Knut Möller

Dynamic versus static respiratory mechanics in acute lung injury and acute respiratory distress syndrome.

Objectives:It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure. Design:Prospective multiple-center study. Setting:Intensive care units of eight German university hospitals. Patients:A total of 28 patients with acute lung injury and acute respiratory distress syndrome. Interventions:None. Measurements:Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation. Main Results:The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure–volume curve, whereas end-expiratory volumes significantly exceeded the static pressure–volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis. Conclusions:Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure–volume curves.

Regional ventilation in cystic fibrosis measured by electrical impedance tomography

BACKGROUND The feasibility of electrical impedance tomography (EIT) as an alternative examination tool in cystic fibrosis (CF) was examined. METHODS 14 CF patients and 14 healthy volunteers were studied. Spirometry and EIT measurements were performed simultaneously. The global inhomogeneity (GI) index was applied to assess the degree of ventilation homogeneity at different levels of maximum inspiratory volume. Ratios of maximum expiratory flow at 25% and 75% of vital capacity (MEF(25)/MEF(75)) were calculated for both global lung and regional areas in EIT images. RESULTS Significant differences among GI values at various lung volumes were found in CF patients (P<0.01) but not in healthy subjects. Global MEF(25)/MEF(75) measured with spirometry and with EIT were highly correlated for all subjects (r(2)=0.69, P<0.01). Significant difference in global MEF(25)/MEF(75) was found between CF patients and healthy volunteers with both spirometer (CF: 0.15±0.09; healthy: 0.46±0.15; P<0.001) and EIT (CF: 0.14±0.09; healthy: 0.42±0.08; P<0.001). Regional airway obstruction was identified in the MEF(25)/MEF(75) maps in CF patients. CONCLUSIONS Compared to the global parameters provided by spirometry, EIT is able to deliver both global and regional information to assess the airway obstruction in CF patients.

Dynamically generated models for medical decision support systems

Doctors applying mechanical ventilation need to find the best balance between benefit and risk for the patient. Mathematical models simulating patients reactions to alterations in the ventilation regime may be employed. A framework is introduced that is able to dynamically combine mathematical models from different model families to form a complex interacting model system. Each of these families consists of submodels differing in complexity of dynamics formulation or anatomical/geometrical resolution. The interaction of model systems reveals qualitatively varying results depending on the complexity of the involved models. Realistic overlaying of respiratory and cardiovascular rhythms can be detected in blood gas concentrations.

A lung area estimation method for analysis of ventilation inhomogeneity based on electrical impedance tomography.

PURPOSE To evaluate a novel method for lung area estimation (LAE method) in electrical impedance tomography (EIT) images as a prerequisite of quantitative analysis of ventilation inhomogeneity. METHODS The LAE method mirrors the lung regions in the functional EIT (fEIT) image and subsequently subtracts the cardiac related areas. In this preliminary study, 51 mechanically ventilated patients were investigated, including 39~patients scheduled for thoracic surgery (test group); 10 patients scheduled for orthopedic surgery without pulmonary disease (control group) and 2 ICU patients undergoing chest computed tomography (CT) examination. EIT data was recorded in all groups. The results of the LAE method were compared to those obtained with the fEIT method and to CT images. RESULTS The lung area size determined with fEIT in control group is S(C,fEIT) = 361 +/- 35 (mean +/- SD) and in test group S(T,fEIT) = 299 +/- 61 (p< 0.01). The sizes estimated with the LAE method in control group S(C,LAE) = 353 +/- 27 and in test group S(T,LAE) = 353 +/- 61 (p=0.41). The result demonstrates that the novel LAE method improves the identification of lung region in EIT images, from which the analysis of ventilation distribution will benefit. The preliminary comparison with CT images exemplary indicates a closer match of the lung area shapes after the LAE than after the fEIT-based analysis. CONCLUSION The LAE method is a robust lung area determination method, suitable for patients with healthy or seriously injured lungs.

Pressure-dependent stress relaxation in acute respiratory distress syndrome and healthy lungs: an investigation based on a viscoelastic model

IntroductionLimiting the energy transfer between ventilator and lung is crucial for ventilatory strategy in acute respiratory distress syndrome (ARDS). Part of the energy is transmitted to the viscoelastic tissue components where it is stored or dissipates. In mechanically ventilated patients, viscoelasticity can be investigated by analyzing pulmonary stress relaxation. While stress relaxation processes of the lung have been intensively investigated, non-linear interrelations have not been systematically analyzed, and such analyses have been limited to small volume or pressure ranges. In this study, stress relaxation of mechanically ventilated lungs was investigated, focusing on non-linear dependence on pressure. The range of inspiratory capacity was analyzed up to a plateau pressure of 45 cmH2O.MethodsTwenty ARDS patients and eleven patients with normal lungs under mechanical ventilation were included. Rapid flow interruptions were repetitively applied using an automated super-syringe maneuver. Viscoelastic resistance, compliance and time constant were determined by multiple regression analysis using a lumped parameter model. This same viscoelastic model was used to investigate the frequency dependence of the respiratory systems impedance.ResultsThe viscoelastic time constant was independent of pressure, and it did not differ between normal and ARDS lungs. In contrast, viscoelastic resistance increased non-linearly with pressure (normal: 8.4 (7.4-11.9) [median (lower - upper quartile)] to 35.2 (25.6-39.5) cmH2O·sec/L; ARDS: 11.9 (9.2-22.1) to 73.5 (56.8-98.7)cmH2O·sec/L), and viscoelastic compliance decreased non-linearly with pressure (normal: 130.1(116.9-151.3) to 37.4(34.7-46.3) mL/cmH2O; ARDS: 125.8(80.0-211.0) to 17.1(13.8-24.7)mL/cmH2O). The pulmonary impedance increased with pressure and decreased with respiratory frequency.ConclusionsViscoelastic compliance and resistance are highly non-linear with respect to pressure and differ considerably between ARDS and normal lungs. None of these characteristics can be observed for the viscoelastic time constant. From our analysis of viscoelastic properties we cautiously conclude that the energy transfer from the respirator to the lung can be reduced by application of low inspiratory plateau pressures and high respiratory frequencies. This we consider to be potentially lung protective.

Positioning of electrode plane systematically influences EIT imaging.

Up to now, the impact of electrode positioning on electrical impedance tomography (EIT) had not been systematically analyzed due to the lack of a reference method. The aim of the study was to determine the impact of electrode positioning on EIT imaging in spontaneously breathing subjects at different ventilation levels with our novel lung function measurement setup combining EIT and body plethysmography. EIT measurements were conducted in three transverse planes between the 3rd and 4th intercostal space (ICS), at the 5th ICS and between the 6th and 7th ICS (named as cranial, middle and caudal) on 12 healthy subjects. Pulmonary function tests were performed simultaneously by body plethysmography to determine functional residual capacity (FRC), vital capacity (VC), tidal volume (VT), expiratory reserve volume (ERV), and inspiratory reserve volume (IRV). Ratios of impedance changes and body plethysmographic volumes were calculated for every thorax plane (ΔIERV/ERV, ΔIVT/VT and ΔIIRV/IRV). In all measurements of a subject, FRC values and VC values differed ≤5%, which confirmed that subjects were breathing at comparable end-expiratory levels and with similar efforts. In the cranial thorax plane the normalized ΔIERV/ERV ratio in all subjects was significantly higher than the normalized ΔIIRV/IRV ratio whereas the opposite was found in the caudal chest plane. No significant difference between the two normalized ratios was found in the middle thoracic plane. Depending on electrode positioning, impedance to volume ratios may either increase or decrease in the same lung condition, which may lead to opposite clinical decisions.

Iterative integral parameter identification of a respiratory mechanics model

BackgroundPatient-specific respiratory mechanics models can support the evaluation of optimal lung protective ventilator settings during ventilation therapy. Clinical application requires that the individual’s model parameter values must be identified with information available at the bedside. Multiple linear regression or gradient-based parameter identification methods are highly sensitive to noise and initial parameter estimates. Thus, they are difficult to apply at the bedside to support therapeutic decisions.MethodsAn iterative integral parameter identification method is applied to a second order respiratory mechanics model. The method is compared to the commonly used regression methods and error-mapping approaches using simulated and clinical data. The clinical potential of the method was evaluated on data from 13 Acute Respiratory Distress Syndrome (ARDS) patients.ResultsThe iterative integral method converged to error minima 350 times faster than the Simplex Search Method using simulation data sets and 50 times faster using clinical data sets. Established regression methods reported erroneous results due to sensitivity to noise. In contrast, the iterative integral method was effective independent of initial parameter estimations, and converged successfully in each case tested.ConclusionThese investigations reveal that the iterative integral method is beneficial with respect to computing time, operator independence and robustness, and thus applicable at the bedside for this clinical application.

Detection of partial endotracheal tube obstruction by forced pressure oscillations

Rapid airway occlusions during mechanical ventilation are followed immediately by high-frequency pressure oscillations. To answer the question if the frequency of forced pressure oscillations is an indicator for partial obstruction of the endotracheal tube (ETT) we performed mathematical simulations and studies in a ventilated physical lung model. Model-derived predictions were evaluated in seven healthy volunteers. Partial ETT obstruction was mimicked by decreasing the inner diameter (ID) of the ETT. In the physical model ETTs of different ID were used. In spontaneously breathing volunteers viscous fluid was applied into the ETTs lumen. According to the predictions derived from mathematical simulations, narrowing of the ETTs ID from 9.0 to 7.0mm decreased the frequency of the pressure oscillations by 11% while changes of the respiratory systems compliance had no effect. In volunteers, a similar reduction (10.9%) was found when 5 ml fluid were applied. We conclude that analysis of pressure oscillations after flow interruption offers a tool for non-invasive detection of partial ETT obstruction.

Analysis of Urinary Stones by Computerized Infrared Spectroscopy

The computerized assessment of infrared spectra of urinary stones with existing programmes such as SEARCH (Lehmann, C. A. et al. (1988) Clin. Chim. Acta 173, 107-116), TWIN or CIRCOM (Hesse, A. et al. (1988) Fresenius Z. Anal. Chem. 330, 372-373) has proved to be unreliable when used for routine urinary stone analysis. A more refined method has to be used in place of simple comparison algorithms. STONES is a new programme for computerized analysis of urinary stones developed with the intention of simulating the former non-computerized analysis procedure. STONES is a rule-based system, which interprets the infrared spectra qualitatively by its rules. A quantitative result is obtained by means of library search. Combining these two methods 93% of the tests were correct with regard to clinical relevance.

TUG Test Instrumentation for Parkinson's disease patients using Inertial Sensors and Dynamic Time Warping.

The Timed Up and Go (TUG) test is a clinical tool widely used to evaluate balance and mobility, e.g. in Parkinson’s disease (PD). This test includes a sequence of functional activities, namely: sit-to-stand, 3-meters walk, 180° turning, walk back, and turn-to-sit. The work introduces a new method to instrument the TUG test using a wearable inertial sen-sor unit (DynaPort Hybrid, McRoberts B.V., NL) attached on the lower back of the person. It builds on Dynamic Time Warping (DTW) for detection and duration assessment of associated state transitions. An automatic assessment to sub-stitute a manual evaluation with visual observation and a stopwatch is aimed at to gain objective information about the patients. The algorithm was tested on data of 10 healthy individuals and 20 patients with Parkinsons disease (10 pa-tients for early and late disease phases respectively). The algorithm successfully extracted the time information of the sit-to-stand, turn and turn-to-sit transitions.