Archves ofdsease n Chldhood 1996;75:F213-F218 F213 CURRNT TOPIC Graphc analyss of pulmonary mechancs n Graphc analyss of pulmonary mechancs n neonates recevng asssted ventlaton Dvson of Women and Chldren, Drectorate of Neonatology, South Cleveland Hosptal, Mddlesbrough Cleveland TS4 3BW S Snha Department of Pedatrc Respratory Care JJ Ncks Department of Pedatrcs (Dvson of Neonatal-Pernatal Medcne), Unversty of Mchgan Medcal Center, Ann Arbor, Mchgan, USA SM Donn Correspondence to: Dr Sunl Snha. Accepted 20 May 1996 Sunl K Snha, Joanne J Ncks, Steven M Donn Wth the ncreasng complexty of neonatal respratory care, pulmonary mechancs testng s emergng as a valuable tool to ad clncal decson makng n the management of ventlated nfants. Although there are as yet no publshed randomsed controlled clncal trals to suggest that pulmonary mechancs testng reduces mortalty or morbdty, t has-n conjuncton wth clncal, radologcal, and blood gas montorng-changed neonatal ventlaton from "good judgment" to "nformed judgment."' It s not surprsng that pulmonary mechancs testng s ncreasngly becomng an essental element n the assessment of patent status, therapeutc evaluaton, and management gudance of nfants wth ventlator dependence. A workng knowledge of pulmonary mechancs also mproves understandng of pulmonary physology and pathophysology and ther responses to mechancal ventlatory support.2 Unfortunately, these graphcal data contnue to be addtonal nformaton that the average bedsde clncan s currently unsure how to nterpret. The purpose of ths artcle s to dssemnate the nformaton on ths technque based on our experence usng the Brd Graphc Montor (Brd Products Corporaton, Palm Sprngs, Calforna, USA), whch n conjuncton wth ts Partner II Montor and VIP Brd Infant/ Pedatrc Ventlator, provdes a contnuous dsplay of graphc representaton of pressure, flow, or volume waveforms, or relatons between pressure-volume and flow-volume n the form of loops. It also gves numercal values for pressure, tdal volume, and flow, and can be used for establshng trends up to 24 hours. All the above graphcs can be obtaned as prntouts when used wth a standard computer prnter. Background Pulmonary mechancs montorng conssts of measurements of several varables whch defne dfferent aspects of lung functon. Although t has become popular to refer to ths type of analyss as "pulmonary functon testng," measurements of functonal resdual capacty (FRC) are not ncluded. Devces are becomng avalable to determne FRC at the bedsde, usng ether ntrogen washout or helum dluton technques, but at present the methodology s extremely expensve and mpractcal. Many dfferent pulmonary mechancs tests can be performed on ventlated nfants; only a few are commonly used n clncal practce. Specfcally, clncans are nterested n the pressure necessary to cause aflow of gas to enter the arway and ncrease the volume of the lungs. From these varables, several other measures of pulmonary mechancs can also be derved, such as pulmonary complance and resstance, and resstve work of breathng (energetcs). Complance s the term used to descrbe the relaton between a change n volume and the pressure requred to produce that change: C=AV/P. It gves nformaton about the elastcty of the lungs. Resstance s a result of frcton of gas flow aganst the ar conductng system. It s roughly measured as the change n transpulmonary pressure dvded by change n ar flow, and s an ndcator of arway functon. The product of complance and resstance determnes the tme constant, whch s a measure of how quckly the lungs can nhale or exhale, or how long t takes for the alveol and proxmal arway pressure to equlbrate. From these mechancs, whch can be dsplayed as numercal values or as graphc sgnals, useful nformaton can be obtaned and used for dagnosng specfc lung pathology, evaluatng dsease progresson, and determnng therapeutc nterventons. Although the facltes to montor arway pressure, tdal volume, and gas flow n ventlated nfants have exsted for some tme, ther applcaton has been mostly lmted to research purposes. However, recent advances n mcroprocessor technology for on-lne analyss of pulmonary mechancs have made such evaluatons easly avalable for bedsde clncal applcaton."4 Indeed, most of the new ventlators ether come wth, or have an opton for, a graphcal dsplay, and ths has become an essental feature of the newer ventlators avalable for nfant/paedatrc use. These on-lne systems obvate the need for nterruptng ventlaton. In addton, because of technologcal advantages, the sensors are very lght n weght and add mnmal dead space to the ventlatory crcut. Ths permts ther applcaton to even the smallest preterm nfants. Putatve advantages The ratonale of pulmonary mechancs testng n ventlated nfants s based on the assumpton that early dentfcaton of pulmonary prob- Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.
F214 Snha, Ncks, Donn Paw 17.50 cm H20 VT 4 ml VT 4ml -1 10 5-0.1 5 Reference Tarrget B cursor currsor Inspratory -5 X flow 2.90 Secondss 2 4 6 5r- I 0.4 1pm I a Sl.1'IJ- 'S'I ' 1 - V., '1 I ^ ss vl>' 'r *rr- T r xpratory flow Fxed nspratory tme Termnaton senstvty off -. Termnaton senstvty on 8 Flow termnated bre Fgure 2 Flow and volume waveforms showng an ntal nspratory tme whch s to long, but whch s subsequently shortened by use of termnaton senstvty mode. / lems, ether nherent or atrogenc, and nsttuton of approprate therapeutc or ventlatory adjustments wll mprove the dysfuncton and/or reduce the ncdence of acute and : chronc lung njures.56 Besdes assessment of acute respratory dstress and evaluaton of mechancal ventlaton, potental benefts of q on-lne pulmonary graphcs nclude assessment of sutablty for weanng, montorng of 10 complex treatments such as extracorporeal membrane oxygen (CMO) or ntrc oxde, and follow up of chronc lung dsease. Methods -I The two forms of respratory graphcs most wdely utlsed n clncal practce are scalar waveforms and loops, both showng smultaneous relatons between pressure, volume, and ar flow. Although the software program whch montors and plots these graphs can also gve numercal values for each varable and calculated parameter, n practce, most of the useful nformaton can be derved by the vsual nspecton of ther morphology. Ths vares, accordng to specfc respratory functon status, provded allowance has been made for errors arsng from naccurate calbraton and artefacts. It s beyond the scope of ths paper to detal the specfc methods through whch the basc sgnals of flow, volume, and pressure undergo transducton to graphc or numerc dsplays, and several references are avalable for such purpose.78 However, the same mcroprocessor based technology used to provde patent trggered ventlaton s used n conjuncton wth hghly senstve transducers generally placed close to the endotracheal tube connector. Respratory waveforms -5 Fgure 1 shows a smultaneous representaton Paw 19.00 cm H2O 25-3-96 11:44 of pressure and volume (A) and volume and Fgure 1 Scalar waveforms for arway pressure (PAW") and tdal volume (VT) (A), ancd flow (B) scalar waveforms durng trgger vent- laton n tme cycled pressure lmted mode. tdal volume and arway flow ( (B). Tme ntervals can be measured usng a reference cursor (dotted lne). A target cursor can be moved to any gven pont on a wave to obta: n The magntude of ndvdual parameters and numercal values ofpressure, volume, or flow at that pont. ther tme nterval can be measured by use of the reference and target cursors. The basc nterpretaton and the clncal applcaton of these waveforms are descrbed below. ath FLOW WAVFORMS Flow may be thought of as the volume of gas delvered per unt of tme. Inspratory flow s plotted above the abscssa (postve) and expratory flow s plotted below the abscssa t~ (negatve). The ntersecton of the abscssa and ordnate occurs at zero flow. The duraton and shape of the nspratory and expratory flow waveform are affected by many factors, nclud- 10 ng type of ventlaton, nspratory:expratory tme rato, mpedance to gas flow durng expraton, and effect of therapeutcs, such as bronchodlators. The major advantage of lookng at flow waveforms s n assessng the amount of flow - that occurs durng the nspratory phase to see f the set nspratory tme s napproprately long. Fgure 2 shows a flow waveform wth 0 fxed nspratory tme. Ths may be needed to acheve a hgh mean arway pressure and better Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.
Pulmonary mechancs n neonates recevng asssted ventlaton F215 10 _ 10 ;-, II Lower resstance F 1LJXDIJX fna, Hgher resstance Fgure 3 Flow waveforms. Upper tracng shows lower expratory resstance. Increased expratory resstance n bottom tracng results n flattened slope and slow return to baselne. nd expratory flow oxygenaton durng the acute stages of respratory dstress syndrome wth stff lungs and short tme constants, but once the lungs become complant and f baby s breathng at a fast rate especally whle on patent trgger ventlaton mode, fxed nspratory tme may prove to be napproprately long. Ths s lkely (mathematcally) to result n nsuffcent expratory tme and contrbute towards ar trappng. In these stuatons, nspratory flow can be lmted by ether shortenng the nspratory tme or nsttutng a mode called "termnaton senstvty" (n VIP Brd Ventlators), whch lmt the nspratory flow to a preset value between 75-95% of peak nspratory flow and then trgger the expratory phase. Ths permts complete synchronsaton. The duraton and shape of expratory flow waveforms depend both on p ~L rarrn A nf lrag :1 Fgure 4 Flow waveform shows ar trappng. xpratoryflow (below baselne) never reaches zero before next breath s ntated. CD 0. 10 40 40 I - I efe f I I ( r Mechancal breath -/"' X4 Spontaneous breath a -\ 1/ -jno %I -- lkx / \ 3"J I XP1k1 Fgure 5 Volume waveform. Top tracng shows neffectve spontaneous breaths; lower tracng shows mproved tdal volume delvery durng spontaneous breathng. OV the resstance and the complance, but because of the small dameter endotracheal tubes used n neonates, resstance s a more mportant determnant for ts confguraton. The expratory waveform can be used to observe changes n expratory resstance (fg 3) and the presence of gas trappng (fg 4). Normally, expratory flow reaches a zero flow state before the next breath begns. If the expratory tme s too short, the expratory flow fals to reach a zero flow state before the next breath starts, thus suggestng that gas trappng may be occurrng. A smlar appearance as shown n fg 4 can happen due to a large leak around the endotracheal tube, but ths can be dfferentated ether by usng the pressure-volume loops whch fal to close, or by montorng the dfference between nspratory and exhaled tdal volume. VOLUM AND PRSSUR WAVFORMS Measurement of tdal volume s becomng ncreasngly mportant n ventlatory management. The desred nspratory tdal volume for a ventlated breath ranges between 5.2-7.2 ml/kg9 whle a tdal volume of 3.5-5 ml/kg for spontaneous breaths generally ndcates sutablty of the nfant for weanng, provded ths s matched by satsfactory arteral blood gases whch reman the best ndex of ventlatory adequacy. Mnute ventlaton (tdal volume x respratory rate) s also currently beng evaluated as one of the predctor of weanng (250-400 ml/kg/mnute),'0 especally n small nfants who have short nspratory tmes and a hgher respratory frequency. Unlke flow waveforms, both the nspratory and expratory phase of volume and pressure waveforms are postve and are therefore plotted above the abscssa. Breath-to-breath varablty and longer term trends are useful n selectng the mode of ventlaton and ndvdualsed adjustments to customse ventlator parameters for each patent. Fgure 5 shows dfferences n tdal volume delvery between mechancal and spontaneous breaths durng synchronsed ntermttent mandatory ventlaton (SIMV). The top waveform ndcates that spontaneous tdal volumes nbetween mechancally delvered breaths are neffectve. On the bottom graph, the nfant has much hgher spontaneous tdal volumes suggestng readness to wean. Pulmonary mechancs loops The loops are commonly used to demonstrate correlatons between arway pressure and volume, and arflow and lung volume (fg 6). Recently, attempts have also been made to produce a flow-pressure loop, but t s not yet avalable n clncal practce. At present, there s no agreed conventon regardng the manner n whch the loops are generated. Some devces draw them clockwse, others antclockwse. Some label upward deflectons as negatve, others postve. The clncan must be famlar wth these dfferences to be able accurately to nterpret the presented data. /\n '. ~~~~~~~~~~~~~~~~~~~~~~~I * Is PRSSUR-VOLUM LOOPS These loops graphcally depct the correlatons between ventlator nspratory pressure and the Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.
F216 Snha, Ncks, Donn 25-15 25 xpraton 0 _- 457-15 L Fgure 7 Pressure-volume relaton. On the left, the complance lne twuld have an axs about 30 above horzontal, ndcatng poor complance. On the rght, the complance ln has mproved to about 45. Ths scenaro mght be seen n an nfant wth respratory dstress syndrome after surfactant admnstraton. Over dsl-tenson Fgure 8 Pressure-volume relaton. Graph on left demonstrates overdstenson. Note mnmal change n volume over the last 20% of the breath. A reducton n ventlatory pressure (rght graph) normalses the loop. changes durng expraton. A lne connectng the ponts of zero flow-changeover from expraton to nspraton-s the complance lne, and the slope of that lne reflects pulmonary complance. Complance can be calculated as 20 Vt (ml) 5 the change n volume dvded by the change n pressure; n practce an estmate of complance can be made by lookng at the slope of the -5 Inspraton 100 complance lne, whch s normally 450 from the horzontal axs. If ths slope s more towards the vertcal axs, complance s mprovng, and Fgure 6 Pulmonary mechancs loops. Left graph dsplays pressure-volume relatons. N nspratory and expratory lmbs, drawn counterclockwse on the Brd Graphc Montor. On conversely s decreasng f ths lne moves the rght s theflow-volume relaton drawn clockwse, n whch arflow to the patent (nspraton) s plotted postvely, arflow awayfrom the patent (expraton) s plotted negatvely. 45r- I 0 70r I.- nearer to the horzontal axs. It must be realsed that slope of the complance lne also depends on the calbratons and can be msleadng f not adjusted properly. Devces such as VIP Brd have an automatc scalng functon whch serves to reference the curve such that normal complance lne should be about 45. Over-dstenson s mpled when lttle or no change n volume occurs as ncreasng pressure s delvered. One way to recognse ths s to look at the termnal (upper) porton of the 25 pressure-volume loop and see how much tdal volume s beng delvered per unt of pressure change. Ths has been schematcally demonstrated n fg 8 created from a test lung for ze nstructve purposes. A useful parameter n ths case s the "C20."" Ths can be estmated by calculatng total complance of the lung n relaton to the complance of the last 20% of the breath (by dvdng tdal volume by peak pressure). If the complance of the last 20% s less than 80% of the total complance, t s suggestve of over-dstenson, and the ventlator should be adjusted to reach a C20 value hgher than 80%. Ths may be accomplshed by decreasng peak nspratory pressure, nspratory tme, and n some nstances, postve end L expratory pressure. 35 35 Pulmonary complance s a measurement of the dstensblty of the lung and s n a sense an -20-20 ndcator of the functon of the lung parenchyma. Dsease processes that make the lung stffer and thus decrease pulmonary complance n neonates nclude surfactant defcency states, pneumona, pulmonary oedema, and pulmonary hypoplasa. Increased resstance, 45r such as that seen n bronchopulmonary dysplasa, causes a "bowng" or hysteress around the complance lne, and s an ndrect measure of the work of breathng (by ether the nfant or the mechancal ventlator) to move j the lungs. Ths resstve work of breathng ~~~~35 35 ncreases as pulmonary mechancs worsen, Crtcal openng -15 pressure -15 reflectng the ncreased amount of energy spent by the nfant or the ventlator to breathe. It can be roughly estmated by the total area covered Fgure 9 Demonstraton ofeffect ofpp on pressure-vlume relaton. Left: nadequa PP results n poor volume ddlvery as pressure ncreases. Rght: hgher PP results ~te by the pressure-volume loop and the area adjacent to ts deflaton lmb and the ordnate. approprate volume delvery wth ncreasng pressure. There are, however, major problems n the nterpretaton of the resstve work of breathng volume of gas enterng the lungs. The horzon- n nfants on ventlaton, as alterng the crcut tal axs represents the degree of postve flow alone can have consderable effects even n pressure exerted by the ventlator and the Nver- the absence of changes n lung mechancs. tcal axs represents the gas volume changefs n The pressure-volume loop may also be used the lungs (fg 7). The lower porton of the kdop to evaluate optmal peak expratory end shows the dynamc relaton between volume pressure (PP) (fg 9). On the left, the loop changes and pressure changes on nspraton, shows a delay n volume delvery despte the upper porton of the loop shows volumne ncreasng pressure, ndcatng nsuffcent Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.
Pulmonary mechancs n neonates recevng asssted ventlaton F217 Vt (ml) -1 50 150 e' 20-150 150 - Fgure 10 Flow-volume loop. Left graph ndcates hgh resstance, demonstrated by lowe flow at a gven volume. Rght graph ndcates lower resstance (such as after bronchodla tor treatment) wth hgherflow at a gven volume. 25 _ blockage wth secretons, meconum aspraton syndrome, and bronchopulmonary dysplasa. Fgure 10 shows flow-volume loops, whch should be rounded and smooth around the horzontal axs lke an egg on ts end. The lower porton of the loop represents nspratory data; 5 the upper lmb descrbes the expratory correlaton. (Because there s no agreed conventon on drecton of nspratory and expratory lmb among varous manufacturers, t s mportant to become famlar wth the drecton of the er flow-volume loop on ndvdual machnes.) A flow-volume loop depcts changes n nspratory and expratory flow aganst volume changes, and s useful n detectng flow restrcton from arway abnormaltes. Under condtons of hgh arway resstance, flow s lower for a gven volume. Flow wll be lower on nspraton wth hgh nspratory resstance, or lower on expraton wth hgh expratory resstance. Z There s a fxed nspratory and expratory resstance assocated wth an endotracheal tube. The example n fg 10 shows ncreased nspratory and expratory flow from left to 15 rght, whch mght be seen after bronchodlator treatment. Ths could also happen as a result of hgh ventlatory pressure or mprovement n lung functon after an mprovement n respratory dstress syndrome. The ablty to dentfy 4- and quantfy endotracheal tube leaks may be facltated through graphc montorng. Leaks can be dentfed both on the volume wave 25 form as well as pressure-volume loops (fg 11). Increased resstance and nterrupton to gas flow from excessve condensaton arsng n the Fgure 11 ndotracheal tube leak. Volume waveform fals to reach baselne at ventlatory crcut or accumulaton of secretons n the arway can be dentfed by the end-expraton. Pressure volume loop fals to close. appearance of jaggedness on graphcs. 30 22 hour trends There are a number of other useful clncal PIP Rentubaton T sucton stuatons, ncludng the assessment of dfferent ventlatory modaltes (such as pressure 28 cm support ventlaton) on pulmonary functon'2 H20 and the assessment of patent-ventlator nteracton (synchronous versus asynchronous),'3 13:21 There s also an advantage n collectng and evaluatng the trends of specfc varables such VT as tdal volume, mnute ventlaton, and 4 frequency for spontaneous and mechancal ml breaths as well as peak pressure, nspratory tme, and mean arway pressure. For example, 11:20 11:40 12:00 12:20 12:40 13:00 13:21 the trend depcted n fg 12 s consstent wth Fgure 12 Trend data, pressure-targeted ventlaton. ven though peak nspratory an nfant on pressure ventlaton where despte pressure s held constant, tdal volume delvery s varable and decreasng. peak nspratory pressure (PIPO remanng constant, tdal volume has gradually decreased. Ths stuaton (as n secondary lung complca- tons durng respratory dstress syndrome) openng pressure. An ncrease n PP, shovvn on the rght, results n an mmedate rse n demands adjustment of ventlatory parameters volume as pressure s delvered. by the operator. Conversely, tdal volume delvery may automatcally ncrease after mprovement n lung complance despte PIP FLOW-VOLUM LOOP Another valuable measurement n the pulm0- beng the same (as after surfactant replace- ment), and cause atrogenc complcatons f nary mechancs profle s the resstance of gas flow through the arways. Ths can be assess ed ventlatory adjustments are not made n tme. by observng the flow-volume loop, wh( c-h graphcally dsplays the relaton betweeen Lmtatons change n volume and change n arflow. Hggh Despte the major advantages of the current arway resstance results n lower flow for a system provdng pulmonary mechancs graphgven volume. Condtons causng hgh ressit- cs, t should be realsed that the nformaton ance n ventlated babes nclude arw; ay provded about the functon of the lungs only obstructon from endotracheal tube knkng ( or complements (and does not substtute) nfor Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.
F218 Snha, Ncks, Donn maton ganed by other means of patent montorng, ncludng clncal sgns and blood gas examnaton. Lke radographs, graphcs should only be taken as suggestve of a condton rather than beng defntve. Pulmonary graphc waveforms can be msshapen because of nherent naccuracy of the measurement system (from calbraton dfferences between nspraton and expraton), or from temporary artefacts arsng from patent poston or mpedance to the gas flow by condensaton n the ventlatory crcut. It s mperatve to correct these ptfalls before acceptng the fndngs of graphcs as a gudance to change ventlatory settngs or assess the patent's clncal condton.'4 In ths partcular respect, the "trends" over a perod of tme seem to be of more value than ndvdual breath analyss. Although there are few data showng a benefcal or more cost effectve approach to neonatal ventlaton by the contnuous use of pulmonary mechancs montorng, the development and mplementaton of ths technology should make some studes feasble n the near future. Concluson On-lne pulmonary graphc analyss represents another major advance n respratory technology whch promses to mprove the safety and effcacy of neonatal mechancal ventlaton. Clncans are now afforded a breath-to-breath vew of pulmonary mechancs and respratory waveforms. Ths permts constant survellance of condtons such as ar trappng before they become clncally obvous, and the fne tunng of ventlator settngs to customse management accordng to the problems and responses of the ndvdual patent. Clncans should aval themselves of ths wndow of opportunty. We thank Brd Products Corporaton for permsson to use the fgures. 1 MacDonald KD, Wrtschafler DD. Contnuous neonatal pulmonary mechancs wth the BICOR CP0 montor. Neo Intensve Care 1992; 5:55-61. 2 MacIntyre NR, Hagus CK. Graphcal Analyss of Flow, Pressure and Volume Durng Mechancal Ventlaton. Rversde, CA: Bear Medcal Systems, 1987. 3 Goldsten M. Contnuous n-lne pulmonary mechancs. Neo Intensve Care 1993;5:42-5. 4 Lloyd JS, Cvetnc WG. Contnuous n-lne respratory montorng n the crtcally ll preterm nfant. Neo Intensve Care 1994;7: 14-16. 5 Fsher JB, Mammel MC, Coleman JM, Bng JR, Boros SJ. Identfyng lung overdstenson durng mechancal ventlaton by usng volume-pressure loops. Pedatr Pulmonol 1988; 5:10-14. 6 Rosen WC, Mammel MC, Bng DR, Boros SJ. Bedsde pulmonary functon testng reduces pneumothoraces durng nfant mechancal ventlaton. Pedatr Res 1989; 4:324. 7 Cunnngham MD. Montorng pulmonary functon. In: Goldsmth JP, Karotkn H, Barker S, eds. Asssted Ventlaton of the Neonate. Phladelpha: WB Saunders Co., 1988: 233-44. 8 Donn SM. Neonatal and Pedatrc Pulmonary Graphc Analyss: Prncples and Clncal Applcatons. Armonk, NY: Futura Publshng Co., 1997. 9 Bhutan V, Abbas S. valuaton of pulmonary functon n the neonate. In: Poln RA, Fox WW, eds. Fetal and Neonatal Physology. Phladelpha: WB Saunders, 1992: 866. 10 Sptzer AR. Mechancal ventlaton. In: Sptzer AR,ed. Intensve Care of the Fetus and Neonate. St Lous, MI: Mosby Year Book, 1996: 558. 11 Comroe JH, Forster RD, Dubos AB, Brscoe WA, Carlsen. The Lung: Clncal Physology and Pulmonary Functon Tests. Chcago: Year Book Mecal Publshng, 1962. 12 Ncks IJ, Becker MA, Donn SM. Bronchopulmonary dysplasa: response to pressure support ventlaton. J Pernatol 1994; 14:495-7. 13 Servant G, Ncks JJ, Donn SM, Bandy KP, Lathrop C, Dechert R, et al. Feasblty of applyng flowsynchronzed ventlaton to very low brthweght nfants. Respratory Care 1992; 37:249-53. 14 Gerhardt TO. Measurement of pulmonary mechancs n the NICU: lmtatons to ts usefulness. Neonatal Respratory Dseases 1995; 5:1. Arch Ds Chld Fetal Neonatal d: frst publshed as 10.1136/fn.75.3.F213 on 1 November 1996. Downloaded from http://fn.bmj.com/ on 24 November 2018 by guest. Protected by copyrght.