Sawtooth pattern on the flow-volume loop

One of the recognized abnormalities of a flow-volume loop is a sawtooth profile due to flow oscillations that are superimposed on either the maximal expiratory or inspiratory flow curve, or the tidal loop.

FVL_Sawtooth_2

Sawtooth pattern on a flow-volume loop

The sawtooth pattern occurs in only a small fraction of patients but it is quite noticeable when you see it. Estimates of the number of individuals with flow oscillation range from 1.4% to 13.4% with the higher estimates being observed primarily with inspiratory loops. Since many spirometry efforts are concentrated solely on exhalation this means that it may frequently go unrecognized. Recently, I had several reports with distinct sawtooth flow-volume loops come across my desk within a short time period and for this reason thought it might be interesting to re-visit this old subject. I call it old only because most of the research on sawtooth profiles was done in the 1970’s and 1980’s and not much has been published since then.

The sawtooth pattern was first noted in the late 1960’s when flow-volume loops came into common clinical use. To some extent these flow oscillations were initially dismissed because they were thought to be noise from either the spirometry equipment, the amplifiers or the X-Y recorder. For this reason many of the published flow-volume loop waveforms of that time were smoothed to remove these oscillations.

The first investigators to recognize flow oscillations as a real phenomena thought that they correlated primarily with Obstructive Sleep Apnea (OSA) and could be used as an aid to diagnosis. Research since then has shown that most patients with OSA do not have a sawtooth pattern on their flow-volume loop and that what a sawtooth pattern usually correlates with instead is with upper-airway abnormalities from a variety of conditions. Investigators have shown that as well as OSA sawtooth flow profiles can occur in:

  • Snorers without OSA
  • Airway tumors
  • Use of inhaled steroids
  • Upper airway stenosis
  • Parkinson’s Disease
  • Neuromuscular disorders with bulbar involvement
  • Leewanhoek’s Disease
  • Burn Injury of the upper airway

The upper airway has a number of structures that are capable of affecting airway resistance.

Upper Respiratory Tract 2

Upper airway (taken from biology-forums.com)

There are two general mechanisms that can cause flow oscillations: fluttering and tremors. Fluttering is partly due to turbulence and partly due to loose tissue. When the structures in the airway are flaccid due to low muscle tone or extra unsupported tissue, it can narrow the airway and when the airway is narrowed turbulence increases. Turbulence in turn causes the loose tissue to flutter and this shows up as flow oscillations. Fluttering is likely the mechanism for flow oscillations in OSA, tumors, burn injuries and other lesions to the upper airway. Flow oscillations are seen more frequently during inspiration is because during inspiration there is negative pressure in the airway which causes the airway to narrow further and increases the likelihood of turbulence. Flow oscillations due to turbulence tend to have a relatively high frequency (>20 Hz).

In extrapyrimidal disorders (Parkinson’s disease) both rhythmic and irregular tremors of the glottic and supraglottic structures have been observed with fiberoptic endoscopes. Vocal cord tremors have been noted in some Parkinson’s patients as well. In motor neuron disease with bulbar involvement tremors can occur in the laryngeal and pharyngeal muscles. Notably these abnormal tremors often persist during breathholding which means they are not caused by airway turbulence but by phasic innervation/denervation of the muscles. Flow oscillations due to tremors tend to be relatively low frequency (4-8 Hz).

Posture can be a factor in the ability to detect flow oscillations. Although most OSA patients with sawtoothing showed this pattern in both the upright and the supine position some OSA patients that did not show sawtoothing while upright did show it while supine.

Like fine art, everybody knows what flow oscillations are when they see them but coming up with an objective definition has been difficult. Various definitions include:

  • “…three or more consecutive peaks and troughs, with an amplitudes of 50 to 500 ml/sec and a maximum width of 10% of FVC occurring … in the middle 80% of the expiratory or inspiratory limb…”
  • “…three or more consecutive peaks and troughs of no greater than 300 ml during the middle half of the vital capacity…”
  • “…a minimum of three oscillations should be present…”
  • “…sawtooth pattern was considered to be present…if at least two out of three readers reported its presence.”
  • “…flow oscillations were defined as changes in the direction of flow (ie, consecutive flow decelerations and accelerations) superimposed on the inspiratory or expiratory flow-volume curves…”

I doubt that these different definitions have made a significant difference in the ability of researchers to assess flow oscillations but without a consensus of some kind it will probably be difficult to develop a computer algorithm to detect them, assuming this ever becomes desirable.

Many of the mechanical and electronic issues that affected the fidelity of early flow-volume loops have long since been overcome. Even though flow oscillations have a relatively high frequency (usually between 4 and 40 Hz) the digital acquisition rate of even simple spirometry systems is usually high enough that flow oscillations can be accurately recorded. The current obstacle towards recognizing flow oscillation has more to do with the resolution used to display or print the flow-volume loop. It doesn’t matter how accurately a flow-volume curve was recorded if it is going to be printed as a 1-inch rectangle. I mention this because my lab’s software came with some sample report formats and one included incredibly tiny flow-volume loops where the overall contour of the loop would have been difficult to discern, let alone any flow oscillations.

The sawtooth profile on flow-volume loops is seen in many diseases primarily involving the upper airway (and in rare instances from synchronous tremors in the respiratory musculature). Although it has a clear association with upper airway obstruction it is not a specific sign of any disease and it’s presence or absence does not tend to indicate a greater or lesser degree of disease severity. For these reasons flow oscillation has limited clinical relevance. The best it can do is to point towards upper airway involvement when this has not been previously suspected and this alone is sufficient reason its presence should be noted when it is noticed.

References:

Amado VM, Costa ACGA, Guiot M, Viegas CA, Tavares P. Inspiratory flow-volume curve in snoring patients with and without obstructive sleep apnea. Braz J Med Biol Res 1999; 32: 407-411.

Bogaard JM, Hovestadt A, Meerwaldt J, Mech FGAvd, Stigt J. Maximal expiratory and inspiratory flow-volume curves in Parkinson’s disease. Am Rev Respir Dis 1989; 139: 610-614.

Hadjikoutis S, Wiles CM. Respiratory complications related to bulbar function in motor neuron disease. Acta Neurol Scand 2001; 103: 207-213.

Haponik EF, Smith PL, Kaplan J, Bleecker ER. Flow-volume curves and sleep-disordered breathing: therapeutic implications. Thorax 1983; 38: 609-615.

Hoffstein V, Wright S, Zamel N. Flow-volume curves in snoring patients with and without obstructive sleep apnea. Am Rev Respir Dis 1989; 139: 957-960.

Krieger J, Weitzenblum E, Vandevenne A, Stierle JL, Kurtz D. Flow-volume curve abnormalities and obstructive sleep apnea syndrome. Chest 1985; 87(2): 163-167.

Neukirch F, Weitzenblum E, Liard R, Korobaeff M, Henry C, Orvoen-Frija E, Kaffmann F. Frequency and correlates of the saw-tooth pattern of flow-volume curves in an epidemiological survy. Chest 1992; 101: 425-431.

Rendleman N, Quinn SF. The answer is blowing in the wind: a pedunculated tomour with saw tooth flow-volume loop. J Laryng Otology 1998; 112: 973-975.

Shore ET, Millman RP. Abnormalities in the flow-volume loop in obstructive sleep apnoea sitting and supine. Thorax 1984; 39: 775-779.

Vincken W, Elleker G, Cosio MG. Detection of upper airway muscle involvement in neuromuscular disorders using the flow-volume loop. Chest 1986; 90(1): 52-57.

Vincken WG, Cosio MG. Flow oscillations on the flow-volume loop: clinical and physiological implications. Eur Respir J 1989; 2: 543-549.

Creative Commons License
PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

2 thoughts on “Sawtooth pattern on the flow-volume loop

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.