Author Daniel J da Costa MD, David W Hsia MD Division of Respiratory and Critical Care Physiology and Medicine Harbor-UCLA Medical Center
Case
A 41-year-old male with a history of prior tracheostomy due to a
motor vehicle accident presented to our emergency department with
dyspnea. Physical examination revealed stridor with otherwise normal
range vital signs. Pulmonary function tests were obtained as seen here.
Question
What is the diagnosis?
A. Fixed upper airway obstruction B. Variable intrathoracic upper airway obstruction C. Left mainstem bronchus obstruction D. Variable extrathoracic airway obstruction
Answer
D: Variable extrathoracic airway obstruction
Discussion
This patient has a flow-volume loop demonstrating a flattening of
both the inspiratory and expiratory limbs raising concern for an upper
airway obstruction. When determining if the obstruction is
intrathoracic or extrathoracic, we need to assess if the flow limitation
is equal or different between inspiration and expiration. To do this,
we can look at the ratio of maximal inspiratory flow at 50% FVC (MIF50%)
to maximal expiratory flow at 50% FVC (MEF50%) (1,2). A ratio near 1
would indicate a fixed obstruction; a higher ratio can indicate
intrathoracic lesions while lower ratios can indicate extrathoracic
lesions, as in this patient. In regards to the incorrect answers, answer
A (fixed airway obstruction) would have the inspiratory and expiratory
limbs limited at the same flow rate as seen in the figure below, panel
A. Answer B (Variable intrathoracic upper airway obstruction) would have
the expiratory flow limb more limited than the inspiratory flow which
is the opposite of which was seen in our patient. Answer C is incorrect
as you would expect a biphasic flow volume loop in both expiratory and
inspiratory limbs.
The physiologic reason for a variable obstruction is attributed to
compression due to changes in transmural pressure (2). During
inspiration, intraluminal pressure become more negative relative to
atmospheric pressure leading to worsening obstruction of flow in the
setting of extrathoracic lesions. An opposite effect happens for
intrathoracic lesions. During expiration, pleural pressure will become
positive when compared to intratracheal pressure, which can lead to
worsening obstruction of flow in the setting of an intrathoracic upper
airway lesion. This patient had a variable extrathoracic upper airway
obstruction due to tracheal stenosis as a late complication of his prior
tracheostomy. In addition to spirometry, this patient had a CT scan
which confirmed stenosis. He subsequently underwent bronchoscopic
intervention and dilation. Post procedure, his flow volume loop
normalized as below, panel B.
References
Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies
for lung function tests. Series "ATS/ERS Task Force: Standardisation of
lung function testing". Eur Respir J 2005; 26: 948-68.
Gamsu G, Borson DB, Webb WR, et al. Structure and function in tracheal stenosis. Am Rev Respir Dis. 1980; 121(3): 519–31.
