Noninvasive monitoring of the maximalflow rates of expiratory and inspiratory(MIF and MEFrespectively)using electrical impedance tomography(EIT)could facilitatean early detection of changes toour respiratory tract’s propertiesin responsetonew circumstances or inresponse totreatment.We aimed to validateEIT-basedmeasurementsforMIFandMEF against spirometryof intubatedhypoxemic patients under controlled breathingor spontaneous breathing.Additionally, the regional distribution ofmaximum airflows can interact withlungpathologies and increasetherisk of additional ventilationinjuries.This is why we wantedtoanalyze the effectsofsettings for mechanical airflow onregions ofMIFas well asMEF.
Methods
We performed a new analysisofdatafrom two prospective, randomized,crossoverstudies.We included patients who wereadmitted to theintensive care unit suffering froman acute respiratory failure(AHRF)and acute respiratory distress syndrome(ARDS)under pressure supportventilatory(PSV, n=10) andventilatory control(VCV, n=20).We measured MIF and MEFwith spirometry and EIT throughout6 different ventilation configurations which were:higherthan. lower supportinPSV and greaterthan. lowerpositive end-expiratory pressure(PEEP)inbothVCV and PSV.Regional airflows were analyzed usingEITfor both dependent and non-dependentlung regions, too.
Results
MIF and impedanztomographie determinedbyEIT werehighly correlated withthe results of spirometry inevery condition(rangebetweenR2 0.629-0.776 and R2 0.606-0.772,respectively, p0.05throughoutall) in accordance with the clinically acceptableboundaries of disagreement.Higher PEEP significantly improvedhomogeneity and consistency in thespreadof MIF and MEFduring ventilation controlled by volumeby increasing airflows within theareas of the lung that are dependent and decreasingthose in non-dependent regions.
Conclusions
EITallows for accurate and noninvasive monitoringforMIFas well asMEF.The current study also supportstheideathat EITcould help guidePSV and PEEPsettings aimedto improve the homogeneity ofspreading and deflating regional airflows.
Introduction
The electrical impedance imaging(EIT)isanon-invasive bedside, radiation-freedynamic lung imaging technique. EIToffers intrathoracic maps oflung impedance changes referenced toan initial value(i.e.,an end-expiratory lung volume measured from athe previousbreath) every20-50 milliseconds].Impedance changes that are measured in the intrathoracic regionwithEIT are linearlyrelated tothe global and regional tidal volumes and this correlation isheld at increasing positive end-expiratorypressure (PEEP) levels [22.Thus,EITprovides a non-invasive continuous bedsidemeasure of regionallung volumevariations duringexhalation and inspiration.
Inspiratory and expiratory airflows are relatedto therate at whichthe lung’s volume as it changesintime.In patients with intubation,they arenormally measured withSpirometers that are connectedto the ventilator circuit either beforetheendotracheal tube , or insidethe ventilator.Global maximal inspiratory andexpiratoryflows(MIF and MEF as well)measured bystandard spirometry depend onthe mechanical properties of the respiratory system(namely lung compliance, lung pressure andresistance to airways) [33.Monitoring ofMIF andthe MEF canbe beneficial to help guideadjustments to the ventilation system(e.g., by selectingthepressure that is positive and associatedwithbettermechanics)or to assesstheeffectiveness of pharmacologic treatments(e.g., increasedMIFand/or MEF in response tobronchodilator drugs) [4(e.g., increased MIF and/or MEF after bronchod.But spirometry is only able to provideglobal measures of MIF andMEF. The heterogeneous spreadofaltered lung mechanics is acharacteristic of acute hypoxemicfailing(AHRF)along with acute respiratory stress syndrome(ARDS) [55.Alveolar damage leads tocollapse of lung units tightlyadjacent to normal-, partiallyor over-inflated ones, potentially yieldingimbalancesacross the region inMIFas well asMEF values.Such imbalances can increasethechance of developing a ventilator-induced lung disease(VILI)via a myriad of mechanisms[6], and settingscreating more homogeneous regional flowcould decrease the risk. Externalspirometry can leadtoaltered patterns in the respiratory system andinaccurate measurementsas well[7].Therefore, a non-invasivebedsidedynamic method to measureboth regional and global MIF as well asMEFvalues couldmake a great contribution tounderstandingAHRF and ARDSpatients’ pathophysiology andfor guiding personalized treatments.
In this study,following preliminary findings from anthe animal model of[8], we soughttoconfirm inan intubatedAHRFandARDS patientsundercontrolledbreathing andEIT-based measurements of spontaneous breathing ofglobal MIF and MEF compared tostandardspirometry.Furthermore, we exploredtheeffect of higher vs. lowerthe levels of pressure support onregionalflows;our hypothesis isthat higherlevels of PEEPand lower pressure support mayyield more homogenous distribution oflocalMIFas 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 thestudyas well as informed consent,as perlocalregulations.Further details regardingthecriteria for inclusion and exclusionforbothstudies are includedinan online supplement to the data(Additionaldocument1.).
Demographic data collection
Wecollected data on sex, ages, Simplified Acute Physiology Score IIvalues, etiology, diagnosis andseverityof ARDS days undermechanical ventilationprior to enrollment in the studyforeachpatient.In-hospital mortality was recordedtoo.
EIT andventilation monitoring
Inall patients, an EIT-specificbelt,comprising 16 evenlyseparated electrodes, was puton the thorax inthefifth or sixthintercostalarea and connected toa commercialEIT monitor (PulmoVista 500, Drager Medical GmbH, Lubeck, Germany).In all phases of the study,EITinformation was generated throughuse of small alternationelectrical currents rotating aroundpatients thorax. These were recorded continuouslyat 20 Hz, and then storedfor offline analysis, asdescribed [12in the previous article [12, 13].As synchronizedEITtracings Airway pressure, as well asairflows fromthe mechanical ventilator wererecorded continuously.
Interventions
More details onthe two protocolsare availablein theonline data supplement(AdditionalFile1).
Briefly, inPSV, in thePSV study,patients were subjected tothe followingsteps of randomized crossover every 20 min:
- 1.
Clinical PEEP support is weak(PSV low)against.higher support forclinical PEEP(PSV high);
- 2.
Clinical supportforthe low level of PEEP(PSV-PEEP low)against.the clinical assistance at higher levels(PSV-PEEP high).
For theVCV study,,the following phasestook placein a randomized order in the crossover,each lasting20 min:
- 1.
A protective VCV when PEEP is low(VCV-PEEP low)in comparison to.VCV to protect at clinicalPEEP+5cmH 2O (VCV-PEEP high).
EIT anddata on ventilation
Based on offline analysis ofEITtracer data collected duringthefinal minutesduring each stage(analysis of10breaths) and analyzed thelocal and global(same-sizeregion of lung that is dependent as well as non-dependent) noninvasive airflowswaveform,like the ones previously reported[89].In short, instantaneous global andregionalinspiratory and expiratoryairflowswere recorded in terms ofvariationsin regional and globalimpedance , which was measured every 50 ms and multiplied by thevolume/tidal impedance ratio ofthestudy phase in question anddivided by 50milliseconds. 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, MIFnondepand MIFdep;MEFglob the MEFglob, MEFnon dep andMEFdep according to) weredetermined and thevalue averaged over5-10respiratorycycles.
