Noninvasive monitoring and monitoring of maximalexpiratory and inspiratory flows(MIF and MEFrespectively)using electrical impedance tomography(EIT)may allow forthe early detection of changes inrespiration’s mechanical parametersin responsetothe new condition orresponse totreatment.The aim of this study was to verifyEIT-basedmeasurementsforMIFas well asMEF against spirometryfor intubatedhypoxemic patients in controlled ventilationand spontaneous breathing.Furthermore, the spatial distribution ofmaximum airflows may interact withlungdiseases and raisetherisk of further ventilatorinjury.We also wantedtostudy the impactofmechanical ventilation settings onregionalMIFas well asMEF.
We performed a new studyofthe dataof two randomised, prospective,cross-sectionalstudies.The study included intubated patientsadmitted to theintensive care unit withsevere hypoxemic and respiratory problems(AHRF)as well as acute respiratory distress syndrome(ARDS)with pressure supportventilation(PSV, n=10) andvolume-controlled ventilation(VCV, n20).We measured MIF and MEFthrough spirometry and EIT inseveral combinations of settings for ventilation withhigherthan. lower supportduringPSV, and higherin comparison to. lowerpressure of positive end-expiratory(PEEP)duringbothPSV and VCV.Regional airflows were assessed byEITin non-dependent and dependentlung regionsas well.
MIF and impedanztomographie measuredbyEIT werestrongly correlated withthose measured using spirometry underevery condition(rangebetweenR2 0.629-0.776 and R2 0.606-0.772respectively, p0.05forall) that was acceptable for clinicallevels of agreement.Higher PEEP significantly improvedhomogeneity in the regionaldistributionof MIF and MEFduring volume-controlled ventilationby increasing airflows in theareas of the lung that are dependent and decreasingthem in non-dependent areas.
EITallows for accurate and noninvasive monitoringofMIFandMEF.The current study also positsthepossibilitythat EITcan help to determinePSV and PEEPsettings aimedto increase homogeneity ofspreading and deflating regional airflows.
The electrical impedance imaging(EIT)isanoninvasive bedside, radiation-freeadvanced lung imaging method that is dynamic. EITproduces maps of the intrathoracicthe changes in lung impedance that are measured againstan initial value(i.e.,the volume of the lungs at the end of expiration frompreviousbreath) every20-50 milliseconds1.Intrathoracic impedance changes measuredwithEIT are linearlycorrelated withthe global and regional volume of tidal, and the correlation issustained at higher positive end-expiratorypressure (PEEP) levels [22.Therefore,EITyields noninvasive bedside continuousmeasurement oflung volumefluctuations duringinspiratory and expiration.
The inspiratory and expiratory flow of air correspondsto thespeed ofthe lung’s volume as it changesovertime.In patients who are intubated,they aretypically measured byan spirometer attachedin the ventilator’s circuit, prior tothetube for endotracheal intubation or withinthe ventilator.Global maximum inspiratory as well asexpiratoryflow(MIF and MEF as well)recorded bystandard spirometry rely onhow the respiratory systems’ mechanical characteristics work(namely the lung compliance as well asresistance of the airway) [33.Monitoring ofMIF andMEF canaid in guiding theventilation settings(e.g. by determiningthelevel of positive pressure associatedwithbettermechanics)and/or to assesstheeffectiveness of treatments with pharmacological agents(e.g. increasingMIFand/or MEF in response tosteroids to bronchodilate the airway) [44.Spirometry, however, only providesgeneral measurements of MIF as well asMEF, whereas heterogeneous distributionofabnormal lung mechanics can be acharacteristic of acute hypoxemicfailing(AHRF)along with acute respiratory stress syndrome(ARDS) [5The spirometry method is not able to detect the heterogeneous distribution.In the event of an alveolar injury, it can lead to thecollapse of lung unitslocated between normal-, part-and excessively inflated units which can result invariationsbetween regionalMIFandMEF values.These imbalances can raisethechance of developing a ventilator-induced lung disease(VILI)by a variety of mechanisms, while settingto achieve more homogeneous regional flowsmay reduce the risk. Externalspirometry is sometimes a causetoaltered respiratory patterns as well asinaccurate measurementsas well[77.Therefore, a non-invasivebedsidedynamic method to determinethe global and regional MIF as well asMEFvalues wouldmake a great contribution tostudyingAHRF and ARDSthe pathophysiology of patients andto aid in the development of personalized treatment.
In the current study,after preliminary results obtained in ananimal models, we aimedtoverify inthe intubatedAHRFas well asARDS patientsreceivingcontrolledbreathing as well asEIT-based breathing tests that are based on spontaneous breathing to measureglobal MIF and MEF compared tothe standardspirometry.Additionally, we investigatedtheeffects of higher vs. lowerthe levels of pressure support onregionalflows;our hypothesis isthat higherPEEPand lower pressure support mayproduce a more homogenous distribution ofMIF andMIFas well asMEF.
Materials and methods
We performed a new analysis of data collected during two prospective randomized crossover studies: in the first (pressure support ventilation (PSV) study) , ten intubated patients recovering from ARDS , 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) , 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 thestudyas well as informed consent,in accordance withlocalguidelines.Additional information abouttheinclusion and exclusion criteriaforthe twostudies are containedinan online data supplement(Additionalfiles1).
Demographic data collection
Werecorded sex and age, Simplified Acute Physiology Score IIscores, etiology, diagnosis andthe severityof ARDS days onmechanical ventilationprior study enrollmentforeachpatient.Mortality in the hospital was reported,as well.
EIT andventilation monitoring
Ineach patient, an EIT-dedicatedbelt,consisting of 16 equallyspaced electrodes, were placedover the thorax aroundthefifth or sixthintercostalspace , and then connected toan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).Throughout all study phases,EITinformation was generated throughthe application of tinyelectrical currents rotating aroundpatient’s thorax, continuously recordedat 20 Hz. These data were storedfor offline analysis as previouslydescribed [1212.Synchronized toEITtracer data Airway pressure, as well asairflows fromthe mechanical ventilator wererecorded continuously.
More details onthe two protocolsare availablein theonline data supplement(AdditionalFile1.).
InthisPSV study,participants underwentthe followingrandomized steps in a crossover which lasted each for 20 minutes:
Support for clinical PEEP is low(PSV low)as compared to.more support atclinical PEEP(PSV high);
Clinical supportatlow PEEP(PSV-PEEP low)against.clinical support at higher PEEP(PSV-PEEP high).
As part of theVCV study,,the following phaseswere executedin crossover randomized order,each lasting20 minutes:
The VCV that protects you at low pressure(VCV-PEEP low)against.VCV to protect at clinicalPEEPplus 5cmH 2O (VCV-PEEP high).
EIT anddata on ventilation
Through offline analysis ofEITtracings obtained duringthelast minutesin each of the phases(analysis oftenbreaths) we determined theboth the global and regional(same-sizeof non-dependent and dependent regions in the lung) noninvasive airflows’ waveforms,similar to what was previously described.The short version is that instantaneous worldwide andregionalexpiratory and inspiratoryairflowswere recorded asvariationsof global and regionalimpedance , which was measured every 50 ms, multiplied by the tidalimpedance/volume ratio obtained fromthestudy phase in question anddivided by 50milliseconds. EIT airflow data werethen converted toL/min (Fig. 1) as well as the maximumMIF and MEF derived from EIT for the global and regional regionsand MEF (MIFglob, MIFnon-depand MIFdep;MEFglob as well as MEFnondep andMEFdep according to) wereidentified , and thevalues were averaged over5-10 consecutivebreathingcycles.