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Annals of Biomedical Engineering,Vol. 29, pp. 60–70, 2001 0090-6964/2001/29~1!/60/11/$15.00Printed in the USA. All rights reserved. Copyright © 2001 Biomedical Engineering Society

Compartmental Analysis of Breathing in the Supine and PronePositions by Optoelectronic Plethysmography

ANDREA ALIVERTI,1,2* RAFFAELE DELLACA,1,2 PAOLO PELOSI,3,4 DAVIDE CHIUMELLO,3

LUCIANO GATTINONI,3 and ANTONIO PEDOTTI,1,2

1Dipartimento di Bioingegneria, Politecnico di Milano, Italy,2Centro di Bioingegneria, Fond. Don Gnocchi IRCCS and Politecnidi Milano, Italy, 3Istituto di Anestesia e Rianimazione, Universita’ di Milano and Servizio di Anestesia e Rianimazione,

Ospedale Maggiore IRCCS, Milano, Italy,4Dipartimento di Scienze cliniche e biologiche, Universita’ degli studi dell’Insubria,Varese, Italy

(Received 7 January 2000; accepted 20 October 2000)

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Abstract—Optoelectronic plethysmography~OEP! has beenshown to be a reliable method for the analysis of chest wkinematics partitioned into pulmonary rib cage, abdominalcage, abdomen, and right and left side in the seated andpositions. In this paper, we extended the applicability of tmethod to the supine and prone positions, typically adoptecritically ill patients. For this purpose we have first developproper geometrical and mathematical models of the chestwhich are able to provide consistent and reliable estimationtotal and compartmental volume variations in these positisuitable for clinical settings. Then we compared chest w~CW! volume changes computed from OEP(DVCW) with lungvolume changes measured with a water seal spirometer~SP!(DVSP) in 10 normal subjects during quiet~QB! and deep~DB!breathing on rigid and soft supports. We found that on a risupport the average differences betweenDVSP andDVCW were24.2%66.2%, 23.0%66.1%, 21.7%67.0%, and24.5%69.8%, respectively, during supine/QB, supine/DB, prone/Qand prone/DB. On the soft surface we obtained20.1%66.0%, 21.8%67.8%, 18.0%611.7%, and 10.2%69.6%, re-spectively. On rigid support and QB, the abdominal compament contributed most of theDVCW in the supine (63.1%611.4%) and prone (53.5%613.1%) positions.DVCW wasequally distributed between right and left sides. In the proposition we found a different chest wall volume distributiobetween pulmonary and abdominal rib cage~22.1%68.6% and24.4%66.8%, respectively! compared with the supine positio~23.3%69.3% and 13.6%63.0%). © 2001 Biomedical En-gineering Society. @DOI: 10.1114/1.1332084#

Keywords—Chest wall, Rib cage, Abdomen, Volume Mesurement, Kinematics.

INTRODUCTION

During breathing, the chest wall changes the voluand shape of both its rib cage and abdominal comp

*Address correspondence to Andrea Aliverti, PhD, Centro di Bingegneria, Politecnico di Milano, P.zza Leonardo da Vinci 32, 201Milano, Italy. Electronic mail: [email protected]

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ment. The measurement of the thoracic and abdomwall shapes allows the computation of the absolute chwall volume and its variations.

Recently, we presented a new method, the basic pciple of which is that the volume variations of the chewall and its different compartments~pulmonary rib cage,abdominal rib cage, and abdomen including their rigand left parts! can be derived from optical measuremenof a finite number of displacements of points on texternal surfaces of the chest wall.8,9,12 These studieshave shown that such a system provides an accuraterobust estimation of the absolute volumes and thvariations during breathing. The system provides nonvasive optoelectronic plethysmography~OEP! to measurechest wall volume changes without a mouthpiece, nclip, or other connection to the patient.

However, previous studies have been mainly decated to the analysis of standing and seated postuAlso, application of this method to physiologicastudies2,16 has shown how compartmental volumchanges are variables of crucial importance for a coplete understanding of respiratory mechanics duringercise.

The aim of the present study is to develop and testfeasibility of the optoelectronic analysis of the chest win constrained postures~supine and prone! typical forcritically ill patients.

METHODS

Optoelectronic Plethysmography

Detailed descriptions of the optoelectronic methodthe analysis of chest wall volume in standing and seaposition have been published previously.8,9,12 Briefly, itis based on an automatic motion analyzer~Elite® system,BTS, Milano, Italy! that uses passive markers composof a thin film of retroreflective paper on plastic hemspheres of 6 mm diam. The markers are placed on

61OPTOELECTRONICPLETHYSMOGRAPHY IN SUPINE AND PRONE

FIGURE 1. Geometrical modeling of thedifferent chest wall compartments in fourdifferent views „supine position …: frontal,sagittal, horizontal, and perspective.Closed points represent the markersplaced on the chest wall and open pointsrepresent virtual markers „see the text ….To allow a better readability the compart-ments are axially separated and theboundary lines are replicated.

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skin by biadhesive hypoallergenic tape. Special telesion ~TV! cameras@solid-state charge-coupled devic~CCDs!# are operating up to 100 frames per second schronized with coaxial infrared flashing light-emittindiodes ~LEDs!. A dedicated parallel processor executreal-time pattern recognition algorithms and compuwith high accuracy the three-dimensional~3D! coordi-nates of the different markers.11 No specific calibration isrequired besides the initial one, performed duringinstallation of the system.

Once the 3D coordinates of the points belongingthe chest wall surface have been acquired, the nextis to compute the volume of the closed surface obtaiby connecting the points to form triangles. Starting frothe measured points a special geometrical modelingthe chest wall was developed to describe the surfacethe whole chest wall and its different compartments.

In the case of the erect and seated positions,when the whole trunk is visible ‘‘on the round,’’ wefound8 that the best markers arrangement is composed86 markers on the chest wall~42 anterior, 34 posteriorand 10 lateral!.

Volume Computing

In the supine and prone positions a hidden part ofchest wall surface lies on the supporting bed. Therefin this case the geometrical model of the chest wconsiders a number of ‘‘virtual’’ points belonging to

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reference plane corresponding to the horizontal planethe bed~Fig. 1!. The definition of the geometry of themodel allows the computation of the enclosed volumand its variations determined by movement by integring over the surface using Gauss’s theorem to obtaivolume integral. In detail, the analytical expressionthe theorem is

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whereS is the surface,V is the volume enclosed byS, Fis an arbitrary vector,n is the normal unit vector at thedifferent points ofS, and ¹ is the divergence operator.

If we choose an arbitrary vector with a unit divegence, Eq.~1! becomes

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and the volume integral is computed by means ofeasier surface integral. Passing from continuous tocrete form, Eq.~2! becomes

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62 ALIVERTI et al.

FIGURE 2. Markers’ positioning and chest wall compartments for optical analysis in the supine „left … and prone „right … posi-tions. „Left … transverse lines: clavicular line; manubrio-sternal joint „angle of Louis …; xiphoid process; lower costal margin;upper abdomen „3 markers …; umbilical level; and anterior superior iliac crest. Axial lines: midsternal line „extended to theabdomen …; two parasternal lines „extended to the abdominal region …; the midpoints of the interval between the midsternal andthe midaxillary lines; and two midaxillary lines. „Right … transverse lines: cervico-thoracic line, scapular line, biangular-scapularline, midline between biangular-scapular line and thoraco-abdominal line, thoraco-abdominal line, midline between thoraco-abdominal line and abdomino-pelvic line, abdomino-pelvic line, and posterior gluteal line. The first two lines are defined by fivemarkers and did not extend laterally to avoid the shoulder regions. The other six lines are defined each by seven markers,placed symmetrically with respect to the vertebral line and extended laterally until the two midaxillary lines. Chest wallcompartments: pulmonary rib cage „RCp…; abdominal rib cage „RCa…; abdomen „AB …. Each compartment is axially split into twoparts, right „r… and left „l….

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where K is the total number of the triangles,Ai is thearea of thei th triangle, andni is the normal unit vectorof the i th triangle.

Considering the geometric model of the whole trusurface it is thus possible to obtain total chest wall vume variations, and, in addition, the contributionsdifferent chest wall compartments to the total volumchange.

After several experimental iterations we have opmized the model to obtain a satisfactory trade off btween accuracy and clinical feasibility. The best arranment consisted of 45 markers in the supine and 52 inprone position. They were positioned on anatomilandmarks as illustrated in Fig. 2. The height of treference plane~the surface on which the subject walying! was estimated as the mean height of the latemarkers minus 5 cm. The positions of the virtual poinwere determined by projecting the lateral markers’ initfrontal coordinates~from the first acquired image frame!onto the reference plane~see Fig. 1!.

Experimental Protocols

We studied a group of 10 volunteer healthy subje~5 men and 5 women!, recruited from the laboratorypersonnel, age 28.064.5 years, weight 65.3611.5 kg andheight 1.7160.09 m. All measurements were performin a room in an intensive care unit~ICU! and the opticalexperimental setup was arranged as shown in Fig. 3.setting of TV cameras was chosen in such a way tha
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the markers placed on the subject were simultaneouseen by at least two cameras in order to reconstruct tthree-dimensional positions and displacements durrespiration by stereo-photogrammetric methods.11

During all the experiments, the subjects performedfollowing respiratory maneuvers in sequence:~1! a pe-riod ~;30 s! of normal quiet breathing~QB!; ~2! fourdeep breaths~DB!; and ~3! belly-in/belly-out isovolume~ISO! maneuvers at the end of a normal expiration. Tsubjects were studied on a therapy bed~Flexicair®, Hill-Rom, Batesville, IN! with two types of support:~a! soft,i.e., air-filled mattress inflated and~b! rigid, i.e., air-filledmattress completely deflated. The two supports wused to evaluate the effects on volume changes recoby OEP due to any downward motions of the chest winto the soft support, which was presumably prevenby the rigid support.

Volume Measurements

During the maneuvers, the subjects breathed througmouthpiece connected to a water seal spiromeequipped with a potentiometer~model 308, SpectrolElectronics, City of Industry, CA, linearity60.25%!.Respitrace bands~Plus R! were applied around the ribcage~RC! and the abdomen~AB! in every subject. Therespiratory inductive plethysmography~RIP! system wasseparately calibrated for the supine and the prone ptions as described in Ref. 27. The analog signals frboth the spirometer and the RIP were sent to an ana


FIGURE 3. Experimental setup for opto-electronic plethysmography in the supineand prone position. Four CCD cameraswere fixed on the ceiling at the vertices ofa rectangle of 2.9 by 1.75 m at about 2.1m of height, above the subject and in-clined downward. The operative cali-brated volume was approximately a rightrectangular parallelepiped of 80 Ã60Ã40 cm able to include the trunk of thesubject lying on the bed.

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to-digital ~A/D! board ~RTI800, Analog Devices, Nor-wood, MA! synchronized with the automatic motion anlyzer and digitally recorded at 100 Hz. Optoelectronplethysmography allowed us to obtain the absolute vumes and their changes, during respiration, of the tchest wall~CW! volume and its compartments. The sampling rate was set at 25 Hz.

The comparison of spirometric measurements wthose of OEP and RIP is not straightforward, becauwith the former, we measure the changes of lungvolume, while with the latter we measure the volumchanges of the thoracic cavity.

As reported in the following discussion, we did napply any correction for humidity, pressure, and teperature changes of the gas between the lung andspirometer.

The base-line drift of the spirometer~SP! was cor-rected as follows. Assuming that the OEP system hasdrift, we computed for each sample the differencestween the OEP chest wall volume and the spiromelung volume. These values were plotted against timea linear regression was computed to estimate the drifthe spirometer. Successively, each sample of the spmetric signal was corrected subtracting the estimadrift from the original signal. In average, we observedmean drift of about 3 ml/s and we found very higregression coefficient values which confirmed thesumption of a linear drift.

Then, for each breath the difference between eexpiratory and end-inspiratoryVL estimated from thespirometer (DVSP) was compared to the correspondinvariation from optical measurement (DVCW). For each

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condition, results have been reported in terms of perceage error~%err!, computed as follows:

%err5DVSP2DVCW

DVSP100. ~4!

In each condition, a Bland–Altman analysis7 was appliedto determine if the difference between the two measuments was related to their magnitude.

Comparison Between OEP and RIP

With the OEP we measured the following compaments~Fig. 2!: the pulmonary rib cage~RCp!, defined asextending caudally from the markers placed at the clicular line ~supine position! or the cervico-thoracic line~prone position! to those placed at the xiphoid level~as-sumed to be the cephalic extremity of the area of apsition of the diaphragm at functional residual capacit!;the abdominal rib cage~RCa!, and the abdomen~AB!.Each compartment was split axially into right and leparts. For the analysis in the supine position, the seration between left and right was obtained by usingaxial column of markers placed on the sternum, wherfor the analysis in prone position we used the markplaced on the vertebrae.

Chest wall volume (VCW) was computed as the sumof the volumes of pulmonary rib cage, abdominal rcage, and abdomen (VRCp, VRCa, and VAB , respec-tively!.

As the RIP allows the measurement of only two copartments~rib cage and abdomen! while the optoelec-tronic system measures three compartments~pulmonary

64 ALIVERTI et al.

TABLE 1. Measurement accuracy, expressed in %err, given by Eq. „4…. All data are expressedas mean value ÁSD and n indicates the number of analyzed breaths.

Rigid support Soft support

Quiet breath Deep breath Quiet breath Deep breath

Supine 24.2%66.2% 23.0%66.1% 20.1%66.0% 21.8%67.8%Position (n570) (n534) (n569) (n535)

Prone 21.7%67.0% 24.5%69.8% 18.0%611.7% 10.2%69.6%Position (n573) (n539) (n574) (n538)

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rib cage, abdominal rib cage, and abdomen! the volumevariations measured with OEP of the abdominal rib ca~not measured by RIP! were assigned either totally to thpulmonary rib cage, or totally to the abdomen, or 50%the pulmonary rib cage and 50% to the abdomen.

Finally, we could not compare the right and left compartments with any available system, and the right–distribution data only refers to the OEP measuremen

Isovolume Maneuvers and Compartmentalization

To compare OEP and RIP compartmentalizationthe supine position, we asked the subjects to performisovolume maneuvers, i.e., to keep the total lung voluconstant keeping the glottis closed, either contractingbelly and expanding the rib cage~belly in! or expandingthe belly and contracting the rib cage~belly out!.

Only 6 subjects were able to correctly perform tisovolume maneuvers. Ideally, the sum of the variatioof the three chest wall compartments~upper rib cage,lower rib cage, and abdomen! should equal zero.

For the time interval in which the spirometer signwas constant, we summed the volume variations ofthree compartments’ OEP signals, we summed thevolume-calibrated RIP signals, and we computedmaximal variations of each sum.

Statistical Analysis

The effects of posture~supine and prone!, maneuver~quiet and deep breathing!, and type of support~rigid andsoft! on measurement accuracy~%err! were analyzedusing a one-way analysis of variance~ANOVA !. Analy-sis included a breath-by-breath comparison of 4breaths in the different conditions~see Table 1!. Differ-ences in the absolute and percentual volume changethe different chest wall compartments~whose mean6SDvalues are reported in Table 2! due to posture and maneuver were also analyzed using a one-way ANOVAPvalues,0.05 were taken as indicating statistical signicance.

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RESULTS

A typical set of experimental tracings is shown in Fi4. Table 1 reports the differences between the variatiof gas volume measured with the spirometer,DVSP, andthe variations of chest wall volume measured by toptoelectronic system,DVCW, during the different ex-perimental conditions:~a! different postures~supine andprone!; ~b! different supports~rigid and soft!; and ~c!different maneuvers~quiet and deep breathing!. Each re-ported value has been computed as the mean6SDof %err of all breaths and all subjects. An examplethe correlation betweenDVSP and DVCW is reported inFig. 5.

The two measures were very close in all experimenconditions except in the prone position with the subjelying on a soft support. For this we found errors18.0%611.7% and 10.2%69.6%, respectively, duringquiet and deep breathing (p,0.001). Excluding thissituation ~prone position on soft support!, we found nostatistical significant effect of posture and maneuver%err. On the other hand, we found a significant effectthe type of support in the supine positio(p,0.01). Finally, the Bland–Altman analysis showethat the optoelectronic system did not introduce any stematic error due to the magnitude of the recorded sigin any situation.

Volume Partitioning

Table 2 shows the volume changes of the differechest wall compartments measured by OEP, in absovalues and as percentages of total chest wall voluvariations, in the supine and prone position during quand deep breathing. We only report data obtained wthe subject lying on the rigid surface, because the laccuracy in lung volume estimation found in the proposition/soft support did not allow a reliable compamental analysis. The absolute values ofVRCp, VRCa,VAB , and their sum at end expiration were obvioushighly dependent on body mass and, in the supine ption, averaged 7.65461.669, 2.93960.896, 7.51962.438, and 18.11364.780 l, respectively. These value


TABLE 2. Compartmental chest wall volume changes measured by OEP. Definition of abbreviations: QB, quiet breathing; DB,deep breathing; DVRCp volume changes of the pulmonary rib cage; DVRCa volume changes of the abdominal rib cage; DVAB volumechanges of the abdomen; and DVCW volume changes of the total chest wall. Data are expressed as mean values „ml …ÁSD. Valuesin brackets represent mean percentage ÁSD, with respect to the volume changes of each compartment „right and left … relative to

the corresponding total compartment.

Supine, rigid, QB Supine, rigid, DB

Right Left Total Right Left Total

DVRCp 87665 85663 1726126 4776198 4706120 9476391(52.067.4) (48.067.4) (23.369.3) (50.866.3) (49.266.3) (34.3611.4)

DVRCa 52630 45624 98653 2376107 2386124 4756228(52.567.6) (47.567.6) (13.663.0) (50.665.0) (49.465.0) (17.165.4)

DVAB 209655 216662 4246116 6306226 6436214 12736437(49.362.1) (50.762.1) (63.1611.4) (49.362.0) (50.762.0) (48.5615.2)

DVCW 3486131 3466122 6946295 13446334 13516326 26956655(50.162.4) (49.962.4) (49.861.9) (50.261.9)

Prone, rigid, QB Prone, rigid, DB

DVRCp 85653 80654 1646106 4136122 3746104 7876224(51.964.6) (48.164.6) (22.168.6) (52.462.2) (47.662.2) (35.3611.6)

DVRCa 94639 84641 178676 270697 248671 5186164(53.565.2) (46.565.2) (22.466.8) (51.763.9) (48.363.9) (22.565.7)

DVAB 190669 201676 3916144 4916242 4986199 9896439(48.762.1) (51.362.1) (53.5613.1) (48.863.2) (51.263.2) (42.2612.4)

DVCW 3696121 3646128 7336242 11746268 11206194 22946456(50.462.4) (49.662.4) (51.062.1) (49.062.1)

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do not exactly correspond to the real values, becauseshape of the nonvisible part of the chest surface wapproximated with a horizontal plane.

The results, reported in Table 2, indicate that:~a! Thepercentage contribution to total chest wall volume of tdifferent compartments that were monitored during qubreathing on a rigid support were dependent on the pture. Particularly, the volume changes of the abdomirib cage were higher (p,0.001) in the prone positionthan in the supine position, and the volume changesthe abdomen were lower in the prone position than insupine position~however, the differences were not sttistically significant! ~Fig. 6!. ~b! During deep breathingthe distribution of compartmental chest wall volumbecomes posture independent.~c! During deep breathingthe percentage of pulmonary rib cage volume chanwere greater (p,0.001) than during quiet breathing, anthe percentage of abdominal volume changes were lo(p,0.01). ~d! The volume changes in the right and leparts of the different compartments were similar andmaximum mean asymmetry for the entire chest wall w2.0%.

The results of the comparison between RIP and ocal compartmentalization during QB are reportedTable 3. In the supine position the slope of the regrsion line between RIP and optical data was closes

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identity when RCa was totally assigned to rib cage.the prone position the best agreement for the abdomwas obtained assigning RCa half to rib cage and halabdomen, while for the rib cage assigning RCa totallythe abdomen.

Isovolume Maneuvers

During the belly-out maneuver, OEP measured inthe considered subjects an increase of the abdomvolume of 5166169 ml on average and a decrease ofrib cage volume~including upper and lower parts! of5136143 ml. The mean discrepancy from the idealequal and opposite changes was, on average, equ3638 ml, or 0.6% of the thoracic volume changes. Fthe RIP signals, the same discrepancy was, on aver3.1%.

During the belly-in maneuver, OEP measured a dcrease in the abdominal volume of 3896165 ml on av-erage. This coincided with a total rib cage volume icrease of 4726206 ml. The mean discrepancy from thideal zero line, on average was 82674 ml or 17.4% ofthe thoracic volume changes. For the RIP signals,same discrepancy was, on average, 16.6%.

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DISCUSSION

Volume displacements of the entire chest wall areasily measured by total body plethysmography~TBP!.The use of this system would be the best validation oour measurements~at least forDVCW), but in this studywe did not consider measurements from TBP for th

FIGURE 4. Experimental tracings for a supine subject on arigid support. VRCp , volume changes of pulmonary rib cage;VRCa , volume changes of abdominal rib cage; VAB , volumechanges of abdomen; VCW , volume changes of total chestwall; VSP , lung volume changes from spirometry „correctedfor drift …; and volume-calibrated signals of rib cage „RC…,abdomen „AB …, and their sum „RC¿AB … from respiratory in-ductive plethysmography.

impossibility to obtain simultaneous measurements wOEP. The wall of the body box, in fact, reflects thinfrared light of the cameras and modifies its pathwaintroducing unacceptable image distortions.

In the past, plethysmographs have been applied torib cage or to abdomen separately,6 but the interpretationof the data was very difficult due to the movement of rcage and abdomen and the clinical applicability wnever demonstrated. Volume displacements of rib cand abdomen are therefore usually estimated indirefrom measurements of linear dimensions or areas. Teniques like respiratory inductive plethysmography24

magnetometry,23 and linear differential transducers17 as-sume that the chest wall is composed of only two coponents~rib cage and abdomen!, i.e., it has two degreesof freedom and that their volumes can be inferredmeasurements of transverse sections or distances.

The modeling of thoraco-abdominal motion is usuabased on the original Konno and Mead study,17 in whichthe rib cage and abdomen must both behave as sicompartments, each with only a single degree of frdom. Ben Haimet al.5 and Lichtensteinet al.20 proposeda mathematical model of chest wall and lung mechanin which the moving part of the chest wall is constituteby the rib cage membrane~formed by the lung-apposerib cage and the diaphragm-apposed rib cage! and theventral abdominal wall membrane. The limits for subehavior are rather narrow and are frequently exceein conditions other than breathing at rest,28 as alreadyindicated in the same study. Both the rib cage anddomen are best modeled as systems of at least two cpartments each.4,10,16,30 The agencies acting to displacthe lung-apposed part of the rib cage are quite differfrom those acting on the diaphragm-apposed part1,15,22,30

so that rib cage distortions are nouncommon.10,16 Furthermore, preliminary data showthat when the rib cage expands, it carries with it that pof the abdominal wall that is immediately subcostwhile the rest of the abdomen may move much les4

Thus, acquisition of volume change from changes

FIGURE 5. Lung gas volume changesfrom spirometry „DVSP… compared tochest wall volume changes from opto-electronic plethysmography „DVCW…

„quiet breathing, supine position, rigidsupport …. „Left … linear regression analysis„solid line, data regression line; dashedline, identity line …; „right … Bland–Altmananalysis.

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diameter or cross-sectional area of a single transvsection are problematic for both the rib cage andabdomen. The change in lung volume~the sum of thevolume changes in these two chest wall compartments! isthus subject to error arising from botcompartments. Three-dimensional x-ray computedmography~CT!19 and magnetic resonance imaging14 al-low accurate measurement of chest wall compartmeHowever, these techniques have a low temporal restion and, also, CT is ionizing.

In the past several other three-dimensional optitechniques18,21,25,26 have been proposed, but they hanot been introduced in clinical practice because of thlow temporal resolution and time-consuming data pcessing procedures. Besides, the boundaries betweedifferent compartments are not easily defined and blandmarks must be manually identified.

We have now demonstrated that optoelectroplethysmography8,9,12 overcomes these problems. It prvides a direct and noninvasive measurement of the enchest wall volume and an accurate computation ofcontribution of the different compartments, i.e., pulmnary rib cage, abdominal rib cage, and abdomen, incling separation between right and left sides. In the presstudy we investigated whether this method, alreadyveloped for the sitting and erect postures, can be scessfully applied to compartmental chest wall analysisconstrained postures as well, i.e., the supine and prpositions commonly used in clinical practice, especiain the ICU setting.

Comparison Among VL Measurements by Spirometryand Optoelectronic Plethysmography

In this study we found a high degree of similariamong the volume changes recorded by OEP and thfrom the reference method~water seal spirometry! in

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most of the conditions tested~supine and prone posturerigid and soft surfaces, and quiet and deep breathing!.

The results reported in Table 1 indicate that the ounsatisfactory condition is the prone position/soft suport. In this case, the error is likely due to the sosupport, that in the prone position let the anterthoraco-abdominal wall sink into the mattress and moduring respiration without being seen by the camerasour protocol we only considered two extreme conditiofor the support~completely rigid and very soft! and fu-ture studies will be performed to systematically chandifferent levels of mattress rigidity and different typesmattresses. This will allow us to identify possible thresolds of acceptance for measurement accuracy ofDVCW

in the prone position. However, in the supine positithe back surface movement contributes to volume dplacement in much lower measure in both these t

FIGURE 6. Tidal volume distribution in the three differentchest wall compartments „RCp, pulmonary rib cage; RCa,abdominal rib cage; and AB, abdomen … during quiet anddeep breathing on rigid support in the supine and proneposition. Data are expressed as mean Á SEM „** pË0.001,supine vs prone; s pË0.01, quiet vs deep; ss pË0.001,quiet vs deep ….

TABLE 3. Comparison between RIP and OEP compartmental volumes. Definition of abbreviations: QB, quiet breathing; ‘‘slope,’’‘‘intercept’’ of the regression line between RIP „x axis … and optical „y axis … estimates of compartmental volume variations; and r 2,squared linear regression coefficient. RC,opt and AB,opt, rib cage and abdominal volumes, respectively, from opticalmeasurements. All data are expressed as mean value ÁSD and they refer to measurements made for the subject on a rigid support.

RC,opt AB,opt

Supine, QB Prone, QB

Rib cage Abdomen Rib cage Abdomen

VRCp¿VRCa VAB Slope 0.98560.322 1.02960.178 1.71660.745 0.83460.374Intercept 0.00460.027 20.00260.034 20.01360.053 20.00360.073

r2 0.92260.057 0.97660.022 0.90860.092 0.98760.012

VRCp¿VRCaÕ2 VRCaÕ2¿VAB Slope 0.82760.296 1.13260.185 1.27760.508 1.00860.374Intercept 0.00660.031 20.00460.040 0.00960.033 20.00760.046

r2 0.92060.075 0.97060.028 0.92760.078 0.98960.013

VRCp VRCa¿VAB Slope 0.66560.276 1.23360.197 0.83260.310 1.18160.380Intercept 0.00760.040 20.00660.053 0.02760.033 20.02060.045

r2 0.90560.100 0.96260.034 0.93760.055 0.98960.013

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limit conditions and does not affect measurement acracy.

In order to explain the small discrepancies in all tconditions, as pointed out by Calaet al.,8 several otherpotential sources of error can be considered, like vations of humidity, pressure, and temperature of thebetween the lung and the spirometer. In order to corrfrom ambient temperature, pressure saturated with wvapor ~ATPS, 20 °C! to body temperature, pressure sarated with water vapor~BTPS! conditions, we shouldhave applied a correction factor of about 1.1. In tsupine position this should have changed the %err fr24.2% to 5.3% on rigid support and from20.1% to9.0% on soft support. However, in our experimental coditions ~closed circuit!, pressure and temperature afterfew breaths cannot be considered at constant ATtherefore the correction factor is likely lower than 1.Considering a correction factor of 1.05, the %err chanto 0.8% on rigid support and to 4.6% on soft suppor

Furthermore, spirometry was carried out during shperiods through closed-circuit rebreathing without C2absorption and with O2 consumption. However, as previously suggested,13 the possible discrepancy betweethe variation of gas in the spirometer and the variationchest wall volume due to these factors is likely neggible.

Possible blood shifts might produce relevant diffeences between variation of gas and chest volumes,ticularly during deep breaths or during positive pressventilation.3 This could be important information in aICU. In the present study, in which only spontaneobreathing was tested, we assumed that changes ofvolume in the lung equal changes in chest wall volumalthough we realize that in particular conditions this asumption does not hold true.

The error in the computation of 3D marker coordnates by the optoelectronic motion analyzer has breported previously for static and dynamic conditionsbe ;0.1 mm for the size of the field of view used for thpresent application,11 so its contribution to total error inchest wall volume reconstruction can be assumed vlow.

Another possible source of discrepancy between Oand the spirometer could be related to the dynamicsponse of the measurement systems. Optoelectronicethysmography is based on a motion analyzer that uCCDs sensors mounted on TV cameras equipped winfrared lighting. The system provides an electronic shter to avoid shape distortion of the markers on the CCThe ‘‘active period’’ of the CCD is only 1 ms, a duratiovery short, which represents the maximum theoretitime for sampling markers’ movement. All subsequeimage processing analysis does not introduce any furdelay or time-dependent effect. Comparing this vawith the frequency content of any respiratory moveme

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we can assume that OEP provides an ‘‘instantaneomeasurement of respiratory kinematics. Furthermore,well known that a water sealed spirometer is affecteda frequency-dependent response, due to its mechanproperties. When the volume changes are slow, thefollows well lung volume changes, while when measuing rapid respiratory maneuvers, the spirometric sigshows phase lags and some overshooting. For thisson, in this study we considered only ‘‘slow kinemaics,’’ to ensure a minimum effect of these dynamicbehaviors. Moreover, for each breath we consideseparately end-expiratory and end-inspiratory chest wand lung volume variations, which is equivalent to corect for possible phase shifts, but not for the effectsthe amplitude.

Isovolume Maneuvers

To assess the accuracy of the compartmentalizaprocedure, we used the isovolume maneuvers~belly-in orbelly-out movement!. Ideally ~absence of blood shiftsand negligible lung gas compression/decompress!changes in thoracic volume should be opposite and eqto changes in abdominal volume, and their sum shobe zero. Considering the overall data the magnitudethe discrepancy from the ideal zero line was 3638 mlduring belly out. This, on average, is a satisfactorysult. The larger discrepancy that we found duribelly-in maneuvers, could be attributed to gcompression/decompression or to possible blood shfrom periphery into the chest and vice versa.

Respiratory Inductive Plethysmography

Respiratory inductive plethysmography is the mocommonly used method for partitioning thoracic and adominal contributions to total volume changes bothnormal subjects and in critically ill patients. Howevethis device assumes the chest wall has only two degof freedom, whereas the optoelectronic method we evaated was designed to analyze kinematics in a chestwith more than two degrees of freedom. Although wused a six-compartment system~pulmonary rib cage, ab-dominal rib cage, and abdomen each divided into rigand left sides! there could be more than a single degrof freedom within the compartments which, in principlshould have little effect on the accuracy with which wcalculate volume changes. Our results indicate that insupine position the thoracic RIP band measures the upand lower thoracic contribution, with a high standadeviation. Thus in the supine position it appears thathealthy young subjects RIP is a reliable method to ptition chest wall into rib cage and abdomen becauseabdominal rib cage behaves as if it were part of tpulmonary rib cage.

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In the prone position this condition does not exist.fact, separate assumptions are required to get theagreement between optoelectronic and RIP data forther the rib cage or the abdomen. In the first case, bresults were obtained by attributing the lower rib cavolume changes to the abdominal band, while insecond case the best results were obtained attribu50% of the lower rib cage volume changes to the thracic band and 50% to the abdominal one. In both cathe standard deviation was generally high. These resare probably due to the functional anatomy of the cosand crural diaphragm, whose insertion on the lowercage is different in the anterior and posterior parts,termining different displacements and shapes. Anotpossible explanation is that they are influenced byshift in the subject’s weight distribution and the externloads caused by the support. RIP measurements reqthat the geometry~shape! of the chest wall that the bandare around be relatively stable as the size changesthe number of degrees of freedom remains the same.results seem to indicate that to assess the lower rib cand abdominal volumes it is mandatory to measure thcomplex shapes. The calibration procedure used forRIP ~Ref. 27! did not allow considerations of deebreathing in our analysis because in this case were abthe validity range and the errors were very high. Finathe results obtained from isovolume maneuvres indicthat OEP and RIP compartimentalization gives smallrors in total chest wall volume variations during belly-and belly-out maneuvers.

Compartmental Chest Wall Volume Changes

As shown in Table 2, the OEP enables the measument of volume variations of the different compartmenthat we choose along the cranio-caudal~pulmonary ribcage, abdominal rib cage, and abdomen! and lateral~right and left! axes. The interpretation of these damust consider that compartmental chest wall voluchanges do not reflect regional lung volume chanbecause it is well known that the value of the shemodulus of the lung is considerably smaller than tvalue of the bulk modulus31 and that the lung is movingfreely within the thoracic cavity. This was demonstratby Venegaset al. using positron imaging.29 The apicalchest wall is mechanically constrained and so voluchanges in the upper half of the lung will by necessityreflected in changes in abdominal rib cage and abdomas the lung expands caudally.

In this context, the capability of the OEP to measusubdivision between right and left chest wall expanscould be useful when considering asymmetries of resratory muscle action and chest wall compliance~i.e., inhemiplegia, paralysis of hemidiaphragm, kyphoscoliosfibrothorax, ankylosing spondylitis, thoracoplasty, etc!,

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while the relationship between asymmetry of lung vetilation and chest wall expansion is questionable and wbe investigated in future studies. In our 10 subjects,expected, most of the chest wall volume change is dtributed in the abdominal compartment, both in the spine and prone position, and it appears equally distruted between right and left parts of the chest wall.

As shown in Fig. 6, the only variation in prone compared to supine is relative to the lower rib cage compament, even if in this posture the subdivision betweupper and lower rib cage is much more critical.

In conclusion, OEP is a method that can be usedstudy, for a wide range of volumes, the kinematics aif combined with pressure measurements, the mechaof the chest wall in the supine and prone positions. Ithighly accurate in the measurement of total chest wvolume variations, allowing the splitting of the compleshape of the chest wall into different compartments. Itcompletely noninvasive and it does not require conntion to the patient. Furthermore, it can be used withousubject-specific calibration, because it provides a dirmeasurement of the volumes in a three-dimensionalerence frame and the calibration is not based on partlar respiratory maneuvers requiring subject cooperatiAll these features make the OEP an attractive toolevaluating mechanical ventilation in critically ill3 orchronic patients, for studying sleep or, in general,analyze patients who are not able to maintain other ptures than supine or prone.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Professor P.Macklem for helpful comments during preparationthis manuscript. This work was supported by the Eupean Commission–BIOMED II program@biomedicaltechnology research project ‘‘BREATH’’~biomedicaltechnology for respiration analysis through optoelectroics!# and MURST–Cofin ’98.

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