Page 257 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 223

TABLE 8-7 Conditions That May Require Lower Tidal Volumes

Condition Examples

Increase of airway pressure requirement ARDS
Pulmonary edema
Increase of lung compliance Emphysema

Decrease of lung volumes Pneumonectomy
© Cengage Learning 2014

When significant gas leakage (.5% of ventilator tidal volume) occurs, the
cause must be identified and corrected. Minor gas leakage and circuit com-
pressible volume loss can be compensated by using a larger tidal volume. Some
ventilators automatically compensate for the compressible volume loss and thus
maintain a stable tidal volume. Other ventilators (e.g., Hamilton Veolar) mea-
sure the volume delivered to the patient at the airway opening. This allows detec-
tion of significant volume loss due to circuit compression factor or gas leakage.
Ventilator circuits are compliant and expand during a positive pressure breath.
The amount of circuit expansion results in a volume that does not reach the
patient but is recorded as part of the expired tidal volume. This volume “lost”
in the ventilator circuit is called the circuit compressible volume and it may
be calculated by following the steps in Table 8-8 (Barnes et al., 1994; Wilkins
et al., 2003).
Once the circuit compressible volume is known, the patient’s corrected tidal
volume can be calculated by:
See Appendix 1 for
example. Corrected Tidal Volume 5 Expired Tidal Volume 2 Circuit Compressible Volume

Pressure Support

Pressure support ventilation (PSV) is used to augment a patient’s breathing effort
by reducing the airflow resistance during spontaneous breathing. The contributing
factors of airflow resistance during mechanical ventilation may include the artifi-
cial airway, ventilator circuit, and secretions. Pressure support (PS) is available in
modes of ventilation that allows spontaneous breathing (e.g., SIMV). The patient
must also be able to breathe spontaneously. The initial pressure support level can be
calculated as follows.
# #
PS level = [(PIP - P plat )/V mach ] * V spon

PS level: Initial pressure support ventilation setting
PIP: Peak inspiratory pressure
P plat : Plateau pressure

Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.