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Q13Q14 Q1Q4Q2[ Critical Care How I Do It ]123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263Setting and Titrating PositiveEnd-Expiratory Pressure646566Scott J. Millington, MD; Pierre Cardinal, MD; and Laurent Brochard, MDABBREVIATIONS: ACP = acutEIT = electrical impedanceexpiratory pressure; Pes = esocare ultrasound; Pplat = plateauindex; RV = right ventricular;volume of lung recruited; VTAFFILIATIONS: From the Uni(S. J. Millington and P. CarResearch Centre, Li Ka Shingchestjournal.org67686970717273747576777879REV 5.6.Descargado para Anonymuso personal exclusivAlthough maintaining some amount of positive end-expiratory pressure (PEEP) seems essen-tial, selecting and titrating a specific level for patients with ARDS remains challenging despiteextensive research on the subject. Although an “open lung” approach to ventilation is popularand has some degree of biological plausibility, it is not without risk. Furthermore, there is noclear evidence-based guidance regarding initial PEEP settings or how to titrate them early in thecourse of the illness. Many busy clinicians use a “one-size-fits-all” approach based on localmedical culture, but an individualized approach has the potential to offer significant benefit.Here we present a pragmatic approach based on simple measurements available on all venti-lators, focused on achieving balance between the potential risks and benefits of PEEP.Acknowledging “best PEEP” as an impossible goal, we aim for a straightforward method toachieve “better PEEP.” CHEST 2022; -(-):---8081KEY WORDS: ARDS; critical care; mechanical ventilation828384858687888990919293949596979899100101Tp1tinafravpAawsmphe application of positive end-expiratoryressure (PEEP) was first described in the930s1,2 and came into common use forreating ARDS in the 1960s.3 In thetervening 50 years, a clear consensusround how to manage PEEP in general, andor patients with ARDS specifically, hasemained elusive.4 The current article offersn approach to selecting an initial PEEPalue and to titrating it over the preliminaryeriod of the acute illness for patients withRDS. Rather than aiming for “best PEEP,”n illusory and impossible-to-achieve goal,e focus on “better PEEP” using a simpletepwise approach centered around easy-to-easure variables and focused on both theotential benefits and harms of PEEP.e cor pulmonale; DP = driving pressure;tomography; PEEP = positive end-phageal pressure; POCUS = point-of-pressure; RII = recruitment-to-inflationTPP = transpulmonary pressure; Vrec == tidal volumeversity of Ottawa/The Ottawa Hospitaldinal), Ottawa, ON, Canada; KeenanKnowledge Institute (L. Brochard), St.Michael’s Hosthe Interdepachard), UniveCORRESPONDtoh.caCopyright � 2Elsevier Inc. ADOI: https://d0 DTD � CHEST4883_proof � 16 February 2022 � 5ous User (n/a) en National Autonomous University of Mexico de Clamente. No se permiten otros usos sin autorización. Copyright ©202Although this pragmatic approach isnecessarily an oversimplification of anextremely complicated topic, it shouldbe viewed through the lens of harmreduction: in many cases, PEEP levels areset, due to lack of time or experience, at anarbitrary level based on local habit andmedical culture. This “one-size-fits-all”approach can be made better via astraightforward approach that offers somepatient-specific adjustments. Although thismethod is based on evidence whereavailable, it places special emphasis onpracticality. A protocol requiring unusualequipment or a significant investment intime will simply be left by the wayside in abusy real-world ICU. The opinionspital, Unity Health Toronto, Toronto, ON, Canada; andrtmental Division of Critical Care Medicine (L. Bro-rsity of Toronto, Toronto, ON, Canada.ENCE TO: Scott J. Millington, MD Q3; email: smillington@022 American College of Chest Physicians. Published byll rights reserved.oi.org/10.1016/j.chest.2022.01.0521102103104105106107108109110:12 am � EO: CHEST-21-3765inicalKey.es por Elsevier en marzo 12, 2022. Para 2. Elsevier Inc. Todos los derechos reservados.Delta:1_given nameDelta:1_surnameDelta:1_given namemailto:smillington@toh.camailto:smillington@toh.cahttps://doi.org/10.1016/j.chest.2022.01.052http://chestjournal.orgQ5111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220expressed here are those of the authors, and otherequally valid approaches exist.Case ExampleA 60-year-old man with type 2 diabetes, hypertension,and obesity is transferred to the ICU with severehypoxemia in the context of a worsening community-acquired pneumonia and is intubated shortly thereafter.Ten minutes’ postintubation, his PaO2 is 80 mm Hg onan FIO2 of 0.7, with a PEEP level set at 8 cm H2O. He isplaced on volume control ventilation with a tidal volume(VT) of 400 mL (6 mL/kg predicted body weight). Withthe patient still under the effects of neuromuscularblockade from the induction of anesthesia, a plateaupressure (Pplat) of 28 cm H2O is measured, yielding astatic lung compliance of 20 mL/cm H₂O. His oxygensaturation level is 94%. His BP drops after intubation,requiring an infusion of norepinephrine (0.2 mg/kg perminute). He has evidence of end-organ dysfunction,including a lactate level of 4.0 mM and a creatinine levelof 225 mM. Should his level of PEEP be adjusted?Basic PrinciplesA full review of the complex pulmonary physiologyaffecting patients with ARDS is beyond the scope of thecurrent article, but two approaches to mechanicalventilation are worth considering briefly. Protective lungventilation5 uses low VT (typically 4-6 mL/kg based onpredicted body weight, reflecting lung size at baseline) toreduce ventilator-induced lung injury by decreasingvolutrauma (hyperinflation-induced shear injury),barotrauma (alveolar rupture and potentialpneumothorax), biotrauma (the release of inflammatorymediators secondary to both processes), and evenatelectrauma (the theoretical shear stress induced by thecyclical collapse and re-opening of alveoli6). The Pplat istypically kept strictly below 30 cm H2O,7 and permissivehypercapnia is often allowed. This approach isconsidered the standard of care for patients with ARDS,but there is good evidence to suggest that it is frequentlynot applied.8 Data suggest that lower tidal volumes maybe of more benefit to patients with reduced lungcompliance, which is of particular interest in the era ofCOVID-19.9,10The open lung concept, first described 30 years ago,11was initially a proposal to use recruitment maneuversfollowed by higher PEEP levels to reduce atelectrauma.An open lung approach should always be combined witha protective lung ventilation strategy, but unfortunatelythere is no agreed upon method to determine ideal PEEP2 How I Do ItREV 5.6.0 DTD � CHEST4883_proof � 16 FDescargado para Anonymous User (n/a) en National Autonomous Univeuso personal exclusivamente. No se permiten otros usos sin autorizacand no consensus as to the role of recruitmentmaneuvers. Higher PEEP levels have been comparedwith lower values in several studies12-14; some secondaryoutcome improvements have been reported, but amortality benefit remains elusive. Such studies aredifficult to interpret for several reasons, includingpatient heterogeneity with respect to the amount ofrecruitable lung. Some patients may recruit additionallung with higherPEEP and benefit from an open lungstrategy, whereas others may not recruit and thereafterbe harmed by higher volumes and pressures. Otherpublications showing benefit are difficult to apply at thebedside as they generally use experimental models oflung injury with highly recruitable lungs,15 differentfrom the physiology often seen in patients with ARDS.Great uncertainty remains with respect to choosing aPEEP value, and safety concerns around “aggressive”recruitment maneuvers persist.13Step 1: Pick a Starting PEEPDeliberations around ideal PEEP levels must begin witha starting value. Two straightforward options exist here,with the priority being to select a safe level in theimmediate postintubation period. The first method,perhaps best named the “Gattinoni” method,16 is thesimpler of the two. Using a standard Berlin definition ofseverity, patients with ARDS and a P/F ratio of 200 to300 would be classified as mild, 100 to 200 as moderate,and < 100 as severe. Initial PEEP values would then beset as follows: 5 to 10 cm H2O for mild, 10 to 15 cm H2Ofor moderate, and 15 to 20 cm H2O for severe. Wesuggest favoring the lower of the two values in mostcases as the initial setting, given that hemodynamicconcerns often predominate immediately post-intubation.Alternatively, a starting PEEP can be selected using thewell-known titration table from the Acute RespiratoryDistress Syndrome Clinical Network (ARDSNet) trial17;this widely known resource is designed for titratingPEEP later in the course of illness, but here it can beborrowed to aid in selecting a starting value. In selectingbetween the low-PEEP and high-PEEP options, wesuggest starting with the low-PEEP table in anticipationof postintubation hypotension. Using this method, apatient requiring an FIO2 of 0.5 (either preintubation orimmediately postintubation) would be started on either8 or 10 cm H2O of PEEP. Of note, both methods oftenyield similar starting values, but the Gattinoni method iseasier and avoids the ultra-high PEEP recommendations(as high as 24 cm H2O) that the ARDSNet table can[ -#- CHE ST - 2 0 2 2 ]ebruary 2022 � 5:12 am � EO: CHEST-21-3765rsity of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para ión. Copyright ©2022. Elsevier Inc. Todos los derechos reservados.Q6221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279recommend if the FIO2 is 1. High levels of PEEP, as a ruleof thumb, should be reserved for patients with diffuseinvolvement (three or four quadrants on chestradiograph or CT imaging) or morbid obesity.18,19280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330Step 2: Think About Potential Harms of PEEPExcessive PEEP can be harmful, with two mainproblems to consider. It is well known for causinghypotension; higher PEEP levels result in increasedintrathoracic pressure, thereafter increasing right atrialpressure and decreasing venous return. Excessive PEEPalso compresses intra-alveolar vessels and increasespulmonary vascular resistance, potentially resulting inright ventricular (RV) dysfunction. Here the net effect isa decrease in left ventricular preload and thereaftercardiac output. However, a “minimal” PEEP is alsoimportant because extensive lung collapse withinsufficient PEEP can cause high pulmonary vascularresistance and RV dysfunction.20 Although the preciserelationship between lung volume and pulmonaryvascular resistance is difficult to ascertain a priori,ventilation:perfusion matching is an important andchallenging aspect of PEEP selection, and a parallelassessment of hemodynamic and ventilator effects isalways warranted.Assessing the harmful hemodynamic effects of PEEP canbe done in several ways, with the most straightforwardbeing a simple assessment of BP. If the patient ishypotensive, and especially if they show evidence of end-organ dysfunction, then consideration should be givento lowering PEEP. This is particularly important if thestarting PEEP level was high, or if hypovolemia, chronicRV dysfunction, or pulmonary hypertension issuspected. In its most straightforward application, thiscan be achieved by lowering PEEP, waiting 5 to 10 min,and reassessing BP.A more precise but labor-intensive approach involvesassessing RV function via point-of-care ultrasound(POCUS) (Fig 1). The phenomenon in which high PEEPresults in RV dysfunction in the context of ARDS isknown as acute cor pulmonale (ACP); this process isoften mitigated by keeping Pplat below 28 cm H2O21 anddriving pressure (DP) below 15 cm H2O22 (see below fora full discussion regarding DP).A detailed review of echocardiographic RV assessment isbeyond the scope of the current paper and is covered indetail elsewhere.23 A basic POCUS RV examination isachievable with modest training and can be performedvia a transthoracic or transesophageal approach.chestjournal.orgREV 5.6.0 DTD � CHEST4883_proof � 16 FebruDescargado para Anonymous User (n/a) en National Autonomous Universityuso personal exclusivamente. No se permiten otros usos sin autorización. Assessing RV size and function is quickly done,reproducible, and repeatable. In the context of shock orhypotension, the right ventricle is assessed, the PEEPlevel is lowered (perhaps by as much as 5 cm H2O tomake ultrasound changes easier to discern), and thePOCUS examination is then repeated. Improvements inRV size or function with lower PEEP levels, especially ifthe BP also improves, suggest the presence of ACP andmay motivate the adoption of a lower PEEP level.Although remaining mindful of the risks of excessivePEEP, it is also important to remember that insufficientPEEP may result in enough de-recruitment to place theright ventricle at risk of circulatory failure.24The second potential harm of higher PEEP to considerinvolves alveolar overdistention and the resultant risk ofvolutrauma and barotrauma. This risk is morepronounced in patients with less recruitable lung, andthere is no clearly agreed upon method or evidence fordetermining the best PEEP to reduce such trauma. Onepragmatic method to screen for potential overdistentioninvolves using best DP measurements, which aredescribed in detail in Step 3. Here DP is used as asurrogate for compliance; better (lower) DP with areduction in PEEP suggests that overdistention mayindeed have been present. Targets to consider includekeeping Pplat < 30 cm H2O (or perhaps 28 cm H2O toavoid ACP21) and DP roughly below 15 cm H2O.25Targeting a lower dead space may also reduce the risk ofexcessive lung distension induced by PEEP. Althoughthe precise measurement of dead space requiresspecialized equipment, the effect of PEEP on PaCO2levels is a reasonable surrogate, provided that otherventilator settings (eg, VT and respiratory rate) are notaltered at the same time. An increase in PaCO2 of 5% to10% (or more) following an increase in PEEP suggestshyperinflation, although this effect can be exacerbatedby hypovolemia.Step 3: Think About Benefits of Higher PEEPHaving made efforts to minimize its deleterious effects,PEEP can thereafter be adjusted to optimize oxygenationand reduce atelectrauma. One mechanism by whichhigher PEEP improves oxygenation is alveolarrecruitment; keeping open previously collapsed (butperfused) lung regions increases the available surfacearea for gas exchange and diminishes intrapulmonaryshunt.Unfortunately, better oxygenation (as assessed either byimproved oxygen saturation levels or a higher P/F ratio)3ary 2022 � 5:12 am � EO: CHEST-21-3765 of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para Copyright ©2022. Elsevier Inc. Todos los derechos reservados.http://chestjournal.organgeloangeloangeloangeloFigure 1 – Q10A-D, Assessment of the right ventricle (RV) according to transthoracic echocardiography. A, Normal parasternal short-axis view, with anormally sized RV (*) and normally positioned interventricular septum (#). B, Normal apical four-chamber view with a normally sized RV (*). C,Parasternal short-axis view showing a severely dilated RV (*) and displaced interventricular septum (#). D, Apical four-chamber view showing aseverely dilated RV (*) and right atrium (#).331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440with higher PEEP does not necessarily translate intoimproved oxygen delivery. PEEP can improve arterialoxygenation yet decrease oxygen delivery by reducingcardiac output26; lower cardiac output preferentiallyreduces perfusion to nonventilated or very poorlyventilated alveoli, thereby decreasing intrapulmonaryshunt.26 As such, improvements in oxygenation aredifficult to definitively interpret unless reliablemeasurements of cardiac output are taken at the sametime. One must also remember that roughly 20% ofpatients have a patent foramen ovale, in which casedifferences in oxygenation may reflect changes in thedegree of intracardiac shunting rather than changes inlung recruitment.27 It is reasonable to use the severity ofhypoxemia as a parameter to help select the initial PEEP(as described in Step 1) and to be cautiously optimisticwhen oxygenation improves with higher PEEP,understanding that PEEP titration may be furtherrefined using other parameters.The second theoretical benefit of more PEEP involves areduction in atelectrauma. This is another longer term4 How I Do ItREV 5.6.0 DTD � CHEST4883_proof � 16 FDescargado para Anonymous User (n/a) en National Autonomous Univeuso personal exclusivamente. No se permiten otros usos sin autorizacand more theoretical benefit, but there is some suggestionthat it may be particularly important for patients withsevere ARDS.18 Although there is no clearly agreed uponmethod for determining the best PEEP to reduceatelectrauma, a stepwise down-titration approach,28 inwhich PEEP levels are first raised to a relatively high level(eg, 10 cm H2O above the initial setting) and then slowlydecreased, is appealing primarily due to its soundphysiological rationale. The hysteresis (differencebetween the inflation and deflation limbs) observed in theclassic pressure-volume curve (Figs 2A, 2B) reflectsdifferences between opening and closing pressures ofindividual lung units. As such, higher PEEP can bethought of as a force used to prevent airway closure andde-recruitment of already opened alveolar units; thepurpose of the subsequent stepwise down-titration is toselect the lowest PEEP level that maintains recruitment.To guide PEEP selection, we suggest calculating the DPat different PEEP levels (Fig 2C), keeping in mindseveral important caveats described here. Assuming theVT is not changed between measurements at different[ -#- CHE ST - 2 0 2 2 ]ebruary 2022 � 5:12 am � EO: CHEST-21-3765rsity of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para ión. Copyright ©2022. Elsevier Inc. Todos los derechos reservados.angeloangeloangeloangeloPaw (cm H2O)Flow (L/s)Pes (cm H2O)PL (cm H2O)−200−1.0−1001020162024281020−0.50.00.530 AOP401.020 25 555045403530Time (s)0 5 10 15Volume20 25 30PressureLIP / AOPABprint&web4C=FPOFigure 2 – A, Mechanical ventilation waveformsshowing (from top to bottom) flow, Paw, Pes, and PLvs time. The third breath is obtained by reducinginspiratory flow to < 10 L/min. The abrupt changein slope of the Paw waveform is AOP, the pressurerequired to reopen the airways. Initially, when Paw islower than the AOP, the inspiratory airflow pres-surizes the ventilator circuit but does not increaselung volume. The lungs begin to inflate once the Pawincreases above AOP (third breath); the slope of thepressure-time curve observed beyond AOP reflects thecompliance of the respiratory system. B, A pressure/volume ventilator waveform showing hysteresis; theLIP is shown, but this is more likely to reflect AOP(the minimal pressure needed to reopen the airwaysor keep them open, as opposed to the optimal level forlung recruitment as is classically taught). C, Simu-lated ventilator waveform showing the effect of aninspiratory hold (*) on Paw (thick arrow); drivingpressure (#) can be calculated as plateau pressure(thin arrow) minus positive end-expiratory pressure(dashed arrow). Adding an inspiratory pause of 0.2 sfollowing every breath allows for continuous moni-toring of plateau and driving pressures. AOP ¼airway opening pressure; LIP ¼ lower infectionpoint; Paw ¼ airway pressure; Pes ¼ esophagealpressure; PL ¼ transpulmonary pressure.441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549PEEP levels (and that there is no intrinsic PEEP),changes in DP can only be explained by changes incompliance:Compliance ¼ VTPplat � PEEPBoth VT and PEEP are known values. Pplat is measuredby performing an end-inspiratory pause, which can beeither be done as a manual maneuver or by adding achestjournal.orgREV 5.6.0 DTD � CHEST4883_proof � 16 FebruDescargado para Anonymous User (n/a) en National Autonomous Universityuso personal exclusivamente. No se permiten otros usos sin autorización. short (0.2 or 0.3 s) pause to the end of each breath tocontinuously monitor Pplat.The formula for DP is [DP ¼ Pplat – PEEP], andtherefore the compliance equation can be simplified to:Compliance ¼ VTDPFinally, with VT, a known and stable value (especiallyif a volume-controlled mode of ventilation is5550ary 2022 � 5:12 am � EO: CHEST-21-3765 of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para Copyright ©2022. Elsevier Inc. Todos los derechos reservados.http://chestjournal.org#*VolumeFlowPressureCprint&web4C=FPOprint&web4C=FPOprint&web4C=FPOFigure 2 – (Continued).551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659selected), the equation can be rearranged as:DP f1ComplianceTherefore, a lower DP value indicates better respiratorysystem compliance. If DP decreases at a higher level ofPEEP, the trial of higher PEEP is considered successful,especially in light of improved oxygenation and absenthemodynamic deterioration. If DP does not change,optimal compliance has potentially been achieved. If DPincreases, then compliance has worsened, possiblyindicating over-distension (as discussed in Step 2earlier), and consideration should be given to loweringPEEP. In simple terms, PEEP is adjusted to achieve thelowest DP possible, keeping in mind other importanttargets such as oxygenationand hemodynamics.Obese patients with ARDS merit special considerationgiven the extra weight applied to their lungs, whichtends to create or worsen atelectasis, especially in thesupine position and under anesthesia. Higher PEEPlevels may be needed to counter the effect of thisimposed load. Accepting a higher than usual Pplat levelfor obese patients may be necessary and is safe insofar asit does not automatically reflect lung overdistention. Alinear relationship between high BMI and end-expiratory esophageal pressure (Pes) has beendescribed29 for patients with ARDS (Pes, described in6 How I Do ItREV 5.6.0 DTD � CHEST4883_proof � 16 FDescargado para Anonymous User (n/a) en National Autonomous Univeuso personal exclusivamente. No se permiten otros usos sin autorizacdetail below, serves to estimate pleural pressure). Therelationship suggests that intrathoracic pressureincreases by approximately 2.5 cm H2O for every10 kg/m2 of BMI beyond 30 kg/m2. Although this hasnot been prospectively validated, this formula mightserve as a quick guide to the amount of extra PEEPneeded or the additional pressure acceptable for obesepatients. Similar accommodation for higher pressuresmay need to be made in other special circ*mstancessuch as abdominal compartment syndrome or majorchest wall deformities, but here evidence-based guidanceis unavailable.Prone positioning, although increasingly popular,presents an additional challenge for PEEP titration. Inthe supine position, PEEP is set essentially as acompromise between the necessary pressure to recruitthe dependent (and recruitable) part of the lung, withoutoverly distending the nondependent (nonrecruitable)part. Interestingly, in the prone position, these twoelements are much more hom*ogeneous, and theirresponse to PEEP is less influenced by gravity.30 Thissuggests that an optimal PEEP level in terms of risk/benefit is present over a much larger range in the proneposition. Therefore, when switching to a prone position,the PEEP level can most likely be left unchanged, orperhaps decreased slightly due to the additionalrecruitment offered by prone positioning.[ -#- CHE ST - 2 0 2 2 ]660ebruary 2022 � 5:12 am � EO: CHEST-21-3765rsity of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para ión. Copyright ©2022. Elsevier Inc. Todos los derechos reservados.TimeVolumeFlowPressurePEEP 5 cm H2OPEEP 15 cm H2OVT releasedExhaled VT recruitmentPredicted exhaled VTExhaled VTSet VTΔ10 cm H2OPlateauPaw (cm H2O)Flow (L/s)1020PEEP 5A B CPEEP 10 PEEP 1530−101print&web4C=FPOprint&web4C=FPOprint&web4C=FPOFigure 3 – Q11 Q12Top: FlowQ16vs time (top) and pressure vs time (bottom) curves obtained on volume-assist control ventilation with constant inspiratory flowat different levels of PEEP. A convex ascending limb (dotted arrow, PEEP 5) may indicate recruitment of alveolar units during inspiration (sug-gesting insufficient PEEP), whereas a concave ascending limb (dotted arrow, PEEP 15) may indicate overdistension (suggesting excessive PEEP).Modified from: Henderson WR, Chen L, Amato MBP, Brochard LJ. Fifty years of research in ARDS. Respiratory mechanics in acute respiratorydistress syndrome. Am J Respir Crit Care Med. 2017;196(7):822-833. Bottom, Ventilator curves illustrating the calculation of the recruitment-to-inflation index. The exhaled VT (red arrow) is measured at higher PEEP, and then the PEEP level is suddenly dropped (in this case by 10 cm H2O),and the total released VT is displayed by the ventilator at the end of exhalation. The predicted exhaled VT (blue arrow) is based on the compliance atlow PEEP multiplied by delta PEEP, and the recruited volume (or exhaled VT recruitment trapped by PEEP; green arrow) is measured aftersubtracting the inspired VT. The recruited volume is then divided by the change in PEEP (here 10 cm H2O) to yield the compliance of the recruitedlung. Finally, the compliance of the recruited lung is indexed to the compliance at the lower PEEP level to yield the recruitment-to-inflation index.These calculations are facilitated by the use of an online calculator (see https://rtmaven.com). PEEP ¼ positive end-expiratory pressure; VT ¼ tidalvolume.chestjournal.org 7661662663664665666667668669670671672673674675676677678679680681682683684685686687688689690691692693694695696697698699700701702703704705706707708709710711712713714715716717718719720721722723724725726727728729730731732733734735736737738739740741742743744745746747748749750751752753754755756757758759760761762763764765766767768769770REV 5.6.0 DTD � CHEST4883_proof � 16 February 2022 � 5:12 am � EO: CHEST-21-3765Descargado para Anonymous User (n/a) en National Autonomous University of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para uso personal exclusivamente. No se permiten otros usos sin autorización. Copyright ©2022. Elsevier Inc. Todos los derechos reservados.https://rtmaven.comhttp://chestjournal.orgQ7771772773774775776777778779780781782783784785786787788789790791792793794795796797798799800801802803804805806807808809810811812813814815816817818819820821822823824825826827828829830831832833834835836837838839840841842843844845846847848849850851852853854855856857858859860861862863864865866867868869870871872873874875876877878879880Caveats in Using Compliance or DP for PEEPTitrationThere are limits when using DP to titrate PEEP levels.Compliance (and DP) can be paradoxically improved byintra-tidal recruitment-derecruitment (the lung openingand closing that occurs within each breath31), and whichcan confound interpretation. Therefore, concluding thatthere is harmful overdistention or derecruitment shouldbe reserved for situations in which different PEEP levelsproduce clear and significant changes in DP.Overdistention may also be assessed by examining theshape of the pressure curve during volume-controlledventilation; an upward concavity of the pressure-timecurve suggests a significant drop in compliance (Fig3A).32In addition, overall respiratory system compliancecomprises contributions from both the lungs and chestwall, and therefore changes in measured compliancemay reflect changes at the level of the chest wall and notnecessarily the lungs. Both recruitment (which increasescompliance) and overdistention (which decreasescompliance) may occur in different parts of the lungssimultaneously when PEEP is increased. Therefore, theoverall compliance of the system may not change (or itmay even increase), whereas some areas of the lungs,especially in nondependent areas, are beingoverdistended30; this problem is not unique to DP andindeed exists for all methods for setting PEEP. Finally, itis unclear how often measurements, DP or otherwise,should be taken and how often PEEP should be re-assessed. Repeated measurements are limited by theneed to have the patient deeply sedated and/orparalyzed, although a technique to measure DP duringspontaneous modes of ventilation has been described.33Step 4: Consider More Advanced OptionsSome centers may have specific advanced equipmentavailable, and some clinicians may have a particularexpertise that allows consideration of other techniques.Evidence in general is lacking, but each has interestingpotential advantages.Esophageal balloon manometry is a minimally invasivetechnique whereby Pes, measured via a modifiednasogastric tube, is used as a surrogate for pleuralpressure. Transpulmonary pressure (TPP), the netdistending pressure exerted on the alveoli, is thenestimated using [TPP ¼ Pplat – Pes]; PEEP can thereafterbe adjusted to achieve a specific TPP. Although thistechnique is straightforwardlyperformed, it is subject to8 How I Do ItREV 5.6.0 DTD � CHEST4883_proof � 16 FDescargado para Anonymous User (n/a) en National Autonomous Univeuso personal exclusivamente. No se permiten otros usos sin autorizacvarious artifacts and assumptions, and evidence ofbenefit has not been shown.34 It seems to be of particularinterest for morbidly obese patients with ARDS, forreducing ventilator dyssynchrony, and in patients withmoderately severe disease.35Electrical impedance tomography (EIT) is based on thedistinct impedance of gas vs various types of tissues,with impedance defined as the inverse of conductivity.Increasing the quantity of air in the lungs, for example,increases impedance. EIT creates a distribution mapsuggesting zones of collapse, normal aeration, andoverdistention. PEEP can thereafter be titrated toachieve the highest electrical impedance in thethorax, suggesting the greatest amount of aerated lung.EIT can also suggest the presence of predominanthyperinflation and be an incentive to reduce PEEP.36This tool is not widely available and merits moreextensive study.The recruitment-to-inflation index (RII) is a novelbedside technique that seeks to estimate, in one breath,the volume of lung recruited (Vrec) when movingabruptly from higher to lower PEEP levels (Fig 3B). Thecompliance of that recruited lung can then be calculatedand indexed to the compliance measured at the lowerPEEP level (estimating the compliance of the “babylung”), yielding the RII. Lower RII values suggest a lackof recruitability and raise concerns that most of the“extra” pressure applied at higher PEEP will serveprimarily to over-distend normal lung segments insteadof recruiting previously collapsed ones. This techniquedoes not require specialized equipment and is easilyperformed (see video on https://rtmaven.com).However, it lacks supporting patient outcome data thusfar (although a randomized controlled trial isongoing37), and although it identifies potentialrecruitability, it is less able to identify a specific targetPEEP level.There are other approaches to PEEP that are welldescribed but may have a different effect than initiallyproposed. For example, setting the PEEP to a levelslightly above the lower inflection point of the pressure-volume curve is a well-known technique described inevery critical care textbook. In reality, beyond beingdifficult to accurately identify, for most cases this lowinflection point represents a somewhat recentlyrediscovered phenomenon known as airway openingpressure (Fig 2A),38,39 which signifies the minimal valuerequired to open the airway rather than the optimalPEEP. It can be measured by a pressure-time curve when[ -#- CHE ST - 2 0 2 2 ]ebruary 2022 � 5:12 am � EO: CHEST-21-3765rsity of Mexico de ClinicalKey.es por Elsevier en marzo 12, 2022. Para ión. Copyright ©2022. Elsevier Inc. Todos los derechos reservados.https://rtmaven.comQ8Q9881882883884885886887888889890891892893894895896897898899900901902903904905906907908909910911912913914915916917918919920921922923924925926927928929930931932933934935936937938939940941942943944945946947948949950951952953954955956957958959960961962963964965966967968969970971972973974975976977978979980981982983984985986987988989990insufflating the lung at a low flow rate (see video on seehttps://rtmaven.com).Finally, very informative studies have used sequentialCT scans to determine a PEEP at which the greatestvolume of lung is recruited. Although CT scans are anexcellent research tool, transporting critically ill patientsto the radiology department is impractical, andcomputation of lung recruitment is complex.ConclusionsFinding the “best” PEEP for a patient with ARDS isimpossible with current technology, and the path to“better” PEEP is far from straightforward. We present apragmatic protocol with three main principles: (1)choose a safe starting PEEP; (2) consider the harms ofhigher PEEP, specifically hemodynamic compromiseand overdistention; and (3) consider the benefits ofhigher PEEP, specifically better oxygenation and lessatelectrauma (when the lung is indeed recruitable). Ourapproach will be correctly criticized as a dramaticoversimplification but should be viewed through a lensof harm reduction in considering the number of patientswith ARDS who currently receive a blind PEEPprescription with no attempt at individualization. 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Todos los derechos reservados.Setting and Titrating Positive End-Expiratory PressureCase ExampleBasic PrinciplesStep 1: Pick a Starting PEEPStep 2: Think About Potential Harms of PEEPStep 3: Think About Benefits of Higher PEEPCaveats in Using Compliance or DP for PEEP TitrationStep 4: Consider More Advanced OptionsConclusionsAcknowledgmentsReferences