects of abnormal diffusion on CO 2 transport
tance. Inflammation causes a widening of the and acid–base metabolism are much less
space between alveoli and blood capillaries as marked. Hypoxia stimulates the respiratory
a result of edema and the formation of connec- neurons, and the resulting increase in ventila-
tive tissue. In interstitial lung fibrosis (→ tion can produce hypocapnia. However, the hy-
p. 74), the connective tissue forces alveoli and poxemia due to abnormal diffusion can only be
blood capillaries apart. It is the distance be- slightly improved by hyperventilation. In the
tween hemoglobin and alveolar gas which example given (→ B3), doubling of the alveolar
matters. Thus, the distance can also be slightly ventilation at unchanged O 2 consumption in-
increased by vessel dilation (inflammation) or creases alveolar O 2 partial pressure by only
anemia. 4 kPa to 17 kPa (30 mmHg to 129 mmHg), but
A diminished diffusion capacity may also be the increased O 2 gradient does not normalize
caused by a reduction of the diffusion area the O 2 saturation of the blood. At the same
(→ A), as after unilateral lung resection, loss of time, respiratory alkalosis develops, despite
alveolar septa (pulmonary emphysema; the abnormal diffusion, because of the in-
→ p. 78), or in loss of alveoli in pneumonia, creased CO 2 removal (→ p. 86). Hypoxemia
pulmonary tuberculosis, or pulmonary fibrosis due to abnormal diffusion can be neutralized
(see above). The diffusion area can also be re- with O 2 -enriched inspiratory air (→ B4). The
duced by alveolar collapse (atelectasis; degree of hypoxemia can be lessened by de-
→ p. 72), pulmonary edema, or pulmonary in- creasing O 2 consumption.
farction (→ p. 80).
Diffusion abnormalities become obvious
when cardiac output is large (→ A), blood
70 flows rapidly through the lungs, and the con-
tact time of blood in the alveoli is thus quite
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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