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Forced oscillation assessment of respiratory mechanics in ventilated patients.

Navajas D, Farré R - Crit Care (2000)

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Unitat de Biofisica i Bioenginyeria, Facultat de Medicina, Institut d'Investigacions Biomèdiques August Pi Sunyer, Universitat de Barcelona, Spain. dnavajas@medicina.ub.es

ABSTRACT
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.

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Respiratory resistance (Rrs) and reactance (Xrs) in a model [7] consisting of a central airway resistance of 5 cmH2O s L-1 plus a bronchial wall elastance of 700 cmH2O L-1 in parallel with a peripheral airway resistance (Rp) and a tissue elastance of 30 cmH2O L-1. Rrs and Xrs are shown for three different values of Rp.
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Figure 7: Respiratory resistance (Rrs) and reactance (Xrs) in a model [7] consisting of a central airway resistance of 5 cmH2O s L-1 plus a bronchial wall elastance of 700 cmH2O L-1 in parallel with a peripheral airway resistance (Rp) and a tissue elastance of 30 cmH2O L-1. Rrs and Xrs are shown for three different values of Rp.

Mentions: The interpretation of Rrs and Xrs measured under nonlinear conditions (Figs 5 and 6) is difficult. However, the results obtained in analogue models, in animals and in patients suggest that FOT is useful for characterising the viscoelas-tic properties of respiratory tissues [7], for assessing inhomogeneities in the respiratory system [5,6,7] and for detecting expiratory flow limitation during mechanical ventilation [16,29]. The interpretation of measured Rrs and Xrs can be performed in terms of models with a pathophysiological interest. For instance, a simple model proposed to analyse Rrs and Xrs in mechanically ventilated COPD patients consists of a central airway resistance plus a shunt elastance due to the bronchial wall, in parallel with peripheral airway resistance and tissue elastance [7]. As shown in Fig. 7, this simple model can mimic the pattern of frequency dependence of Rrs and Xrs during end-inspiratory and end-expiratory pauses (Fig. 5) by means of an increase in the peripheral resistance due to airway closure during expiration. Figure 7 shows that, in inhomogeneous models, a change in airway properties, for example an increase in peripheral resistance, modifies both Rrs and Xrs. Nevertheless, it should be stressed that so far there is no solid evidence in favour of using a specific model to interpret FOT data in mechanically ventilated patients. Consequently, the values of Rrs and Xrs measured at certain frequencies are probably the best indices for monitoring the mechanical status and evolution of the ventilated patient in clinical practice.


Forced oscillation assessment of respiratory mechanics in ventilated patients.

Navajas D, Farré R - Crit Care (2000)

Respiratory resistance (Rrs) and reactance (Xrs) in a model [7] consisting of a central airway resistance of 5 cmH2O s L-1 plus a bronchial wall elastance of 700 cmH2O L-1 in parallel with a peripheral airway resistance (Rp) and a tissue elastance of 30 cmH2O L-1. Rrs and Xrs are shown for three different values of Rp.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC137264&req=5

Figure 7: Respiratory resistance (Rrs) and reactance (Xrs) in a model [7] consisting of a central airway resistance of 5 cmH2O s L-1 plus a bronchial wall elastance of 700 cmH2O L-1 in parallel with a peripheral airway resistance (Rp) and a tissue elastance of 30 cmH2O L-1. Rrs and Xrs are shown for three different values of Rp.
Mentions: The interpretation of Rrs and Xrs measured under nonlinear conditions (Figs 5 and 6) is difficult. However, the results obtained in analogue models, in animals and in patients suggest that FOT is useful for characterising the viscoelas-tic properties of respiratory tissues [7], for assessing inhomogeneities in the respiratory system [5,6,7] and for detecting expiratory flow limitation during mechanical ventilation [16,29]. The interpretation of measured Rrs and Xrs can be performed in terms of models with a pathophysiological interest. For instance, a simple model proposed to analyse Rrs and Xrs in mechanically ventilated COPD patients consists of a central airway resistance plus a shunt elastance due to the bronchial wall, in parallel with peripheral airway resistance and tissue elastance [7]. As shown in Fig. 7, this simple model can mimic the pattern of frequency dependence of Rrs and Xrs during end-inspiratory and end-expiratory pauses (Fig. 5) by means of an increase in the peripheral resistance due to airway closure during expiration. Figure 7 shows that, in inhomogeneous models, a change in airway properties, for example an increase in peripheral resistance, modifies both Rrs and Xrs. Nevertheless, it should be stressed that so far there is no solid evidence in favour of using a specific model to interpret FOT data in mechanically ventilated patients. Consequently, the values of Rrs and Xrs measured at certain frequencies are probably the best indices for monitoring the mechanical status and evolution of the ventilated patient in clinical practice.

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Unitat de Biofisica i Bioenginyeria, Facultat de Medicina, Institut d'Investigacions Biomèdiques August Pi Sunyer, Universitat de Barcelona, Spain. dnavajas@medicina.ub.es

ABSTRACT
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.

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