Not known Facts About impedanztomographie

Noninvasive monitoring of the maximalexhalatory and inspiratory flow(MIF and MEFrespectively)through electrical impedance imaging(EIT)could facilitatean early detection of changes tothe mechanical properties of the respiratory systemin responsetothe new condition orresponse totreatment.Our goal was to testEIT-basedmeasurementsofMIFas well asMEF against spirometryin intubatedhypoxemics during controlled ventilationand breathing spontaneously.Moreover, regional distribution ofmaximum airflows can interact withlungpathology and increasethechance of further ventilationinjuries.So, we also triedtoexamine the effectofdifferent settings for mechanical ventilation onthe regional distribution ofMIFas well asMEF.

Methods

We performed a new analysisofdatafrom two prospective, randomized,crossoverstudies.Intubated patients wereadmitted to theunit for intensive care withan acute respiratory failure(AHRF)and acute respiratory stress syndrome(ARDS)with pressure supportventilatory(PSV, n10) andVCV, n=20).(VCV, n=20).We evaluated MIF and MIFby spirometry and EIT duringthe six ventilation combinations withhigherthan. lower supportduringPSV and greaterin comparison to. lowerpressure of positive end-expiratory(PEEP)duringbothVCV and PSV.Regional airflows were measured byEITin dependent and non-dependentlung regions.

Results

MIF and impedanztomographie  measuredthroughEIT werehighly correlated withthose measured using spirometry underthe entire range of conditions(rangebetweenR2 0.629-0.776 and R2 0.606-0.772respectively, p0.05acrossall) in accordance with the clinically acceptablelimits of agreement.A higher PEEP was able to improvehomogeneity and consistency in thedistributionof MIF and MEFwhen ventilation is controlled in volume,by increasing airflows within theaffected lung regions while reducingthose in non-dependent regions.

Conclusions

EIToffers accurate, non-invasive monitoringforMIFas well asMEF.This study also suggeststhepossibilitythat EITcould guidePSV and PEEPparametersto increase homogeneity ofexpanding and deflating regional airflows.

Introduction

Electric impedance tomography(EIT)isanoninvasive bedside, radiation-freedynamic lung imaging technique. EIToffers intrathoracic maps ofthe changes in lung impedance that are measured againsta baseline(i.e.,the volume of the lung at the end of expiration fromthe previousbreath) every20-50 milliseconds [11.Intrathoracic impedance changes measuredthroughEIT are linearlylinked toglobal and regional tidal volume andmaintained as the positive end-expiratorypressure (PEEP) levels [22.Therefore,EITproduces a noninvasive bedside continuousmeasure of regionallung volumefluctuations duringinspiration and exhalation.

The inspiratory and expiratory flow of air correspondsto thevelocity at whichlung volume that changesovertime.In patients who are intubated,they arenormally measured withSpirometers that are connectedto the ventilator circuit beforetheendotracheal tube , or insidethe ventilator.Global maximum inspiratory andexpiratoryflow(MIF and MEF respectively)that are measured usingstandard spirometry depend onthe mechanical properties of the respiratory system(namely, lung compliance andairway resistance) [33.Therefore, monitoring ofMIF andMEF couldbe useful to guidebreathing settings(e.g. by choosingthelevel of positive pressure associatedwithbettermechanics)and/or to assesstheefficacy of pharmacologic treatments(e.g. the increase inMIFor MEF afterthe use of bronchodilator medications) [4The results of these tests are a good indication of the effectiveness of bronchodil.Spirometry, however, only providesgeneral measures of MIF andMEF, and heterogeneous distributionofmodified lung mechanics is acharacteristic of acute hypoxemic respirationdysfunction(AHRF)and the acute respiratory distress disorder(ARDS) [55.Alveolar damage leads tocollapse of lung unitsbordering normal-, partial- and over-inflated units that could result in the formation ofan imbalancewithin regionalMIFandMEF values.These imbalances may increasethedanger of ventilator-induced respiratory injury(VILI)through multiple mechanisms[6, 7], whilecreating more homogeneous regional flowcan reduce it. Externalclassic spirometry sometimes leadstoaltered respiratory patterns as well asinaccurate measures,as well[77.Therefore, a non-invasivebedsidedynamic method of measuringboth regional and global MIF as well asMEFvalues wouldbe a significant addition toknowingAHRF and ARDSsufferers' pathophysiology, andto provide personalized treatment.

In this study,with the help of preliminary data obtained from ananimal models[8], we set outtoverify inan intubatedAHRFas well asARDS patientswho are receivingcontrolledbreathing andEIT-based measurements of spontaneous breathing ofglobal MIF and EEF againstthe standardspirometry.Furthermore, we exploredtheeffects of higher vs. lowerPEEP andpressure levels inregionalflow patterns;it is our beliefthat higherPEEPand lower pressure support mightproduce a more homogenous distribution ofregions ofMIFas well asMEF.

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. Theethics committee ofSan Gerardo Hospital, Monza, Italy, approved thestudieswith informed consent obtainedin accordance withlocalregulations.Additional details onthecriteria for inclusion and exclusionforbothstudies are containedinan online supplement to the data(Additionalfile1.).

Demographic data collection

Wegathered sex, gender, age, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andthe severityof ARDS days onmechanical ventilationprior to enrollment in the studyforeverypatient.In-hospital mortality was recorded,too.

EIT andmonitoring of ventilation

Inall patients, an EIT-specificbelt,consisting of 16 equallyspaced electrodes, was positionedwithin the thorax, inthesixth or fifthintercostalregion and connected withan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).During all study phases,EITdata were generated through theuse of small alternationelectrical currents that rotated around thepatients thorax. These were recorded continuouslyat 20 Hz. These data were storedfor offline analysis as previouslydescribed in [12and 13.As synchronizedEITtracer data as well as airway pressure andairflows froman airway ventilator arecontinuously recorded.

Interventions

More information onthe two protocolsare availablein theonline data supplement(Additionaldata supplement file1).

InthisPSV study,patients were subjected tothe followingrandomized steps which lasted each for 20 minutes:

1. 1. Low support at clinical PEEP(PSV _low)against.more support atclinical PEEP(PSV _high);
2. 2.  Clinical supportforthe low level of PEEP(PSV-PEEP _low)compares to.medical support for higher PEEP(PSV-PEEP _high).

In theVCV study,instead,the following phaseswere performedin a randomized order for crossovers,each lasting20 min:

1. 1. A protective VCV when PEEP is low(VCV-PEEP _low)against.VCV to protect at clinicalPEEPand 5cmH ₂O (VCV-PEEP _high).

EIT anddata on ventilation

From offline analysis of theEITtracer data collected duringthelast few minutesfrom each study phase(analysis oftenbreaths) and analyzed theglobally and regional(same-sizeregion of lung that is dependent as well as non-dependent) noninvasive airflows' waveshape,like the ones previously reported[88.In short, instantaneous global andregionalexpiratory and inspiratoryairflowswere measured asvariationsin global and regionalimpedance , which was measured every 50 ms when multiplied with the tidalvolume/tidal-impedance ratio fromthesame study phase 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 MIFdep,MEFglob the MEFglob, MEFnon dep andMEFdep for MEFglob, MEFnon-dep and MEFdep) wereidentified , and theresults were averaged over5-10respiratorycycles.