Single-breath method for assessing the viscoelastic properties of the respiratory system

Determination of Respiratory Parameters by Means of Hurst Exponents of the Respiratory Sounds and Stochastic Processing Methods

OBJECTIVES

System approach to the human respiratory system and input/output signals which characterize the system properties were not explored in detail in the literature. The aim of this study is to propose a combination of methods to investigate the indirect relationship between the fractal properties of Respiratory Signals (RS) and Respiratory Sound Signals (RSS) and the clinically measured respiratory parameters.

METHODS

We used Hurst exponent to reveal the fractal properties of RS and RSS and to estimate the pressures in the respiratory system. The combination of well-known statistical signal processing methods and optimization were applied to the experimentally acquired 23 records. Pearson correlation coefficient and Bland-Altman analysis were the chosen validation methods.

RESULTS

Considerable amounts of Hurst exponent values of RSS were found to be between 0.5 and 1, which means increasing trend or decreasing trend can be seen in RSS with fractional Gaussian process properties. Results of the pressure estimator revealed that internal pressure due to tissue viscoelasticity is higher than the pressure due to static elasticity. Feature power and skewness also provided distinctive results for all recordings.

CONCLUSION

Hurst exponent values of the RSS are fruitful representation of the signals which bring the underlaying system characteristics into the surface. We illustrated that required number of sensors can be reduced in the feature calculation to ease implementation effort on the hardware of the handheld devices.

SIGNIFICANCE

Bland-Altman plots were very successful to demonstrate the connection between the sets of measured respiratory parameters and calculated features.

Gas distribution in a two-compartment model during volume or pressure ventilation: role of elastic elements

The results of the studies on pulmonary gas distribution during constant-flow controlled-volume inflation (VCV) and inspiratory constant pressure inflation (PCV) in experimental studies are conflicting. In a mathematical model, with the characteristics of two lung compartments including tissue viscoelastic properties, pulmonary gas distribution was tested by simulating PCV and VCV at same inflation volumes. The compartmental distributions of the tidal volume were compared during CMV and PCV in different configurations obtained by changing the elastic and viscoelastic properties in each compartment, but maintaining the same total values of respiratory mechanics measured in patients. In all instances PCV resulted in a slightly higher air-trapping than in VCV mode. Heterogeneous elastic properties diverted most of the tidal volume towards the less compromised compartment. However, both ventilatory modes provided similar compartmental gas distribution, but during VCV compartmental peak pressures were higher in the sicker compartment respect to PCV. The use of PCV could grant a less remarkable pressure variability able to reduce the potential ventilator-associated lung injury. Moreover, the parameters measured during an end-inspiratory pause could not pinpoint unique characteristics for each configuration.

Effects of a Sigh on the Respiratory Mechanical Properties in Ali Patients

OBJECTIVES

The application of sighs during baseline ventilation was found to improve alveolar recruitment and oxygenation in patients with acute respiratory distress syndrome (ARDS). The present investigation evaluates if respiratory mechanics can be modified by a sigh.

METHODS

Ten consecutive patients with acute lung injury (ALI) admitted to the University Hospital Intensive Care Unit the were studied during mechanical ventilation. Three sighs were administered to sedated-paralyzed patients during the measurement period. Respiratory mechanics were studied in regular breaths immediately before and after a sigh provided that a steady-state had been reached and by the airway pressure-time curve profile to evaluate the lung recruitment. Viscoelastic constants (elastic, resistive, and time), as well as elastance and resistances, were determined by the single breath method. Arterial blood gases were also determined pre- and post-sigh.

RESULTS

Elastic and resistive components of viscoelasticity decreased after a sigh (20 and 21%, respectively). As a result, the pressure required to overcome viscoelasticity and mechanical inhomogeneities also decreased in these patients (17%). The mechanical changes were associated with alterations in PaO(2).

CONCLUSIONS

The sigh is useful to diminish viscoelastic impedance in ALI patients, thus allowing a smaller inflation pressure. Under the present experimental conditions it seems that viscoelastic mechanical alterations precede their elastic and resistive counterparts.

Effects of viscoelasticity on volume distribution in a two-compartmental model of normal and sick lungs

Among the models describing respiratory mechanics none has been published with the characteristics of two lung compartments including the viscoelastic properties. We used such a model to describe the inspiratory compartmental volume distribution under homogeneous and inhomogeneous conditions. The present mathematical model was tested against actual data and proven accurate. The volume distribution was studied using data from normal subjects and from patients with COPD and ARDS. In a normal lung, changes in viscoelastic constants in one compartment can modify substantially the volume distribution diverting more or less gas to the other compartment. In diseased compartments, the increase of viscoelasticity increased the difference between the compartments and the opposite was true in the less affected compartment. In conclusion, the viscoelastic properties are of paramount importance in determining gas distribution in normal and sick lungs.

Sigh: tool to determine the respiratory viscoelastic properties

OBJECTIVE

In mechanically ventilated patients a high fraction of the pressure can be dissipated to overcome the viscoelastic components of the respiratory system. Recently it was demonstrated that sigh improved oxygenation in mechanically ventilated ARDS patients. We evaluated if, in acute lung injury (ALI) patients, the sigh can be used to measure the respiratory viscoelastic properties.

METHODS

Ten consecutive normal subjects undergoing general anaesthesia for minor abdominal surgery and ten ALI patients admitted to the ICU, were studied. Three sighs were administered every minute during the measurement period. The viscoelastic constants (E2, R2 and tau2) were determined by (i) a series of end-inflation airway occlusions (multiple breath method, MBM) and (ii) fitting the time course of the slow decay in pressure during end inspiratory pause of the sigh (sigh method, SM). The results were compared by means of the limits of agreement as modified for small sample sizes.

RESULTS

Viscoelastic parameters were similar to those obtained in other studies. In normal subjects the mean differences (+/- SEM) of tau2, R2, and E2 given by the SM and the MBM were 0 +/- 0.04 s, 0.37 +/- 0.20 cmH2O L(-1) s, and 0.21 +/- 0.26 cmH2O L(-1), respectively. The mean differences (+/- SEM) of tau2, R2, and E2 in ALI patients were 0.02 +/- 0.02 s, 0.45 +/- 0.31 cmH2O L(-1) s, 0.34 +/- 0.36 cmH2O L(-1), respectively. No lack of agreement could be detected between the two methods in all variables in normal subjects and ALI patients.

CONCLUSIONS

The long inflation time characteristic of the sigh allowed the determination of the viscoelastic constants by means of a simpler and faster method. Moreover it does not require very small tidal volumes, which can increase reabsorption atelectasis in ALI patients and can improve alveolar recruitment and oxygenation in these patients.