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An Unbiased View of impedanztomographie

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Noninvasive monitoring and monitoring of maximalflow rates of expiratory and inspiratory(MIF and MEFrespectively)using electrical impedance tomography(EIT)could enableearly recognition of changes inthe mechanical properties of the respiratory systemduetonew conditions or inresponse totreatments.We sought to confirmEIT-basedmeasurementsforMIFandMEF against spirometryof intubatedhypoxemic patients under controlled ventilationand breathing spontaneously.Furthermore, the distribution of regionalmaximum airflows can interact withlungpathology and increasetherisk of further ventilatorinjury.Therefore, we also soughttodescribe the effectsofventilation settings that affectregions ofMIFandMEF.

Methods

We conducted a fresh analysisofdatain two prospective, randomized,cross-sectionalstudies.Intubated patients wereadmitted to theintensive care unit suffering fromsevere hypoxemic and respiratory problems(AHRF)as well as acute respiratory distress syndrome(ARDS)under pressure supportventilation(PSV, n10) andthe volume-controlled ventilation(VCV, n20).The MIF and the MIF were measuredby spirometry and EIT duringthe six ventilation combinations withhighervs. lower supportinPSV, and highervs. lowerpressure of positive end-expiratory(PEEP)duringbothPSV and VCV.The regional airflows were evaluated byEITfor both dependent and non-dependentlung regions, too.

Results

MIF and impedanztomographie measuredbyEIT weretightly correlated withthose measured using spirometry underall conditions(rangeofR2 0.629-0.776 and R2 0.606-0.772respectively, p0.05inall) in accordance with the clinically acceptablelimits of agreement.The higher PEEP levels significantly improveduniformity in thespreadof MIF and MEFduring volume-controlled ventilation,by increasing airflows to thedependent lung regions , and decreasingthe levels in non-dependent lung regions.

Conclusions

EITgives accurate, non-invasive measurementofMIFas well asMEF.The study also proposesthenotionthat EITcould help guidePSV and PEEPset-upsto improve the homogeneity ofdistending and deflating regional airflows.

Introduction

Electric impedance tomography(EIT)can be described asanon-invasive bedside, radiation-freemethod for imaging the lung dynamically. EITgives intrathoracic maps ofthe changes in lung impedance that are measured againstan initial value(i.e.,an end-expiratory lung volume measured from athe previousbreath) every20-50 milliseconds [11.Changes in intrathoracic impedance as measuredviaEIT are linearlycorrelated withthe global and regional volume of tidal, and the correlation issustained at higher positive end-expiratorypressure (PEEP) levels [2].Thus,EITproduces a noninvasive bedside continuousmeasure of regionallung volumefluctuations duringinspiration and expiration.

Inspiratory and expiratory airflows relateto thevelocity ofthe lung’s volume as it changesovertime.In patients who are intubated,they aretypically measured usingan spirometer connectedto the ventilator circuit prior totheendotracheal tube or withinthe ventilator.Global maximal inspiratory andexpiratoryflow(MIF and MEF respectively)recorded bystandard spirometry depend uponthose mechanical parameters of respiratory function(namely the lung compliance as well asairway resistance) [33.Monitoring ofMIF andthe MEF canbe useful to guidebreathing settings(e.g. selectingtheappropriate pressure level that correspondswithbettermechanics)and/or to measuretheefficacy of various pharmacologic treatments(e.g. increasingMIFand/or MEF in response tosteroids to bronchodilate the airway) [4The results of these tests are a good indication of the effectiveness of bronchodil.Spirometry can only provideglobal measures of MIF andMEF. However, heterogeneous distributionofaffected lung mechanics is thecharacteristic of acute hypoxemicdysfunction(AHRF)as well as acute respiratory distress syndrome(ARDS) [5It is also a sign of ARDS.Alveolar damage leads tocollapse of lung units tightlythat are bordered by normal-, partial– and over-inflated units and can causean imbalancebetween regionalMIFas well asMEF values.This can increasetherisk of a ventilator-induced lung injury(VILI)through multiple mechanisms[6], while settingsthat create more homogenous regional flowcould decrease the risk. Externalclassic spirometry sometimes leadstoaltered respiratory patterns andinaccurate measurementsas well[7].Thus, a noninvasivebedsidedynamic method to determineregional and global MIF andMEFvalues canmake a great contribution toknowingAHRF and ARDSpatients’ pathophysiology andfor guiding personalized treatments.

The present studywith the help of preliminary data obtained from ananimals[8], we aimedtoverify inintubatedAHRFas well asARDS patientsundergoingcontrolledbreathing andEIT-based spontaneous breathing measures ofMIF and global MIF againstthe standardspirometry.Furthermore, we exploredtheeffects of higher. lowerpressure supports on theregionalflows;Our hypothesis wasthat higherlevels of PEEPand lower pressure support willgive a more homogenous distributionthe regionalMIFas well asMEF.

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 theresearch,as well as informed consent,as perlocalguidelines.Additional information onthecriteria for inclusion and exclusionforthe twostudies are providedina data supplement online(Additionalfiles1.).

Demographic data collection

Werecorded sex and age, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andseverityof ARDS days onmechanical ventilationprior study enrollmentforeverypatient.The death rate at the hospital was documented,as well.

EIT andventilation monitoring

Ineach patient, EIT-dedicatedbelt,which contained 16 equallyspaced electrodes was placedclose to the thorax area atthesixth or fifthintercostalspaces and connected toan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).During all study phases,EITdata were generated through theapplying smallelectrical currents that circulated around thepatients thorax. These were recorded continuouslyat 20 Hz, and then storedfor offline analysis, asexplained [12and 13.As synchronizedEITtracer data the airway pressure as well asairflows ofventurism wererecorded continuously.

Interventions

More information aboutthe two protocolsare availablein thedata supplement online(AdditionalFile1).

Briefly, inPSV, in thePSV study,patients were subjected tothe followingsteps of crossover, each lasting 20 min:

  1. 1.

    Clinical PEEP support is weak(PSV low)against.more support atPEEP in the clinical setting(PSV high);

  2. 2.

    Clinical supportformoderate PEEP(PSV-PEEP low)as opposed to.help from clinical specialists at higher pressure(PSV-PEEP high).

For theVCV study,insteadthe following phaseswere performedin crossover randomized order,each lasting20 min:

  1. 1.

    The VCV that protects you at low pressure(VCV-PEEP low)as compared to.VCV that is protective at clinicalPEEPand 5cmH 2O (VCV-PEEP high).

EIT andventilation data

Analyzing offline theEITtracer data collected duringthelast few minutesof each phase(analysis oftenbreaths) We measured theglobally and regional(same-sizeof non-dependent and dependent regions in the lung) noninvasive airflows’ waveshape,as previously described[8in [8.].In short, instantaneous global andregionalinspiratory and expiratoryairflowswere recorded asvariationsin the regional and globalimpedance , measured every 50 milliseconds, multiplied by the tidalvolume/tidal impedance ratio ofthesame study phase , anddivided by 50ms. EIT airflow data werethen transformed from mL/msec toL/min (Fig. 1) as well as the maximumMIFs from EITs that were global and regionaland MEF (MIFglob MIFglob, MIFnon-dep,and MIFdepMEFglob, MEFnon-dep andMEFdep and MEFdep, respectively) wereidentified , and thevalues were averaged over5-10respiratorycycles.

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