Types of Wheeze
Wheeze is classified into two main acoustic types: monophonic (single-note) and polyphonic (multiple-note), distinguished by their frequency characteristics, with monophonic wheeze typically indicating localized airway obstruction and polyphonic wheeze suggesting more diffuse airway narrowing as seen in asthma.
Acoustic Classification
By Number of Tones
- Monophonic wheeze: Single musical note, constant frequency throughout the respiratory cycle, suggests focal airway obstruction (e.g., tumor, foreign body, or single airway narrowing) 1
- Polyphonic wheeze: Multiple simultaneous musical notes at different frequencies, indicates widespread airway involvement typical of asthma or COPD 2
By Pitch/Frequency
The American Thoracic Society defines wheeze by frequency characteristics 3:
- High-pitched wheeze: Dominant frequency ≥400 Hz, the classic "wheeze" sound 3
- Low-pitched continuous sounds (rhonchi): Dominant frequency ≤200 Hz, often confused with wheeze but acoustically distinct 3
Important distinction: In infants, what parents and clinicians call "wheeze" is often actually ruttles—low-pitched (<600 Hz), continuous rattling sounds with irregular, non-sinusoidal waveforms lacking musical features. These differ fundamentally from true wheeze, which has sinusoidal waveforms with distinct spectral peaks 4. This misclassification leads to overestimation of wheeze prevalence in epidemiological studies of young children.
By Timing in Respiratory Cycle
Expiratory Wheeze
- Most common type, occurs during exhalation
- Appears late in respiratory cycle (mean 58% of maximal tidal expiratory flow) 5
- Pitch typically falls during the breath (mean decrease 187 Hz) 5
- Frequency range: 669 ± 100 Hz 5
- Indicates expiratory flow limitation
Inspiratory Wheeze
- Less common, occurs during inhalation
- Appears around maximal tidal inspiratory flows 5
- More variable pitch pattern than expiratory wheeze 5
- Frequency range: 710 ± 76 Hz 5
- Suggests different production mechanism than expiratory wheeze, possibly severe obstruction or extrathoracic airway involvement
Biphasic Wheeze
- Present in both inspiration and expiration
- Indicates more severe airway obstruction 6
Clinical Context Classification
By Associated Conditions 2
Specific cough/wheeze pointers (indicate underlying disease requiring investigation):
- Wet/productive wheeze: Suggests protracted bacterial bronchitis, bronchiectasis, or aspiration
- Dry wheeze with specific pointers: Consider asthma, but investigate for cardiac anomalies, interstitial lung disease, foreign body if other red flags present
Quality-based recognition 2:
- Barking/brassy wheeze: Tracheomalacia, croup
- Stridor-like wheeze: Upper airway obstruction, vocal cord dysfunction
- Musical wheeze: Classic asthma
Key Clinical Pitfalls
Wheeze timing matters for severity: Wheeze during tidal breathing (not just forced exhalation) indicates severe airflow limitation—it only appears after expiratory flow limitation is reached 5
Absence of wheeze doesn't exclude obstruction: Physical examination may be normal between asthma episodes; spirometry is essential 6. Patients' perception of obstruction is highly variable 6
Wheeze intensity/pitch doesn't correlate with FEV1: The duration of wheeze (tw/ttot—proportion of respiratory cycle occupied by wheeze) correlates best with obstruction severity, though too scattered for precise FEV1 prediction 3
Not all wheeze is asthma: Differential diagnosis includes foreign body, vocal cord dysfunction, vascular rings, COPD, heart failure, pulmonary embolism 6, 7
Infant wheeze is often viral and transient: Most young children with viral-induced wheeze have symptom remission by age 6 years 6. Only those with positive asthma predictive index (parental asthma, atopic dermatitis, OR ≥2 of: allergic rhinitis, >4% eosinophilia, wheeze apart from colds) progress to persistent asthma 8