impedanztomographie – An Overview

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Non-invasive monitoring of maximalinspiratory and expiratory flows(MIF and MEF,respectively)by electrical impedance tomography(EIT)may allow foran early detection of changes tophysical properties and functions of respiratory systemsduetonew circumstances or inresponse totreatments.The aim of this study was to verifyEIT-basedmeasurementsforMIFas well asMEF against spirometryin intubatedhypoxemic patients during controlled ventilationand spontaneous breathing.Moreover, regional distribution ofmaximum airflows can interact withlungdiseases and increasethechance of further ventilationinjuries.Therefore, we also soughttoinvestigate the effectsofventilation settings that affectregion-wideMIFandMEF.

Methods

We carried out a new analysisofthe datain two prospective, randomizedcross-sectionalstudies.We included patients who wereadmitted to theintensive care unitacute hypoxemic respiratory failure(AHRF)and acute respiratory stress syndrome(ARDS)undergoing pressure supportventilatory(PSV, n10) andVCV, n=20).(VCV, n20).We assessed MIF and MIFusing spirometry, EIT andthe six ventilation combinations withhigherthan. lower supportinPSVand higherin comparison to. lowerpositive end-expiratory pressure(PEEP)inbothVCV and PSV.Regional airflows were analyzed usingEITin both non-dependent and dependentlung regions.

Results

MIF and impedanztomographie determinedbyEIT wereclosely correlated tothe spirometry measurements duringevery condition(rangeinR2 0.629-0.776 and R2 0.606-0.772respectively, p<0.05forall) that was acceptable for clinicalboundaries of disagreement.A higher PEEP was able to improveuniformity in thedistributionof MIF and MEFduring ventilation controlled by volumeby increasing airflows within thedependent lung regions and loweringthe levels in non-dependent lung regions.

Conclusions

EIToffers accurate, non-invasive monitoringforMIFandMEF.The current study also positsthepossibilitythat EITcould provide guidance forPSV and PEEPset-upsto ensure homogeneity inthe regional airflows that are extending and deflating.

Introduction

Electronic impedance tomography(EIT)isanon-invasive, bedside, radiation-free,technology for lung imaging that is dynamic. EITproduces maps of the intrathoraciclung impedance changes that are linked toan initial value(i.e.the volume of the lungs at the end of expiration fromthe previousbreath) every20-50 ms ([1].The changes in intrathoracic impedance measuredviaEIT are linearlylinked tothe global and regional tidal volumes andheld at increasing positive end-expiratorypressure (PEEP) levels [22.This means thatEITproduces a noninvasive bedside continuousmeasure of regionallung volumefluctuations duringinspiration and exhalation.

Inspiratory and expiratory airflows are relatedto thevelocity at whichlung volume fluctuationsovertime.In patients with intubation,they aretypically measured usingan spirometer connectedto the ventilator’s circuit prior tothetube for endotracheal intubation or withinthe ventilator.Global maximum inspiratory andexpiratoryflows(MIF and MEF as well)as measured bystandard spirometry depend uponhow the respiratory systems’ mechanical characteristics work(namely lung compliance, lung pressure andresistance to airways) [33.Therefore, monitoring ofMIF andMEF couldhelp to determineadjustments to the ventilation system(e.g. by determiningtheappropriate pressure level that correspondswithimprovedmechanics)and/or to evaluatetheefficacy of pharmacologic treatments(e.g. increasingMIFor MEF followingthe use of bronchodilator medications) [44.However, spirometry provides onlyglobal measures of MIF andMEF, and heterogeneous distributionofimpaired lung mechanics is acharacteristic of acute hypoxemicfailure(AHRF)and acute respiratory distress syndrome(ARDS) [55.Alveolar damage leads tocollapse of lung structureslocated between normal-, part-and excessively inflated units that could result in the formation ofvariationswithin regionalMIFandMEF values.These imbalances may increasethechance of developing a ventilator-induced lung disease(VILI)through a variety of ways[6, 7], whilethat create more homogenous regional flowcould decrease the risk. Externalspirometry may leadtoaltered patterns in the respiratory system andincorrect measurements,as well[7].Therefore, a non-invasivebedsidedynamic method to measurethe global and regional MIF as well asMEFvalues canbe a significant addition tostudyingAHRF and ARDSpatients’ pathophysiology andto provide personalized treatment.

In the present study,using preliminary data from ananimal models[8], our goal wastotest the validity ofventilatedAHRFas well asARDS patientsreceivingcontrolledbreathing as well asEIT-based breathing tests that are based on spontaneous breathing to measureglobal MIF and MEF compared tostandardspirometry.Moreover, we exploredtheimpact of higher vs. lowerPEEP andpressure support levels onregionalflow patterns;the hypothesis we have isthat higherlevels of PEEPand lower pressure support willproduce a more homogenous distribution ofregions ofMIFandMEF.

Materials and methods

Studyof the population

We performed a new analysis of data collected during two prospective randomized crossover studies: in the first (pressure support ventilation (PSV) study) [9], ten intubated patients recovering from ARDS [10], lightly sedated (RASS – 2/0), undergoing PSV and admitted to the intensive care unit (ICU) of the university-affiliated San Gerardo Hospital, Monza, Italy, were enrolled; and in the second (volume-controlled ventilation (VCV) study) [11], twenty intubated, deeply sedated and paralyzed patients with AHRF (i.e., PaO2/FiO2 <=300, PEEP >=5 cmH2O, acute onset, no cardiac failure) or ARDS admitted to the same ICU were enrolled. Theethics committee atSan Gerardo Hospital, Monza, Italy, approved thestudies,and informed consent was obtainedin accordance withlocalguidelines.Additional details ontheinclusion and exclusion criteriaforthe twostudies are containedinthe online data supplement(AdditionalFile1.).

Demographic data collection

Wecollected sex, age, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andthe severityof ARDS days onmechanical ventilationprior to study enrollmentforeachpatient.The death rate at the hospital was documented,too.

EIT andventilation monitoring

Ineach patient, EIT-dedicatedbelt,comprising 16 evenlyspaced electrodes, was positionedover the thorax aroundthesixth or fifthintercostalregion and connected witha commercialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).In all phases of the study,EITdata were generated byuse of small alternationelectrical currents which rotated around thean individual’s thorax. They were continuously recordedat 20 Hzand storedfor offline analysis, in accordance with the procedure previouslyexplained [12and 13.In sync withEITtracer data, airway pressure andairflows ofthe mechanical ventilator wererecorded continuously.

Interventions

More details onthe two protocolscan be foundin theonline data supplement(Additionaldata supplement file1.).

Briefly, inthisPSV study,patients were subjected tothe followingrandomized steps with each one lasting 20 minutes:

  1. 1.

    Low support for PEEP in clinical(PSV low)against.higher support atPEEP at the clinic(PSV high);

  2. 2.

    Clinical supportatlow PEEP(PSV-PEEP low)compares to.clinical support at higher PEEP(PSV-PEEP high).

For theVCV study,insteadthe following phaseswere performedwith randomized crossoverseach lasting20 mins:

  1. 1.

    Secure VCV during low PEEP(VCV-PEEP low)in comparison to.VCV to protect at clinicalPEEPand 5cmH 2O (VCV-PEEP high).

EIT andventilation data

Through offline analysis ofEITtracer results obtained inthefinal minutesfrom each study phase(analysis oftenbreaths), we measuredglobally and regional(same-sizeregion of lung that is dependent as well as non-dependent) noninvasive airflows’ waveforms,similar to what was previously described[8in [8.].In short, instantaneous global andregionalexpiratory and inspiratoryairflowswere recorded asvariationsof global and regionalimpedance measured every 50 ms when multiplied with the tidalimpedance/volume ratio obtained fromthesame study phase anddivided by 50ms. EIT airflow data werethen transformed from mL/msecL/min (Fig. 1) and the maximumEIT-derived regional and global MIFand MEF (MIFglob MIFglob, MIFnondepand MIFdepMEFglob with MEFnon-dep, MEFglob andMEFdep as well as MEFdep and MIFnon-dep) wereidentified . Thevalue averaged over5-10 consecutiverespiratorycycles.

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