Page 558 - Clinical Application of Mechanical Ventilation

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hinder the weaning process. Any large P (A-a) O gradient (.350 mm Hg) should be
2
corrected prior to the weaning trial.

Pulmonary Reserve

A patient’s pulmonary reserve may be assessed by measuring the vital capacity (VC)
and maximum inspiratory pressure (MIP). The VC and MIP maneuvers require
active patient cooperation, and therefore these two measurements are effort-
dependent. Proper explanation, vigorous coaching, and allowance of an equilibra-
tion period to stimulate active respiratory drive are the prerequisites for valid and
meaningful measurements.
Vital Capacity. The vital capacity (VC) reflects a patient’s pulmonary reserve as it
includes three lung volumes: inspiratory reserve volume, tidal volume, and expi-
ratory reserve volume. VC measures the maximum amount of lung volume that
the patient can exhale following maximal inspiration. Typically the patient is in-
structed to breathe in as deeply as possible and exhale all the air into a spirometer.
Unlike the forced vital capacity obtained in the pulmonary function lab, this VC
For a successful weaning maneuver does not require forceful exhalation. For successful weaning, the patient
outcome, the patient should
have a VC .10 mL/Kg. should have a VC of greater than 10 mL/kg (Boles et al., 2007; Pierson, 1982, 1983;
Tahvanainen et al., 1983).

Maximum Inspiratory Pressure. The maximum inspiratory pressure (also called nega-
tive inspiratory force) is the amount of negative pressure that the patient can gener-
ate in 20 sec when inspiring against an occluded measuring device (negative pres-
sure manometer) (Marini et al., 1986). If the patient is alert, explain the procedure
and encourage the patient to attempt to inspire as forcibly as possible. In some
mechanically ventilated patients, a waiting period without assisted ventilation may
be needed to induce mild hypoxia and hypercapnia for the best inspiratory efforts.
In addition, the duration of airway occlusion is an important factor in determining
the accuracy (individual therapists) and reliability (between therapists) of the MIP
measurements (Soo Hoo, 2002).
Weaning will likely be The MIP is considered a measure of ventilatory muscle strength, and weaning
successful if the patient can will likely be successful if the patient can generate an MIP of at least 230 cm H O.
generate an MIP of at least 2
230 cm H 2 O. The results of 11 studies indicate that the MIP averaged 237 cm H O for weaned
2
patients versus only 230 cm H O for nonweaned patients (Jabour et al., 1991).
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Pulmonary Measurements

Static compliance, airway resistance, and deadspace to tidal volume (V /V ) ratio
T
D
are three measurements that are not dependent on a patient’s cooperation or effort.
They are used to indicate the amount of pulmonary workload that is needed to sup-
port spontaneous ventilation. In general, low compliance, high airway resistance,
and high V /V ratio all contribute to an increased workload. When these undesir-
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able conditions reach the patient’s threshold, they may hinder the weaning process
and outcome.

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