Manipulation of ventilator settings to prevent active expiration against positive pressure inflation

Transcription

1 Archives f Disease in Childhd, 1985, 60, Manipulatin f ventilatr settings t prevent active expiratin against psitive pressure inflatin D FIELD, A D MILNER, AND I E HOPKIN Department f Nenatal Medicine and Surgery, City Hspital, Nttingham SUMMARY Recent publicatins have suggested that in infants receiving artificial ventilatry supprt a particular pattern f interactin between spntaneus breaths and ventilatr inflatins (active expiratin against each ventilatr inflatin) may be imprtant in the prductin f pneumthraces. We have lked at patterns f interactin frm 47 preterm infants studied n 51 ccasins. We fund that active expiratin against the ventilatr ccurred n a ttal f 16 ccasins. This pattern was prevented n 14 ccasins by altering the ventilatr settings. In tw ther babies, the pattern persisted but neither baby develped a pneumthrax. Recently, Greenugh et all described varius patterns f breathing they have bserved in infants f less than 34 weeks' gestatin wh were receiving artificial ventilatin. They related ne specific pattern f breathing, ccurring during artificial ventilatin, with the develpment f pneumthrax. This pattern they describe as 'Active expiratry effrt against each ventilatr inflatin'.' During the past 15 mnths we have develped a system that allws us t ptimise the ventilatry supprt given t ur sickest preterm infants. T d this we have been recrding ventilatr inflatin pressure, esphageal pressure, tidal vlume, and transcutaneus xygen tensin (TcP) and carbn dixide tensin (TcPc) during a series f changes in ventilatr settings. In the curse f ur studies we have becme aware f the deleterius effect that the infants' wn respiratry effrts can have n the applied ventilatin. We als bserved that the infants' wn pattern f respiratin culd ften be mdified by changing the ventilatr settings. We decided t determine hw ften active expiratin against the ventilatr ccurred and hw ften we culd ablish this pattern by changes in ventilatr settings. Classificatin Greenugh et all described five patterns f interactin: (1) Apnea. () Synchrny-spntaneus respiratin synchrnised with ventilatr inflatins. (3) Hering-Breuer-each ventilatr inflatin induces a temprary cessatin f the infant's respiratry effrts. (4) Active expiratin against the ventilatr. (5) Augmented respiratin-this was seen nly at rates belw 15 breaths per minute and therefre need nt be cnsidered here. Inflatin pressure (0 cm H0) 01 pressure (4OcmH0) Tidal vlume (40ml) L- L- Ss Figure Inflatin pressure, esphageal pressure, and tidal vlume trace frm an infant shwing a mixed pattern f interactin between spntaneus breaths and ventilatr inflatins. It can be seen that the infant's wn respiratry effrts are ccurring at varius phases f the ventilatr cycle Arch Dis Child: first published as /adc n 1 Nvember Dwnladed frm n 18 Octber 018 by guest. Prtected by

2 Manipulatin f ventilatr settings t prevent active expiratin against psitive pressure inflatin 1037 We wish t add a sixth categry. In this the infant prduces a mixed pattern f interactin with spntaneus breaths ccurring thrughut the ventilatr cycle (Figure). Subjects and methds Subjects. Frty seven infants with a clinically based diagnsis f the idipathic respiratry distress syndrme, were studied n 51 ccasins during the first week f their illness. The mean birthweight was 1*47 kg, and the range 0-68 t -73 kg. The mean gestatin was 30-4 weeks, and the range 4 t 40 weeks. Mean age at the time f study was - days. All babies were receiving intermittent psitive pressure ventilatin (IPPV) with ventilatr rates f between 0 and 100 breaths per minute at the time f the study. Draeger Babylg ventilatrs (time cycled pressure limited) were used exclusively during these studies. Nne f the babies had a pneumthrax at the time f study, and each was studied in his incubatr. Apparatus. Measurements were perfrmed as fllws: (1) Inflatin pressure-a wide bre needle was inserted int the infant's endtracheal tube at the pint where it entered the muth and cnnected t an SE Labs 4-86 pressure transducer. Calibratin was perfrmed using a water manmeter. Frequency respnse, measured by bursting ballns in a sealed bttle t which the pressure transducer was cnnected, was satisfactry with a 63% rise time f 6-0 msec. () Oesphageal pressure-an esphageal balln was passed using standard techniques and cnnected t a pressure transducer calibrated as abve. Signals btained were used nly t btain a qualitative estimate f the infant's respiratry effrts. (3) Tidal vlume-a Fleisch type 0 pneumtachgraph was cnnected between the patient manifld f the ventilatr circuit and the endtracheal tube. A cnstant bias flw thrugh the pneumtachgraph was achieved by cnnecting a suctin pump (Airshields Diapump) via a 1 gauge needle between the distal prtin f the pneumtachgraph and the infant's endtracheal tube. As the pressure gradient acrss the needle was in the rder f 900 cm H0, the bias flw was nt affected by the ventilatr cycling pressure. Prvided that the ventilatr circuit was cmpensated fr the lss f the bias flw, ventilatr perfrmance was nt affected. The flw signal was cnverted t a vlume signal by electrnic integratin against time. Calibratin was perfrmed by injecting and withdrawing knwn vlumes f air. The pneumtachgraph was fund t give a linear respnse t flws up t 30 1 per minute. The 63% time f the pneumtachgraph, tubes, and differential pressure transducer was 10-3 msec. (4) Bld gases-bld gas trends were recrded with a Radimeter TcP and TcPc apparatus. These were used ver the perid f the study t prvide a measure f change; hwever, values were always checked against an arterial bld gas sample taken during the curse f the study. All signals were recrded n paper (Devices recrder) and retained n tape (RACAL tape recrder). Visual display was als available n an SE Labs scillgraph. Technique. All babies were clinically stable at the time f study and n ventilatr settings determined by the medical staff, wh were guided by bld gas values. A perid f 15 minutes was allwed fr stabilisatin. Our initial measurements were made n the babies riginal settings (that is, thse chsen by the clinical staff) during a 14 minute study perid. Transcutaneus P and Pc values were recrded at tw minute intervals while the predminant pattern f interactin was defined using the classificatin abve. At the end f 14 minutes the first f a series f ventilatr changes were made. Each change cmprised an alteratin f the rate, r inspiratry: expiratry (I:E) rati, while peak inspiratry pressure and psitive end expiratry pressure remained cnstant. An attempt was made t study each baby at fast (60 r mre breaths per minute) and slw (30 r less breaths per minute) rates, with data n physilgical and reversed I:E ratis at each rate. Thus, we aimed t study each individual baby at I:E ratis f : 1, 1: 1, and 1:-rates f apprximately 30 and 100 breaths per minute: six settings. Each change was fllwed by a 14 minute perid f stabilisatin. If any ventilatr change was accmpanied by an appreciable deteriratin, measurements at that particular setting were abandned. In additin, the need t perfrm rutine care fr the baby ften restricted the number f ventilatr changes that culd be perfrmed n each infant. Results Data were cllected n 30 settings, with at least fur readings n each baby. The mean (range) fr the initial ventilatr setting ver the 51 studies, that is that chsen as ptimal by the clinical staff, were: peak inspiratry pressure 19 cm H0 (1 t 8); psitive end expiratry pressure 1*4 cm H0 (0 t 4); inspiratry time 0*6 secnds (0-3 t 1.5); Arch Dis Child: first published as /adc n 1 Nvember Dwnladed frm n 18 Octber 018 by guest. Prtected by

3 1038 Field, Milner, and Hpkin expiratry time 0*7 secnds (0- t -8); ventilatry rate 56*4 breaths per minute (0 t 100); inspired xygen 63.% (1 t 100). Mean and range f TcP and TcPc n these settings were 58 mmhg (33 t 103) and 50*1 mmhg (9 t 101), respectively. Thrughut ur studies nly tw infants were recrded as having a bld ph f 7-. The frequencies f the varius patterns f interactin ccurring n these settings are given in Table 1. On each ccasin, the predminant pattern f interactin was recrded. Active expiratin against the ventilatr was dcumented, hwever, if we bserved this pattern ver three cnsecutive breaths r during five inflatins in any 30 secnd perid. The baby's initial pattern f interactin was recrded as active expiratin against the ventilatr n eight ccasins. The mean (range) f the initial ventilatr settings fr these infants were: peak pressure 19-6 cm H0 (1 t 8); psitive end expiratry pressure 1-1 cm H0 (0 t 3); inspiratry time 0-6 secnds (0.4 t 1.0); expiratry time 0-55 secnds (3 t 1.0); ventilatry rate 63 breaths per minute (45 t 100); inspired xygen cncentratin 78% (4 t 100), TcP 7 mmhg (65 t 103), TcPc 57 mmhg (33 t 101). Active expiratin against the ventilatr was prevented by changing the settings in six f these eight babies. This was achieved by increasing the ventilatr rate in fur studies and by changing frm a reverse t a physilgical I:E rati (1:1 r less) in a further tw studies. Details are given in Table. It can be seen that these changes caused transcutaneus gas readings t remain stable r t imprve despite the reductin in mean airway pressure that ccurred in thse studies where a physilgical I:E rati was used. The remaining tw babies wh shwed active expiratin against the ventilatr n their initial settings cntinued t shw this pattern thrugh all cmbinatins f ventilatr settings. In these, the pattern ccurred fr at least 5% f the study perid. Neither baby develped a pneumthrax. On eight ther ccasins the baby's riginal pattern f interactin was altered t that f active expiratin against the ventilatr during the series f ventilatr changes. On five ccasins this ccurred as a result f reducing the ventilatr rate and n three ccasins after the use f a reversed I:E rati. Details f these eight events are given in Table 3. The TcP and Pc values given in Tables and 3 fr infants displaying active expiratin against the ventilatr indicate that bld gas values were well maintained. During active expiratin against the ventilatr, hwever, and when there was a mixed pattern f interactin, TcP values shwed nticeable fluctuatin bth abve and belw the baseline, which n ccasins did nt becme evident fr smetime during bservatin. Tw infants included in the study subsequently develped a pneumthrax: neither were making respiratry effrts when studied. A further baby actually suffered a pneumthrax during a perid f mnitring when making synchrnised respiratry effrts. Discussin Our results shw that in mst babies active expiratin against the ventilatr can be prevented by changing the ventilatr setting, either by the use f faster ventilatr rates (60 breaths per minute) r mre nrmal I:E ratis (1:1 r less). This may explain the findings in a previus cntrlled trial which shwed a decreased risk f air leak assciated with the use f a ventilatr rate f 60 breaths per minute cmpared with 30 breaths per minute.3 Active expiratin against the ventilatr is an extremely disruptive pattern f interactin, and is certainly a marker f pr ventilatr adjustment. In ur studies, it did nt seem t be a cnsistent pattern, and was usually accmpanied by episdes in which the baby and the ventilatr intermittently synchrnised t prduce large inflatins in a chatic fashin. It seems likely that these excessive and rapid fluctuatins in lung vlume are mre dangerus than cnsistent active expiratin against the ventilatr, where tidal exchange is reduced and the transpulmnary pressure change is small. Similar fluctuatins, but f smaller magnitude, were seen during the mixed pattern f interactin. In ur study, we made n attempts t study every baby wh develped respiratry distress within the first few hurs f birth. Our aim was merely t investigate the pssibility f cntrlling babies' patterns f interactin with the ventilatr. In this wrk we have used Draeger Babylg ventilatrs Table 1 Frequency f bserved patterns f interactin n riginal ventilatr settings between ventilatr inflatins and spntaneus respiratin Pattern Synchrny Apnea Hering-Breuer Active expiratin Mixed Ttal against the ventilatr Number Arch Dis Child: first published as /adc n 1 Nvember Dwnladed frm n 18 Octber 018 by guest. Prtected by

4 Manipulatin f ventilatr settings t prevent active expiratin against psitive pressure inflatin 1039 *_ -.t *_.k *Mt Cq.t s *t: *_ *t c-) *s; z c E.t Z k.t : *_,. X Cq t w L. - a -S -.- E z - C! zi :. 1- WI "t - 'IC 00 vn r- s r c~~~~..0~~~~. JM.~~~~~~~~~~~~~~ CZ H4 E.ll.-K. E,. IE (6- C: z.j C:,,,a 'i (..j - E "M r, (Z 'Z z,.3 14i i "I r- r- 1D 00 en W) W t v~~~~~n 'I OOO O~~~~~~~' c OOa ; > D a Arch Dis Child: first published as /adc n 1 Nvember Dwnladed frm tu s;! W.st Q.- st t Ea) w D Y s: C 0 O r O< O < n 18 Octber 018 by guest. Prtected by

5 1040 Field, Milner, and Hpkin (time cycled, pressure limited) thrughut. We have shwn previusly that ver the range f ventilatr rates used in the study (0 t 100 breaths per, minute) that this machine is capable f wrking satisfactrily withut appreciable alteratin t the wavefrm r perfrmance.4 5 We feel that Greenugh et al have identified an imprtant marker fr pr ventilatr adjustment. The risks attached t this pattern f interactin have yet t be quantified. We als d nt knw whether the imprtance f the patterns is influenced by maturity, pstnatal age, r the extent f lung pathlgy, but cnsider that the pattern can be mdified by alteratin in ventilatr settings, pssibly aviding the need fr paralysis. References 1 Greenugh A, Mrley C, Davis J. Interactin f spntaneus respiratin with artificial ventilatin in preterm babies. J Pediatr 1983;103: Greenugh A, Mrley CJ, Davis JA, Wd S. Pancurnium prevents pneumthraces in ventilated premature babies wh actively expire against psitive pressure ventilatin. Lancet 1984;i: Heicher DA, Kirshching DS, Harrd JR. Prspective clinical cmparisn f tw methds fr mechanical ventilatin f nenates: rapid rate and shrt inspiratry time versus slw rate and lng inspiratry time. J Pediatr 1981;98: Field DJ, Milner AD, Hpkin IE. The effect f inspiratry time n tidal vlume when using intermittent psitive pressure ventilatin and high frequency psitive pressure ventilatin. Arch Dis Child 1985;60: Field DJ, Milner AD, Hpkin IE. Calculatin f mean airways pressure during nenatal intermittent psitive pressure ventilatin (IPPV) and high frequency psitive pressure ventilatin (HFPPV). Pediatric Pulmnlgy 1985;1: Crrespndence t Prfessr A D Milner, Department f Nenatal Medicine, City Hspital, Nttingham NG5 1PB. Received 16 June 1985 Arch Dis Child: first published as /adc n 1 Nvember Dwnladed frm n 18 Octber 018 by guest. Prtected by