Noninvasive monitoring and monitoring of maximalexpiratory and inspiratory flows(MIF and MEF,respectively)using electrical impedance tomography(EIT)may allow forearlier detection of changes inbreathing system mechanical characteristicsduetochanges in conditions orresponse totreatments.We aimed to validateEIT-basedmeasurementsofMIFas well asMEF against spirometryfor intubatedhypoxemic patients during controlled ventilationand spontaneous breathing.Moreover, regional distribution ofmaximum airflows can interact withlungdisease and increasethelikelihood of additional ventilationinjury.Therefore, we also soughttoinvestigate the effectsofmechanical ventilation settings onregion-wideMIFas well asMEF.
The present study was a reanalysisofdataof two randomised, prospective,cross-sectionalstudies.We included patients who wereadmitted to theintensive care unit (ICU) withan acute respiratory failure(AHRF)and acute respiratory distress syndrome(ARDS)undergoing pressure supportbreathing(PSV, n10) andventilatory control(VCV, n=20).We evaluated MIF and MIFby spirometry and EIT duringvarious combinations of the ventilation setting which were:highervs. lower supportinPSV, and higherthan. lowerpressure of positive end-expiratory(PEEP)inbothPSV and VCV.Regional airflows were measured byEITin both dependent and non-dependentlung regions.
MIF and impedanztomographie measuredthroughEIT werestrongly correlated withthe spirometry measurements duringany condition(rangebetweenR2 0.629-0.776 and R2 0.606-0.772,respectively, p0.05acrossall), with clinically acceptablelevels of agreement.A higher PEEP level significantly increasedhomogeneity in the regionallocationof MIF and MEFduring volume-controlled ventilation,by increasing airflows independent lung regions , and decreasingthe levels in non-dependent lung regions.
EITis a precise, noninvasive way to monitor the healthofMIFas well asMEF.The current study also positstheideathat EITcould help guidePSV and PEEPset-upsto increase homogeneity ofthe regional airflows that are extending and deflating.
The electrical impedance imaging(EIT)can be described asanon-invasive, bedside, radiation-free,technology for lung imaging that is dynamic. EITcan provide maps of intrathoraciclung impedance fluctuations that are referenced tothe baseline(i.e.,an end-expiratory lung volume measured from apreviousbreath) every20-50 milliseconds [11.Impedance changes within the intrathoracic area measuredthroughEIT are linearlycorrelated withthe global and regional volume of tidal and this correlation iskept at increasing positive expiratorypressure (PEEP) levels [22.So,EITprovides a non-invasive continuous bedsidemeasurement oflung volumefluctuations duringinspiratory and expiration.
Inspiratory and expiratory airflows correspondto therate at whichthe lung’s volume as it changesintime.In patients who are intubated,they aretypically measured usingan spirometer connectedto the ventilator circuit prior totheendotracheal tube or withinthe ventilator.Global maximal inspiratory andexpiratoryflows(MIF and MEF and MEF, respectively)determined bystandard spirometry are influenced byhow the respiratory systems’ mechanical characteristics work(namely lung compliance, lung compliance andairway resistance) [33.Thus, monitoringMIF andthe MEF canbe useful to guideventilatory settings(e.g. by selectingthepressure that is positive and associatedwithbettermechanics)and/or to measuretheeffectiveness of pharmacologic treatments(e.g. increasingMIFand/or MEF in response tobronchodilator drugs) [44.However, spirometry only yieldsgeneral measures of MIF andMEF, while heterogeneous distributionofaltered lung mechanics is acharacteristic of acute hypoxemic respirationinsufficiency(AHRF)or acute respiratory distress(ARDS) [5The spirometry method is not able to detect the heterogeneous distribution.Alveolar damage leads tocollapse of lung cellslocated between normal-, part-or over-inflated ones which can result inunbalanceswithin regionalMIFandMEF values.These imbalances may increasetherisk of ventilator-induced lung injury(VILI)via a myriad of mechanisms, while settingsobtaining more homogenous regional flowsmight reduce it. Externalspirometry can leadtoaltered patterns of breathing andinaccurate measures,as well[77.Therefore, a non-invasivebedsidemethod for measuringboth regional and global MIF as well asMEFlevels couldprovide a useful tool forlearning aboutAHRF and ARDSsufferers’ pathophysiology, andto aid in the development of personalized treatment.
The present studyusing preliminary data from ananimal model, we soughttovalidate inan intubatedAHRFas well asARDS patientsreceivingcontrolledventilation andEIT-based breathing tests that are based on spontaneous breathing to measureMIF and MIF global againststandardspirometry.In addition, we examinedtheeffects of higher. lowerthe levels of pressure support onregionalflows;our hypothesis isthat higherPEEPand lower pressure support mayproduce a more homogenous distribution ofregionalMIFas 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. Theethical committee ofSan Gerardo Hospital, Monza, Italy, approved thestudy,with informed consent obtainedaccording tolocalregulations.Further information regardingtheinclusion and exclusion criteriaforthe twostudies are includedinan online data supplement(Additionalfiles1.).
Demographic data collection
Wecollected data on sex, ages, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andthe severityof ARDS, days onmechanical ventilationprior study enrollmentforeverypatient.The death rate at the hospital was documented,too.
EIT andmonitoring of ventilation
Inall patients, an EIT-specificbelt,comprising 16 evenlyspaced electrodes was placedwithin the thorax, inthefifth or sixthintercostalspace , and then connected toan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).In all phases of the study,EITdata were recorded byapplication of small alternateelectrical currents that rotated around thehis thorax. Data were recordedat 20 Hzand storedfor offline analysis as previouslydescribed in [12in the previous article [12, 13].As synchronizedEITtracer the airway pressure as well asairflows fromventurism werecontinuously recorded.
More details onthe two protocolsare availablein theonline data supplement(Additionaldocument1).
In short, inthePSV study,participants underwentthe followingrandomized steps that lasted for 20 minutes each:
Support for clinical PEEP is low(PSV low)against.greater support at theclinical PEEP(PSV high);
Clinical supportformoderate PEEP(PSV-PEEP low)against.medical support for higher PEEP(PSV-PEEP high).
In theVCV study,insteadthe following phaseswere performedin a randomized order in the crossover,each lastingfor 20 minutes:
A protective VCV when PEEP is low(VCV-PEEP low)in comparison to.the protective VCV atPEEP+ 5cmH 2O (VCV-PEEP high).
EIT anddata on ventilation
From offline analysis of theEITtracings obtained duringthelast minutesat the end of every phase(analysis oftenbreaths) We measured theglobal and regional(same-sizeregion of lung that is dependent as well as non-dependent) noninvasive airflows in the form of a waveformsimilar to the one described previously[8in [8.].The short version is that instantaneous worldwide andregionalexpiratory and inspiratoryairflowswere measured asvariationsof the global and regionalimpedance measured every 50ms and multiplied by thevolume/tidal impedance ratio fromthestudy phase in question anddivided by 50milliseconds. EIT airflow data werethen transformed from mL/msec toL/min (Fig. 1), and the maximumEIT-derived regional and global MIFand MEF (MIFglob MIFglob, MIFnon-dep,and MIFdepMEFglob, MEFnon-dep , andMEFdep, respectively) weredetermined and thevalue averaged over5-10 consecutiverespiratorycycles.