Background In the acute respiratory distress syndrome (ARDS),positive end-expiratory pressure (PEEP) may decrease ventilator-inducedlung injury by keeping lung regions open that otherwise wouldbe collapsed. Since the effects of PEEP probably depend on therecruitability of lung tissue, we conducted a study to examinethe relationship between the percentage of potentially recruitablelung, as indicated by computed tomography (CT), and the clinicaland physiological effects of PEEP.
Methods Sixty-eight patients with acute lung injury or ARDSunderwent whole-lung CT during breath-holding sessions at airwaypressures of 5, 15, and 45 cm of water. The percentage of potentiallyrecruitable lung was defined as the proportion of lung tissuein which aeration was restored at airway pressures between 5and 45 cm of water.
Results The percentage of potentially recruitable lung variedwidely in the population, accounting for a mean (±SD)of 13±11 percent of the lung weight, and was highly correlatedwith the percentage of lung tissue in which aeration was maintainedafter the application of PEEP (r2=0.72, P<0.001). On average,24 percent of the lung could not be recruited. Patients witha higher percentage of potentially recruitable lung (greaterthan the median value of 9 percent) had greater total lung weights(P<0.001), poorer oxygenation (defined as a ratio of partialpressure of arterial oxygen to fraction of inspired oxygen)(P<0.001) and respiratory-system compliance (P=0.002), higherlevels of dead space (P=0.002), and higher rates of death (P=0.02)than patients with a lower percentage of potentially recruitablelung. The combined physiological variables predicted, with asensitivity of 71 percent and a specificity of 59 percent, whethera patient's proportion of potentially recruitable lung was higheror lower than the median.
Conclusions In ARDS, the percentage of potentially recruitablelung is extremely variable and is strongly associated with theresponse to PEEP.
Acute respiratory distress syndrome (ARDS) is a clinical syndromecharacterized by inflammatory pulmonary edema, severe hypoxemia,stiff lungs, and diffuse endothelial and epithelial injury.1,2Mechanical ventilation is often implemented in these patientsto restore adequate oxygenation. However, it has become evidentover the past two decades that mechanical ventilation itselfcan augment or cause pulmonary damage that is indistinguishablefrom that caused by ARDS.3 As a consequence, the therapeutictarget of mechanical ventilation in patients with ARDS has shiftedfrom the maintenance of "normal gas exchange"4 to the protectionof the lung from ventilator-induced lung injury.5,6,7
The lung-protection strategy combines the use of higher levelsof positive end-expiratory pressure (PEEP) (greater than 12to 15 cm of water) and low tidal volumes to prevent regionaland global stress and strain on the lung parenchyma.8,9,10 Ventilationat low tidal volumes alone has been shown to increase survivalamong patients with acute lung injury or ARDS,11 and the additionof higher PEEP to low tidal volumes did not further increasesurvival.12 In patients with low levels of recruitable lung(i.e., lung tissue in which aeration can be restored),13,14,15,16however, the application of higher levels of PEEP may be moreharmful than beneficial, since it will serve only to increaseinflation of lung regions that are already open, increasingthe stress and strain on these regions.17 It follows that knowledgeof the capacity of the lung to become and remain recruited shouldbe a prerequisite for a rational determination of the levelsof PEEP to be applied.
Using computed tomography (CT) to analyze the entire lung inpatients with ARDS, we measured the percentage of lung thatcan be recruited, termed "potentially recruitable lung," byincreasing airway pressures.18,19 We also investigated the relationshipbetween the percentage of lung that can be recruited by thismaneuver and the changes in physiological respiratory variablesduring mechanical ventilation with lower or higher PEEP.
Methods
Patients
The patients were studied from June 2003 through January 2005at four university hospitals. The study was approved by theinstitutional review board of each hospital, and written informedconsent was obtained according to the national regulations ofthe participating institutions (consent was delayed in Italyuntil after the patients had recovered from the effects of sedation,obtained from a legal representative in Germany, and obtainedfrom the next of kin in Chile; for details see the Supplementary Appendix,available with the full text of this article at www.nejm.org).
Patients were enrolled if they met the standard criteria foracute lung injury: a ratio of the partial pressure of arterialoxygen to the fraction of inspired oxygen (PaO2:FIO2) of lessthan 300, the presence of bilateral pulmonary infiltrates onthe chest radiograph, and no clinical evidence of left atrialhypertension (defined by a pulmonary-capillary wedge pressureof 18 mm Hg or less, if measured).20 The exclusion criteriawere an age of less than 16 years, pregnancy, and chronic obstructivepulmonary disease, according to the patient's medical history.The underlying cause of acute lung injury or ARDS was recordedby each institution, but no specific classifications were defineda priori. Patients with healthy lungs and patients with unilateralpneumonia who underwent CT for clinical purposes from April2001 through June 2005 were retrospectively selected from fivehospitals and included in the study for comparison (Figure 1and the Supplementary Appendix).
In the study group, a recruitment maneuver was performed immediately before application of each PEEP level. In the comparison groups, patients with bilateral pneumonia were excluded from the analysis to limit the possible confounding factors caused by the partial overlapping between patients with less severe acute lung injury or ARDS and patients with bilateral pneumonia (see the Supplementary Appendix for further details). Therefore, only patients with unilateral pneumonia, who by definition did not meet the inclusion criteria for acute lung injury or ARDS, were included. The group with a lower percentage of potentially recruitable lung includes patients with potentially recruitable lung values at or below the overall median of 9 percent, and the group with a higher percentage of potentially recruitable lung includes patients with values above the median.
PEEP Trial
The clinical characteristics of the patients, respiratory variables,and ventilator settings were recorded before the study. Immediatelybefore each step of the PEEP trial, as well as before each CTsession, a recruitment maneuver — that is, a sustainedinflation of the lungs to higher airway pressures and volumesthan are obtained during tidal ventilation — was performedin which the patient underwent ventilation for two minutes inthe pressure-controlled mode at an inspiratory plateau pressureof 45 cm of water, a PEEP of 5 cm of water, a respiratory rateof 10 breaths per minute, and a 1:1 ratio of inspiration toexpiration.21,22 After the recruitment maneuver, PEEP at a levelof 5 or 15 cm of water was randomly applied (Figure 1). Thetidal volume (8 to 10 ml per kilogram of predicted body weight),FIO2, and respiratory rate were identical to the values usedin everyday clinical treatment. After 20 minutes, the systemicarterial and central venous pressures and blood gas tensions,minute ventilation, and inspiratory plateau pressure were recorded.The dead-space fraction and the end-tidal partial pressure ofcarbon dioxide were measured with a CO2SMO monitor (Novametrix).Standard formulas were used to calculate the right-to-left intrapulmonaryshunt fraction, alveolar dead-space fraction, and respiratory-systemcompliance (see the Supplementary Appendix).
Computed Tomography
The CT scanner was set as follows: collimation, 5 mm; interval,5 mm; bed speed, 15 mm per second; voltage, 140 kV; and current,240 mA. A whole-lung CT scan was performed at an inspiratory-plateaupressure of 45 cm of water during an end-inspiratory pause (rangingfrom 15 to 25 seconds) and thereafter at PEEP values of 5 and15 cm of water applied in a random order during an end-expiratorypause (ranging from 15 to 25 seconds). Immediately before eachCT scan was obtained, a recruitment maneuver was performed,as described above (Figure 1); the ventilator settings wereotherwise kept identical to those used during the PEEP trial.The patients included in the comparison groups underwent onlyone CT of the whole lung, for diagnostic purposes. The cross-sectionallung images were processed and analyzed by a custom-designedsoftware package, as described previously19 (see the Supplementary Appendix).Briefly, the outline of the lungs was manually drawn in eachimage, excluding the hilar vessels, by investigators unawareof the airway pressure applied. Specific lung weight was assumedto be equal to 1, and the total lung weight was calculated fromthe physical density of the lung expressed in Hounsfield units.Similarly, the tissue weights of lung regions with differentdegrees of aeration were calculated. The regions were classifiedas nonaerated (density between +100 and –100 Hounsfieldunits), poorly aerated (density between –101 and –500Hounsfield units), normally aerated (density between –501and –900 Hounsfield units), and hyperinflated (densitybetween –901 and –1000 Hounsfield units). The percentageof potentially recruitable lung was defined as the proportionof the total lung weight accounted for by nonaerated lung tissuein which aeration was restored (according to CT) by an airwaypressure of 45 cm of water from an airway pressure of 5 cm ofwater.
Statistical Analysis
Comparison of prestudy clinical variables, respiratory physiologicalvariables, and CT results was performed by one-way analysisof variance or Student's t-test in the case of variables thatwere normally distributed; by the Kruskal–Wallis test,the Wilcoxon test, or two-way analysis of variance on a rank-sumtest in the case of variables that did not appear normally distributedon graphic inspection; and by the chi-square test or Fisher'sexact test in the case of qualitative variables. When analysisof variance revealed a significant difference, Bonferroni'st-test or Dunn's test was used, as appropriate, to correct formultiple comparisons. Mortality rates were analyzed by the chi-squaretest. Mortality rates were based on the number of deaths occurringin the intensive care unit (ICU) among patients with acute lunginjury or ARDS and the number of deaths occurring in the hospitalamong patients with unilateral pneumonia. Multiple backwardlogistic-regression analysis was used to investigate the possibleassociation between outcome and the percentage of potentiallyrecruitable lung, as well as other measurements used to estimatethe severity of the systemic illness and of the lung injury.The Hosmer–Lemeshow goodness-of-fit test and the C statisticwere used to verify the adequacy of the models.
To obtain a bedside estimate of the percentage of potentiallyrecruitable lung using only physiological respiratory measurements,we measured the changes in the PaO2:FIO2, the partial pressureof arterial carbon dioxide (PaCO2), the percentage of alveolardead space, and respiratory-system compliance associated withincreasing the PEEP from 5 to 15 cm of water while minute ventilationand FIO2 were held constant. An increase in the PaO2:FIO2, adecrease in the PaCO2 or alveolar dead space, or an increasein respiratory-system compliance was defined as a positive response,and any change in the opposite direction was defined as a negativeresponse, irrespective of the magnitude of the change. P valuesof less than 0.05 were considered to indicate statistical significance.All reported P values are two-sided. Data are expressed as means(±SD) and 95 percent confidence intervals when appropriate.
Results
A total of 68 patients were enrolled in the study: 19 had acutelung injury without ARDS, and 49 had ARDS (Figure 1 and Table 1).The overall mortality rate in the ICU among the study populationwas 28 percent. The percentage of potentially recruitable lung,as assessed by CT, varied widely within the study population(Figure 2); the average was 13±11 percent of the totallung weight (95 percent confidence interval, 10 to 16 percent;median, 9 percent), corresponding to an absolute weight of 217±232g of recruitable lung tissue (95 percent confidence interval,161 to 273; median, 134).
Figure 2. Frequency Distribution of Patients According to the Percentage of Potentially Recruitable Lung (Panel A) and CT Images at Airway Pressures of 5 and 45 cm of Water from Patients with a Lower Percentage of Potentially Recruitable Lung (Panel B) and Those with a Higher Percentage of Potentially Recruitable Lung (Panel C).
Panel A shows the frequency distribution of the 68 patients in the overall study group according to the percentage of potentially recruitable lung, expressed as the percentage of total lung weight. Acute lung injury without ARDS was defined by a PaO2:FIO2 of less than 300 but not less than 200, and ARDS was defined by a PaO2:FIO2 of less than 200. The percentage of potentially recruitable lung was defined as the proportion of lung tissue in which aeration is restored at airway pressures between 5 and 45 cm of water. Panel B shows representative CT slices of the lung obtained 2 cm above the diaphragm dome at airway pressures of 5 cm of water (left) and 45 cm of water (right) from a patient with a lower percentage of potentially recruitable lung (at or below the median value of 9 percent of total lung weight). Lung injury developed in the patient after an episode of severe acute pancreatitis (PaO2:FIO2, 296 at an airway pressure of 5 cm of water; PaCO2, 34 mm Hg; and respiratory-system compliance, 44 ml per centimeter of water). The percentage of potentially recruitable lung was 4 percent, and the proportion of consolidated lung tissue was 33 percent of the total lung weight. Panel C shows representative CT slices of the lung obtained 2 cm above the diaphragm dome at airway pressures of 5 cm of water (left) and 45 cm of water (right) from a patient in the group with a higher percentage of potentially recruitable lung. Lung injury developed in the patient after an episode of severe pneumonia (PaO2:FIO2, 106 at a PEEP of 5 cm of water; PaCO2,58 mm Hg; and respiratory-system compliance, 25 ml per cm of water). The percentage of potentially recruitable lung was 37 percent, and the proportion of consolidated lung tissue was 27 percent of the total lung weight.
Functional Anatomy According to CT Findings and Response to PEEP
The study population was divided into quartiles according tothe percentage of potentially recruitable lung (Figure 3A);the average values were 2±4 percent of total lung weightin quartile 1 (range, –9.2 to 5.7 percent), 7±1percent in quartile 2 (range, 5.8 to 9.4 percent), 14±3percent in quartile 3 (range, 9.5 to 18.6 percent), and 28±10percent in quartile 4 (range, 18.7 to 59.3 percent). In eachgroup, the increase in airway pressure from 5 to 15 to 45 cmof water induced a progressive increase in the percentage ofhyperinflated and normally aerated lung tissue (P<0.01 forboth variables), paralleled by a decrease in the percentageof nonaerated lung tissue (P<0.01). In contrast, in all fourgroups, about 24 percent of the lung could not be recruited,even at an airway pressure of 45 cm of water.
Figure 3. Lung Recruitment in Response to Changes in Airway Pressure in Patients with Acute Lung Injury or ARDS, According to the Percentage of Potentially Recruitable Lung.
Panel A shows the proportion of total lung tissue classified as nonaerated, poorly aerated, normally aerated, and hyperinflated in response to three values of airway pressure in patients with different percentages of potentially recruitable lung. The study population was divided into quartiles of 17 patients each according to the percentage of potentially recruitable lung. Nonaerated lung tissue was also divided into potentially recruitable tissue (nonaerated tissue in which aeration was restored at airway pressures between 5 and 45 cm of water) and consolidated tissue (tissue remaining nonaerated despite an airway pressure of 45 cm of water). The asterisk denotes P<0.01 for the comparison with patients with a very low or low percentage of potentially recruitable lung (first and second quartiles), the daggers P<0.01 for the comparison with patients in the other quartiles, the double dagger P<0.01 for the comparison with patients with a very low percentage of potentially recruitable lung (first quartile), the section marks P<0.05 for the comparison with an airway pressure of 45 cm of water in patients within the same quartile, the paragraph mark P<0.01 for the comparison with airway pressures of 15 and 45 cm of water for patients within the same quartile, and the double slashes P<0.05 for the comparison with a PEEP value of 15 cm of water for patients within the same quartile. Panel B shows lung recruitment induced by increasing PEEP from 5 to 15 cm of water in the overall study population — that is, the decrease in nonaerated lung tissue between PEEP values of 5 and 15 cm of water, as a function of the percentage of potentially recruitable lung; both values are expressed as proportions of the total lung weight measured at a baseline PEEP of 5 cm of water (r2=0.72, P<0.001, slope=0.52 and y intercept=1.03). A linear function (y=ax+y0) was used.
The decrease in the percentage of nonaerated lung tissue asPEEP was raised from 5 to 15 cm of water was highly correlatedwith the percentage of potentially recruitable lung (r2=0.72,P<0.001) (Figure 3B). The near-constant fraction of the percentageof potentially recruitable lung that remained recruited at aPEEP of 15 cm of water was about 50 percent, irrespective ofits absolute percentage, as indicated by the slope of the plotin Figure 3B.
Clinical Characteristics and Overall Severity of Lung Injury
We divided the patients into two groups according to the percentageof potentially recruitable lung: at or below the median valueof 9 percent of total lung weight or greater than the medianvalue for the study population. In the prestudy period, thetwo groups had similar clinical characteristics with regardto age, severity of illness (as assessed by the Simplified AcutePhysiology Score [SAPS II]23), daily fluid balance, and numberof days of mechanical ventilation before the beginning of thestudy (Table 1). The tidal volume and PEEP level used clinicallyfor mechanical ventilation were similar in the two groups. Incontrast, at baseline, patients in the group with a higher percentageof potentially recruitable lung had a lower PaO2:FIO2 (P=0.008),a higher PaCO2 (P=0.04), and lower respiratory-system compliance(P=0.02) than those in the group with a lower percentage ofpotentially recruitable lung (Table 1). Acute lung injury orARDS resulting from sepsis was more frequent among patientsin the group with a lower percentage of recruitable lung (P=0.02),whereas acute lung injury or ARDS resulting from pneumonia wasmore frequent among patients in the group with a higher percentage(P=0.01) (Table 1 and the Supplementary Appendix).
The association between the percentage of potentially recruitablelung and the severity of the overall lung injury was examinedat a PEEP of 5 cm of water. The total lung weight was greater(P<0.001), the proportion of nonaerated lung tissue was higher(P=0.001), the PaO2:FIO2 was lower (P<0.001), the respiratory-systemcompliance was lower (P=0.002), the PaCO2 was higher (P=0.02),the percentage of dead space was higher (P=0.002), the shuntfraction was higher (P=0.008), and the mortality rate was higher(P=0.02) in the group with a higher percentage of potentiallyrecruitable lung than in the group with a lower percentage ofpotentially recruitable lung (Table 2).
Table 2. Baseline Characteristics, Functional Anatomy According to CT Findings, and Mortality Rates in Patients with Healthy Lungs, Patients with Unilateral Pneumonia, and Patients with Acute Lung Injury or ARDS and with Lower or Higher Percentages of Potentially Recruitable Lung.
An association was observed between the percentage of potentiallyrecruitable lung and the risk of death (Figure 4A). To investigatewhether there was an association between mortality and othermeasurements of severity of illness, a multivariate analysisfor independent predictors of mortality was performed. The SAPSII score was included as a marker of the overall severity ofthe systemic illness, and the percentage of potentially recruitablelung, the percentage of nonaerated lung tissue, the PaO2:FIO2,the PaCO2, and the respiratory-system compliance at a PEEP of5 cm of water were included as markers of the severity of lunginjury. The SAPS II score and the percentage of potentiallyrecruitable lung appeared to be independently associated withan increased risk of death (P=0.47 by the Hosmer–Lemeshowgoodness-of-fit test; C=0.78); the odds ratios for each one-pointincrease in the SAPS II score and in the percentage of potentiallyrecruitable lung were 1.08 (95 percent confidence interval,1.02 to 1.15) and 1.08 (95 percent confidence interval, 1.01to 1.14), respectively.
Figure 4. Mortality in Relation to the Percentage of Potentially Recruitable Lung (Panel A) and Pulmonary Anatomy According to CT Findings in Patients with Healthy Lungs, Patients with Unilateral Pneumonia, and Patients with Acute Lung Injury or ARDS (Panel B).
Panel A shows the mortality rate in the ICU (mean length of stay, 29±27 days; range, 2 to 163) among patients with acute lung injury or ARDS (P=0.34 by the Hosmer–Lemeshow goodness-of-fit test; C=0.72). Results are shown for quartiles of 17 patients each according to the percentage of potentially recruitable lung. Panel B shows the weights of total lung tissue and nonaerated lung tissue in 39 patients with healthy lungs, 34 patients with unilateral pneumonia, 34 patients with acute lung injury or ARDS with a lower percentage of potentially recruitable lung (at or below the overall median value of 9 percent), and 34 patients with acute lung injury or ARDS with a higher percentage of potentially recruitable lung. Data from the patients with healthy lungs and unilateral pneumonia were obtained from a whole-lung CT obtained for diagnostic purposes. Data from patients with acute lung injury or ARDS were obtained from a whole-lung CT performed at a PEEP of 5 cm of water. Solid lines represent mean values of total lung weight, and dashed lines mean values of nonaerated lung-tissue weight. Asterisks denote P<0.01 for the comparison with patients with healthy lungs; daggers denote P<0.01 for the comparison between the groups of patients with acute lung injury or ARDS and a higher percentage of potentially recruitable lung and the other three groups.
We further characterized these findings as specific for acutelung injury or ARDS by retrospectively evaluating a group of39 patients with healthy lungs and a group of 34 patients withunilateral pneumonia (20 patients who were breathing spontaneouslyand 14 patients who were undergoing mechanical ventilation)(see the Supplementary Appendix) and comparing the findingswith those of the study population (Figure 1). Among patientswith either healthy lungs or unilateral pneumonia, the totallung weight and the proportion of nonaerated lung tissue werelower and the proportion of normally aerated lung tissue washigher than among the overall population of patients with acutelung injury or ARDS (P<0.01 for all comparisons). However,the greatest differences between patients with unilateral pneumoniaand patients with acute lung injury or ARDS was among the patientsin the latter group who had a higher percentage of recruitablelung. The functional anatomy, as determined by CT, appearedto be very similar in patients with unilateral pneumonia andpatients with acute lung injury or ARDS and a lower percentageof recruitable lung (Table 2 and Figure 4B).
Prediction of the Percentage of Potentially Recruitable Lung
To provide a bedside estimate of the percentage of potentiallyrecruitable lung, we initially hypothesized that in patientswith a higher percentage of potentially recruitable lung, atleast two of the following three changes in respiratory variableswould occur when PEEP was increased from 5 to 15 cm of water:an increase in the PaO2:FIO2, a decrease in the PaCO2, or anincrease in the respiratory-system compliance. However, thepower of this test to predict which patients had a higher percentageof potentially recruitable lung had a sensitivity of 71 percentand a specificity of 59 percent. A post hoc analysis was usedto evaluate other combinations of different physiological respiratoryvariables that were tested as predictors of the percentage ofpotentially recruitable lung. Among these combinations, a PaO2:FIO2of less than 150 at a PEEP of 5 cm of water had a sensitivityof 74 percent and a specificity of 79 percent. The combinationof variables that yielded the best results appeared to be thepresence of at least two of the following: a PaO2:FIO2 of lessthan 150 at a PEEP of 5 cm of water, any decrease in alveolardead space, and an increase in respiratory-system compliancewhen PEEP was increased from 5 to 15 cm of water (sensitivity,79 percent; specificity, 81 percent) (see the Supplementary Appendix).
Discussion
CT revealed that the percentage of potentially recruitable lungvaried widely among patients with acute lung injury or ARDS,from a negligible fraction to more than 50 percent of the totallung weight. Furthermore, we demonstrated that the effect ofPEEP on lung recruitment was closely associated with the percentageof potentially recruitable lung and that the percentage of potentiallyrecruitable lung was itself highly correlated with the overallseverity of lung injury.
In clinical practice, lung recruitment is usually considereda useful strategy.8,9,24 For this reason, it has been suggestedthat the condition of patients with a high percentage of potentiallyrecruitable lung is better than that of patients with a lowerpercentage of potentially recruitable lung, given the presenceof similar degrees of lung injury. Surprisingly, among our patients,a higher percentage of potentially recruitable lung correlatedwith markedly poorer gas exchange and respiratory mechanics,a greater severity of lung injury, and a higher mortality rate,even though the severity of their systemic illness at studyentry, as assessed by the SAPS II score, was similar in patientswith higher and those with lower percentages of potentiallyrecruitable lung (see the Supplementary Appendix). An associationbetween the percentage of potentially recruitable lung and theseverity of lung injury, although unexpected, appears logical.In healthy lungs, the percentage of potentially recruitablelung is close to 0 percent, because the alveolar units are usuallynot collapsed. When ARDS affects the lungs, the extent of theinflammatory pulmonary edema is linked to the likelihood ofgravity-dependent alveolar collapse18,25 and thus to the percentageof potentially recruitable lung. It is tempting to speculatethat the "core disease" is reflected by the unrecruitable lungtissue at 45 cm of water (about 24 percent of the total lungweight), whereas the extent of the surrounding inflammatoryreaction26 is reflected by the collapsed but openable lung tissue— that is, the potentially recruitable lung.
The use of respiratory physiological variables that can be measuredat the bedside to ascertain the percentage of potentially recruitablelung was less specific and sensitive than expected. However,we think that analysis of CT findings can identify the increasein aeration of previously collapsed lung regions ("anatomical"lung recruitment19), whereas changes in respiratory physiologicalvariables are specifically related to "functional" recruitmentof lung tissue to participation in gas exchange — thatis, to an improvement in the overall ventilation–perfusionratio. The anatomical and the functional lung recruitment cancoincide only if the restoration of aeration of pulmonary units,as detected by CT, occurs in association with the absence ofa change in perfusion of the same units. Our data support thehypothesis that anatomical and functional lung recruitment areat least partially dissociated.
We believe that knowledge of the percentage of potentially recruitablelung may be important for establishing the therapeutic efficacyof PEEP. Setting levels of PEEP independently of the percentageof potentially recruitable lung, which was the strategy usedby Brower et al.,12 may offset the possible benefits of PEEP.Our data show that the use of higher PEEP levels in patientswith a lower percentage of potentially recruitable lung provideslittle benefit and may actually be harmful. To determine whetherdifferent levels of PEEP may affect the outcome among patientswith acute lung injury or ARDS, a formal study will be necessary,but it should be limited to patients with a higher percentageof potentially recruitable lung. Although the use of higherPEEP levels seems appropriate in these patients, it should beformally tested. Since about 60 percent of lung parenchyma isalready open to aeration in patients with a higher percentageof potentially recruitable lung, this portion of the lung maybe unnecessarily exposed to increased stress and strain withthe use of higher PEEP levels.27 While we wait for such a studyto be performed, in our daily practice we limit the use of PEEPlevels of more than 15 cm of water to patients with a higherpercentage of potentially recruitable lung28 and of PEEP levelsbelow 10 cm of water to those with a lower percentage of potentiallyrecruitable lung.
Dr. Gattinoni reports having received consulting and lecturefees from KCI; Dr. Ranieri reports serving as a consultant toMaquet and having received grant support from Tyco; and Dr.Quintel reports having received consulting fees from Siemens/Maquet,Novalung, Dräger Medical, Abbott Laboratories, and Gambro.No other potential conflict of interest relevant to this articlewas reported.
We are indebted to Angelo Colombo, M.D., Ph.D., of the TerapiaIntensiva Neuroscienze, Fondazione Istituto di Ricovero e Curaa Carattere Scientifico (IRCCS)–Ospedale Maggiore Policlinico,Mangiagalli, Regina Elena di Milano, Milan, for statisticaladvice; to Pietro Biondetti, M.D., Marco Lazzarini, M.D., BenedettaFinamore, M.D., and Cristian Bonelli of the Dipartimento diRadiologia, Fondazione IRCCS–Ospedale Maggiore Policlinico,Mangiagalli, Regina Elena di Milano, Milan, for technical assistancewith analysis of CT findings; to Milena Racagni, M.D., LauraLandi, M.D., Alice D'Adda, M.D., Serena Azzari, M.D., SoniaTerragni, M.D., Federico Polli, M.D., Paola Cozzi, M.D., GiulianaMotta, M.D., Federica Tallarini, M.D., Cristian Carsenzola,M.D., and Monica Chierichetti, M.D., of the Istituto di Anestesiologiae Rianimazione, Fondazione IRCCS–Ospedale Maggiore Policlinico,Mangiagalli, Regina Elena di Milano, Università degliStudi di Milano, Milan, for help with the data analysis; toFerdinando Raimondi, M.D., of the I Servizio di Anestesia eRianimazione, Ospedale Luigi Sacco di Milano, Milan; AntonioPesenti, M.D., of the Dipartimento di Medicina Perioperatoriae Terapia Intensiva, Azienda Ospedaliera S. Gerardo di Monza,Università degli Studi Milano–Bicocca, Milan; RobertoFumagalli, M.D., of the Dipartimento di Anestesia e Rianimazione,Ospedali Riuniti di Bergamo, Università degli Studi Milano-Bicocca,Milan; and Danilo Radrizzani, M.D., of the Dipartimento EmergenzaUrgenza, Ospedale Civile di Legnano, Legnano, Italy, for theircooperation in retrieving data for control groups; to the studypatients for their participation; and to the physicians andnursing staff of the participating units for their valuablecooperation.
Source Information
From the Istituto di Anestesiologia e Rianimazione, Fondazione Istituto di Ricovero e Cura a Carattere Scientifico, Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Università degli Studi di Milano, Milan (L.G., P.C., M.C., D.C.); the Dipartimento di Anestesia, Azienda Ospedaliera San Giovanni Battista–Molinette, Università degli Studi di Torino, Turin, Italy (V.M.R.); Anaesthesiologie II, Operative Intensivmedizin, Universitatsklinikum Gottingen, Gottingen, Germany (M.Q., S.R.); the Dipartimento di Medicina Perioperatoria e Terapia Intensiva, Azienda Ospedaliera San Gerardo di Monza, Università degli Studi Milano–Bicocca, Milan (N.P.); and the Departamentos de Anestesiologia y Medicina Intensiva, Facultad de Medicina, Pontificia Universidad Catolica de Chile, Santiago, Chile (R.C., G.B.).
Address reprint requests to Prof. Gattinoni at the Istituto di Anestesiologia e Rianimazione, Fondazione IRCCS–Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Università degli Studi di Milano, Via F. Sforza 35, Milan 20122, Italy, or at gattinon{at}policlinico.mi.it.
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Lung Recruitment in Patients with ARDS
Borges J. B., Carvalho C. R.R., Amato M. B.P., Kacmarek R. M., Villar J., Dixon B., Rouby J.-J., Puybasset L., Lu Q., Gattinoni L., Caironi P., Ranieri V. M.
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N Engl J Med 2006;
355:319-322, Jul 20, 2006.
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