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Kraft et al

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

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

/MIF

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

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

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.

0.0

0.2

0.4

0.6

0.8

1.0

Change in FEV1/PEF

Months

Figure 2.

Change in FEV1/PEF postoperatively. FEV1/PEF

similarly changes in a unique linear fashion after intervention

(

< .0001).

177