Patient-Ventilator Interactions: Optimizing Conventional...

Patient-Ventilator Interactions:Optimizing Conventional Ventilation Modes

Neil R MacIntyre MD FAARC

IntroductionFeatures Available on Conventional Modes That Enhance Synchrony

Breath and Mode Design FeaturesMonitoring Techniques for Synchrony

Using These Conventional Features at the Bedside to Improve Patient-Ventilator Interaction

Optimizing Breath TriggeringOptimizing Flow DeliveryOptimizing Breath CyclingThe Distribution of Ventilator Breaths May Affect Synchrony

Summary

Assisted (interactive) breathing is generally preferred to controlled breaths in patients on mechan-ical ventilators. Assisted breaths allow the patient’s respiratory muscles to be used, and ventilatorymuscle atrophy can be prevented. Moreover, the respiratory drive of the patient does not have tobe aggressively blunted. However, interactive breaths need to be synchronized with the patient’sefforts during the trigger, the flow delivery, and the cycling phases. Asynchrony during any of thesecan put an intolerable load on the respiratory muscles, leading to fatigue and the need for a highlevel of sedation or even paralysis. Current ventilation modes have a number of features that canmonitor and enhance synchrony, including adjustment of the trigger variable, the use of pressure-targeted versus fixed-flow-targeted breaths, and manipulations of the cycle variable. Cliniciansneed to know how to use these ventilation mode and monitor them properly, especially understand-ing the airway pressure and flow graphics. The clinical challenge is synchronizing ventilator gasdelivery with patient effort. Key words: mechanical ventilation; patient-ventilator synchrony; ventila-tion modes. [Respir Care 2011;56(1):73–81. © 2011 Daedalus Enterprises]

Introduction

Breaths delivered by a mechanical ventilator can be con-sidered as either controlled or assisted.1 A controlled breath

means that the machine determines the rate, the inspiratorytime, and the tidal volume (VT), so the patient does no work.With controlled breaths, sedation and paralysis may be neededto suppress patient activity that might interfere with breath

Neil R MacIntyre MD FAARC is affiliated with the Division of Pulmo-nary and Critical Care Medicine, Duke University Medical Center,Durham, North Carolina.

Dr MacIntyre presented a version of this paper at the 46th RESPIRATORY

CARE Journal Conference, “Patient-Ventilator Interaction,” held March 19-21, 2010, in Cancun, Quintana Roo, Mexico.

Dr MacIntyre has disclosed a relationship with CareFusion.

Correspondence: Neil R MacIntyre MD FAARC, Division of Pulmonaryand Critical Care Medicine, Duke University Hospital, Box 3911, DurhamNC 27710. E-mail: [email protected].

RESPIRATORY CARE • JANUARY 2011 VOL 56 NO 1 73

delivery. There is also a risk of diaphragmatic muscle struc-tural derangement and atrophy (ventilator-induced diaphrag-matic dysfunction).2,3 In contrast, in an assisted breath thepatient interacts in some way with the ventilator, so the workof breathing is shared between the ventilator and the pa-tient.4-7 The level of patient work is minimal if all the patientdoes is trigger the breath, or the patient work can be consid-erable if the ventilator’s flow delivery and/or pressure appli-cation is minimal.

With assisted breaths, sedation needs may be less and pa-tient muscle function may recover more rapidly,5,6 but thisrequires that the ventilator’s flow and pressure delivery aresynchronized with the patient’s effort during all 3 breathphases: initiation, delivery, and termination. Triggering de-lays, inadequate flow delivery, or mismatching of the end ofinspiration and the beginning of expiration can place a con-siderable load on the patient5-7 and lead to muscle fatigue,discomfort, and an increasing need for sedation or even par-alytics. The importance of patient-ventilator asynchrony hasbeen shown in several studies.8-11 In one, substantial asyn-chrony was associated with much longer duration of mechan-ical ventilation and even a trend toward a worse mortality.8

The clinical challenge, then, is to provide ventilatory supportthat is in accordance with patient effort: this is the concept ofsynchronizing mechanical ventilation to the patient’s respi-ratory drive.

Features Available on Conventional ModesThat Enhance Synchrony

Breath and Mode Design Features

As noted above, ventilator breaths can be described bywhat initiates the breath, what governs gas flow during thebreath, and what terminates the breath (the trigger, target, andcycle variables, respectively). Conventional ventilators todaydeliver 5 basic breath types (Table 1 and Fig. 1):

• The volume-control breath is time-triggered, flow-tar-geted, and volume-cycled.

• The volume-assist breath is effort-triggered, flow-tar-geted, and volume-cycled.

• The pressure-control breath is time-triggered, pressure-targeted, and time-cycled.

• The pressure-assist breath is effort-triggered, pressure-targeted, and time-cycled.

• The pressure-support breath is effort-triggered, pressure-targeted, and flow-cycled.

Three of these breath types can be considered assistedor interactive breaths: volume-assist, pressure-assist, andpressure-support.

The mechanical ventilation modes are the ways in whichthese 5 basic breaths can be delivered to the patient (Ta-ble 2). The volume assist-control mode provides volume-control and volume-assisted breaths. If the machine breathrate is set very high, the mode becomes virtually onlyvolume control; in contrast, if the mandatory breath rate isset very low and patient triggers most of these breaths, themode essentially becomes a pure volume-assist mode. Sim-ilarly, the pressure assist-control mode has a combinationof pressure-control and pressure-assisted breaths. Again,depending upon the set machine rate, the breaths can bepredominantly pressure-control or predominantly pressure-assist. Synchronized intermittent mandatory ventilation(SIMV) modes mix either (1) volume-assist and volume-control breaths with pressure-support or spontaneousbreaths, or (2) pressure-assist and pressure-control breathsinterspersed with pressure-support and/or spontaneousbreaths. Finally, pressure-support ventilation (PSV) can bea stand-alone mode in which only pressure-support breathsare given. As a side note, the novel mode airway pressurerelease ventilation (APRV) is really a form of SIMV inthat APRV provides essentially a pressure-assist-controlbreath with a very long inspiratory time and allows spon-taneous breathing during both the inflation and deflationphase.

In applying these conventional ventilation modes, sev-eral design features can be used to improve patient-venti-lator synchrony. For breath triggering the clinician canchoose either a pressure trigger or a flow trigger, and canset the trigger as sensitive as possible. When breath trig-gering is impeded by intrinsic PEEP, the clinician can alsoadd extrinsic or circuit PEEP to overcome that impedance.During flow delivery the clinician can choose either flowor pressure targeting. With flow targeting the flow is fixed.With pressure targeting the flow will adjust to maintain atarget pressure. Pressure-targeted breaths also have addi-tional features that control the rate of pressure rise, andalgorithms that can adjust the pressure target to compen-sate for the endotracheal tube. For breath cycling, the cli-nician can set the volume (in volume-control and volume-assist breaths), the inspiratory time (in pressure-controland pressure-assist breaths), or the inspiratory flow cutoff(in pressure-support breaths) to optimize synchrony. Fi-

Table 1. Trigger, Target, and Cycle Criteria of the 5 Basic BreathTypes

Breath Type Trigger Target Cycle

Volume control Timer Flow VolumeVolume assist Effort Flow VolumePressure control Timer Pressure TimePressure assist Effort Pressure TimePressure support Effort Pressure Flow

PATIENT-VENTILATOR INTERACTIONS: OPTIMIZING CONVENTIONAL VENTILATION MODES

74 RESPIRATORY CARE • JANUARY 2011 VOL 56 NO 1

nally, the distribution of various breath types in variousmodes (ie, the number of controlled vs assisted breaths,and the number of assisted vs supported vs spontaneousunsupported breaths) can also be clinician-selected, basedon various clinical goals. How these features can be usedto optimize synchrony is described in more detail below.

Monitoring Techniques for Synchrony

There are a number of ways to monitor patient-ventila-tor interactions on current mechanical ventilator mod-els.11-15 Importantly, clinical observation should always beat the forefront of monitoring. This includes not only mon-itoring vital signs, but also assessing the patient’s comfort.While comfort may be difficult to define, discomfort ischaracterized by anxiety, diaphoresis, nasal flaring, andjust “looking miserable.” A useful technique is to place

one’s hand on the chest to feel patient effort and thenobserve the ventilator’s behavior, looking particularly fortriggering delays, missed efforts, flow starvation, and mis-matching of effort and breath cycling. Clinical monitoringalso involves evaluating the patient’s metabolic status (es-pecially gas exchange and acid-base balance), and assur-ing that pain is adequately managed.

On the ventilator itself, understanding of the airwaypressure and flow graphics is critical.11-13 Classical signsof asynchrony are airway pressure graphics that depictmarked negative deflections during the triggering process,marked negative pressure during the flow delivery, andboth positive and negative airway pressure graphics depictdeflections during the cycling process.5,6,11-13 Indeed, whenthe area under the airway pressure graphic during an as-sisted breath is compared to a controlled breath, one cancalculate the amount of patient work being done or patient

Fig. 1. Airway pressure, flow, and volume tracings illustrating the 5 basic breath types in Table 1. The solid lines indicate clinician-setparameters. The dotted lines indicate variable effects determined by respiratory-system mechanics and effort. TI � inspiratory time.(Adapted from Reference 1, with permission.)

Table 2. Ventilation Modes and the 5 Basic Breath Types and Spontaneous Unassisted Breaths

Mode

Breath Types Available

VolumeControl

VolumeAssist

PressureControl

PressureAssist

PressureSupport

Spontaneous

Volume assist control � �

Pressure assist control � �

Volume SIMV � � � �

Pressure SIMV � � � �

Pressure support �

SIMV � synchronized intermittent mandatory ventilation



load being borne during all 3 phases of interactive breaths.12

Esophageal and transdiaphragmatic pressure can also beused for the same purpose.

Flow graphics can also give clues to problems withpatient-ventilator asynchrony.5,6,11-13 Spikes in flow or slug-gish inspiratory flow often indicate that flow is not insynchrony with patient efforts. Expiratory flow is alsoimportant to evaluate. Reversals of flow during expirationmay signify patient inspiratory efforts during expiration.Expiratory flow that does not return to the zero baselinebefore the next breath is delivered might reflect the devel-opment of intrinsic PEEP.11

Using These Conventional Features at the Bedsideto Improve Patient-Ventilator Interaction

As noted above, the clinician has several tools to im-prove patient-ventilator synchrony. Importantly, as syn-chrony improves in one aspect of breath delivery (eg, trig-gering), patient dyspnea, and thus respiratory drive, maydecrease, making other aspects of breath delivery easier tosynchronize.16-18 Similarly, as asynchrony worsens in oneaspect of breath delivery, dyspnea, and thus respiratorydrive, may increase, making other aspects of breath deliv-ery more difficult to synchronize. Another example is cy-cling asynchrony worsening air-trapping and thus trigger-ing. Simply put: “synchrony begets synchrony, andasynchrony begets asynchrony.”

Optimizing Breath Triggering

The clinician should choose the trigger sensor (flow vspressure) that is most sensitive and responsive to patienteffort.19 Importantly, some ventilators have both types ofeffort sensors present and will respond to whichever signalis detected first. With either sensor, the clinician shouldadjust the triggering sensitivity to be as sensitive as pos-sible without causing auto-triggering. Evaluation of theairway or esophageal pressure graphic before ventilatorflow hits the target value allows quantification of the trig-gering load on the patient.19 The more insensitive andunresponsive the trigger is, the more negative will be thepressure-time profile. Indeed, the triggering load can bequantified either as work or pressure-time product via anal-ysis of these tracings.

If intrinsic PEEP is producing a triggering load (a prob-lem often seen in patients with substantial air-flow ob-struction), clinical observation will reveal a marked delaybetween the initiation of patient effort and the beginningof flow delivery, and possibly missed efforts.20-22 Graph-ically there may be transient reductions in expiratory flow,due to efforts that did not trigger breaths. The airwaypressure graphic, however, is often unremarkable becausethe patient’s efforts are insufficient to overcome the in-

trinsic PEEP and thus are not detected in the ventilatorcircuit. In contrast, an esophageal pressure tracing willclearly show the elevated intrathoracic pressure from in-trinsic PEEP and the ineffective patient efforts to over-come that pressure.

The first way to address this is to try reducing the in-trinsic PEEP as much as possible by reducing minute ven-tilation (eg, reduce controlled ventilation, reduce the ven-tilation needs driving the patient efforts), lengthening theexpiratory time, or improving the airway mechanics.23 Inaddition, the triggering load from intrinsic PEEP can bereduced by applying a judicious amount of circuit PEEP,which serves to narrow the difference between circuit andintrinsic PEEP.20-22,24,25 This could be guided by an esoph-ageal pressure tracing, with the goal of providing around70–80% of measured intrinsic PEEP as circuit PEEP.25 Ifan esophageal balloon is not available, an alternative ap-proach is to empirically titrate PEEP and follow the pa-tient’s response. If the application of PEEP is benefitingthe patient, the delay between effort and ventilator trig-gering will shorten and the patient will be observed to bemore comfortable. Ironically, the breathing frequency mayactually increase because more breaths that were previ-ously missed are now being triggered. An important signto look for is the amount of pressure required for the VT.As long as the applied PEEP is less than the intrinsicPEEP, this inspiratory pressure/VT relationship will notchange. Excessive PEEP (above the intrinsic PEEP), how-ever, will either drive up the end-inspiratory pressure (involume-targeted ventilation) or reduce the VT (in pressure-targeted ventilation).

Optimizing Flow Delivery

Once the breath is triggered, the clinician can chooseeither a fixed gas flow or a variable gas flow deliverypattern, as described above. The fixed-flow breath allowsthe clinician control over the size of the VT, but becausethe flow is fixed, there is generally little opportunity forthe patient to alter that flow. As a consequence, a numberof studies over the last 20 years have suggested that flowasynchrony is common with volume-assisted breaths.26-28

The classic clinical sign of flow asynchrony is the airwaypressure graphic literally being “sucked down” during theassisted breath, as compared to the controlled breath(Fig. 2).26,27,29 If an esophageal pressure tracing is avail-able, marked negative deflections are prominent through-out gas delivery.

There are, however, things the clinician can do to im-prove synchrony with a fixed-flow breath. Because thesize of the VT can impact synchrony, the VT itself can beadjusted30 Indeed in the ARDS Network trial that showedthe superiority of a VT of 6 mL/kg versus 12 mL/kg, therewas a provision to increase the VT to 8 mL/kg (provided



the plateau pressure remained below 30 cm H2O) to im-prove synchrony.31 In addition to VT magnitude, the mag-nitude and shape (sinusoidal vs square vs decelerating) ofthe flow can be adjusted to enhance synchrony.16-18,32 In-spiratory-time adjustments can also be made with the ap-plication of an inspiratory pause (although this may causeair-trapping in patients with airway obstruction). Thesemanipulations are usually applied with a trial-and-errorapproach while constantly monitoring the airway pressuregraphic and patient comfort.

An alternative to flow-targeted breaths is the pressure-targeted breath. As noted above, the pressure-targetedbreath has variable flow, and this may match up betterwith patient effort under many circumstances (Fig. 3). Thepressure-targeted breath also has several features that canenhance synchrony. One is adjustment of the rate of pres-sure rise to the target.33-36 This setting goes by many names,including rise time, pressure slope, and inspiratory per-cent, and is essentially an adjustment of the initial flowcharacteristics of the pressure-targeted breath and how fastthe ventilator attempts to get to the pressure target. Inpatients with very aggressive respiratory drive, a very rapidinitial flow may synchronize easier; in contrast, in patientswith very weak inspiratory drive a much slower pressurerise may be needed to enhance synchrony. A way to titrate

this adjustment is simply to look at the airway pressuregraphic and try to create a smooth square wave of pres-sure.35 Another approach is simply to look at the size ofthe VT as a marker of when the patient is synchronizing

Fig. 2. Examples of 3 levels of patient-ventilator flow asynchrony seen in pressure-time, flow-time, and volume-time tracings during assistedbreaths (dashed lines). The dotted pressure lines represent control breaths. In the left column the patient-ventilator flow asynchrony is mild:the assisted-breath inspiratory pressure is visibly below the control-breath inspiratory pressure, but never less than 50% of it. In the middlecolumn the patient-ventilator flow asynchrony is moderate: the assisted-breath inspiratory pressure at some point falls below 50% of thecontrol-breath inspiratory pressure, but never below the baseline (expiratory) pressure. In the right column the patient-ventilator flowasynchrony is severe: at some point the assisted-breath inspiratory pressure goes below the baseline (expiratory) pressure. (Adapted fromReference 29, with permission.)

Fig. 3. Enhancing flow synchrony with a variable flow, pressure-tar-geted breath. In the left column, a flow-targeted breath is deliveredbut the flow is inadequate for patient demand and asynchrony ismanifest by the deeply coved airway-pressure profile (solid arrow). Incontrast, the pressure-targeted breath (right column) is set to delivera similar VT, but the variable flow of the pressure-targeted breath(dashed arrow) synchronizes better with patient effort, providing asmoother, more constantly positive airway pressure profile. (Adaptedfrom Reference 29, with permission.)



with the best pressure-slope setting: the larger the VT, themore the patient is probably working with the ventilatorflow delivery (Fig. 4).35

Another feature of pressure-targeted breaths is the abil-ity to compensate for the diameter of the endotrachealtube.37,38 Called by a number of proprietary names, thisfeature adjusts the pressure profile on the airway to createthe target pressure profile at the end of the endotrachealtube. This results in a higher than set pressure target in thecircuit but results in the desired square wave of pressure ormore consistent CPAP application in the trachea (Fig. 5).

Importantly, the level of assistance supplied by pressuretargeting will also affect synchrony in several ways. First,in the presence of air-trapping, ventilatory support reduc-tion can reduce the air-trapping and thereby improve trig-gering. On the other hand, in patients without air-trapping,as the level of assistance is reduced (ie, the inspiratorypressure settings are reduced), patient respiratory drivemay increase and the potential for flow asynchrony mayincrease.39 Reducing the inspiratory pressure is often doneas part of the weaning effort as the patient recovers fromrespiratory failure. It is important to point out, however,that evidence-based reviews of the weaning process, doneover the last 20 years, have never shown an advantagefrom gradual reduction in support, versus simply provid-ing stable support and doing daily spontaneous breathing

trials.40 Thus, the notion of protocolized or automated re-duction in inspiratory pressure in between spontaneousbreathing trials must be challenged as a desirable strategy.It may be that, if one does daily spontaneous breathingtrials, the level of support in between the trials is irrele-vant, so aggressive pressure-reduction strategies may onlyincrease the need for sedation from asynchrony.

A critical question is whether clinical studies supportthe notion that pressure targeting provides better synchronythan flow targeting. The weight of evidence suggests thatindeed this is true, as most studies have shown benefit orat least comparability.29,41-44 There are times, however,when the pressure-targeting strategy may not be better. Forexample, at a very low pressure setting, patient assistanceis minimal, so patient work may be higher than duringflow-targeted breaths.45 However, this is often an inten-tional pressure reduction for the purpose of weaning.

There also is the risk that excessive pressure could causecentral apneas during sleep.46,47 Another concern with pres-sure support is in obstructive-lung-disease patients, inwhom the low initial flow caused by high airway resis-tance and a low flow-cycle criteria with pressure supportresults in excessively long inspiratory time and consequentair-trapping.48 Some studies have also pointed out that thebenefit of pressure targeting seems to be less when theflow-targeted strategy uses a very high inspiratory flow.32,49

Fig. 4. Examples of different pressure rise time adjustments in 3 pressure-support breaths. In column A the rise time is set very low, so theinitial flow and the rise to the pressure target are slow. In column B the rise time is set faster, so the initial flow and rise to the pressure targetare faster. In column C the rise time is set very fast, and the airway pressure tracing shows “ringing” from what is probably excessive flowfor the patient’s demand. Moreover, the esophageal pressure tracing shows premature patient expiratory effort to terminate the breath.Among these 3 rise-time settings, the one in column B appears to be optimal.



One concern about pressure targeting is that VT controlis lost.50 One way to address this is to use a feedback modethat allows the clinician to set the target VT and thenautomatically adjusts the ventilator’s pressure-targeting al-gorithm. While this has theoretical appeal, it is possiblethat a patient with a high anxiety or pain drive to breathemay create a large VT that then results in inappropriatelowering of the inspiratory pressure.51 This is more com-mon with a pressure-regulated volume-control mode andvolume support than it is with adaptive support ventilation,in which the feedback control mechanism also includes anoptimal VT and inspiratory-expiratory ratio.52

Optimizing Breath Cycling

There are several tools to improve cycling synchrony.As noted above, the size of the VT in volume-cycled ven-

tilation can affect synchrony, often with larger VT satis-fying the patient’s respiratory drive.30 With pressure tar-geting the clinician has the option of using a set inspiratorytime with pressure-assisted breaths, or, on most modernventilators, of adjusting the flow-cycling criteria in pres-sure support.33 These features, as noted above, may beparticularly important in the patient with obstructive lungdisease receiving a pressure-targeted mode.

The Distribution of Ventilator Breaths May AffectSynchrony

The pattern of support may also be important in enhanc-ing synchrony. Specifically, when more than one breathtype is being delivered, the patient’s respiratory controllerin the brainstem is unable to predict what the next breathwill be, and the potential for asynchrony may go up. Inessence, the load sensed by the brainstem during one typeof breath will probably alter the effort pattern response forthe next breath. This has been well documented in volume-targeted SIMV,53 in which, as more and more spontaneousbreaths without ventilator assistance are allowed, respira-tory drive goes up, which then translates to less synchronyduring the flow-targeted volume-assisted breaths. Usingpressure support during these spontaneous breaths may re-duce some of this effect. However, it would seem that thebest clinical strategy is to provide as consistent a breath pat-tern as possible (ie, avoiding SIMV and supplying the fewestpossible controlled backup breaths with assist-control modes).This is the most direct approach to stabilizing respiratorydrive, which, conceptually, makes it easier to synchronize theventilator and the patient.

Summary

Conventional ventilation modes have features that can mon-itor and enhance synchrony, including adjustment of the trig-ger variable, the choice of pressure-targeted versus flow-tar-geted breaths, and manipulation of the cycle variable.Clinicians need to know how to use these modes and monitorthem properly, especially understanding airway pressure andflow graphics. Newer ventilation modes and strategies, dis-cussed in other contributions to this Journal Conference, whichinvolve more sophisticated flow interactive strategies andnovel sensing mechanisms such as neuromuscular signals,hold promise for improving synchrony in the future, in pa-tients with very active respiratory drive.

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Fig. 5. Effects of an automated endotracheal-tube-compensationsystem on a pressure-support breath. Column A shows a typicalpressure-support breath. Note that the airway (circuit) pressurecurve has the desired square wave shape, but this is distorted bythe endotracheal-tube resistance to a more gradually rising pres-sure profile in the trachea. In contrast, with an automated endo-tracheal-tube-compensation system (pressure support plus auto-matic tube compensation, column B), the airway (circuit) pressureis automatically adjusted to create the desired square wave ofpressure in the trachea.



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43. Cinnella G, Conti G, Lofaso F, Lorino H, Harf A, Lemaire F, Bro-chard L. Effects of assisted ventilation on the work of breathing:volume- controlled versus pressure-controlled ventilation. Am J Re-spir Crit Care Med 1996;153(3):1025-1033.



44. Kallet RH, Campbell AR, Alonso JA, Morabito DJ, Mackersie RC.The effects of pressure control versus volume control assisted ven-tilation on patient work of breathing in acute lung injury and acuterespiratory distress syndrome. Respir Care 2000;45(9):1085-1096.Erratum in: Respir Care 2000;45(11):1416.

45. Kreit JW, Capper MW, Eschenbacher WL. Patient work of breathingduring pressure support and volume-cycled mechanical ventilation.Am J Respir Crit Care Med 1994;149(5):1085-1091.

46. Parthasarathy S, Tobin MJ. Effect of ventilator mode on sleep qual-ity in critically ill patients. Am J Respir Crit Care Med 2002;166(11):1423-1429.

47. Toublanc B, Rose D, Glerant JC, Francois G, et al. Assist-controlventilation vs. low levels of pressure support ventilation on sleep qualityin intubated ICU patients. Int Care Med 2007;33(7):1148-1154.

48. Jubran A, Van de Graaff WB, Tobin MJ. Variability of patient-ventilator interaction with pressure support ventilation in patientswith chronic obstructive pulmonary disease. Am J Respir Crit CareMed 1995;152(1):129-136.

49. Kallet RH, Campbell AR, Dicker RA, Katz JA, Mackersie RC. Workof breathing during lung-protective ventilation in patients with acutelung injury and acute respiratory distress syndrome: a comparisonbetween volume and pressure-regulated breathing modes. Respir Care2005;50(12):1623-1631.

50. Jaber S, Delay JM, Matecki S, Sebbane M, Eledjam JJ, Brochard L.Volume-guaranteed pressure-support ventilation facing acute changesin ventilatory demand. Intensive Care Med 2005;31(9):1181-1188.

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53. Marini JJ, Smith TC, Lamb VJ. External work output and forcegeneration during synchronized intermittent mechanical ventilation.Effect of machine assistance on breathing effort Am Rev Respir Dis1988;138(5):1169-1179.

Discussion

Epstein: Neil, you showed datafrom the study by Jubran1 that foundthat pressure support led to cyclingproblems in patients with COPD,but wasn’t that only at fairly highlevels of pressure support, above15 cm H2O?

1. Jubran A, Van de Graaff WB, Tobin MJ.Variability of patient-ventilator interactionwith pressure support ventilation in patientswith chronic obstructive pulmonary disease.Am J Respir Crit Care Med 1995;152(1):129-136.

MacIntyre: It requires a fair amountof pressure support to really magnifythis issue. But you know, Scott, I stillthink it’s important to assess any timeyou have someone with an obstruc-tive disease in pressure support, justto make sure you don’t run into trou-ble. When it happens, you get longinspiratory times and clear air-trapping, and it’s just a question ofbeing aware of it. You’re right that itcertainly becomes more obvious andmore manifest if you’re using a perhapsexcessive level of pressure support.

Epstein: I’m not an advocate of it,but the cycle criteria of just 5 L/minmay not be an ideal setting.

MacIntyre: My conclusion is not toabandon pressure support in those pa-

tients, but to recognize this possibilityand to recognize that you have a lot oftools that you can use to fix it. Maybea lower pressure support: certainly ad-justing the cycle criterion. What I dois put the patient on pressure assistand set the inspiratory time.

Epstein: I have no particular opin-ion about automated weaning or theSmartCare system, but I think it is im-portant to note that the Australianstudy1 came from a very different set-ting; I think those units had 24-hour-a-day attending coverage, and a muchhigher caregiver-to-patient ratio thanin the Brochard study.2 So maybe theywere not studying quite the same pa-tient population.

1. Rose L, Presneill JJ, Johnston L, Cade JF.A randomised, controlled trial of conven-tional versus automated weaning from me-chanical ventilation using SmartCare/PS.Intensive Care Med 2008;34(10):1788-1795.

2. Lellouche F, Mancebo J, Jolliet P, RoeselerJ, Schortgen F, Dojat M, et al. A multi-center randomized trial of computer-drivenprotocolized weaning from mechanical ven-tilation. Am J Respir Crit Care Med 2006;174(8):894-900.

MacIntyre: I think it’s absolutelycritical in looking at any clinical study(and I’m preaching to the choir here)to recognize the potential for 3 fatalflaws:

1. Picking the wrong populationthat either would not benefit from orwould be harmed by the intervention.I think large studies that take all com-ers are guilty of that.

2. Picking an end point that doesn’tmake any sense, or trying to use aphysiologic end point that may haveno relationship whatsoever to out-come.

3. Failing to make sure your con-trol group is a fair control group.

With those thoughts in mind, I’dlike to address a few recently pub-lished automated weaning studies.We’ve had the weaning guidelines for9 years now, and I get asked to updatethem from time to time, but I reallyhave no interest in updating them be-cause I don’t think anything that newhas come out. Clearly, the guidelinesstate that the daily SBT [spontaneousbreathing trial] is the way to managethese patients, and to be aggressiveabout doing those. If you’re going tosell me on SmartCare or another au-tomated system, it’s got to be com-pared to a control group that gets adaily SBT. I’m not sure you need one-to-one nursing to do that, what youneed is a team who understands thedaily SBT requirement and does it.

Kacmarek: I agree with everythingyou said, Neil. But that SmartCare trialdemonstrated at least equivalence toSBTs. It showed that in the average



ICU throughout theworld the Smart-Care was a better approach because adaily SBT is not normally done in theaverage ICU. In my own institution,where we have harped and harped ondoing daily SBTs, they are not alwaysdone every day. If a ventilation modeensures that the patient gets weanedas quickly as with a therapist-drivenprotocolized SBT, that mode is prettygood to me. The study does presentevidence in support of SmartCare.

MacIntyre: But the notion is thatSmartCare tells you to do a daily SBT.A cheaper, simpler way to do it wouldbe to put up a sign that reads “Do aDaily SBT.”

Kacmarek: But SmartCare will dothe SBT for you, so you don’t have todo anything. The closed-loop con-trolled approach is currently availablein Europe; it’s not yet approved in theUnited States. SmartCare will decreasepressure support to the level that meetscriteria for the SBT and does the SBTautomatically, then puts them back onpressure support, and then tells you ifthe patient is ready to be discontinuedfrom ventilatory support.

MacIntyre: I like the part about tell-ing you and doing an automatic SBTfor you. My problem is that it firstmust reduce (wean) the pressure sup-port to a minimal level before it pro-ceeds to an SBT. Remember that the 2large weaning trials1,2 in the 1990sshowed that weaning often delayedconducting an SBT. My point is that Ithink that you may end up wasting aday or more weaning the pressure sup-port when you could’ve just done anSBT that day. One of the assumptionsof SmartCare is that you’ve got to weanthe pressure support to a certain levelbefore you can do an SBT, and I wouldchallenge that assumption.

1. Esteban A, Frutos F, Tobin MJ, Alía I, Sol-sona JF, Valverdu I, et al. A comparison offour methods of weaning patients from me-chanical ventilation: the Spanish Lung Fail-

ure Collaborative Group. N Engl J Med1995;332(6):345-350.

2. Brochard L, Rauss A, Benito S, Conti G,Mancebo J, Rekik N, Gasparetto A, Le-maire F. Comparison of three methods ofgradual withdrawal from ventilatory sup-port during weaning from mechanical ven-tilation. Am J Respir Crit Care Med 1994;150(4):896-903.

Kacmarek: Neil, I agree with you100%. Daily SBTs is the way every-body should be doing it, but it is notbeing done. So whether or not we be-lieve it’s the best approach, if this au-tomated mode is equivalent to the bestapproach, then it’s pretty good. That’swhat you said, and that is what thedata indicate.

MacIntyre: If I could design the sys-tem, it would ask 4 basic questions:

1. Is the respiratory status stable orimproving?

2. Is gas exchange at a reasonablelevel? And you can define whateverreasonable level you want.

3. Are the hemodynamics stable?An easy way to do that is to ask ifthey’re on pressors.

4. Will they take a spontaneousbreath?

If they meet those 4 criteria, I thinkthey should have an SBT. If they havea machine that will do that for me, I’llget one.

Chatburn: Wouldn’t you make anexception for patients who have failedmultiple SBTs and are ventilator-de-pendent? In those patients doesn’t itlook like you have to more graduallyreduce the support?

MacIntyre: Rob, you’re exactlyright. The population you just men-tioned used to be very uncommon, butit’s certainly becoming more common.I had the privilege of being involvedwith a prolonged-mechanical-ventila-tion consensus group, and that groupagreed with you and said that if thepatient has had multiple SBT failuresand you’re looking at 2 or 3 weeks onthe ventilator, maybe daily SBT

“flogs” are not in that patient’s bestinterest. The consensus from thatgroup is interesting: they said that whatyou need to do is to find out whatlevel of support they’re on, get it to atleast below 50% of that level, and thenrestart the SBT attempts. I agree withyou: the prolonged ventilator-depen-dent patient is a more challenging one.

Kallet: Neil, I want to touch onsomething you said earlier that I thinkis extremely important to this confer-ence. You talked about mixing breathtypes and how that might confuse thepatient, and this is something that wediscovered in one of our work-of-breathing studies.1 My impression isthat with dual modes such as Auto-Flow and APRV [airway pressure re-lease ventilation], if the breathing en-vironment is constantly changing(either directly trying to adapt to thepatient’s demand or irrespective of it),that it can increase the work of breath-ing and increase central respiratorydrive.

I thought back to the times whenI’ve gotten the wind knocked out ofme playing sports. You’re lying onthe ground gasping for air and yourbrain is throbbing with an incredibledrive to breathe! But in that situation,you only have only one external sig-nal to deal with: an airway openingpressure that is ambient and constant.If you’re getting support from a ven-tilator, if that signal’s constantlychanging, trying to adapt to you, youmay get into this feedback loop thatjust spins off.

What I didn’t expect to find in ourstudy was that volume control with ahigh flow would be better, and nextbest was pressure control. That’s whenI started to think that maybe if you hitthe patient in distress with a constantvolume or constant airway pressure,and a constant inspiratory time, that itgives them something to lock into. Itmay not apply to weaning, but whenthe patient is in distress, we might begetting too smart for ourselves tryingto find modes that constantly adapt to



the patient, when, ironically, the pa-tient is trying to find something toadapt to!

1. Kallet RH, Campbell AR, Dicker RA, KatzJA, Mackersie RC. Work of breathing dur-ing lung-protective ventilation in patientswith acute lung injury and acute respira-tory distress syndrome: a comparison be-tween volume and pressure-regulatedbreathing modes. Respir Care 2005;50(12):1623-1631.

MacIntyre: Rich, that makes a lotof sense to me. I would point out onething, though, about respiratory drive.I’ve been intrigued over the last sev-eral years by a number of papers thathave pointed out that none of us has ahomogenous constant tidal volume.It’s natural to have some variability,and that volume variability has bene-fits in terms of gas distribution. Hav-ing said that, I think having a some-what reliable delivery pattern withevery breath from the ventilator doesmake some sense.

Branson: I agree with Bob that thepromise of closed-loop weaning isconsistency and preventing people go-ing off in their different directions. I’mwith you. There were limitations tothat study, but I think that’s reallywhere the advantage is. And if we weresitting here with the marketing man-agers of a company, the next questionwould be, “So, what would you bewilling to pay for that?” I think it’s$5,000 to $6,000 to add SmartCare toyour Drager ventilator. The questionis, depending on how many ventila-tors you have, what’s the total cost/potential benefit of that in your par-ticular ICU?

Kacmarek: Two days in the ICUover a year and you’d save the money.Just 2 days, not 2 patients.

MacIntyre: At our place, our edu-cation coordinator Bob Campbell rou-tinely monitors the things that I thinkare important: how often do we ex-ceed plateau pressure limits, how of-

ten do we exceed the 6-mL/kg tidalvolume limit, and how often do we dothe daily SBTs when they’re indi-cated? I’m the first to admit that we’renot 100% on any of those parameters,but we’re working on it.

Parthasarathy: If there’s that vari-ability or that issue in ivory tower uni-versities, then it is certainly going tobe an issue in community-based hos-pitals. This is where I think we needto move from evidence-based medi-cine (derived from randomized con-trolled trials) to translating suchknowledge to the patient’s bedside.There is this whole field of implemen-tation science or cost-effectivenessmedicine. These are not physiologyresearchers or critical care research-ers: they are health-services research-ers. So if someone were to do a studywherein they study implementationscience in that they study the effect ofSmartCare on bedside implementationof such principles in a communityrather than tertiary-care hospital, thenthe odds are extremely good that theywill show improvement and the cost-effectiveness of such a system. I thinkthat because we have the equivalencein one particular setting that doesn’tmean that the SmartCare should neversee the light of day again.

MacIntyre: I agree to a certain ex-tent. I think machines that can remindus to do things like that are important.Does it have to be something that in-corporates pressure-support weaningand then automatically does it, or couldthere be a simpler $500 addition suchas a decision-support tool on your ven-tilator that lights up a section of thedisplay in fancy colors and indicatesthat this needs to be done, that yourtidal volume’s too high, that yourPEEP is off the table, or that youhaven’t done your SBT.

Sassoon: In the graph that youshowed us of volume control and pres-sure control, there’s a concavity to-ward the Y axis on the airway-pres-

sure waveform with volume control.In that case, if you just raise the flowrate, wouldn’t that just take care of it,similar to pressure control?

MacIntyre: It does sometimes. Wefrequently put patients on volume as-sist and many of them do just fine; Idon’t want to make it sound like it’s apublic health hazard, that volume con-trol is harming people.

Branson: SIMV [synchronized in-termittent mandatory ventilation] is apublic health hazard.

Sassoon: Have you used pressure-control volume-assured ventilation? Inmy experience that seems to be a bet-ter mode than just plain pressure con-trol.

MacIntyre: We kind of like the pres-sure-regulated volume control. I’mvery anxious to hear Rich’s presenta-tion tomorrow, because he’s collectedsome interesting data on that. It doeshave some logic to it; it tries to staymore or less on the ARDS [acute re-spiratory distress syndrome] Networkalgorithm. There’s some logic, if youwant to use pressure-targeted modes,that putting that feedback mode fea-ture into it might help you stay withinyour target ranges a little better. I knowRich has some data that suggest that’snot always the case, and I think we’llhave some fun talking about that.

Kallet: Actually, I’m speaking onAPRV tomorrow.

MacIntyre: Well, I’m going to makeyou talk about it, and PRVC [pressure-regulated volume control] as well.

Kallet: OK.

MacIntyre: Rich has shown someinteresting data, and we actually seethis too in the ARDS Network data.When we looked at when and wherewe were violating 6 mL/kg (and all 12sites do violate 6 mL/kg), it’s almost



always happening at very low levelsof support in the so-called weaningphase, when they often are on pres-sure support and their respiratory drivehas recovered, and they should prob-ably be extubated a lot faster thanwe’re doing it. At least in our institu-tion, most of our tidal-volume viola-tions are at very low levels of pressuresupport or pressure assist, and perhapswe need that reminder to get the tubeout a little faster.

Kallet: I think the problem with dualmodes like APRV is just that. The

problem happens when the patientwants a higher tidal volume than thetarget; it’ll give it to them but it thinksthe compliance is improving, so itbrings down the pressure.

We’ve all but outlawed it at SanFrancisco General Hospital. We had alot of problems when we used dual-mode AutoFlow as our default mode.In a 6-month period alone, I found4 patients in overt shock on AutoFlowwith a peak pressure of 12 cm H2Oand a pH of around 7.15. Two of thesepatients were in cardiogenic shock, soit didn’t exactly help their after-load!

The rule of thumb is basically thatif your peak pressure on someonewho’s very sick is less than 20 cm H2O,you need to take a closer look. If yourpeak pressures on a dual mode are inthe mid-20s to mid-30s, I don’t have aproblem with it at all. It’s a clinicianproblem, the same way that asyn-chrony is primarily a clinician prob-lem, in that a lot of times the clinicianis not recognizing that there’s a prob-lem going on.

MacIntyre: My last comment is thatwe have not outlawed PRVC at Duke.



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