The classic model of the respiratory system (RS) is comprised of a Newtonian resistor in series with a capacitor and a viscoelastic unit including a resistor and a capacitor. The flow interruption technique has often been used to study the viscoelastic behavior under constant inspiratory flow rate. To study the viscoelastic behavior of the RS during complete respiratory cycles and to quantify viscoelastic resistance (Rve) and compliance (Cve) under unrestrained conditions, we developed an iterative technique based on a differential equation. We, as others, assumed Rve and Cve to be constant, which concords with volume and flow dependency of model behavior. During inspiration Newtonian resistance (R) was independent of flow and volume. During expiration R increased. Static elastic recoil showed no significant hysteresis. The viscoelastic behavior of the RS was in accordance with the model. The magnitude of Rve was 3.7 +/- 0.7 cmH2O.l-1 x s, i.e., two times R. Cve was 0.23 +/- 0.051 l/cmH2O, i.e., four times static compliance. The viscoelastic time constant, i.e., Cve.Rve, was 0.82 +/- 0.11s. The work dissipated against the viscoelastic system was 0.62 +/- 0.13 cmH2O x 1 for a breath of 0.56 liter, corresponding to 32% of the total energy loss within the RS. Viscoelastic recoil contributed as a driving force during the initial part of expiration.

Respiratory mechanics, using flow interruption, was previously studied during the complete breath in healthy ventilated man, with numerical techniques relieving constraints regarding flow pattern. The classical linear model of nonNewtonian behaviour was found to be valid. The present study was extended to subjects with critical lung disease.Subjects with acute lung injury (ALI; n=2), acute respiratory distress syndrome (ARDS; n=4), and chronic obstructive pulmonary disease (COPD; n=3) were studied with and without positive end-expiratory pressure (PEEP). Functional residual capacity (FRC) was measured with sulphur hexafluoride (SF 6 ) wash-out.The static pressure-volume (P-V) curve was linear at zero end-expiratory pressure (ZEEP), but nonlinear at PEEP. Its hysteresis was nonsignificant. In ALI/ ARDS, PEEP increased lung volume by distension and recruitment, but only by distension in COPD. In ALI/ARDS, resistance was increased, at ZEEP. In COPD, resistance became extremely high during expiration at ZEEP. In ALI/ARDS at ZEEP, non-Newtonian behaviour, representing tissue stress relaxation and pendelluft, complied with the classical linear model. At PEEP, the non-Newtonian compliance became volume-dependent to an extent correlated to the nonlinearity of the static P-V curve. In COPD, non-Newtonian behaviour was adequately explained only with a model with different inspiratory and expiratory behaviour.The classical model of the respiratory system is valid in ALI/ARDS at ZEEP. More advanced models are needed at PEEP and in COPD. Eur Respir J., 1996, 9, 262-273 The flow interruption method has been used for studies both in ventilated humans and animals. The analysis has been based on a classical model. This model includes a Newtonian resistor (R) representing mainly airway resistance [1], a Newtonian capacitor (C) representing the static pressure-volume (P-V) relationship and a unit, that comprises a resistor (Rve) and a capacitor (Cve) ( fig. 1), which reflects non-Newtonian behaviour due to ventilation inhomogeneity (pendelluft) and tissue viscoelasticity (stress adaptation). It is well-known that these two phenomena cannot be distinguished on the basis of pressure and flow measured at the airway opening [1,2]. It will be discussed below that Rve and Cve probably represent tissue viscoelasticity to a greater extent than pendelluft in the present context. Rve and Cve will together be denoted the viscoelastic unit, to be considered in a broad sense. Due to methodological limitations, most studies have been limited to inspiration and constant flow [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Recently, we modified the flow interruption technique to improve accuracy of data acquisition and analysis [17]. Rve and Cve were estimated with a numerical iterative method. This method is not limited to a particular flow pattern. This enabled an analysis of the complete respiratory cycle. In normal subjects, the static P-V curve was linear. The static P-V curve did not show any hysteresis over the tidal volume...

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Twelve healthy pigs were ventilated with high frequency jet ventilation via a Mallinckrodt HiLo jet tube. The expired gas was led to a conventional ventilator and CO2 analyzer which were used to measure CO2 elimination. There was no bias flow, so that the jet entrained only expired gas, i.e. rebreathing occurred. Frequency was varied between 2 and 11 Hz and the duration of inspiration, as a fraction of the ventilatory cycle (Ti/Ttot), from 5 to 20%. The minute ventilation, Vjet, delivered by the jet ventilator was adjusted to maintain a constant PaCO2. At 2 Hz and a Ti/Ttot of 5%, Vjet was of the same magnitude as ventilation during conventional intermittent positive pressure ventilation, and the total dead space fraction, VD/VT was 0.32. Both increasing frequency at a constant Ti/Ttot, and increasing Ti/Ttot at a constant frequency, increased VD/VT which was maximal (0.8) at 11 Hz and a Ti/Ttot of 20%. When entrainment was blocked, tidal jet volume had to be greatly increased. The continuous measurement of CO2 elimination was found to be useful for maintaining isocapnia when the jet ventilator setting was changed.

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