INACCURACY OF OESOPHAGEAL PRESSURE FOR PLEURAL PRESSURE ESTIMATION IN SUPINE ANAESTHETIZED SUBJECTS

Transcription

1 Br. J. Anaesth. (1983), 55, 585 INACCURACY OF SOPHAGEAL PRESSURE FOR PLEURAL PRESSURE ESTIATION IN SUPINE ANAESTHETIZED SUBJECTS G. B. DRUOND AND A. D. G. WRIGHT SUARY Oesophageal pressure wa measured, usng a balloon and catheter system, at three or four postons n the oesophagus of eght supne subjects anaesthetzed wth 1-1.5% halothane n 67% ntrous oxde. Arway pressure and the dfference between arway and oesophageal pressures were recorded durng occluson of nspraton, occluson of expraton and occluson of expraton followed by nspratory occluson. Lung volume could not change apprecably durng these manoeuvres, so the dfference between arway and oesophageal pressure should not have changed. The change n oesophageal pressure was less than the change n arway pressure n 86 of 90 measurements. The change n oesophageal pressure was exp eased as a fracton of the change n arway pressure: the marmm fracton was obtaned n each patent, and the mean of these maxmum values was 82%. Ths suggests that changes n the dfference between arway and oesophageal pressures wll overestmate the change n transpulmonary pressure durng artfcal ventlaton n supne subjects, and that hng complance would be underestmated. To measure the separate mechancal propertes of the lung and chest wall, pleural surface pressure must be known. Oesophageal pressure s wdely used as an ndrect ndex of pleural surface pressure and seems vald n conscous man (Agoston, 1972) and nfants (Beardsmore et al., 1980). Oesophageal pressure has been used to nvestgate the changes n respratory mechancs assocated wth the nducton of anaesthesa n supne subjects. Lung volume and complance are decreased, and may be responsble for altered dstrbuton of ventlaton and mpared gas exchange. However, the mechansms of the mechancal changes are stll hypothetcal (Schmd and Rehder, 1981). Unless oesophageal pressure does provde a relable estmate of pleural surface pressure n anaesthetzed subjects, measurements of respratory mechancs durng anaesthesa by these means may be msleadng (Sykes, 1982). Arway occluson durng an nspratory attempt can be used to assess the relatonshp of oesophageal and pleural pressures n anaesthetzed subjects. Durng the nspratory attempt, pleural pressure wll decrease as the nspratory muscles contract. If the arway s occluded, the lungs can expand only by a small amount, caused by expanson of the gas already wthn them. Consequently, transpulmo- G. B. DRUOND, _A.,.B., CH.B.. F.F.AJI.OS.; A. D. G. WRIGHT,.B., CH.B., F.F.AJI.C.S.; Department of Anaesthetcs, Royal Infrmary, Ednburgh EH3 9YW. nary pressure wll change only slghtly, and arway pressure should change almost equally wth pleural pressure. Thus, smultaneous measurement of arway and oesophageal pressures (or the dfference between arway and oesophageal pressure) should show whether a change n oesophageal pressure s a good ndex of a change n pleural pressure. PATIENTS AND ETHODS Eght patents were studed n the supne poston durng stable anaesthesa for varcose ven surgery; detals are gven n table I. They breathed 1-1.5% halothane n 67% ntrous oxde from a T-pece system connected through a large-bore tap to a one-way valve (Ambu Hesse) and tracheal tube. Arway pressure (.? ) was measured from a connecton to the valve usng a pressure transducer (DC cromanometer, Furness Control) and recorded wth a Sex F F F Age (yr) TABLE I. Pattnts studed Heght (cm) Weght (% expected) The acmllan Press Ltd 1983

2 586 BRITISH JOURNAL OF ANAESTHESIA chart recorder (Devces X2). Oesophageal pressure (Poe«) was measured usng a latex rubber balloon (5cm long, 6.5mm dameter, walls 0.1mm thck) connected to a catheter (80 mm long, 1 mm nternal dameter). Ths catheter and the arway pressure tubng were connected to the opposte sdes of a second pressure transducer so that the dfference (P,w - Poo) was measured and recorded. Several balloons were used n the course of the experment. The pressure volume characterstcs of each balloon were determned wth the balloon suspended n ar, and a note taken of the volume range over whch added ar caused the least ncrease n balloon pressure. A volume of ar close to the lower end of ths range was placed n the balloon when makng measurements. One balloon was suspended n ar n a closed chamber and the pressure dfference between the balloon catheter and the chamber measured usng an nclned plane manometer for chamber pressures from to Pa. The oesophageal catheter was advanced n the oesophagus untl an ncrease n balloon pressure was noted wth nspraton, ndcatng that the balloon was n the stomach, and then wthdrawn untl the pressure swngs reversed. It was then wthdrawn a further 5 cm and the frst set of measurements taken. Further measurements were made at two or three more 5-cm ntervals cranally n the oesophagus. Each set of measurements was performed as follows: durng expraton through the nonrebreathng valve, the tap at the nspratory port was turned to occlude the followng nspraton. P w and (P«- Poa) were recorded at a chart speed of Smms" 1. The tap was opened n the followng expraton and an nterval of at least 10 breaths allowed before the next occluson was performed. Fve occlusons were performed. The mean changes n P m and (P w - P^ from the start to fnsh of the nspratory attempt were measured and used to calculate the rato (change n Poe,)/(change n P«), (A Poo/A P.,), for occluson of nspraton (). Fgure 1 shows dagrammatcally how, f lung volume does not change, arway and oesophageal pressure should decrease equally from A to B durng the nspratory attempt (sold lnes). The second manoeuvre used was occluson of the expratory port of the valve durng nspraton (fg. 1, pont C). At end-nspraton, P w was atmospherc and (Ptw-Poo) was ncreased, because of the ncreased lung volume and transpulmonary pressure. Durng the occluded expraton, arway pressure ncreased as the nspratory muscles relaxed Tdal volume Occluson Occluson FIG. 1. Dagrammatc representaton of the manoeuvres studed. P n = pressure n arway;? <*, -= pressure n oesophagus. The sold lnes represent the deal values f oesophageal pressure and pleura! pressure were changng dentcally and the dotted lne represents the proposed "underestmate" of pleural pressure change by oesophageal pressure. A-B = occluson of nspraton; C-D = occluson of expraton; D-E = occluson of nspraton after occluson of nspraton.

3 SOPHAGEAL AND PLEURAL PRESSURES IN ANAESTHESIA 587 and pleural pressure ncreased. As lung volume dd not change, (Ptm-Poa) should not have changed durng ths occluded expraton () (to pont D, fg. 1). The subsequent nspratory attempt, whch was taken wth lung volume at (FRC + the precedng tdal volume), was also occluded ( ). Agan, lung volume dd not change and (P^-Pon) should not have changed (D to E, fg. 1). Fve of these manoeuvres were done and mean pressure at ponts C, D and E calculated, and AP oc JAP n calculated for C to D and D to E. The deal relatonshp between P w and P^, durng these manoeuvres can be shown dagrammatcally (fg. 2). The sold lnes represent ths deal relatonshp, whle the broken lnes are typcal of the expermental results. If (.? - Poo) remans constant, then the lnes A-B and D, C, E should have a slope of 1. If Poa changes less than P w, then the slope wll be less. Fnally, ntrous oxde was wthdrawn from the nspred gas mxture and after 2 mn a reservor bag was attached to the open lmb of the T-pece and the fresh gas nlet connected to the expratory port of the non-rebreathng valve. In ths way, progressve rebreathng resulted, and caused ventlatory stmulaton, wth ncreases n tdal volume. Occlusons of nspraton were performed at 10-breath ntervals untl arway pressure at the end of the nspratory attempt had become approxmately twce the ntal value. Rebreathng was then stopped, ntrous oxde re-ntroduced nto the fresh gas supply, and the oesophageal balloon wthdrawn 5 cm. Fve mnutes were allowed for stablzaton and occluson pressure was then measured at breath ntervals untl constant occluson values were obtaned. The measurements (,, and rebreathng) were then repeated. Studes were undertaken at three or four postons n the oesophagus, accordng to the tme avalable durng surgery. The ratos of A P^/A P n were compared for dfferent manoeuvres and dfferent postons usng the Wlcoxon text for pared values, and mean values obtaned for comparson after logarthmc converson, snce the logarthms of these ratos were more lkely to be normally dstrbuted. RESULTS The relatonshp between balloon volume and nternal pressure, wth the balloon suspended n ar, s shown n fgure 3. The "workng range" of volume was taken as -0.2 to +0.1 ml, relatve to the volume at zero pressure. Fgure 4 shows the change n pressure dfference from nsde to outsde the balloon, as external pressure was altered. Ths ndcates that, over ths "workng range" of volumes, correspondng to nternal balloon pressures from 40 to +10 Pa, the relatonshp of nternal and external pressures was lnear. A systematc dscrepancy of about 2% exsted between changes n appled (external) and measured (nternal) pressure. oes Balloon pressure (kpa) Q.2, A aw 'S Balloon volume (ml) FIG. 2. Ideal relatonshps between arway pressure and oesophageal pressure ndcated by the sold lnes for occluson of nspraton (from A to B) and occluson of expraton (C to D) followed by occluson of nspraton (from D to E). The nterrupted lne ndcates the observed "underestmate" of pleural pressure FIG. 3. Pressure-volume relatonshp for balloon, suspended n

4 588 BRITISH JOURNAL OF ANAESTHESIA Balloon pressure - External pressure (Pa) 20 III, for each poston, patent and manoeuvre. The mean rato was slghtly greater durng the occluded expraton (0.60) than occluded nspraton (0.53) or occluded nspraton after occluded expraton (). Comparson of the ratos obtaned at each poston n each subject for occluded expraton and nspraton at the same lung volume ( and ) showed that the rato was sgnfcantly greater (P < 0.05). In ndvdual subjects n whom Pp s to be estmated by means of /*«,, t may be possble to adjust the balloon poston to obtan a maxmal rato of oesophageal to arway pressure change. Takng the maxmal values for the rato observed n each subject, usng the manoeuvre, a mean rato of External pressure (Pa) FIG. 4. Pressure dfference from nsde to outsde the balloon, wth changes n external pressure, at three balloon volumes n the "workng range". Alteraton of balloon volume by greater amounts, up to 0.3 ml, durng the patent studes, resulted n obvous changes n balloon pressure. However, no dfference could be detected n the changes n pressure measured durng the dfferent manoeuvres, suggestng that even f small errors n absolute pressure were nduced, satsfactory measurement of the change n pressure could be obtaned. Table II gves mean values for arway pressure and for oesophageal pressure, calculated from values of (P., - Poo) and P n, for all the balloon postons measured. These values are shown n dagram form (fg. 5), usng a plot as n fgure 2, wth each lne of plots showng measurements, from caudal to cranal postons n the oesophagus of each patent. ean oesophageal pressure at end-expraton vared consderably from poston to poston wthn the oesophagus of each patent, and to a lesser extent from patent to patent. The pressure at the second poston, 10 cm above the daphragm was partcularly varable. ean pressure was greater at ths pont (0.90, SD 0.32 kpa) and sgnfcantly greater than at the thrd poston (0.41 kpa). The change n.?«, durng the sovolume manoeuvres (,, ) was only greater than, or equal to, the change n P n n four of the 90 measurements. The rato of these changes s shown n table Caudal Cranal FIG. S. Each horzontal row of plots shows the relatonshp of PQC (vertcally) to P mw (horzontally), n a sngle patent, n postons n the oesophagus from caudal to cranal, for the manoeuvres studed.

5 SOPHAGEAL AND PLEURAL PRESSURES IN ANAESTHESIA 589 TABLE II. Arway and oesophageal pressures (Pa) at 5-cm ntervals abovt daphragm, = the most caudal poston. A - end-expraton; B = end of occluded nspraton (the manoeuvre); C=end of tdal nspraton; D=end of occluded expraton (the manoeuvre); E=end of a subsequent occluded nspraton ( manoeuvre). Arway pressures at A and C were always zero P.W I P.. P<*. Patent No. Poston A B B C D D E E v v v v v v O TABLE III. Rato of change n oesophageal pressure to change n arway pressure durng occluson of nspraton (), occluson of expraton () and occluson of expraton and nspraton ( ), at each oesophageal poston n each pattnt Poston 1 Poston 2 Poston 3 Poston 4 Patent

6 590 BRITISH JOURNAL OF ANAESTHESIA was obtaned, wth a range from to The maxmal ratos were not found at the same poston n the oesophagus n dfferent subjects. The mean rato for all the subjects was 0.74, 0.56 and at the 5-, 10- and 15-cm postons n the oesophagus. Often, an oesophageal balloon s adjusted to obtan a mnmal statc (end-expratory) pressure. The mean rato APo^/AP,,, obtaned by takng the ratos where P^, was least, was In only two of eght subjects was the rato less than mnvmfll at the poston where statc P,*, was mnmal and the dfference between the ratos obtaned at the ponts of mnmum?<*, and the maxmum ratos observed n each patent were not statstcally sgnfcant. However, n only four of eght patents was the mnmal rato found mmedately above the daphragm. Because only sngle values for P n and (P,*, - P w ) were obtaned at the end of the rebreathng manoeuvres, mean values could not be used, and the varaton was consderable. No systematc dfference could be detected between the rato A P^/ A!> for the manoeuvres at the start and end of rebreathng despte a doublng of the A P n. DISCUSSION easurement The oesophageal balloon technque s not of great value n the estmaton of absolute values of pleural pressure, although pressure changes can be measured satsfactorly. The oesophagus tself forms part of the recordng system and ts elastc behavour and the effects of contaned flud and ar may nfluence transmsson of pressure from the pleural cavty (Agoston, 1972). lc-eml, ead and Turner (1964) emphaszed the mportance of usng small volumes of ar n the balloon and used 2-cm long balloons to measure local oesophageal pressure. In nfants, satsfactory pressure recordngs were obtaned usng thn-walled balloons 5 cm long and 0.76 cm dameter, smlar to those used n the present study. Wth a workng range of volume of less than 0.4 ml, the occluson test gave ratos of oesophageal to arway pressure change from 94 to 103% and accurate statc measurements were obtaned over a pressure range of ± 3kPa (Beardsmore et al., 1980). When the balloon s n place n the oesophagus, the pressure-volume relatonshp dffers from the relatonshp found n ar (ead et al., 1955; lc- Eml et al., 1964). For accurate measurements of absolute oesophageal pressure, lc-emh' and colleagues (1964) suggested measurement at several dfferent balloon volumes and extrapolaton of the pressures obtaned to zero balloon volume. An external pressure gradent, such as s produced by suspendng a balloon vertcally n water, dsplaces ar wthn the balloon to a small regon wthn t and ntroduces an error n the measurement of statc pressure. Dstenson of the materal of the wall n ths small regon ntroduces a dfference n pressure from nsde to outsde the balloon. The range of balloon volumes over whch accurate absolute pressure measurements can be made s decreased, but pressure changes are stll accurately measured (Lemen, Benson and Jones, 1974). In contrast to the studes of lc-eml and co-workers (1964), the balloon was not vertcal n the present study. Nevertheless, pressure gradents of magntude smlar to the vertcal gradent n water were detected occasonally: n three subjects, a pressure change of 0.9 kpa was found between balloon postons 5 cm apart. Consequently, an error of up to 0.1 kpa could have been ntroduced n the measurement of the statc pressure between these ponts. If lung volume s held constant, then (P n - Poet) should not change when P n s altered (lc-eml, ead and Turner, 1964). Two factors could ntroduce dscrepances between changes n P n and P^,. Frst, an appled pressure change wll cause rarefacton or compresson of the gas n the lungs, and the small change n lung volume wll lead to a small change n transpulmonary pressure. In the present study, the lung volume would have been small, and the pressure changes were small, whch would decrease the magntude of ths effect. However, f lung complance were decreased by anaesthesa, the change n transpulmonary pressure would be greater for a gven change n lung volume. Assumng an FRC of 2 ltre and a small lung complance of 0.5 ltre kpa" 1, the change n oesophageal pressure would be only 4% greater than arway pressure. Second, transmsson of alveolar pressure to the upper arway s nfluenced by the resstance and complance of the arways. In addton, gas flow may occur between lung regons, f the lung s not homogeneous. In patents wth chronc arways obstructon, the change n arway pressure 100 ms after the onset of an occluded nspraton s 30% less than oesophageal pressure change (arazzn et al., 1978). However, f upper arway complance s small, and the complance of the mouth and pharynx are excluded by ntubaton, the tme constant of the arway system wll be neglgble (Jaeger, 1982). In

7 SOPHAGEAL AND PLEURAL PRESSURES IN ANAESTHESIA 591 addton, our measurements were made at the end of the nspratory or expratory phase, when ths dynamc effect should have been mnmal. There was a small dscrepancy between appled and measured pressure change when the oesophageal balloon used n the present study was tested (fg. 4). Ths was caused by the relatvely great volume dsplacement of the nclned plane manometer that was used to measure accurately the small pressure dfference across the balloon. The volume dsplacement of the mcromanometers used n patent measurements was too small to be measured, so the dscrepancy would have been less n the patent measurements, wth a neglgble error n estmaton of oesophageal pressure change. These consderatons ndcate that although some errors may be present n estmates of absolute oesophageal pressure, pressure changes should be measured accurately to wthn 5%. Results Clearly, great attenton to absolute pressure measurements n the oesophagus s not justfed. Prevous workers have observed artefacts n oesophageal pressure measurement at low lung volumes (Knowles, Hong and Rahn, 1959) and that changes n oesophageal pressure were less than changes n arway pressure when measurements were made n the upper one-thrd of the oesophagus (lc-eml, ead and Turner, 1964). In supne subjects, restng lung volume decreases on nducton of anaesthesa (Rehder, Sessler and arsh, 1975). Ths decrease occurs rapdly and s probably caused by an abolton of daphragmatc tone (Froese and Bryan, 1974; uller et al., 1979; Bergman, 1982). Thus, oesophageal pressures are lkely to be greater than n conscous subjects. In the present study, the mean ncrease n arway pressure durng the manoeuvre was 0.52 ± 1.1 kpa. Ths s almost as much as the decrease durng the manoeuvre (0.71 ±0.18kPa), and t s possble that expratory muscle acton was present whch would further decrease FRC. Ths was confrmed by the observaton of postve arway pressures n expraton when both nspraton and expraton were occluded, and by nspecton and palpaton of the abdomnal muscles. Consequently, end-expratory lung volume would be even less, and oesophageal pressures would be greater than at relaxed FRC. In sx curarzed patents durng artfcal ventlaton n the supne poston, Wldsmth (1973) noted that oesophageal pressure was postve n 33 of 36 observatons at dfferent postons. In the present study, a subatmospherc pressure was found on only one occason. However, n contrast to the fndngs of Wldsmth (1973), the most postve pressures were not found n the most caudal poston. In the present study, the balloon was postoned relatve to the daphragm rather than relatve to the teeth, so the most caudal poston reported n the two studes may not be comparable. Westbrook and colleagues (1973) found oesophageal pressures from about 2 to 4 cm H2O n supne subjects anaesthetzed wth thopentone, and stated that (P m -.?«,) dd not change durng arway occluson. Changes n pleural pressure durng respraton are not unform throughout the pleural space. Especally f only one group of nspratory muscles s actve, changes n oesophageal pressure may not reflect an "average" of the pleural pressure change. Daphragmatc actvty alone generates greater changes n the daphragmatc pleural space, and ntercostal actvty alone produces greater changes n pressure n the upper costal pleural space (D'Angelo, Sant'Ambrogo and Agoston, 1974). Such changes may be transmtted to the alveol, snce pressure swngs n the occluded lower lobe bronchus of anaesthetzed dogs can be made to be greater or smaller than n the occluded arway by phrenc nerve stmulaton (Brown, Scharf and Ingram, 1982). organ, Downs and Weled (1980) measured ntrapleural pressure drectly wth a transducertpped catheter n anaesthetzed dogs and found that pleural pressure change was 9% greater than arway pressure change durng occluson of nspraton. Lung gas rarefacton, decreased transmsson of pressure to the upper arways, and the above expermental fndng, all ndcate that oesophageal pressure should change more than arway pressure. However, n the present study changes n oesophageal pressure were almost always less, often consderably so, than the changes n arway pressure. Snce the oesophageal balloon can accurately measure pressure change, ths fndng suggests that n such patents, oesophageal pressure change s a poor ndex of "average" pleural pressure change. Three factors may be responsble. Frst, because of actve expraton, dependent lung zones and medastnal structures may mpar transmsson of changes n pleural pressure from other regons of the pleural space. Second, because the daphragm s lkely to have been the only actve muscle durng

8 592 BRITISH JOURNAL OF ANAESTHESIA nspraton (Tusewcz, Bryan and Froese, 1977), the pressure changes n the pleural space are lkely to. have been greatest on the daphragmatc surface, rather than unformly dstrbuted. Thrd, pressure change n expraton was not caused by passve recol of the elastc structures of the respratory system alone, but also by abdomnal muscle contracton. The slghtly greater rato of oesophageal to arway pressure change durng occluson of expraton may reflect a more unform contrbuton of passve recol to pleural pressure change. Lung volume durng the manoeuvre was greater than the lung volume durng the manoeuvre. The and manoeuvres, and the rebreathng manoeuvres, were studed to detect a possble nfluence of lung volume on transmsson of pressure changes to the pleural space. In some subjects, tdal volume was measured by means of a Wrght resprometer, and was rarely greater than 200 ml, so the effect of lung volume would have been small. No dfference could be demonstrated n the rato of the changes n oesophageal and arway pressures durng the,, and rebreathng manoeuvres, so t s possble that the sgnfcantly greater rato for the manoeuvre was caused by an nfluence other than that of tdal volume. Ths study ndcates that measurement of the mechancal propertes of the lung and chest wall n supne anaesthetzed subjects could be erroneous f oesophageal pressure s used to assess pleural pressure. In spontaneously breathng subjects, the ncrease n transpulmonary pressure durng nspraton s a result of a decrease n pleural pressure, and arway pressure s lttle changed. If oesophageal pressure underestmates the pleural pressure change, then derved complance wll be erroneously large. In contrast, when arway pressure s ncreased to nflate the lungs artfcally, both arway pressure and pleural pressure ncrease. The arway pressure change s greater, and transpulmonary pressure (P.w -.Pp) ncreases. If the change n P*. s not as great as the change n pleural pressure, the change n (P n - P<») wll be greater than the change n (P re Pp). Consequently, lung complance wll be underestmated f t s calculated from (P w - P^). Smlarly, (P w - P^ at a gven lung volume durng lung nflaton, as an ndex of lung recol pressure, s lkely to be erroneously large. Westbrook and colleagues, studyng artfcally ventlated supne subjects, reported that lung complance was decreased by anaesthesa and that recol pressure at 50% control TLC was ncreased (Westbrook et al., 1973). easurement of the rato of oesophageal and arway pressure change durng sovolume manoeuvres, at dfferent lung volumes, may allow a correcton to be made when changes n P*, are used n measurements of lung mechancs n supne anaesthetzed subjects. REFERENCES Agoston, E. (1972). echancs of the pleural space. Physol. Ker.,52, 57. Beardsmore, C. S., Helms, P., Stocks, J., Hatch, D. J., and Slverman,. (1980). 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9 SOPHAGEAL AND PLEURAL PRESSURES IN ANAESTHESIA 593 Tusewcz, K., Bryan, A. C, and Froese, A. B. (1977). Contrbutons of changng rbcagc-daphragm nteracton to the ventlatory depresson of halothane anesthesa. Anathesology, 47, 327. Westbrook, P. R., Stubbs, S. E., Sesslcr, A. D.,Rehder, K.,and Hyan, R. E. (1973). Effects of anesthesa and muscle paralyss on respratory mechancs n normal man. /. Appl. Phyol., 34, 81. Wldsmth, J. A. W. (1973). The effect of posture on the measurement of oesophageal pressure n the curarzed subject. Br. J. Anaath., 4S, LA ESURE DE LA PRESSION SOPHAGIENNE EST UN AUVAIS REFLET DE LA PRESSION INTRAPLEURALE CHEZ DES SUJETS ANESTHESIES EN DECUBITUS DORSAL RESUE La presson oesophagenne a et mesurce a l'ade d'un systeme de catheter a ballonnet, en tros ou quatre ponts de l'oesophage, chez hut sujets en decubtus dorsal, anestheses par de 1-1,5% l'halothane dans 67% de protozyde d'azote. La presson dans les voes aerennes, et la dfference entre le pressons dans les voes aerennes et dans l'oesophage, ont etc enregstrees au cours d'une nspraton avec occluson, d'une occluson a l'expraton et d'une occluson a l'ezpraton suve d'une occluson a l'nspraton. Le volume puhnonare ne pouvat pas se modfer de faeon nette au cours de ces manoeuvres, auss la dfference entre la presson dans les voes aerennes et la presson dans l'oesophage n'aurak-elle pas du changer. La modfcaton de la presson oesophagenne etat nfereure 4 celle de la presson dans les voes aerennes dans 86 mesures sur 90. La modfcaton de la presson oesophagenne etat exprmee comme une fracton de la modfcaton de la presson dans les voes aerennes: la fracton mn-rmm a etc obtenue chez chaque patent et la moyenne de ces valeurs maxmales etat de 82%. Cec suggere que des modfcatons de la dfference entre les pressons dans les voes aerennes et dans l'oesophage pussent entraner une suresumaton des modfcatons de la presson transpulmonare au cours de la ventlaton artfcelle chez des sujets en decubtus dorsal et que la complance pulmonare pusse etre sous-estmee. UNGENAUIGKEIT DER OSOPHAGEALEN DRUCKESSUNG ZUR BESTIUNG DES PLEURALEN DRUCKES" BEI AUF DE RUCKEN LIEGENDEN HUNDEN ZUSAENFASSUNG Be acht legenden Teren, de mt 1-1,5% Halothan n 67% Lachgas narkotsen wurden, wurde der osophageale Druck mt enem Ballon-und Kathetersystem n dre oder ver Postonen gemessen. Der Druck n den Luftwegen und dessen Dfferenz zum osophagealen Druck wurde be Okkluson der Enatmung, der Ausatmung und der Ausatmung mt nachfolgender nspratorscher Okkluson gemessen. Das Lungenvolumen konnte sch wfhrend deser anfver nur unbedeutend andern, so dab der Untersched zwschen den Drucken n den Luftwegen und m Osophagus sch ncht andern durfte. De Veranderung des osophagealen Drucks war jedoch be 86 von 90 essungen gernger als de des Drucks n den Luftwegen. De Veranderung des osophagealen Drucks wurde als Bruch der Veranderung des Luftwegedrucks ausgedruckt, wobe der ttelwert der grobten Bruchzahlen von jedem Ter 82% ergab. Wahrend kunstlcher Beatmung legender Tere werden also Veranderungen n der Dfferenz zwschen Luftwege- und Osophagusdrucken de Veranderungen m transpulmonalen Druck uberschatzen und de Lungencomplance unterschatzen. INEXACTTTUD DE LA PRESION ESOFAGAL EN LA ESTIACION DE LA PRESION PLEURAL EN PACIENTES ANESTESIADOS EN POSICION SUPINA SUARIO Se mdo la preson csofagal utlzando un sstema dc sonda y cateter en tres o cuatro puntos del esofago de ocho pacentes anestesados en posc6n supna, con halotano al 1-1,5% en odo ntroso al 67%. La preson del are y la dferenca entre las presones del conducto de are y del esofago se mderon durante la ocluson de la aspracon, ocluson de la expracon y ocluson de la expracon seguda por ocluson de la aspracon. Durante estas manobras el volumen pulmonar no pudo varar de forma aprecable, por lo que la dferenca entre la pres6n del conducto de are y la del esofago no deben cambarse. El cambo de la preson en el esofago fue nferor al de la preson en el conducto de are en 86 de 90 medcones efectuadas. El cambo de la pres6n en el esofago se expreso como una f racc6n del cambo de la pres6n en el conducto de are: en cada uno de los pacentes se obtuvo la fraccon maxma y la meda de estos valores mtfxmos fue del 82%. De esto puede deducrse que los cambos en la dferenca de presones entre el esofago y el conducto de are haran que le sobreestmen los cambos en la preson transpulmonar durante la ventlacdn artfcal en pacentes en poston supno, y que la conformdad pulmonar se nfraestmara.