Forced oscillation technique for the evaluation of severe sleep apnoea/hypopnoea syndrome: a pilot study

The forced oscillation technique (FOT) is a noninvasive method of potential clinical interest for quantitatively assessing airway mechanics during sleep. We investigated the applicability of FOT as a diagnostic tool for noninvasive assessment of airflow obstruction in patients with sleep apnoea/hypopnoea syndrome (SAHS) during sleep. In seven patients previously diagnosed with severe SAHS (mean+/-SD apnoea/ hypopnoea index (AHI) 67+/-14) we performed a full polysomnography (PSG) together with on-line measurement of respiratory impedance (IZI) using FOT. For each patient we determined: 1) number of respiratory events conventionally detected by full PSG, those obtained by FOT and their degree of concordance; and 2) the characteristics and values of IZI during the respiratory events. FOT was well tolerated and easily applied in conjunction with a conventional sleep setup. The mean number of respiratory events x h(-1) detected by PSG and FOT were 55+/-16 and 58+/-17, respectively, with a strong concordance. IZI increased from a baseline of 11+/-4 to 50+/-20 cmH2O x L(-1) x s during apnoea (mean+/-SD). In all but one patient intermittent increases of IZI occurred immediately before each obstructive apnoea. In four patients, the increases of IZI developed at end-expiration whereas in two others occurred during inspiration. During hypopnoea most of the patients showed decreases of IZI during expiration. In conclusion, forced oscillation technique can be used as a noninvasive and complementary tool for the diagnosis of respiratory events and provides an on-line quantitative approach for continuous monitoring of airflow obstruction during sleep in patients with sleep apnoea/hypopnoea syndrome.

Assessment of airflow obstruction during CPAP by means of forced oscillation in patients with sleep apnea

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.

Accuracy of thermistors and thermocouples as flow-measuring devices for detecting hypopnoeas

The aim of this work was to assess the accuracy of thermistors/thermocouples as devices for detecting hypopnoeas in sleep studies. Conventional thermistor/thermocouples were studied with a respiratory model allowing the simulation of inspiratory (22 degrees C) and expiratory (37 degrees C) flows. The thermistor signal (V'th) was compared with a pneumotachograph (V'th): 1) for sinusoidal and square-wave airflows (+/-0.05 to +/-0.8 L.s(-1), 10-20 breaths.min(-1) (bpm)); 2) when changing the distance from the thermistor to the nose (0-20 mm); and 3) when doubling the section of the nostrils. The thermistor was strongly nonlinear and flow reductions (hypopnoeas) were underestimated: a 50% reduction in V' (+/-0.5 L.s(-1), 15 bpm, sinusoidal) resulted in only an 18% reduction in V'th. V'th depended considerably on the airflow pattern: for V'=+/-0.5 L.s(-1), V'th increased by 100% from sinusoidal (20 bpm) to square-wave (10 bpm). For V'=+/-0.5 L.s(-1), 15 bpm, sinusoidal flow, V'th increased by 79% when the distance thermistor-nose varied from 20-0 mm, and V'th decreased by 37% when doubling the nose section. We concluded that thermistor/thermocouples are inaccurate flow-measuring devices when used at the airflow conditions typical of sleep studies. Their use for quantifying hypopnoeas may lead to considerable underdetection of these respiratory events.

Analog circuit for real-time computation of respiratory mechanical impedance in sleep studies

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.

A system to generate simultaneous forced oscillation and continuous positive airway pressure

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.

Gas compression artefacts when testing peak expiratory flow meters with mechanically-driven syringes

Mechanically-driven syringes used to test peak expiratory flow (PEF) meters must produce the American Thoracic Society (ATS) standard waveforms with PEF accuracy of 2%. However, gas compression within the syringe could result in significant PEF inaccuracy when testing high resistance meters. The gas compression artefact was investigated in a mechanical syringe (PWG; MH Custom Design & Mfg L.C., Midvale, Ut, USA) of 13.6 L connected to a standard range mini-Wright PEF meter (Clement Clarke International, Harlow, UK). Scaled versions of the ATS standard waveform No. 24, with peak flows of 750 and 450 L x min(-1), were discharged through the PEF meter from different starting piston positions to vary syringe volume (Vsyr). The PEF recorded by the meter decreased linearly with increasing Vsyr. PEF decreased by 0.31 and 0.27% per litre for the ATS standard waveforms with PEF of 750 and 450 L x min(-1), respectively. The target PEF computed from piston displacement overread the actual PEF delivered into the PEF meter by approximately 4% when Vsyr = 13.6 L. Overreading fell to approximately 1% when Vsyr was reduced to 3.62 L. Therefore, gas compression error in commercially available large mechanical syringes can exceed the 2% inaccuracy limit when testing high resistance portable PEF meters. Measurements can be corrected for gas compression by linearly extrapolating PEF recordings to zero volume.

Gas compression artefacts when testing peak expiratory flow meters with mechanically-driven syringes

Mechanically-driven syringes used to test peak expiratory flow (PEF) meters must produce the American Thoracic Society (ATS) standard waveforms with PEF accuracy of 2%. However, gas compression within the syringe could result in significant PEF inaccuracy when testing high resistance meters. The gas compression artefact was investigated in a mechanical syringe (PWG; MH Custom Design & Mfg L.C., Midvale, Ut, USA) of 13.6 L connected to a standard range mini-Wright PEF meter (Clement Clarke International, Harlow, UK). Scaled versions of the ATS standard waveform No. 24, with peak flows of 750 and 450 L x min(-1), were discharged through the PEF meter from different starting piston positions to vary syringe volume (Vsyr). The PEF recorded by the meter decreased linearly with increasing Vsyr. PEF decreased by 0.31 and 0.27% per litre for the ATS standard waveforms with PEF of 750 and 450 L x min(-1), respectively. The target PEF computed from piston displacement overread the actual PEF delivered into the PEF meter by approximately 4% when Vsyr = 13.6 L. Overreading fell to approximately 1% when Vsyr was reduced to 3.62 L. Therefore, gas compression error in commercially available large mechanical syringes can exceed the 2% inaccuracy limit when testing high resistance portable PEF meters. Measurements can be corrected for gas compression by linearly extrapolating PEF recordings to zero volume.

Estimation of random errors in respiratory resistance and reactance measured by the forced oscillation technique

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.

Estimation of random errors in respiratory resistance and reactance measured by the forced oscillation technique

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.

Inspiratory dynamic obstruction detected by forced oscillation during CPAP. A model study

Assessment of upper airway mechanics in patients with obstructive sleep apnea/hypopnea (OSA) can be carried out qualitatively from indirect signals (flow pattern, snoring, strain gauges, inductance plethysmography) or quantitatively by means of invasive estimation of esophageal pressure. The forced oscillation technique (FOT) is a noninvasive method of potential interest for quantitatively assessing airway obstruction in the sleeping patient. The aim of this work was to ascertain in a model study whether FOT could provide an index of airway obstruction when applied at the conditions of total and partial occlusions similar to the ones found in patients with OSA. An airway analog closely mimicking upper airway collapsibility was constructed and mechanically characterized by the relationship between its flow, upstream and downstream pressures as well as by means of FOT. We simulated total collapse (apnea), different levels of partial collapse with flow limitation (hypopnea), and release of airway obstruction when the collapsible analog was used as an artificial upper airway in a spontaneously breathing subject submitted to continuous positive airway pressure (CPAP) up to 14 cm H2O.s/L. The results showed that the amplitude of airway impedance measured by FOT was a suitable index to detect obstruction in collapsible segments. We concluded from this realistic model study that FOT could be a valuable tool for quantitatively assessing airway obstruction in patients with OSA treated with CPAP. This noninvasive technique is potentially useful both in studying upper airway mechanics in detail and in automatically monitoring airway obstruction in routine studies.

Effect of expiratory flow limitation on respiratory mechanical impedance: a model study

Large phasic variations of respiratory mechanical impedance (Zrs) have been observed during induced expiratory flow limitation (EFL) (M. Vassiliou, R. Peslin, C. Saunier, and C. Duvivier. Eur. Respir. J. 9: 779-786, 1996). To clarify the meaning of Zrs during EFL, we have measured from 5 to 30 Hz the input impedance (Zin) of mechanical analogues of the respiratory system, including flow-limiting elements (FLE) made of easily collapsible rubber tubing. The pressures upstream (Pus) and downstream (Pds) from the FLE were controlled and systematically varied. Maximal flow (Vmax) increased linearly with Pus, was close to the value predicted from wave-speed theory, and was obtained for Pus-Pds of 4-6 hPa. The real part of Zin started increasing abruptly with flow (V) > 85% Vmax and either further increased or suddenly decreased in the vicinity of Vmax. The imaginary part of Zin decreased markedly and suddenly above 95% Vmax. Similar variations of Zin during EFL were seen with an analogue that mimicked the changes of airway transmural pressure during breathing. After pressure and V measurements upstream and downstream from the FLE were combined, the latter was analyzed in terms of a serial (Zs) and a shunt (Zp) compartment. Zs was consistent with a large resistance and inertance, and Zp with a mainly elastic element having an elastance close to that of the tube walls. We conclude that Zrs data during EFL mainly reflect the properties of the FLE.

Assessment of respiratory pressure-volume nonlinearity in rabbits during mechanical ventilation

The volume dependence of respiratory elastance makes it difficult to recognize actual changes in lung and chest wall elastic properties in artificially ventilated subjects. We have assessed in six anesthetized, tracheotomized, and paralyzed rabbits whether reliable information on the static pressure-volume (PV) curve could be obtained from recordings performed during step variations of the end-expiratory pressure without interrupting mechanical ventilation. Pressure and flow data recorded during 5- and 10-hPa positive-pressure steps were analyzed in the time domain with a nonlinear model featuring a sigmoid PV curve and with a model that, in addition, accounted for tissue viscoelastic properties. The latter fitted the data substantially better. Both models provided reasonably reproducible coefficients, but the PV curves obtained from the 5- and 10-hPa steps were systematically different. When the PV curves were used to predict respiratory effective elastance, the best predictor was the curve derived from the 10-hPa step with the viscoelastic model: unsigned differences averaged 8.6 +/- 11.1, 26.9 +/- 36.4, and 5.5 +/- 5.8% at end-expiratory pressures of 0, 5, and 10 hPa, respectively. This approach provides potentially useful, although not highly accurate, estimates of respiratory effective elastance-volume dependence.

Dynamic elastance and tissue resistance of isolated liquid-filled rat lungs

The effect of the surface forces of the alveolar air-liquid interface on the dynamic behavior of lung tissue was investigated in five isolated liquid-filled rat lungs. The lungs were subjected to 0.04-Hz sinusoidal oscillation (1.5-ml tidal volume) at lung volume (VL) levels ranging from volume at zero pressure (V0) + 4 ml to V0 + 10 ml. Oscillations were performed at each VL after inflation of the lungs from V0. Alveolar pressure (PA) was measured with an alveolar capsule attached to the visceral pleura. Dynamic elastance (Edyn), tissue resistance (Rti), and hysteresivity [eta = Rti omega/Edyn, where omega is angular frequency (2 pi x frequency)] were computed from PA and VL changes. Edyn was 59.6 +/- 4.3 Pa/ml at V0 + 4 ml and varied little up to V0 + 7 ml. Thereafter, Edyn increased markedly with VL, reaching 102 +/- 16 Pa/ml at V0 + 10 ml. No significant difference was found between elastance computed from PA and that computed from pressure recorded at the airway opening. Rti was 35.2 +/- 3.6 Pa.s.ml-1 and exhibited a VL dependence similar to that of Edyn. As a result, eta was 0.16 and did not vary significantly in the explored VL range. We conclude that PA can be reliably measured in the liquid-filled lung by means of alveolar capsules. In the liquid-filled lung, Edyn was smaller than and eta was similar to values reported for air-filled lungs. Hence, surface tension accounts for a considerable part of elastance and Rti of the air-filled lung within the volume range of normal breathing.

Servocontrolled generator to measure respiratory impedance from 0.25 to 26 Hz in ventilated patients at different PEEP levels

Assessing respiratory impedance (Zrs) in ventilated patients over a wide frequency band, ranging from breathing rates to typical forced oscillation frequencies, during end-expiratory pauses at different positive end-expiratory pressures (PEEP) is of potential interest to assess a patient's respiratory mechanics. Zrs measurements under these conditions are not possible with the present variants of the forced oscillation technique. The aim of this work was to design a forced oscillation generator operating from spontaneous breathing frequencies whilst withstanding PEEP. To this end, we constructed a generator based on a servocontrolled loudspeaker. This allowed the loudspeaker cone to remain at its resting position regardless of the external PEEP applied. The system was optimized by using a mechanical analogue. The clinical applicability of the servocontrolled generator was assessed by measuring Zrs in mechanically-ventilated chronic obstructive pulmonary disease (COPD) patients during end-expiratory pauses at different transrespiratory pressures. The forced oscillation generator designed may be easily applicable in practice since it is small and light. The system is able to withstand transrespiratory pressures of up to 17 hPa and allows the application of forced oscillation of sufficient amplitude ( > 2 hPa peak-to-peak, 0.25-26 Hz) to obtain reliable respiratory resistance and reactance data. The servocontrolled generator permits the assessment of respiratory mechanics over a wide frequency band ranging from breathing frequencies to the most typical forced oscillation frequencies during end-expiratory pauses at PEEPs within the conventional range.

Lung and respiratory impedance at low frequency during mechanical ventilation in rabbits

We have tested in eight rabbits the feasibility of measuring respiratory (Zrs) and lung (ZL) impedances in the low-frequency domain, including below the breathing frequency (fb), during conventional mechanical ventilation (CMV). The animals were tracheotomized and ventilated with a tidal volume (VT) of 20 ml at a fb of 1 Hz. The excitation signal was provided by a flow generator connected in parallel with the ventilator; it included six components ranging from 0.45 to 14.8 Hz, which met the neither-sum-nor-difference criterion of B. Suki and K. Lutchen (IEEE Trans. Biomed. Eng. 39: 1142-1151, 1992) to minimize the influence of nonlinearities. Zrs and ZL were also measured at the same mean lung volume and with the same excitation signal both during apnea and when the ventilator signal was replaced by a sine wave with the same VT and fb (SMV). The real parts (Re) of both Zrs and ZL, as well as the effective elastances, were significantly larger during apnea than during CMV and SMV over the whole frequency range. Re(Zrs) and Re(ZL) were similar during CMV and SMV above fb but they were lower during CMV at 0.45 Hz. The latter difference seems to be related to the presence of harmonics of fb and of additional frequency components due to pulse amplitude modulation. We conclude that, because of nonlinearities, it is feasible to measure Zrs and ZL during CMV only at and above fb.

Salbutamol inhibits pulmonary effects of platelet activating factor in man

Inhaled platelet-activating factor (PAF) provokes considerable pulmonary gas exchange disturbances in normal man and in patients with mild asthma, similar to those observed in acute severe asthma. To further examine the mechanisms involved in PAF-induced ventilation-perfusion (VA/Q) mismatch, eight healthy, non-atopic, nonsmoking subjects were studied after administration of PAF aerosol (24 micrograms). They had been previously treated with inhaled salbutamol (300 micrograms) in a randomized, double-blind, cross-over, placebo-controlled design. After placebo, PAF provoked a fall in total arterial white cell count with a rebound leukocytosis. As shown in a previous study, an overall index of VA/Q inequality (DISP R-E*, 1.64 +/- 0.10) showed a threefold increase (P < 0.006) that accounted for the increase (79%) in AaPO2 (p < 0.04) after PAF, while the respiratory system resistance (Rrs) rose by 16% (p < 0.02). In contrast, after pretreatment with salbutamol inhaled PAF had no effects on pulmonary gas exchange, Rrs, or white cell count; facial flushing and cough were also hindered. The results are consistent with the hypothesis that salbutamol inhibits PAF-induced venoconstriction in both the airway and pulmonary microcirculation.

T model partition of lung and respiratory system impedances

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.

Inhaled platelet-activating factor worsens gas exchange in mild asthma

To investigate the potential effects of inhaled platelet-activating-factor (PAF) (12 micrograms) to perturb pulmonary gas exchange in bronchial asthma, six patients (mean +/- SE, 23 +/- 2 yr) with intermittent asthma (FEV1, 90% predicted) were studied before and 5, 15, and 45 min after challenge. Circulating white blood cells, respiratory system resistance (Rrs), systemic and pulmonary hemodynamics, and respiratory and inert pulmonary gas exchange were measured. Five minutes after PAF leukocytes fell, Rrs increased (by 27%). PaO2 decreased (by 15 mm Hg), and AaPO2 increased (twofold) (p < 0.05 each). Ventilation-perfusion (Va/Q) distributions worsened in a pattern similar to that commonly observed in patients with moderate to severe asthma. Dispersions of pulmonary blood flow (log SD Q) and of alveolar ventilation (log SD V), and an overall index of Va/Q heterogeneity (DISP R-E*) increased significantly (123% for DISP R-E*; p < 0.05, each). Gas exchange indices and Rrs were still minimally abnormal at 15 min but returned towards baseline at 45 min. Ventilatory and hemodynamic variables remained unaltered throughout the study. These results suggest that endogenous PAF may be implicated in the arterial blood gas abnormalities shown during exacerbations of bronchial asthma.

Human lung impedance from spontaneous breathing frequencies to 32 Hz

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.

Respiratory input impedance up to 256 Hz in healthy humans breathing foreign gases

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.

Validity of the esophageal balloon technique at high frequencies

The reliability of the esophageal balloon technique in measuring high-frequency changes in pleural pressure (Ppl) was investigated in six normal subjects by studying the amplitude ratio (A) and phase angle (phi) of esophageal (Pes) and mouth (Pm) pressures during airway occlusion and while pseudorandom pressure variations (2-32 Hz) were applied to the chest. The measurements were made with a common esophageal balloon-catheter system connected to a high-impedance piezoresistive transducer. When the cheeks were firmly supported, A averaged 1.08 +/- 0.063 at 2 Hz and 1.06 +/- 0.11 at 32 Hz. Pes increasingly led Pm with increasing frequency, and phi averaged 20.8 +/- 4.0 degrees at 32 Hz. Washing the airways with 80% He-20% O2 reduced phi by 50%. When the cheeks were not supported, A exhibited a strong positive frequency dependence, averaging 1.71 +/- 0.34 at 32 Hz, whereas phi increased much faster below 20 Hz and tended to decrease afterward. Because the esophageal transfer function Pes/Ppl = (Pes/Pm)/(Ppl/Pm), we could estimate Pes/Ppl by computing for individual subjects the pressure difference between the pleura and the mouth based on the lung and upper airway wall properties that were measured separately. The results suggest that the ratio of Pes and Ppl remains close to unity from 2 to 32 Hz, but Pes lags slightly behind Ppl (phi equals about -7 degrees at 32 Hz).

Optimized estimation of respiratory impedance by signal averaging in the time domain

The spontaneous breathing of a subject during measurements of respiratory impedance (Zrs) by the forced oscillation technique (FOT) induces errors that result in biased impedance estimates, especially at low frequencies. Although in standard measurements this bias may be avoided by using special impedance estimators, there are two applications of FOT for which such estimators are not useful: when a head generator is used and when measurements are made during intubation. In this paper we describe a data-processing procedure for unbiased impedance estimation for all FOT setups. The proposed estimator (Z) was devised for pseudorandom excitation and is based on time-domain signal averaging before frequency analysis. The performance of estimator Z was first analyzed by computer simulation of a head generator setup and a setup including an endotracheal tube to measure (2-32 Hz) a resistance-inertance-elastance model mimicking Zrs of a healthy subject. Second, Z was assessed during real measurements in 16 healthy subjects. The results obtained in the simulation (e.g., error in elastance was reduced from 15.6% with most conventional estimators to 3.3% with Z in simulation of head generator setup) and in the measurements in subjects (differences of less than 1.6% between Z and a reference) confirmed the theoretical lack of bias of Z and its practical suitability for the different FOT setups. In addition to its applicability in the situations in which no other unbiased estimators are available, estimator Z is also advantageous in most conventional applications of FOT, since it requires much less computing time and thus allows on-line Zrs measurements.

Compared responses of rat lungs to step volume changes and to sinusoidal forcing

Lung pressure-volume hysteresis of cat lungs has been found by Hildebrandt (J. Appl. Physiol. 28, 365-372, 1970) to be 20-50% larger than predicted from stress adaptation data on the basis of a viscoelastic model. We have reinvestigated this phenomenon in isolated rat lungs with a different approach, in which the approximation inherent to using a model is avoided : Lung transfer function was derived from the digitally-computed Laplace transform of the pressure decay following a step volume change and used to predict lung pressure-flow relationship in the frequency domain. The latter was expressed in terms of lung effective resistance (Rlc) and effective elastance (Elc), and compared to the observed values (Rl and El) in the frequency range 0.01-0.5 Hz. The measurements were made in 5 lungs at a transpulmonary pressure (Pl) of 0.5 kPa and in 5 others at a Pl of 0.8 kPa. Rl was found to be 23-41% larger than Rlc at Pl = 0.5 and 29-51% larger at Pl = 0.8. El did not differ significantly from Elc at Pl = 0.5 but was 14-28% larger at Pl = 0.8. These results are in good agreement with previous findings. The differences between Rl and Rlc are proportional to the reciprocal of frequency and, thus, correspond to a rate-independent dissipation. They are consistent with a yield stress of 3-6 Pa.

Airways impedance during single breaths of foreign gases

The changes in airways resistance (Raw) and inertance (Iaw) during single inspirations of pure methane, helium, neon, and ethane at a flow of 0.1 l/s were measured in six healthy subjects by use of a forced-oscillation technique. Raw and Iaw were computed from respiratory transfer impedance obtained at a frequency of 20 Hz by applying pressure oscillations at the chest and measuring flow at the mouth with a bag-in-box system. Compared with the air data, the changes of Iaw after inhalation of 500 ml of gas averaged -41.1% with methane, -82.8% with helium, -25.8% with neon, and +4.8% with ethane. These changes were slightly less than the changes in gas density (-45%, -86%, -31%, and +5%, respectively). The inhaled volumes at which 50% of the changes had occurred (V50) did not differ significantly among gases and were approximately 100 ml. For Raw the data were more noisy than for Iaw; they were discarded in two subjects because of a strong and irreproducible volume dependence in air. Consistent differences were seen between the remaining subjects, one of whom exhibited a predominant viscosity dependence of Raw, one a predominant density dependence, and two an intermediate pattern. V50s were larger for Raw than for Iaw, indicating a more peripheral distribution of Raw. For Raw, V50s were lower with helium than with methane, in agreement with the notion that density-dependent resistance is located mainly in the large airways. The results suggest that some information on the serial distribution of Raw and Iaw may be derived from impedance measurements with foreign gases.