Poor expiratory effort is known to over estimate traditional measures of airways obstruction (MMEFR% & FEV1/VC). We investigated if four different non-traditional indices of the shape of the flow-volume curve would accurately reflect the severity of airways obstruction in the presence of poor effort.
Ten subjects were recruited among our patients and hospital personnel to perform at least 4 maximal and 4 incomplete spirometric efforts. In addition to traditional spirometric indices,we calculated decay coefficient, area ratio (area bounded by flow-volume curve and a line from peak flow to FEV6, divided by the total area under the curve), and percent predicted flow and volume at which the slope of the normalized flow volume curve is -1(figure 1). For maximal efforts, we plotted MMEFR% & FEV1/FVC against each of these four indices, to generate prediction equations. We plotted the actual MMEFR% (or FEV1/FVC) and MMEFR% (or FEV1/FVC) predicted from each of the new indices against the standard (MMEFR% or FEV1/FVC from the maximal efforts).
As expected, MMEFR% is systematically overestimated when efforts are incomplete. Although there was considerable scatter, for incomplete efforts, volume at slope-1 could predict MMEFR% at maximal effort more accurately then could MMEFR% at incomplete effort(figure 2, p ≪ 0.05). There was also a trend toward improved prediction of FEV1/FVC.
Volume at slope-1 is a potentially useful measure of airways obstruction, and may prove to be more accurate then traditional measures of airways obstruction in the presence of incomplete expiratory efforts, when these traditional measures, specifically MMEFR% & FEV1/VC are grossly overestimated.
A common problem with PFT analysis is quantification of airways obstruction in those with poor expiratory efforts. A new index, percent predicted volume at slope -1 may ultimately assist in interpretation of studies that would otherwise be considered uninterpretable due to incomplete effort. At this point, a higher powered study looking at varied expiratory efforts in those with known airways obstruction is needed to test what we have found.
Julie Linek, No Financial Disclosure Information; No Product/Research Disclosure Information