4.Discussion
To our knowledge, this is the first systematic review and meta-analysis
of IOS parameters to identify the risk of acute attack or loss of
control in children with asthma. Our research involved a total of 615
cases from 6 studies. We selected common IOS parameters and analyzed
them. The results showed that R5, X5, AX, and Fres may better predict
asthma exacerbations or loss of control.
Asthma is a common disease in pediatrics. The goal of care is to
maximize the safety of treatment by achieving and maintaining the
control of disease at the lowest level and dose of inhaled
corticosteroids (ICS). Studies have shown that childhood asthma control
is closely related to adult asthma control, lung function, and chronic
obstructive pulmonary disease (COPD)[21]. An acute
attack of asthma refers to the sudden occurrence of symptoms such as
wheezing, coughing, shortness of breath, chest tightness, or the acute
exacerbation of the original symptoms, which can even lead to dyspnea,
asphyxia and life-threatening. Therefore, timely and objective
indicators are needed for early identification and prevention. Pulmonary
ventilation function monitoring is an important means for future risk
assessment of asthma, but it has certain limitations. IOS has been
widely clinically studied because of its simple operation and its
ability to measure the impedance of the respiratory system, which
consists of reactance and resistance. Previous trials suggested that IOS
may be a reliable non-invasive method to assess asthma control in
children[22, 23].
Resistance can be conceptualized as the energy required for sound waves
to pass through the airways and inflate the lung. R5 represents the
total airway resistance. The included five trials found that R5
increased in asthma exacerbations[14-17, 19]. Only
Smith et al did not report the difference in R5 between the two
groups[17, 19]. Our study showed that R5 had a
good correlation with the acute attack of asthma. Besides, Galant et al
held that R5 increased significantly when peripheral airway obstruction
occurred[24]. The reactance of the respiratory
system, expressed in Xrs, consists of the inert and elastic properties
of the respiratory system. X5 is defined as the peripheral elastic
resistance, providing important information about the peripheral
respiratory tract. When small airway obstruction and lung compliance
decrease, the negative value will increase
significantly[25]. Our study found a good
correlation between X5 and asthma loss of control. Schulze et al found
that X5 represents hyperinflation[16].
Tirakitsoontorn et al also reported X5 may be well suited to identify
peripheral airway impairment phenotype in the clinical
setting[18]. Galant et al further confirmed that
X5 decreased significantly in the uncontrolled asthma
group[26]. Fres also known as resonant frequency
refers to the frequency when the elastic recoil of the airways and lungs
is equal and opposite to the inertance of the system, that is, the
reactance is zero. Moreover, based on the mechanical properties of
airways, Fres is the dividing point between large and small
airways[24]. Our result revealed that Fres had a
certain significance in predicting the acute attack of asthma, but there
was large heterogeneity (I2=85.1%,p=0.000). This may
be contributed to the small number of trials and the large age span
between the trials. AX means the area under the reactance curve between
5 Hz (X5) and the Fres value, similar to X5, which also can provide
information about distal airway obstruction[27].
Our research found a significant correlation between AX and acute
asthma.
Peripheral airways have much smaller lumina than central (large)
airways, and inflammation/edema in the walls of peripheral airways can
be expected to have a proportionately larger effect on lumen size than
inflammation/edema in larger airways. The above indicators reveal that
peripheral airway impairment may be associated with uncontrolled asthma.
Besides, previous studies have also reported that peripheral airway
impairment is clinically related to asthma exacerbation, uncontrolled
asthma, loss of control, and asthma severity[22,
23]. However, our study did not find the role of the R5-20 indicator
in predicting the loss of asthma control.R5-20 refers to the resistance
of the peripheral airways, which was also inconsistent with previous
research. This may be related to the age and height of the population
included in the study. When the peripheral airway was obstructed, R5 was
significantly elevated and R20 was not changed. However, R5 decreased
linearly with the growth of height[28].
Our meta-analysis had some limits. First, the severity of asthma
exacerbations or uncontrolled asthma and baseline treatment varied from
study to study, which may affect the results. In addition, other factors
like race, age, sex, height can also bring out the heterogeneity. These
indicate that a large number of randomized trials are still needed to
confirm the potential relationship between IOS parameters and acute
attack of asthma. Our research also had some advantages. For instance,
it is the first systematic review and meta-analysis about the predictive
value of the IOS common parameter in asthma exacerbations. And all
trials were of high quality.