Page 567 - Clinical Application of Mechanical Ventilation

Weaning from Mechanical Ventilation 533

CAUSES OF WEANING FAILURE

Aside from the pathological conditions that lead to the need for mechanical ventila-
Weaning failure is gener- tion, weaning failure may occur when the work of spontaneous breathing becomes
ally related to (1) increase of
airflow resistance, (2) decrease too great for the patient to sustain. Weaning failure is generally related to (1) increase
of compliance, or (3) respira- of airflow resistance, (2) decrease of compliance, or (3) respiratory muscle fatigue.
tory muscle fatigue.
Increase of Airflow Resistance

Normal subjects using an endotracheal (ET) tube have an increase of 54% to 240% in
the work of breathing, depending on the size of the ET tube and ventilator flow rate
2
(Fiastro et al., 1988). An 8-mm ET tube has a cross-sectional area of 50 mm , which
2
is slightly smaller than the average cross-sectional area of the adult glottis (66 mm ),
the narrowest part of the airway (Kaplan et al., 1991). To minimize the effects of an
artificial airway on airflow resistance, ET tubes of size 8 or larger should be used when
ET tubes of size 8 or it is appropriate to the patient’s size. In addition, the ET tube may be cut to about an
larger should be used to
reduce the airflow resistance. inch from the patient’s lips to minimize the airflow resistance contributed by the length
of the ET tube. The cut section of the ET tube should be displayed prominently so that
others would not presume that the ET tube had been moved deep into the brochus.
Other strategies for decreasing airway resistance can easily be done by periodic
monitoring of the ET tube for kinking or obstructions by secretions, or other
devices attached to the ET tube such as a continuous suction catheter, heat and
moisture exchanger, or end-tidal CO monitor probe. Endotracheal suctioning to
2
remove retained secretions and use of bronchodilators to relieve bronchospasm have
also been used successfully to reduce the airflow resistance.

Decrease of Compliance

Abnormally low lung or thoracic compliance impairs the patient’s ability to
maintain efficient gas exchange. Low compliance makes lung expansion difficult
Low lung or thoracic and, it is a major contributing factor to respiratory muscle fatigue and weaning
compliance makes lung ex-
pansion difficult, and it is a failure.
major contributing factor to In situations where the compliance gradually decreases (e.g., ARDS), the resultant
respiratory muscle fatigue and
weaning failure. refractory hypoxemia and increased work of breathing may lead to muscle fatigue
and ventilatory failure. When this occurs to a patient undergoing a weaning trial,
a return to the mechanical ventilator is almost inevitable. Table 16-8 shows some
examples that lead to a decreased compliance measurement.

Respiratory Muscle Fatigue

Respiratory work is a product of transpulmonary pressure (P ) and tidal volume
TP
(V ). Studies have been done to evaluate the relationship between the work of
T
breathing and a patient’s ability to sustain adequate spontaneous ventilation.
Work of breathing 5 P 3 V T
TP

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