Computerised breath sounds analysis was used to assess the response of infants with wheeze and ruttles to the drug ipratropium bromide. A mean decrease in breath sounds intensity occurred in infants with ruttles after five minutes (20.5 dB), but not until 20 minutes in those with wheeze (8.1 dB). This differential response may be related to different underlying pathophysiology.
- breath sounds analysis
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The incidence of “wheeze” in early childhood appears to have increased over recent years. We have recently shown that many children with “wheeze” in the first year of life do not wheeze but exhibit a type of respiratory sound known as a ruttle.1 Ruttles are lower in pitch than wheeze, with a continuous rattling quality and lacking musical features. Characteristically parents are able to feel this noise as a vibration over the baby's back.
Classical wheeze is characterised acoustically by a sinusoidal waveform with distinct peaks in the power spectrum. Ruttles are represented by a more variable, non-sinusoidal waveform with diffuse, irregular peaks in the power spectrum display.2 These acoustic characteristics would suggest that the underlying pathophysiology differs in these two conditions. Wheeze is believed to originate from oscillation in large airways in response to small airways obstruction. The underlying mechanisms causing ruttles are unclear. They may represent excessive secretions or alterations in airways tone.
The objective of the study was not to assess benefits of treatment with ipratropium bromide, but to use the drug as a tool to investigate the differential response of ruttles and wheeze, in order to support the hypothesis that the sounds arise from a different mechanism.
Symptomatic infants with wheeze or ruttles were recruited from paediatric respiratory clinics, from inpatients admitted with acute viral induced respiratory illness, or from infants with ongoing noisy breathing in the community. Infants with bronchiolitis were not included. Approval for the study was obtained from the local ethics committee and informed consent was obtained from the infants' parents.
Infants for entry into the study were carefully selected by one of the investigators (MLE) and confirmed using breath sounds analysis by a second investigator (HEE). Only infants with clear characteristics of either wheeze or ruttles were selected as we wished only to include typical examples for the purpose of defining the response to ipratropium bromide. Infants with any other lung sounds or a combination of sounds were excluded from the study.
In total, 13 babies were selected: seven with wheeze (four boys, three girls; age range 7–10 months) and six with ruttles (two boys, four girls; age range 8–10 months).
Using a contact sensor attached to the right upper area of the chest anteriorly, we recorded lung sounds for a period of up to one minute. Air flow at the mouth was measured simultaneously using a pneumotachograph and facemask. The acoustic signals were analysed using a Fast Fourier Transformation technique. A customised computer program, RALE (Respiratory Acoustics Laboratory Environment, Pixsoft, Winnipeg) was used for data acquisition, analysis, and display.3 At expiratory flow rates of 0.1 l/s/kg, recordings were analysed for each infant before, and 5 and 20 minutes after administration of 40 μg ipratropium bromide through a spacer device.
We have previously defined the spectral characteristics of wheeze and ruttles.2 The dominant peak frequencies of both sounds were below 300 Hz. Frequencies less than 150 Hz are largely influenced by interference from the cardiovascular system.4 We therefore calculated the average sound intensity for the band of frequency 150–300 Hz.
Figure 1 shows a fall in breath sound intensity during expiration within five minutes of administering ipratropium bromide to the babies with ruttles. Mean change from baseline to 5 minutes was 20.5 dB. There was a further mean decrease in intensity from 5 to 20 minutes of 7.3 dB. This pattern of results was consistent for all subjects with ruttles.
In the babies with wheeze, there was a slight mean increase in intensity of 0.24 dB observed at 5 minutes; however, there was a mean decrease in average intensity at 20 minutes of 8.1 dB. Most of the overall response was accounted for by three of the subjects. Overall, the infants with wheeze had higher baseline intensity than those with ruttles.
Using acoustic analysis, we were able to identify a reduction in intensity of both wheeze and ruttles in symptomatic infants after inhalation of ipratropium bromide. The magnitude of the change was greater in those infants with ruttles. The time course of action appears to be different with a clear reduction in breath sounds intensity evident at 5 minutes in infants with ruttles but not until 20 minutes in those with wheeze.
The slow onset of action seen in the infants with wheeze is consistent with the known bronchodilator effects of ipratropium bromide, which peak at approximately 30 minutes.5 It is believed that the bronchodilator effects, which are mediated through blockade of cholinergic receptors, predominantly affect larger airways.6 The rapid onset of action in infants with ruttles would suggest that the improvement is not attributable to bronchodilation and that another mechanism is likely to be involved. It is possible that the response of ruttles may have been influenced by coughing or clearance of secretions. Ipratropium bromide can influence both airways secretions and airways tone, and its effect on ruttles may be mediated through one of these mechanisms.
Breath sounds analysis provides a non-invasive method for objective assessment of infants. No advice can be made about the treatment of symptoms from this observational study, which has simply identified a differential response to an inhaled anticholinergic in infants with ruttles and wheeze. The response of ruttles is earlier than the time course expected for a bronchodilator effect, suggesting that ruttles have a different pathogenesis from wheeze. It may be important to highlight the differences between wheezes and ruttles for future clinical and epidemiological studies of infants.