Flow-volume curves: 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.