Forced oscillation assessment of respiratory mechanics in ventilated patients
© BioMed Central Ltd on behalf of the copyright holder 2000
Published: 20 December 2000
The forced oscillation technique (FOT) is a method for non-invasively assessing respiratory mechanics that is applicable both in paralysed and non-paralysed patients. As the FOT requires a minimal modification of the conventional ventilation setting and does not interfere with the ventilation protocol, the technique is potentially useful to monitor patient mechanics during invasive and noninvasive ventilation. FOT allows the assessment of the respiratory system linearity by measuring resistance and reactance at different lung volumes or end-expiratory pressures. Moreover, FOT allows the physician to track the changes in patient mechanics along the ventilation cycle. Applying FOT at different frequencies may allow the physician to interpret patient mechanics in terms of models with pathophysiological interest. The current methodological and technical experience make possible the implementation of portable and compact computerised FOT systems specifically addressed to its application in the mechanical ventilation setting.
Keywordsforced oscillation technique monitoring respiratory impedance respiratory resistance airway mechanics
The mechanical properties of the respiratory system in paralysed and mechanically ventilated patients are commonly measured by means of occlusion techniques  or by multilinear regression analysis . Respiratory resistance and elastance are computed from pressure and flow recorded at the airway opening under the assumption that the contribution of the patient's respiratory muscles is nil, so that airway pressure coincides with transpulmonary pressure. Although these measurements can readily be performed on paralysed patients, their application during assisted ventilation in patients with active inspiratory muscles is not possible. The only conventional procedure for assessing respiratory mechanics when the patient's respiratory muscles are active is to employ an oesophageal balloon catheter. However, the use of an oesophageal balloon is not practicable in applications during mechanical ventilation such as noninvasive ventilation or weaning.
FOT is a method for noninvasively assessing respiratory mechanics during mechanical ventilation  and is applicable both in paralysed [4,5,6,7] and nonparalysed patients [8,9,10]. FOT is based on the application of a small pressure oscillation (about 2 cmH2O) at the airway opening by means of an external generator, and on recording the oscillation pressure and flow . When employed in non-paralysed patients, forced oscillation is applied at a frequency (more than 2 Hz) that is much higher than the patient's breathing rate. At such high frequencies the activity of the muscle pump, which operates at the breathing frequency and its first harmonics, is negligible. Under this assumption the only driving pressure at the forced oscillation frequency is the pressure applied at the mouth by the external generator. Consequently, FOT applied at a sufficiently high frequency allows the noninvasive assessment of respiratory mechanics in patients with artificial respiratory support regardless of the degree of activity of their respiratory muscles.
Oscillatory resistance and reactance
To assess oscillatory mechanics by FOT, the recorded signals (Fig. 2) are filtered to isolate the 5 Hz oscillatory signals from the low-frequency breathing components. The patient's mechanics is characterised by means of his/her respiratory impedance, which quantifies the relationship between oscillatory pressure and flow. As explained elsewhere , respiratory impedance is a complex quantity represented by two components: respiratory resistance (Rrs) and reactance (Xrs). Rrs is define as the quotient between the oscillatory pressure component in phase with flow and the oscillatory flow amplitude. Similarly, Xrs is computed from the oscillatory pressure component out of phase with oscillatory flow and the oscillatory flow amplitude .
Rrs and Xrs measured by FOT can be interpreted by means of mechanistic models of the respiratory system mechanics. For instance, the respiratory system can be modelled by a simple resistance (R), an inertance (I) and an elastance (E), in an R-I-E model. According to the definition of Rrs and of Xrs, it follows  that for a given oscillation frequency (f) the oscillatory resistance coincides with the resistance value of the simple R-I-E model (Rrs = R). Moreover, the reactance of the R-I-E model is Xrs = 2πfI – E/(2πf). Therefore, when applied to a homogeneous R-I-E model, FOT results in a Rrs value that is independent of frequency, whereas Xrs depends on frequency and accounts for the elastic (E) and inertial (I) properties. Accordingly, measuring Rrs and Xrs at different frequencies and fitting the R-I-E model to the data make it possible to derive the values of the parameters R, I and E that characterise the mechanical properties of the patient's respiratory system. The pattern of frequency dependence of Rrs and Xrs described by the R-I-E homogeneous model is observed in healthy subjects . Nevertheless, when FOT is applied in ventilated patients this simple model is no longer applicable owing to the complexity of the patient's respiratory system [5,6,7]. For this application, respiratory impedance data should be interpreted in terms of models that take into consideration the different mechanisms that determine the behaviour of the respiratory system at the oscillatory frequencies investigated [7,12,13]. In particular, modelling should include coefficients that account for the expected serial and/or parallel inhomogeneities  and the viscoelasticity of respiratory tissues . In this regard, FOT has the advantage that the frequency band explored can be modified depending on the pathophysiological aim of the measurement. For instance, if interest is focused on studying respiratory tissue properties, low-frequency (less than 2 Hz) impedance data would be more sensitive . By contrast, measurements at higher frequency would afford more sensitive data on airway characteristics . A frequency of 5 Hz or higher should be used when applying a simple sinusoidal oscillation (Fig. 1) for monitoring respiratory mechanics along the ventilation cycle [5,15,16].
The basic FOT set-up shown in Fig. 1 can be implemented by means of different technical approaches. The main practical issues are the type of oscillation generator, the procedure for measuring pressure and flow, and the type of oscillatory signal employed to measure Rrs and Xrs.
As shown in Fig. 1, the forced oscillation generator should be able to withstand the high positive ventilator pressure during inspiration and should be able to generate the forced oscillation pressure . In the absence of a FOT generator specially designed to withstand high positive pressures, the technique was employed by disconnecting the patient from the ventilator for a few seconds and applying pressure oscillation with a loudspeaker attached to the inlet of the endotracheal tube . Another method for applying FOT in ventilated patients with a simple loudspeaker-in-box system is to connect it to the expiratory outlet of the ventilator . During the inspiratory phase the expiratory valve of the ventilator is occluded with the result that the loudspeaker is not subjected to the high pressures applied to the patient. During the expiratory phase the valve is opened and the pressure oscillation generated by the loudspeaker is transmitted to the patient through the expiratory tubing. This simple method, which can be applied to both paralysed and nonparalysed patients, allows measurements only during expiration. The application of FOT during all the ventilation cycle and for any external positive end-expiratory pressure (PEEP) applied is currently possible by means of two procedures. One of the generators employed  consists of a loudspeaker-in-box system including a pneumatic filter that allows the device to withstand positive pressures. This procedure has the advantage of being very simple but its main drawback is that forced oscillation can be applied only for frequencies greater than 5 Hz, which excludes the physiological frequency band close to the breathing rate. To overcome this limitation, a more complex FOT generator based on a servocontrolled loudspeaker-in-box system has been described . This allows the extension of forced oscillation measurements to frequencies close to the breathing rate (0.25 Hz). It is of interest to note that future technical developments could enable the ventilator to generate the oscillation signal superimposed on the ventilation waveform [10,20], which would considerably simplify the routine application of FOT.
An important issue in the assessment of respiratory mechanics by FOT in ventilated patients is the measurement of flow and pressure. For practical reasons the pressure transducers are placed at a certain distance from the inlet of the endotracheal tube. Consequently, the effects of the tubes connecting the transducers to the pneumotachograph, and the pneumotachograph to the airway opening, on the frequency response of the sensors  should be taken into consideration and corrected . Although flow measurement is readily performed by a pneumotachograph, a good common-mode rejection ratio in the differential pressure transducer connected to the pneumotachograph should be ensured  or data should be corrected . This requirement, which is general in FOT measurements, is particularly relevant in this application given that the impedance values characterising ventilated patients are high. Increasing the resistance of the pneumotachograph, which could reduce potential artefacts [23,24], is not practicable because this would promote auto-PEEP. Pressure measurement in ventilated patients is influenced by the nonlinear pressure-flow relationship in the endotracheal tube. The most direct way of rounding off the artefacts caused by tube nonlinearities is to measure pressure at the tracheal level instead of at the endotracheal tube inlet. This procedure can be performed by using a special tube incorporating a catheter  or by introducing a catheter into the trachea through the endotracheal tube. In both cases, the tube nonlinearity does not affect the measurements because the recorded pressure does not include the pressure drop across the tube. These two procedures for directly recording tracheal pressure suffer from the practical drawback that they require the use of a special endotracheal tube or the introduction of an extra catheter, respectively. For routine purposes, it is more practical to measure pressure at the inlet of the endotracheal tube and to correct for its nonlinear effects in the measured impedance .
Assessment of the frequency dependence of Rrs and Xrs provides information for a better characterisation of the patient's respiratory mechanics. Determining Rrs and Xrs at several frequencies with sinusoidal oscillation (Fig. 2) would require several measurements at the different frequencies of interest, which would not be practicable in routine. Alternatively, it is possible to characterise respiratory mechanics in greater detail by measuring Rrs and Xrs at different frequencies simultaneously in a single measurement. For this purpose, a complex signal containing several frequencies is employed instead of a sinusoidal oscillation . The multifrequency signal might consist of a random noise, of a fundamental frequency plus its harmonics (for example 2, 4, 6, 8 Hz) or of a more complex combination of optimised frequencies, which is particularly useful in the case of marked nonlinearities in the respiratory system . The analysis of the complex multifrequency signals recorded and the computation of Rrs and Xrs at the different frequencies are performed automatically by means of a computer. Conventional data processing also provides an index of impedance reliability .
Application of FOT in ventilated patients
The basic approach to the assessment of respiratory mechanics by FOT is the application of a sinusoidal oscillation (Fig. 2). This procedure is particularly suitable for computing an index of respiratory impedance to monitor the changes in the patient's respiratory mechanics within the ventilation cycle during invasive and noninvasive ventilation [5,8]. Moreover, sinusoidal oscillation is also useful for assessing patient mechanics during noninvasive ventilation with continuous positive airway pressure, pressure support, volume control or proportional assist ventilation [8,9,10,28]. Given its simplicity, sinusoidal oscillation is a potential tool for routinely determining the response to different pharmacological treatments or interventions during mechanical ventilation (such as postural change , aspiration, application of PEEP [19,28], weaning).
FOT is useful for characterising the respiratory mechanics in patients subjected to noninvasive or invasive mechanical ventilation. The main advantage of this technique is that it is readily and continuously applicable during mechanical ventilation, obviating the need for disturbing or modifying the ventilation pattern. Moreover, it allows automatic and on-line monitoring of Rrs and Xrs to assess the dependence of the patient's respiratory mechanics on pressure, flow and volume as well as to detect expiratory flow limitation. Continuous FOT monitoring of the status and evolution of the ventilated patient provides updated information that is potentially useful for resetting the ventilator. Although at present the application of FOT in ventilated patients has been mainly confined to research, current methodological and technical experience enables the application of portable and compact computerised FOT systems during mechanical ventilation [30,31].
This work was supported in part by Comisión Interministerial de Ciencia y Tecnología (CICYT, SAF99-0001) and Dirección General de Enseñanza Superior e Investigación Científica (DGESIC, PM98-0027).
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