COMPLIANCE
Compliance is the ability of the lungs and thorax to expand or it is the expansibility of lungs and thorax. It is defined as the change in volume per unit change in the pressure.
Significance of Determining Compliance
Determination of compliance is useful as it is the measure of stiffness of lungs. Stiffer the lungs, less is the compliance.
NORMAL VALUES
Compliance is expressed by
two ways:
1. In relation to
intraalveolar pressure
2. In relation to
intrapleural pressure.
Compliance in Relation to Intra-alveolar Pressure
Compliance is the volume increase in
lungs per unit increase in the intraalveolar
pressure.
1.
Compliance of lungs and thorax together:
2.
130 mL/1 cm H2O pressure
2. Compliance of lungs
alone:
220 mL/1 cm H2O
pressure.
Compliance in
Relation to Intrapleural Pressure
Compliance is the volume
increase in lungs per unit decrease in the intrapleural pressure.
1. Compliance of lungs and
thorax together:
100 mL/1 cm H2O
pressure
2. Compliance of lungs
alone:
200 mL/1 cm H2O
pressure. Thus, if lungs could be removed from thorax, the expansibility (compliance)
of lungs alone will be doubled. It is because of the absence of inertia and restriction
exerted by the structures of thoracic cage, which interfere with expansion of
lungs.
Specific Compliance
The term
specific compliance is introduced to assess the stiffness of lung tissues more
accurately. Specific compliance is the compliance per liter of lung volume. It is usually reported for expiration at functional residual capacity. It is the compliance divided by functional residual capacity.
Specific compliance Compliance of lungs
of lungs = Functional residual capacity
Functional residual
capacity is the volume of air present in lungs at the end of normal expiration.
TYPES OF COMPLIANCE
Compliance is of two
types:
1. Static compliance
2. Dynamic compliance.
1. Static
Compliance
Static compliance is the
compliance measured under static conditions, i.e. by
measuring pressure and
volume when breathing does
not take place. Static compliance is the pressure required to overcome
the elastic resistance of
respiratory system for a given tidal volume under zero flow
(static) condition.
2. Dynamic
Compliance
Dynamic compliance is the
compliance measured during dynamic conditions, i.e.
during breathing.
Static Compliance Vs
Dynamic Compliance
In healthy subjects, there
is little difference between static and dynamic compliance. In patients with
stiff
lungs, the dynamic compliance
decreases while little change occurs in the static compliance.
MEASUREMENT OF
COMPLIANCE
Measurement of Static
Compliance
To measure the static
compliance, the subject is asked to inspire air periodically at regular steps
from
a spirometer. In each
step, a known volume of air is inspired. At the end of each step, intrapleural
pressure is measured by means of an esophageal balloon. Then,
the air is expired in steps until the volume returns to original preinspiratory
level. Intrapleural pressure is measured at the end of each step.
Values of volume and
pressure are plotted to obtain a curve, which is called pressure-volume
curve. From this curve compliance can be calculated. This curve also
shows the difference in inspiration and expiration.
Measurement of
Dynamic Compliance
Dynamic compliance is
measured during normal breathing. It is measured by determining the lung volume
and esophageal pressure (intrapleural pressure) at the end of inspiration and
expiration when the lungs are apparently stationary.
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