Page 655 - Clinical Application of Mechanical Ventilation

Case Studies 621

In the emergency room, she was tachypneic with inspiratory and expiratory
Hypoxemia is severe since wheezes, had diffuse rhonchi, and dry crackles throughout all lung fields. She
the PaO 2 is only 59 mm Hg
with a non-rebreather mask. was placed on oxygen with a non-rebreather mask at 10 L/min and given one
®
Proventil nebulizer treatment. A stat blood gas was done and she was started
on IV aminophylline. Her respiratory frequency at that time was 28/min, with the
following blood gas results: (P of 640 mm Hg at 4,400 ft elevation maintains a
B
normal PaO of approx. 70 mm Hg).
2
pH 7.48
PaCO 2 34 mm Hg
PaCO 2 of 34 mm Hg PaO 59 mm Hg
indicates hyperventilation is 2 -
needed to maintain borderline HCO 3 24.2 mEq/L
oxygenation. Hb 12.8 g %
SpO 2 91%
Mode Non-rebreather mask
Oxygen 10 L/min
f Spontaneous 28/min

Four hours later her frequency had increased to 36/minute and she had
Increase in frequency received medication nebulizer treatments every hour with Proventil . However,
®
from 28 to 36/minute is an
ominous sign in the progres- inspiratory and expiratory wheezes persisted. A follow-up blood gas revealed:
sion of acute asthma.
pH 7.43
PaCO 2 36 mm Hg
PaO 2 58 mm Hg
-
HCO 23.2 mEq/L
3
PaO 2 does not show Hb 12.8 g %
much improvement in spite
of increase in spontaneous SpO 2 91%
frequency.
Mode Non-rebreather mask
Oxygen 10 L/min
f Spontaneous 36/min

Indications

Subsequent blood gases indicated that the patient’s ventilatory status quickly deteri-
orated and she became unable to maintain prolonged hyperventilation. Her PaCO
2
began rising toward normal (35 to 45 mm Hg).
The PaCO rising from a low level (by hyperventilation) toward normal is indicative
2
of muscle fatigue. Impending ventilatory failure is likely if not treated aggressively.
In asthma, the increased work of breathing is related to a combination of bron-
chospasm, airway inflammation, and mucus accumulation in the airway. The
changes in the airway increase the airflow obstruction, airway resistance, and
work of breathing. They also perpetuate degranulation of the mast cells caus-
ing release of histamine and other harmful chemical substances. Asthmatic pa-
tients may not respond to inhaled bronchodilators and eventually may show signs
of exhaustion with hypersomnolence, progressive hypoxemia, and ultimately
hypercapnea.

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