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The aim of this study was to establish the ability to estimate separate airway and tissue properties from transfer respiratory impedance (Zrs,tr) data in the presence of airway obstruction. Zrs,tr, thoracic gas volume (TGV) and airway resistance (Raw,pleth) were measured in the presence of obstruction and after use of a bronchodilator (BD) in 13 normal or asymptomatic asthmatic adults and 28 children with symptomatic asthma. An analytical approach was used to solve the equations of a simplified variant of DuBois' model, including airway resistance (Raw*) and inertance (Iaw), tissue compliance (Ct) and resistance (Rt) and pulmonary gas compliance (Cg). The equations of the model could not be reliably solved in four children before BD. Mean Raw,pleth was not different from mean Raw* in adults before (mean +/- SEM) (3.4 +/- 0.5 vs 3.1 +/- 0.3 hPa.s.L-1) or after BD (1.4 +/- 0.2 vs 1.8 +/- 0.2 hPa.s.L-1), or in children after BD (2.9 +/- 0.3 vs 3.2 +/- 0.2 hPa.s.L-1, respectively). In children before BD, Raw* was significantly underestimated compared with Raw,pleth (3.8 +/- 0.4 vs 5.4 +/- 0.6 hPa.s.L-1). Overall, a significant positive correlation was found between the difference [Raw,pleth - Raw*] and Raw,pleth (r = 0.82). In adults, BD induced a decrease in Raw* and Rt, an increase in Ct, and no change in Iaw. In children after BD, there was no significant change in Raw* or Ct, whilst Rt decreased and Iaw increased. Taking Raw,pleth as the gold standard, it is concluded that coherent estimation of parameters of DuBois' model may be obtained from combined Zrs,tr and TGV measurements in normal subjects and moderately obstructed adults, but not in children with significant airway obstruction. This seems to be due to the systematic under-estimation of Raw*.

Respiratory resistance (Rrs) and elastance (Ers) are commonly measured in artificially-ventilated patients or animals by multiple linear regression of airway opening pressure (Pao) versus flow (V') and volume (V), according to the first order model: Pao = P0 + Ers.V + Rrs.V', where P0 is the static recoil pressure at end-expiration. An alternative way to obtain Rrs and Ers is to derive them from the Fourier coefficients of Pao and V' at the breathing frequency. A potential advantage of the second approach over the first is that it should be insensitive to a zero offset on V' and to the corresponding volume drift. The two methods were assessed comparatively in six tracheotomized, paralysed and artificially ventilated rabbits with and without adding to V' an offset equal to 5% of the mean unsigned flow. The 5% flow offset did not modify the results of Fourier analysis, but increased Rrs and Ers from linear regression by 15.8 +/- 4.6% and 4.55 +/- 0.64%, respectively. Without additional offset, differences between the two methods averaged 30.2 +/- 14.0% for Rrs and 9.3 +/- 6.2% for Ers. The differences almost completely disappeared (2.47 and 0.61%, respectively) when the flow signal was zero-corrected using the assumption that inspired and expired volumes were the same. After induced bronchoconstriction, however, Ers was still slightly larger by linear regression than by Fourier analysis, which may result from nonlinearities and/or frequency dependence of the parameters. We conclude that the regression method requires zero flow correction and that Fourier analysis is an attractive alternative.

Respiratory resistance (Rrs) was measured by the forced oscillation technique at 10, 20 and 30 Hz in 54 healthy subjects. The sinusoidal pressure oscillations were applied around the head, rather than at the mouth, so as to minimize transmural pressure across extrathoracic airway walls and the corresponding artefact (Peslin et al., J Appl Physiol, 1985, 59, 1790-1795). The flow (V') and volume (V) dependences of Rrs during the respiratory cycle were analysed by least square regression according to: Rrs = K1 + 2.K2.[V']#- K3.V, where K1 and K2 are Rohrer's constants, and where K3 expresses the (negative) volume dependence of Rrs. The analysis was made separately on the inspiratory and expiratory phases. A good fit was usually found between the data and the model, with a root-mean-square error averaging 15% of the mean Rrs at 10 Hz. At all frequencies K2 and K3 were substantially and significantly larger, and K1 slightly lower during expiration than during inspiration. Rrs, K1 and K3 were minimum at 20 Hz, while K2 exhibited a strong positive frequency dependence. The decrease of Rrs from 10 to 20 Hz was entirely explained by the variations of its linear component, and its increase from 20 to 30 Hz was largely due to its flow dependent component. Both the phasic variations and the frequency dependence of the coefficients suggest that the model is purely descriptive and that coefficients K2 and K3 reflect a number of phenomena, including the variations in glottic aperture during the respiratory cycle.

Total respiratory input (Zin) and transfer (Ztr) mechanical impedances were measured from 4-30 Hz in 9 patients with severe chronic obstructive pulmonary disease (COPD) and in 12 healthy subjects. Zin was obtained by applying a pressure input around the head to minimize transmural pressure across extrathoracic airway walls, and Ztr was obtained with a pressure input at the chest. In agreement with previous studies total respiratory compliance and inertance were decreased in patients, while effective input resistance was increased and exhibited a negative frequency dependence. Effective transfer resistance (Re(Ztr)) was also increased at all frequencies, and, in some patients, the Re(Ztr)-frequency curve was sigmoid in shape, which was never seen in normals. When Ztr was analysed with a six-coefficient monoalveolar model featuring tissue properties, alveolar gas compliance, and airways properties, the model fitted the data less closely in patients than in normals and, in the former, provided unrealistic coefficients. Such was also the case with a bialveolar model. A better fit with more realistic values for the coefficients was obtained in selected patients with a model where central and peripheral (Rp) airway resistance were separated by a shunt representing airway wall compliance (Cb): Cb was found to range from 0.029-0.062 l.kPa-1 and Rp represented 44-81% of total airway resistance.

The validity of a new method for measuring thoracic gas volume (TGV) was studied in 69 children, 4-16 yrs old, including twelve normal children and 57 children with an obstructive (n = 38) or restrictive (n = 19) respiratory disease. The method consisted of applying very slow (0.05 Hz) sinusoidal variations of ambient pressure around the body (delta Pam = 40 cmH2O peak to peak) and studying the relationship between delta Pam and the resulting gas displacement at the mouth (Vaw): TGVapc = PB.delta Vaw/delta Pam.cos phi, where PB is barometric minus alveolar water vapour pressure and phi the phase angle between Pam and Vaw. Functional residual capacities derived from TGVapc (FRCapc) were compared to the values obtained by plethysmography (FRCplet) and by helium dilution (FRCdil). FRCapc did not differ significantly from FRCplet in either the entire group (1.75 +/- 0.62 l vs 1.79 +/- 0.45 l) or in the patient subgroups. However, with the new method a trend to slightly lower FRCs was seen in patients with the most obstruction (p less than 0.05). FRCdil was significantly lower than both FRCapc and FRCplet (p less than 0.001), particularly in children with obstruction. Significant correlations were found between the three methods (p less than 0.001). On the other hand, the method investigated requires that the subject breathe very regularly for a period of several minutes. This was rarely achieved, so that the reproducibility of the measurements was unacceptably low. At present, the method cannot be recommended for routine use in 4-16 yr old children.

A new method of studying the linearity of gas flowmeters was tested on different models of Fleisch pneumotachographs. The method applies a steady flow to the test flowmeter, which is increased in a stepwise manner by adding a constant flow-increment. This is achieved using two flow sources in parallel. The method does not require any reference flow channel and may be implemented with standard laboratory equipment. Using this method, the gain of Fleisch pneumotachographs, whatever their size, decreased by about 2-3% from low flows to about 40% of their nominal full scale (FS), and then increased almost linearly with increasing flow. The error was 8-13% at 200% FS. The following equation was devised to correct the data at high flow: Vc = Vt (1-K (Vt-S] where Vc and Vt are the corrected and measured flow respectively, K a gain correction factor and S a flow threshold below which no correction is needed. Applying this correction with suitable coefficients, the maximal error was below 3% from 0-200% FS.