Relative Humidity in Incubators

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1 Arch. Dis. Childh., 1968, 43, 172. Reltive Humidity in Incubtors ELIZABETH BARDELL, JILL FREEMAN, nd E. N. HEY From the Medicl Reserch Council Group on Respirtion nd Energy Metbolism in the Newborn, The London Hospitl Medicl College, London E.1 In previous pper Hey nd Murice (1968) hve reported the effect of chnges of reltive humidity on the het production nd het loss in norml nked bbies in specilly constructed metbolic chmber under controlled environmentl conditions. In order to evlute the clinicl relevnce of these observtions we hve studied the control of reltive humidity tht could be chieved in three commercil incubtors. A mens of ssessing the opertive environmentl temperture in such incubtors hs lredy been published (Hey nd Mount, 1967). The mount of wter tht cn be contined s vpour in ir depends on the temperture of the ir (Fig. 1 hevy line, 1% reltive humidity). By definition the bsolute humidity is the mount of wter present s vpour in given weight or volume of ir, nd the reltive humidity (RH) is the mount of wter vpour ctully present, expressed s percentge of the mximum mount tht could be present t tht temperture. The ir in hospitl wrds is usully bout 5% RH (Fig. 1 point A); if this ir is wrmed from bout 2 to 3 C. without the ddition of extr wter vpour, its bsolute humidity will remin constnt but its reltive humidity will fll to 25% (Fig. 1 point B). For this reson wrmir incubtors contin humidifying devices. Air cn contin more wter thn indicted by the hevy 1% RH line in Fig. 1 t ny given temperture only s liquid droplets, nd these droplets of mist will only remin in suspension for limited time, depending on their size. Avery, Glin, nd Nchmn (1967) hve recently reviewed the use of mist therpy in peditrics. This specilized spect of humidity control ws not covered in the present study. Mteril nd Methods Tests were undertken in temperturecontrolled room on three empty incubtors of different design, n Received June 3, Oxygenire Series III nturl convection incubtor, New Oxygenire Nursing forcedconvection incubtor, nd Stndrd C86 Isolette forcedconvection incubtor. Tempertures were mesured with 36 s.w.g. thermocouples nd recorded on 12chnnel Cmbridge D.E. Recorder t oneminute intervls. Incubtor ir ws smpled from 5 cm. bove the centre of the mttress nd its wter content monitored continuously with wet nd dry thermocouple ssembly of the type described by McLen (1963), through which ir ws drwn t 1 cm./sec. The whole ssembly ws mounted inside the incubtor. Regulr cross checks were performed using smll spirtion psychrometer (Henni, Jegenstorf, Switzerlnd) inside nd outside the incubtors. The discrepncy between the two redings ws never more thn 2%. The opertive environmentl temperture within the incubtor ws mesured using the methods described by Hey nd Mount (1967). The rte t which fresh wrd ir ws being drwn into the incubtors (I) ws ssessed by mesuring the equilibrium frctionl concentrtion of oxygen chieved._ 5* 4 v L s g 2 C~ E Air temperture 3 I I I 1 /o 75/o 4 C. 1 F. FIG. 1.Reltion between bsolute humidity, reltive humidity, nd ir temperture (brometric pressure 76 mm.hg). 172 Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

2 v E._., Reltive Humidity in Incubtors 173 the centre of the mttress ws 32 C. The humidity in the incubtor ws mesured when the humidity Isolette reservoir ws empty nd t four different settings of the humidity shutter or control knob when the reservoir ws full of wter. It ws found tht the 1 Series IIt./ rnge of control vilble in the New Oxygenire incubtor ws smll, nd the level of humidity /,,_.~ New Oxygenire chieved in the other two incubtors ws not linerly relted to the control setting (Fig. 2). Becuse the....,." ^".....^""... Dry Min. 'i2, Control setting FIG. 2.Reltion between control setting nd RH chieved inside three commercil incubtors when room RH is 5%. inside (x) when known mount of pure oxygen (y) ws lso dded ech minute. The rte ws then clculted from the reltion: I = y(1x)/(x21). Rtes of between 1 nd 5 cubic metres per hour were encountered. Assessment of Three Stndrd Incubtors Control. Ech of the incubtors ws plced in turn in room t temperture of 2 C. with RH held t 5±3% (men±sd), nd the incubtor djusted so tht the temperture of ir 5 cm. bove 7.._ : Control 1/4 on IF humidity reservoir is not completely closed off by the control mechnism in the New Oxygenire nd Isolette incubtors, relly low humidity could only be chieved by drining the humidity reservoir of ll wter (Fig. 2). When the control setting ws ltered, n increse in humidity ws quickly chieved in ll three incubtors, but decrese took very much longer (e.g. Fig. 3). When the incubtor doors were opened (even briefly) chnges occurred in incubtor humidity in the direction of tht existing in the room (Fig. 3). (Similr brisk chnges in the percentge of oxygen present could be detected when door ws opened while high oxygen level ws being mintined within the incubtor.) Some minor fluctutions in humidity occurred in the Series III Incubtor in time with the cycling of the thermostt (Fig. 3); no such fluctutions could be detected in the other two incubtors. Effect of temperture. The mximum nd minimum humidity chieved ws mesured t different room nd incubtor ir tempertures with room humidity held t 5 ±3%. It ws found tht the mgnitude of the difference between room nd incubtor ir temperture ffected the minimum Hnd port open 4 Control 'off' Jr Heter on: m m m m m m m m m I Hours FIG. 3.Record of RH within n Oxygenire Series III incubtor (incubtor ir 32 C., room 2 C., nd RH 5%). Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

174 75. 1 5 25 _. 75 :2 re C 1 5' cm 25. o 1* 75 5 25 %W _ = = _.. bwu Room humidity. These tests were then repeted with the room humidity t 9 +4%.

the two Oxygenire Incubtors (Fig. 4). Added oxygen. The ddition of 2 to 4 litres dry oxygen per minute from cylinder hd no significnt effect on the generl level of incubtor humidity.

3 _. 75 :2 re C 1 5' cm 25. o 1* %W _ = = _.. bwu Room humidity. These tests were then repeted with the room humidity t 9 +4%. It ws found tht the rise in room humidity influenced the minimum humidity chieved in ll three incubtors in predictble mnner nd lso the mxi ' ; A '' ' Brdell, Freemn, nd Hey mum humidity obtinble in the two Oxygenire Incubtors (Fig. 4). Added oxygen. The ddition of 2 to 4 litres dry oxygen per minute from cylinder hd no significnt effect on the generl level of incubtor humidity. Higher oxygen concentrtions cn most conveniently nd economiclly be chieved by leding oxygen into smll Perspex hood plced over the infnt's hed, but the infnt will of course be brething exceedingly dry gs unless this oxygen is first wrmed nd then humidified (Wrley nd Girdner, 1962). IllllllllllllslllllllllllllllllllllllllllilllllllllllllllllllllSS Humidity levels with n infnt present. Tests were repeted when premture infnt ws being nursed within the incubtor, to see if the humidity ws incresed under these conditions. The differences observed were smll, nd suggested tht the presence of bby never incresed the humidity by more thn4%. This grees resonbly New Oxygeneire with theoreticl predictions bsed on estimtes of the rte t which fresh ir is drwn into the incubtors. A relly wet npkin significntly incresed the child's totl evportive het loss, but not the humidity within the incubtor. Isolette C Temperture ( C.) Difference between room nd incubtor ir temperture FIG. 4.Shded res indicte the rnge over which reltive humidity cn be controlled inside three different commercil incubtors when room RH is 5% (=) or 9o% (I 1). The lines represent the minimum humidity obtinble when ll wter is removedfrom the humidity tnks nd room RH is 5% (... ) or 9% ( ). The mount by which incubtor temperture exceeds room temperture is plotted on the horizontl xis. humidity chieved in ll three incubtors, nd lso the mximum humidity chieved in the Series III Incubtor (Fig. 4). r Men rdint temperture. Chnges in reltive humidity produced only very smll nd rther vrible chnges in the men rdint temperture. Condenstion of wter on to the sides of the incubtors led to slight rise of surfce temperture which ws probbly due to the relese of ltent het from the wter. However, vritions in ir temperture lso occurred: in the Isolette incubtor, in prticulr, ir temperture t the inlet fell severl degrees when the ir ws routed over the humidifying wter try; though men ir temperture within the incubtor ws little ffected, rdint surfce tempertures ner the inlet fell in consequence. Accessories. An Oxygenire humidity 'tomiser' ws fitted to the bck wll of the Series III Incubtor nd tested. With through flow of 21./ min. from cylinder this ccessory ws cpble of producing 1% RH in the incubtor independent of the min humidity control setting. However, this input of cold ir into nturl convection incubtor produced 3 C. fll in ir temperture bove the mttress. The Isolette ws lso tested with its 'Vpojette' ccessory. The effect on incubtor ir temperture ws negligible (except when the heter ws full on) becuse of the compensting effect of the thermostt, but with the Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

4 min humidity control open, the 'Vpojette' ccessory only incresed humidity further 7%. Wter level. Vrition in the mount of wter present in the humidity tnk in the two Oxygenire incubtors (within the limits set by the mrked mximum nd minimum wter levels) cused no chnges in humidity. No such working limits for the wter level were mrked on the Isolette*. In this incubtor wter hd to be visible more thn n inch deep in the trnsprent filling receptcle before pprecible quntities entered the humidity try, nd when the wter level ws low it ws impossible to rise incubtor humidity bove 55%. It is our experience tht ll three incubtors re often used with less thn the minimum effective mount of wter present in the humidity tnk, nd tht this is not pprecited becuse no relible mens is provided of mesuring humidity. Mesurement of incubtor humidity. The Series III Oxygenire Incubtor ws the only incubtor fitted with hygrometer; in the other incubtors no mens of mesuring internl humidity ws provided, even s n optionl extr. The series III direct reding pper hygrometer ws tested on receipt nd found to be ccurte to within 3% except for n hour or two fter becoming sturted t 1% RH, but those in constnt wrd use soon becme inccurte for lck of servicing or djustment (though the mjority of the nursing stff hd not pprecited this). The hygrometer ws screwed through one of the Perspex wlls. In this position the dil ws esy to red, but, becuse it ws fixed to reltively cold outer wll, the instrument ws in cold lyer of boundry ir where the reltive humidity ws higher thn in the min body of the incubtor. Thus, with n ir temperture of 34 C. nd n RH of 75% in the body of the incubtor (Fig. 1 point C) in room t 2 C. the reltive humidity recorded close to the outer wlls pproched 1% (Fig. 1 point D) becuse the temperture here ws below 29 C. Visibility. Becuse the incubtor wll temperture is influenced s much by the temperture of the room s by the temperture of the ir within, it follows tht misting nd condenstion will occur on these reltively cool surfces long before the reltive humidity in the min body of the incubtor reches 1%, unless the difference between room nd incubtor ir temperture is smll. Thus, visibility through the wlls of the Series III incubtor is frequently obscured by condenstion during * We understnd tht the firm intend to provide these in future. 4 Reltive Humidity in Incubtors 175 routine use, even though it cn be shown tht the ir inside is not more thn threequrters sturted. After some time wter strts to run down the side wlls nd even the mttress becomes wet. The problem is even more cute in those incubtors cpble of producing higher internl humidity. Assessment of n Experimentl Doublewlled Incubtor In the belief tht the greter insultion offered by seled 'double glzing' might ese the problem of condenstion nd consequent loss of visibility, n experimentl doublewlled cnopy ws mde for the Isolette incubtor. A thin sheet of Perspex ws plced few mm. inside the min Perspex cnopy: ll the wlls nd dome were so covered except for the four hnd ports, nd the edges of the inner wll were seled to the min cnopy ll round leving cvity of trpped ir between the two Perspex sheets. The outer surfces of the modified cnopy were esily clened; it ws thought tht the inner surfces would not require ttention if the seling ws sufficiently thorough. The therml environment provided within this experimentl cnopy ws then compred with tht chieved in the Stndrd Isolette C86 incubtor nd the incidence of condenstion studied. Air speed ws mesured directly nd found to be low (41 cm./sec.) immeditely bove the mttress. The reltion between verge surfce temperture nd room nd incubtor ir temperture is indicted in Fig. 5, nd the effect of room temperture on the men rdint temperture within the two incubtors is summrized in Fig. 5b. The therml environment provided by the Stndrd Isolette incubtor is seen to be very similr to tht provided by the two Oxygenire Incubtors studied previously (Hey nd Mount, 1967); the discrepncy between ir temperture nd men rdint temperture is pproximtely hlved by the use of seled double cnopy. When humidity within the incubtor ws high, condenstion occurred on to the cold prts of the cnopy in both models, but the greter insultion provided by seled cvity wll reduced the incidence of this problem. Discussion Where no ccurte mens of mesuring RH in the incubtor is vilble the findings summrized in Fig. 2 nd 4 cn be used to predict pproximte humidity, but where ccurcy is required the ctul RH should of course be mesured. For this purpose directreding hygrometer conforming to British Stndrd 3292(196) would suffice, s long s cre is tken not to plce it in cold lyer of Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

176 U _ ^ u.2 4.o c E 4. c._ 4. 8 3 32 31 3 29 28 () Incubtor ir 32 C. _ 27 1 15 2 Room temperture (CJ 25 3 (b) FIG. 5.

5 176 U _ ^ u.2 4.o c E 4. c._ () Incubtor ir 32 C. _ Room temperture (CJ 25 3 (b) FIG. 5.() Effect of room temperture on surfce tempertures inside Stndrd Isolette incubtor, nd inside prototype with doublewlled cnopy. (b) Effect of room temperture on men rdint temperture in these two incubtors when ir temperture inside is mintined t 32 C. boundry ir. Any such hygrometer would require regulr servicing nd periodic reclibrtion, prticulrly if the hygrometer is left permnently in situ, becuse dirt nd chemicls ffect the sensing element. It hs been shown tht, unless n ccessory nebulizer is used, the reltive humidity in the three incubtors tested cn only be controlled nd vried Brdell, Freemn, nd Hey between bout 3 nd 85% RH, though there re significnt differences in the performnce of the three models studied. From the previous pper (Hey nd Murice, 1968), it cn be seen tht vrition of humidity within this rnge hs comprtively little effect on n infnt's het loss or energy metbolism, prticulrly if the surroundings re wrm nd the infnt's het production miniml. The effect on the infnt's wter blnce is probbly lso mrginl, though O'Brien, Hnsen, nd Smith (1954) hve shown tht lrge chnges of humidity ffect wter blnce nd hemoconcentrtion in infnts strved for the first 3 dys of life. The effect of humidity on bcteril contmintion in incubtors is poorly documented, though the dnger of the humidity reservoir nd other dmp surfces becoming colonized with pseudomons is now widely recognized. None of the incubtors tested provided RH of 95%, s required by the current British Stndrd for incubtors 361(1965), without uxiliry equipment. However, the British Stndrd requirement _ in this respect is not very precise nd is probbly in need of revision. It remins, of course, most importnt to be ble to mintin high inspired ir humidity when nursing n infnt who hs been trcheostomized or intubted. Here the inspired ir should idelly be fully sturted t body temperture whtever the humidity of the rest of the ir in the incubtor my be. Air fully sturted t 29 C. (Fig. 1 point D) will be only 65% sturted t body temperture (Fig. 1 point E). The loss of visibility due to the condenstion tht occurs when internl humidity exceeds 75% RH is of considerble consequence, since ese of observtion is one of the principl resons why incubtors re used in preference to heted cots or devices such s Chrlotte's box. The use of seled ir gp in the wll of the Perspex cnopy reduced the incidence of this problem nd lso decresed the discrepncy between ir temperture nd men rdint temperture within the incubtor under norml wrd conditions. Mesurements of het flow through, nd temperture grdient cross, the cnopy wlls confirm tht this improvement is due to the therml insultion being greter thn tht fforded by similr thickness of solid Perspex. The number of ir: surfce interfces hs been incresed from 2 to 4, nd this would be expected to bout double the therml insultion provided by the cnopy, nd pproximtely hlve the net difference between internl ir nd rdint tempertures. Fig. 5 shows tht these predictions re fulfilled. The exct extent of insultion provided will be determined by the width of the gp between Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

6 Reltive Humidity in Incubtors 177 the two Perspex wlls which ws not constnt in the prototype cnopy tested. The optimum gp is probbly nerly 1 cm., but minor devitions re unlikely to mke detectble difference. The use of wll stnding further wy from the min cnopy, with free circultion of wrm incubtor ir over both its surfces, s suggested by Hey nd Mount (1966), is more effective in reducing the discrepncy between ir temperture nd men rdint temperture, but does nothing to combt condenstion. Thus, choice between these two methods of modifying the design of wrm ir incubtors to control rdint het loss should depend in prt on clinicl decision s to the importnce of nursing infnts in more thn 75% humidity. Summry The rnge of reltive humidity (RH) tht cn be chieved in three stndrd commercil incubtors hs been investigted under vriety of conditions. The rnge of control ws sometimes limited, nd often not linerly relted to the control setting. In none of the incubtors tested ws it possible to chieve mximum RH of 95% (s currently required by the British Stndrd specifiction), even under optimum conditions, without the use of ccessory equipment. A mens of mesuring humidity ws only provided with one incubtor, nd, since this hygrometer ws fixed to the cnopy in the boundry lyer of reltively cold ir, the instrument gve flsely high reding. Mrked condenstion on to the cold Perspex wlls obscured visibility when incubtor ir ws more thn threequrters sturted under norml wrd conditions; the provision of doublewlled cnopy with seled cvity reduced the incidence of this problem s well s hlving the discrepncy between ir temperture nd men rdint temperture within the incubtor. We re grteful to Professor K. W. Cross nd Dr. L. E. Mount for dvice, nd to AirShields (UK) Ltd. nd to Oxygenire Ltd., for coopertion nd for the lon of equipment. The experimentl doublewlled cnopy ws mde for us by AirShields (UK) Ltd. We re prticulrly grteful to Dr. A. E. Hnwell nd to Dr. G. R. Bll medicl engineering consultnts with these two firms, for their constructive criticism nd help. REFERENCES Avery, M. E., Glin, M., nd Nchmn, R. (1967). Mist therpy. Peditrics, 39, 16. British Stndrd 3292 (196). Specifiction for direct reding hygrometers. British Stndrds Institution, London. 361 (1965). Specifiction for electriclly heted incubtors for bbies. British Stndrds Institution, London. Hey, E. N., nd Murice, N. P. (1968). Effect of humidity on production nd loss of het in the newborn bby. Arch. Dis. Childh., 43, 166., nd Mount, L. E. (1966). Temperture control in incubtors. Lncet, 2, 22., nd (1967). Het losses from bbies in incubtors. Arch. Dis. Childh., 42, 75. McLen, J. A. (1963). Mesurement of cutneous moisture vporiztion from cttle by ventilted cpsules. J. Physiol. (Lond.), 167, 417. O'Brien, D., Hnsen, J. D. L., nd Smith, C. A. (1954). Effect of supersturted tmospheres on insensible wter loss in the newborn infnt. Peditrics, 13, 126. Wrley, M. A., nd Girdner, D. (1962). Respirtory distress syndrome of the newbornprinciples of tretment. Arch. Dis. Childh., 37, 455. Arch Dis Child: first published s /dc on 1 April Downloded from on 25 July 218 by guest. Protected by copyright.

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