MEASUREMENT OF AIRWAY OBSTRUCTION
Airway Resistance
Of the standard techniques used to evaluate airway obstruction,
Raw measurements appear to be the most direct. The technique is rapid and noninvasive
and requires merely that a subject pant once or twice per second through a mouthpiece
and with a noseclip in place. During normal breathing, a major fraction of the resistance
to airflow resides in the nose, pharynx, and larynx and can mask changes taking place
in the lungs. The use of a mouthpiece bypasses the nose to minimize the effects
of the upper airway on the measurement. The panting maneuver is used to keep the
larynx dilated and to reduce its influence on the total resistance to airflow. The
measurements of Raw require the subject to sit in a constant-volume body plethysmograph
("body box"), which also permits recording of the thoracic gas volume and thereby
provides an accurate appraisal of the effects of lung volume on Raw. Lung volume
is estimated by use of Boyle's law to relate changes in box pressure and mouth pressure,
and Raw is calculated from changes in box pressure and flow.[10]
The subjects initially pant against a closed mouthpiece, usually at end expiration.
Thoracic gas volume (i.e., FRC) is calculated from the relationship of box pressure
to mouth pressure ( Fig. 26-10
).
The shutter in the mouthpiece is then opened, and continued panting inscribes the
relationship between box pressure and flow at the mouth to derive Raw. The upper
limit of normal Raw is usually considered to be 2 cm H2
O in 1 second.
To eliminate passive changes in Raw as a result of differences in lung volume, the
reciprocal of Raw, Gaw, is calculated (see Fig.
26-5
). The Gaw is usually divided by the lung volume at which the measurement
is made (usually FRC) to obtain specific Gaw. The coefficient of variation (standard
deviation/mean × 100) in normal baseline values for a single subject is usually
small (<10%). Specific Gaw is a
Figure 26-10
Diagram of the constant-volume body plethysmograph used
to measure airway resistance and lung volume. When a subject pants against an obstructed
mouthpiece (shutter closed), box pressure (PB
) is plotted against mouth
pressure (Pm
). Changes in PB
are converted to changes in lung
volume by calibration of the box with known volumes of added gas and observation
of PB
changes. As the subject pants through the open mouthpiece, flow
(V) replaces Pm
on the plot, and airway resistance is computed from the
relationship between PB
and V.
highly reproducible measurement that can identify changes in the caliber of the intrapulmonary
airways. However, a considerable portion of normal airway resistance resides in
the upper airways and can be significantly increased with head flexion, which reduces
the caliber of the hypopharynx.[11]
[12]
It is important that patients position themselves as erectly as possible when using
the mouthpiece in the body box.
