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Review - Respiratory Care

ReviewAirway pressure release ventilation : What Do We Know?Ehab G Daoud MD, Hany L Farag MD, and Robert L Chatburn MHHS RRT-NPS FAARCI ntroductionTerminologyAPRV Versus BIPAPAPRV Versus Conventional VentilationAPRV Versus Other Non-Conventional VentilationAPRV SettingsP High/P LowT High/T LowSettings AdjunctsPressure Support and Automatic Tube CompensationSynchronizationAPRV and WeaningAdvantages of APRVS pontaneous VentilationLong Inflation TimeLung-Protective BenefitsAPRV and MortalityDisadvantages of APRVS pontaneous BreathingWork of BreathingTidal Volume and Minute VentilationPresent and FutureConclusionsAirway pressure release ventilation ( aprv ) is inverse ratio, pressure controlled, intermittent man-datory ventilation with unrestricted spontaneous breathing. It is based on the principle of open lungapproach. It has many purported advantages over conventional ventilation , including alveolarrecruitment, improved oxygenation, preservation of spontaneous breathing, improved hemodynam-ics, and potential lung-protective effects.

Introduction Airway pressure release ventilation (APRV) was first described and introduced to clinical practice over 20 years ago.1 It …

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1 ReviewAirway pressure release ventilation : What Do We Know?Ehab G Daoud MD, Hany L Farag MD, and Robert L Chatburn MHHS RRT-NPS FAARCI ntroductionTerminologyAPRV Versus BIPAPAPRV Versus Conventional VentilationAPRV Versus Other Non-Conventional VentilationAPRV SettingsP High/P LowT High/T LowSettings AdjunctsPressure Support and Automatic Tube CompensationSynchronizationAPRV and WeaningAdvantages of APRVS pontaneous VentilationLong Inflation TimeLung-Protective BenefitsAPRV and MortalityDisadvantages of APRVS pontaneous BreathingWork of BreathingTidal Volume and Minute VentilationPresent and FutureConclusionsAirway pressure release ventilation ( aprv ) is inverse ratio, pressure controlled, intermittent man-datory ventilation with unrestricted spontaneous breathing. It is based on the principle of open lungapproach. It has many purported advantages over conventional ventilation , including alveolarrecruitment, improved oxygenation, preservation of spontaneous breathing, improved hemodynam-ics, and potential lung-protective effects.

2 It has many claimed disadvantages related to risks ofvolutrauma, increased work of breathing, and increased energy expenditure related to spontaneousbreathing. aprv is used mainly as a rescue therapy for the difficult to oxygenate patients withacute Respiratory distress syndrome (ARDS). There is confusion regarding this mode of ventilation ,due to the different terminology used in the literature. aprv settings include the P high, T high, P low, and T low . Physicians and Respiratory therapists should be aware of the different waysand the rationales for setting these variables on the ventilators. Also, they should be familiar withthe differences between aprv , biphasic positive airway pressure (BIPAP), and other conventionaland nonconventional modes of ventilation . There is no solid proof that aprv improves mortality;however, there are ongoing studies that may reveal further information about this mode of venti-lation. This paper reviews the different methods proposed for aprv settings, and summarizes thedifferent studies comparing aprv and BIPAP, and the potential benefits and pitfalls for words: airway pressure release ventilation ; biphasic positive airway pressure ; ARDS/ALI; ventilator-induced lung injury; acute Respiratory failure.

3 [Respir Care 2012;57(2):282 292. 2012 Daedalus En-terprises]282 RESPIRATORYCARE FEBRUARY2012 VOL57 NO2 IntroductionAirway pressure release ventilation ( aprv ) was firstdescribed and introduced to clinical practice over 20 became commercially available in the is a mode of mechanical ventilation that is best de-scribed as a partial ventilatory support,3,4and is based onthe open lung ,6 The primary goals of this modewere to provide both safety and comfort: safety in thatadequate or superior ventilatory support is provided with-out dangerously high applied pressures, thus minimizingthe risk of ventilator-induced lung injury (VILI), and with-out depressing hemodynamics, while comfort in that un-restricted spontaneous breathing would be allowed, whichis a feature unavailable in conventional ventilatory modes,thus minimizing patient-ventilator asynchrony. Despite itstheoretically attractive advantages over other conventionalmodes of ventilation , and its availability in most of thenew commercially available ventilators, aprv is still notused routinely in clinical practice in North America.

4 APRVis used more frequently in is still mostlythought of as an alternative rescue mode for the difficult tooxygenate patient with acute lung injury (ALI) and acuterespiratory distress syndrome (ARDS).8 TerminologyAPRV is classified as pressure controlled intermittentmandatory ventilation , and is typically applied using in-verse inspiratory-expiratory (I:E) such, thereare both mandatory breaths (ie, machine-triggered and ma-chine-cycled), as well as spontaneous breaths (ie, patient-triggered and patient-cycled) (Fig. 1). The mandatorybreaths applied by aprv are time-triggered, pressure -tar-geted, time-cycled breaths (depending on the ventilator,trigger and cycle events may be synchronized with patientbreathing signals). Spontaneous breaths can occur bothduring and between mandatory breaths. Because APRVhas historically been viewed as alternating levels of CPAP,the amplitude of the time-triggered mandatory breath iscalled P high instead of inspiratory pressure , and theduration is called T high instead of inspiratory , the expiratory pressure is called P low and theexpiratory time ( release time) is called T low.

5 1,8,10 The confusion regarding this mode of ventilation arisesfrom the different terminology used in the literature, asauthors seldom provide adequately explicit definitions forthe terms they use when describing various modes of issue has been studied by Rose and Hawk-ins,11in an attempt to identify the definitional criteria ofboth aprv and biphasic positive airway pressure (BIPAP)modes. They concluded: Ambiguity exists in the criteriathat distinguish aprv and BIPAP. The other source ofconfusion arises from the ventilator manufacturers choiceof terminology. Different names have been coined for thesame mode, such as BiLevel (Covidien), aprv (Dra ger),Bi-Vent (Maquet), BiPhasic (CareFusion), and DuoPAP(Hamilton). aprv Versus BIPAPBoth modes clearly have the same general pattern ofairway pressures as intermittent mandatory ventilation , withtime-triggered, pressure -targeted, and time-cycled manda-tory breaths according to the preset values of T high andT low (Fig.)

6 2). Both modes allow unrestricted spontaneousbreathing both during and between mandatory breaths, butthere is more time for them to occur during mandatorybreaths with aprv . aprv typically uses extreme inverseI:E ratios, while BIPAP usually does aprv usually keeps the duration of the T low at seconds,while there is no restriction on the T low in ,8,12 The authors are affiliated with the Respiratory Institute, The ClevelandClinic, Cleveland, authors have disclosed no conflicts of : 1. A: pressure (green) and volume (black)/time curve in airwaypressure release ventilation ( aprv ). Shown in the figure: 2 fullmandatory breaths (not triggered by muscle effort), and 2 full spon-taneous breaths (triggered by muscle effort, in blue) on the top ofthe mandatory ones. Muscle pressure (blue): the long arrow rep-resents the T high, and the short one represents the T low of themandatory breath. B: Flow (black) and volume (green)/time curvein aprv .

7 T high is the start of the inspiratory flow to the start ofexpiratory flow, T low is the start of expiratory flow to the begin-ning of the next inspiratory flow. The 2 red intersecting lines are at50% of the peak expiratory :WHATDOWEKNOW?RESPIRATORYCARE FEBRUARY2012 VOL57 NO2283A recent Review of 50 published studies of both modes11showed that 78% of the studies described aprv , while22% described BIPAP. Extreme inverse I:E ( 2:1) wasused in 46% of the studies describing aprv . None of thestudies describing BIPAP used an I:E 2:1. Twenty-onepercent of the aprv studies used I:E of 1:1 to 2:1, com-pared to 9% of studies describing BIPAP. Thirty-one per-cent of the aprv studies used I:E of 1:1, compared to64% of the BIPAP studies. Finally, only 5% of the aprv studies used I:E less than 1:1, compared to 27% of theBIPAP studies. The mean reported inspiratory time (T high)was seconds for aprv , compared to seconds forBIPAP.

8 Conversely, the mean reported expiratory time(T low) was nearly 3 times longer in the BIPAP studies,compared to aprv ( s and s, respectively). Themean P high used in the aprv studies was 6 cm H2 Ohigher, but not statistically significant, compared to theBIPAP , despite the difference in P high and T highsettings, the mean P low was similar in both aprv andBIPAP, at cm those 2 modes have never been comparedhead to head in an animal or human trial to determine ifone is superior to the other. In a bench study using a lungsimulator to compare both modes, with equivalent settingsin an ARDS lung model, we found that aprv suppliedhigher mean airway pressure (Paw) but lower minute ven-tilation (V E) than Versus Conventional VentilationSeveral studies15 22have compared aprv to conven-tional mechanical ventilation in humans with ALI/ARDS,but most of them are weakened by the small number ofpatients and by being short-term observations studies.

9 Con-sistently, most of them have shown improvement in oxy-genation, but none of them has shown mortality benefitsin the aprv group. Putensen and colleagues18random-ized 30 trauma patients to either aprv or pressure con-trolled continuous mandatory ventilation (PC-CMV); theyhave shown that the aprv group had shorter ventilatordays (15 vs 21) and ICU days (23 vs 30). According tothe authors, those findings were explained by the improve-ment in hemodynamics and the less sedation and no par-alytic agents used in the aprv group. This study is weak-ened by the small number of patients (15 in each group),by the fact that only 20% of the aprv group had ARDS,compared to 74% in the PC-CMV group, and by the factthat the 2 groups had different sedation protocols. A recentretrospective study23with a small number of patients, com-pared aprv to volume controlled intermittent mandatoryventilation (VC-IMV); beside improved oxygenation, itshowed a trend to lower mortality in the aprv group, butdid not reach statistical significance.

10 A larger internationalretrospective study7using propensity score to compare bothAPRV and BIPAP to conventional ventilation confirmedthe significantly improved oxygenation with both modes,but failed to show any mortality recent study24in 63 adult trauma patients requiringmechanical ventilation for greater than 72 hours showedthat aprv has a similar safety profile as the low tidalvolume ventilation . aprv patients tended to have increasedventilator days ( d vs d), ICUstay ( d vs d), and ventilator-associated pneumonia (VAP) ( vs ).This may be explained by the significant initial worsephysiologic derangement demonstrated by Acute Physiol-ogy and Chronic Health Evaluation II (APACHE II) scoresin the aprv group ( vs ). aprv Versus Other Non-Conventional VentilationAPRV shares some similar features with other non-con-ventional modes of ventilation (eg, inverse ratio venti-lation,andhigh-frequencyoscillator yventilation[HFOV]).


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