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The Fact About impedanztomographie That No One Is Suggesting

The Fact About impedanztomographie That No One Is Suggesting

Noninvasive monitoring and monitoring of maximalinspiratory and expiratory flows(MIF and MEF,respectively)using electrical impedance tomography(EIT)may allow forrapid recognition of changes inthe mechanical properties of the respiratory systemin responsetonew conditions or inresponse totreatments.The aim of this study was to verifyEIT-basedmeasuresofMIFandMEF against spirometryin intubatedhypoxemic patients under controlled ventilationas well as spontaneous breathing.Furthermore, the distribution of regionalmaximal airflows may be influenced bylungdiseases and increasethelikelihood of additional ventilationinjury.We also wantedtoanalyze the effectsofdifferent settings for mechanical ventilation onlocalMIFandMEF.

Methods

We carried out a new analysisofdatafrom two prospective, randomizedcross-sectionalstudies.Intubated patients wereadmitted to theintensive care unit withchronic hypoxemic respiratory dysfunction(AHRF)and acute respiratory distress syndrome(ARDS)under pressure supportventilatory(PSV, n=10) andventilatory control(VCV, n=20).We assessed MIF and MIFusing spirometry, EIT andsix different combinations of ventilation settings that werehighervs. lower supportduringPSV and greaterin comparison to. lowerpressure of positive end-expiratory(PEEP)forbothVCV and PSV.Regional airflows were analyzed usingEITin non-dependent and dependentlung regions, too.

Results

MIF and impedanztomographie determinedbyEIT wereclosely correlated tothe spirometry measurements duringall conditions(rangefromR2 0.629-0.776 and R2 0.606-0.772respectively, p<0.05inall) in accordance with the clinically acceptablelimits of agreement.The higher PEEP levels significantly improvedhomogeneity of theareaof MIF and MEFwhen ventilated with volume control,by increasing airflows in thedependent lung regions , and decreasingthem in non-dependent areas.

Conclusions

EITprovides accurate noninvasive monitoringofMIFas well asMEF.The present study also generatesthepossibilitythat EITcould be able to guidePSV and PEEPsettingto increase homogeneity ofthe regional airflows that are extending and deflating.

Introduction

Electric impedance tomography(EIT)isanon-invasive bedside, radiation-freeactive lung imaging technique. EITprovides intrathoracic maps oflung impedance changes that are linked tothe baseline(i.e.the volume of the lungs at the end of expiration frompreviousbreath) every20-50 milliseconds [11.Impedance changes within the intrathoracic area measuredusingEIT are linearlycorrelated withregional and global tidal volume and this correlation iskept at increasing positive expiratorypressure (PEEP) levels [22.Thus,EITproduces a noninvasive bedside continuousmeasure of regionallung volumechanges duringinspiration and expiration.

Inspiratory and expiratory flows correspondto thespeed oflung volume that changesintime.In intubated patients,they areusually measured withan spirometer attachedin the ventilator’s circuit, prior totheendotracheal tube , or insidethe ventilator.Global maximum inspiratory andexpiratoryflow(MIF and MEF (MIF and MEF, respectively)determined bystandard spirometry are influenced bythose mechanical parameters of respiratory function(namely lung compliance, lung pressure andairway resistance) [3(3).Thus, monitoringMIF andthe MEF canbe helpful in guidingairway settings(e.g. by selectingthepressure level positive associatedwithimprovedmechanics)or to assesstheefficacy of pharmacologic treatments(e.g., increasedMIFand/or MEF afterthe use of bronchodilator medications) [4The results of these tests are a good indication of the effectiveness of bronchodil.But spirometry is only able to provideglobal measurements of MIF andMEF, and heterogeneous distributionofabnormal lung mechanics can be acharacteristic of acute hypoxemicdysfunction(AHRF)along with acute respiratory stress syndrome(ARDS) [5The spirometry method is not able to detect the heterogeneous distribution.Alveolar damage leads tocollapse of lung structuresadjacent to normal-, partiallyand over-inflated units and can causeunbalanceswithin regionalMIFas well asMEF values.This can increasethelikelihood of suffering from ventilator-induced pulmonary injury(VILI)via a myriad of mechanisms[6, 7], whilefor more uniform regional flowcould reduce the risk. Externalclassic spirometry sometimes leadstoaltered patterns of breathing andinaccurate measurementsas well[7].Thus, a noninvasivebedsidedynamic method to determineglobal and regional MIF andMEFvalues couldprovide a useful tool forlearning aboutAHRF and ARDSthe pathophysiology of patients andto guide personalized treatments.

In the present studyusing preliminary data from anthe animal model of[8], we set outtoconfirm inintubatedAHRFandARDS patientsundergoingcontrolledbreathing andspontaneous breathing EIT-based measures ofMIF and global MIF againststandardspirometry.Moreover, we exploredtheeffects of higher. lowerpressure levels inregionalflows;we believethat higherPEEPand lower pressure support willresult in a more uniform distribution ofMIF andMIFandMEF.

Materials and methods

StudyPopulation

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. Theethical committee ofSan Gerardo Hospital, Monza, Italy, approved thestudies,while informed consent was soughtin line withlocalregulations.Further details regardingthecriteria for inclusion and exclusionforthe twostudies are includedinthe online data supplement(Additionaldata file1.).

Demographic data collection

Wecollected sex, age, Simplified Acute Physiology Score IIscores, etiology, diagnosis andthe severityof ARDS, days onmechanical ventilationprior study enrollmentforeachpatient.The mortality rate in hospital was also recordedas well.

EIT andventilation monitoring

Ineach patient, EIT-dedicatedbelt,with 16 equallyspaced electrodes, was positionedaround the thorax atthesixth or fifthintercostalregion and connected withan commercialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).Throughout all study phases,EITdata were generated byapplication of small alternateelectric currents that rotated aroundyour thorax. The data were recorded continuouslyat 20 Hzand storedfor offline analysis, in accordance with the procedure previouslydescribed [12].The data were synchronized withEITtracer data, airway pressure andairflows ofventurism werecontinuously recorded.

Interventions

Further details aboutthe two protocolsare availablein theonline data supplement(Additionaldocument1.).

Briefly, inThePSV study,patients had to undergothe followingrandomized steps, each lasting 20 min:

  1. 1.

    Clinical PEEP support is weak(PSV low)against.more support atPEEP at the clinic(PSV high);

  2. 2.

    Clinical supportforthe low level of PEEP(PSV-PEEP low)compares to.clinical support at higher PEEP(PSV-PEEP high).

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

  1. 1.

    Protection VCV in low-PEEP(VCV-PEEP low)as compared to.VCV with a protective function atPEEPplus 5cmH 2O (VCV-PEEP high).

EIT andventilation data

Based on offline analysis ofEITtracings obtained duringthefinal minutein each of the phases(analysis of10breaths), we measuredglobal and regional(same-sizeindependent and dependent lung areas) noninvasive airflows’ waveform,as previously described[8].The short version is that instantaneous worldwide andregionalinspiratory and expiratoryairflowswere measured asvariationsin global and regionalimpedance measured every 50ms which were multiplied by tidalintensity ratio forthestudy phase in question anddivided by 50milliseconds. EIT airflow data werethen transformed from mL/msecL/min (Fig. 1) and the highestMIFs from EITs that were global and regionaland MEF (MIFglob, MIFnon-depand MIFdepMEFglob, MEFnon-dep andMEFdep, respectively) wereidentified , and thevalue averaged over5-10respiratorycycles.

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