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.