Discussion
Recent advances in breath sound analyses have been
remarkable10, 11). In clinical practice, attempts have
been made to use breath sound analyses for the obstructive evaluation of
airway narrowing23, 24), and the technique is
improving18, 22). Since the measurement of breath
sounds is non-invasive, simple and can be obtained with effortless
breath, studies are underway concerning its application for the
evaluation of respiratory diseases in children16) and
infants14) who cannot perform normal lung function
tests.
In the present study, of note, the breath sound spectrogram of
expiration in acute bronchiolitis was found to be characteristic. As
previously mentioned25, 26), the breath sounds of
acute phase of acute bronchiolitis show marked power that is not
observed in the breath sound spectrogram of healthy children27) or children with mild exacerbation of
asthma16). According to reports of the
pathophysiology28), epithelial damage and desquamation
that introduce clusters of desquamated epithelial cells and viscous
sputum29) are severe in the patients with acute
bronchiolitis. Thus, a remarkable increase in the expiratory sound power
of infants with acute bronchiolitis may be induced by severe stenosis of
airways associated with these causes.
In this report, attending physicians described the presence of rhonchi,
which are low-pitched continuous-expiratory sounds30),
as a respiratory adventitious sound during expiration in many patients
detected on auscultation. Widely, acute bronchiolitis is known to cause
wheezes2), which are high-pitched
continuous-expiratory sounds30), but our data included
fewer patients with wheezes than previous reports on breath
sounds25, 26). Typical rhonchi and wheezes are
continuous musical sounds31) that are easy to identify
as a sinusoidal waveform in the breath sound
spectrogram27). However, based on examination of
breath sound spectrograms, the expiratory sounds may differ from the
waveform sounds of rhonchi and wheezes which commonly found in asthmatic
children. We suspect that the non-uniform, band-like sounds in the
low-pitched area seen in Figure 1 were heard as rhonchi. The sound
spectrum of the expiration showed several repetitive peaks (Figure 2),
which we presumed to be a pulse train or “complex repetitive
waveforms”, as suggested by Tal, et al.25). We will
continue to investigate the cause of such characteristic sound in
infants with acute bronchiolitis.
Due to the increased power of the expiration, the E/I values were
extremely large, showing a significant decrease in the recovery period.
The E/I MF values in the recovery period were almost the same as those
in healthy infants (the median value of E/I MF in 26 healthy 1-month-old
infants who visited our hospital=0.05, private data). The airway
stenosis in the acute bronchiolitis28, 29) may be
stronger than that in the acute exacerbation of
asthma32), although these results cannot be simply
compared, as the airway portion with stenosis may differ between these
conditions, and the subjects with acute bronchiolitis were younger than
asthmatic children on average9, 33).
Despite these uncertainties, we speculate that our outstanding finding
will prove useful for the diagnosis of acute bronchiolitis in infants.
Although this study was a single-institutional, short-term study, we
were able to limit the target to patients with RSV-induced acute
bronchiolitis within the same season in an environment where pediatric
respiratory tract infections are almost nonexistent due to the COVID-19
epidemic. Acute bronchiolitis should produce special sounds different
from bronchoconstriction induced by smooth muscle
constriction33), which may be particularly meaningful
in distinguishing this condition from infantile asthma.
According to previous reports of adults, the E/I LF value in healthy
adults is 0.36, which is equivalent to that in infants, although the E/I
MF value is large at 0.2717). This difference was
perplexing, as the I MF value of infants (87.2 dB) was larger than that
of adults (57.3 dB) 17). The cause of this result is
speculated to be dependent on the difference in airway diameter,
respiratory flow rate (L/sec) and respiration pattern between adults and
infants.
The values of the four sound spectrum curve indices in the inspiratory
sounds13, 16) (A3/AT,
B4/AT, RPF75 and
RPF50) being significantly increased during the recovery
period is attributed to the presence of bronchial constriction during
the acute phase, based on previous breath sound analysis
results15). In the present study, respiratory symptoms
have improved in all patients at the second visit, and there were no
adventitious sounds with auscultation. Thus, the changes in breath
sounds during the recovery period are considered to indicate direct
improvement of the airways.
More interestingly, a significant correlation was found between the E/I
MF and SpO2, and even the severity score of
bronchiolitis19). Although no correlation was found
with the length of hospital stay, the correlation with the severity
score used in clinical practice seems to be helpful for creating more
objective severity scores. A number of studies have reported that
RSV-induced acute bronchiolitis was associated with recurrent wheezing
in later childhood3) and that the rate of development
of asthma was not small4). Our results seems to be
important for establishing more precise severity scores with an
objective markers.
One limitation of our report is that we were unable to clearly
demonstrate the specificity of the acute bronchiolitis due to the lack
of data on the healthy age-matched infants (1-3 months old). Thus, we
hope to explore this a topic in the future. Furthermore, we were not
able to indicate whether or not the same findings would be obtained in
other cases of infantile wheezing diseases with
dyspnea33). However, this problem is not easily
resolved, as the infants with acute bronchiolitis are mainly
<6 months old9), and asthma, which is a
representative of childhood wheezing diseases with dyspnea, cannot be
diagnosed at this period33, 34). In a study of older
children with asthma, it was reported that an intentional loud breathing
increases the power of respiration35). There is no
doubt that the breathing at rest in infants with acute bronchiolitis is
greater than that in normal infants, so the specificity of breath sounds
of acute bronchiolitis must be evaluated under such situation. However,
the apparent significant increase in the expiration power is very clear,
and we hope to evaluate this issue in the future.
Many discussions have been held concerning the effective treatment of
acute bronchiolitis. According to the previous reports, inhaled
β2 agonists have been shown to be ineffective, while
inhaled adrenaline, inhaled saline, systemic steroids and their
combination have been reported to be effective5-7). We
hope to revisit our evaluation of the advantages of a breath sound
analysis13, 16). Objective data over time using breath
sound analyses may facilitate a clear evaluation of the effect of
pharmacotherapy. As there are no clinical objective lung function tests
available for infants33, 34), our findings are
considered clinically meaningful. We are now considering a method to
objectively evaluate the effects of drugs using a breath sound analysis.
Our results confirmed the specificity of the breath sounds in
RSV-induced acute bronchiolitis. It has been reported that the breath
sound directly reflects the condition of airways 13,
16). Our results seem to be useful for differentiating the diagnosis of
acute bronchiolitis from other wheezing diseases, creating a more
accurate severity scores and determining the objective effect of
treatment.