Poor performance
Occasionally it is impossible to obtain correctly performed,
reproducible forced expiratory maneuvers. In that case it is
best to make a note of this; you may also record the FEV1 and the FVC, but do add a note to the effect that they derive
from unsatisfactory FVC maneuvers. It is to be noted that in
epidemiological surveys the prevalence of respiratory pathology
is greater among those who cannot perform spirometric tests
satisfactorily than among those who can (see literature).
In the first illustration the forced expiratory maneuver was
stopped prematurely. The subject’s FVC will certainly
be underestimated, as may be the FEV1.
Data derived from such curves should not be interpreted. Judging
from the straight descending portion of the curve, however,
airway obstruction is unlikely in this subject.
Another example of poor performance in all respects. This
subject should be instructed to exhale as forcibly and as long
as possible. While performing the maneuver the subject should
be loudly encouraged to blow out as hard and as completely as
possible. Neither the shape of the curves nor data derived from
any curve are suitable for interpretation.
This subject did not blow out hard at the start of the maneuver,
so that the first 10% of the FVC were not produced with maximal
force. The maneuver should be performed again, and the subject
instructed to blow out after a maximal inspiration as hard and
as completely as possible. Maybe the mouthpiece leaked at the
start of the forced expiration, or the patient just halted briefly.
At any rate data derived from this curve cannot be reliably
interpreted. Even so the shape of the curve (fairly straight
descending portion) is not suggestive of expiratory airway obstruction.
Do pay your technician a compliment for the patience and endurance
in trying to obtain acceptable flow-volume curves in this patient,
even though the efforts were unsuccessful. Occasionally a patient
defeats even the most experienced technician. If the FVC or
FEV1 from one of the
curves would be in a normal range, this at least precludes significant
airway obstruction or a restrictive ventilatory defect. If,
on the other hand, the ‘best’ values are below the
normal range, this should not be used as reliable evidence of
lung pathology.
Do make sure that the poor performance is not due to pain, or due to stress incontinence.
Greater variability related
to pathology in general population:
Kellie SE, Attfield MD, Hankinson JL, Castellan
RM. Spirometry variability criteria - association with respiratory
morbidity and mortality in a cohort of coal miners. Am J Epidemiol
1987; 125: 437-444.