HSC Section 6 Nov2016 Green Book

Kraft et al

evaluation. Previous studies have demonstrated that the number reported by the computer for PIF and FIF50% did not represent the largest value in 50% and 69% of cases, respectively. 30,31 It is necessary to look at all loops or to establish a protocol for maximal inspiratory effort to miti- gate this. Finally, some data suggest that the test may not be adequately sensitive for mild stenosis. Miller and Hyatt 32 reported that the trachea would have to be narrowed to 8 mm (~ 80%) prior to creating detectable changes in flow. The retrospective nature of this study is an inherent limi- tation. As this is a single surgeon experience, the surgical technique was fairly consistent between patients. However, there was some variability in selecting the initial dilation size. The radius of the fluid conduit being one of the great- est influences on airflow has the potential to affect postop- erative PFT values. Our data suggest that PIF may be influenced by dilation size. In addition, pulmonary function data were not collected at consistent time points during fol- low-up. Our data demonstrate that restenosis occurs in a linear manner that appears to be unique for each patient. It will be important in future studies to standardize the time and manner in which PFTs are administered in order to mit- igate these confounding variables. Conclusion In the isolated subglottic stenosis model, PEF, PIF, FEV1/ PEF, and FIF50% demonstrate a significant change after endoscopic intervention. The postoperative rate of change in the PEF and FEV1/PEF is linear but appears to be unique to each patient. PIF and FIF50% also change as restenosis occurs, but the lack of standardized protocols makes the rate of change more difficult to predict. These pilot data suggest that the change in PEF, FEV1/PEF, PIF, and FIF50% can be used to assess outcomes in the endoscopic management and treatment of iSGS. Furthermore, the rate of change in these values (PEF and FEV1/PEF, in particular) could potentially be used as an objective means of comparing the efficacy of different techniques or adjuvant therapies. Moving forward, prospective studies should focus on establishing uniform time points and protocols for collecting PFT data. Given the low incidence of the entity and the unique characteristics of each stenosis, garnering adequate numbers and long-term follow- up will likely require multi-institutional collaboration. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Figure 2.  Change in FEV1/PEF postoperatively. FEV1/PEF similarly changes in a unique linear fashion after intervention ( P < .0001).

diameters (15, 10, 8, and 6 mm) in line with a spirometer. PEF and PIF were found to be the most sensitive measures by which to assess changes in airway resistance. Nouraei et al 22 performed a similar experiment in 2007 in which the resistors were designed to generate a greater resistance to inspiratory flow than expiratory flow. The MEF 50 /MIF 50 and the ratio of the areas under the expiratory/inspiratory curves were the most sensitive and specific parameters. Both the location of a stenosis and its characteristics (length, radius, boundary conditions) affect airflow, making every stenosis unique. In this study, we elected to specifi- cally look at iSGS as a model of fixed obstruction. We chose to exclude glottic and tracheal stenosis from our analysis, both of which have some component of variability due to the contribution of Bernoulli forces and compression of membranous tracheal wall, respectively. Not surprising, we found that the PFT values that changed after intervention were the same as those reported by Wasserman et al. We suspect that an examination of PFT values affected by inter- vention for glottic or tracheal stenosis would yield results more closely resembling those recently reported by Nouraei and colleagues for a variable obstructive model. In our evaluation, we examined the effect of radial inci- sion and dilation on PFT values. Pulmonary function tests are an attractive means of evaluating stenosis patients because the study is inexpensive; widely available; and, unlike current techniques using CT for computational fluid dynamic studies, avoids the need for ionizing radiation. Whereas CT examines the structural component of stenosis, PFTs are a physiologic measure of the effect of stenosis on airflow and the patient’s actual respiratory function. There are limitations to the PFT, the primary issue being that the study is dependent on patient effort. In addition, not all stud- ies are conducted uniformly. Spirometry software is pro- grammed to select the loop with the best expiratory effort for

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