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Recently, "Technical standards for respiratory oscillometry" was published, which reviewed the physiological basis of oscillometric measures and detailed the technical factors related to equipment and test performance, quality assurance and reporting of results. Here we present a review of the clinical significance and applications of oscillometry. We briefly review the physiological principles of oscillometry and the basics of oscillometry interpretation, and then describe what is currently known about oscillometry in its role as a sensitive measure of airway resistance, bronchodilator responsiveness and bronchial challenge testing, and response to medical therapy, particularly in asthma and COPD. The technique may have unique advantages in situations where spirometry and other lung function tests are not suitable, such as in infants, neuromuscular disease, sleep apnoea and critical care. Other potential applications include detection of bronchiolitis obliterans, vocal cord dysfunction and the effects of environmental exposures. However, despite great promise as a useful clinical tool, we identify a number of areas in which more evidence of clinical utility is needed before oscillometry becomes routinely used for diagnosing or monitoring respiratory disease., Competing Interests: Conflict of interest: D.A. Kaminsky reports personal payments made as faculty speaker for Cardiorespiratory Diagnostics Seminar from MGC Diagnostics, Inc. outside the submitted work. Past Chair of ATS Proficiency Standards for Pulmonary Function Laboratories Committee, unpaid. Conflict of interest: S.J. Simpson has nothing to disclose. Conflict of interest: K.I. Berger has nothing to disclose. Conflict of interest: P. Calverley reports receiving consulting fees paid by Phillips Respironics for advisory work on a novel COPD ventilator. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events received from Phillips Respironics, outside the submitted work. Conflict of interest: P.L. de Melo reports patent 28727 issued. Conflict of interest: R.J. Dandurand reports grants or contracts paid to the institution from AstraZeneca, Boehringer-Ingelheim, Covis Pharma, Grifols, MGC Diagnostics, Teva Pharma, Thorasys, and Vyaire, outside the submitted work. Speaking payment from Novartis for L'oscillométrie en clinique: qu'ajoute-t-elle aux évaluations pulmonaires?, 18 September 2019, Boehringer-Ingelheim for L'oscillométrie: vieille physiologie avec un avenir brilliant, 17 November 2020, and Latin American Respiratory Physiology Society for Oscillometry in Asthma and COPD: Interpretation Strategies, 14 November 2020, outside the submitted work. Chairman, Oscillometry Harmonisation Study Group, an international committee academic and industry experts working to standardise oscillometry devices, and Chairman, Respiratory Effectiveness Group Technologies Working Group, Cambridge, U.K. (https://www.regresearchnetwork.org). Conflict of interest: R.L. Dellacà reports royalties or licenses from Restech, Philips and Vyaire. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events received from Restech and Philips, outside the submitted work. Support for attending meetings from Philips and Vyaire. Patent issued, owned and licensed from Politecnico di Milano University. Member of the Board of Directors for Restech. Stocks owned for Restech. Free loan of equipment for studies received from Vyaire and Restech. Conflict of interest: C.S. Farah has nothing to disclose. Conflict of interest: R. Farré has nothing to disclose. Conflict of interest: G.L. Hall has nothing to disclose. Conflict of interest: I. Ioan has nothing to disclose. Conflict of interest: C.G. Irvin received consulting fees from Medical Graphics Corporation, outside the submitted work. Conflict of interest: D.W. Kaczka reports support for the present manuscript from University of Iowa. Grants or contracts from Dept of Defence and NIH, outside the submitted work. Consulting fees received from ZOLL Medical, Inc. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events received from CHEST, ASME, Medical Society of New Zealand, and Johns Hopkins University, outside the submitted work. US patent 10,675,423 B2 (patent on MFOV technique, inventor) and PCT patent pending, patent on MFOV technique pending. Stock or stock options held for OscillaVent, Inc. Loan of ventilator for other projects from ZOLL Medical Inc. Conflict of interest: G.G. King reports grants or contracts from Restech Italy, NHMRC, Boehringer Ingelheim, CycloPharm, GlaxoSmithKline, Menarini, MundiPharma, Philanthropic individuals and societies, outside the submitted work. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events received from AstraZeneca, Boehringer Ingelheim, Chiesi, Cipla, CycloPharm, GlaxoSmithKline, Menarini, MundiPharma and Novartis, outside the submitted work. Leadership or fiduciary role in other board, society, committee or advocacy group for ERS Technical Standards for Respiratory Oscillometry. Conflict of interest: H. Kurosawa reports receiving a grant from CHEST Co. Ltd. Royalties or licence for CHEST Co. Ltd. Payment or honoraria for lectures received from CHEST Co. Ltd. Nippon, Boehringer Ingelheim, Novartis, and Teijin Pharma, outside the submitted work. Conflict of interest: E. Lombardi reports grants or contracts from Restech and Sanofi, outside the submitted work. Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events received from Angelini, Chiesi, GSK, Novartis and Sanofi, outside the submitted work. Participation on a Data Safety Monitoring Board or Advisory Board for GSK and Novartis. Conflict of interest: G.N. Maksym reports grants or contracts from National Research Council of Canada, Cyclomedica Inc. Australia, and Lung Association of Nova Scotia, outside the submitted work. Accommodation Expenses received from Thorasys, Thoracic Medical Systems Inc. for attending European Society Meeting 2019. Patents planned, issued or pending for Method and system to acquire oscillometry measurements, owned by Thorasys, Thoracic Medical Systems Inc. Stock or stock options held for Thorasys, Thoracic Medical Systems Inc. Conflict of interest: F. Marchal has nothing to disclose. Conflict of interest: E. Oostveen has nothing to disclose. Conflict of interest: B.W. Oppenheimer has nothing to disclose. Conflict of interest: P.D. Robinson has nothing to disclose. Conflict of interest: M. van den Berge reports grants or contracts from GlaxoSmithKline, Novartis, Astra Zeneca, Roche, and Genentech, outside the submitted work. Conflict of interest: C. Thamrin report grants or contracts from Restech SRL and THORASYS Thoracic. Equipment on loan for research studies from Restech SRL and THORASYS Thoracic, outside the submitted work., (Copyright ©The authors 2022.)

Marfan syndrome (MFS) is a genetic disorder caused by mutations in the FBN1 gene that codifies for fibrilin-1. MFS affects elastic fiber formation and the resulting connective tissue shows abnormal tissue laxity and organization. Although an increased prevalence of obstructive sleep apnea among patients with MFS has been described, the potential effects of this genetic disease on the collapsible properties of the upper airway are unknown. The aim of this study was to assess the collapsible properties of the upper airway in a mouse model of MFS Fbn1((C1039G/+)) that is representative of most of the clinical manifestations observed in human patients. The upper airway in wild-type and Marfan mice was cannulated and its critical pressure (Pcrit) was measured in vivo by increasing the negative pressure through a controlled pressure source. Pcrit values from MFS mice were higher (less negative) compared to wild-type mice (-3.1±0.9cmH2O vs. -7.8±2.0cm H2O) suggesting that MFS increases the upper airway collapsibility, which could in turn explain the higher prevalence of OSA in MFS patients., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Obstructive sleep apnea, which is the most prevalent sleep breathing disorder, is characterized by recurrent episodes of upper airway collapse and reopening. However, the mechanical properties of the upper airway are not directly measured in routine polysomnography because only qualitative sensors (thermistors for flow and thoraco-abdominal bands for pressure) are used. This review focuses on two techniques that quantify upper airway obstruction during sleep. A Starling model of collapsible conduit allows us to interpret the mechanics of the upper airway by means of two parameters: the critical pressure (Pcrit) and the upstream resistance (Rup). A simple technique to measure Pcrit and Rup involves the application of different levels of continuous positive airway pressure (CPAP) during sleep. The forced oscillation technique is another non-invasive procedure for quantifying upper airway impedance during the breathing cycle in sleep studies. The latest developments in these two methods allow them to be easily applied on a routine basis in order to more fully characterize upper airway mechanics in patients with sleep breathing disorders.

Study Objectives: To determine whether the vibratory mechanical stimulus due to snoring induces upper-airway inflammation in an in-vivo rat model., Design: Prospective controlled animal study., Setting: University laboratory., Patients or Participants: Sixteen male Sprague-Dawley rats (250-300 g)., Interventions: The upper trachea of 8 rats was cannulated, and the upper airway was subjected to vibration (60 Hz; +/- 10 cm H2O) with a periodic pattern consisting of 1 second of vibration followed by 3 seconds of no vibration. This snoring-like vibration was applied for 3 hours. The animals breathed spontaneously through a cannula in the lower trachea. In a control group (8 rats), the animals were similarly instrumented, but no upper-airway vibration was applied., Measurements and Results: The effect of vibration was assessed by measuring the vibration-induced increase in gene expression of the pro-inflammatory cytokine tumor necrosis factor-alpha and of the neutrophil attractant chemokine macrophage inflammatory protein-2 in the soft-palate tissue. Real-time reverse-transcription polymerase chain reaction measurement of mRNA showed that vibration induced a significant overexpression of both tumor necrosis factor-alpha and macrophage inflammatory protein-2: 6.01-fold +/- 2.47-fold (p = .005) and 2.38-fold +/- 0.54 -fold (p = .021) increase when compared with control (mean +/- SEM)., Conclusions: The mechanical stimulus of vibration per se triggers an early proinflammatory process in the upper airway.

Increased upper airway collapsibility in the sleep apnea/hypopnea syndrome (SAHS) is usually interpreted by a collapsible resistor model characterized by a critical pressure (Pcrit) and an upstream resistance (Rup). To investigate the role played by the upstream segment of the upper airway, we tested the hypothesis that breathing different gases would modify Rup but not Pcrit. The study was performed on 10 patients with severe SAHS (apnea-hypopnea index: 59 +/- 14 events/hour) when breathing air and helium-oxygen (He-O2) during non-REM sleep. The continuous positive airway pressure that normalized flow (CPAPopt) was measured. Rup and Pcrit were determined from the linear relationship between maximal inspiratory flow VImax and nasal pressure (PN):VImax = (PN - Pcrit)/Rup. Changing the breathing gas selectively modified the severity of dynamic (CPAPopt, Rup) and static (Pcrit) obstructions. CPAPopt was significantly (p = 0.0013) lower when breathing He-O2 (8.44 +/- 1.66 cm H2O; mean +/- SD) than air (10.18 +/- 2.34 cm H2O). Rup was markedly lower (p = 0.0001) when breathing He-O2 (9.21 +/- 3.93 cm H2O x s/L) than air (15.92 +/- 6.27 cm H2O x s/L). Pcrit was similar (p = 0.039) when breathing He-O2 (4.89 +/- 2.37 cm H2O) and air (4.19 +/- 2.93 cm H2O). The data demonstrate the role played by the upstream segment of the upper airway and suggest that different mechanisms determine static (Pcrit) and dynamic (Rup) upper airway obstructions in SAHS.

Noninvasive measurement of respiratory resistance during nasal ventilatory support could be useful to assess the mechanical status of the patient and to optimize the ventilator settings. The aim was to investigate whether the forced oscillation technique (FOT) applied through a nasal mask allows reliable noninvasive estimation of respiratory resistance (Rrs) in patients with severe chronic respiratory disease. FOT Rrs (5 Hz) and lung resistance (R(L)) measured simultaneously from spontaneous breathing signals by an oesophageal balloon were compared in eight patients with chronic obstructive pulmonary disease and in six patients with a restrictive ventilatory defect due to chest wall disease. Measurements were performed in sitting and supine postures during application of nasal continuous positive airway pressure (CPAP): 4, 8 and 12 cmH2O in obstructive patients and 4 cmH2O in restrictive patients. In the restrictive patients Rrs and R(L) (in cmH2O x s x L(-1)) were virtually coincident: mean+/-SD, 12.6+/-6.1 and 11.6+/-6.6 (r=0.96) in sitting and 9.7+/-3.1 and 10.2+/-3.3 (r=0.92) in supine posture, respectively. In the obstructive patients (CPAP = 4 cmH2O), Rrs slightly underestimated R(L): mean+/-SD, 11.5+/-5.9 and 14.4+/-16.8 (r=0.92) in sitting and 15.0+/-9.8 and 21.1+/-12.6 (r=0.96) in supine posture, respectively. Similar results were found at CPAP = 8 and 12 cmH2O. The results obtained in patients with resistance values in the range typically found in nasal ventilatory support suggest that forced oscillation technique could be valuable to noninvasively estimate a patient's respiratory mechanical resistance.

The forced oscillation technique is useful in detecting changes in upper airway obstruction in patients with sleep apnoea undergoing continuous positive airway pressure (CPAP) ventilation. The aim of this study was to implement and evaluate a method for estimating respiratory impedance (Zrs) from the pressure and flow recorded at the inlet of the CPAP tubing. The method is based on correcting impedance measured at the inlet of the CPAP tubing (Zi) for the effect of the tubing and the exhalation port. The method was evaluated in mechanical analogues and in a healthy subject. Sinusoidal oscillation of 5, 10 and 20 Hz were superimposed on CPAP (5-15 cmH2O). At 5 Hz, the changes in airflow obstruction were substantially underestimated by Zi. Furthermore, Zi exhibited a negative dependence on Zrs at 20 Hz. The assessment of Zrs was greatly improved after correcting Zi for the effects of the CPAP tubing and the exhalation port. Zrs was well estimated at low frequencies, reaching very high values during total occlusion (>60 cmH2O x s x L(-1) at 5-10 Hz). These results indicate that changes in airflow obstruction can be detected using the forced oscillation technique from pressure and flow recorded on the continuous positive airway pressure device. This facilitates the clinical application of the forced oscillation technique for monitoring upper airway patency during sleep.

Forced-oscillation total respiratory resistance (Rrs) has been shown to underestimate spontaneous breathing lung resistance (RL,sb) in patients with airway obstruction, probably owing to upper airway shunting. The present study reinvestigates that relationship in seven severely obstructed chronic obstructive pulmonary disease patients using a technique that minimizes that artefact. Rrs at 8 and 16 Hz was computed for each successive forced oscillation cycle. Inspiratory and expiratory RL,sb were obtained by analysing transpulmonary pressure (Ptp) with a four-coefficient model, and compared to Rrs over the same periods. "Instantaneous" values of RL,sb were also obtained by computing the dynamic component of Ptp, and compared to simultaneous values of Rrs. In both respiratory phases, good agreement between Rrs and RL,sb was observed up to RL,sb values of approximately 15 hPa x s(-1) x L(-1) at 8 Hz and 10 hPa x s(-1) x L(-1) at 16 Hz. Instantaneous Rrs and RL,sb varied systematically during the respiratory cycle, exhibiting various amounts of flow- or volume-dependence in the seven patients; the amplitudes of their variations were significantly correlated, but Rrs was much more flow-dependent than RL,sb in three patients. Also, Rrs exceeded RL,sb at end-expiration in three instances, which could be related to expiratory flow limitation. In conclusion, total respiratory resistance is reliable up to much higher levels of airway obstruction than previously thought, provided upper airway shunting is avoided.

The respiratory mechanics of artificially ventilated chronic obstructive pulmonary disease (COPD) patients were investigated by means of the forced oscillation (FOT) and the end-inspiratory airway occlusion (AOT) techniques. FOT was applied to measure respiratory resistance (Rrs) and reactance (Xrs) from 0.25-16 Hz. Maximum (Rmax) and minimum (Rmin) resistances, static elastance (Est) and time constant (T) were computed by AOT. FOT and AOT data were interpreted with models featuring airway wall shunt, tissue viscoelasticity and parallel inhomogeneity. Rrs* and Xrs*, predicted from the AOT data, were computed and compared with Rrs and Xrs measured by FOT. Rrs and Xrs (hPa x s x L(-1)) decreased from 31.2+/-10.3 to 5.9+/-4.6 and increased from -20.3+/-7.1 to -8.0+/-4.4 from 0.25-16 Hz, respectively. Central resistance (Rc) and peripheral resistance (Rp) (in hPa x s x L(-1)), and shunt elastance (Esh) and tissue elastance (Et) (in hPa x L(-1)) were 4.4+/-5.4, 28.4+/-153, 723+/-393 and 31.8+/-10.1, respectively. Rmin, Rmax and Est were 18.4+/-5.9, 28.4+/-12.8 and 18.1+/-4.2 respectively, and T=0.76+/-0.25 s. The frequency dependence of predicted Rrs* and Xrs* differed markedly from that of measured Rrs and Xrs. The use of different models to interpret the measured data suggests that both airway and tissue properties determined the frequency dependence of respiratory resistance and respiratory reactance in ventilated chronic obstructive pulmonary disease patients at the investigated frequencies (0.25-16 Hz).

The forced oscillation technique (FOT) is a noninvasive method to measure respiratory resistance (Rrs) potentially useful for monitoring upper airway obstruction in patients with obstructive sleep apnea/hypopnea syndrome (SAHS). The aim of this work was to test the clinical suitability of FOT in assessing dynamic changes in airflow obstruction in patients with SAHS during continuous positive airway pressure (CPAP) and to investigate the CPAP dependence of Rrs. Forced oscillation (5 Hz) was applied to six male patients with SAHS submitted to CPAP titration procedure. Esophageal pressure was measured with a balloon-tipped catheter. Mid-inspiratory resistance (Rrs,i), mid-expiratory resistance (Rrs,e), and esophageal pressure swings (deltaPes) were computed for the respiratory events recorded at each CPAP level. Rrs,i decreased markedly and significantly from 36.0 +/- 4.0 cm H2O x s/L (mean +/- SEM) at baseline CPAP (4 cm H2O) to 13.1 +/- 2.8 cm H2O x s/L at optimal CPAP (11.3 +/- 0.4 cm H2O). Rrs,e showed a faster decrease with increasing CPAP reaching normal values at approximately 8 cm H2O. Rrs,i was strongly correlated (r2 = 0.94) with deltaPes. Our results suggest that FOT can be used as an alternative to the esophageal balloon for assessing airflow obstruction in patients with SAHS and for CPAP titration. Moreover, FOT allows us to detect phasic changes in resistance within the breathing cycle.

The aim of this work was to develop a low-cost circuit for real-time analog computation of the respiratory mechanical impedance in sleep studies. The practical performance of the circuit was tested in six patients with obstructive sleep apnea. The impedance signal provided by the analog circuit was compared with the impedance calculated simultaneously with a conventional computerized system. We concluded that the low-cost analog circuit developed could be a useful tool for facilitating the real-time assessment of airway obstruction in routine sleep studies.

Assessment of airway obstruction in patients with obstructive sleep apnoea (OSA) subjected to continuous positive airway pressure (CPAP) may be carried out using the forced oscillation technique (FOT). To facilitate routine application of forced oscillation (FO) in sleep studies, our aim was to design a system capable of generating CPAP and applying FOT simultaneously. We constructed a prototype CPAP + FO generator by connecting a specially designed electromagnetic valve in parallel with a conventional blower. The capacity of the prototype to generate forced oscillation (5 Hz +/- 1 hPa) was tested by connecting it to a model simulating spontaneous breathing. The response of the prototype for target CPAPs of 5, 10 and 15 hPa and imposed sinusoidal breathing with peak flow up to 0.75 L x s(-1) was excellent when compared with that reported for commercially available CPAP generators. The applicability of the prototype was tested by applying it to assess airway obstruction in four patients with OSA during sleep. We conclude that the generator designed is able to apply continuous positive airway pressure and forced oscillation simultaneously. The system could be useful for automatic and noninvasive assessment of airway obstruction in patients with obstructive sleep apnoea subjected to continuous positive airway pressure. Future development of the generator may be helpful in implementing a set-up for automatic titration of continuous positive airway pressure.

The forced oscillation technique (FOT) allows the measurement of respiratory resistance (Rrs) and reactance (Xrs) and their associated coherence (gamma2). To avoid unreliable data, it is usual to reject Rrs and Xrs measurements with a gamma2 <0.95. This procedure makes it difficult to obtain acceptable data at the lowest frequencies of interest. The aim of this study was to derive expressions to compute the random error of Rrs and Xrs from gamma2 and the number (N) of data blocks involved in a FOT measurement. To this end, we developed theoretical equations for the variances and covariances of the pressure and flow auto- and cross-spectra used to compute Rrs and Xrs. Random errors of Rrs and Xrs were found to depend on the values of Rrs and Xrs, and to be proportional to ((1-gamma2)/(2 x N x gamma2))1/2. Reliable Rrs and Xrs data can be obtained in measurements with low gamma2 by enlarging the data recording (i.e. N). Therefore, the error equations derived may be useful to extend the frequency band of the forced oscillation technique to frequencies lower than usual, characterized by low coherence.

The aim of this work was to demonstrate that the three compartments of the lung T network and the chest wall impedance (Zcw) can be identified from input and transfer impedances of the respiratory system if the pleural pressure is recorded during the measurements. The method was tested in six healthy volunteers in the range of 8-32 Hz. The impedances resulting from the decomposition confirm the adequacy of the monoalveolar structure commonly used in healthy subjects. Indeed, the T shunt impedance is well modeled by a purely compliant element, the mean compliance [0.038 +/- 0.081 (SD) l/kPa], which coincides within 9.5 +/- 6.3% of the alveolar gas compressibility derived from thoracic gas volume (0.036 +/- 0.011 l/kPa). The results obtained provide experimental evidence that the alveolar gas compression is predominantly isothermal and that lung tissue impedance is negligible throughout the whole frequency range. The shape of Zcw is consistent with a low compliance-low inertance pathway in parallel with a high compliance-high inertance pathway. We conclude that the proposed method is able to reliably identify the T network featuring the lung and Zcw.

Lung impedance (ZL) was measured from 0.1875 to 32 Hz in spontaneously breathing healthy subjects by spectral analysis of the pressure and flow signals generated simultaneously by the muscular generator of breathing and by a forced oscillation system. This method did not require cooperation from the subject to perform panting or special ventilatory maneuvers and therefore allowed us to analyze the frequency dependence of lung resistance, reactance, and elastance (-2 pi.frequency.reactance) at the physiological conditions of normal breathing. Resistance and elastance parameters were also computed by multiple linear regression of the time-domain pressure and flow data on a simple resistance-elastance model. Resistances and elastances computed at the breathing frequency by spectral analysis and by multiple linear regression were similar (nonsignificant differences < 4 and 10%, respectively). The results obtained when comparing ZL from the breathing component (0.1875-0.75 Hz) of the recorded signals and from the forced oscillation component (2-32 Hz) were fairly consistent. ZL (0.1875-10 Hz) was interpreted in terms of a model consisting of an airway compartment, including a resistance and an inertance, in series with a viscoelastic tissue compartment (J. Hildebrandt. J. Appl. Physiol. 28: 365-372, 1970) characterized by two parameters. The model analysis provided parameter values (resistance 2.49 +/- 0.58 hPa.l-1.s, inertance 1.70 +/- 0.29 Pa.l-1.s2, Hildebrandt parameters 4.87 +/- 2.28 and 0.73 +/- 0.99 hPa/l) consistent with the hypothesis that lung tissue in healthy humans during spontaneous breathing behaves as a viscoelastic structure with a hysteresivity of approximately 0.10.

Currently available data concerning respiratory input impedance (Zrs) at frequencies up to 300 Hz indicate that Zrs is determined mainly by the airways and, in particular, the gas compressibility in the airways and the airway wall compliance. Hence, measurements of Zrs when breathing gases with different physical properties would be useful in investigating airway mechanics and the role of acoustic propagation. Zrs measured with a standard generator (Zst) and corrected for the upper airway shunt (Zrs*) were measured in nine healthy subjects breathing air or a gas mixture consisting of 20% O2 and 80% He or SF6. The frequency band was extended up to 256 Hz for air and He-O2 and up to 128 Hz for SF6-O2. Zrs exhibited a similar pattern for the three gases, with a shift toward low frequencies as the gas density increased. Moreover, the resonance peaks tended to be narrower and higher as the gas density increased. The second frequency of resonance for He-O2, air, and SF6-O2 were 220, 180, and 50 Hz, respectively, for Zrs* and were systematically higher for Zst. Zrs* and Zst data were interpreted in terms of a tricompartmental model that partitioned the airways into two segments: a central one featuring the acoustic propagation in the airways and a peripheral one that included bronchial wall elasticity (Farré et al. J. Appl. Physiol. 67: 1973-1981, 1989). The model was able to interpret the gas dependence of Zrs* but not that of Zst. The influence of the gas physical properties on both Zrs* and Zst confirms that total Zrs at high frequencies is basically that of the airways and that the second resonance is related mainly to the gas compressibility in the airways.

The mechanical impedance of the respiratory system Zrs is usually measured by forced excitation while the patient breathes spontaneously. Pressure and flow signals due to breathing contaminate the excitation signals, leading to a poor signal-to-noise ratio (SNR) and thus to errors in impedance estimation, especially at low frequencies (up to 8 Hz). To enhance SNR in the recorded signals we designed an infinite impulse response digital filter for the frequent case in which the excitation is pseudorandom. The algorithm is based on narrowband second-order bandpass elements centred at the excitation frequencies. The performance of the filter was assessed in a simulation study by superposing forced excitation signals (2-32 Hz) from a reference model and the signals of breathing recorded from 16 subjects. When compared with a conventional high-pass filtering, the devised filtering resulted in an increase in SNR which was almost constant over the whole frequency band: 6.30 +/- 0.98 dB (mean +/- SD). This improvement in SNR was reflected in an increase in the number of subjects for which the corresponding coherence y2 attained a value greater than the conventional threshold of acceptability (y2 = 0.95). At the lowest frequency (2 Hz) only two (12.5 per cent) simulated subjects had y2 greater than or equal to 0.95 with the conventional high-pass filtering. By contrast, when using the devised comb filter the number of subjects with y2 greater than or equal to 0.95 increased up to 13 (81 per cent). The results obtained suggest that this filter may be useful to improve SNR and thus Zrs estimation.

The usual setup for measuring respiratory input impedance requires a differential pressure transducer attached to a pneumotachograph. As, up to now, no data correction procedure has been devised to account for transducer asymmetry, a highly symmetrical transducer is required to obtain reliable impedance data. In this communication, a general model for the measuring system is presented. Its main feature is that differential pressure transducers are modeled as two input-one output systems. From the theoretical model, we defined a dynamic calibration and data correction procedure. This was tested using highly asymmetrical transducers (common-mode rejection ratio between 45 and 27 dB) to measure the impedance of two respiratory analogs. The latter were linear resistance (R), inertance (I), compliance (C) series models simulating a normal subject (R = 3.47 hPa.s.l-1, I = 1.45 Pa.s2.l-1, C = 18.6 ml.hPa-1) and an obstructive patient (R = 11.15 hPa.s.l-1, I = 1.28 Pa.s2.l-1, C = 18.5 ml.hPa-1). Results obtained applying the devised procedure (errors in R, I, and C always less than 4 percent) show that respiratory input impedance can be adequately measured if data are corrected for transducer asymmetry.

To minimize the flow-dependent effects caused by an endotracheal tube during impedance measurements, we recorded pressure inside the tube at its distal end. We used a commercial endotracheal tube with a lumen built into its wall with the opening located near the outlet of the tube. We characterized the effect of the tube by means of an effective transfer function (H). We measured H from 0.25-32 Hz on a mechanical analogue by using pseudorandom excitation with different peak-to-peak flow amplitudes (Vpp). For an 8 mm internal diameter (ID) endotracheal tube the modulus of H measured with Vpp 0.2 l.s-1 was 1.00 at 0.25 Hz and increased with the frequency to 1.40 at 32 Hz. The phase factor was close to zero (less than 5 degrees) over the whole frequency band. The modulus of H changed less than 5% and the phase factor less than 3 degrees when Vpp was increased from 0.2 to 0.8 l.s-1. We evaluated the method on five mechanical analogues with increased resistance or elastance and with a different tracheal area. The mean normalized distance in the complex plane over the whole frequency band (dz) between the analogue impedance and the estimated value from intubation was always less than 5%. Finally, the method was tested on an active analogue which superimposed a high-amplitude (up to 1.4 l.s-1 peak-to-peak) low-frequency (0.25 or 0.33 Hz) sinusoidal flow onto excitation: dz was always less than 4.3%.