Josef Guttmann

Electrical impedance tomography for verification of correct endotracheal tube placement in paediatric patients: a feasibility study.

Endotracheal tubes (ETTs) are frequently used in paediatric anaesthesia. Correct placement is crucial. The aim of this study was to evaluate electrical impedance tomography (EIT) for guiding and confirmation of paediatric ETT placement. In a retrospective analysis of stored EIT data, distribution of ventilation between left and right lung was used to verify correct paediatric ETT placement.

A method to measure mechanical properties of pulmonary epithelial cell layers

The lung has a huge inner alveolar surface composed of epithelial cell layers. The knowledge about mechanical properties of lung epithelia is helpful to understand the complex lung mechanics and biomechanical interactions. Methods have been developed to determine mechanical indices (e.g., tissue elasticity) which are both very complex and in need of costly equipment. Therefore, in this study, a mechanostimulator is presented to dynamically stimulate lung epithelial cell monolayers in order to determine their mechanical properties based on a simple mathematical model. First, the method was evaluated by comparison to classical tensile testing using silicone membranes as substitute for biological tissue. Second, human pulmonary epithelial cells (A549 cell line) were grown on flexible silicone membranes and stretched at a defined magnitude. Equal secant moduli were determined in the mechanostimulator and in a conventional tension testing machine (0.49 ± 0.05 MPa and 0.51 ± 0.03 MPa, respectively). The elasticity of the cell monolayer could be calculated by the volume-pressure relationship resulting from inflation of the membrane-cell construct. The secant modulus of the A549 cell layer was calculated as 0.04 ± 0.008 MPa. These findings suggest that the mechanostimulator may represent an adequate device to determine mechanical properties of cell layers.

The dynamics of carbon dioxide equilibration after alterations in the respiratory rate

Manual or automated control of mechanical ventilation can be realized as an open or closed-loop system for which the regulation of the ventilation parameters ideally is tuned to the dynamics and equilibration time of the biological system. We investigated the dynamic, transient state and equilibration time (teq) of the CO2 partial pressure (PCO2) after changes in the respiratory rate (RR). In 17 anaesthetized patients without known history of lung disease, respiratory rate was alternately increased and decreased and end-tidal CO2 partial pressures (PetCO2) were measured. Linear relations were found between ΔRR and PetCO2 changes (ΔPetCO2 = 0.3 − 1.1 ΔRR) and between ΔRR and teq for increasing and decreasing RR (teq(hypervent) = 0.5 |ΔRR|, teq(hypovent) = 0.7 |ΔRR|). Extrapolation of the transition between two PCO2 steady-states allowed for the prediction of the new PCO2 steady-state as early as 0.5  teq with an error <4 mmHg. At bedside or in automated ventilation systems, the linear dependencies between ΔRR and ΔPCO2 and between ΔRR and teq as well as early steady-state prediction of PCO2 could be used as a guidance towards a timing and step size regulation of RR that is well adapted to the biological system.

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

Objectives: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. Design:Prospective animal study. Setting:Animal research laboratory. Subjects:Male Wistar rats. Interventions: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. Measurements and Main Results: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). Conclusions: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.

Time and volume dependence of dead space in healthy and surfactant-depleted rat lungs during spontaneous breathing and mechanical ventilation

Volumetric capnography is a standard method to determine pulmonary dead space. Hereby, measured carbon dioxide (CO2) in exhaled gas volume is analyzed using the single-breath diagram for CO2. Unfortunately, most existing CO2 sensors do not work with the low tidal volumes found in small animals. Therefore, in this study, we developed a new mainstream capnograph designed for the utilization in small animals like rats. The sensor was used for determination of dead space volume in healthy and surfactant-depleted rats (n = 62) during spontaneous breathing (SB) and mechanical ventilation (MV) at three different tidal volumes: 5, 8, and 11 ml/kg. Absolute dead space and wasted ventilation (dead space volume in relation to tidal volume) were determined over a period of 1 h. Dead space increase and reversibility of the increase was investigated during MV with different tidal volumes and during SB. During SB, the dead space volume was 0.21 ± 0.14 ml and increased significantly at MV to 0.39 ± 0.03 ml at a tidal volume of 5 ml/kg and to 0.6 ± 0.08 ml at a tidal volume of 8 and 11 ml/kg. Dead space and wasted ventilation during MV increased with tidal volume. This increase was mostly reversible by switching back to SB. Surfactant depletion had no further influence on the dead space increase during MV, but impaired the reversibility of the dead space increase.

Investigation of alveolar stability in the rat lung using transthoracic endoscopy.

Transthoracic endoscopy was used to investigate mechanical stability of subpleural alveoli during repeated recruitment manoeuvres and at different plateau pressures between these manoeuvres in healthy rats ventilated at ZEEP. Images of subpleural alveoli were continuously recorded and alveolar size was measured frame by frame. Preliminary results show that the size of subpleural alveoli is constant over a wide pressure range and that repeated recruitment manoeuvres do not lead to further enlargement of alveoli. Higher plateau pressures lead to slight enlargement of subpleural alveoli.

Dynamic Hysteresis Behaviour of Respiratory System Mechanics

The static pressure-volume (PV) curve of the respiratory system is characterized by hysteresis behaviour. Determination of separate inspiratory and expiratory compliance is required to analyse this phenomenon during the dynamic situation of mechanical ventilation. In five piglets expiratory flow was linearized (flow-controlled expiration, FLEX) to allow for compliance estimation separately for inspiration and expiration. Expiratory compliance was higher than inspiratory compliance along the entire intratidal course, converging at higher volumes. At higher PEEP levels expiratory and inspiratory compliance tended to run more in parallel. We conclude that the analysis of the separate inspiratory and expiratory compliance profiles allows for indicating unfavourable mechanical ventilation settings.

Quality index control system for shape identification of intratidal compliance-volume curve with fuzzy logic

One of the goals of mechanical ventilation is to sustain alveolar recruitment while avoiding excessive lung overinflation as well as underinflation [1]. To achieve this goal the patient should be ventilated within that range of the PV-loop, where the compliance of the respiratory system is maximal. In the compliance-volume curve six shape categories can be identified [2]. Our quality index control system with fuzzy logic evaluates the quality and reliability of the compliance shape identification. If the quality index is evaluated as “o.k.”, “good” or “very good” the identified shape will be considered, otherwise the identified compliance shape will be neglected. The quality index control system helps to obtain maximal compliance avoiding errors by the compliance shape identification.

The shape of intratidal resistance-volume and compliance-volume curves in mechanical ventilation - an animal study.

In respiratory system mechanics, the shape of the intratidal pulmonary compliance-volume curve can be used to detect atelectasis or overdistension in the mechanically ventilated lung and thus to op ...

Mechanical properties of human lung cells after mechanostimulation

Experimental mechanostimulation of soft biologic tissue is a widely used method to analyze cellular reactions to stress or strain. While there are plenty different methods to stimulate cells or tissues and thereby to analyze intracellular and extracellular processes, it remains difficult to analyze the forces produced by cells or tissues while cells resist the applied strain. We previously described a mechanostimulator [1] for strain application to soft biologic samples. Here we show how lung epithelial cells that are cultivated on self-made highly flexible RGD-peptide surface treated PDMS membranes behave under increasing surface-deflection. We reveal that a decrease in stiffness (increase in compliance) of pulmonary epithelial cell layers after stimulation comes along with a change in cell adherence and loss of cell-cell junc-