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Considerations To Know About impedanztomographie

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Noninvasive monitoring of the maximalflow of expiratory and inspiratory(MIF and MEFrespectively)using electrical impedance tomography(EIT)could allowrapid recognition of changes inrespiration’s mechanical parametersduetothe new condition orresponse totreatments.We aimed to validateEIT-basedmeasurementsofMIFas well asMEF against spirometryof intubatedhypoxemics during controlled ventilationas well as spontaneous breathing.Additionally, the regional distribution ofmaximal airflows might interact withlungdiseases and raisetherisk of additional ventilationinjury.This is why we wantedtostudy the impactofmechanical ventilation settings onregion-wideMIFas well asMEF.

Methods

The present study was a reanalysisofthe dataof two randomised, prospective,crossoverstudies.We included patients who wereadmitted to theunit for intensive care withsevere hypoxemic and respiratory problems(AHRF)or acute respiratory distress(ARDS)undergoing pressure supportbreathing(PSV, n=10) andVolume-controlled ventilation(VCV, n20).We assessed MIF and MIFvia spirometry as well as EIT oversix different combinations of ventilation settings withhighervs. lower supportinPSV, and highersupport vs. lowerpressure of positive end-expiratory(PEEP)forbothPSV and VCV.Regional airflows were measured byEITin both dependent and non-dependentlung regionsas well.

Results

MIF and impedanztomographie measuredthroughEIT werestrongly correlative withthose measured using spirometry underevery condition(rangebetweenR2 0.629-0.776 and R2 0.606-0.772,respectively, p0.05onall) which was within clinically acceptableboundaries of disagreement.Higher PEEP significantly enhancedhomogeneity in the regionallocationof MIF and MEFwhen ventilation is controlled in volume,by increasing airflows within theaffected lung regions while reducingthem in the non-dependent ones.

Conclusions

EITis a precise, noninvasive way to monitor the healthofMIFas well asMEF.The present study also generatesthepossibilitythat EITcould help guidePSV and PEEPset-upsat ensuring homogeneity whenthe regional airflows that are extending and deflating.

Introduction

EI tomography(EIT)isanoninvasive bedside, radiation-freemethod for imaging the lung dynamically. EITgives intrathoracic maps oflung impedance fluctuations that are referenced toa baseline(i.e.,an end-expiratory lung volume measured from athe previousbreath) every20-50 ms ([11.Changes in intrathoracic impedance as measuredviaEIT are linearlyrelated tothe global and regional tidal volumes andmaintained at increasing positive end-expiratorypressure (PEEP) levels [22.This means thatEIToffers a noninvasive continuous bedsidemeasurement oflung volumechange duringinspiration and exhalation.

Inspiratory and expiratory flows correspondto thespeed ofthe lung’s volume as it changesovertime.In intubated patients,they aretypically measured usinga spirometer appliedto the ventilator’s circuit prior tothetube for endotracheal intubation or withinthe ventilator.Global maximal inspiratory andexpiratoryflow(MIF and MEF as well)that are measured usingstandard spirometry depend uponphysical properties in the respiratory system(namely lung compliance, lung compliance andairway resistance) [3The latter is a function of lung compliance and airway resistance.Therefore, monitoring ofMIF andEMF couldbe useful to guideairway settings(e.g., by selectingthepressure level positive associatedwithbettermechanics)and/or to measuretheefficacy of various pharmacologic treatments(e.g. increasingMIFand/or MEF afterbronchodilator drugs) [4The results of these tests are a good indication of the effectiveness of bronchodil.Spirometry, however, only providesglobal measurements of MIF andMEF. The heterogeneous spreadofaltered lung mechanics is acharacteristic of acute hypoxemicbreakdown(AHRF)as well as acute respiratory distress syndrome(ARDS) [5].The damage to the alveoli causesan elongation of lung tissueslocated between normal-, part– and over-inflated units which can result indifferencesacross the region inMIFas well asMEF values.These imbalances can raisethedanger of ventilator-induced respiratory injury(VILI)through multiple mechanisms[6], and settingscreating more homogeneous regional flowcan reduce it. Externalspirometry can leadtoaltered patterns in the respiratory system andincorrect measurements,too[7].Therefore, a non-invasivebedsidemethod for measuringglobal and regional MIF andMEFlevels wouldprovide a useful tool forunderstandingAHRF and ARDSpatients’ pathophysiology as well asto guide personalized treatments.

The present studyusing preliminary data from anthe animal model of[8], our goal wastoverify inan intubatedAHRFandARDS patientsreceivingcontrolledbreathing andspontaneous breathing EIT-based measures ofglobal MIF and MEF againstthe standardspirometry.Additionally, we investigatedtheeffect of higher vs. lowerPEEP andpressure levels inregionalflow patterns;we believethat higherlevels of PEEPas well as lower pressure support wouldproduce a more homogenous distribution ofregionalMIFandMEF.

Materials and methods

Studypopulations

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 ofSan Gerardo Hospital, Monza, Italy, approved thestudies,while informed consent was soughtaccording tolocalregulations.Further details regardingthecriteria for inclusion and exclusionforbothstudies are containedina data supplement online(Additionalfiles1).

Demographic data collection

Werecorded sex, age, Simplified Acute Physiology Score IIscores, etiology, diagnosis andthe severityof ARDS, days onmechanical ventilationprior study enrollmentforeverypatient.In-hospital mortality was recordedtoo.

EIT andmonitoring of ventilation

Ineach patient, an EIT-dedicatedbelt,containing 16 equallyspaced electrodes was placedon the thorax inthesixth or fifthintercostalspace and connected toan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).Throughout all study phases,EITdata were recorded byapplying smallelectric currents that rotated aroundhis thorax. Data were recordedat 20 Hz, and then storedfor offline analysis as previouslymentioned [1213].In sync withEITtracer data as well as airway pressure andairflows generated byMechanical ventilators wererecorded continuously.

Interventions

More details onthe two protocolscan be foundin thedata supplement online(Additionalto file1.).

Briefly, inthePSV study,patients had to undergothe followingrandomized steps in a crossover, each lasting 20 min:

  1. 1.

    Low support at clinical PEEP(PSV low)in comparison to.more support atPEEP in the clinical setting(PSV high);

  2. 2.

    Clinical supportatmoderate PEEP(PSV-PEEP low)in contrast to.medical support for higher PEEP(PSV-PEEP high).

As part of theVCV study,instead,the following phaseswere conductedin a randomized order in the crossover,each lasting20 minutes

  1. 1.

    Protection VCV in low-PEEP(VCV-PEEP low)against.VCV that is protective at clinicalPEEP+5cmH 2O (VCV-PEEP high).

EIT anddata on ventilation

Through offline analysis ofEITtracer results obtained inthefinal minuteduring each stage(analysis oftenbreaths), we measuredlocal and global(same-sizeof non-dependent and dependent regions in the lung) noninvasive airflowswaveform,similar to what was previously described[8in [8.].In brief, instantaneous globalregionalinspiratory and expiratoryairflowsare measured asvariationsof the global and regionalimpedance measured every 50 ms which were multiplied by tidalimpedance/volume ratio obtained fromthesame study stage anddivided by 50ms. EIT airflow data werethen transformed from mL/msec toL/min (Fig. 1) and the maximumMIF and MEF derived from EIT for the global and regional regionsand MEF (MIFglob MIFglob, MIFnon-dep,and MIFdepMEFglob with MEFnon-dep, MEFglob andMEFdep, respectively) weredetermined and thevalues averaged acrossfive to 10 consecutiverespiratorycycles.