Page 568 - Clinical Application of Mechanical Ventilation

534 Chapter 16

TABLE 16-8 Clinical Conditions That Decrease the Compliance

Type of Compliance Clinical Conditions

T Static compliance Atelectasis
ARDS

Tension pneumothorax
Obesity

Retained secretions in lungs

T Dynamic compliance Bronchospasm

Kinking of ET tube
Airway obstruction

Retained secretions in airways
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The transpulmonary pressure is increased in conditions of low compliance or high
airway resistance. Normally a threshold work value of 1.6 kg.m/min or less is needed
before ventilator-dependent patients can be weaned and assume adequate spontaneous
breathing. Conditions leading to an increased workload such as low compliance and
high airflow resistance may lead to respiratory muscle fatigue and eventual ventilatory
failure. A threshold work value of 1.7 kg.m/min or higher is associated with failure to
wean from mechancial ventilation (Tobin et al., 1990; Vassilakopoulos et al., 1996).
Prolonged full ventilatory support and muscle disuse may lead to respiratory muscle
dysfunction and diaphragmatic atrophy. The cellular mechanism for the rapid onset
of mechanical ventilation-induced (MV-induced) diaphragmatic atrophy is unclear.
Studies have shown MV-induced oxidative stress is an important contributor to MV-
induced proteolysis and contractile dysfunction (Betters et al., 2004; Levine et al.,
2008). Other factors that may contribute to muscle weakness include inadequate oxy-
gen delivery (low O content or cardiac output), insufficient nutrition or electrolyte
2
imbalance, especially hypokalemia, hypophosphatemia, hypocalcemia, and hypomag-
nesemia (Knochel, 1982).
Retraining of atrophied muscles may be accomplished by short T-tube trials that
improve respiratory muscle strength. Pressure support ventilation may also be tried
as it increases diaphragmatic endurance (Hess et al., 1991).

TERMINAL WEANING

Terminal weaning is defined as withdrawal of mechanical ventilation that results in
the death of a patient. This differs from withholding of mechanical ventilation in
which the patient is not placed on any mechanical ventilatory support.

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