Non-invasive monitoring of maximalinspiratory and expiratory flows(MIF and MEF,respectively)by electrical impedance tomography(EIT)could enablethe early detection of changes inthe mechanical properties of the respiratory systemduetonew conditions or inresponse totreatment.Our goal was to testEIT-basedmeasuresforMIFandMEF against spirometryinhypoxemics during controlled ventilationand spontaneous breathing.Furthermore, the distribution of regionalmaximum airflows can interact withlungdiseases and increasetherisk of additional ventilationinjury.Thus, we also aimedtoinvestigate the effectsofventilation settings that affectthe regional distribution ofMIFandMEF.
Methods
We performed a new studyofthe datataken from two prospective, randomized,crossoverstudies.The study included intubated patientsadmitted to theintensive care unit suffering fromchronic hypoxemic respiratory dysfunction(AHRF)as well as acute respiratory distress syndrome(ARDS)under pressure supportbreathing(PSV, n=10) andVCV, n=20).(VCV, n20).The MIF and the MIF were measuredthrough spirometry and EIT inseveral combinations of settings for ventilation such ashigherthan. lower supportinPSVand higherthan. lowerpressure of positive end-expiratory(PEEP)inbothPSV and VCV.The regional airflows were evaluated byEITin non-dependent and dependentlung regionsas well.
Results
MIF and impedanztomographie determinedthroughEIT weretightly correlated withthe spirometry measurements duringevery condition(rangebetweenR2 0.629-0.776 and R2 0.606-0.772respectively, p0.05forall) which was within clinically acceptablelimitations of agreement.A higher PEEP level significantly increasedhomogeneity and consistency in thepatternof MIF and MEFwhen ventilation is controlled in volume,by increasing airflows to thezones of the lung that are dependent, and reducingthose in non-dependent regions.
Conclusions
EIToffers accurate, non-invasive monitoringofMIFandMEF.The current study also positsthenotionthat EITcan help to determinePSV and PEEPsettingto ensure homogeneity indistending and deflating regional airflows.
Introduction
Electrical impedance tomography(EIT)isanon-invasive, bedside, radiation-free,active lung imaging technique. EITgives intrathoracic maps oflung impedance variations that are compared tothe baseline(i.e.the volume of the lungs at the end of expiration frompreviousbreath) every20 to 50 milliseconds].Intrathoracic impedance changes measuredthroughEIT are linearlylinked tothe volume of the tidal system in both regions, and the correlation iskept at increasing positive expiratorypressure (PEEP) levels [22.So,EITgives a noninvasive bedside continuousmeasure of regionallung volumechanges duringthe process of inspiration and expiration.
The inspiratory and expiratory flow of air correspondsto thespeed oflung volume changeintime.In patients who are intubated,they aretypically measured usingan spirometer that is connectedin the ventilator’s circuit, prior tothetube for endotracheal intubation or withinthe ventilator.The global maximum inspiratory andexpiratoryflow(MIF and MEF (MIF and MEF, respectively)recorded bystandard spirometry depend uponthe mechanical properties of the respiratory system(namely lung compliance, lung volume andresistance to airways) [3(3).Therefore, monitoring ofMIF andMEF maybe beneficial to help guideadjustments to the ventilation system(e.g. by choosingtheappropriate pressure level that correspondswithbettermechanics)and/or to measuretheefficacy of various pharmacologic treatments(e.g. increasingMIFor MEF followingan bronchodilator drug) [4(e.g., increased MIF and/or MEF after bronchod.Spirometry, however, only providesglobal estimates of MIF andMEF, whereas heterogeneous distributionofaffected lung mechanics is thehallmark of acute hypoxemic respiratorydysfunction(AHRF)as well as acute respiratory distress syndrome(ARDS) [5].In the event of an alveolar injury, it can lead to thecollapse of lung structuresbordering normal-, partialor over-inflated ones which can result invariationsin regionalMIFas well asMEF values.These imbalances can raisethechance of developing a ventilator-induced lung disease(VILI)by a variety of mechanisms[6], whereas settingsthat create more homogenous regional flowcan reduce it. Externalspirometry can leadtoabnormal respiratory patterns andincorrect measurements,as well[77.Therefore, a non-invasivebedsidedynamic method to measureboth regional and global MIF as well asMEFvalues couldmake a great contribution tostudyingAHRF and ARDSsufferers’ pathophysiology, andto aid in the development of personalized treatment.
In the current study,with the help of preliminary data obtained from ananimal model[8], we aimedtotest the validity ofpatients who are intubatedAHRFas well asARDS patientsreceivingcontrolledventilation as well asEIT-based measurements of spontaneous breathing ofglobal MIF and MEF compared tothe standardspirometry.Furthermore, we exploredtheeffects of higher. lowerpressure supports on theregionalflow patterns;our hypothesis isthat higherlevels of PEEPand lower pressure support mightgive a more homogenous distributionregionalMIFas 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 atSan Gerardo Hospital, Monza, Italy, approved thestudywith informed consent obtainedin line withlocalregulations.Additional information abouttheinclusion and exclusion criteriaforthe twostudies are includedinan online data supplement(Additionaldata file1.).
Demographic data collection
Wecollected sex, age, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andthe severityof ARDS days onmechanical ventilationprior to study enrollmentforeachpatient.Mortality in the hospital was reported,too.
EIT andventilation monitoring
Ineach patient, an EIT-dedicatedbelt,which contained 16 equallyspaced electrodes, was positionedclose to the thorax area atthefifth or sixthintercostalspace , and then connected toan industrialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).Through all phases of the studyEITdata were generated byusing small, alternateelectrical currents that rotated around thean individual’s thorax. They were continuously recordedat 20 Hzand storedfor offline analysis, as previouslyexplained [12and 13.The data were synchronized withEITtracer data as well as airway pressure andairflows fromMechanical ventilators wererecorded continuously.
Interventions
More information onthe two protocolscan be foundin theonline data supplement(Additionalfile1).
In short, inPSV, in thePSV study,the patients went throughthe followingrandomized steps in a crossover which lasted each for 20 minutes:
- 1.
The support level at PEEP for clinical patients is low(PSV low)contrasts with.greater support atPEEP at the clinic(PSV high);
- 2.
Clinical supportforvery low levels of pressure(PSV-PEEP low)as opposed to.clinical support at higher PEEP(PSV-PEEP high).
In theVCV study,insteadthe following phaseswere conductedwith randomized crossoverseach lasting20 minutes
- 1.
A protective VCV when PEEP is low(VCV-PEEP low)vs.VCV to protect at clinicalPEEP+ 5cmH 2O (VCV-PEEP high).
EIT anddata on ventilation
Based on offline analysis ofEITtracings obtained duringthefinal minutein each of the phases(analysis of10breaths), we measuredboth the global and regional(same-sizedepended and not-dependent regions of the lung) noninvasive airflows in the form of a waveformas described in the past[88.In brief, instantaneous globalregionalinspiratory and expiratoryairflowswere recorded in terms ofvariationsin global and regionalimpedance measured every 50ms which were multiplied by tidalvolume/tidal-impedance ratio fromthesame study phase , anddivided by 50ms. EIT airflow data wereconverted from mL/msec intoL/min (Fig. 1) and the maximalEIT-derived regional and global MIFand MEF (MIFglob MIFglob, MIFnon-dep,and MIFdepMEFglob, MEFnon-dep andMEFdep, respectively) wereidentified and thevalue was averaged over5-10breathingcycles.
