Determination of the Optimal Pressure Support Level Evaluated by Measuring Transdiaphragmatic Pressure*

Transcription

1 Determnaton of the Optmal Pressure Support Level Evaluated by Measurng Transdaphragmatc Pressure* 'Iomomasa Kmura, M.D.;t [un Takezawa, M.D., F.C.C.P;+ Kmtosh Nshwak, M.D.;t and Yasuhro Shmada, M ~ D F.C.C.R., The purpose of ths study was to determne the optmum pressuresupport(ps)n sxpatentswth respratoryfalure. Esophageal pressure (Pe), gastrc pressure (Pg), arway pressure, and transdaphragmatc pressure (Pd), obtaned by subtractng Pe from Pg, were measured usng a newly developed multlumnal nasogastrc catheter. For each patent, dfferent PS levels were selectedevery 20 mnutes, and measurements were made at each PS level. We de6ned the optmum PS level as the level that showed the mnmum Pe value. Respratoryrate (RR) decreasedand tdal volume (VT)ncreased wth an ncrease n PS level. RR and VT at the optmum PS were 19.7±5.5 breaths per mnute and 11.7 ± 4.5 mllkg, respectvely Pd decreased lnearly wth ncreasng PS level n all patents. Mean Pd at the optmum Ps was 4.2 ± 1.2 em 1Is0. Based on the relatodshp between Pd and PS level, we constructed an equaton to estmate the optmum PS level as follows: Optmum PS level= ([Pd durngt-pece mode] - 4)/0.8. We conclude that Pd measurement s helpful for ttratng the requred PS level. (Cheat 1991; 100:112-17) Paw =arway pressure; Pd =transdaphragmatc pressure; Pe=esophageal pressure; Pg=gastrc pressure; Ppl=ntrapleural pressure; PS =pressure support; PSV=pressure support ventlaton; SIMV =synchronzed ntermttent mandatory ventlaton M echancal ventlaton wth pressure support (PS) can be characterzed as a pressure assst durng a spontaneous nspraton that decreases mposed work of breathng.!" Dependng on the level of nspratory pressure assst, ventlatory muscle effort s ether partally or totally elmnated.v' However, an excess PS may be smlar to total asssted mechancal ventlaton wth potental hypernflaton; an nadequate PS cannot reduce work of breathng and could result n respratory muscle fatgue. A smple way to determne the optmum PS s to use the pressure requred to obtan the slowest regular respratory rate (RR). 2 The second method s to obtan a predetermned tdal volume (VT) used n a synchronzed ntermttent mandatory ventlaton (SIMV) mode." Both methods were termed maxmum pressure support ventlaton (PSVmaJ by Maclntyre." Another method s to fnd the PS level at whch measured lung work and mposed work are least." Concernng the mposed work, the optmum PS s regarded as the level at whch the total ventlatory system resstance to gas flow s nullfed. 5 6 However, n none of those methods can the changes n daphragmatc actvty wth alterng PS be evaluated. From the vewpont of respratory muscle fatgue, the optmum level of PS *From the ICU and Department of Anesthesology, Nagoya Unversty Hosptal, Nagoya, Japan. t Assstant Professor. tassocate Professor. I Professor. Manuscrpt receved August 8; revson accepted December 18. &prnt requests: Dr. Kmura, leu, Nagoya Unversty Hosptal, TsufUmt.d-cho, Showa-1cu, Nagoya 466, japan 112 s, theoretcally, the PS that mantans daphragmatc actvty wthout fatgue.v Daphragmatc contractlty cannot be assessed drectly, and hence, the best ndrect means s to obtan transdaphragmatc pressure (Pd) by subtractng ntrapleural pressure (Ppl) from gastrc pressure (Pg). 8 The magntude of Pd could be a smple and useful marker of nspratory effort for a gven patent wth severe respratory dsease undergong pressure support ventlaton (PSV). The purpose of ths study was to clarfy the usefulness of Pd measurement for ttratng PS level by comparng Pd wth RR and VTat varous PS levels. MATERIALS AND METHODS Sx patents requrng mechancal ventlatory support (72OOa, Purtan- Bennett) were ncluded n ths study. Before conductng ths study, nformed consent was obtaned from each patent followng the gudelnes set by the nsttutonal human ethcs commttee. Clncal and respratory data for each patent are shown n Table 1. Crtera for selecton were as follows: (1) a spontaneous RR >8 breaths per mnute when mechancal ventlatory support was dscontnued; (2) stable chest roentgenogram and PaCO s <50 mm Hg; (3) hemodynamc stablty as evdenced by a systolc blood pressure >90 mm Hg and a regular cardac rhythm. Flo, and the SIMV rate were kept constant. Patents' spontaneous ventlatory rates exceeded the backup mandatory RR; patents were placed n a semrecumbent poston, and no sedatves or narcotcs were admnstered durng the course of the study. Durng mechancal ventlaton" Pg and esophageal pressure (Pe) were smultaneously measured usng transducers (DT-4812, Spectramed) attached by sde arms to a newly developed multlumnal nasogastrc tube (15 FG, custom made, Sumtomo Bakelte" Japan). The nasogastrc tube has fve lumna for sump, thermstor, suctonng, and montorng Pg and Pe (Fg 1). The dstal openng of the tube was placed n the stomach and the proxmal openng was Determnaton of Optmal Pressure Support UMtI(Kmura et ai)

2 Table I-ClncalGnd BsaprtJlory Indca ofstutlg ftjtenta* Patent No.1 P(A-a)O., Flo., S I M ~ RR, Sex/Age, yr mmhg If, mm" mn:" Dagnoss lif HF, pneumona 2IMI TAA,ARDS ~ TAA,ARDS 4IMI6O TAA,ARDS vrn: Pneumona 6/F/ Pneumona PH *HF=heart falure; TAA=thoracc aortc aneurysm; ARDS =adult respratory dstress syndrome; PH =pulmonary hemorrhage. TEMPERATURE SENSOR CONNEcroR DISTAL END OF GASTRIC PRESSURE DISTAL END OF ESOPHAGEAL PRESSURE PROXIMAL END OF GASTRIC PRESSURE FIGURE I. Scheme of the newly developed multlumnal nasogastrc catheter. PROXIMAL END OF ESOPHAGEAL PRESSURE placed n the mdthrd of the esophagus. The poston of openngs for Pe and Pg was adjusted by chest and abdomnal roentgenograms and confrmed further by each pressure tracng characterstc. The lumna for montorng Pg and Pewere frst flushed wth physologc sodum chlorde soluton and then kept patent wth a slow and constant nfuson of sodum chlorde soluton, 3 mllb. Arway pressure (Paw) was measured at the dstal end of the endotracheal tube. Pe, Pg, and Paw were contnuously dsplayed on a bedsde montor (BSM 8500, Nhon kohden, Japan) and recorded on a multchannel thermal recorder (WS-800R, Nhon kohden, Japan). Pd was calculated by subtractng Pe from Pg at the pont of the negatve peak n the Petracng. The varyng levels of PS were set for each patent. Each level of PS was used for a tral perod of 20 mnutes. Arteral blood gas tensons were analyzed (wth an ABL 300, Radometer, Copenhagen). Changes n PaO. and PaCO. ( ~PaO t and ~PaCOJ were calculated as the dfference between the value of P a or O ~ PaCO t obtaned at the end of the each test perod and the value obtaned durng the lowest PS level. All recordngs and measurements were obtaned durng the last two mnutes of each tral. RESULTS Changes n arteral blood gastensons are shown n Fgure 2. Alterng PS level, 4 P a O and ~ h 4 PaC0 2 vared from -14 to 36 mm Hg and from - 3 to 5 mm Hg, respectvely We could not fnd a defnte correlaton between changes n arteral blood gases and (mmhg) 40 cs'.. Q. 0 <I -40 M e a s u r e ~ e n t s of VTwere made wth a hot-wre 80wmeter (Mn sensor, Mnato, Japan) by ntegratng expratory flows n the ventlatory crcut. A d d t durng o n a la l one-mnute ~ perod of removal from mechancal ventlatory support on a T-pece mode n 100 percent oxygen, Pd as the unasssted spontaneous ventlatory effort was measured n each patent. Statstcal analyss was performed usng a lnear regresson ~ technque to determne the relatonshp between ventlatory varables and levels of P S ~ All values are expressed as mean ± SD D. -5 FIGURE 2. Changes n PaO. and PaCO. compared wth the value of PaO. or PaCO. durng the lowest PS level. CHEST I 100 I 1 I JUL'(

3 o -20 PS5 PS15 FIGURE 3. Recordngsof Pavv, Pg, and Pe from fve 1Mb of PSV n sx' patents. Pawn was deleted because arway was opened. Pe n was shfted ~ p w at a the r dend of nspraton. PSV levels. Typcal tracngs of Pe, Pg, and Paw Huctuatons at dfferent levels of PS are shown n Fgure 3 (patent 6). The magntude of the negatve deflecton of Pe decreased wth an ncrease n PS, resultng n a concomtantdecrease n Pd. An excessvely hgh level ofps caused a postve deflecton ofpe. Ths phenomenon was observed n two patents (patents 3 and 6). A postve dehecton of Pe suggests an excess PS at the end of n s p rthus, a ~ owe n. de&ned the optmum PS level as the fs that showed the mnmal negatve or postve deflecton of Pe durng the late nspratory phase; ths was assumed to cause the least nspratory effort. The relatonshp between Pd and PS levels n sx patents s shown n Fgure 4. Pd decreased lnearly wth PS. The mean value of Pd at the optmum PS was 4.2±1.4 cm H 20 and ranged from 2.7 to 6.8 cm Fgure 5 shows the relatonshp between RR and PS. RR decreased lnearly wth ncreasng PS. RR at C2 20 :I:. E u..., ~ C o, FIGURE 4. Changes n Pd n sx patents recevng varyng levels of P S Double ~ crcles ndcate the selected ponts at the optmum PS level.., o FIGURE 5. RR at dfferent levels of PSV n sx patents. Double crcles ndcate the selected ponts at the optmum PS level. 114 DeIIrn*udonof OptmalPr8s8u..SUpportlJMtI (Kmura et aj)

4 ... > o FIGURE 6. VT at dfferent levels of P S Double ~ crcles ndcate VT at the optmum PS; hatched area, the range of vr at PSV mu' the optmum PS ranged from 13 to 26 breaths per mnute, and the mean value ofrr at the optmum PS was 19.7±5.5 breaths per mnute. VT ncreased wth ncreasng PS level as demonstrated n Fgure 6. VT at the optmum PS vared wdely from 6.4 to 18.8 ml/kg n each patent. The mean VTat the optmum PS was 11.7 ± 4.5 mllkg. The range ofvt at PSVmax recommended by MacIntyre2 s marked n the hatched area n Fgure 6. The values of VT at the optmum PS (Table 2) were dfferent from VTat PSVmax. We calculated the slope and probablty of the relatonshps between respratory parameters and PS levels (Table 2). The optmum PS and respratory varables at the optmum PS for each patent are shown n Table 3. Table 3-Optmum PS and Reapratory Varables Patent Optmum PS, RR, VT, Pd, No. cmh 20 mn:" ml/kg em H Mean SD DISCUSSION PSV has been wdely used for the purpose of assstng spontaneous breathng by decreasng nspratory work ofbreathng.v-? A problem wth PSV s the need to ndvdualze the PS requrement for each patent. RR, VT, the magntude of nspratory work, and total respratory system resstance have been used as ndces for optmzng PS.2-6 Our results showed that the optmum PS could be readly determned by measurng Pd. MacIntyre and Leatherman-" defned the optmum PS as the ventlator settng at whch RR was lowest or VT was 10 to 12 mllkg. The VT at the optmum PS dd not necessarly concde wth the values reported by MacIntyre and Leatherman snce we ttrated the PS level only to mnmze Pd reflectng daphragmatc contracton. We suggest that the optmum PS could be fne-tuned by measurng Pd as well as by gaugng other respratory parameters to reduce the patent's nspratory effort. Measurement of Pd mght be the most accurate method of assessng daphragmatc functon." Pd s lnearly related to the actve daphragmatc tenson n dogs," and ts waveform s reported to be related to electromyographc sgns of daphragmatc fatgue.!" Therefore, we chose Pd as a marker of nspratory effort generated by daphragmatc shortenng. In our study, before each pressure measurement was taken, gastrc contents were suctoned to avod a baselne shft ofpd. Pd measurement at the vtal capacty was Table 2-RelatoRBhpa between Respratory Varables and PS Pd RR VT Patent No. Slope Probablty Slope Probablty Slope Probablty Mean SD CHEST I 100 I 1 I J U 1991 L ~ 115

5 PS FIGURE 7. Schematc llustraton of the regresson lne between Pd and level ofps. A ndcates optmal PS level; B, Pd durng T-pece mode; X, PS level; and Y,Pd. not measured snce measurement of Pd.n.x requres cooperaton of the patent.'! Varablty of Pd.n.x resultng from nadequate cooperaton shows the nherent operatonal dffcultes n Pd.nu measurement under clncal settngs. Therefore, we selected Pd as a marker of nspratory effort. Pd decreased lnearly n accordance wth the ncrease n PS. As shown n Fgure 7, we express the relatonshp between Pd and PS level by usng the followng equaton (a): Y=AX+B, (a) where A, B, X, and Y are slope, Pd durng T-pece mode, PS level, and Pd, respectvely Thus, the equaton (a) can be rewrtten as follows: Y= X (PS level)+ Pd durng T-pece mode, (b) where the slope (-0.8) s taken from Table 2. Our result ndcates that Y s assumed to be 4 when the optmum PS s appled (Table 3). Rearrangng the equaton (b), we get equaton (c): PSopttmum = ( P d -~ 4)/0.8. n t (c) Therefore, fpd durng T-pece mode s determned, the optmum PS could be determned by calculatng equaton (c). A further prospectve study s needed for justfcaton of equaton (c) n clncal settngs. Cauton must be pad when applyng the equaton (c)to the patentwth severe respratory muscle fatgue or paralyss. Pd cannot be generated durng daphragmatc fatgue or paralyss. Therefore, the calculated optmal PS level may be too small compared wth the requred PS level. In such a case we recommend PSVmax when ttratng PS level. VT and RR are also lnearly correlated wth PS as follows: PSoptmum =(Rl\pont -19.7)/1.32, (d) where Rl\pont s the spontaneous RR durng T-pece 111 mode. Ptmum = ( VTspont)/O. 24, (e) where VTspont s VT durng T-pece mode. These equatons would not necessarly elmnate the need for the measurements of Pe and Pg. Pd s reduced by ncreasng Ps and converged to about 4 cm H 20 n almost all patents (Fg 3). However, the values of SD n VTand RR were greater than the SD n Pd (Table 3). Thus, we regard the relaton between Pd and PS as more mportant than the relatons between VT or RRand PS. PSV can reduce the patent's work of breathng to vrtually zero. 2 However, an excess PS shfts Pe upward and wll not load the respratory muscles suffcently, resultng n atrophy of the respratory muscles. An ncreased VTdue to hgh PS level mght requre longer expraton tme to expel the nspred volume, resultng n a decrease n venous return. An excess PS could be judged by a postve deflecton of Pe from the baselne level. Compresson of the esophagus aganst the heart due to excess PS mght have resulted n an ncrease of cardac oscllatons of Pe tracng as shown n Fgure 3. Thus, postve deflecton of Pe wth ncreased cardac oscllaton also suggests excess PS. Snce Pd s mnmzed when applyng hgh PS level, nspratory work s reduced and essentally the patent s bengventlated completely by the PS mode. Ths mode of ventlaton s dfferent from pressurelmted ventlaton wth regard to the ntaton of the expraton. The expraton durng PSV starts ether at the pont of a decrease of How to 25 percent of the ntal maxmal How or a decrease of demand Howto 5 Umn. Pressure-lmted ventlaton termnates the nspraton when Paw reaches the predetermned pressure value. Patents who receve PSV can be weaned from the optmum PS by decreasng the nspratory pressure assst to progressvelylower levels as ventlatoryfalure resolves. Weanng s accomplshed by gradually reducng the nspratory pressure level, whch allows the patent to acqure an ncreasngly greater proporton of hs ventlatory work. The adequacy of weanng can be confrmed by addng conventonal respratory varables such as RR and arteral blood gas analyss. Weanng s complete when PSV level s just that requred to overcome the resstance mposed by the tracheal cannula and demand valve.' If the weanng process s too rapd compared wth the recoverng process of respratory functons, tachypnea, abdomnal paradox, sweatng, and CO 2 retenton would appear, leadng to respratory muscle fatgue. Inapproprate weanng mght cause a rapd ncrease n Pd due to augmented nspratory effort" or a rapd decrease n Pd, whch means patent's ntolerance wth resultant respratory muscle fatgue. l3 Thus, Pd measurement may have a sgnfcant nfluence on the course of Determnaton of Optmal Pressure SUpportLevel(Kmura et 81)

6 weanng from P S ~ Pd measurement, orgnally, requres Ppl measurement. In practce a measurement of pressure nsde the esophagus s a useful approxmaton to Ppl14 and can be obtaned by havng a patent swallow a catheter wth a small balloon. We have developed a relable and nexpensve multpurpose nasogastrc catheter. The catheter has lumna for montorng Pe and Pg n addton to the ordnary sump and avods the ntroducton of an addtonal ntragastrc balloon. Ths method gves measurements of Pd that correlate well wth the esophagogastrc balloon method.p"? Usng our new catheter, measurements of Pe and Pg were carred out wth the mnmum of patent dscomfort. In the semrecumbent poston, however, the weght of the medastnal contents wll produce an artfact n the measurement of Pe. 18 Therefore, Pe mght be falsely hgh and Pd mght be falsely low However, Pd measured n the mddle thrd of the esophagus, where we placed the openng hole of the Pe lne, does not vary markedly wth body posture. 19 Furthermore, the downward force exerted by the medastnum would nfluence all measured Pe evenly throughout ths study wthout alterng Pd fluctuaton and the slope between Pd and PS level. Therefore, the topographc change of Pd tracng was regarded as neglgble. In summary, Pd measurement s helpful for ttratng PS level. The optmum PS level can be determned by mnmzng Pd. ACKNOWLEDGMENT: We thank Sumtomo Bakelte Co Ltd, Japan, for kndly supplyng the newly developed multlumnal nasogastrc catheter. REFERENCES 1 Fastro JF, Habb M Quan ~ SF. Pressure support compensaton for nspratory work due to endotracheal tubes and demand contnuous postve arway pressure. Chest 1988; 93: MacIntyre NR. Respratory functon durng pressure support ventlaton. Chest 1986; 89: MacIntyre NR, Leatherman NE. Ventlatory muscle loads and the frequency-tdal volume pattern durng nspratory pressureasssted (pressure-supported) ventlaton. Am Rev Respr Ds 1990; 141: Kacmarek RM. The role of pressure support ventlaton n reducng work of breathng. Respr Care 1988; 33: Mta JF. Pressuresupportwth the Purtan-Bennett7200 (letter), Respr Care 1985; 30:213 6 Chatburn RL. Estmatng approprate pressure support levels (letter), Response, Mta JF. Respr Care 1985; 30: Brochard L, HarfA, Lorno H, Lemare F. Inspratory pressure support prevents daphragmatc fatgue durng weanng from mechancal ventlaton. Am Rev Respr Ds 1989; 139: Loh L, Goldman M, Newsom Davs J. The assessment of daphragmatc functon. Medcne 1977; 56: Km MJ, Druz WS, Danon J, Machnach w Sharp JT. Mechancs of the canne daphragm. J Appl PhysolI976; 41: Levne S, Gllen M. Daphragmatc pressure waveform can predct electromyographc sgns of daphragmatc fatgue. J Appl Physoll987; 62: Laporta D, Grassno A. Assessment of transdaphragmatc pressure n humans. J Appl Physoll985; 58: Pourrat JL, Lamberto C, Hoang PH, Fourner JL, Vasseur B. Daphragmatc fatgue and breathng pattern durng weanng from mechancal ventlaton n COPD patents. Chest 1986; 90: Swartz MA, Marno PL. Daphragmatc strength durngweanng from mechancal ventlaton. Chest 1985; 88: Mlc-Eml J, Mead J, Turner JM, Glauser EM. Improved technque for estmatng pleural pressure from esophageal balloons. J Appl Physoll964; 19: Asher MI, Coates AL, Collnger JM, Mlc-Eml J. Measurement of pleural pressure n neonates. J Appl Physol 1982; 52: Coates AL, Davs GM, Vallns ~Outerbrdge EW Lqudflled esophageal catheter for measurng pleural pressure n preterm neonates. J Appl Physoll989; 67: Lemare F, Teboul JL, Cnott L, Gotto G, Abrouk F, Steg G, et ala Acute left ventrcular dysfuncton durng unsuccessful weanng from mechancal ventlaton. Anesthesology 1988; 69: Mead J, Gaensler EA. Esophageal and pleural pressures n man, uprght and supne. J Appl Physoll959; 14: Mlc-Eml J, Mead J, Turner JM. Topography of esophageal pressure as a functon of posture n man. J Appl Physol 1964; 19: CHEST I 100 I 1 I JUL"f,