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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Computed tomography (CT) assessment of positive end-expiratory pressure (PEEP)-induced alveolar recruitment is classically achieved by quantifying the decrease in nonaerated lung parenchyma on a single juxtadiaphragmatic section (Gattinoni's method). This approach ignores the alveolar recruitment occurring in poorly aerated lung areas and may not reflect the alveolar recruitment of the entire lung. This study describes a new CT method in which PEEP-induced alveolar recruitment is computed as the volume of gas penetrating in poorly and nonaerated lung regions following PEEP. In 16 patients with acute respiratory distress syndrome a thoracic spiral CT scan was performed in ZEEP and PEEP 15 cm H2O. According to the new method, PEEP induced a 119% increase in functional residual capacity (FRC). PEEP-induced alveolar recruitment was 499 ± 279 ml whereas distension and overdistension of previously aerated lung areas were 395 ± 382 ml and 28 ± 6 ml, respectively. The alveolar recruitment according to Gattinoni's method was 26 ± 24 g and no correlation was found between both methods. A significant correlation was found between PEEP-induced alveolar recruitment and increase in PaO2 only when recruitment was assessed by the new method (Rho = 0.76, p = 0.003), suggesting that it may be more accurate than Gattinoni's method.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Thoracic computed tomography (CT) enables an accurate
evaluation of the volume of gas and tissue present in the lungs
of patients with acute respiratory distress syndrome (ARDS)
(1). Using CT, it is possible to assess the pulmonary distribution of the increase in gas volume resulting from tidal ventilation and positive end-expiratory pressure (PEEP) (2) and to
separate PEEP-induced lung overdistension from alveolar recruitment (3, 4). In the mid-1990s, Gattinoni and coworkers
measured PEEP-induced alveolar recruitment on a single juxtadiaphragmatic CT section by quantifying the decrease in
nonaerated lung parenchyma characterized by CT attenuations ranging between 
100 and +100 Hounsfield units (HU) (2, 5, 6). This approach ignores the alveolar recruitment occurring in poorly aerated lung areas characterized by CT attenuations ranging between 100 HU and 500 HU and, therefore,
tends to underestimate PEEP-induced alveolar recruitment.
In addition, by considering a single CT section, it may underestimate or overestimate alveolar recruitment occurring in the
entire lung. In fact, PEEP-induced alveolar recruitment may
transform nonaerated lung areas into poorly aerated lung areas and poorly aerated lung areas into normally aerated lung
areas. Because of these complex gas transfers between the different lung compartments, it appears quite difficult to quantify
alveolar recruitment using a global approach. Clinically, PEEP-induced alveolar recruitment can be defined as the volume of
gas penetrating in poorly and nonaerated alveolar structures following the administration of PEEP, whereas PEEP-induced
alveolar distension can be defined as the volume of gas penetrating in previously normally aerated alveolar structures. The
aim of the present study was to describe a new CT method for
assessing PEEP-induced alveolar recruitment based on this simple definition.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Inclusion Criteria
Sixteen consecutive hypoxemic patients with ARDS defined according to criteria proposed by the American-European Consensus Conference (7) were prospectively studied within the first 10 d of their acute lung disease. Inclusion criteria were PaO2 < 200 mm Hg at fraction of inspired oxygen (FIO2) 1 and zero end-expiratory pressure (ZEEP) and absence of left ventricular failure defined as a pulmonary capillary wedge pressure (PCWP) > 18 mm Hg and/or a left ventricular ejection fraction < 50% as estimated by transesophageal echocardiography. Exclusion criteria were untreated pneumathorax and bronchopleural fistula at inclusion in the study. Informed consent was obtained from the patients' next of kin.
Cardiorespiratory Measurements and Study Design
During the study period, all patients were sedated and paralyzed with a continuous intravenous infusion of fentanyl, midazolam, and vecuronium. Patients were ventilated using controlled mechanical ventilation (César Ventilator, Taema, France). For each patient, tidal volume was limited to values < 8 ml/kg and respiratory rate was increased by the physician in charge in order to achieve PaCO2 values between 30 and 50 mm Hg without generating auto-PEEP (8). An inspiratory time of 33% and FIO2 1 were maintained throughout the study period. All patients were monitored using a fiberoptic thermodilution pulmonary artery catheter (Baxter Healthcare Co., Irvine, CA) and radial or femoral arterial catheters.
Cardiorespiratory parameters were measured and recorded as recently described (9). Pressure-volume curves of the respiratory system were measured in ZEEP conditions according to a recently described technique (10). Inflation, starting and quasistatic respiratory compliances, and the lower inflection point were determined according to Gattinoni and coworkers (11). In each patient, a thoracic CT scan was obtained at end expiration in ZEEP conditions and at a PEEP of 15 cm H2O. In our institution, the thoracic CT scan is considered an integral part of the respiratory management of patients with ARDS. Hemodynamic and respiratory parameters were measured in ZEEP and PEEP conditions within a few hours of the thoracic CT scan in 5 patients and at the same moment in 11 patients.
Computed Tomography Measurement of Lung Volumes
Acquisition of the CT sections. Each patient was transported to the
Department of Radiology (Thoracic Division) by two intensivists. Lung scanning was performed from the apex to the diaphragm using a
spiral Tomoscan SR 7000 (Philips, Eindhoven, The Netherlands) as
previously described (1). Acquisition of spiral sections was obtained
at end expiration
at a lung volume equal to functional residual capacity (FRC)in ZEEP, the patient being disconnected from the
ventilator (FRC-PEEP), and 15 min after applying a PEEP of 15 cm
H2O, the connecting piece between the Y piece and the endotracheal
tube being clamped at end expiration (FRC-PEEP). Airway pressure
was monitored during the CT scan acquisition in PEEP to ensure that
a pressure of 15 cm H2O was actually applied. All CT sections were
recorded on an optical disk for later computerized analysis.
Measurement of lung, gas, and tissue volumes. The lung volume was computed as the total number of voxels present in a given region of interest times the volume of the voxel. The lung is composed of gas and tissue. The respective volumes of gas and tissue were measured according to a previously described analysis (4), based on the tight correlation existing between the CT attenuation and the physical density (12). Total lung volume, the volume of gas and tissue, the fraction of gas, and the weight of tissue were measured using a specifically designed Software (Lungview) according to a method previously described (13, 14).
Computed Tomography Measurement of Alveolar Recruitment
Gattinoni's method. As recommended by Gattinoni and coworkers
(2), alveolar recruitment was assessed on a single CT section located
1 cm above the diaphragmatic cupola and computed as the decrease in
the weight of nonaerated lung parenchyma between ZEEP and PEEP
(Figure 1). PEEP-induced alveolar recruitment (RECALV) was expressed as percentage of variation of the weight of the nonaerated
lung parenchyma: RECALV (%) = (WZEEP WPEEP)/WZEEP, where
WZEEP is the weight of the nonaerated lung parenchyma in ZEEP
conditions and WPEEP is the weight of the nonaerated lung parenchyma in PEEP conditions.
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The new method. PEEP-induced alveolar recruitment was assessed on the entire lung and defined as the increase in the volume of gas penetrating in nonaerated and poorly aerated lung regions following PEEP administration. The CT analysis was performed on each CT section from the apex to the diaphragm.
In a first step, the following analysis was performed on each CT
section obtained in ZEEP conditions. On the screen of the computer
on which the CT image was displayed, the left and right lung boundaries were manually delineated. Then, normally, poorly and nonaerated lung areas were visualized on the screen of the computer using a
color encoding system integrated in the Lungview software. Each
nonaerated voxel characterized by a CT attenuation ranging between
100 and +100 HU was colored in white; each poorly aerated voxel
characterized by a CT attenuation ranging between 500 and 100
HU was colored in light gray; and each normally aerated voxel characterized by a CT attenuation ranging between 500 and 900 HU
was colored in dark gray. As shown in Figure 1, the color encoding
served to separate two regions of interest on each CT section: normally aerated lung regions and poorly or nonaerated lung regions. As
shown in Figure 2, the roller ball of the computer was used to delineate with a dashed line poorly and nonaerated lung regions by following the limits given by the color encoding.
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In a second step, left and right lung boundaries were manually delineated on each CT section obtained in PEEP conditions. Then the
two CT sections obtained in ZEEP and PEEP conditions and corresponding to the same anatomical level were simultaneously displayed
on the screen of the computer. By referring to anatomical landmarks
such as pulmonary vessels, fissures, and segmental bronchi, the limit
between the two regions of interest was manually redrawn on the CT
section in PEEP according to the previous delineation performed in
ZEEP (Figure 2). As shown in Figure 3, in patients with diffuse CT
hyperattenuations the delineation of poorly and nonaerated lung regions coincided with the delineation of lung boundaries because of the
lack of any normally aerated lung areas in ZEEP conditions. In each
of the two regions of interest delineated in ZEEP and PEEPnormally aerated lung region and poorly and nonaerated lung regions
the volumes of gas and tissue were measured.
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PEEP-induced alveolar recruitment was computed as the increase in
gas volume within the poorly and nonaerated lung regions (VGas PEEP VGas ZEEP) following PEEP administration (alveolar recruitment in
absolute values) divided by the FRC measured in ZEEP conditions
(FRCZEEP): RECALV (%) = (VGas PEEP VGas ZEEP)/FRCZEEP. Alveolar recruitment was normalized to FRC in order to take into account
the degree of loss of aeration before PEEP administration. It is obvious
that for a given PEEP-induced alveolar recruitment expressed in absolute values, the physiological impact on gas exchange varies markedly according to the initial reduction of FRC. The efficiency of PEEP-induced alveolar recruitment (EFREC) expressed as a percentage was
computed as the ratio between the increase in gas volume within
poorly and nonaerated lung regions and the increase in FRC on the
whole lung between PEEP and ZEEP: EFREC = VGas PEEP VGas ZEEP)/
(FRCPEEP FRCZEEP), or EFREC = RECALV/(RECALV + DISTALV);
where VGas ZEEP and VGas PEEP are the volumes of gas present within
poorly and nonaerated lung regions in ZEEP and PEEP conditions,
FRCZEEP and FRCPEEP are the end-expiratory volumes of gas present
in the entire lung in ZEEP and PEEP conditions, REC is alveolar recruitment (in absolute values), and DISTALV is PEEP-induced alveolar distension.
PEEP-induced alveolar distension and overdistension were defined as the increase in gas volume within normally aerated lung regions. PEEP-induced alveolar overdistension was defined as the increase in lung volume characterized by a CT attentuation ranging between 1000 and 900 HU (3). PEEP-induced alveolar distension was defined as the increase in lung volume characterized by a CT attenuation ranging between 900 and 500 HU.
Statistical Analysis
Because of their normal distribution, respiratory and hemodynamic
parameters were compared before and after the administration of
PEEP using a paired Student's t test. In contrast, the effects of PEEP
on lung volumes of gas and tissue were assessed using a Wilcoxon test
because the distribution of these parameters was not normal. All correlations were made by means of Spearman correlation rank analysis.
The correlations between two variables were accepted when the nonparametrical correlation coefficient (Rho) was 
0.5. Agreement between both methods aimed at measuring alveolar recruitment was
tested using the Bland and Altman method (15). The statistical analysis was performed using Statview 5.0 (SAS Institute Inc., Cary, NC).
All data are expressed as mean ± SD unless otherwise specified. The
significance level was fixed at 0.05.
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Study Population
Sixteen patients with primary ARDS (4 females and 12 males, mean age 49 ± 18 yr) were studied. ARDS was related to postoperative pulmonary infection (n = 7), bronchopulmonary aspiration in the postoperative period (n = 5), and lung contusion (n = 4). The mean delay between the onset of ARDS and the inclusion in the study was 5 ± 3 d. The lung injury severity score (16) was 3 ± 0.7, the quasistatic respiratory compliance measured in ZEEP conditions was 39 ± 10 ml/cm H2O, the starting compliance was 33 ± 16 ml/cm H2O, and the inflation compliance was 46 ± 17 ml/cm H2O. A lower inflection point was present in 15 patients, with a mean value of 6.2 ± 3.2 cm H2O (extremes 2 to 14 cm H2O). Twelve patients had septic shock, defined according to reference criteria (17). All patients with septic shock were treated with norepinephrine at a mean dose of 3.3 ± 2.9 mg/h. Cardiorespiratory parameters in ZEEP conditions are summarized in Table 1. The overall mortality rate was 37.5%.
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PEEP-induced Alveolar Recruitment and Distension
As shown in Table 2, PEEP induced a 119% increase in FRC (p < 0.001) without modifying the volume of lung tissue and the lung weight. The increase in FRC had two components: PEEP-induced alveolar recruitment in previously poorly and nonaerated lung regions and distension or overdistension of previously normally aerated lung regions. PEEP-induced alveolar recruitment was equivalent to a 56% increase in FRC-ZEEP and represented half of the increase in FRC, the other half being a distension of previously normally aerated lung areas. As shown in Table 3, PEEP induced a 10% decrease in the tissue volume of the juxtadiaphragmatic CT section.
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PEEP-induced alveolar recruitment was assessed either on
the entire lung (new method) or on a single juxtadiaphragmatic CT section (Gattinoni's method). According to the new
method, Vgas PEEP Vgas ZEEP was 499 ± 279 ml, representing
a 56 ± 20% increase in FRCZEEP, whereas the alveolar recruitment was 26 ± 24 g according to Gattinoni's method, representing a mean decrease of 42 ± 41% in nonaerated tissue
weight. As shown in Figure 4, there was neither correlation
(Rho = 0.273, p = 0.18) nor agreement between the two
methods. This result held true when alveolar recruitment was expressed in absolute values. PEEP-induced lung distension
was 395 ± 382 ml, representing an increase of 95 ± 40% in the
volume of gas present in normally aerated lung regions in
ZEEP. As shown in Table 2, PEEP-induced overdistension of
the normally aerated parenchyma was 23 ± 67 ml.
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Relationships between PEEP-induced Alveolar Recruitment, Arterial Oxygenation, and Pulmonary Hemodynamics
Significant relationships were found between PaO2 and 
S/T
measured in ZEEP conditions and FRCZEEP and more tightly
with the volume of total lung tissue (VTISSUE): PaO2 (mm
Hg) = 0.7 FRCZEEP (L) + 77 (Rho = 0.5, p = 0.05) and S/T
(%) = 0.09 FRCZEEP (L) + 49 (Rho = 0.5, p = 0.05); PaO2
(mm Hg) = 0.1 VTISSUE (L) + 271 (Rho = 0.8, p = 0.02)
and S/T (%) = 0.02 VTISSUE (L) + 18 (Rho = 0.6, p = 0.03).
As shown in Table 1, PEEP induced a significant increase in
PaO2 and a significant decrease in S/T. As shown in Figure
5, a significant correlation was found between PEEP-induced
alveolar recruitment and increase in PaO2 only when alveolar
recruitment was assessed using the new method.
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In ZEEP, a significant relationship was found between mean pulmonary artery pressure (MPAP) and the volume of nonaerated lung parenchyma to overall lung volume/ratio (Rho = 0.64, p = 0.002). No correlation was found between MPAP and FRCZEEP or the volume of lung tissue. As shown in Table 1, PEEP significantly increased the pulmonary vascular resistance index (PVRI) and central venous pressure and significantly decreased cardiac index (CI), mean arterial pressure, and oxygen delivery index. No correlations were found between PEEP-induced alveolar distension and increase in MPAP or PVRI and decrease in CI.
Factors Influencing the Efficiency of PEEP-induced Alveolar Recruitment
As shown in Figure 6 the efficiency of PEEP-induced alveolar recruitment, defined as the ratio between alveolar recruitment and increase in FRC induced by PEEP, was highly correlated with the proportion of poorly and nonaerated lung parenchyma in ZEEP conditions (Rho = 0.95, p < 0.0001). It was negatively correlated to static respiratory compliance (Crs) (Rho = 0.67, p = 0.002) and positively correlated to the value of the lower inflection point (Rho = 0.575, p = 0.009).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In this study, a new CT method is described for assessing PEEP-induced alveolar recruitment in patients with ARDS and compared with the method proposed by Gattinoni and coworkers (2). This new method is based on a regional CT analysis that quantifies the volume of gas penetrating in poorly and nonaerated lung regions following the administration of PEEP. It requires a CT analysis of the entire lung and also enables an assessment of PEEP-induced alveolar distension and overdistension occurring in normally aerated lung regions. Alveolar recruitment as assessed by this new method is well correlated with PEEP-induced improvement in PaO2.
Comparison of the Two Methods
Neither correlation nor agreement was found between the new method and the one proposed by Gattinoni and coworkers in 1995 (2). Gattinoni's method is based on two fundamental assumptions: (1) the alveolar recruitment assessed on a single juxtadiaphragmatic CT section is representative of the overall alveolar recruitment occurring in the entire lung; and (2) the alveolar recruitment occurring in nonaerated lung areas is representative of the alveolar recruitment occurring in poorly aerated lung regions. Recently, it was shown that in patients with ARDS, PEEP-induced alveolar recruitment as assessed by Gattinoni's method (extended to all CT sections) decreases along a cephalocaudal axis and that in the caudal lung regions alveolar recruitment is minimum (13). In another study, it was even shown that an alveolar derecruitment could be observed in diaphragmatic lung regions (1). As a consequence, it is highly unlikely that PEEP-induced alveolar recruitment assessed on a single juxtadiaphragmatic CT section can be representative of the alveolar recruitment of the entire lung.
Another source of error associated with Gattinoni's method results from the PEEP-induced increase in the cephalocaudal dimension of the lung. As previously reported (1, 13) the overall amount of lung tissue was not modified by PEEP in the present study whereas a 10% reduction of lung tissue was observed in the juxtadiaphragmatic CT section. This paradoxical result may be explained by a PEEP-induced cephalocaudal expansion of the lungs: if the thickness of each CT section remains constant, then the amount of tissue per CT section automatically decreases according to the increase in lung size. In the present study, to analyze the entire lung, two or three additional CT sections had to be performed in PEEP as compared with ZEEP. As a consequence, a decrease in the weight of the nonaerated lung parenchyma detected on a single juxtadiaphragmatic CT section following PEEP administration does not always reflect a true alveolar recruitment. The amount of tissue present on a single basal CT section may also be influenced by the redistribution of pulmonary blood flow (18). It has recently been reported that PEEP may overdistend upper lobes while recruiting lower lobes (4). As a consequence, pulmonary blood flow may be diverted from upper lobes toward lower lobes resulting in a regional increase in lung tissue that may conceal alveolar recruitment-induced decrease in the weight of nonaerated lung parenchyma.
The systematic underevaluation of alveolar recruitment in the poorly aerated lung regions represents another limitation of the method proposed by Gattinoni. Very likely, PEEP-induced alveolar recruitment transforms nonaerated lung regions into poorly aerated lung regions and poorly aerated lung regions into normally aerated lung regions. This may explain why the overall volume of poorly aerated lung parenchyma remained unchanged in the present study confirming the results of several previous studies (3, 4, 6). In contrast with Gattinoni's method, the new method takes into account the alveolar recruitment occurring in poorly aerated lung regions. With the color encoding system included in Lungview, the lung parenchyma can be easily partitioned into two compartments: a potentially recruitable compartment composed of poorly and nonaerated lung regions and an already recruited compartment composed of normally aerated lung regions. This regional analysis enables PEEP-induced alveolar recruitment occurring in the recruitable compartment to be differentiated from PEEP-induced distention occurring in the normally aerated compartment.
Limitations of the New Method
The new method is based on a regional analysis of PEEP- induced reaeration of insufficiently aerated lung regions. In ZEEP conditions, using a color encoding software it is easy to separate lung regions characterized by a loss of aeration from normally aerated lung parenchyma. By referring to anatomical landmarks such as bronchi and vessels that are visible only in the parts of the lung that are normally aerated, the same lung regions can be delineated in PEEP conditions allowing the assessment of reaeration of insufficiently aerated lung regions. However, this analysis cannot be performed separately on poorly and nonaerated lung regions because anatomical landmarks are not visible in the absence of normal pulmonary aeration. As a consequence, the new method does not permit us to separate PEEP-induced alveolar recruitment occurring in poorly and nonaerated lung regions.
In a limited number of patients, the very heterogeneous distribution of the loss of aeration within the lungs does not permit appropriate delineation between normally and insufficiently aerated lung areas in PEEP conditions. As shown in Figure 7, multiple islets of normally aerated lung parenchyma can be surrounded by large areas of poorly aerated lung parenchyma rendering the delineation of the same zones of interest in ZEEP and PEEP conditions very difficult. This morphological pattern, although rarely observed in patients with ARDS, precludes any attempt of assessing PEEP-induced alveolar recruitment using the new method.
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Along these lines, another potential limitation of the new
method is related to the respective dimensions of voxels and
alveolar structures (alveolar ducts, alveolar sacs, and alveoli).
Recent studies in humans (19) and animals (20) with ARDS
have shown that alveolar structures remaining normally aerated are distended by mechanical ventilation delivering tidal
volumes 10 ml/kg. The effect of reducing tidal volume on alveolar dimensions is not known. If the administration of low
tidal volumes does not prevent the distension of alveolar
structures remaining aerated, then the mean alveolar volume
should be around 0.11 mm3 assuming that a single distended
alveolar structure is a sphere with an internal diameter of 0.6 mm
(19). Because in the present study the volume of the voxel was
ranging between 1.7 and 4.7 mm3, a single voxel could therefore contain one to five distended alveolar structures. If the
administration of low tidal volumes < 8 ml/kg in the present
studyprevents airspace enlargement, then the voxel could
contain between 120 and 350 alveolar structures, assuming
that the mean diameter of a normally sized alveolar structure is around 0.3 mm (21). As a consequence, a voxel with a
CT attenuation of 300 HU could contain either 105 normally
and 245 nonaerated alveolar structures or 350 poorly aerated
alveolar structures. In the first situation, the gas penetrating
the voxel following PEEP could distend normally aerated
alveolar structures without recruiting nonaerated alveolar
structures, resulting in an overestimation of PEEP-induced alveolar recruitment. Although theoretically possible, this possibility appears unlikely. Histologically, ARDS is made of large
areas of nonaerated and poorly aerated lung parenchyma corresponding to alveolar structures totally or partially filled with edema and/or inflammatory cellscoexisting with normally aerated lung regions. The limit between normally and
insufficiently aerated lung areas is most often well defined and
the histological pattern of multiple islets of normally aerated
alveolar structures scattered within large areas of insufficiently
aerated lung parenchyma is exceptionally observed. As a consequence, the error related to the respective dimensions of
voxels and alveolar structures concerns mainly the zone of delineation between normally and poorly or nonaerated lung parenchyma, that is less than 1% of the lung parenchyma analyzed. To increase the accuracy of PEEP-induced alveolar
recruitment assessed by the new method, the size of the voxel
should be reduced as much as possible. Voxels as small as 0.2 mm3 can be obtained by using ultrafast spiral CT scanners of
the last generation, which provide a matrix of 1024 × 1024.
Gas Exchange, Pulmonary Hemodynamics, and Alveolar Recruitment
In the present study, FIO2 1 was used. As a consequence, impairment in arterial oxygenation measured in ZEEP conditions and improvement in oxygenation parameters observed in PEEP were exclusively dependent on pulmonary shunt (nonaerated lung areas) and not on venous admixture (poorly aerated lung areas).
In ZEEP conditions, the degree of arterial oxygenation impairment correlated with the loss of lung aeration and with the volume of total lung tissue. With Gattinoni's method, no correlation was found between PEEP-induced alveolar recruitment and improvement in arterial oxygenation. Very likely, this lack of correlation was related to the poor accuracy of the method for assessing PEEP-induced alveolar recruitment occurring in shunt regions. In contrast, a significant correlation was found between PEEP-induced improvement in arterial oxygenation and alveolar recruitment as assessed using the new method. This result appears logical. In ARDS, arterial hypoxemia is essentially related to the presence of nonaerated lung regions remaining perfused. PEEP basically improves arterial oxygenation by reestablishing some degree of aeration in these shunt regions. By founding the new method on the assessment of the reaeration of insufficiently aerated lung regions, it is not surprising to have found a significant correlation between PEEP-induced alveolar recruitment and improvement in arterial oxygenation.
In this study, a good correlation was found between the initial loss of pulmonary aeration and the efficiency of PEEP- induced alveolar recruitment. In fact, it has been shown that the distribution of PEEP-induced increase in gas volume within the lung parenchyma is markedly dependent on disparities in regional lung compliances (13), which, in turn, are determined by the regional loss of gas (1). Therefore, it appears logical that the greater the extension of poorly and nonaerated lung regions in ZEEP, the lower the disparities in regional lung compliances and the greater the efficiency of PEEP-induced alveolar recruitment. In addition, the efficiency of PEEP-induced alveolar recruitment was positively correlated to the lower inflection point. These results agree with the previous findings of Vieira and coworkers (4). According to these authors, the patients, whose ARDS is characterized by diffuse lung CT attenuations, a marked lower inflection point on the lung P-V curve, and a low respiratory compliance, respond to increasing PEEP levels by a progressive and sustained alveolar recruitment. In contrast, the patients, whose ARDS is characterized by lobar lung CT attenuations involving predominantly the lower lobes, the absence of a lower inflection point on the lung P-V curve and a moderate decrease in respiratory compliance respond to increasing PEEP levels by an overdistension of previously aerated lung regions.
In conclusion, the alveolar recruitment defined as the penetration of gas within poorly and nonaerated lung regions following the administration of PEEP can be quantified using a CT analysis of the entire lung. In contrast with previous CT methods, it takes into account the alveolar recruitment occurring in poorly and nonaerated lung regions, quantifies PEEP-induced alveolar distension occurring in normally aerated lung regions, and is correlated with PEEP-induced improvement in arterial oxygenation. Further studies are required to determine whether this CT method is correlated with methods based on the determination of alveolar recruitment from P-V curves obtained in ZEEP and PEEP conditions.
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Footnotes |
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Correspondence and requests for reprints should be addressed to J. J. Rouby, Réanimation Chirurgicale Pierre Viars, Department of Anesthesiology, La Pitié- Salpêtrière Hospital, 47-83 boulevard de l'Hôpital, 75013 Paris, France. E-mail: jjrouby.pitie{at}invivo.edu
(Received in original form April 27, 2000 and in revised form October 17, 2000).
Dr. Malbouisson was the recipient of a scholarship provided by the French Ministery of Foreign Affairs (Ref. 23344471).The following members of the CT scan ARDS Study Group participated in this study: I. Goldstein, E. Mourgeon, and P. Coriat, Unité de Réanimation Chirurgicale Pierre Viars, Department of Anesthesiology, Hôpital de La Pitié-Salpêtrière, Paris, France; Maria T. Bugalho, Department of Critical Care Medicine, Hospital de Santarem, Santarem, Portugal; P. Grenier and P. Cluzel, Department of Radiology, Hôpital de la Pitié-Salpêtrière, Paris, France; J. Richecoeur, General ICU, Pontoise Hospital, Pontoise, France; P. Gusman, Department of Anesthesiology, Santa Casa, de Misericordia, Juiz de Fora, MG, Brasil; F. Préteux and C. Fetita, Institut National des Télécommunications, Evry, France.
This article has an online data supplement, which is accessible from this issue's table of contents online at www.atsjournals.org
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References |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Richecoeur J,
Lu Q,
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