Page 559 - Clinical Application of Mechanical Ventilation

Weaning from Mechanical Ventilation 525

Static Compliance. The static lung compliance is measured by dividing the patient’s
For a successful weaning
outcome, the compliance tidal volume (measured at the airway opening) by the difference in the plateau pres-
should be .30 mL/cm H 2 O. sure and the PEEP. The lower the compliance, the greater the work of breathing will
be. The minimal compliance value consistent with successful weaning is 30 mL/cm
H O or greater (Hess et al., 1991).
2
The static lung compliance may be calculated as follows:

DV
C =
See Appendix 1 ST DP
for example.
C : Static lung compliance in mL/cm H O
ST
2
DV: Corrected tidal volume in mL
DP: Pressure change (P PLAT - PEEP) in cm H O
2

Airway Resistance. The airway resistance can be estimated by dividing the differ-
ence in the peak inspiratory pressure and the plateau pressure (H O) by the con-
2
stant inspiratory flow (L/sec). The normal range for airway resistance is 0.6–2.4 cm
H O/L/sec (Burton et al., 1991) and higher for ventilator patients because of the
2
associated pathological conditions (e.g., bronchospasm) and tubing resistance (e.g.,
endotracheal tube, ventilator circuit).
Although no critical weaning value for airway resistance has been established for
mechanical ventilation, the work of breathing is directly related to the degree of air-
way resistance. The endotracheal (ET) tube contributes significantly to the airway
resistance. The effect of resistance through the tube can be minimized by ensuring
that the ET tube is not kinked or the suction catheter of a continuous suction sys-
tem is not protruding into the tube. Since retained secretions and bronchospasm
contribute to the airway resistance, the patient’s airways and lungs should be suc-
tioned as needed. Use of bronchodilators may be helpful to reduce the airway resis-
tance in conditions of reversible bronchospasm.
pressure support ventilation Pressure support ventilation (PSV) has been used extensively to reduce the elas-
(PSV): A mode of ventilation in tic and nonelastic airflow resistance and to augment the spontaneous tidal volume.
which the patient’s spontaneous
tidal volume is augmented by the The resulting spontaneous tidal volume is directly related to the pressure support
application of a preset pressure level. PSV may be used in the spontaneous breathing mode or in conjunction with
plateau to the patient’s airway
during the inspiratory phase of a other modes of ventilation that include spontaneous breathing (e.g., SIMV). PSV
spontaneous breath.
cannot be used in modes of ventilation that do not allow spontaneous breathing.
Since airflow resistance is highly variable among patients and at different stages of
mechanical ventilation, the pressure support level must be monitored and adjusted
The initial PSV level should accordingly. The initial PSV level should be titrated until a desired spontaneous
be titrated until a desired
spontaneous tidal volume tidal volume is reach (e.g., 10 to 15 mL/kg) or until the spontaneous frequency
is reach (e.g., 10 to 15 mL/ decreases to a target value (e.g., #25/min) (MacIntyre, 1987).
kg) or until the spontaneous
frequency decreases to a target
value (e.g., #25/min). Deadspace/Tidal Volume (V /V ) Ratio. The deadspace to tidal volume (V /V ) ratio
D
T
D
T
indicates the amount of each breath that is “wasted” or not being perfused by pul-
monary circulation. The higher the V /V ratio, the greater the minute volume
T
D
demand will be. The V /V ratio can be calculated as the partial pressure of arterial
D
T
carbon dioxide minus the mean partial pressure of the carbon dioxide in the exhaled

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