Forced Oscillation Technique for Monitoring the Respiratory Status of Children with Cystic Fibrosis: A Systematic Review

Spirometry is considered the gold standard method for monitoring lung function of patients with cystic fibrosis (CF) but it requires patients’ cooperation and therefore it is not useful for the majority of preschool-aged children. Oscillometry is an alternative modality for lung function monitoring that requires minimal cooperation and can be applied in children as young as 3 years of age. Furthermore, it generates lesser aerosol compared to spirometry, an issue that is of considerable importance in the COVID-19 era. The aim of this review was to present the existing clinical data regarding the application of oscillometry in children and adolescents with CF. The method seems to have acceptable feasibility and repeatability. However, there is conflicting data regarding the correlation of oscillometry values with the clinical symptoms of CF patients either in clinically stable or in exacerbation periods. Furthermore, it is not clear to what extent oscillometry measurements correlate with the spirometry indices. Based on current evidence, spirometry cannot be substituted by oscillometry in the monitoring of the respiratory status of children and adolescents with CF.

Perioperative changes in respiratory impedance in lobectomy and their clinical impact

Background

Respiratory function declines after lung resection. However, perioperative changes in respiratory impedance and their clinical significance are unclear. The forced oscillation technique can measure respiratory impedance during quiet breathing and possibly early after surgery. We investigated respiratory impedance changes before and after lung lobectomy and examined the correlation of impedance with clinical factors.

Methods

We prospectively included patients who underwent lobectomy between February 2018 and March 2020 and measured respiratory impedance by forced oscillation preoperatively and postoperative days 1 and 7. We statistically analyzed changes in perioperative forced oscillation measurements and their correlation with clinical factors, including subjective symptoms. The modified British Medical Research Council scale and the chronic obstructive pulmonary disease (COPD) assessment test were used for scoring subjective symptoms.

Results

Forty-four subjects were included, in whom respiratory impedance could be measured from postoperative day 1. The respective mean values for forced oscillation measurements preoperatively and at postoperative days 1 and 7 were as follows: respiratory resistance, 5 Hz: 2.28, 2.77, and 2.75; respiratory resistance, 20 Hz: 2.00, 2.36, and 2.32; difference in respiratory resistance at 5 and 20 Hz: 0.28, 0.40, and 0.43; respiratory reactance, 5 Hz: -0.31, -0.65, and -0.56; resonant frequency: 7.45, 10.41, and 9.81; and low-frequency reactance area: 1.33, 3.27, and 2.84. These changes were statistically significant (P<0.01). Besides the difference in respiratory resistance at 5 and 20 Hz, all other measurements on postoperative day 7 were relatively weakly correlated with the modified Medical Research Council scale score at this time point (all P<0.05). Respiratory complications correlated with the respiratory resistance difference, respiratory reactance, and resonant frequency on day 7 (R =0.415, -0.421, and 0.441), while the latter also correlated with postoperative hypoxemia on day 1 (R =0.433).

Conclusions

Respiratory impedance was measurable even early after surgery and significantly changed postoperatively. As the sample size was small and appeared to be biased, assessing respiratory impedance and clinical factors in detail was difficult. Since respiratory impedance is suggested to be associated with clinical factors that affect the postoperative course, it is necessary to accumulate cases and observe them over longer periods.

Airway closure is the predominant physiological mechanism of low ventilation seen on hyperpolarized helium-3 MRI lung scans

Hyperpolarized helium-3 MRI (³He MRI) provides detailed visualization of low- (hypo- and non-) ventilated lungs. Physiological measures of gas mixing may be assessed by multiple breath nitrogen washout (MBNW) and of airway closure by a forced oscillation technique (FOT). We hypothesize that in patients with asthma, areas of low-ventilated lung on ³He MRI are the result of airway closure. Ten control subjects, ten asthma subjects with normal spirometry (non-obstructed), and ten asthmatic subjects with reduced baseline lung function (obstructed) attended two testing sessions. On visit one, baseline plethysmography was performed followed by spirometry, MBNW, and FOT assessment pre and post methacholine challenge. On visit two, ³He MRI scans were conducted pre and post methacholine challenge. Post methacholine the volume of low-ventilated lung increased from 8.3% to 13.8% in the non-obstructed group (P = 0.012) and from 13.0% to 23.1% in the obstructed group (P = 0.001). For all subjects, the volume of low ventilation from ³He MRI correlated with a marker of airway closure in obstructive subjects, Xrs (6 Hz) and the marker of ventilation heterogeneity Scond with r² values of 0.61 (P < 0.001) and 0.56 (P < 0.001), respectively. The change in Xrs (6 Hz) correlated well (r² = 0.45, p < 0.001), whereas the change in Scond was largely independent of the change in low ventilation volume (r² = 0.13, P < 0.01). The only significant predictor of low ventilation volume from the multi-variate analysis was Xrs (6 Hz). This is consistent with the concept that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.NEW & NOTEWORTHY This study introduces a novel technique of generating high-resolution 3D ventilation maps from hyperpolarized helium-3 MRI. It is the first study to demonstrate that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.

Bronchodilator Response Predicts Longitudinal Improvement in Small Airway Function in World Trade Center Dust Exposed Community Members

The evolution of lung function, including assessment of small airways, was assessed in individuals enrolled in the World Trade Center Environmental Health Center (WTC-EHC). We hypothesized that a bronchodilator response at initial evaluation shown by spirometry or in small airways, as measured by forced oscillation technique (FOT), would be associated with improvement in large and small airway function over time. Standardized longitudinal assessment included pre and post bronchodilator (BD) spirometry (forced vital capacity, FVC; forced expiratory volume in 1 second, FEV₁) and FOT (resistance at 5 Hz, R₅; resistance at 5 minus 20 Hz, R_5–20). Longitudinal changes were assessed using linear mixed-effects modelling with adjustment for potential confounders (median follow-up 2.86 years; 95% measurements within 4.9 years). Data demonstrated: (1) parallel improvement in airflow and volume measured by spirometry and small airway function (R₅ and R_5–20) measured by FOT; (2) the magnitude of longitudinal improvement was tightly linked to the initial BD response; and (3) longitudinal values for small airway function on FOT were similar to residual abnormality observed post BD at initial visit. These findings suggest presence of reversible and irreversible components of small airway injury that are identifiable at initial presentation. These results have implications for treatment of isolated small airway abnormalities that can be identified by non-invasive effort independent FOT particularly in symptomatic individuals with normal spirometry indices. This study underscores the need to study small airway function to understand physiologic changes over time following environmental and occupational lung injury.

Respiratory function in healthy Emirati children using forced oscillations

BACKGROUND

The forced oscillation technique (FOT) allows the quantification of respiratory function, does not require active cooperation and as such is ideally suited for use in young children. The application of the FOT in non-Caucasian populations is limited and it remains unclear if current reference ranges for the FOT in Caucasian children are appropriate for children in the Arabian Peninsula. This study explored the use of the FOT in healthy school-aged children in the United Arab Emirates (UAE).

METHODS

Lung function using FOT was assessed in healthy Emirati children aged 4-12 years. Using a validated questionnaires general medical and respiratory histories were obtained. Prediction equations for resistance (Rrs), reactance (Xrs), and area under reactance curve (AX) were calculated and compared to previous studies.

RESULTS

FOT was successfully obtained in 291 healthy Emirati children. Linear regression modelling including weight, height, age, and gender in transformed FOT outcomes demonstrated that height was the strongest predictor of FOT outcomes (P < 0.001). Using the equated prediction equation Z-scores were calculated. FOT outcomes in Emirati children did not match previously published reference equations in Caucasians children (P < 0.001).

CONCLUSION

FOT measurements were feasible in Emirati school-children. New FOT reference equation in Emirati children were derived. FOT reference equation in Emirati children was different from published equations in Caucasians.

Isolated small airway reactivity during bronchoprovocation as a mechanism for respiratory symptoms in WTC dust-exposed community members

INTRODUCTION

Small airway dysfunction occurs following WTC dust exposure, but its role in producing symptoms is unclear.

METHODS

Methacholine challenge (MCT) was used to assess the relationship between onset of respiratory symptoms and small airway abnormalities in 166 symptomatic WTC dust-exposed patients. Forced oscillation testing (FOT) and respiratory symptoms were assessed during MCT. FOT parameters included resistance at 5 and 20 Hz (R5 and R20 ) and the R5 minus R20 (R5-20 ).

RESULTS

Baseline spirometry was normal in all (mean FEV1 100 + 13% predicted, mean FEV1 /FVC 80 + 4%). MCT revealed bronchial hyperreactivity by spirometry in 67 patients. An additional 24 patients became symptomatic despite minimal FEV1 change (<5%); symptom onset coincided with increased R5 and R5-20 (P > 0.001 vs. baseline). The dose-response of FOT (reactivity) was greater compared with subjects that remained asymptomatic (P < 0.05).

CONCLUSIONS

FOT during MCT uncovered reactivity in small airways as a mechanism for respiratory symptoms in subjects with inhalational lung injury. Am. J. Ind. Med. 59:767-776, 2016. © 2016 Wiley Periodicals, Inc.

Breathing-related changes of respiratory resistance in vocal cord dysfunction

Vocal cord dysfunction induced by exercise in children with uncontrolled asthma was identified by laryngoscopy. The paradoxical adduction of the vocal cords was also indicated by the breathing-related changes of the forced oscillation respiratory resistance showing prominent increase during inspiration and a large positive difference between inspiration and expiration. The breathing-related changes of respiratory resistance offer thus a useful first-line technique to diagnose vocal cord dysfunction.

Airway obstruction, upper airway artifact and response to bronchodilator in asthmatic and healthy children

BACKGROUND

The forced oscillation technique (FOT) may be affected by the upper airway artifact in children with airway obstruction. The aim was to test the impact of the artifact on FOT ability to diagnose asthma in children as recommended by current guidelines.

METHODS

The FOT was performed in 58 asthmatics and 27 controls before and after salbutamol. Respiratory resistance (Rrs) was measured at 8 Hz with a standard generator (SG), and a head generator (HG) that minimizes the upper airway artifact. The response to salbutamol was computed as change in respiratory resistance (Δ%Rrs) and admittance (Δ%Ars) that almost cancels the effect of the upper airway artifact.

RESULTS

Rrs and Δ%Rrs were significantly larger in patients than controls by HG (respectively P < 0.001 and P = 0.002) but not SG, while Δ%Ars was larger in asthma than control by both (P < 0.04). Best discriminators between patients and controls were Rrs or Δ%Rrs by HG and Δ%Ars by SG.

CONCLUSION

In asthmatic children, the upper airway artifact significantly impacts FOT measurements. The diagnostic value may be improved by minimization of the shunt, such as the computation of Δ%Ars.

Comparison of two devices for respiratory impedance measurement using a forced oscillation technique: basic study using phantom models

Since commercial forced oscillation technique (FOT) devices became available, they have been widely used for physiological assessments, mainly of obstructive lung diseases. However, it is not known whether the impedance values measured with different devices are identical. In this study, two FOT devices-the impulse oscillometry system (IOS) and the MostGraph (MG)-were compared using phantom models. The resistance values varied up to 10% from estimated values in both devices. Additionally, there was a difference in frequency dependence for the resistance between the devices. The reactance values measured with MG were higher than those measured with IOS. The effects of ventilation on the measured impedance values were higher for IOS than for MG, especially at lower frequencies. We concluded that the devices do not always generate identical impedance values. Thus, differences between the devices should be taken into consideration when evaluating clinical data.

A modeling exploration of how synaptic feedback to descending projection neurons shapes the activity of an oscillatory network

Rhythmic activity which underlies motor output is often initiated and controlled by descending modulatory projection pathways onto central pattern generator (CPG) networks. In turn, these descending pathways receive synaptic feedback from their target CPG network, which can influence the CPG output. However, the mechanisms underlying such bi-directional synaptic interactions are mostly unexplored. We develop a reduced mathematical model, including both feed-forward and feedback circuitry, to examine how the synaptic interactions involving two projection neurons, MCN1 and CPN2, can produce and shape the activity of the gastric mill CPG in the crab stomatogastric nervous system. We use simplifying assumptions that are based on the behavior of the biological system to reduce this model down to 2 dimensions, which allows for phase plane analysis of the model output. The model shows a distinct activity for the gastric mill rhythm that is elicited when MCN1 and CPN2 are co-active compared to the rhythm elicited by MCN1 activity alone. Furthermore, the presence of feedback to the projection neuron CPN2 provides a distinct locus of pattern generation in the model which does not require reciprocally inhibitory interactions between the gastric mill CPG neurons, but is instead based on a half-center oscillator that occurs through a tri-synaptic pathway that includes CPN2. Our modeling results show that feedback to projection pathways may provide additional mechanisms for the generation of motor activity. These mechanisms can have distinct dependence on network parameters and may therefore provide additional flexibility for the rhythmic motor output.

Vitamin A deficiency alters airway resistance in children with acute upper respiratory infection

OBJECTIVE

To assess whether vitamin A deficiency alters the recovery of total respiratory resistance (TRR) following acute upper respiratory tract infection (URI).

METHODS

This is a case control study of children, age 4-6 years and grouped as: URI, (n = 74), URI and wheezing, (URI-wheezing, n = 52), and healthy controls (n = 51). Vitamin A and total respiratory resistance (TRR) were assessed using the modified relative dose response (MRDR) and forced oscillometry, respectively.

RESULTS

Children with URI and URI-wheezing had lower retinol, 32.4 ± 13.12 and 18.3 ± 6.83 µg/dl respectively, compared to controls, 56.9 ± 29.82 µg/dl (ANOVA, P < 0.001). The MRDR was elevated in children in the URI or URI-wheezing groups 0.066 ± 0.045 and 0.021 ± 0.021, respectively, compared to controls 0.007 ± 0.006 (ANOVA, P < 0.0001). The TRR in the URI and URI-wheezing groups differed from controls. During convalescence, the TRR failed to decline in the URI-group only when the MRDR was >0.06. In the URI-wheezing group, TRR declined independently of retinol and MRDR.

CONCLUSION

Vitamin A contributes to preservation of airway function during and in recovery after upper respiratory infection in children.

Issues determining direct airways hyperresponsiveness in mice

Airways hyperresponsiveness (AHR) is frequently a primary outcome in mouse models of asthma. There are, however, a number of variables that may affect the outcome of such measurements and the interpretation of the results. This article highlights issues that should be kept in mind when designing experiments using AHR as an outcome by reviewing techniques commonly used to assess AHR (unrestrained plethysmography and respiratory input impedance using forced oscillations), discussing the relationship between structure and function and, then exploring how the localization of AHR evolves over time, how the airway epithelium may affect the kinetics of methacholine induced AHR and finally how lung volume and positive end expiratory pressure (PEEP) can be used as tools assessing respiratory mechanics.

Measuring the lung function in the mouse: the challenge of size

Measurement of the effects of drugs, mediators and infectious agents on various models of lung disease, as well as assessment of lung function in the intact mouse has the potential for significantly advancing our knowledge of lung disease. However, the small size of the mouse presents significant challenges for the assessment of lung function. Because of compromises made between precision and noninvasiveness, data obtained may have an uncertain bearing on the mechanical response of the lung. Nevertheless, considerable recent progress has been made in developing valid and useful measures of mouse lung function. These advances, resulting in our current ability to measure sophisticated indices of lung function in laboratory animals, are likely to lead to important insights into the mechanisms of lung disease.

Clinical applications of forced oscillation techniques (FOT) in patients with bronchial asthma

BACKGROUND

Forced oscillation technique (FOT) is a method to characterize the mechanical properties of the respiratory system over a wide range of frequencies. Its most important advantage is to require minimal cooperation from the subject. This study was performed to evaluate the usefulness of the FOT applications in patients with bronchial asthma by estimating the associations between asthma severity and FOT parameters, and the relationships between FOT and spirometry parameters.

METHODS

216 patients with asthma were enrolled in this study. Patients were classified into 3 different groups according to their symptoms and pulmonary functions. Respiratory impedance, resistance (at 5 Hz, 20 Hz, 35 Hz) and resonant frequency were measured by FOT. FEV1, FVC and MMEF were measured with conventional spirometry.

RESULTS

There were significant differences of resonant frequency, resistance at 5 Hz and 20 Hz, resistance difference at 5 Hz and 20 Hz according to asthma severity (p < 0.05, respectively). Resonant frequency, resistance at 5 Hz, and impedance were significantly correlated with FEV1 (r = -0.55, -0.48, -0.49, p < 0.05, respectively), and with MMEF in patients with normal pulmonary function (r = -0.37, -0.35, -0.34, p < 0.05, respectively). Resistance at 5 Hz had similar reproducibility compared to FEV1 (resistance at 5 Hz, r = 0.78 vs FEV1, r = 0.79).

CONCLUSION

FOT is a useful and alternative method to evaluate the clinical status of bronchial asthma. Further studies will be needed to clarify its value for a wide range of clinical applications.