Page 44 - Clinical Application of Mechanical Ventilation

10 Chapter 1

Clinical Ranges. For critically ill patients, the dynamic compliance is between 30 and
40 mL/cm H O and static compliance is between 40 and 60 mL/cm H O (Barnes
2
2
et al., 1994). It is lower in intubated patients, depending on the internal diameter of
the ET tube. Refer to Table 1-2 for the method to measure compliance. The equa-
tions for static compliance (C ) and dynamic compliance (C DYN ) are:
ST
Corrected Tidal Volume
C ST = (Plateau Pressure - PEEP)

See Appendix 1 for
example. Corrected Tidal Volume
C DYN =
(Peak Inspiratory Pressure - PEEP)

Compliance and the Work of Breathing

Since compliance is inversely related to pressure change (work of breathing), a de-
crease in compliance means an increase in the work of breathing. In a clinical set-
ting, acute respiratory distress syndrome (ARDS) and atelectasis are two causes of
increased work of breathing.
If the plateau pressure and peak inspiratory pressure (work of breathing) in the
previous equations are held unchanged, a decrease in compliance will cause a de-
crease in volume. This is because compliance and volume change in the equations
are directly related. In a clinical setting, hypoventilation usually results when a pa-
tient is unable to compensate for the decrease in compliance by increasing and
maintaining a higher level of work of breathing. In low compliance situations, such
as ARDS, pulmonary fibrosis, and kyphoscoliosis, the decrease in minute ventila-
tion is characterized by decreased tidal volumes and increased frequencies—a sign
of volume restriction.

Effects on Ventilation and Oxygenation

Abnormal compliance impairs the gas exchange mechanism. When an abnormally
low or high compliance is uncorrected and prolonged, muscle fatigue may occur
and lead to the development of ventilatory and oxygenation failure (Rochester,
1993). Ventilatory failure develops when the patient’s minute ventilation cannot
keep up with the CO production. Oxygenation failure usually follows when the
2
cardiopulmonary system cannot supply the oxygen needed for metabolism and pro-
longed increase in the work of breathing.

DEADSPACE VENTILATION

deadspace ventilation: Ventila- Deadspace ventilation is defined as wasted ventilation or a condition in which
tion in excess of perfusion; wasted ventilation is in excess of perfusion. There are three types of deadspace: anatomic,
ventilation.
alveolar, and physiologic.

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