Forced Expiratory Maneuvers
Despite the specificity and sensitivity of Raw measurements, airway
obstruction is more commonly evaluated by measurements of maximum forced expiration.
The indices obtained from forced expiration, unlike Raw, are determined by a complex
interrelationship of flow-resistive properties of intrathoracic airways and elastic
recoil of the lung. The simplest of such measurements is the peak expiratory flow,
which is conveniently measured with a variable orifice flow meter. The peak flow
occurs early in a forced expiration, when flow limitation has not occurred in the
airways; flow therefore depends greatly on effort and on the subject's cooperation.
However, because variation for the measurement in the same subject is surprisingly
low, peak expiratory flow is a fairly reproducible test of airway function.
Another extensively used indirect measure of airway dimensions
is the FEV1
. During the first 25% of an FVC
maneuver, flow reflects dimensions of the airways between the alveoli and the mouth
and is effort dependent. Although the physiologic parameters governing the remaining
flow are complex, FEV1
, like peak flow, is simple and reproducible and
therefore is a useful index of airway function. The measurement is subject to day-to-day
variability, which is greater in patients with obstructive airway disease than in
normal patients.[13]
These changes in FEV1
must exceed 15% to signify bronchodilation or constriction. An even greater potential
variability applies to measurements of FEF25%–75%
. It is also necessary
to account for the possibilities of negative effort dependence and, more importantly,
the changes in FVC that may occur. For example, in patients with bronchodilation,
FVC may increase but may produce a misleading decrease in FEF25%–75%
.
[14]
The measurement therefore should be adjusted
to the same absolute lung volume (i.e., the same segment of FVC below TLC) ( Fig.
26-11
).
Traditionally, the response to bronchodilators is expressed as
the percentage of change in FEV1
from a baseline value. Healthy normal
subjects and those with very mild obstruction typically exhibit a minimal increase
in FEV1
(<5%). Likewise, patients with severe baseline obstruction respond poorly because of accompanying secretions and airway edema. The most dramatic improvement occurs in patients with moderate obstruction; in these patients, the response to bronchodilators follows a bell-shaped distribution.
[15]
Because of the variability of response patterns,
reliance on FEV1
changes may underestimate the efficacy of bronchodilator
therapy. The spirometric inspiratory capacity (IC),
which reflects the degree of lung hyperinflation as a result of airway obstruction,
has been suggested as a more useful alternative.[16]
Additional assessment of the flow-resistive properties of the
airways can be obtained from maximum expiratory flow-volume (MEFV) curves, which
illustrate the relationship between airflow and lung volume during an FVC maneuver
(see Fig. 26-6
). A typical
response when bronchoconstriction is induced consists of diminished flows throughout
the sole MEFV curve envelope ( Fig.
26-12
). Ventilatory flows and FVC usually decrease, and RV increases.
Expiratory flows must be measured at the same reference lung volume. This is usually
at a fixed percentage of the baseline or normal FVC and requires that all curves
be superimposed at TLC.
In normal subjects, full inflation to TLC may remove the bronchoconstriction
induced by mechanical stimuli or drugs, whereas in asthmatics, an increase in bronchomotor
tone may accompany the same deep inspiration. To overcome these variable effects
of a full inspiration on bronchial tone, flows can be measured with partial expiratory
flow-volume curves. In this case, the maximum forced expiration is started at or
slightly above the middle of the FVC ( Fig.
26-13
). In all cases, the partial expiratory flow-volume curve is followed
by a full inhalation to TLC and a maximum forced exhalation to RV to obtain a reference
MEFV curve. Flows are usually measured between 20% and 40% of the VC above the RV.
Because partial expiratory flow-volume curves are unaffected by changes in upper
airway resistance, they are sensitive to the change in the intrapulmonary airways
Figure 26-11
Forced expiratory spirograms before (A)
and after (B) bronchodilator therapy. Notice that
the forced vital capacity (FVC) is increased in B,
but flow over its midportion (FEF25%–75%
) is decreased unless adjusted
to the same volume (i.e., the portion of the FVC below total lung capacity [TLC]).
The artifact results from the increased FVC as a result of bronchodilation. If
FEF25%–75%
is measured over the same volume segment as in A,
the value increases.
and have been suggested as a useful alternative to Raw for detecting bronchoconstrictor
and bronchodilator responses.
