Authors Kunal Jakharia, MD, Fellow, Division of Pulmonary Diseases and Critical Care Medicine, University of North Carolina, Chapel Hill M. Leigh Anne Daniels, MD MPH, Clinical Instructor, Division of Pulmonary Diseases and Critical Care Medicine, University of North Carolina, Chapel Hill
Case
Introduction
A 52-year-old male with hypertension, chronic kidney disease, COPD, and obesity (BMI of 31 kg/m2)
presented to a primary care physician to establish care. He endorsed
good compliance with ICS/LABA plus LAMA for his presumed clinical
diagnosis of COPD, despite one exacerbation in the last year. He
reported mild morning headaches and some daytime fatigue. He denied any
voice changes or hoarseness.
Spirometry performed pre-and post-bronchodilator is shown below.
Question
In addition to obstructive pathology, what do these PFTs suggest?
A. Variable intrathoracic obstruction B. Obstructive sleep apnea C. Fixed upper airway obstruction D. Poor bronchodilator response
Answer
B. Obstructive sleep apnea
Discussion
This patient’s flow volume (FV) loop has a classic saw-tooth pattern
with oscillations in both the inspiratory and expiratory curves. It was
previously thought that this saw-tooth pattern could be attributed to
artifacts in the resonance frequency from the spirometry equipment (1).
Later, these flow oscillations were seen more commonly in patients with
sleep-disordered breathing and those with upper airway abnormalities.
Although the sensitivity of the saw-tooth pattern for OSA is low (11%),
some studies demonstrated specificities as high as 94% (2) while others
failed to replicate it (3). However, it has also been described in 31%
of individuals who snore but do not carry a diagnosis of sleep apnea, as
well as in 10% of normal individuals (2). The presence of this pattern
has not shown to correlate with AHI or other metrics of severity in OSA
(3).
The oscillations are defined as a reproducible sequence of
alternating decelerations and accelerations of flow creating a
‘saw-tooth’ pattern; this pattern is superimposed on the patient’s
baseline contour of the FV loop (4). They can occur in any portion of
the FV loop. The saw-tooth pattern can also be present in a variety of
conditions affecting the upper airway. Fluttering and tremors are
thought to be the two general mechanisms causing these oscillations on
the FV loops. Turbulence is caused by flaccidity of upper airway
structures due to low muscle tone or increased unsupported adipose
tissue. This leads to fluttering of the redundant tissue during
inspiration and expiration causing oscillations (4). Tremors of glottis
and supraglottic structures can also produce oscillations of the FV
loop, however they are generally low in frequency. These tremors are
generally neurological in origin and do not arise from turbulence.
Common causes of the saw-tooth pattern due to turbulence (4):
Obstructive sleep apnea
Upper airway tumors/obstruction
Upper airway burn injuries (due to damage to the supporting structures of upper airways)
Tracheobronchomalacia
Snorers without OSA
Common causes of saw-tooth pattern due to tremors (4):
Parkinson's disease
Neuromuscular disorders with bulbar involvement
Laryngeal dyskinesia
van Leeuwenhoek’s disease (rare disorder characterized by rapid, involuntary diaphragmatic contractions)
Posture can be an important factor in saw-toothing of the FV loop.
The sensitivity of this pattern to detect OSA increased from 11% in
sitting position to nearly 40% in supine position (5). This re-enforces
the concept that loose unsupported tissue plays a role in the
oscillations.
A similar pattern can be seen when a patient frequently coughs
(Figure 2) or tries to produce a sound during the maneuver. However, it
is not reproducible in each attempt. In contrast, the saw-tooth pattern
is reproducible in the causes listed above and does not impact the
accuracy of the PFT either.
Our patient underwent a polysomnogram and was diagnosed with moderate
obstructive sleep apnea. An ENT evaluation ruled out laryngeal and
vocal cord dysfunction and upper airway tumors.
References
Zamel N. Flow volume curve: the "saw-tooth" sign. Eur J Respir Dis 1986; 69:73.
Katz I, Zamel N, Slutsky AS, et al. An evaluation of flow-volume
curves as a screening test for obstructive sleep apnea. Chest 1990;
98:337.
Ashraf M, Shaffi SA, BaHammam AS (2008) Spirometry and flow-volume
curve in patients with obstructive sleep apnea. Saudi Med J 29:198–202
Vincken WG, Cosio MG. Flow oscillations on the flow-volume loop:
clinical and physiological implications. Eur Respir J 1989; 2: 543-549.
Shore ET, Millman RP. Abnormalities in the flow-volume loop in
obstructive sleep apnea sitting and supine. Thorax 1984; 39: 775-779.
