quantitative computed tomography in porcine lung injury with

Quantitative computed tomography in porcine lung injurywith variable versus conventional ventilation: Recruitment andsurfactant replacement*

M. Ruth Graham, MD, FRCPC; Andrew L. Goertzen, PhD; Linda G. Girling, BSc; Talia Friedman, MD;Ryan J. Pauls, MD; Timothy Dickson, MD; Ainsley E. G. Espenell, MD; W. Alan C. Mutch, MD, FRCPC

I ncorporating variability into me-chanical ventilation improveslung mechanics, gas exchange, in-flammatory mediators, and histo-

logic evidence of lung injury in acuterespiratory distress syndrome (ARDS)models (1–7). The primary mechanismproposed to account for the improvement

in compliance and decrease in inflationpressures observed is enhanced recruit-ment (8). Two mathematical models pre-dict a volume benefit with variable inputpressure, but direct corroboration is lack-ing (9, 10).

The first goal of this study was toconfirm biologically variable ventilation

(BVV) enhanced recruitment in a porcineoleic acid (OA) ARDS model using quan-titative computed tomography (CT).

The second goal was to compare sur-factant replacement during BVV to con-ventional ventilation (CMV) in the sameinjury model. The integral role of surfac-tant inactivation in ARDS and its primary

*See also p. 1839.From the Department of Anesthesia (MRG, LGG,

RJP, TD, AEGE, WACM) and PET/CT Program (ALG),Department of Radiology (TF), University of Manitoba,Winnipeg, Manitoba, Canada.

Supported, in part, by an award from the D. ElaineAndison Foundation (Winnipeg, Manitoba, Canada), anoperating grant from the Manitoba Institute for ChildHealth (Winnipeg, Manitoba, Canada), and a research

grant from the University of Manitoba Department ofAnesthesia (Winnipeg, Manitoba, Canada). BLES Bio-chemicals (London, Ontario, Canada) provided the sur-factant preparation at cost.

The patent for the BVV-related software and hard-ware, previously held by Dr. Mutch, is currently held bythe University of Manitoba. The University and Dr.Mutch stand to gain with commercialization of theproduct. Dr. Mutch received three patents for life

support devices. The remaining authors have not dis-closed any potential conflicts of interest.

For information regarding this article, E-mail:[email protected]

Copyright © 2011 by the Society of Critical CareMedicine and Lippincott Williams & Wilkins

DOI: 10.1097/CCM.0b013e3182186d09

Objectives: Biologically variable ventilation improves lungfunction in acute respiratory distress models. If enhanced recruit-ment is responsible for these results, then biologically variableventilation might promote distribution of exogenous surfactant tononaerated areas. Our objectives were to confirm model predic-tions of enhanced recruitment with biologically variable ventila-tion using computed tomography and to determine whether sur-factant replacement with biologically variable ventilation providesadditional benefit in a porcine oleic acid injury model.

Design: Prospective, randomized, controlled experimental an-imal investigation.

Setting: University research laboratory.Subjects: Domestic pigs.Interventions: Standardized oleic acid lung injury in pigs ran-

domized to conventional mechanical ventilation or biologicallyvariable ventilation with or without green dye labeled surfactantreplacement.

Measurements and Main Results: Computed tomography-de-rived total and regional masses and volumes were determined atinjury and after 4 hrs of ventilation at the same average low tidalvolume and minute ventilation. Hemodynamics, gas exchange, andlung mechanics were determined hourly. Surfactant distribution wasdetermined in postmortem cut lung sections. Biologically variableventilation alone resulted in 7% recruitment of nonaerated regions(p < .03) and 15% recruitment of nonaerated and poorly aeratedregions combined (p < .04). Total and normally aerated regionalvolumes increased significantly with biologically variable ventilation,

biologically variable ventilation with surfactant replacement, andconventional mechanical ventilation with surfactant replacement,while poorly and nonaerated regions decreased after 4 hrs of venti-lation with biologically variable ventilation alone (p < .01). Biologi-cally variable ventilation showed the greatest improvement (p <.003, biologically variable ventilation vs. all other groups). Hyperaer-ated regional gas volume increased significantly with biologicallyvariable ventilation, biologically variable ventilation with surfactantreplacement, and conventional mechanical ventilation with surfac-tant replacement. Biologically variable ventilation was associatedwith restoration of respiratory compliance to preinjury levels andsignificantly greater improvements in gas exchange at lower peakairway pressures compared to all other groups. Paradoxically, gasexchange and lung mechanics were impaired to a greater extentinitially with biologically variable ventilation with surfactant re-placement. Peak airway pressure was greater in surfactant-treated animals with either ventilation mode. Surfactant wasdistributed to the more caudal/injured lung sections with biolog-ically variable ventilation.

Conclusions: Quantitative computed tomography analysis con-firms lung recruitment with biologically variable ventilation in aporcine oleic acid injury model. Surfactant replacement withbiologically variable ventilation provided no additional recruit-ment benefit and may in fact be harmful. (Crit Care Med 2011; 39:1721–1730)

KEY WORDS: acute respiratory distress syndrome; mechanicalventilation; variable ventilation; surfactant replacement

1721Crit Care Med 2011 Vol. 39, No. 7

role in maintaining alveolar stability (11)make restoration of surfactant functionan important goal of management. Howbest to accomplish this remains elusive asevidenced by two negative meta-analyses(12, 13) of surfactant replacement inARDS. Barriers to effective replacementinclude inhomogeneous surfactant deliv-ery and ongoing surfactant inactivation.BVV offers theoretical advantages in bothregards. Recruitment maneuvers pro-mote more homogeneous surfactant dis-tribution but increase the risk of overd-istension (14). BVV may provide a similarrecruitment benefit without overall in-creases in airway pressure. Additionalbenefits of variable ventilation strategiesinclude a twofold increase in endogenoussurfactant, attenuated alveolar fluid proteincontent, and a decrease in inflammatorymediators (4, 5, 15), which may attenuateinflammatory-mediated inhibition of deliv-ered surfactant. We hypothesized that aer-ation, gas exchange, and lung mechanicswould be most improved when surfactantadministration was combined with BVV.

METHODS

Experimental Protocol. All animals weretreated following University of Manitoba Re-search Ethics Board Guidelines. A total of 28animals were studied in four groups: BVValone (n ! 6), CMV alone (n ! 6), BVV withsurfactant replacement (BVVs; n ! 8) andCMV with surfactant replacement (CMVs; n !8). Farm-bred pigs (10–15 kg) were anesthe-tized and ventilated with an Esprit intensivecare ventilator (Respironics, Palo Alto, CA)capable of delivering either CMV or BVV. In-strumentation, data acquisition, and induc-tion of OA injury have been reported previ-ously (2). Pre-OA ventilation was maintainedwith a tidal volume (VT) of 10 mL/kg, f ! 25,a positive end-expiratory pressure (PEEP) of 5cm H2O, and FIO2 ! 35%. OA was infused untilPaO2 "70 mm Hg and compliance decreased#40% from the baseline. PEEP was then in-creased to 10 cm H2O. We used a sustainedincrease in PaO2 to 80–100 mm Hg with in-creased PEEP as evidence of a stable but re-cruitable injury. Ventilation was then main-tained in both groups using a lower VT (7.5mL/kg), f ! 30, PEEP ! 10 cm H2O, and FIO2! 0.35. VT ! 7.5 mL/kg instead of the ARD-SNet (National Heart, Lung, and Blood Insti-tute, National Institutes of Health, Bethesda,MD) recommended 6 mL/kg. This was re-quired to compensate for added equipmentdead space. Normal saline was administered at10 mL/kg/hr. After acquisition of a baseline CTscan postinjury, the animals were randomizedto either BVV or CMV at the same minuteventilation and PEEP level for 4 hrs. Hemody-

namics, gas exchange, and lung mechanicswere determined hourly.

BVV Ventilation. BVV has been describedpreviously (2). Briefly, breath-to-breath con-trol of the Esprit ventilator is provided by amodulation file and laptop computer. Themodulation file is derived from the breath-to-breath variability in frequency obtained from a2-hr end-tidal carbon dioxide recording in ahealthy, spontaneously ventilating, awake sub-ject. Functions were developed to convert ven-tilator rate to voltage scaled to the chosenfrequency. Voltage to the rate controller of theventilator is altered on the basis of the nextinstantaneous rate. Output to the control fre-quency is updated every 5 msecs. Minute ven-tilation is set, and the ventilator functions as avolume divider: changes in frequency result inreciprocal changes in VT. The file contains1,589 breaths and loops over to repeat itself. Agraph showing the BVV breath-to-breath VTvariability and the frequency distribution his-togram of VT with a mean of 7.5 mL/kg isshown in Figure 1.

CT Imaging. A spiral CT was obtained atbaseline OA injury and after 4 hrs of ventila-tion. The animals were transported to the CTscanning facility with ventilation unchanged.Lung scanning was performed from the apexto diaphragm using a Biograph 16 positronemission tomography/CT system (Siemens,Munich, Germany). Images were acquired inspiral mode using a 120-kVp, 150-mAs proto-col and reconstructed with a B80s kernel, slicewidth of 1.0 mm, and slice increment of 0.7mm. Acquisition of the CT sections requiredapproximately 13 secs, and they were obtainedwith a respiratory pause at PEEP ! 10 cmH2O. All CT sections were archived to an op-tical disk for later analysis.

The lung CT images were analyzed follow-ing the method of Gattinoni (16), which clas-sifies lung regions according to the degree ofaeration as measured by the Hounsfield unitvalues: regions between $1000 and $900Hounsfield units were considered hyperaer-ated, those between $900 and $500 normallyaerated, those between $500 and $100 poorlyaerated, and those greater than $100 nonaer-ated. Regions of interest were drawn in theimages for the lung regions, excluding thehilar vessels, in every 25th slice (i.e., at17.5-mm intervals) to create a total volume ofinterest. The CT number values for the voxelsin the volume of interest were extracted andhistogrammed for the total volume of interest,allowing the calculation of the percentage oflung volume that fit each of the four classifi-cations. CT slices were analyzed by both massand volume. Total regional volume was di-vided into tissue and gas volumes using stan-dard calculations based on the regional den-sity per Gattinoni et al (16). Recruitment wasdefined as the change in mass of the nonaer-ated region plus poorly aerated region relativeto the total lung mass.

Surfactant Administration. After the base-line CT image was obtained, 4 mL/kg naturalbovine surfactant (bovine lipid extract surfac-tant) (BLES Biochemicals, London, Ontario,Canada) containing surfactant proteins B andC (equivalent to 108 mg/kg/dose) and labeledwith green dye (17) was given to those animalsrandomized to surfactant replacement. Thesurfactant was instilled over 30 mins througha catheter placed distal to the end of the en-dotracheal tube with uninterrupted ventila-tion. The time from CT scan to surfactantinstillation averaged 45 mins. In control ani-mals, a catheter was positioned identically butno surfactant delivered. All animals were ro-tated from supine to both lateral positions inrandom order to maximize bilateral surfactantdistribution. The catheter was then withdrawnand ventilation continued at the same minuteventilation and PEEP level for 4 hrs. Hemody-namics, gas exchange, and lung mechanicswere determined hourly.

Surfactant Distribution. At the end of theexperimental period, the animals were eutha-nized, the trachea clamped at end expiration,and the lungs removed and divided. The rightlung was used to determine wet/dry weightratios. The left lung was processed to deter-mine the distribution of green dye labeledsurfactant. With the airway clamped at endexpiration, the lung was cut into transversesections at 2-cm intervals from base to apex.The cut surfaces were immediately photo-graphed with a tripod-mounted camera sys-tem. The image distances and light settingswere identical in all studies. Using Photoshop(Adobe, San Jose, CA), we determined the dis-tribution of green dye in each cut section as apercentage of total surface area. The imagewas analyzed using a red filtered channel pro-cessor. In this mode, green elements appearblack, background red elements are elimi-nated, and all nongreen elements in the imageappear white. The cut surface was outlined,and the fraction of the total pixels in theoutlined image that were black (i.e., surfactantcontaining) was determined. The area subten-ded by large airways was subtracted from theoutlined image. The observer was blinded tothe experimental group assignment. Dye dis-tributions from the two cut surfaces per ver-tical height were analyzed separately and theresults averaged.

Stress Index. In control animals, the stressindex was measured during constant flow perGrasso et al (18), fitting the constant flowportion of the airway pressure (Paw) trace tothe power equation Paw ! a(inspiratorytime)b % C using a curve fitting program(NCSS Statistical and Power Analysis Soft-ware, Kaysville, UT). The exponent b, whichdescribes the shape of the airway pressuretrace, defines the stress index. The averagestress index of 3–5 breaths per animal wasdetermined at baseline before OA administra-tion (VT ! 10 mL/kg, PEEP ! 5 cm H2O),post-OA (VT ! 7.5 mL/kg, PEEP ! 10 cm H2O,CMV in both groups), and 4 hrs post-OA. To

1722 Crit Care Med 2011 Vol. 39, No. 7

allow comparison at equivalent tidal excur-sions, the average stress index of 3–5 breathswith VT between 7 and 8 mL/kg was deter-mined in animals ventilated with BVV.

Statistical Analysis. Data were analyzed byrepeated measures analysis of variance usingleast squares means test matrices to identifydifferences within and between groups fromgroup & time (G & T), group & surfactant(G & S), and group & surfactant & time (G &S & T) interactions. A mixed effects model wasused for analysis of CT regional aeration. Bon-ferroni’s correction was applied where ap-propriate. Regional aeration vs. compliancewas correlated by linear regression analysis.In all circumstances a p value of !.05 cor-rected for multiple comparisons was consid-ered significant.

RESULTS

Airway Pressures and Compliance.Ventilation variables and lung mechanicsare shown in Table 1. Airway pressures,VT, and compliance were similar betweengroups at baseline and after OA injury.BVV was associated with lower peak Paw

at all times after OA. Surfactant adminis-tration resulted in higher peak and meanPaw values with either ventilation mode.Compliance decreased to a similar extentwith OA injury in all groups. BVV was as-sociated with a significantly greater resto-ration of compliance to control values by 4hrs compared to all other groups. Compli-ance improved to a significantly greaterextent in the BVVs compared to CMVsgroup but remained significantly less thanwith BVV at 2 and 4 hrs.

Blood Gases. Arterial blood gases, ve-nous admixture, and dead space areshown in Table 2. After OA injury, BVVresulted in significantly greater restora-tion of PaO2 compared to all other groupspostsurfactant administration, and PaO2decreased and the shunt fraction in-creased to a significantly greater extentwith BVVs compared to all other groups.At 2 and 4 hrs, the restoration of PaO2 wassignificantly less with BVVs compared toBVV and not different between CMVs andCMV. PaCO2 was significantly lower with

BVV and CMVs at the surfactant/shamtime period but not different betweengroups thereafter. These results are re-flected similarly in dead space/VT.

Hemodynamics. Hemodynamic dataare shown in Table 3. Heart rate re-mained significantly higher in the BVVsgroup compared to all other groups at 2and 4 hrs. The mean arterial pressureshowed a significant time effect, decreas-ing from baseline at 4 hrs in all groupswith no between group differences. Car-diac output was higher in the CMVsgroup at baseline, fell significantly withOA in all groups, and remained lower inthe CMVs group at 4 hrs. The pulmonaryartery pressure increased significantlywith OA injury, to a greater extent in thetwo control groups compared to the twosurfactant groups, but was not differentbetween groups at any other time period.A small but significant increase in tem-perature was seen with OA administra-tion. Thereafter, temperature rose signif-icantly in both surfactant groups and tothe greatest extent with BVVs at 4 hrs.Wet to dry weight ratios were not signif-icantly different between groups.

CT Regional Aeration. CT scans wereobtained in five BVV, six CMV, sevenBVVs, and six CMVs animals. Regionalaeration was analyzed by mass, total vol-ume, and calculated tissue and gas vol-umes. The results are shown in Table 4.

Mass. Total lung mass did not differbetween groups at baseline OA injury andwas unchanged over 4 hrs. G & T inter-actions were significant for hyperaeratedregions (p ! .0001). G & S & T interac-tions were significant for normally (p !.05) and nonaerated (p ! .05) regions.Regional mass was unchanged with CMVover time. With BVV alone, the masses ofthe normally aerated (p ! .008) and hy-peraerated (p " .001) regions increasedwhile those of the poorly (p ! .03) andnonaerated (p ! .003) regions decreasedat 4 hrs. BVVs and CMVs were not asso-ciated with any change in regional massexcept for a significant decrease in poorlyaerated mass with BVVs at 4 hrs (p !.02). The tissue volume results mirroredthe mass results.

Total Volumes. G & T, G & S, and G &S & T interactions were highly signifi-cant (p " .01). Total lung volume was notdifferent between groups at baseline OAinjury. After 4 hrs, total lung volume wasunchanged with CMV, but increased sig-nificantly with BVV (p " .001) and to alesser extent in both surfactant-treatedgroups (BVVs, p " .04; CMVs, p " .03).

Figure 1. (Top) Graph of instantaneous tidal volume (VT) vs. time generated from the biologicallyvariable ventilation modulation file used in the experiment with average VT ! 7.5 mL/kg. The entirefile contains 1,589 breaths. The noise signal demonstrates fractal characteristics, which distinguishesthis variability pattern from computer-generated random variability files. (Bottom) Frequency histo-gram of VT generated from the above modulation file. VT values of #6 but "9 mL/kg constitute 85%of breaths delivered.


This increase in total lung volume couldbe accounted for by a significant increasein the normally aerated region with BVV(p " .001), BVVs (p " .01), and CMVs(p " .006) and in the hyperaerated re-gions with BVV and BVVs (p " .001). At 4hrs, the increase in normally aerated re-gions with BVV alone exceeded that ofboth groups given surfactant (p " .0002).This was accompanied by a significantdecrease in the poorly aerated andnonaerated regions with BVV alone at 4hrs (p " .001). The total volume of thenonaerated region was significantlygreater in both surfactant groups com-pared to BVV alone (p ! .03).

Gas Volumes. G & T, G & S, and G &S & T interactions were highly signifi-cant (p " .01) except for poorly aeratedregions. Total gas volume was unchangedwith CMV alone but increased signifi-cantly with BVV (p " .0001), BVVs (p !.002), and CMVs (p ! .009) at 4 hrs com-pared to baseline OA injury. The increasein total gas volume was associated with

significant increases in the normally aer-ated (BVV, p " .0001; BVVs, p ! .01;CMVs, p ! .007) and hyperaerated re-gional (BVV, p ! .002; BVVs, p ! .0002;CMVs, p ! .05) gas volumes. Poorly aer-ated regional gas volumes were not sig-nificantly different from baseline lung in-jury with any ventilation mode. Thenonaerated gas volume decreased signif-icantly with BVV alone (p ! .004). Figure5, top, shows that the change in normallyaerated gas volume correlated signifi-cantly with the change in complianceover 4 hrs.

Recruitment. In Figure 2, using thechange in nonaerated regional massalone, BVV alone was associated with asignificant recruitment of 7% (p " .03).If poorly and nonaerated regions arecombined, BVV was associated with a15% recruitment, significantly greaterthan that of all other groups (p " .04).Figure 3 shows that recruitment corre-lated less well than the change in nor-

mally aerated gas volumes with thechange compliance over 4 hrs.

Green Dye Labeled Surfactant Distri-bution. The average distribution of greendye by vertical height in the lung isshown in Figure 4 . The total area underthe dye distribution curve is 230 mm2

with the maximum percent green dye at 8cm for BVV vs. 196 mm2 with the maxi-mum percent green dye at 10 cm forCMV. In all sections, green dye distrib-uted in a patchy manner. Overall, surfac-tant was present in 15.7% of the totallung surface with BVVs vs. 10.2% withCMVs (nonsignificant difference by un-paired t analysis). The G & S & D inter-action was significant (p ! .047). In bothgroups, the majority of green dye labeledsurfactant distributed to the midlungzones, but the curve is shifted to themore caudal lung segments with BVVcompared to CMV. Figure 5, a (CMVs)and b (BVVs), demonstrates the differ-ence in the distribution of green dye

Table 1. Ventilation variables, airway pressure, and lung mechanics

Variable BaselinePost-Oleic

AcidPostsurfactant

or Sham 2 hrs 4 hrs

Group &Time

Interaction

Group &SurfactantInteraction

Group &Surfactant & Time

Interaction

Tidal volume (mL)CMV 10.0 (0.2) 7.4 (0.3) 7.4 (0.3) 7.5 (0.4) 7.5 (0.4) NS NS NSBVV 10.3 (0.2) 7.4 (0.2) 7.3 (0.4) 7.3 (0.5) 7.2 (0.5)CMVs 9.9 (0.5) 7.4 (0.3) 7.3 (0.4) 7.4 (0.4) 7.4 (0.4)BVVs 10.2 (0.6) 7.8 (0.5) 7.8 (0.7) 7.7 (0.8) 7.7 (0.8)

Minute volume (L/min)CMV 4.7 (0.4) 4.5 (0.3) 4.6 (0.4) 4.8 (0.6) 4.8 (0.4)b NS .001 .04BVV 4.8 (0.5) 4.5 (0.5) 4.5 (0.6) 4.5 (0.5) 4.4 (0.6)CMVs 4.5 (0.4)b 4.6 (0.5) 4.4 (0.4) 4.5 (0.4) 4.5 (0.4)BVVs 4.6 (0.6) 4.5 (0.5) 4.4 (0.5) 4.6 (0.4) 4.6 (0.4)

Peak airway pressure(cm H2O)

CMV 20.0 (1.0) 28.2 (1.4)a 26.7 (1.7) 26.0 (1.4) 25.4 (2.5) ".001 ".001 NSBVV 20.9 (1.1) 28.7 (2.3)a 24.7 (1.8) 24.0 (1.9) 23.7 (2.5)CMVs 20.2 (1.7) 30.6 (2.6)a 32.4 (3.2)c 29.6 (2.6)c 29.0 (2.8)c

BVVs 19.6 (1.3) 29.3 (2.3)a 30.4 (1.7)c 27.1 (2.9)c 25.9 (2.7)c

Mean airway pressure(cm H2O)

CMV 8.8 (0.3) 13.9 (0.3) 14.0 (0.8) 13.7 (0.5) 14.2 (1.5) NS ".01 NSBVV 8.9 (0.4) 14.0 (0.8) 14.0 (0.8) 14.0 (0.7) 13.9 (0.9)CMVs 8.7 (0.3) 14.8 (0.7) 15.8 (1.1)c 14.6 (0.6)c 14.9 (0.9)c

BVVs 8.7 (0.4) 14.5 (0.4) 15.9 (1.2)c 14.9 (0.6)c 14.7 (0.9)c

Static compliance(mL/cm H2O/kg)

CMV 1.11 (0.09) 0.66 (0.07)a 0.75 (0.10) 0.84 (0.09) 0.86 (0.12) ".001 ".001 .003BVV 1.06 (0.09) 0.66 (0.07)a 0.94 (0.12)b 1.12 (0.13)b 1.21 (0.19)b

CMVs 1.09 (0.15) 0.63 (0.07)a 0.59 (0.08) 0.66 (0.08) 0.69 (0.10)b

BVVs 1.19 (0.14) 0.68 (0.11)a 0.61 (0.14) 0.88 (0.22) 1.00 (0.23)Stress index

CMV 1.07 (0.04) 1.18 (0.14)a 1.12 (0.04)BVV 1.13 (0.04) 1.2 (0.08)a 1.0 (0.09)b

CMV, conventional mechanical ventilation; BVV, biologically variable ventilation; CMVs, CMV with surfactant replacement; BVVs, BVV with surfactantreplacement; NS, not significant.

Ventilation variables and lung mechanics (SD) are shown for each time period and ventilation mode.Data shown as mean (SD). ap ".05 compared to baseline, bp " .05 compared to other groups, cp " .05 surfactant vs. control groups.


stained surfactant at the same verticalheights in two representative animals.

Stress Index. Data from all analyzedbreaths fit the power relationship with R2

values #0.999. The stress index increasedto similar extents with OA lung injury inboth groups (Table 1). After 4 hrs, atcomparable VT, the stress index was un-changed with CMV and decreased to pre-injury levels with BVV (p ! .003 vs. post-OA, p ! .02 vs. CMV).

DISCUSSION

The results of the present study cor-roborate previously documented superiorlung mechanics and gas exchange withBVV (1, 2, 19). Quantitative CT analysis ofregional aeration provides confirmationfor model predictions of BVV-induced re-cruitment but refutes our a priori hy-pothesis that surfactant administrationwith BVV confers additional benefit.

Recruitment. Previous studies basevariable-ventilation-induced recruitmenton changes in oxygenation, lung compli-ance, pressure–volume curves, and redis-tribution of blood flow (3, 4, 6, 8, 20–22).CT analysis provides direct confirmationfor these findings. Gattinoni et al (16)

proposed a CT definition of PEEP-induced recruitment to be the change inmass of the nonaerated region relative tothe total mass in a single juxtadiaphrag-matic CT slice. Malbouisson et al (23)suggested that effective recruitment—that which contributes to improvementsin gas exchange—should include poorlyaerated regions. They defined PEEP-induced recruitment as the degree ofpenetration of gas within nonaerated andpoorly aerated lung regions in delineatedlung slices and added PEEP-induced al-veolar distension vs. overdistension (gaspenetration in normally aerated regionsand hyperaerated regions relative to func-tional residual capacity, respectively) totheir analysis. As we could not delineateidentical lung slices with CT images ob-tained 4 hrs apart, we used the change intissue mass of the poorly and nonaeratedregions of the entire lung relative to thetotal lung mass as an index of globalrecruitment over 4 hrs of ventilation cou-pled with regional gas volume determina-tions. If the Gattinoni definition is ap-plied to the entire lung, BVV-inducedrecruitment is small but significant at7%. If nonaerated and poorly aerated re-

gions are combined, BVV-induced re-cruitment doubles to 15%. This modestrecruitment was associated with com-plete recovery of both gas exchange andcompliance coupled with a 20% in-crease in normally aerated region gasvolume and a 3% increase in hyperaer-ated region gas volume (indexed toFRC).

The recruitment-associated change innormally aerated gas volume would ap-pear to be most relevant with respect tolung mechanics as Figure 3 shows thatthe change in compliance correlated bestwith normally aerated gas volume (p !.001) and less well with recruitment perse (p ! .02). The change in neither ofthese regions correlated with oxygen-ation (R2 ! 0.004, not significant). Infact, PaO2 increased in the conventionallyventilated group when no change in re-gional aeration or compliance occurred,suggesting that a vigorous hypoxic pul-monary vasoconstrictor response waspresent in this porcine model. PaO2 maytherefore be a poor index of underlyingmechanics under these circumstances asrestoration of regional perfusion balancemay partially restore oxygenation in the

Table 2. Arterial blood gases, venous admixture, and dead space


AcidPostsurfactant

or Sham 2 hrs 4 hrs

Group &Time

Interaction



Interaction

PaO2 (mm Hg)CMV 139.4 (8.9) 87.9 (3.5)a 97.9 (20.4) 117.8 (12.6) 123.3 (16.0) NS .001 ".001BVV 139.9 (9.3) 88.7 (6.7)a 127.4 (14.2)b 136.8 (14.1)b 136.2 (9.9)b

CMVs 139.5 (8.9) 96.2 (12.2)a 96.2 (14.0) 118.6 (16.8) 124.2 (17.9)BVVs 140.6 (10.9) 89.1 (12.5)a 71.7 (11.5)b 105.9 (18.0)b 114.9 (16.2)

PaCO2 (mm Hg)CMV 39.3 (2.5) 56.5 (5.7)a 60.5 (5.2) 55.3 (3.1) 50.8 (6.9) NS .009 ".001BVV 41.8 (3.8) 56.5 (2.5)a 52.8 (4.5)b 48.5 (3.4) 46.9 (3.6)CMVs 40.0 (5.3) 56.6 (6.5)a 51.1 (6.9)b 48.7 (6.1) 45.9 (6.9)BVVs 40.6 (5.1) 55.6 (6.7)a 61.7 (4.4) 52.5 (3.3) 45.9 (3.5)

pHCMV 7.45 (0.03) 7.3 (0.04)a 7.23 (0.06) 7.25 (0.13) 7.30 (0.08) NS .00 .01BVV 7.44 (0.05) 7.3 (0.02)a 7.31 (0.08) 7.35 (0.04) 7.36 (0.05)CMVs 7.4 (0.03) 7.3 (0.04)a 7.33 (0.08) 7.35 (0.09) 7.37 (0.1)BVVs 7.42 (0.01) 7.28 (0.06)a 7.21 (0.13) 7.30 (0.05) 7.31 (0.02)

Qs/Qt (%)CMV 3.3 (1.1) 19.6 (6.6)a 17.7 (7.2)b 11.1 (4.6) 9.7 (4.1) .01 ".001 ".001BVV 5.4 (2.1) 18.7 (4.2)a 9.0 (8.2) 5.0 (3.1)b 4.8 (2.0)b

CMVs 5.3 (2.3) 13.00 (5.0)a 11.6 (5.4) 6.7 (3.4) 5.9 (3.5)BVVs 4.7 (1.2) 19.2 (8.3)a 34.1 (9.0)b 12.0 (6.4) 9.7 (4.7)

VD/tidal volume (%)CMV 58.0 (1.2) 75.6 (0.9)a 76.8 (4.9) 72.5 (4.4) 72.2 (5.6) ".001 NS NSBVV 59.4 (3.6) 75.3 (4.6)a 72.8 (4.5) 68.3 (4.8) 67.3 (5.2)CMVs 55.8 (4.7) 72.5 (5.3)a 73.3 (4.4) 72.0 (6.3) 70.1 (5.9)BVVs 57.5 (3.7) 72.1 (3.4)a 73.2 (5.2) 67.2 (5.2) 62.8 (6.1)

CMV, conventional mechanical ventilation; BVV, biologically variable ventilation; CMVs, CMV with surfactant replacement; BVVs, BVV with surfactantreplacement; Qs/Qt, shunt flow/total flow; VD, dead space; NS, not significant.

Arterial blood gases, calculated venous admixture, and dead space (SD) are shown for each time period and ventilation mode.ap ".05 compared to baseline, bp " .05 compared to other groups.


face of unchanged aeration. Supportingthis contention, Gama de Abreu et al (24)document redistribution of pulmonaryblood flow without significant changesin aeration with noisy pressure supportventilation.

Conventional investigations of re-cruitment during lung injury employ ei-ther increasing PEEP or recruitment ma-neuvers (25, 26). Karmrodt et al (27) andMartynowicz et al (28) show that PEEP#15 cm H2O is required to significantlydecrease the nonaerated region. Our re-

sults confirm that simply varying venti-lator input induces recruitment at signif-icantly lower average peak airwaypressures than either increasing PEEP orrecruitment maneuvers. In Figure 1,breaths of #12 mL/kg (significantlysmaller than a recruitment maneuver)occur twice within the file. At a rate of 30breaths/min, this translates into approx-imately two larger breaths per minute.Concern that these intermittent largerBVV breaths may induce stress/strainover longer periods of ventilation is not

borne out by Spieth et al (6), who showdecreased inflammatory mediators andless histologic evidence of lung injury, atleast up to 6 hrs, but demonstration oflong-term safety and sustained benefitwith variable ventilation would requiretrials over days/weeks. In contrast, how-ever, the efficacy of standard recruitmentmaneuvers regarding sustained improve-ments in oxygenation and compliance isquestionable (29), and repeated sustainedinflations of this magnitude may contrib-ute to alveolar capillary disruption and

Table 3. Hemodynamics


AcidPostsurfactant

or Sham 2 hrs 4 hrs

Group &Time

Interaction



Interaction

Heart rate (beats/min)CMV 155.0 (30) 200.0 (41)a 224.3 (25) 198.8 (31) 192.5 (35) NS ".001 .049BVV 160.3 (38) 210.0 (17)a 197.2 (25) 174.8 (32) 175.8 (27)CMVs 169.5 (28) 215.5 (20)a 178.5 (35) 194.4 (39) 170.3 (44)BVVs 170.1 (33) 209.8 (37)a 212.8 (31) 210.8 (39)c 208.4 (61)c

Mean arterial pressure(mm Hg)

CMV 98.7 (21.2) 96.7 (14.3) 90.8 (13.1) 86.0 (15.0) 84.3 (14.8) NS ".001 NSBVV 103 (7.6) 93.7 (13.1) 97.8 (9.6) 94.9 (14.2) 84.8 (7.7)CMVs 91.5 (11.8) 94.3 (8.0) 80.5 (9.1) 92.6 (9.6) 82.8 (11.5)BVVs 89.6 (12.0) 86.4 (16.4) 87.5 (19.0) 83.9 (13.2) 83.6 (17.3)

Cardiac output (L/min)CMV 2.9 (0.4) 2.2 (0.7)a 2.1 (0.7) 2.0 (0.7) 2.4 (0.4) NS ".001 ".048BVV 3.0 (0.7) 2.2 (0.5)a 1.8 (0.5) 1.4 (0.4) 1.8 (0.5)b

CMVs 3.2 (0.7)a 1.9 (0.8)a 1.4 (0.5) 1.6 (0.5) 1.7 (0.6)b

BVVs 3.1 (0.6) 2.5 (0.9)a 2.1 (0.7) 2.0 (0.5) 1.8 (0.6)Pulmonary artery pressure

(mm Hg)CMV 19.8 (4.9) 35.7 (3.6) 33.8 (3.8) 29.6 (3.6) 30.3 (4.1) NS ".001 NSBVV 19.7 (2.6) 37.0 (3.0) 32.0 (3.6) 28.4 (4.0) 29.0 (3.7)CMVs 17.6 (2.9) 33.0 (6.0)c 31.9 (5.4) 31.8 (5.8) 30.6 (5.0)BVVs 18.3 (3.1) 32.9 (6.5)c 36.6 (5.0) 30.9 (3.5) 29.6 (4.0)

Pulmonary vascular resistance(cm H2O/L/min)

CMV 3.6 (1.4) 12.5 (3.8)a 12.0 (3.3) 9.4 (2.7) 8.8 (2.1) NS ".001 NSBVV 4.0 (1.2) 13.2 (3.9)a 13.8 (5.4) 14.2 (5.6) 12.1 (5.2)CMVs 3.6 (1.1) 14.2 (4.2)a 17.7 (4.7) 17.2 (5.7) 15.5 (6.1)BVVs 3.9 (1.5) 11.6 (4.6)a 15.7 (6.3) 12.6 (6.2) 13.3 (5.4)

Pulmonary capillary wedgepressure (mm Hg)

CMV 8.5 (1.4) 9.6 (2.2) 9.4 (1.8) 9.7 (1.6) 9.8 (1.5) NS NS NSBVV 8.3 (1.0) 8.0 (0.8) 8.1 (0.9) 8.5 (1.0) 8.5 (1.3)CMVs 7.7 (1.6) 9.5 (1.3) 9.3 (1.0) 8.6 (0.9) 9.6 (1.3)BVVs 8.1 (1.0) 7.7 (1.1) 7.6 (1.1) 8.6 (1.2) 8.6 (1.4)

Temperature (°C)CMV 36.5 (0.4) 37.3 (0.6)a 37.3 (0.7) 37.0 (0.5) 36.8 (0.6) ".03 ".001 .02BVV 36.8 (0.5) 36.8 (0.9) 36.7 (1.1) 36.8 (0.8) 37.0 (0.5)CMVs 37.0 (0.9) 37.5 (1.1)a 37.3 (1.0) 37.3 (1.0)c 37.6 (0.9)c

BVVs 37.1 (1.0) 37.8 (0.9)a 37.9 (0.9)c 38.1 (0.7)c 38.4 (0.9)c

Postmortem wet weight/dry weight

CMV 7.0 (1.0) NSBVV 6.6 (1.0) NSCMVs 7.3 (0.6) NSBVVs 7.4 (1.0) NS

CMV, conventional mechanical ventilation; BVV, biologically variable ventilation; CMVs, CMV with surfactant replacement; BVVs, BVV with surfactantreplacement; NS, not significant.

Hemodynamics and core temperature (SD) are shown for each time period and ventilation mode.ap ".05 compared to baseline, bp " .05 compared to other groups, cp " .05 surfactant vs. control groups.


release of inflammatory mediators (30).Although less injurious RM techniquesare under investigation (31, 32), BVV mayprovide an effective alternative.

Two models have been proposed toaccount for volume gain with variableventilation strategies based on the non-linear pressure–volume dynamics of thedegassed or injured lung. Suki et al (10)invoke the principle of stochastic reso-

nance—noise enhancement of an inputsignal to augment output in a nonlinearsystem. The second model generalizesthis finding to the pressure volume curveof the ARDS lung on the basis of themathematical principle of Jensen’s in-equality (9). Both models invoke the con-vex shape of the reinflation pressure–volume curve at low inflation pressureswhereby the volume gained by the impo-

sition of larger than average breaths isgreater than the volume lost with smallerthan average breaths. This situation ap-plies with variable ventilation strategiesusing low VT in ARDS where ventilationoccurs over the lower convex portion ofthe pressure–volume curve. Applicationof this principle to the pressure–volumecurve obtained from an OA-injured pigusing the pressure variability obtainedfrom a BVV file yields a calculated 12%increase in volume at the same averageinput pressure, consistent with the 15%BVV-induced recruitment seen in thepresent study.

Hyperaeration. Previously docu-mented persistence of hyperaerated re-gions despite the use of lower VT in ARDSis confirmed in the present study (33).Hyperaerated regions account for approx-imately 14% of the total gas volume and1.2% of the lung mass at baseline OAinjury and persist with BVV, BVVs, andCMVs at 4 hrs. The persistence of hype-raeration despite lower average peak Pawand restoration of both compliance andthe stress index to baseline with BVValone suggests that separation of recruit-ment from hyperinflation is challengingin a focal injury lung model. Supporting

Figure 2. Bar graph displaying percent recruitment of the nonaerated region, poorly aerated region,and nonaerated plus poorly aerated regions combined (defined as the change in mass of each regionover the total mass from baseline injury to 4 hrs) for each ventilation mode. Lines delineate the SD.An open star indicates p " .05 vs. other groups. BVV, biologically variable ventilation; CMV,conventional mechanical ventilation.

Table 4. Total and regional lung and gas volumes for each ventilation mode at baseline oleic acid lung injury and after 4 hrs of ventilation

Region andVentilation Mode

Mass (g) Total Volume (mL) Tissue Volume (mL) Gas Volume (mL)

Post-OA 4 hrs Post-OA 4 hrs Post-OA 4 hrs Post-OA 4 hrs

TotalCMV 306.75 (64.1) 287.3 (45.5) 815.7 (112.1) 816.7 (146.8) 303.1(61.4) 295.3 (58.1) 520.3 (88.4) 522.6 (77.7)BVV 289.22 (78.1) 272.64 (70.1) 813.7 (76.6) 917.0 (61.1)a 283.7 (70.7) 276.4 (58.2) 518.8 (54.9) 626.9 (93.2)a,b

CMVs 310.0 (65.4) 312 (48.4) 847.4 (83.9) 901.7 (101.4)a 299.4 (63.4) 310.2 (43.2) 536.5 (74.5) 577.6 (95.1)a

BVVs 301.0 (50.0) 291.7 (51.0) 835.6 (102.1) 881.3 (96.8)a 296.6 (47.8) 291.7 (45.3) 527.3 (86.0) 573.9 (94.1)a

Normally aeratedCMV 157.0 (19.8) 152.9 (25.4) 539.7 (97.7) 552.5 (76.7) 131.0 (23.9) 134.5 (19.3) 404.7 (73.3) 414.3 (57.5)BVV 151.8 (12.5) 176.6 (24.5)a 539.0 (67.1) 671.3 (119.9)a,b 134.8 (16.8) 167.8 (30.0)a,b 404.3 (50.3) 503.5 (89.9)a

CMVs 153.6 (17.3) 166.3 (23.7) 539.1 (77.9) 593.0 (104.9)a 135.0 (19.4) 148.3 (26.2) 404.8 (58.2) 444.8 (78.7)a

BVVs 157.0 (19.8) 160.0 (23.0) 543.3 (93.0) 587.8 (104.1)a 135.9 (23.3) 147.1 (26.1)a 407.6 (69.8) 441.3 (78.3)a

Poorly aeratedCMV 110.2 (98.7) 98.72 (32.5) 156.8 (64.8) 152.5 (54.8) 126.4 (48.5) 134.5 (19.3) 39.2 (16.2) 38.2 (13.7)BVV 87.9 (40.9) 66.48 (51.7)a 134.7 (59.4) 98.5 (80.1)a 101.0 (44.1) 77.7 (57.2) 33.7 (14.9) 25.9 (19.1)CMVs 109.8 (44.9) 103.1 (31.4) 167.2 (65.6) 158.8 (45.2) 118.9 (53.3) 119.1 (33.9) 48.3 (21.6) 39.7 (11.3)BVVs 107.7 (87.7) 87.7 (37.6) 165.7 (59.5) 133.4 (52.5)a 124.4 (44.7) 100.0 (39.4)a 41.5 (14.9) 30.3 (11.4)

NonaeratedCMV 34.6 (17.3) 33.4 (25.9) 27.1 (4.5) 22.5 (4.5) 37.8 (33.2) 29.2 (23.0) 2.7 (0.7) 2.3 (0.9)BVV 46.0 (23.9) 23.9 (35.5)a 43.9 (4.5) 22.4 (4.5)a,b 39.4 (38.1) 20.2 (30.2)a 4.4 (4.2) 2.3 (3.4)a

CMVs 42.9 (25.3) 38.5 (30.0) 40.9 (4.1) 36.5 (4.1)b 36.8 (22.0) 32.8(26.0) 4.1 (2.4) 3.6 (2.9)BVVs 32.8 (19.6) 39.6 (26.4)a 31.2 (3.8) 37.7 (3.8)b 28.1 (17.2) 34.0 (23.7) 3.1 (1.9) 3.8 (1.9)

HyperaeratedCMV 3.3 (0.9) 2.9 (0.8) 81.8 (28.0) 75.4 (25.3) 8.0 (2.5) 7.4 (2.3) 73.7 (25.2) 67.9 (22.9)BVV 3.5 (1.0) 4.7 (1.8)a 84.9 (18.8) 105.8 (33.5)a 8.5 (1.9) 10.6 (3.3)a 76.5 (16.8) 95.2 (30.1)a

CMVs 3.7 (1.1) 4.2 (1.7) 88.0 (22.2) 99.4 (36.1) 8.8 (2.2) 9.9 (3.6)a 79.2 (25.7) 89.4 (32.5)a

BVVs 3.4 (1.1) 4.4 (1.4)a 83.2 (28.7) 104.9 (34.0)a 8.4 (2.9) 10.6 (3.3)a 75.2 (25.7) 95.4 (30.0)a

CMV, conventional mechanical ventilation; BVV, biologically variable ventilation; CMVs, CMV with surfactant replacement; BVVs, BVV with surfactantreplacement; OA, oleic acid.

Interactions are presented in Results section.ap " .05 compared to post-OA, bp " .05 compared to other groups.


Figure 4. Graph of surfactant distribution, determined as the percentage of cut surface area stainedgreen vs. vertical height in centimeters above the lung base. Each point represents the average for eachgroup. Lines represent the SD. Squares represent biologically variable ventilation (BVV), and diamondsrepresent conventional mechanical ventilation (CMV). *Significance group times distance interaction.The distribution of green dye labeled surfactant is shifted to the more basal segments with BVVcompared to CMV.

Figure 5. (Top) Representative example of greendye labeled surfactant distribution in one con-ventional mechanical ventilation (CMV) lung.Sections obtained from the apical region (16 cmabove the base) (A), midlung region (10 cm abovethe base) (B), and basal region (2 cm above thebase) (C). Patchy lobular distribution of surfac-tant with areas of both atelectasis and aerationare evident. In this example, no green dye labeledsurfactant is present in the basal region and dis-tribution of surfactant is equivalent in the mid-lung and apical regions. (Bottom) Representativeexample of green dye labeled surfactant distribu-tion in one biologically variable ventilation (BVV)lung. Sections obtained at equivalent distancesabove the lung base as in the CMV example.Increased total surfactant is present in this cutsection compared to that in the top panel. Patchylobular distribution and greater degree of aera-tion are evident. In this example, minimal sur-factant is present in the apical region. The ma-jority of green dye labeled surfactant is present inthe midlung section, and more surfactant is pres-ent in the basal section compared to that of theCMV example.

Figure 3. Percent change in compliance from baseline to 4 hrs plotted against the percent change inthe normally aerated region (top) and percent recruitment of both nonaerated and poorly aeratedregions (bottom) for all animals: (filled squares) conventional mechanical ventilation, (open squares)conventional mechanical ventilation with surfactant replacement, (filled diamonds) biologically vari-able ventilation, (open diamonds) biologically variable ventilation with surfactant replacement.


this, Carvalho et al (34) were unable tofind a level of PEEP at which recruitmentwas maximized where nontidal hyperin-flation did not occur.

Surfactant Replacement. Despite thetheoretical advantages of coupling surfac-tant replacement and BVV, we could notdemonstrate a significant recruitment,mechanical, or gas exchange benefit inthis porcine OA model. Although totalgas volume was improved with bothCMVs and BVVs compared to CMV alone,BVVs was not superior to CMVs, and nei-ther was it as beneficial as BVV alone.Importantly, peak inflation pressureswere greater in both surfactant groups,and paradoxically, gas exchange and lungmechanics were impaired to a greater ex-tent in the immediate period followingsurfactant administration with BVVs. Theparadoxic increase in nonaerated and hy-peraerated regions with BVVs, with initialworsening of gas exchange and an in-crease in airway pressures, suggests that,in this exudative OA injury model, sur-factant administration coupled with BVVmay, in fact, be detrimental.

Surfactant Distribution. Analysis ofgreen dye labeled surfactant is a novelapproach to determine surfactant distri-bution grossly. Krause et al (17) havepreviously shown that green dye bindinghas no effect on surfactant function, ox-ygenation, or ventilation and preservesits color and adherence to exogenous sur-factant during standard microscopy. Theinability to discriminate on the basis ofthe density of the green marker or distin-guish between active and inactivate sur-factant is a limitation of the analysis. Thetheoretical recruitment advantage of BVVto promote more homogeneous surfac-tant distribution was not borne out byeither gross examination of green dye la-beled surfactant or CT imaging. Diemelet al (35) demonstrated histologicallypreferential distribution of fluorescentlylabeled surfactant to underinflated andaerated units but not to atelectatic areas.The increase in total gas volume and nor-mally aerated regional gas volume witheither BVVs or CMVs associated with nosignificant recruitment of poorly ornonaerated regions is consistent withthese results.

Although delivered surfactant was notmore homogeneous with BVV (Fig. 3),the shift in surfactant distribution to themore caudal (and more injured) lungwith BVV may paradoxically account forthe lack of benefit observed. Preferentialdelivery of surfactant to areas of greater

injury may increase exposure of deliveredsurfactant to inflammatory exudate, re-sulting in greater inactivation. Alterna-tively, recruitment of atelectatic areas be-fore surfactant delivery may mobilizeedema fluid into the airways with surfac-tant inhibition at this site. Grossly, weobserved dense green plugs in both largeand small airways in most lung slicesexamined, consistent with this hypothe-sis. Confirmation would require an anal-ysis of surfactant biophysical propertiesand function.

Surfactant Inactivation. In uninjuredguinea pig lung, variable ventilation in-creased endogenous surfactant levelstwofold and attenuated release of inflam-matory mediators (15). In a saline lavagemodel, Bellardine and colleagues (4)show significantly fewer neutrophils inbronchoalveolar lavage fluid with variableventilation. Using variable pressure sup-port ventilation and saline lavage, Spiethet al (6) documented an attenuation oflung histologic damage without increasesin gene expression or release of proin-flammatory markers of lung injury. Sa-line lavage provides a model of surfactantdepletion with less inflammatory re-sponse than OA injury. Our negative re-sults suggest that these salutary effectsmay be less apparent in an OA modeldue to the greater amount of inflamma-tory edema present. Supporting this,Boker et al (2) report much higher in-flammatory mediator interleukin-6concentrations with OA (2000 –7000 pg/mL) compared to saline lavage models(250 –1400 pg/mL) (15). Finally, wecannot exclude the possibility that en-dogenous– exogenous surfactant inhib-itory interactions may have occurred.Direct measurement of inflammatorymediators and surfactant functionwould be required to confirm/refutethese arguments.

CONCLUSION

CT analysis provides definitive evi-dence for modest but effective recruit-ment of atelectatic and poorly aeratedlung regions with BVV in this porcineARDS model. Surfactant replacementconferred no additional benefit. BVV plussurfactant was associated with a worsen-ing of respiratory system compliance, ox-ygenation, and shunt associated with anincrease in the total volume of thenonaerated and hyperaerated regions de-lineated by CT. Of the four ventilationstrategies studied, BVV alone was the

most effective in restoring gas exchangeand lung mechanics and demonstratedthe most positive effect on lung recruit-ment over the short 4-hr period of thisexperiment.

ACKNOWLEDGMENTS

We gratefully acknowledge GeraldStelmack, PhD, and Andrew J. Halayko,PhD (Department of Physiology and In-ternal Medicine, University of Manitoba,Winnipeg, Manitoba, Canada), for assis-tance with the Photoshop (Adobe, SanJose, CA) image analysis, Brenden Du-four, MSc (University of Manitoba, Win-nipeg, Manitoba, Canada), for statisticalanalysis, and Jonathan Mutch (OakheadOntario College of Art and Design, To-ronto, Ontario, Canada) for preparationof the lung images for publication.

REFERENCES

1. Lefevre GR, Kowalski SE, Girling LG, et al:Improved arterial oxygenation after oleicacid lung injury in the pig using a computer-controlled mechanical ventilator. Am J Re-spir Crit Care Med 1996; 154:1567–1572

2. Boker A, Graham MR, Walley KR, et al: Im-proved arterial oxygenation with biologicallyvariable or fractal ventilation using low tidalvolumes in a porcine model of acute respira-tory distress syndrome. Am J Respir CritCare Med 2002; 165:456–462

3. Arold SP, Mora R, Lutchen KR, et al: Variabletidal volume ventilation improves lung me-chanics and gas exchange in a rodent modelof acute lung injury. Am J Respir Crit CareMed 2002; 165:366–371

4. Bellardine CL, Hoffman AM, Tsai L, et al:Comparison of variable and conventionalventilation in a sheep saline lavage lung in-jury model. Crit Care Med 2006; 34:439–445

5. Ingenito EP, Arold S, Lutchen K, et al: Ef-fects of noisy ventilation (NV) and open lungventilation (OLV) on lung mechanics, gasexchange, and surfactant content and prop-erties. Am J Respir Crit Care Med 2001; 163:A483

6. Spieth PM, Carvalho AR, Pelosi P, et al: Vari-able tidal volumes improve lung protectiveventilation strategies in experimental lunginjury. Am J Respir Crit Care Med 2009;179:684–693

7. Thammanomai A, Majumdar A, Bartolak-Suki E, et al: Effects of reduced tidal volumeventilation on pulmonary function in micebefore and after acute lung injury. J ApplPhysiol 2007; 103:1551–1559

8. Mutch WA, Harms S, Ruth Graham M, et al:Biologically variable or naturally noisy me-chanical ventilation recruits atelectatic lung.Am J Respir Crit Care Med 2000; 162:319–323

9. Brewster JF, Graham MR, Mutch WA: Con-


vexity, Jensen’s inequality and benefits ofnoisy mechanical ventilation. J R Soc Inter-face 2005; 2:393–396

10. Suki B, Alencar AM, Sujeer MK, et al: Life-support system benefits from noise. Nature1998; 393:127–128

11. Poynter SE, LeVine AM: Surfactant biologyand clinical application. Crit Care Clin 2003;19:459–472

12. Davidson WJ, Dorscheid D, Spragg R, et al:Exogenous pulmonary surfactant for thetreatment of adult patients with acute respi-ratory distress syndrome: results of a meta-analysis. Crit Care 2006; 10:R41

13. Adhikari N, Burns KE, Meade MO: Pharma-cologic treatments for acute respiratory dis-tress syndrome and acute lung injury: Sys-tematic review and meta-analysis. TreatRespir Med 2004; 3:307–328

14. Krause MF, Jakel C, Haberstroh J, et al: Al-veolar recruitment promotes homogeneoussurfactant distribution in a piglet model oflung injury. Pediatr Res 2001; 50:34–43

15. Arold SP, Suki B, Alencar AM, et al: Variableventilation induces endogenous surfactantrelease in normal guinea pigs. Am J PhysiolLung Cell Mol Physiol 2003; 285:L370–L375

16. Gattinoni L, Caironi P, Pelosi P, et al: Whathas computed tomography taught us aboutthe acute respiratory distress syndrome?Am J Respir Crit Care Med 2001; 164:1701–1711

17. Krause MF, Orlowska-Volk M, Hendrik ER, etal: A new simple method of staining exoge-nous surfactant in experimental research.Eur Respir J 2000; 15:949–954

18. Grasso S, Stripoli T, De Michele M, et al:ARDSnet ventilatory protocol and alveolarhyperinflation: Role of positive end-expira-tory pressure. Am J Respir Crit Care Med2007; 176:761–767

19. Funk DJ, Graham MR, Girling LG, et al: A

comparison of biologically variable ventilationto recruitment manoeuvres in a porcine modelof acute lung injury. Respir Res 2004; 5:22

20. Mutch WA, Eschun GM, Kowalski SE, et al:Biologically variable ventilation prevents de-terioration of gas exchange during prolongedanaesthesia. Br J Anaesth 2000; 84:197–203

21. Mutch WA, Harms S, Lefevre GR, et al: Bio-logically variable ventilation increases arte-rial oxygenation over that seen with positiveend-expiratory pressure alone in a porcinemodel of acute respiratory distress syn-drome. Crit Care Med 2000; 28:2457–2464

22. Thammanomai A, Hueser LE, Majumdar A,et al: Design of a new variable-ventilationmethod optimized for lung recruitment inmice. J Appl Physiol 2008; 104:1329–1340

23. Malbouisson LM, Muller JC, Constantin JM,et al: Computed tomography assessment ofpositive end-expiratory pressure-induced al-veolar recruitment in patients with acute re-spiratory distress syndrome. Am J Respir CritCare Med 2001; 163:1444–1450

24. Gama de Abreu M, Spieth PM, Pelosi P, et al:Noisy pressure support ventilation: A pilotstudy on a new assisted ventilation mode inexperimental lung injury. Crit Care Med2008; 36:818–827

25. Lu Q, Malbouisson LM, Mourgeon E, et al:Assessment of PEEP-induced reopening ofcollapsed lung regions in acute lung injury:Are one or three CT sections representativeof the entire lung? Intensive Care Med 2001;27:1504–1510

26. Constantin JM, Jaber S, Futier E, et al: Re-spiratory effects of different recruitment ma-neuvers in acute respiratory distress syn-drome. Crit Care 1909; 12:R50

27. Karmrodt J, Bletz C, Yuan S, et al: Quantifi-cation of atelectatic lung volumes in twodifferent porcine models of ARDS. Br J An-aesth 2006; 97:883–895

28. Martynowicz MA, Walters BJ, Hubmayr RD:Mechanisms of recruitment in oleic acid-injured lungs. J Appl Physiol 2001; 90:1744–1753

29. Pelosi P, Gama de Abreu M, Rocco PR: Newand conventional strategies for lung recruit-ment in acute respiratory distress syndrome.Crit Care 2010; 14:210

30. Steimback PW, Oliveira GP, Rzezinski AF, etal: Effects of frequency and inspiratory pla-teau pressure during recruitment manoeu-vres on lung and distal organs in acute lunginjury. Intensive Care Med 2009; 35:1120–1128

31. Rzezinski AF, Oliveira GP, Santiago VR, et al:Prolonged recruitment manoeuvre improveslung function with less ultrastructural dam-age in experimental mild acute lung injury.Respir Physiol Neurobiol 2009; 169:271–281

32. Odenstedt H, Lindgren S, Olegård C, et al:Slow moderate pressure recruitment maneu-ver minimizes negative circulatory and lungmechanic side effects: Evaluation of recruit-ment maneuvers using electric impedancetomography. Intensive Care Med 2005; 31:1706–1714

33. Terragni PP, Rosboch G, Tealdi A, et al: Tidalhyperinflation during low tidal volume ven-tilation in acute respiratory distress syn-drome. Am J Respir Crit Care Med 2007;175:160–166

34. Carvalho AR, Spieth PM, Pelosi P, et al: Abil-ity of dynamic airway pressure curve profileand elastance for positive end-expiratorypressure titration. Intensive Care Med 2008;34:2291–2299

35. Diemel RV, Walch M, Haagsman HP, et al: Invitro and in vivo intrapulmonary distributionof fluorescently labeled surfactant. Crit CareMed 2002; 30:1083–1090


quantitative computed tomography in porcine lung injury with

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