0071643192.pdf

(Barré) #1
THORACIC AND RESPIRATORY

DISORDERS

■ A-a gradient =PiO 2 – (PaO 2 – PaCO 2 /8).
■ Assuming sea level, an FiO 2 of 0.21 and 37°C, PiO 2 becomes 150 mmHg.
■ A conservative estimate of a normal A-a gradient is 4 +age (years) / 4.

TREATMENT


■ All patients with hypoxemia should be treated with supplemental O 2. How-
ever, be careful with chronic COPD patients whose respiratory drive is O 2
dependent. No improvement with supplemental O 2? Suspect anatomic shunt.
■ Treat underlying cause.


HYPERCARBIA

Hypercarbia is elevated CO 2 in the blood (PaCO 2 >45 mmHg). It is nearly
always a result of alveolar hypoventilation from a variety of disease processes.
In rare cases, this can be the result of exogenous CO 2 poisoning (dry ice, volcanic
eruption).


ETIOLOGIES


See Table 10.5.


TREATMENT


■ Treatment depends on underlying cause (eg, narcan in opiate-induced
hypoventilation).
■ In all cases, provide supplemental O 2 and support ventilation via bag-valve
mask ventilation, noninvasive positive-pressure ventilation, or intubation
(depending on cause).


Hypoxemia due to shunt does
not correct with 100% O 2
therapy.

TABLE 10.4. Etiologies of Hypoxemia


CAUSE MECHANISM DISEASESTATES COMMENTS

Reduced O 2 is replaced by Enclosed spaces, fire Normal alveolar-arterial
inspired O 2 other gases or High altitude, air travel (A-a) gradient
low total O 2

Diffusion Reduction in diffusion Interstitial lung disease An uncommon cause of
abnormality capacity →low PaO 2 hypoxemic respiratory failure

Hypoventilation ↓Minute ventilation results See Table 10.5. Normal A-a gradient
in↑PaCO 2 and↓PaO 2

Ventilation-perfusion Altered ratio of perfusion Pulmonary embolus, pulmonary ↑A-a gradient. PaO 2 corrects
(V/Q) mismatch to ventilation hypertension, COPD, asthma with supplemental O 2

Shunt Physiologic shunt: Perfusion ARDS, pneumonia ↑A-a gradient. PaO 2 does not
to nonventilated lung. correct with supplemental O 2
Anatomic shunt: Pulmonary AVM, congenital
Communication between heart disease, patent foramen
the arterial and venous ovale with right-to-left flow
systems.
Free download pdf