0071643192.pdf

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.