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A new instrument for the measurement of tissue pO2 of human fetal scalp
FETUS AND NEWBORN A new instrument for the measurement of tissue p02 of human fetal scalp ADRIAN WALKER, B.Sc. LYN PHILLIPS, Dip . A . I . M . L.T . LEN POWE* CAR L WOO D, F. R . C S", . M, R . C , 0 c:, , Melbollme. Victoria, AIIJtra/in MenJurement o//etal scalp tiuue pO, lIIay be a wa)' o/monitoring /etal oxygenation continuouIly . A membrane-colJered {iuIh-type oxygen electrode was del'eloped for this purpose , This electrode measured qualitative changes in tissue pO,. Movement artifacts were reduced by the membrane coating and by the matching of electrode diameter to mean intercapillary distance, the electrode then monitoring mean tissue pO,. Matemal inhalation of 50 per cent oxygen increased scalp tissue pO,. Following normal birth the scalp tissue pO, increased. Neonatal inspiration of 100 per cent oXYRen produced a marked increase in scalp tissue pO,. In one baby born in a poor clinical condition, reeol'ery was mirrored by challRes in scalp tissue pOt. In 2 patients with slowing of the fetal heart rate during contractions, scalp tissue pO, decera.red ., inlll/talleously. Because the electrode cannot be considered accurately quantitative the information obtained has only been useful ill elucidating fetal oxygenation in rellllion to particular tl'ents. At pre.rent it has no use in clinical obstetric.'"
lllent, the "oxygen cathode" or polarographic Illethod is the one best suited to continuous recording in vivo. Daneel, the discoverer (\898) of the oxygen cathode, showed that a small DC voltage applied to platinum dectrodes in a medium containing oxygen resulted in electrolysis of the dissolved oxygen, the current being approximately proportional to the oxygen tension (Fig. 1). Davies and Brink ( 1942), using platinum electrodes, applied this principle to measurement of oxygen tension in animal tissues. Since then the use of the oxygen cathode has increased. There are three fundamental types of oxygen electrode: 1. The "flush" -type electrode has the
F ETA L H Y I' 0 X I A is responsible for a large proportion of stillbirths and neonatal deaths and also accounts for brain damage in a significant number of babies who survive. Collection of fetal scalp bloocP enables measurement of fetal blood pO., but the information obtained is only relevant to the time of collection. However, a lIIeasurement of scalp tissue pO. might enablt' continuous monitoring of fetal oxygenation. Of all known methods of p02 measureFrom the Department of Obstetrics alld Gynaecology. MOllash University Medical School. Queen Victoria Memorial Hospital. *Titron Inltruments. Melbourne. Victoria, Australia.
63
64
Walker et 01.
V
Am,
VOLT
I
10< p02
ACTIVI
ILierRODI
. . . . UNCI
IUCtRODI
Fig. 1. Diagrammatic representation of the prin-
ciple of pO, measurement. When a potential difference V is maintain .. d betwt'en two electrodes as shown oxygen is electrolytically reduced at the active eh'ctrodc surface giving rise to a direct current I. Commonly used electrode materials are gold or platinum for the active electrode and silver for the referent'c cl("ctrode, The reaction i. maintainl."d by diffusion of oxygl."n to thl." active rlectrode, the rdt'n"uce electrode providing (,Icctrical continuity, When V = 0,6 volt the lII('asurem('nt is essentially sp!'cific for oXyl(!'n in a biologic lIIedium, the current I then bring directly proportional to tht' oXY,gen tension,
cathode or active area flush with the insulating material which normally provides mechanical support. It effectively measures p02 direC'tly in the medium in combination with a physically separate reference electrode, 2. The "recessed" -type electrode has the active area recessed in the insulating material. Used in combination with a separate reference electrode it effectively measures pOz indirectly in the volume of the recess. 3. The "Clark" or "combined" -type electrode has both the active and reference electrodes combined in the one physical structure and effectively measures p02 indirectly in a microdiffusion chamber separated from the medium by a thin nonconducting but oxygen-permeable membrane. The choice of a particular type of electrode involves consideration of its absolute
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January 1, 1968 Dbs!, & Gyne<:.
accuracy. sp""d of response, long-term stahility and susceptibility to "poisoning," size, and tIll' pmsihility of inaccuracy due to tissue damal!t'. till' validity of in vitro calibratioll, and the technical difficlllty involved in its ma 1111 f a I'! II f(', Flush-t~v'" electrodes ha\'c been most popiliaI' in tissue p02 measurements. They are easily mad,· and maintained and have the fast response tll'('l'ssary for following rapid changes, Th"y have been criticized because they an' unstable, susceptible to poisoning, and quantitatin'ly inaccuratc due to depeIlllence Oil diffusion conditions and to the degree of tissue damage caused by insertion. IlIIpro\'"d stability and resistance to poisoning lIlay be achieH"(1 by using gold as the cathode lIlatl'rial,2 a fixed DC biasing potential, grinding a fresh electrodc surface before each experiment,' and coating the surface with oxyg"11 permeable materials to prevent both contamination and erroneously high current \'ahu"s due to movement of fluid past the surface! Extrapolation from in vitro calibration to in vivo lIIeaSUrellH'nt has been questioned. Charlton (personal comllHlIlication) has stated that no quantitative Illl"asurement of oxygen concentration is possible without knowledge of diffusion conditions and fluid InO\'l:"lIIel1t in the region of mcasurement. However, Cater and Silver," after experiments on "model" tissues of compressed cotton wool and artificial extraceJluiar fluid and on perfused tissul", were satisfied that this extrapolation was satisfactory, especially with electrodes less than 100 p. in diameter. Jamieson and van den Brenk 2 have claimed that damage produced by insertion of e1ectrodl's into soft tissues causes greater error than that of calibration, thus obviating any advantages of more complicated electrodes. "Recessed" -type electrodes have advantages in that measurement is virtuaJly unaffected by fluid movement and diffusion conditions because oxygen is drawn from within the recess. However, their use in continuous in vivo recording is precluded by their very slow respon!lt' to a rapid change in oxygen
VolulIl< 100
Numhcr 1
tension and by the necessity of reading thc electrolysis currcnt at a precise tillle after application of biasill~ potential. Clark-typc electrodes have the advantages of recessed electrodes, being virtually unaffected by diffusion conditions and fluid movemcnt but without the disadvantages of slow responsc tilllC and illlenllittent current reading requirements. The protection afforded by thc membrane makes thelll suitable for a medium such as blood where poisoning may occur. Electrodes of this type havc been developed with excellent accuracy, stability, reproducibility, and rt'sponse tillles,5, a but the technical difficulty of achieving these characteristics together with a high dt'gree of mechanical strength is great. It was decided to develop an electrode system for continuous recording of oxygen tension in the fetal scalp embracing as JlIany important characteristics as possiblc. The flush-type electrode protected by a suitable coating material was chosen. It offered a reasonable compromise bl'twl'cn the necessities of accuracy, stability, speed of rl'sponse, and easc of mechanical adaptation to a difficult experimental environment.
Methods Construction of electrodes. * Because needle electrodes were difficult to kcep in thc scalp they were mounted in a modified wound clip (Fig. 2). This has the following advantages: 1. Both the referenct~ and active elcctrodes are contained in a single unit. 2. It has the mechanical strength necessary to withstand thl' stn's5 of insertion. 3. Insertion of the c1l'ctrode is not difficult with specially made forceps. 4. Once inserted, the electrode remains firmly in position. The existing points of a 14 mm. wound clip are cut off. Holes are drilled through the clip maintaining the angle of the original point. A short-bevel size 18 needle bent to shape is soldered into each side of the clip. ·Availahle (rom Tieron InltruPlf'nt., Gf'Ol'ge St. Sand .. riDlLham. Melbourne, Victoria, AUltraiia .
Oxygen electrode for tissue pOz measurement
65
Fig. 2. The pO, electrode. Both active and reference electrodes mounted on a 14 mm. wound clip. (Propper Manufacturing Company.)
A 2 inch length of 60 p. gold wire (active electrode) is soldered to a length of 0.03 inch enameled copper wire, taking care that the diameter of the copper wire is not greatly increased by the solder joint. The gold wire and soldered joint are coated four times with Araldite 985 E (3 parts resin, 1 hardener, 1 solvent), curing for 5 minutes at 180° C. after each application. This coating is necessary to ensure good insulation. A 60 p. silver wire is treated in the same way. The two prepared wires are threaded down separate nylon tubes (external diameter, 0.63 mm.). The needles in the clip are filled with Araldite F (CY 205). (Equal parts of resin and hardener to 1/100 part accelera tor.) Before this hardens the nylon tubing containing the gold wire is pushed through one needle until it protrudes about one inch f rom the point. A second nylon tube containing a silver wire is pushed through the second needle in the same way. Araldite CY 205 is injected into the tubing through a size 25 needle until it passes the gold-copper wire joint. Small glass stoppers are placed in the two protruding tubes to prevent leakage during curing. The Araldite is cured after 5~ hours at 90° C. Protective tubes of polythene are threaded over the nylon tubes and slipped over the sections of the needles protruding from
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Walker et 01.
the back of the clip. The two lengths of nylon tubing protruding from the points of the needles are cut flllsh with the be\"(' I using a vcry sharp scalpel blade. This exposes tht' active area of the gold and sih'er wires. The gold electrode is ground lightly "ith a diamond disk and cleaned in ether bdore coating with a layer of dichiorodilllethyl hydrosilicatc which protects the elect rod I' from poisoning and reduCt,s artifact without significantly lowering the response til1le. The silver electrode is wated with silver chloride electrolytically using a carbon cathode and 0.1 N HCl solutioll. The electrode is aged hy leaving it ill normal saline for some hours with a bias potential applied. Behavior of the electrode. When the I'iI'Ctrode is moved in a hOlllogeneolls 1l1l'diuIll a sharp hut tplIlporary increase in current results. Oxygen is consullll'd hy the elt'ctrode so that the oxygen cOllcentratioll is
Fig. 3. Subcutaneous capillary vessels in the scalp of a stillborn fetug. Mean intercapillary distance, 76 JJ (S.D. ± 24.6). (x250.)
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January I, 1968 Dbsl. & Gynec.
lower close to the tip than in the surrounding tissue. The steepness of this oxygen con('t'ntratioll gradiellt determines the current magnitude.; If the electrode is moved, its tip is shifted closl'I' to the fl'gion of high concentratioll, the gradient is increased, and the ClllTI'nt rises. Steady state conditions are reI'stahlishl'd within 5 seconds as oxygen consUlllption by the electrode regains equilibriulIl with the surrounding medium. \\,llI'n the electrode is Illoved in tissue a ~harp but temporary increase in current results. This finding in an inhomogeneous oxygen ('11\'ironllll'nt delllands further explanation as one might expect movement in relation to capillaries would have a random chance of producing a rise or fall in current. In order to dl'terllline the conditions in which the l'll'ctrode works, the anatomy of the capillaries in the suhcutaneous tissue of the hUlIlan fetal scalp was examined. Sections of scalp f rOIl1 a stillborn fetus were examined microscopically. Capillaries could be distinguished rl'adily in the subcutaneous laycr (Fig. 3 I. They were generally oriented wrtically at right angles to the skin surface, but adjacent capillaries were linked by nulIlerous branches arrangl,d parallel with the surfan'. Tell areas 111 the skin wcre selected at randolll and the in tercapillary distance betwcen vessels oriented vertically was measurt'd using an eyepiece grid. The mean intercapillary distann' was 76 fL (S.D. ± 24.6). An electrode receives 95 per cent of its oxygen from within six t·lt·ctrode diameters. s Thus the 60 It electrode used in this study will "see" approximately 360 It into the tissue and should receive oxygcn from a volume containing at least twenty capillaries mostly oriented parallel to the plane of its active surface. Thus the electrode will monitor mean tissue pO" regardless of the variations of pO" between capillaril's. As far as Illeasurement of p02 is concerned, the scalp anatomy and the nature of the present electrode enable it to behave as though it were in a homogeneous environment. Thus the explanation for the rise of current when the electrode is moved in tis-
Volume 100 Numht'r I
sue is the sallie as for movelllent in a homogeneous mediullI . Movl'llIent of the electrode should result in only an erroneous rise in current, not a fall. For this n'aSOll, any decrease of pO" during a period of patient movement was considered valid, but a rapid rise was not. Long-term electrode stability in vitro is reasonable. Current readings in air-equilibrated saline show It'SS than 10 per cellt variation over 8 hours. However, romparison of calibration currents before and after an in vivo experilllent can show changes of up to 50 per cent. The electrodes are also susceptible to "stirring" artifact. Extremely vigorous stirring of the calibration llIedium increases electrode current by 50 per cent and sharp movement of the electrode in an unstirred medium results in a characteristic spike of current of the sallie dt'gree. Response to a sudden change in oxygen concentration reaches 90 per cent of the final level within 5 seconds. Experimental procedure. The p02 electrode is calibrated in normal saline equilibrated with known gas mixtures and sterilized in 1/2,000 aqueous Hibitane. The wound clip housing the oxygen electrode is attached to the fetal scalp by means of specially made forceps. This can be done only when the membranes are ruptured and the cervix is 3 em. dilated. Care in handling the electrode is required as it may break under mechanical stress. The fetal pO. is usually recorded with
Oxygen electrode for tissue p02 measurement
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fetal heart rate' and intrauterine pressure. The current reading from the p02 electrode llIay take up to one hour to stabilize. Apart from the continllolls n~cord of p02 change on the ()fl'ncr recorder, actual current readings are also taken every 15 minutes and 1lI0VeIllents of the patient noted on the chart. The oxygen electrode is disconnected fmm the recorder at the time of delivery and reconnected immediately after. The J1t'onate is breathed with 100 per cent oxygen by mask and if the scalp p02 increases, the electrode is then known to be still functioning. If excessive caput is present at the site of the electrode, it is reattached to a normal area of scalp to compare current readings. Whenever possible, the electrode is recalibrated after the experiment. The validity of the experiment is best judged by the similarity between pre- and postexperimental calibration curves and response to neonatal inspiration of 100 per cent oxygen. Thirty-nine experiments have been conducted of which 19 have provided information . These have demonstrated a relationship between fetal scalp tissue p02, on one hand, and on the other, normal contractions, normal and abnormal fetal heart rate, general anesthesia , and maternal and neonatal oxygen inspiration. Results
Maternal inhalation of 50 per cent oxygen. Patient D. P. had a nomlal labor and de-
Fig. 4. Fetal scalp pO, recorded on a patient during normal labor.
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.... m.
Fig. 5. Rise of f('tal scalp pO, during a pt'riod of maternal inspiration of 50 per c.. nt oxygen. F.. tal scalp capillary blood pO, increased from 14 to 26 mm.Hg.
Fig. 6. Scalp pO, increase after delivery. A further increase in pO, occurred wht'n the nf'onat .. breathed 100 per cent oxygen.
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January 1, 1968 Ohsl. & Gynec.
livery resulting in the birth of a healthy bahy (Apgar i." l'terint' contractions and fptal heart rate were normal. A representaative pt'riod of the fetal scalp p02 shows slight changl's with normal contractions (Fig. 4 I. These slight rises cannot yet be in tl'rprl'ted. The patient breathed a 50:50 nitrous oxide-oxygen mixture for 5 minutes during labor. The fetal s('alp p02 rose within one minute of breathing oxygen (Fig. 5) and a maximulll len'l was reached in approximately 4 minutes. This level was more than twice the resting len·1. When oxygen breathing ceased, scalp pO~ returned to the resting level within one minute. The recording which followed was similar to that prior to oxygen inhalation. Fetal blood collections were taken just before and during the period of oxygen breathing, the fetal blood p02 rising from a resting lewl of 14 mm. Hg to 26 mm. Hg. Change from fetal to neonatal existence. Patient R. P. had a normal labor and delin'ry, the Apgar scon' of the baby at 2 minutes being i. Fetal scalp p02 showed slight increases during contractions which were also accornpani,·d hy a small decrease in heart rate (Fig. 6). After delivery the neonatal scalp pO~ was three times the fetal scalp lew I. After a neonatal rt'sting level had bt'en established, the neonate breathed 100 per cent oxygen by mask, and an immediatt' increas(' in the neonatal scalp p 0 2
P.l Ampo.IO" goA
6
Go
Fig. 7. Readings of scalp pO, recorded bl'fore and after delivery. Scalp pO, incrf'ased as the neonatal condition improved.
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followed. The neonatal scalp p02 returned to basal level when the mask was rellloved. The experiment was repeated with similar results. Patient P. Z. gave birth to a baby with an Apgar score of 2 following a normal labor and delivery. For 3 minutes after birth, the scalp tissue p02 remained at a similar level to that before birth (Fig. 7). When artificial ventilation of the lungs was carried out with 100 per cent oxygen, the scalp tissue p02 began to rise immediately and had increased fivefold after 3!/:1 minutes of oxygen administration. At this time, the baby's condition had improved, its color, tone, and pulse rate were normal, and spontaneous respiration had comlllenced. After removal of the endotracheal tube, the scalp tissue pO, remained high. Slowing of fetal heart rate. In Patient D. B. marked slowing of the heart rate occurred with contractions (Fig. 8). Delivery was norIllal, the Apgar score at 2 minutes being 6. The bradycardia was associated with a fall in fetal scalp pOz, the degree of change being proportional to the strength of the contraction. Both the bradycardia and decrease in p02 commenced with the onset of the contraction but the pOz was slower than the heart rate in returning to the normal level. Patient E. P. was delivered of a normal baby at 35 weeks with an Apgar score of 7. The pregnancy was complicated by preeclampsia and because of this labor was induced. Fetal heart rate slowed with uterine contractions and coincident with these changes scalp tissue pOz decreased (Fig. 9). General anesthesia. Patient M. K. was delivered by cesarean section after failure of progress in labor. The Apgar score of the baby at 2 minutes was 6. The fetal recording showed mild slowing of heart rate and slight rises in scalp p02 with contractions (Fig. 10). When general anesthesia began, two effects were noted. First, the slight rises in pOz with contractions were abolished. Second, a gradual fall in pOz commenced and continued until delivery. This fall was accompanied by fetal heart rate slowing. A
Oxygen electrode for tissue pOl measurement
69
Fig. 8. Slowing of the fetal heart rate and fall in f('(al scalp pO, occurring with contractions.
Fig. 9. Slowing of the fetal heart rate and fall ill scalp pO, occurring with contractions.
subsequent neonatal experiment showed a characteristic response to 100 per cent oxygen. Comment
Due to unknown diffusion conditions in the fetal scalp tissue, the electrode susceptibility to stirring artifact, and the variability of calibration currents, conclusions as to the absolute value of tissue p02 have not been drawn. Results have been quoted in terms of measured electrode current, but conclusions have only been made on a qualitative basis. However, in 5 cases where comparison of electrode measurements and fetal blood pOz measurements was possible, electrode measurements indicated scalp tissue p02 values of 7, 3, 9, 9, and 17 mm. Hg
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loUt
------------------------------
Fig. 10. Decrease in f(' tal s calp pO, and f("(al hear! rat(' durin\( cesarean s!'ction under general anesthesia.
compared with corresponding scalp capillary blood pO ~ levels of 26, 12, 25 , 13, and 23 mm. Hg. The rise of fetal scalp tissue pO~ with maternal inhalation of 50 per cent oxygen ( Patient D. P.) indicated that oxygen transport to fetal tissue may be increased in normal patients. Although only olle successful experiment has been completed measuring scalp tissue p02 during OXY,gen administration, the association of the raised tissue pO~ with an 85 per cent increase in fetal capillary blood pO~ adds credence to the result . Furthermore, in a series of 30 experilllents where the mother inhaled high oxygen gas mixtures, the fetal capillary blood p02 usually rose by a similar degree to the present experiment (unpublished data). The rise in fetal scalp tissue pO, following normal delivery (Patient R. P.) may reflect improved scalp circulation resulting from increase in fetal oxygenation coincident with the onset of respiration. Engstrom and associates fi have shown that iliac artery p02, which probably represents the blood pO~ supplying fetal tissue, increases within the first five minutes after birth and then slowly increases further over one hour. In the presence of caput succedaneum tissue pOz may be lowered by the associated circulatory stasis or by impaired diffusion of oxygen from capillary to tissue. However, in the experiment described the prompt increase on two separate occasions of scalp tissue p 0 2 when the baby was breathed with 100
per C(, lIt oxygl'n suggests that local circulatory stasis was not presellt and that the mark ed incrt'.lse of tissue p02 which ocCUlTed after the OIlSt:'t of respiration was refI('ctill~ illlpwn'd oxy~enation. In Patient P. Z. the sralp tissue p02 changes mirrored the changes in the clinical condition of the lIeonatt', no change in tissue p02 occurring in the first few minutes after birth when the haby remained apllcic and flaccid but a rapid increase ()('clIITcd as the baby's condition \\'as impro\'l'd by intubation and 100 per Cl'nt oxygen . COlltinuous lIIeasurement of tissue p02 in Patient D. B. showed a pattern of fall of pOl similar to the pattern of bradycardia o('currinl\' dUI;ng contractions. A similar experinwnt was carril'd out in Patient E. P., when a decrease of scalp tissue p02 was al\'ain coincident with slowing of the heart rate during a contraction. Th('se two studies pro\'itlc evidence that marked decrease of heart ratl' during contractions is sometimes associatt'd with interruption of gaseous interchange hNwt:'en tht:' lIIother and fetus. In tht:' presence of a normal fetal heart rate during contractions, scalp p02 docs not have any dl'finite changt:'o WI' wish to thank Mrs. Dana Jamieson, Dr. D. R. Cat!'r. and Dr. G. Charlton for their help and ad\'irl' on oxyg .. n III l'aSII t'eml'nt , and Dr. W. Newman. Miss Loris McKinnon, Miss Dianne Braid, Mr, David Walk('r, and Dr. Peter Paterson for .heir hdp in prl'paring the electrodes and carrying out the expf'riments. We wish to thank
Volume 100 Number 1
the Amalgamated Dental Company for their technical help. We also wish to thank Profl'ssor C. Schofield who prepared the s('ctions of fetal scalp and pro-
Oxygen electrode for tissue p02 measurement
vidcd us with m!'asurements of intercapillary distance. Financial assistallc(' from the Felton Bequest is gratefully acknowledged.
REFERENCES
1. Saling, E. H .: Micro Blood Analysis. A Method for Examination and Supervision of the Fetal Condition During Labor, Address given at Yale University School of Medicinc, 1964. 2. Jamieson, Dana, and van dcn Brenk, H. A. S.: J. App\. Physio\. 20: 514, 1965. 3. Cater, D. B.. and Silver, 1. A.: Microelcctrodt's and Electrodes Used in Biology, in Ives, D. J. G., and Janz, G. j. , editors: Reference Electrodes, New York, 1961, Academic Press, Inc . 4. Cater, D . B.: The Measurement of pO, in
71
5. 6. 7.
8. 9.
Tissues, ill Abstracts of Symposium on Oxygen in the Animal Organism, London, 1963, Bedford College. Charlton, G., Read, David, and Read, John: J. App\. Physio\. 18: 1247,1963. Fait, 1: J. App\. Physiol. 19: 326, 1964 Davies, Philip W.: The Oxygen Cathode, ill Nastuk, William L., editor: Physical Techniques in Biological Research, New York, 1962, Academic Press, Inc., vol. IV. Hon, E. H .: Connecticut Med. 24:289, 1960. Engstrom, Lars, Kalberg, Petter, Rooth, Gosta. and Tunel1, Ragnar: Acta paediat. scandinav., Suppl., 1966.
lllent, the "oxygen cathode" or polarographic Illethod is the one best suited to continuous recording in vivo. Daneel, the discoverer (\898) of the oxygen cathode, showed that a small DC voltage applied to platinum dectrodes in a medium containing oxygen resulted in electrolysis of the dissolved oxygen, the current being approximately proportional to the oxygen tension (Fig. 1). Davies and Brink ( 1942), using platinum electrodes, applied this principle to measurement of oxygen tension in animal tissues. Since then the use of the oxygen cathode has increased. There are three fundamental types of oxygen electrode: 1. The "flush" -type electrode has the
F ETA L H Y I' 0 X I A is responsible for a large proportion of stillbirths and neonatal deaths and also accounts for brain damage in a significant number of babies who survive. Collection of fetal scalp bloocP enables measurement of fetal blood pO., but the information obtained is only relevant to the time of collection. However, a lIIeasurement of scalp tissue pO. might enablt' continuous monitoring of fetal oxygenation. Of all known methods of p02 measureFrom the Department of Obstetrics alld Gynaecology. MOllash University Medical School. Queen Victoria Memorial Hospital. *Titron Inltruments. Melbourne. Victoria, Australia.
63
64
Walker et 01.
V
Am,
VOLT
I
10< p02
ACTIVI
ILierRODI
. . . . UNCI
IUCtRODI
Fig. 1. Diagrammatic representation of the prin-
ciple of pO, measurement. When a potential difference V is maintain .. d betwt'en two electrodes as shown oxygen is electrolytically reduced at the active eh'ctrodc surface giving rise to a direct current I. Commonly used electrode materials are gold or platinum for the active electrode and silver for the referent'c cl("ctrode, The reaction i. maintainl."d by diffusion of oxygl."n to thl." active rlectrode, the rdt'n"uce electrode providing (,Icctrical continuity, When V = 0,6 volt the lII('asurem('nt is essentially sp!'cific for oXyl(!'n in a biologic lIIedium, the current I then bring directly proportional to tht' oXY,gen tension,
cathode or active area flush with the insulating material which normally provides mechanical support. It effectively measures p02 direC'tly in the medium in combination with a physically separate reference electrode, 2. The "recessed" -type electrode has the active area recessed in the insulating material. Used in combination with a separate reference electrode it effectively measures pOz indirectly in the volume of the recess. 3. The "Clark" or "combined" -type electrode has both the active and reference electrodes combined in the one physical structure and effectively measures p02 indirectly in a microdiffusion chamber separated from the medium by a thin nonconducting but oxygen-permeable membrane. The choice of a particular type of electrode involves consideration of its absolute
J,
January 1, 1968 Dbs!, & Gyne<:.
accuracy. sp""d of response, long-term stahility and susceptibility to "poisoning," size, and tIll' pmsihility of inaccuracy due to tissue damal!t'. till' validity of in vitro calibratioll, and the technical difficlllty involved in its ma 1111 f a I'! II f(', Flush-t~v'" electrodes ha\'c been most popiliaI' in tissue p02 measurements. They are easily mad,· and maintained and have the fast response tll'('l'ssary for following rapid changes, Th"y have been criticized because they an' unstable, susceptible to poisoning, and quantitatin'ly inaccuratc due to depeIlllence Oil diffusion conditions and to the degree of tissue damage caused by insertion. IlIIpro\'"d stability and resistance to poisoning lIlay be achieH"(1 by using gold as the cathode lIlatl'rial,2 a fixed DC biasing potential, grinding a fresh electrodc surface before each experiment,' and coating the surface with oxyg"11 permeable materials to prevent both contamination and erroneously high current \'ahu"s due to movement of fluid past the surface! Extrapolation from in vitro calibration to in vivo lIIeaSUrellH'nt has been questioned. Charlton (personal comllHlIlication) has stated that no quantitative Illl"asurement of oxygen concentration is possible without knowledge of diffusion conditions and fluid InO\'l:"lIIel1t in the region of mcasurement. However, Cater and Silver," after experiments on "model" tissues of compressed cotton wool and artificial extraceJluiar fluid and on perfused tissul", were satisfied that this extrapolation was satisfactory, especially with electrodes less than 100 p. in diameter. Jamieson and van den Brenk 2 have claimed that damage produced by insertion of e1ectrodl's into soft tissues causes greater error than that of calibration, thus obviating any advantages of more complicated electrodes. "Recessed" -type electrodes have advantages in that measurement is virtuaJly unaffected by fluid movement and diffusion conditions because oxygen is drawn from within the recess. However, their use in continuous in vivo recording is precluded by their very slow respon!lt' to a rapid change in oxygen
VolulIl< 100
Numhcr 1
tension and by the necessity of reading thc electrolysis currcnt at a precise tillle after application of biasill~ potential. Clark-typc electrodes have the advantages of recessed electrodes, being virtually unaffected by diffusion conditions and fluid movemcnt but without the disadvantages of slow responsc tilllC and illlenllittent current reading requirements. The protection afforded by thc membrane makes thelll suitable for a medium such as blood where poisoning may occur. Electrodes of this type havc been developed with excellent accuracy, stability, reproducibility, and rt'sponse tillles,5, a but the technical difficulty of achieving these characteristics together with a high dt'gree of mechanical strength is great. It was decided to develop an electrode system for continuous recording of oxygen tension in the fetal scalp embracing as JlIany important characteristics as possiblc. The flush-type electrode protected by a suitable coating material was chosen. It offered a reasonable compromise bl'twl'cn the necessities of accuracy, stability, speed of rl'sponse, and easc of mechanical adaptation to a difficult experimental environment.
Methods Construction of electrodes. * Because needle electrodes were difficult to kcep in thc scalp they were mounted in a modified wound clip (Fig. 2). This has the following advantages: 1. Both the referenct~ and active elcctrodes are contained in a single unit. 2. It has the mechanical strength necessary to withstand thl' stn's5 of insertion. 3. Insertion of the c1l'ctrode is not difficult with specially made forceps. 4. Once inserted, the electrode remains firmly in position. The existing points of a 14 mm. wound clip are cut off. Holes are drilled through the clip maintaining the angle of the original point. A short-bevel size 18 needle bent to shape is soldered into each side of the clip. ·Availahle (rom Tieron InltruPlf'nt., Gf'Ol'ge St. Sand .. riDlLham. Melbourne, Victoria, AUltraiia .
Oxygen electrode for tissue pOz measurement
65
Fig. 2. The pO, electrode. Both active and reference electrodes mounted on a 14 mm. wound clip. (Propper Manufacturing Company.)
A 2 inch length of 60 p. gold wire (active electrode) is soldered to a length of 0.03 inch enameled copper wire, taking care that the diameter of the copper wire is not greatly increased by the solder joint. The gold wire and soldered joint are coated four times with Araldite 985 E (3 parts resin, 1 hardener, 1 solvent), curing for 5 minutes at 180° C. after each application. This coating is necessary to ensure good insulation. A 60 p. silver wire is treated in the same way. The two prepared wires are threaded down separate nylon tubes (external diameter, 0.63 mm.). The needles in the clip are filled with Araldite F (CY 205). (Equal parts of resin and hardener to 1/100 part accelera tor.) Before this hardens the nylon tubing containing the gold wire is pushed through one needle until it protrudes about one inch f rom the point. A second nylon tube containing a silver wire is pushed through the second needle in the same way. Araldite CY 205 is injected into the tubing through a size 25 needle until it passes the gold-copper wire joint. Small glass stoppers are placed in the two protruding tubes to prevent leakage during curing. The Araldite is cured after 5~ hours at 90° C. Protective tubes of polythene are threaded over the nylon tubes and slipped over the sections of the needles protruding from
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the back of the clip. The two lengths of nylon tubing protruding from the points of the needles are cut flllsh with the be\"(' I using a vcry sharp scalpel blade. This exposes tht' active area of the gold and sih'er wires. The gold electrode is ground lightly "ith a diamond disk and cleaned in ether bdore coating with a layer of dichiorodilllethyl hydrosilicatc which protects the elect rod I' from poisoning and reduCt,s artifact without significantly lowering the response til1le. The silver electrode is wated with silver chloride electrolytically using a carbon cathode and 0.1 N HCl solutioll. The electrode is aged hy leaving it ill normal saline for some hours with a bias potential applied. Behavior of the electrode. When the I'iI'Ctrode is moved in a hOlllogeneolls 1l1l'diuIll a sharp hut tplIlporary increase in current results. Oxygen is consullll'd hy the elt'ctrode so that the oxygen cOllcentratioll is
Fig. 3. Subcutaneous capillary vessels in the scalp of a stillborn fetug. Mean intercapillary distance, 76 JJ (S.D. ± 24.6). (x250.)
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January I, 1968 Dbsl. & Gynec.
lower close to the tip than in the surrounding tissue. The steepness of this oxygen con('t'ntratioll gradiellt determines the current magnitude.; If the electrode is moved, its tip is shifted closl'I' to the fl'gion of high concentratioll, the gradient is increased, and the ClllTI'nt rises. Steady state conditions are reI'stahlishl'd within 5 seconds as oxygen consUlllption by the electrode regains equilibriulIl with the surrounding medium. \\,llI'n the electrode is Illoved in tissue a ~harp but temporary increase in current results. This finding in an inhomogeneous oxygen ('11\'ironllll'nt delllands further explanation as one might expect movement in relation to capillaries would have a random chance of producing a rise or fall in current. In order to dl'terllline the conditions in which the l'll'ctrode works, the anatomy of the capillaries in the suhcutaneous tissue of the hUlIlan fetal scalp was examined. Sections of scalp f rOIl1 a stillborn fetus were examined microscopically. Capillaries could be distinguished rl'adily in the subcutaneous laycr (Fig. 3 I. They were generally oriented wrtically at right angles to the skin surface, but adjacent capillaries were linked by nulIlerous branches arrangl,d parallel with the surfan'. Tell areas 111 the skin wcre selected at randolll and the in tercapillary distance betwcen vessels oriented vertically was measurt'd using an eyepiece grid. The mean intercapillary distann' was 76 fL (S.D. ± 24.6). An electrode receives 95 per cent of its oxygen from within six t·lt·ctrode diameters. s Thus the 60 It electrode used in this study will "see" approximately 360 It into the tissue and should receive oxygcn from a volume containing at least twenty capillaries mostly oriented parallel to the plane of its active surface. Thus the electrode will monitor mean tissue pO" regardless of the variations of pO" between capillaril's. As far as Illeasurement of p02 is concerned, the scalp anatomy and the nature of the present electrode enable it to behave as though it were in a homogeneous environment. Thus the explanation for the rise of current when the electrode is moved in tis-
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sue is the sallie as for movelllent in a homogeneous mediullI . Movl'llIent of the electrode should result in only an erroneous rise in current, not a fall. For this n'aSOll, any decrease of pO" during a period of patient movement was considered valid, but a rapid rise was not. Long-term electrode stability in vitro is reasonable. Current readings in air-equilibrated saline show It'SS than 10 per cellt variation over 8 hours. However, romparison of calibration currents before and after an in vivo experilllent can show changes of up to 50 per cent. The electrodes are also susceptible to "stirring" artifact. Extremely vigorous stirring of the calibration llIedium increases electrode current by 50 per cent and sharp movement of the electrode in an unstirred medium results in a characteristic spike of current of the sallie dt'gree. Response to a sudden change in oxygen concentration reaches 90 per cent of the final level within 5 seconds. Experimental procedure. The p02 electrode is calibrated in normal saline equilibrated with known gas mixtures and sterilized in 1/2,000 aqueous Hibitane. The wound clip housing the oxygen electrode is attached to the fetal scalp by means of specially made forceps. This can be done only when the membranes are ruptured and the cervix is 3 em. dilated. Care in handling the electrode is required as it may break under mechanical stress. The fetal pO. is usually recorded with
Oxygen electrode for tissue p02 measurement
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fetal heart rate' and intrauterine pressure. The current reading from the p02 electrode llIay take up to one hour to stabilize. Apart from the continllolls n~cord of p02 change on the ()fl'ncr recorder, actual current readings are also taken every 15 minutes and 1lI0VeIllents of the patient noted on the chart. The oxygen electrode is disconnected fmm the recorder at the time of delivery and reconnected immediately after. The J1t'onate is breathed with 100 per cent oxygen by mask and if the scalp p02 increases, the electrode is then known to be still functioning. If excessive caput is present at the site of the electrode, it is reattached to a normal area of scalp to compare current readings. Whenever possible, the electrode is recalibrated after the experiment. The validity of the experiment is best judged by the similarity between pre- and postexperimental calibration curves and response to neonatal inspiration of 100 per cent oxygen. Thirty-nine experiments have been conducted of which 19 have provided information . These have demonstrated a relationship between fetal scalp tissue p02, on one hand, and on the other, normal contractions, normal and abnormal fetal heart rate, general anesthesia , and maternal and neonatal oxygen inspiration. Results
Maternal inhalation of 50 per cent oxygen. Patient D. P. had a nomlal labor and de-
Fig. 4. Fetal scalp pO, recorded on a patient during normal labor.
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.... m.
Fig. 5. Rise of f('tal scalp pO, during a pt'riod of maternal inspiration of 50 per c.. nt oxygen. F.. tal scalp capillary blood pO, increased from 14 to 26 mm.Hg.
Fig. 6. Scalp pO, increase after delivery. A further increase in pO, occurred wht'n the nf'onat .. breathed 100 per cent oxygen.
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January 1, 1968 Ohsl. & Gynec.
livery resulting in the birth of a healthy bahy (Apgar i." l'terint' contractions and fptal heart rate were normal. A representaative pt'riod of the fetal scalp p02 shows slight changl's with normal contractions (Fig. 4 I. These slight rises cannot yet be in tl'rprl'ted. The patient breathed a 50:50 nitrous oxide-oxygen mixture for 5 minutes during labor. The fetal s('alp p02 rose within one minute of breathing oxygen (Fig. 5) and a maximulll len'l was reached in approximately 4 minutes. This level was more than twice the resting len·1. When oxygen breathing ceased, scalp pO~ returned to the resting level within one minute. The recording which followed was similar to that prior to oxygen inhalation. Fetal blood collections were taken just before and during the period of oxygen breathing, the fetal blood p02 rising from a resting lewl of 14 mm. Hg to 26 mm. Hg. Change from fetal to neonatal existence. Patient R. P. had a normal labor and delin'ry, the Apgar scon' of the baby at 2 minutes being i. Fetal scalp p02 showed slight increases during contractions which were also accornpani,·d hy a small decrease in heart rate (Fig. 6). After delivery the neonatal scalp pO~ was three times the fetal scalp lew I. After a neonatal rt'sting level had bt'en established, the neonate breathed 100 per cent oxygen by mask, and an immediatt' increas(' in the neonatal scalp p 0 2
P.l Ampo.IO" goA
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Fig. 7. Readings of scalp pO, recorded bl'fore and after delivery. Scalp pO, incrf'ased as the neonatal condition improved.
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followed. The neonatal scalp p02 returned to basal level when the mask was rellloved. The experiment was repeated with similar results. Patient P. Z. gave birth to a baby with an Apgar score of 2 following a normal labor and delivery. For 3 minutes after birth, the scalp tissue p02 remained at a similar level to that before birth (Fig. 7). When artificial ventilation of the lungs was carried out with 100 per cent oxygen, the scalp tissue p02 began to rise immediately and had increased fivefold after 3!/:1 minutes of oxygen administration. At this time, the baby's condition had improved, its color, tone, and pulse rate were normal, and spontaneous respiration had comlllenced. After removal of the endotracheal tube, the scalp tissue pO, remained high. Slowing of fetal heart rate. In Patient D. B. marked slowing of the heart rate occurred with contractions (Fig. 8). Delivery was norIllal, the Apgar score at 2 minutes being 6. The bradycardia was associated with a fall in fetal scalp pOz, the degree of change being proportional to the strength of the contraction. Both the bradycardia and decrease in p02 commenced with the onset of the contraction but the pOz was slower than the heart rate in returning to the normal level. Patient E. P. was delivered of a normal baby at 35 weeks with an Apgar score of 7. The pregnancy was complicated by preeclampsia and because of this labor was induced. Fetal heart rate slowed with uterine contractions and coincident with these changes scalp tissue pOz decreased (Fig. 9). General anesthesia. Patient M. K. was delivered by cesarean section after failure of progress in labor. The Apgar score of the baby at 2 minutes was 6. The fetal recording showed mild slowing of heart rate and slight rises in scalp p02 with contractions (Fig. 10). When general anesthesia began, two effects were noted. First, the slight rises in pOz with contractions were abolished. Second, a gradual fall in pOz commenced and continued until delivery. This fall was accompanied by fetal heart rate slowing. A
Oxygen electrode for tissue pOl measurement
69
Fig. 8. Slowing of the fetal heart rate and fall in f('(al scalp pO, occurring with contractions.
Fig. 9. Slowing of the fetal heart rate and fall ill scalp pO, occurring with contractions.
subsequent neonatal experiment showed a characteristic response to 100 per cent oxygen. Comment
Due to unknown diffusion conditions in the fetal scalp tissue, the electrode susceptibility to stirring artifact, and the variability of calibration currents, conclusions as to the absolute value of tissue p02 have not been drawn. Results have been quoted in terms of measured electrode current, but conclusions have only been made on a qualitative basis. However, in 5 cases where comparison of electrode measurements and fetal blood pOz measurements was possible, electrode measurements indicated scalp tissue p02 values of 7, 3, 9, 9, and 17 mm. Hg
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loUt
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Fig. 10. Decrease in f(' tal s calp pO, and f("(al hear! rat(' durin\( cesarean s!'ction under general anesthesia.
compared with corresponding scalp capillary blood pO ~ levels of 26, 12, 25 , 13, and 23 mm. Hg. The rise of fetal scalp tissue pO~ with maternal inhalation of 50 per cent oxygen ( Patient D. P.) indicated that oxygen transport to fetal tissue may be increased in normal patients. Although only olle successful experiment has been completed measuring scalp tissue p02 during OXY,gen administration, the association of the raised tissue pO~ with an 85 per cent increase in fetal capillary blood pO~ adds credence to the result . Furthermore, in a series of 30 experilllents where the mother inhaled high oxygen gas mixtures, the fetal capillary blood p02 usually rose by a similar degree to the present experiment (unpublished data). The rise in fetal scalp tissue pO, following normal delivery (Patient R. P.) may reflect improved scalp circulation resulting from increase in fetal oxygenation coincident with the onset of respiration. Engstrom and associates fi have shown that iliac artery p02, which probably represents the blood pO~ supplying fetal tissue, increases within the first five minutes after birth and then slowly increases further over one hour. In the presence of caput succedaneum tissue pOz may be lowered by the associated circulatory stasis or by impaired diffusion of oxygen from capillary to tissue. However, in the experiment described the prompt increase on two separate occasions of scalp tissue p 0 2 when the baby was breathed with 100
per C(, lIt oxygl'n suggests that local circulatory stasis was not presellt and that the mark ed incrt'.lse of tissue p02 which ocCUlTed after the OIlSt:'t of respiration was refI('ctill~ illlpwn'd oxy~enation. In Patient P. Z. the sralp tissue p02 changes mirrored the changes in the clinical condition of the lIeonatt', no change in tissue p02 occurring in the first few minutes after birth when the haby remained apllcic and flaccid but a rapid increase ()('clIITcd as the baby's condition \\'as impro\'l'd by intubation and 100 per Cl'nt oxygen . COlltinuous lIIeasurement of tissue p02 in Patient D. B. showed a pattern of fall of pOl similar to the pattern of bradycardia o('currinl\' dUI;ng contractions. A similar experinwnt was carril'd out in Patient E. P., when a decrease of scalp tissue p02 was al\'ain coincident with slowing of the heart rate during a contraction. Th('se two studies pro\'itlc evidence that marked decrease of heart ratl' during contractions is sometimes associatt'd with interruption of gaseous interchange hNwt:'en tht:' lIIother and fetus. In tht:' presence of a normal fetal heart rate during contractions, scalp p02 docs not have any dl'finite changt:'o WI' wish to thank Mrs. Dana Jamieson, Dr. D. R. Cat!'r. and Dr. G. Charlton for their help and ad\'irl' on oxyg .. n III l'aSII t'eml'nt , and Dr. W. Newman. Miss Loris McKinnon, Miss Dianne Braid, Mr, David Walk('r, and Dr. Peter Paterson for .heir hdp in prl'paring the electrodes and carrying out the expf'riments. We wish to thank
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the Amalgamated Dental Company for their technical help. We also wish to thank Profl'ssor C. Schofield who prepared the s('ctions of fetal scalp and pro-
Oxygen electrode for tissue p02 measurement
vidcd us with m!'asurements of intercapillary distance. Financial assistallc(' from the Felton Bequest is gratefully acknowledged.
REFERENCES
1. Saling, E. H .: Micro Blood Analysis. A Method for Examination and Supervision of the Fetal Condition During Labor, Address given at Yale University School of Medicinc, 1964. 2. Jamieson, Dana, and van dcn Brenk, H. A. S.: J. App\. Physio\. 20: 514, 1965. 3. Cater, D. B.. and Silver, 1. A.: Microelcctrodt's and Electrodes Used in Biology, in Ives, D. J. G., and Janz, G. j. , editors: Reference Electrodes, New York, 1961, Academic Press, Inc . 4. Cater, D . B.: The Measurement of pO, in
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5. 6. 7.
8. 9.
Tissues, ill Abstracts of Symposium on Oxygen in the Animal Organism, London, 1963, Bedford College. Charlton, G., Read, David, and Read, John: J. App\. Physio\. 18: 1247,1963. Fait, 1: J. App\. Physiol. 19: 326, 1964 Davies, Philip W.: The Oxygen Cathode, ill Nastuk, William L., editor: Physical Techniques in Biological Research, New York, 1962, Academic Press, Inc., vol. IV. Hon, E. H .: Connecticut Med. 24:289, 1960. Engstrom, Lars, Kalberg, Petter, Rooth, Gosta. and Tunel1, Ragnar: Acta paediat. scandinav., Suppl., 1966.