Page 555 - Clinical Application of Mechanical Ventilation

Weaning from Mechanical Ventilation 521

sustain an adequate ventilation. The generally accepted ventilatory weaning criteria
include a PaCO of less than 50 mm Hg with normal pH, a vital capacity of greater
2
than 10 mL/kg, a spontaneous V of greater than 5 mL/kg, a spontaneous fre-
T
quency of less than 35/min, an f/V of less than 100 breaths/min/L, and a minute
T
ventilation of less than 10 L with satisfactory blood gases.
PaCO . The partial pressure of carbon dioxide in the arterial blood (PaCO ) is a reli-
2
2
The PaCO 2 should be able indicator of the patient’s ventilatory status. Weaning from mechanical ventilation
between 35 and 45 mm Hg
and the pH between 7.35 and should be attempted only when the PaCO is less than 50 mm Hg with a compen-
2
7.45 (for COPD patients, the
PaCO 2 may be around 50 mm Hg sated pH (non-COPD patient).
with a pH near 7.35). In patients with normal lung functions, the PaCO should be within the normal
2
range of 35–45 mm Hg and the pH should be between 7.35 and 7.45. However,
in patients with COPD, the acceptable PaCO may be slightly higher and the pH
2
slightly lower, depending on the patient’s baseline normal values prior to mechani-
cal ventilation (Millbern et al., 1978).
Vital Capacity and Spontaneous Tidal Volume. The mechanical condition of the lungs
may be evaluated by measuring the vital capacity and spontaneous tidal volume.
The vital capacity and It is generally accepted that the minimal vital capacity and spontaneous tidal vol-
spontaneous tidal volume ume consistent with successful weaning are 10 mL/kg and 5 mL/kg, respectively
that correlate with successful
weaning are 10 mL/kg and (Pierson, 1982, 1983; Tahvanainen et al., 1983). The results of 11 studies indicate
5 mL/kg, respectively.
that spontaneous tidal volume averaged 368 mL in weaned patients but only aver-
aged 277 mL in nonweaned patients (Jabour et al., 1991).
If the patient has been receiving full ventilatory support, it is advisable to allow
the patient to breathe spontaneously for 3 min under close observation prior to mea-
suring the vital capacity and spontaneous tidal volume. An equilibration period is
needed to obtain the spontaneous effort based on the patient’s actual respiratory
requirement.
Unlike spontaneous tidal volume, vital capacity requires active patient effort and
The vital capacity ma- cooperation. Vital capacity measures the maximal amount of volume that the patient
neuver is effort-dependent.
Proper teaching and coaching can expire following a maximal inspiration. For this reason, its validity is effort-
are required for accurate and dependent, and proper teaching and coaching are required for accurate measure-
valid measurements.
ments. Poor effort or inability to follow commands may result in lower than actual
vital capacity measurement.
Spontaneous Frequency. For a successful weaning outcome, the spontaneous fre-
quency should be less than 35/min while the corresponding PaCO should be less
2
than 50 mm Hg (Pierson, 1983; Tahvanainen et al., 1983). A frequency of greater
than 35/min is associated with rapid shallow breathing. This breathing pattern in-
Rapid shallow breathing creases deadspace ventilation and is highly ineffective for gas exchange during spon-
increases deadspace ventila-
tion and is highly ineffective taneous breathing. A moderate to significant increase in spontaneous frequency after
for gas exchange during discontinuation of mechanical ventilation is a sign of impending weaning failure
spontaneous breathing.
(Jabour et al., 1991).
As with the spontaneous tidal volume measurement, the patient should be
allowed to breathe spontaneously for 3 min prior to measuring the spontane-
ous frequency. This allows the patient ample time to normalize the breathing
pattern, and thus is more reflective of the patient’s response to the respiratory
requirement.

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