Page 250 - Clinical Application of Mechanical Ventilation

216 Chapter 8

compensate for the gas exchange deficiencies. However, if the underlying pathology
is not corrected in time, ventilatory failure will ensue when muscle fatigue occurs
as a result of prolonged, excessive work of breathing. At this time, the PaCO will
2
rise and the pH will fall.
If the early clinical signs indicate that a patient is in impending ventilatory failure,
it is appropriate to initiate mechanical ventilation. Early intervention is done to cor-
rect hypoxemia and acidosis imposed on the major organs and to reduce the stress
placed on the cardiopulmonary system. There are several objective measurements
that can be used to determine whether the patient is in impending ventilatory fail-
ure. These measurements are discussed below.
Assessment of Impending Ventilatory Failure. Development of impending ventilatory
Assessment of
impending ventilatory failure failure is dependent on the balance of metabolic needs and work of breathing.
relies on the measurements When metabolic needs cause excessive work of breathing, impending ventilatory
related to the lung functions
(i.e., V T , f, V E , VC, MIP). failure is likely. Since the work of breathing is carried out entirely by the respiratory
system, assessment of impending ventilatory failure relies solely on the measure-
ments related to the lung functions (i.e., tidal volume, frequency, minute volume,
vital capacity, and maximal inspiratory pressure). Table 8-4 shows the factors for
the assessment of impending ventilatory failure.
Tidal volume. A spontaneous tidal volume of less than 3 to 5 mL/kg is indicative
of impending ventilatory failure.
Frequency (f). A spontaneous frequency of greater than 30/min may indicate im-
pending ventilatory failure (Byrd et al., 2010).
Minute volume. If the patient’s spontaneous minute volume is greater than
10 L/min, then impending ventilatory failure is likely. Although it may appear that

TABLE 8-4 Assessment of Impending Ventilatory Failure

Parameter Threshold

Tidal volume ,3 to 5 mL/kg
Frequency and pattern .30/min
Labored or irregular
respiratory pattern
Minute ventilation .10 L/min

Vital capacity ,15 mL/kg

Maximum inspiratory pressure (MIP) ,220 cm H O
2
PaCO trend Increasing to over
2
50 mm Hg
Vital signs Increase in heart rate,
blood pressure
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