Saw-Tooth Pattern on Flow-Volume Loop: Upper Airway Obstruction
A saw-tooth pattern on spirometry flow-volume loop indicates upper airway obstruction and should prompt immediate evaluation for upper airway pathology, including obstructive sleep apnea, tracheal masses, or other structural lesions causing airway collapsibility. 1
Mechanism and Significance
The saw-tooth sign represents flow oscillations caused by rapid intermittent changes in driving pressure or airway resistance during inspiration and expiration, reflecting upper airway collapsibility rather than lower airway disease. 1 This pattern is fundamentally different from the smooth flow-volume curves seen in COPD or asthma, where the European Respiratory Society guidelines describe end-expiratory curvilinearity (particularly with aging) or generalized concavity without oscillations. 2
Critical Diagnostic Pitfall in Smokers and Older Adults
In patients with smoking history or known COPD, a saw-tooth pattern should never be dismissed as simply representing obstructive lung disease. 3 A case report documented a patient with emphysema and tobacco abuse who presented with severe COPD symptoms refractory to standard therapy; the initial flow-volume loop showed what appeared to be distal airway obstruction consistent with severe COPD, but the patient actually had a large endotracheal mass causing the abnormality. 3 This led to significant diagnostic delay because clinicians attributed the spirometric findings to the patient's known COPD rather than recognizing the atypical upper airway pattern. 3
Specific Evaluation Required
When you identify a saw-tooth pattern, you must:
- Perform direct visualization of the upper airway through laryngoscopy or bronchoscopy to identify structural lesions, masses, or dynamic airway collapse 1, 3
- Evaluate for obstructive sleep apnea, as flow oscillations are widely described in this condition and represent a common cause of the saw-tooth sign 1
- Assess for variable intrathoracic or extrathoracic obstruction by examining both inspiratory and expiratory limbs of the flow-volume loop for plateau patterns, though these can be mimicked by poor effort if not reproducible across multiple maneuvers 2
Pattern Recognition from Guidelines
The European Respiratory Society's standardization of spirometry emphasizes that flow-volume loop shapes must be repeatable across three or more maneuvers for valid interpretation, particularly when evaluating for upper airway obstruction, as poor effort can mimic plateau effects. 2 The guidelines provide specific examples showing that variable intrathoracic obstruction produces expiratory flow limitation, while variable extrathoracic obstruction affects the inspiratory limb, and fixed obstruction creates plateau effects on both limbs. 2
Clinical Context Matters
In older adults with smoking history undergoing routine spirometry, the presence of a saw-tooth pattern is particularly concerning because:
- Age-related changes typically produce smooth end-expiratory curvilinearity, not oscillations 2
- Standard COPD patterns show smooth concave expiratory curves without flow oscillations 2
- The combination of respiratory symptoms refractory to standard COPD therapy plus atypical spirometric patterns demands investigation for alternative diagnoses 3
Visual inspection of the flow-volume loop is essential, and clinicians must recognize that flow oscillations should immediately trigger evaluation for upper airway pathology rather than being attributed to lower airway disease. 1