Page 563 - Clinical Application of Mechanical Ventilation

Weaning from Mechanical Ventilation 529

TABLE 16-5 Clinical Criteria and Thresholds Related to SBT Failure

PaO # 60 mm Hg on F O $ 50%
2
I
2
SaO , 90% on F O $ 50%
I
2
2
PaCO . 50 mm Hg or an increase in PaCO . 8 mm Hg from baseline of SBT
2
2
pH , 7.32 or a decrease in pH $ 0.07 from baseline of SBT
f/V . 100 breaths/min/L (rounded from 105)
T
f . 35 breaths/min or increase by $50% from baseline of SBT
Heart rate . 140 beats/min or increase by $20% from baseline of SBT
Systolic BP . 180 mm Hg or increase by $20% from baseline of SBT

Systolic BP , 90 mm Hg
Presence of cardiac arrhythmias
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spontaneous tidal volume corresponds with a decreased spontaneous frequency. If
the patient tolerates the weaning process well, the pressure support level is gradu-
ally decreased by 3 to 6 cm H O increments until a level of close to 5 cm H O is
2
2
reached. Extubation may be considered when the patient’s blood gases and vital
signs remain satisfactory (Tobin et al., 1990).
Automatic Tube Compensation. Automatic tube compensation (ATC) is a mode in the
Evita 4 ventilator (Dräger Medical) that reduces the airflow resistance imposed by
the artificial airway (endotracheal or tracheostomy tube). It allows the patient to
have a breathing pattern as if breathing spontaneously without an artificial airway.
This type of compensation may facilitate breathing efficacy and reduce the work of
breathing throughout the weaning process.

Other Modes of Partial Ventilatory Support

SIMV can be used to alleviate the need to alternate the patient on T-tube and
ventilatory support. Using SIMV to shift the work of breathing from the ventila-
tor to the patient is accomplished by progressively reducing the mandatory SIMV
frequency (usually 1 to 3 breaths per minute at each step). Arterial blood gases (or
SpO ) may be measured after 30 min or more at that setting (Tobin et al., 1990).
2
If the pH remains near normal (above 7.30 or 7.35) (Downs et al., 1974; Milbern
et al., 1978), the SIMV frequency is further reduced in steps until a frequency of
The pace of SIMV 2 to 4/min is reached. The pace of SIMV weaning is dictated by the patient’s clini-
weaning is dictated by the cal condition and tolerance.
patient’s clinical condition and
tolerance. Sometimes SIMV and PSV are used together in patients who have failed the spon-
taneous breathing trial or have done poorly with SIMV or PSV alone. Under these

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