Forced Oscillometry
Forced oscillometry (FO) uses measurement of pressure and flow in
response to an oscillatory pressure wave introduced at the airway
opening to measure respiratory system impedance (Z) [1]. Z is a
generalized concept of resistance that incorporates the in phase and out
of phase relationships between pressure and flow. The in phase element
of Z is resistance (R), while the out of phase element is represented by
reactance (X). R is composed of the forces associated with frictional
losses in the airways and the lung parenchyma. X is composed of an
inertive element (Xi), which represents the inertive forces of the
respiratory system, and a capacitant element (Xc), which reflects the
visco-elastic properties of the lung. At low frequencies, Xc
predominates over Xi. Because FO does not require a maximal forced
expiratory maneuver, it is easier to obtain measurements from young
children compared to spirometry. X and R at low frequency may also
reflect small airways function better than spirometric measures such as
the forced expiratory volume in 1 second (FEV1).
There is increasing evidence that the lungs are affected by diabetic
microangiopathy. Anik, et al performed FO and spirometry on children
with type 1 diabetes mellitus (T1DM) and compared the results to data
from age matched healthy controls [2]. R at 5, 10, and 20 Hz (R5,
R10, and R20 respectively) was significantly higher in children with
T1DM compared to controls, and the forced expiratory flow at 75% of
forced vital capacity (FEF75) and between 25-75% of
forced vital capacity (FEF25-75) were both significantly
lower. Lung function was worse in children with poorly controlled T1DM
compared to well-controlled T1DM. These results demonstrate subclinical
impairment of lung function in children with T1DM and suggest that
monitoring of lung function in T1DM should be a part of T1DM care.
Zheng, et al studied whether impulse oscillometry (IOS), a form of FO,
and the fraction of exhaled nitric oxide (FeNO) could be used to predict
future asthma exacerbations in preschool children [3]. They were
able to identify a cutoff value for the area under the reactance curve
(AX), a measure of total low frequency X, that predicted future loss of
asthma control, while FeNO did not have strong predictive value in this
patient population. Their report suggests that IOS could be used to
monitor preschool children for future risk of asthma exacerbation.
Lundberg, et al investigated the feasibility and correlation between
spirometry and IOS in 6 year old children born extremely premature
[4]. The success rate of IOS was significantly higher in both term
and preterm children compared to spirometry (93% vs 60%). There was
moderate correlation between spirometry measurements and IOS
measurement, suggesting that IOS could be considered as an alternative
test in children who cannot perform spirometry.
Veneroni, et al used FO to compare respiratory mechanics in preterm
infants treated with continuous positive airway pressure (CPAP) versus
CPAP with sustained inflations of 25 cmH2O for 15 seconds [5].
Infants presented with highly heterogeneous degrees of lung aeration at
birth, limiting their ability to use FO to assess ventilation
strategies. However, this pilot study did demonstrate the feasibility of
obtaining FO measurements at birth in preterm infants.