Analysis Of Respiratory System Mechanics Research Articles

VALIDATION OF THE LEAST SQUARES FITTING METHOD (LSF) DURING NAVA AND PSV VENTILATION

The Least Squares Fitting (LSF) is a computerized method of analysis of respiratory system mechanics. It is based on applying a regression analysis for every sample points of the loop of pressure, flow and volume by fitting the equation Paw= Rrs× V' + VT/Crs+ PEEPtot during inspiration [1]. This technique has been already validated in Controlled Mechanical Ventilation (CMV) and at high level of Pressure Support Ventilation (PSV) [2]. However this method gives unreliable values of resistance (Rrs) and elastance (Ers) in presence of inspiratory muscle activity and in absence of an adequate neuromuscular coupling. We reasoned that NAVA (Neurally-Adjusted Ventilatory Assist) ventilation should assure a better neuromuscular coupling compared to PSV and hence the coefficient of determination (CD) of the above equation should be much higher during NAVA ventilation.

Endoscopic Imaging to Assess Alveolar Mechanics During Quasi-static and Dynamic Ventilatory Conditions in Rats With Noninjured and Injured Lungs*

Although global respiratory mechanics are usually used to determine the settings of mechanical ventilation, this approach does not adequately take into account alveolar mechanics. However, it should be expected that the ventilatory condition (quasi-static vs. dynamic) and lung condition (noninjured vs. injured) affect alveolar mechanics in a clinically relevant way. Accordingly, the aim of this study was to investigate alveolar mechanics during quasi-static and dynamic ventilatory maneuvers in noninjured and injured lungs. We hypothesized that alveolar mechanics vary with ventilatory and lung conditions. Prospective animal study. Animal research laboratory. Male Wistar rats. Alveolar mechanics (derived from alveolar size and airway pressure) were determined in noninjured (n = 9) and in lungs lavaged with saline (n = 8) at quasi-static (low flow at a peak pressure of 40 cm H₂O) and dynamic ventilatory maneuvers (increase and decrease in positive end-expiratory pressure from 0 to 15 and back to 0 cm H₂O in steps of 3 cm H₂O). Alveoli were recorded endoscopically and alveolar mechanics were extracted using automated tracking of alveolar contours. The increase in alveolar size during quasi-static maneuvers was significantly greater than during dynamic maneuvers in noninjured (mean difference 18%, p < 0.001) but not in injured lungs (mean difference 3%, p = 0.293). During dynamic maneuvers, slope of the intratidal alveolar pressure/area curve (reflecting distensibility) decreased with increasing positive end-expiratory pressure (p = 0.001) independent of lung condition (noninjured and injured lungs). In contrast, independent of positive end-expiratory pressure but dependent on lung condition, the maximal tidal change in alveolar size was greater by an average of 40% in injured compared with noninjured lungs (p = 0.028). Alveolar mechanics during mechanical ventilation differed between quasi-static and dynamic conditions and varied with lung condition. Our data thus confirm that analysis of respiratory system mechanics under dynamic conditions is preferable to analysis during static conditions.

Reliability of pressure measurements via balloon catheters is high: an evaluation of six esophageal catheters

Reliable measurement of esophageal pressure is a prerequisite for analysis of respiratory system mechanics in spontaneous breathing patients. For that purpose, various types of balloon catheters exist that differ in material, size and shape. In physical models we studied the quality of pressure measurement via six different balloon catheters, three of them containing a second balloon for measurement of gastric pressure.

Estimating the intratidal compliance profile with the new gliding-SLICE method

At the bedside, but even in most research work, the analysis of respiratory system mechanics is limited to quasi-static conditions excluding any insight into what happens during the breath. The new gliding-SLICE method helps looking into the breath. It is a further development of the SLICE method for calculating compliance and resistance of subsequent intratidal volume ranges (slices) of the pressure-volume (PV) loop by multiple linear regression analysis in a continuous way. This allows for detecting intratidal compliance and resistance nonlinearity during ongoing ventilation. Our objective was to determine whether the nonlinear intratidal compliance profile hints at what level to set PEEP and tidal volume (VT) to make lung ventilation protective.

Estimating intratidal nonlinearity of respiratory system mechanics: a model study using the enhanced gliding-SLICE method

In the clinical situation and in most research work, the analysis of respiratory system mechanics is limited to the estimation of single-value compliances during static or quasi-static conditions. In contrast, our SLICE method analyses intratidal nonlinearity under the dynamic conditions of mechanical ventilation by calculating compliance and resistance for six conjoined volume portions (slices) of the pressure–volume loop by multiple linear regression analysis. With the gliding-SLICE method we present a new approach to determine continuous intratidal nonlinear compliance. The performance of the gliding-SLICE method was tested both in computer simulations and in a physical model of the lung, both simulating different intratidal compliance profiles. Compared to the original SLICE method, the gliding-SLICE method resulted in smaller errors when calculating the compliance or pressure course (all p < 0.001) and in a significant reduction of the discontinuity error for compliance determination which was reduced from 12.7 ± 7.2 cmH2O s L−1 to 0.8 ± 0.3 cmH2O s L−1 (mathematical model) and from 7.2 ± 3.9 cmH2O s L−1 to 0.4 ± 0.2 cmH2O s L−1 (physical model) (all p < 0.001). We conclude that the new gliding-SLICE method allows detailed assessment of intratidal nonlinear respiratory system mechanics without discontinuity error.

Continuous monitoring of tracheal pressure including spot-check of endotracheal tube resistance

During mechanical ventilation, the resistance of the endotracheal and tracheostomy tube (ETT) highly influences analysis of respiratory system mechanics and imposes additional work of breathing for...

Continuous monitoring of tracheal pressure including spot-check of endotracheal tube resistance

During mechanical ventilation, the resistance of the endotracheal and tracheostomy tube (ETT) highly influences analysis of respiratory system mechanics and imposes additional work of breathing for the spontaneously breathing patient which both can be circumvented by applying the automatic tube compensation (ATC) mode. In the ATC mode, tracheal pressure (ptrach) is continuously calculated on the basis of measured flow and airway pressure using predetermined tube specific coefficients. However, as during long-term ventilation the ETT might become partially obstructed by secretions or tube kinking, the predetermined coefficients are no longer valid rendering calculation of ptrach inaccurate. We propose an easy-to-handle maneuver for the bedside determination of current tube coefficients in the tracheally intubated patient. Based on check-spot measurement of ptrach, current tube coefficients are determined by a least-squares fit procedure valid for the partially obstructed ETT with the indwelling pressure-measuring catheter (PMC). To correct for the removal of the PMC, the relationship between tube coefficients with and those without indwelling PMC has been determined in a laboratory investigation. Accuracy of the procedure was determined during artificial ETT obstruction by comparing calculated with measured ptrach. Correspondence between calculated and measured ptrach has been found excellent. We conclude that by adopting this bedside procedure periodically, accurate calculation of ptrach is guaranteed and the advantages of the ATC mode are ensured even in long-term ventilatory support.

Volume dependence of respiratory system resistance during artificial ventilation in rabbits

The volume dependence of respiratory resistance (Rrs), usually observed during normal breathing, is expected to be accentuated during expiratory flow limitation (EFL). In order to quantify this dependence we studied the pressure, flow, and volume data obtained from eight New Zealand rabbits, artificially ventilated at different levels of applied expiratory pressure (0-10 hPa), before and during histamine i. v. infusion. EFL was provoked by lowering the expiratory pressure and was detected by the application of an additional negative expiratory pressure and by forced oscillations. The analysis of respiratory system mechanics was performed by multiple regression, using the classical linear first-order model and also a nonlinear model, accounting for volume dependence of Rrs. Both models satisfactorily fitted the data in the absence of EFL. The nonlinear model proved to be more appropriate in the presence of EFL. The coefficient expressing the volume dependence of Rrs (Rvd) was significantly more negative during EFL. Rvd values were highly correlated with the fraction of the tidal volume left to be expired at the onset of EFL. A threshold Rvd value of -1,000 (hPa x s x l(-2)) detected EFL with high sensitivity and specificity. We conclude that a strongly negative volume dependence of Rrs is a reliable and noninvasive index of EFL during artificial ventilation.

Grundlagen der Lungenmechanik unter Beatmung

The study of lung mechanics deals with the pressures acting on the respiratory system (lungs and chest wall) and the changes in volume that they produce. In mechanical ventilation, the technical gas delivery system (ventilator, respiratory tubing system, gas humidifier) and the respiratory system of the patient form a connected pneumatic system of high complexity. The respiratory system produces a mechanical impedance is composed of an elastic and a non-elastic, i.e., resistive part. The corresponding indices describing respiratory mechanics are compliance and flow resistance. Based on the equation of motion of the respiratory system, the methods of analyzing semistatic compliance and resistance during mechanical ventilation are described. Quantitative analysis of respiratory system mechanics (a) is a prerequisite for the understanding of the complex patient-ventilator interaction, (b) provides important clinical information on pulmonary function and the course of disease, and (c) allows the physician at the bedside to adjust the ventilatory settings to the needs of the individual patient.

Analysis of respiratory mechanics during artificial ventilation.

Mechanical or artificial ventilation is the most important life-saving therapeutic instrument in modern intensive care medicine. The ventilator takes on the convective transport of the respiratory gas, i.e. delivery of oxygen and removal of carbon dioxide. The technical gas delivery system (ventilator, respiratory tubing system, gas humidifier) and the respiratory system (lungs and thorax) of the patients form a connected pneumatic system of high complexity. The respiratory system produces a mechanical impedance to ventilator output. Impedance is composed of an elastic, a non-elastic, i.e. resistive, and an inertive part. The corresponding indices describing respiratory mechanics are compliance, flow resistance and inertance. Based on the equation of motion of the respiratory system, several methods of analysing respiratory mechanics during mechanical ventilation are described. Quantitative analysis of respiratory system mechanics (a) is a prerequisite for the understanding of the complex patient-ventilator interaction, (b) provides important clinical information on pulmonary function and the course of disease, and (c) allows the physician at the bedside to adjust the ventilatory settings to the needs of the individual patient.