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Advanced Modes of MechanicalVentilationImplications for Practice

AACN Advanced Critical Care

Volume 17, Number 2, pp.145–160

© 2006, AACN

Louise Rose, MN, Adult Ed Cert, BN, ICU Cert, Dip Nurs

Although life-saving technology, mechani-cal ventilation can exacerbate acute lung

injury and worsen patient outcomes if not applied appropriately. The primary goal of allventilator modes is to maintain adequate oxy-genation and ventilation, reduce the work ofbreathing, and improve patient comfortthroughout the duration of respiratory failure.This has led to the development of a variety of ventilation modes and applications that potentially reduce complications and shortenthe duration of mechanical ventilation, result-ing in improved patient outcomes.

Recent advances in ventilator modes have focused on 3 key areas. First, modes havebeen developed that incorporate guaranteedcontrol of both pressure and volume settingsso as to obtain the benefits of both styles ofventilation. An example of this type of dualcontrol mode is pressure regulated volumecontrol (PRVC). Second, pressure style modeshave been developed that allow spontaneousbreathing to occur during both high-and

Mechanical ventilation is one of the most

commonly applied interventions in inten-

sive care units. Despite its life-saving role,

mechanical ventilation is associated with

additional risks to the patient and additional

healthcare costs if not applied appropri-

ately. To decrease risk, new ventilator

modes continue to be developed with the

goal of improving patient outcomes.

Advances in ventilator modes include dual

control modes that enable guaranteed tidal

volume and inspiratory pressure, pressure-

style modes that permit spontaneous

breathing at high- and low-pressure levels,

and closed-loop systems that facilitate ven-

tilator manipulation of variables based on

measured respiratory parameters. Clini-

cians need to develop a thorough under-

standing of these modes including their ef-

fects on underlying respiratory physiology

to be able to deliver safe and appropriate

patient care.

Keywords: closed-loop ventilation, mechan-

ical ventilation, weaning

A B S T R A C T

From RMIT University, Bundoora, VIC, Australia.

Reprint requests to Louise Rose, Critical Care Course Coor-

dinator, RMIT University; PhD candidate, The University of

Melbourne, The Royal Melbourne Hospital, PO Box 71,

Bundoora, VIC, Australia, 3083 ([email protected]).

low-pressure settings. This facilitates sponta-neous breathing in the early phase of acutelung injury, as well as during the weaning period. Examples of these modes includebiphasic positive airway pressure (BIPAP) andairway pressure release ventilation (APRV).Third, advances in microcomputer technologyhave enabled the development of advancedclosed-loop systems that allow ventilator, asopposed to clinician, manipulation of ventila-tor variables based on feedback from patientcharacteristics.

This article will discuss advances in ventila-tor modes in these 3 key areas with particularemphasis on appropriate ventilator settings,advantages and disadvantages, their particular

ROSE AACN Advanced Cri t ical Care

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effects on oxygenation and ventilation, andthe monitoring priorities for clinicians.

Guaranteed Control ofPressure and VolumeTraditionally, volume control modes have beenfavored for management of acute respiratoryfailure because of the ability to guarantee a pre-set tidal volume (VT) and minute ventilation(V

.E) enabling straightforward manipulation of

ventilation in response to changes in PaCO2.However, volume control modes do not pro-vide any limit of peak airway pressures. Thismay result in high peak airway pressures asso-ciated with changes in the patient’s complianceand resistance, causing alveolar overdistensionand barotrauma. In contrast, pressure controlventilation (PCV) allows control, or limitationof, the peak inspiratory pressure and inspira-tory time with no guarantee of VT.

Increasingly, PCV is utilized as a protectivelung strategy, particularly in patients with acutelung injury (ALI) and acute respiratory distresssyndrome (ARDS).1 Patients with ALI or ARDShave a higher proportion of diseased, collapsed,or consolidated lung regions that take a longertime to open and thus may require a longer inspiratory time. During PCV, the inspiratorypressure and time are set by the clinician andthe ventilator adjusts the inspiratory flow toachieve and maintain the set inspiratory pres-sure based on the compliance and resistance ofthe patient. Setting adequate inspiratory time isessential in all of the pressure control-stylemodes to enable the preset pressure to reachboth healthy and diseased lung units.2

The primary advantage of PCV is the abilityto accurately determine and maintain peak air-way pressure and inspiratory time, thus reducing the risk of lung injury. The variableand decelerating inspiratory flow pattern ofPCV enables a more rapid alveolar filling andmore even gas distribution, resulting in im-proved gas exchange, decreased work ofbreathing, and prevention of overdistension inhealthy alveoli.2–4 The primary disadvantage ofPCV is the inability to maintain a fixed VT andV.E due to changes in respiratory compliance

and resistance and activation of respiratorymuscles. This requires the clinician to set ap-propriate high and low V

.E and VT alarms, in

addition to close monitoring of gas exchangevia arterial blood gas (ABG) analysis and endtidal CO2 (etCO2) monitoring, to identify thepresence of hypercapnia.

Due to the identified disadvantages of volumeand pressure controlled modes, dual controlmodes have been developed to provide the bene-fits of both volume and pressure control whileavoiding the shortcomings of single controlmodes. Dual control modes include complexclosed-loop systems that switch between pres-sure control and volume control either within asingle breath or between individual breaths basedon measured patient characteristics (Table 1).

In dual control within-a-breath modes, suchas volume assured pressure support (VAPS)(BIRD 8400 STi, Viasys, Palm Springs, Cali-fornia, USA) and pressure augmentation (PA)(Bear 100, Viasys, Palm Springs, California,USA), the ventilator switches from pressurecontrol to volume control or pressure supportto volume control during the inspiratory phaseof individual breaths based on the patient’s inspiratory effort and ability to achieve the cli-nician-set minimum VT. This enables mainte-nance of a minimum VT and is meant to reducework of breathing. VAPS may also be referredto as volume-assisted pressure support.

In these modes, the clinician is required to setthe respiratory frequency, peak flow, pressuresupport, and the minimum desired VT toachieve adequate gas exchange. The clinicianadjusts the peak flow to achieve an appropriateinspiratory time and I:E ratio that facilitates gasexchange and does not induce gas trapping. Set-ting of the initial level of pressure support maybe guided by the plateau pressure obtained in avolume control breath at the minimum desiredVT. In addition, positive end expiratory pressure(PEEP), fraction of inspired oxygen (FIO2), andtrigger sensitivity are set by the clinician.

The within-a-breath modes may be usedduring mandatory or pressure-supportedbreaths. They are designed to combine thebenefits of the flow characteristics of PCVwith a guaranteed VT. When used with pres-sure supported breaths, VAPS and PA preventthe occurrence of ineffective ventilation as theminimum desired VT works as a safety net.5

Small clinical studies of the within-a-breathmodes have found a reduction in work ofbreathing.6,7 This finding was attributed to thehigher and more rapid delivery of inspiratoryflow as opposed to the guaranteed VT.

Appropriate setting of pressure support is animportant aspect of this mode to be consideredby clinicians. Appropriate pressure supportwill ensure the patient’s inspiratory effort isaugmented to achieve the set minimum VT. If

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the pressure support is set too low, the breathwill change to a volume control breath, result-ing in an increased proportion of mandatorybreath that increases the potential for impairedV/Q match and patient-ventilator dyssyn-chrony.8 Ongoing monitoring of the style ofbreath delivery is required by clinicians to ensure optimum ventilation is delivered.

Dual control between-breath modes arepressure limited and either flow-cycled or time-cycled. Cycling refers to the criterion used toterminate inspiration. Flow cycled, dual con-trol between-breath modes combine the fastinitial flow of pressure support with the con-stant V

.E and VT of volume control. An exam-

ple of this style of mode is Volume SupportVentilation (VSV; Servo 300, Siemens MedicalSystems, Solna, Sweden). This mode is similarto pressure support ventilation (PSV) in thatthe patient determines the respiratory fre-quency, inspiratory time, and flow. The peakpressure is adjusted to ensure the delivery ofthe target VT based on the patient’s compliancemeasured during the previous breath. There-fore, VT is used as the feedback control to

adjust pressure support.5 This is equivalent toPSV with VT again functioning as a safety net.

On commencement of VSV, the ventilatordelivers 4 test breaths of incremental pressurewhile measuring VT and calculating compli-ance.9 The peak pressure is adjusted by the ven-tilator to ensure the delivery of the target VT

using the lowest possible inspiratory pressure.Pressure adjustments are made based on the pa-tient’s compliance measured during the previ-ous breath. All breaths are pressure limited bythe high-pressure alarm set by the clinician. Afurther feature of this mode is the automatic reduction in pressure support that takes placewhen the patient’s lung mechanics improve. Thisoccurs because the pressure support is adjustedbased on the patient’s respiratory complianceduring the previous breath. If the patient’s com-pliance is improving, the VT target will be metwith lower levels of pressure support. Theoreti-cally, VSV can also be used as a weaning modeby clinician-reduction of the target VT.

Reports of clinical experience of VSV havedescribe VT instability, patient ventilator dys-synchrony, and failure to wean.9–11 In patients

Table 1: Dual Control Modes of Mechanical Ventilation

Model Ventilator Model

Within-a-Breath

Volume-Assured Pressure Support (VAPS); BIRD 8400 STi, TBird, Avea (Viasys, Palm Springs,

Volume-Assisted Pressure Support Calif)

Pressure Augmentation (PA) Bear 1000 (Viasys, Palm Springs, Calif)

Breath-to-Breath, Pressure-Limited, Flow Cycled

Volume Support (VSV) Servo 300 (Siemens Medical Systems, Solna, Sweden)

Variable Pressure Support Venturi (Cardiopulmonary Corporation, New Haven,

Conn)

Breath-to-Breath, Pressure-Limited, Time Cycled

Mandatory Rate Ventilation Pressure Regulated Cesar and Horus (Taema, Anthony, France) Servo 300

Volume Control (PRVC)

Autoflow Dräger, Evita 2, 4, XL (Dräger Medical, Lübeck, Germany)

Volume Control Plus Puritan Bennett, 840 (Puritan Bennett, Pleasanton,

Calif)

Variable Pressure Control Venturi

Combination Modes

Adaptive Support Ventilation (ASV) Gallileo (Hamilton Medical, Rhäzuns, Switzerland)

Automode Servo 300

with rapid shallow breathing, a well-recognized sign of respiratory failure,12 VSVtends to reduce the pressure support setting asopposed to increasing it, thus worsening thepatient’s respiratory distress. Clinicians needto closely monitor the patient’s respiratoryrate, ABGs, and work of breathing includinguse of accessory muscles to identify respiratorydistress and revert to either conventional PSVor a mandatory mode, such as synchronized intermittent mandatory ventilation (SIMV) orassist control (AC).

Time cycled, pressure limited, dual controlbetween-breath modes, such as PRVC, may bepatient or time triggered. These modes enablethe ventilator to adjust inspiratory flow accord-ing to patient flow demand combined withmaintenance of a constant VT, ensuring ade-quate ventilation. The pressure limit is adjustedusing the clinician-set desired VT as the negativefeedback control. PRVC (Servo 300, SiemensMedical Systems, Solna, Sweden), Autoflow(Dräger Evita 4, Dräger Medical, Lübeck, Germany), Variable Pressure Control (Venturi,Cardiopulmonary Corporation, New Haven,Connecticut, USA) are all examples of time cy-cled, pressure limited between-breath dual con-trol.5,13 Dual control between-breath modes aredescribed as equivalent to a bedside clinicianadjusting the pressure limit of each breath ac-cording to the previous VT.14 The primary ad-vantage of these modes is the reduction in peakinspiratory pressures associated with a deceler-ating flow pattern, as opposed to the constantflow pattern commonly associated with volumecontrol ventilation, combined with the guaran-teed delivery of V

.E regardless of changes in

patient compliance, ensuring adequate oxygena-tion and ventilation.15 In addition, automaticweaning of the inspiratory pressure will occur ifthe patient’s compliance improves.

In the few reported clinical studies, the onlysignificant benefit of PRCV compared to vol-ume control ventilation (VCV) was the reduc-tion of peak inspiratory pressure, suggestingpotential benefits in terms of prevention ofalveolar distention and barotrauma.16–18 How-ever, a reduction in peak inspiratory pressureis a predictable finding when comparing theconstant flow pattern of VCV with the decel-erating flow pattern of PRVC. When compar-ing PRVC to conventional PCV, no differencesin peak inspiratory pressures were found.17

Automode (Servo 300A, Siemens MedicalSystems, Solna, Sweden) combines PRVC and

VSV in the one mode to enable seamless wean-ing from pressure control to pressure supportor volume control to volume support whilemaintaining a guaranteed VT.19 In the apneicpatient, pressure limited, time cycled breaths,are delivered. Once the patient triggers 2 con-secutive breaths, the ventilator switches to VSVto support the patient’s spontaneous efforts. Inthe one identified clinical study,20 Automodewas compared to SIMV in postoperative pa-tients without respiratory failure. Notably theduration of ventilation was shorter and thenumber of ventilator manipulations by staffwere fewer in the Automode group, suggestinga potential reduction in clinician workload.However, further clinical studies need to beconducted to evaluate this mode further.

Modes that ensure a guaranteed VT and limitpeak airway pressure potentially provide addi-tional patient protection from alveolar distensionand barotrauma caused by high peak airwaypressures. However, clinicians need to closelymonitor and respond to triggering of the peakairway pressure alarm. The maximum pressureused by the ventilator to achieve the desired VT is 5 cmH2O below the high pressure alarm. Activa-tion of this alarm may indicate the presence of se-cretions or worsening compliance and resistanceand thus requires the clinician to perform a respi-ratory assessment and review the desired VT set-ting. Moreover, these modes may prevent periodsof hypoventilation and worsening respiratorydistress due to inadequate ventilatory supportarising from changing respiratory parameters.This may be particularly pertinent in units withinsufficient nursing resources or levels of expert-ise to provide timely recognition and interventionfor changing respiratory parameters.

Partial Ventilatory SupportScientific studies have shown spontaneousbreathing during mechanical ventilation prevents atelectasis and promotes alveolar recruitment.21–23 This results in an improved ventilation/perfusion (V/Q) match and hencepromotes effective gas exchange.8 Furthermore,spontaneous ventilation reduces the need for sedation and muscle relaxants, resulting in reduced durations of ventilation and inten-sive care (ICU) stay and ventilator-associated complications.23–25

Radiologic studies indicate gas is directed todependent well-perfused regions of the lungsduring spontaneous breathing due to the move-ment of the posterior muscular sections of the

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diaphragm.24 Conversely, full ventilatory sup-port causes dependent atelectasis and reducedfunctional residual capacity. This is due to theupward displacement of the diaphragm by intra-abdominal pressure, with resultant preferential ventilation of the anterior lung regions. Therefore, full ventilatory support results in ventilation of nondependent lung areas and perfusion of dependent areas, creat-ing a worsening V/Q match and impaired oxygenation. In addition, positive pressureventilation produces hemodynamic effects,such as reduced venous return, cardiac output,and thus decreased organ perfusion, whichhave long been recognized.24

The gas distribution pattern of spontaneousbreathing has also been shown to occur in par-tial ventilatory support, resulting in improvedV/Q matching.26 Modes, such as BIPAP andAPRV, may be considered as partial ventila-tory support modes when applied in a sponta-neously breathing patient. These modes do notsupport the patient’s own inspiratory efforts;rather, application of continuous positive air-way pressure (CPAP) facilitates recruitment ofatelectic alveoli and decreases the elastic work

of breathing.23 Increasingly, partial ventilatorysupport is being used during the acute phase ofrespiratory failure as well as during weaning.25

Biphasic Intermittent PositiveAirway Pressure Biphasic Positive Airway Pressure, also know asBilevel, Bivent, and DuoPAP, was first describedby Benzer and Baum and has been available forover twenty years.21,27 BIPAP was originally described as a combination of pressure control-style ventilation with unrestricted spontaneousbreathing during both high- and low-pressuresettings (Figure 1). This is in contrast to tradi-tional PCV, which does not allow spontaneousbreathing at the upper pressure level.28 BIPAP isalso described within the literature as a CPAPsystem with time cycling between 2 clinician-setpressure levels.22 BIPAP can therefore be viewedas a progression of monophasic CPAP and wasinitially regarded as a weaning mode. In addi-tion, BIPAP differs from PSV given that sponta-neous breaths are allowed by the ventilatorwithout synchronized support, as opposed tosupport of each spontaneous breath, available inPSV.27 However, while BIPAP may be categorized

Figure 1: Airway pressure graphs during 3 different modes of mechanical ventilation: continuous positive

airway pressure (CPAP), pressure controlled ventilation, and biphasic positive airway pressure (BIPAP).

Reprinted with permission of Dräger Medical, Lübeck, Germany.


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as a partial ventilatory mode suitable for wean-ing, if a patient is not breathing spontaneously,BIPAP is equivalent to time-cycled PCV.

In recent studies, by combining sponta-neous breathing with pressure control-styleventilation, BIPAP has been shown to be bene-ficial in patients during the acute phase of ALIand ARDS by improving dependent gas distri-bution.29 Therefore, BIPAP can be used fordifferent phases of the patient’s course of mechanical ventilation and can be consideredas a flexible, universal mode that covers thespectrum of respiratory failure.21

During BIPAP, unrestricted spontaneousbreathing throughout the ventilator cycle isachieved via an active expiratory valve thatopens during inspiration, as well as the tradi-tional expiratory opening. The active expira-tory valve enables continuous control of air-way pressure and compensates for pressurevariations normally found in the ventilator circuit.30 If the airway pressure drops belowthe set level due to spontaneous inspiration,gas is rapidly supplied to ensure return to thepreset pressure level.21

Careful setting of pressure levels is requiredin BIPAP to avoid shear stress due to repetitivealveolar opening and closing and lung distension due to inappropriate low-pressure(Plow) and high-pressure (Phigh) settings respec-tively.24 The difference in the 2 pressure set-tings, which can be viewed as 2 levels of func-tional residual capacity (FRC), determines theVT. The pressure difference creates a drivingpressure to permit gas to enter the lung unitsand represents the difference between airwaypressure (Paw) and alveolar pressure (Palv).The VT decreases with poor respiratory compli-ance and equally increases with improved res-piratory compliance. Therefore, VT is derivedfrom the product of the driving pressure andthe patient’s compliance.21 VT targets shouldaim towards 6 mL per kg of ideal body weightin patients with ALI/ARDS based on improvedoutcomes shown in the ARDSnet study.31

Recommended SettingsBIPAP requires the setting of upper and lowerpressures (Phigh and Plow) and their respectivetime durations (Thigh and Tlow). Hörmann andassociates22 recommend initially setting the Plow

equivalent to PEEP. The Phigh should be set at alevel equivalent to the inspiratory plateaupressure obtained if the patient is already ventilated in another mode. If the patient’s

ventilation is to be initiated on BIPAP, the Phigh

setting may be set 12 to 16 cm H2O above thePlow setting, adjusted according to the patient’scompliance and resultant VT.

BIPAP routinely implies time cycling phasesthat approximately maintain a 1:1inspiratory:expiratory (I:E) ratio. Careful con-sideration of the patient’s underlying patho-physiology is required when adjusting settings.The duration of both of the predefined time cy-cling phases (Thigh and Tlow) may be adjusted in-dependently of the 2 pressure levels. Longer Thigh

and shorter Tlow times may be beneficial to pa-tients with ALI/ARDS to encourage alveolar re-cruitment, improve oxygenation, and facilitatespontaneous breathing.21 In contrast, patientswith chronic obstructive pulmonary disease(COPD), experience hyperexpansion and donot require an increased FRC. Extending theThigh will result in further gas trapping, increasedwork of breathing, and worsening compliancein these patients. Rather, this patient group re-quires a long Tlow to enable full expiration andprevent gas trapping and auto-PEEP. Figure 2shows how the gas flow waveforms may be uti-lized by the clinician to check the appropriate-ness of the Thigh and Tlow in order to prevent gastrapping. In addition, as with other pressurecontrol-style modes of ventilation, careful andcontinuous monitoring is required to identifypotential changes in patient compliance withthe resultant effect on gas flow and VT.

Weaning of Biphasic Positive Airway PressureWeaning BIPAP is achieved by reducing thepressure difference between the 2 pressure set-tings, followed be a reduction in rate. Themode can then be changed to CPAP with aCPAP level equivalent to the mean airway pres-sure when on the reduced BIPAP settings.32

However, some patients may be switched di-rectly to CPAP and not require weaning of thepressure difference or respiratory rate. TheCPAP level may then be weaned to a minimumof 5 to 7 cm H2O based on the patient’s oxy-genation status. BIPAP can also be combinedwith pressure support in the weaning phase tofurther augment the patient’s spontaneous in-spiratory effort.

Mode TerminologyThe use of the term BIPAP in European litera-ture originally created misunderstanding inNorth America. The term BiPAP® is reserved


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for noninvasive, positive pressure ventilationavailable on Respironics ventilators (Respiron-ics, Murraysville, Pa). Ventilator companiesare prevented from using the term BIPAP bylaw in North America and this has led to theuse of terms such as Bilevel on the Puritan Ben-nett 840 ventilators (Puritan Bennett, Pleasan-ton, Calif) and Bivent on the Servo 300 range.In addition, Airway Pressure Release Ventila-tion (APRV) was also introduced as a newmode around the same time as BIPAP, produc-ing further misunderstanding.30

Definitional differences of BIPAP andAPRV are confusing within the literature.Hedenstierna and Lattuada8 state that APRV isfrequently called BIPAP in Europe. However,while being a continuum of the same concept(ie, allowing spontaneous breathing duringtime-cycled changes in airway pressure), thereare some distinct differences (Table 2). BIPAPconventionally maintains the high-pressure

setting for more traditionally acceptable inspi-ratory times, whereas APRV always implies aninverse ratio.32,33 Therefore, APRV can be re-garded as a special setting or extension of BI-PAP.21 APRV can be used to maintain a highmean airway pressure for the majority of therespiratory cycle with only brief pressure re-leases. APRV is usually reserved for patients inthe more acute phase of ALI/ARDS, whereasBIPAP can be viewed as a mode applicable forpatients with acute and resolving ALI who aremoving into the weaning phase.21 Nonetheless,BIPAP can be applied with varying time andpressure durations that enable it to mimic othermodes of ventilation.

Airway Pressure Release VentilationAirway pressure release ventilation was origi-nally described as CPAP with an intermittentrelease phase.34,35 During APRV, mechanicalventilation occurs though a time-cycled switch-ing between 2 pressure levels, with sponta-neous breathing occurring during any phase ofthe ventilatory cycle in a manner equivalent toBIPAP.36 However, when used in apneic pa-tients, APRV is identical to pressure-controlledinverse ratio ventilation. The higher continu-ous airway pressure level (Phigh) is designed tomaintain adequate lung volumes and promotealveolar recruitment and oxygenation, whereasthe lower pressure level (Plow) enables alveolarventilation and carbon dioxide (CO2) removal.37

The estimated upper and lower inflectionpoints of the volume pressure curve are used asa guide to the 2 pressure levels.38 The upper in-flection point is the part of the curve where thepressure continues to rise, but the volume nolonger increases. The lower inflection pointmarks the point on the curve where the volumestarts to increase as pressure is applied. Duringspontaneous breathing, the patient can controlthe rate and duration of both spontaneous inspi-ration and expiration.37

APRV differs conceptually from conven-tional modes of ventilation. In order to achievetidal ventilation, conventional modes elevateairway pressures from a low baseline pressure,whereas APRV uses a short deflation to alower pressure level then returns to a highbaseline pressure.39 The temporary pressure release facilitates removal of intrapulmonarygas, which is then replaced once the higherpressure level is reestablished and augmentsthe patient’s own minute ventilation.32 During

Figure 2: Checking Thigh and Tlow using the flow

waveform. The arrow indicates failure of the

expiratory flow to return to zero prior to inspiration

representing gas trapping. Reprinted with

permission of Dräger Medical, Lübeck, Germany.


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APRV, the peak airway pressures never exceedthe Phigh, and airway pressures decrease ratherthan increase to deliver VT.40 As with BIPAP,APRV can be regarded as the 2 levels of FRC,with the inspiratory level of FRC maintainedfor a prolonged duration without risk ofoverdistension. This differs conceptionallyfrom conventional ventilation that superim-poses the VT on the FRC (Figure 3).21

Recommended SettingsIn APRV, the Phigh is maintained for an ex-tended Thigh and the Plow is maintained for a

short release duration or Tlow. In the originaldescription of APRV, the Thigh and Tlow were 1.8and 1.3 seconds respectively indicating a mod-erate inverse ratio.35 An extension of the origi-nal description of APRV is well described byHabashi.37–39 This application of APRV utilizesa Thigh that is extended by 4 to 6 seconds to im-prove recruitment of alveoli by increasing themean airway pressure. The mean airway pressure correlates with mean alveolar vol-ume, which contributes to the increased surface area available for gas exchange.37 TheTlow may be reduced to as little as 0.2 seconds

Figure 3: Volume/time diagrams comparing functional residual capacity in airway pressure release ventila-

tion and intermittent mandatory ventilation. Reprinted with permission of Dräger Medical, Lübeck, Germany.

Table 2: Classifications and Characterisitics of Biphasic Positive Airway Pressure andAirway Pressure Release Ventilation

Name of Mode Characteristics

CMV-BIPAP, equivalent to PCV Time cycled, pressure controlled; No spontaneous breathing

BIPAP Time cycled, pressure controlled; Spontaneous breathing at both

pressure levels; Phigh long enough to allow spontaneous breathing

at upper level; I:E ratio approximately 1:1

BIPAP � PS Pressure support provides additional inspiratory assistance of

spontaneous efforts at the lower pressure level; Used with longer

Tlow settings during weaning

APRV (traditional) Time cycled, pressure controlled; Phigh longer then Plow;

Spontaneous breathing occurs at upper level; If no spontaneous

breathing is equivalent to PCV-inverse ratio

APRV (extended) Time cycled, pressure controlled; Thigh extended to 4 to 6 seconds;

Tlow reduced to �0.8 seconds; Spontaneous breathing at upper level

BIPAP, biphasic positive airway pressure; APRV, airway pressure release ventilation; CMV, controlled mandatory ventilation; PCV, pressure-controlled ventilation; I:E, inspiratory/expiratory; Phigh, pressure high; PS, pressure support; Thigh, time high; Tlow, time low.


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(default setting of 0.8 sec) in order to termi-nate the expiratory flow early, causing reten-tion of lung volume referred to as end-releaselung volume.37 This will prevent de-recruitmentassociated with the lack of PEEP due to main-tenance of an end-expiratory volume.38

This application of APRV requires a Phigh

setting based on the inspiratory plateau pres-sure or a setting of 30 to 35 cm H2O. The rec-ommended Plow setting is 0 cm H2O. Setting thePlow at 0 cm H2O facilitates rapid expiratorygas flow driven by lung recoil, which reducesthe time required for release and providesmore time for Phigh.38 Use of a higher Plow settingwill reduce expiratory gas flow and result ingas trapping and CO2 retention.39

The short release time may result in the de-velopment of intrinsic PEEP in patients withdelayed alveolar emptying. However, in somepatients, intrinsic PEEP may beneficially improve gas exchange due to alveolar recruit-ment. Nonetheless, excessive intrinsic PEEPwill result in alveolar hyperinflation.24,38 Thismandates careful assessment of the expiratoryportion of the flow waveform (Figure 2) anddifferentiation between restrictive or obstruc-tive lung disease when selecting the Tlow. Pa-tients with restrictive lung disease may tolerateTlow settings of 0.2 to 0.8 seconds, whereas pa-tients with obstructive disease may require alonger Tlow (0.8 to 1.5 seconds) to prevent dele-terious levels of intrinsic PEEP.37 In a study byNeumann and associates41 comparing differentThigh and Tlow settings in APRV, an increase inPaCO2 was only noted in patients with COPDwhen the Tlow was reduced to 0.5 seconds. Notably, the study did not identify significantdifferences in oxygenation when extending theThigh despite the increase in mean airway pres-sure. However, patients were only maintainedon the settings for 20 minutes, which arguablyis not enough time to produce dramaticchanges in oxygenation. The study also notedan increase in the proportion of spontaneousbreathing with the extended Thigh setting. Thisindicates the Thigh setting has to be long enoughto facilitate spontaneous breathing at the upperpressure level.

Advantages and DisadvantagesAPRV has been shown to be an effective modein the management of recruitable diseases, suchas ALI and ARDS, that are characterized by atelectasis and V/Q mismatch.38 Spontaneousbreathing during APRV produces a better

distribution of gas to dependent lung regions, improving alveolar recruitment.37 However, arecent evidence-based review of mechanicalventilation in sepsis-induced ALI and ARDSrecommends APRV should only be used in con-trolled clinical trials or as a rescue therapy.42

Increased resources may be required totrain staff in the principles of APRV.38 In or-der to gain confidence with APRV, cliniciansneed good knowledge of flow waveforms inorder to be able to assess the appropriatenessof Thigh and particularly Tlow settings. Closemonitoring of minute ventilation is requiredto respond appropriately to changes resultingfrom alterations in compliance and resist-ance.32 In addition, the management of pa-tients on APRV with higher pressure levelsmay produce periods of respiratory instabil-ity during procedures that warrant the use oftransport ventilators without APRV capabil-ity and manual resuscitation bags.38 Clini-cians are required to carefully monitor tidalvolumes and gas exchange during theseprocedures. In addition, transportation of pa-tients out of the ICU should only be consid-ered when the clinical outcomes outweigh theassociated risks.

Clinical Studies of BiphasicPositive Airway Pressure andAirway Pressure ReleaseVentilationCareful interpretation of research findings forBIPAP and APRV is required due to previousmisunderstanding with the terminology. Examination of the inspiratory and expiratorytimes cited in the study will help determinewhich method of partial ventilatory support(BIPAP, APRV [traditional], APRV [extendedtechnique]) is being referred to.

Clinical studies have compared these 2modes to a variety of other ventilation modesincluding SIMV, PSV, and inverse ratio volumecontrol. Significant findings of studies on BI-PAP and APRV have shown improvements inV/Q match and oxygenation, a reduction inthe work of breathing and peak inspiratorypressures.22,26,28,36,43,44 In a recent study, Putensenand associates25 compared APRV (traditional)with PCV-CMV in 30 trauma patients at riskof developing ARDS. The APRV groupdemonstrated consistently better oxygenationand hemodynamic parameters plus reductionsin sedation and inotrope use, and the dura-tions of ventilation and ICU stay.


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Studies on partial ventilatory support modeshave shown a reduced effect on the cardiac out-put with these modes compared to controlledmechanical ventilation.26,29,45,46 Furthermore,studies have examined if the hemodynamic benefits identified with spontaneous breathingduring APRV using the traditional approachcan also be seen in spontaneous breathing withPSV with equivalent minute ventilation and air-way pressures. Putenson and associates29 foundimproved V/Q match in APRV with sponta-neous breathing; however, no improvementswere noted in the PSV group. In addition, Her-ing and associates47 examined the effects ofAPRV with and without spontaneous breathingon renal perfusion in patients with ALI. Thestudy identified a statistically significant im-provement in renal perfusion and function inthe spontaneously breathing patients.

Unlike the dual control modes (PRVC,VSV), BIPAP and APRV do not provide guar-antee of a minimum VT. Therefore, cliniciansare required to monitor VT, respiratory fre-quency, and ABGS to identify and respond tochanges in these parameters caused by alter-ations in resistance and compliance. The Phigh

setting and respiratory frequency may be ad-justed either up or down to increase or de-crease V

.E. Furthermore, increasing the Thigh

may assist in improving gas exchange in pa-tients exhibiting refractory hypoxemia.

Another clinically significant aspect of thesemodes is the ability to maintain unrestrictedspontaneous breathing during both the inspi-ratory and expiratory phase of ventilation.This results in improved patient-ventilatorsynchrony and thus reduced sedation require-ments. Patients are able to participate in activ-ities, such as physiotherapy and respiratorymuscle training, that potentially reduce the du-ration of ventilation and incidence of ventila-tor-associated pneumonia. Clinicians need tobalance the need to reduce sedation to facili-tate spontaneous breathing while preventingincreased anxiety and agitation associatedwith increased awareness of the ICU environ-ment and critical illness.

Optimal and Knowledge-based ControlAdaptive Support VentilationAdaptive support ventilation (ASV) is a newmode available on the Galileo ventilator(Hamilton Medical, Rhäzüns, Switzerland).This mode provides full or partial ventilatory

support during the initiation, maintenance, orweaning phases of mechanical ventilation.Clinicians are required to set the ideal bodyweight, desired percentage of V

.E (20% to

200%), FIO2, PEEP, and maximal inspiratorypressure alarm. A desired V

.E setting of 200%

is equivalent to a V.E of 200 mL/min/kg and

represents full ventilatory support. Reductionof the desired V

.E percentage setting by the cli-

nician facilitates weaning of the patients.Thus, the desired V

.E percentage enables the

clinician to titrate ventilation to provide fullmechanical support or encourage spontaneousbreathing with the aim of weaning and extu-bation. The ventilator delivers 5 test breathsthat determine the initial respiratory fre-quency, VT, pressure limit of mandatory andspontaneous breaths, inspiratory time ofmandatory breaths, and I:E ratio when spon-taneous breathing is absent.5 The ventilatordetermined settings maintain the predeter-mined desired V

.E percentage set by the clini-

cian. Titration of the desired V.E percentage

requires clinician decision input to enable pro-gression to readiness for extubation.

ASV has been described as a mode incorpo-rating PCV and PSV, with automatic adapta-tion of respiratory rate and pressure levels.48 Inthe absence of spontaneous ventilation, ASVdelivers SIMV style PCV, using lung mechanicsmeasured during each breath to guide adjust-ment of ventilator settings.49 During sponta-neous respiratory effort, the ventilatorswitches from PCV to PSV and reverts to PCVif the patient’s V

.E drops below the guaranteed

minimum. The level of pressure support is alsoadapted to provide adequate tidal volumes according to the desired V

.E percentage set. If

the patient develops rapid shallow breathingindicative of inadequate ventilatory support,the pressure support is automatically in-creased, thus providing real-time adaptationof ventilator support according to the patient’srespiratory demands and preventing the devel-opment of respiratory distress.

The majority of clinical studies of ASV have been conducted in cardiothoracic patients.48,50,51 This is remarkable consideringthe complexity of the mode and the perceivedstraightforwardness of ventilatory manage-ment in this patient group. These studiesfound ASV was safe and resulted in durationsof ventilation comparable to those publishedin other studies of early extubation postcardiacsurgery. A notable finding in one study48 was


155

the reduction in apnea and high-pressurealarms in the ASV group that the authors sug-gested could potentially improve the workingenvironment of nurses.

Proportional Assist VentilationProportional assist ventilation (PAV) is a spon-taneous mode designed to deliver supportbased on continuous measurement of the pa-tient’s ventilatory parameters. There are no settargets of pressure, volume, or flow; rather, theairway pressure is increased or decreased inproportion to the patient effort via positivefeedback control using respiratory elastanceand resistance as the feedback signals.15,52 Thepatient’s respiratory drive determines the res-piratory rate and inspiratory time while FIO2

and PEEP are set by the clinician. The othersettings required are the percentage of volumeassist and the percentage of flow assist. Thepercentage of volume assist overcomeselastance and the percentage of flow assistovercomes resistance. These assist settings areroutinely set at 80%.5

Proportional assist ventilation has been described as the only mode to be primarily de-signed on a physiological basis rather than thetechnical abilities of ventilators.53 Proportionalassist ventilation is available as ProportionalPressure Support (PPS) on Dräger Evita 4 andXL ventilators (Dräger Medical, Lübeck, Germany). One proposed advantage of PAV isthe ability to respond to rapid changes in ventilatory effort associated with increasedelastance and resistance that occurs in patientswith respiratory failure. This enables respira-tory muscle unloading during increases in inspi-ratory effort and improves patient comfortdue to a reduction in the work of breathing.52–54

Negative aspects associated with PAV includedifficulties with accurate measurement of elas-tance and resistance in spontaneously breath-ing patients or those receiving partial support,and the confounding effects imposed by endo-tracheal tube resistance and the presence ofauto-PEEP.5,55,56 Difficulty in determining appropriate settings for percentage of volumeand flow assist has discouraged the adoptionof this mode into mainstream clinical practice.

Knowledge-based System(Smartcare)Various other systems for titration of mechani-cal ventilation and weaning have been devel-

oped and investigated as a potential solutionto inefficient weaning and alternative to traditional methods. One system that has beenmade available for commercial application isthe knowledge-based system (Smartcare),available on the Dräger XL ventilator.57,58

The knowledge-based ventilation systemdiffers from the other modes describedabove.57–60 The Smartcare™ application usesseveral inputs to enable titration of pressuresupport in response to changes in patient respiratory rate, VT, and etCO2 in order tomaintain the patient in a “respiratory zone ofcomfort.” The patient’s respiratory rate is themost influential parameter, as it reflects the res-piratory muscles’ ability to adapt to changes inworkload.60 VT and etCO2 are incorporated intothe system to ensure safety.

The application was designed for weaningin the presence of spontaneous ventilation using the pressure support mode. To com-mence Smartcare™ weaning simply requiresactivation of the Smartcare™ option on theventilator; no other setting changes are re-quired. The Smartcare™ system acquires dataon the patient’s current respiratory status andits time course, establishes a respiratory statusdiagnosis, determines the intervention, andacts on the ventilator to adjust pressure support.57 The Smartcare™ system samples patient parameters over periods of 2 minutesand adjusts the pressure support level accord-ingly. Acceptable parameter ranges are notedin Table 3.

The modification of pressure support alsotakes into account the patient’s breathing

Table 3: Acceptable Ranges for VentilatorParameters with Smartcare, RespiratoryZone of Comfort

Parameter Range

Respiratory rate 15 and 30 breaths/min

34 breaths/min if a neuro-

logical disorder is present

Tidal volume �300 mL in patients weighing

�55 kg

�250 mL in patients weighing

�55 kg

End tidal carbon �55 mm Hg

dioxide (EtCO2 ) �65 mm Hg in patients with

chronic CO2 retention


156

pattern history. If the pressure support is below 15 cm H2O, it will be decreased by 2 cmH2O once the patient’s ventilation has beenstable within the “respiratory zone of com-fort” for 30 minutes; if the pressure supportlevel is higher than 15 cm H2O, it will be de-creased by 4 cmH20 if the ventilation has beenacceptable for the last 60 minutes. When therespiratory rate exceeds 30 breaths/min, thepressure support is increased by 2 cm H2O andincreased by 4 cm H2O when the respiratoryrate exceeds 36 breaths/min. The system alsotolerates transient instabilities for 2 or 4 minutes depending on the level of pressuresupport. When the patient has tolerated thelowest level of pressure support for 1 to 2 hoursdepending on the initial pressure support setting, the Smartcare™ system suggests thepotential for disconnecting from the ventilator,termed “readiness for separation.” It is thenup to the clinician to determine whether thepatient is ready for extubation or if there areadditional factors that mandate the ongoingrequirement for intubation.

The key aspect of this technology is the abil-ity to monitor the patient’s respiratory status inreal time, which potentially encourages a rapid,safe, and efficient weaning process and mini-mizes possible complications, such as respira-tory distress, increased work of breathing, andimpaired ventilation and oxygenation. Further-more, the Smartcare™ system has the potentialto reduce the duration of ventilation and wean-ing. A recent study61 of patients receiving me-chanical ventilation for greater than 24 hoursfound a halving of the weaning duration andsignificant reductions in the duration of total ventilation, ICU stay, and use of non-invasive ventilation post extubation.

The benefits of this application may bemost evident in units with barriers to ventila-tion decision-making processes, such as poorstaffing ratios and levels of experience. TheSmartcare™ program potentially reduces thenumber of clinician decisions required to weanthe patient to the point of extubation. How-ever, the decision to activate the Smartcare™program to commence the weaning process remains that of the clinician. Assessment ofrespiratory parameters that indicate weaningreadiness is performed by the clinician prior toinitiation of the program. This requires knowl-edge and recognition of both objective andsubjective indices of weaning readiness andtheir thresholds.

Implications of Advanced Modesfor CliniciansAdvanced modes of ventilation have a numberof direct implications for clinical practice. Incorporation of new modes into practice requires intense education programs and expe-rienced members of staff to facilitate appropri-ate and safe mechanical ventilation for all patients. Education programs need to focus onthe clinical applications of modes includingappropriate settings, weaning techniques, andassessment of patient-ventilator interaction.Advanced modes require high levels of expert-ise in arterial blood gas and waveform analy-sis, combined with knowledge of current scientific recommendations for managementof the various patient groups that require mechanical ventilation. Clinicians require excellent assessment skills and knowledge of respiratory parameters and weaning indices in order to assess the appropriateness of a ventilator mode’s ability to meet the ongoingneeds of the patient.

Dual control modes that guarantee VT whilemaintaining set peak inspiratory pressuresprovide an additional safety feature for pa-tients and busy clinicians who possibly may beunable to respond to changes in a patient’sventilation status in a timely manner. Modesthat offer real-time monitoring and adjust-ment of ventilator parameters may prevent in-appropriate ventilation, that leads to increasedrates of ventilator-associated complications towhich clinicians are required to respond andmanage. Ventilator-initiated as opposed to clinician-initiated titration of ventilator para-meters arguably may overcome barriers totimely ventilation decision making, such as unavailability of experienced staff and over-stretched resources.

Another potential advantage of some advanced modes is a reduction in ventilationand ICU stay durations. These outcomes pro-duce significant patient benefits relating to re-duced complications associated with mechani-cal ventilation and ICU admission. However, itmust be noted that the favorable reductions inventilation and ICU durations also alter thestaff workload profile. Nonventilated patientsfrequently entail a lower staff/patient ratio despite potentially requiring a higher numberof interventions. For example, the confused,combative patient requiring non-invasive ven-tilation often requires more clinical interven-tions than the stable mechanically ventilated


157

patient. Reduced durations of stay result in increased levels of patient throughput, whichmay place additional stressors on the clinicalworkforce. These issues require considerationfrom an organizational perspective to informcurrent workforce discussions.

SummaryAdvances in computer technology and scien-tific knowledge of lung pathophysiology con-tinue to produce new and sophisticated venti-lator modes designed to facilitate the mostappropriate and safe method of delivering mechanical ventilation. Advanced modes com-bine pressure control-style ventilation withguaranteed volume or spontaneous breathingat upper and lower pressure levels and facili-tate ventilator recognition and adaptation topatient parameters. Increasingly, ICU clini-cians need to develop a sophisticated level ofunderstanding of mechanical ventilation andaccompanying respiratory physiology to en-sure safe and effective management of patientsventilated by these modes. Although physio-logical benefits have been identified, there arefew reported large clinical studies that evalu-ate the efficacy of these modes; hence, further research is required

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