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Maternal, fetal, and newborn tissue PO2 in sheep measured with galvanic oxygen electrodes
BIOCHEMlCAL
MEDICINE
33.
l80-
188
(1985)
Maternal, Fetal, and Newborn Tissue PO, in Sheep Measured with Galvanic Oxygen Electrodes’ M. E.
TOWELL,
J.
JOHNSON,
1. LYSAK,
Received
II.
August
F‘. SAYLER.
1.
AND
S. P. BESSMA~
19X3
The galvanic oxygen sensor, developed by Bessman and Schultz (1) which forms the basis of their glucose sensor for the artificial pancreas (2). has proved to be a useful tool to examine oxygenation in animal tissues (3). We have nou had an opportunity to study maternal and fetal tissue PO, in pregnant ewes under a variety of conditions including labor and delivery. This report deals with the performance of galvanic PO2 electrodes implanted in maternal and fetal tissue5 in two pregnant ewes in late gestation. The electrodes continued to function throughout the course of labor and delivery and in the newborn period afterdelivery. METHODS Galvanic oxygen electrodes. The galvanic oxygen electrode consists of a hilvet cathode and lead anode housed in an acrylic disc 1 cm wide and 3 mm deep covered with a polypropylene membrane. The electrodes were constructed as previously described (I) and were calibrated prior to implantation. Initially. we used a two-point calibration, i.e.. in nitrogen-equilibrated saline (PO2 = 0 mm Hg) and in saline equilibrated with air (PO-, = 150 mm Hg). Our present technique is to calibrate the electrode over the range of PO, values found in living tissues using a three-point calibration in saline equilibrated with 1. 2.5, and 5%~oxygen. All calibrations are performed in a water-jacketed tonometer maintained at a temperature of 39°C. i.e.. a temperature close to that of maternal and fetal tissues in the pregnant ewe. Electrode output was measured with specially constructed microammeters in our initial experiments. We now find it more convenient to measure the output by connecting the terminals of the electrode to the input of a continuous recording
’ Supported by grants David Berk Foundation.
from B.C. Heart Foundation and Ontario Heart l-oundatton. and Glycolysis Fund of the University of Southern California. I80
0006-2944185 Copyright .411 nghtr
$3.00 Q 1985 by Acadrrmc Pre\\. of reproduction m any firm
Inc. rcwrred
Minnie
,mtl
OXYGEN
PRESSURE IN NEWBORN
SHEEP
181
polygraph (Beckman R612) with a 220 kQ resistor in parallel in the circuit. With appropriate amplification, the output of the cell can be measured in millivolts and displayed on the polygraph. Postcalibration is performed after removal of the electrode from the tissues, using the same technique as for precalibration (Fig. 1). We have used a variety of techniques for sterilization including cold sterilization in benzalkonium chloride I:750 solution and gas sterilization with ethylene oxide. We now prefer to use gamma irradiation with 3 Mrad. This dose can be relied upon to sterilize the contents of the cell and does not alter the calibration slope. Intravascular oxygen electrode. A commercially available oxygen electrode (Searle venous oxygen probe) was used to measure arterial PO1 in the fetus. This electrode consists of a standard Clark-type oxygen electrode in the tip of a double-lumen catheter 1.5 m long. It is connected to an ammeter, the output of which is recorded on a continuous recording polygraph via an interfacing unit which electrically isolates the subject from the recorder. Calibration is performed in viva by obtaining a blood sample through the double-lumen catheter, measuring PO2 in vitro at 39°C with a Radiometer PO, electrode, and adjusting the gain of the ammeter if necessary. Calibration was performed at least once a day in this study. After removal from the fetus, zero output of the electrode was checked with the electrode immersed in nitrogen-equilibrated saline at 39°C. Preparation of animals and implantation of electrodes. The electrodes were implanted in two pregnant ewes, one of unknown gestational age and the other at 130 days gestation (term = 147 days). The surgical techniques used to implant the electrodes and catheters were carried out with full aseptic precautions and have been previously described (4S). Ewe 1. Silastic catheters were introduced into the fetal carotid artery and jugular vein, and the maternal femoral artery. Galvanic oxygen electrodes were implanted on the fetal sternomastoid muscle, and maternal omentum (peritoneal cavity), and muscle (groin). Samples of maternal and fetal blood were obtained EIectrode#1391
5’ t’o
,‘5- io
is
jo PO?
j, mmtig
FIG. 1. Pre- and postcalibration slopes for galvanic electrode implanted on fetal dura mater in ewe 2. Postcalibration 18 days after precalibration.
182
TOWELL
ET AL..
intermittently and PO2 was measured in vitro using a Radiometer PO, electrode at 39°C. Ewe 2. An intravascular oxygen electrode (Searle venous 0: probe) was in. traduced into the carotid artery of the fetus. The catheter portion of the doublelumen catheter was thereafter continuously infused with a solution of heparin 20 units/ml at a rate of 1.8 ml/hr until the time of delivery. A galvanic oxygen electrode was implanted on the surface of dura mater overlying the cerebral cortex. Fetal electrocorticogram (FECOGI leads of Teflon-coated braided stainlesssteel wire (Cooner AS 632) were implanted over parietal dura on either side ot the midline. Similar leads were sutured into the myometrium of the uterine wall to record electromyographic (EMG) activity. An open-ended catheter was fixed to the fetal chest wall to record amniotic fluid pressure. Delitlery c?f‘lamhs. Both ewes went into spontaneous labor. l:we I delrveicti spontaneously on the third postoperative day (gestation unknown) while ewe :I was delivered by Caesarean section after 8 hr of labor on the I I th postoperative day (141 days gestation). Catheters were tied off immediately before birth and electrode wires cut close to the point of emergence on the mother’s flank. They were carried with the fetus as it delivered and connections to recording equipment subsequently reestablished to monitor the newborn lamb. RESULTS Stubility of’ gul~wzic oxygen rlrctrotlrs. Calibration values fbr the galvanic oxygen electrodes implanted in ewe I showed little change over the h-day period. pre- and postcalibration values being very similar (Table I). Four-point calibration lines for the galvanic oxygen electrode implanted in NC 2 are shown in Fig. I. There was a linear response over the PO, range from 0 to 35 mm Hg. The slope of the pre- and postcalibration lines was almost identical. However, the intercept changed and postcalibration output of the cell was lower for the same PO? values resulting in downward displacement of the line. These results are typical for galvanic electrodes which appear to be functioning
Results .~
TABLE 1 of Pre- and Postcalibration of Galvanic 0, Electrodes Hg) and in Saline Equilibrated with Air (PO. : Oxygen
At PO, Electrode
site
Precalib. 0 (I 0
Maternal muscle Maternal peritoneum Fetal muscle ” Postcalibration
z= 0 mm Hg
is 6 days
after
precalibration.
Po5tcalib. 0 0 I .4
in O,-FI-er Solution (fJ07 IS0 mm Hg) at YC,
ammeter I’,; scakl
readings
ti ,nm
OXYGEN
PRESSURE IN NEWBORN
183
SHEEP
Tissue reaction to implantation of oxygen electrodes. The tissue from which
the electrodes were removed appeared healthy with little or no tissue reaction to the electrode. After removal of the dural electrode (ewe 2) the blood vessels of the pia mater and cerebral cortex could clearly be seen through the thin dura. This electrode had been implanted a total of 16 days. Ewe 1. Recordings of maternal and fetal tissue POz were made daily from this ewe until delivery which occurred spontaneously on the third day after implantation of electrodes. Recordings were also obtained from the newborn lamb after delivery. The lamb which weighed 2.08 kg appeared healthy at birth but did not survive more than 48 hr. Basal values for arterial PO, and tissue PO? (mean & SD) are shown in Table 2. Maternal tissue PO, was higher in peritoneum (omentum) than muscle. Fetal tissue POz declined from a mean of 11.5 mm Hg on the first day to 2.6 mm Hg on the third day after implantation, when the ewe went into labor. It rose to 25.5 mm Hg in the newborn lamb 3 hr after delivery, i.e., to values comparable to those found in adult tissues. Response of the tissue was challenged daily by administration of 100% oxygen to the ewe for periods of 15 or 30 min. Results of experiments on the second and third days after implantation of electrodes are shown in Figs. 2 and 3. There was an increase in tissue PO1 in both mother and fetus on each occasion. A bolus of epinephrine given to the ewe intravenously during hyperoxia (Fig. 3) led to a temporary decrease in maternal tissue PO1 with little or no effect on arterial blood PO, in the mother or blood and tissue PO? in the fetus. The response of the newborn lamb to 100% oxygen breathing is shown in Fig. 4. Note that central venous POz was higher than tissue PO? before birth but lower after birth, and the response to oxygen breathing was augmented after birth. TABLE 2 Basal Values for Blood and Tissue PO? following Implantation of Electrodes in Ewe I” First day
Second day
Third day
Mean 2 SD (mm Hg)
n
Mean + SD (mm Hg)
II
Mean 2 SD (mm Hg)
Mother Arterial blood Muscle Peritoneum
80.5 6.4 k 2.68 42.3 k 2.92
1 94 23
85.0 13.3 t 2.69 27.8 k 5.32
I 25 96
82.5 @O/85) 2 17.7 ir 5.08 52 -Not measured-
Fetus Arterial blood Venous Muscle
15.5 8.0 11.5 ” 1.26
I 1 92
18.0 14.5 5.1 t 1.65
I 1 96
19.5 t 19/20) 16.0 2.6 + 1.10
2 I 52
61.5 22.0 25.5 k 6.39
I 1 32
Newborn Arterial blood Venous Muscle
” n = No. of blood samples or No. of observations of tissue PO? at I-min intervals.
,I
184
TOWELL
0
!
ET AL
..W....“..,..~*i-. --
Perltoneol
llllllllllllllllHllllll
o
0
10
20
30
40
MlWkS FIG. 17. Effect of 100% O2 breathing by ewe 1 on maternal and fetal PO, in blood and tissue on the second postoperative day. Tissue PO2 obtained at I-min intervals from continuous records.
Ewe 2. The intravascular PO2 electrode continued to function until delivery (11 days). The galvanic tissue PO? electrode functioned throughout the period of implantation (16 days) and was removed from the newborn lamb on the fifth day of life. The lamb weighed 3.60 kg and appeared vigorous and healthy until it was euthanized on the fifth day of life.
FIG. 3. Effect of 100% 0, breathing by ewe I on maternal and fetal PO2 in blood and ussue on the third postoperative day when the ewe was in labor. Tissue PO: obtained at I-min intervals from continuous records. Note the effect of epinephrine 200 pg (5 pg/kg) bolus iv to ewe at *
OXYGEN
PRESSURE IN NEWBORN
SHEEP
185
Fro. 4. Effect of 100% 0: breathing by newborn lamb (ewe I) on blood and tissue PO, 3 hr after birth. Tissue PO, obtained at I-mm intervals from continuous records.
Recordings of fetal arterial and tissue PO2 as well as a variety of other variables were recorded during labor in this ewe (Fig. 5). Fetal dural PO* ranged from 1.3 to 2.7 mm Hg during labor. Fetal arterial PO* ranged from 16 to 23 mm Hg and there was a close temporal relationship between fluctuations in arterial and tissue POz despite wide disparity in absolute values. The relationship of these changes in fetal arterial and tissue PO* to a wide variety of other variables such as fetal electrocortical activity and uterine activity is currently under investigation.
30
mina
5. Part of continuous recording from ewe 2 during labor showing fetal arterial POz, fetal dural PO,, fetal electrocorticogram (FECOG), uterine electromyogram (EMG), and amniotic fluid pressure. Note close correspondence between fetal arterial and dural PO,: also relationship between bursts of uterine EMG activity and decline of PO,. Arrows indicate change in position of ewe from standing to sitting J and sitting to standing T FIG.
186
TOWELL
ET AL
Response of dural PO, in the newborn lamb to 100% oxygen administratnm. on the second and fifth days of life is illustrated in Fig. 6. Dural PO, ranged from 3.4 to 9.1 mm Hg on the second day, and increased by 10 mm Hg with oxygen administration. By the fifth day, dural PO: had risen to a basal level ot about 16 mm Hg and oxygen administration led to a further increase of I6 mm Hg. Dural PO1 fell to near zero values when the lamb was euthanized. COMMENTS The design of the galvanic oxygen electrode used in this study has remarned essentially similar to that of the original description (1) with only minor modifications. We have previously reported that over 50% of electrodes implanted in maternal and fetal tissues of the pregnant ewe for periods of up to 73 days will have acceptable recalibration values when removed from the tissues (5). We consider that our present technique of calibrating electrodes with gas mixtures of low oxygen concentration is the most appropriate for electrodes which are to be used for implantation in living tissue. Downward displacement of the calibration slope after implantation (Fig. I) is typical for these electrodes and may represent an aging process. This report contains the first description of tissue PO, recordings obtained from fetal lambs with chronically implanted galvanic oxygen electrodes which have been followed through labor and delivery and into the newborn period. Basal tissue PO2 values found in our fetal lambs were lower than average values reported by other investigators in fetal monkeys (6), fetal lambs (7), and human fetuses (8). Fetal tissue POz during labor was less than 3 mm Hg despite arterial blood values within the normal range of 16 to 23 mm Hg. Furthermore, PO, was similar in both fetuses despite differences in location of the electrode and in the interval since implantation. The rise of PO? after birth to values which have been reported in adult tissues (3) suggests that the low fetal values measured during labor cannot be attributed to technical problems such as electrode failure, tissue trauma. or oxygen conNewborn
2
Lamb
41
42
IS
days
old
min.
40 5
days
-~
~._
Old
r”“‘i 0
\-.,
o”f PE
_ ‘\ ‘\ --
_-
---
0k 43 15min.
FIG. 6.
Dural PO, in newborn lamb delivered from ewe 2. Note effect of 100% Oz breathing for 15 min. Arrows indicate the following events: (1) lamb taken out of cage (note slight decrease in tissue POz): (2) lamb fell asleep (note increase in tissue POz); (3) iamb euthanized (note tissue PO1 fell to near zero level).
OXYGEN
PRESSURE
IN
NEWBORN
SHEEP
187
sumption of the electrode itself. Postimplantation calibration results suggest that the electrodes were responding satisfactorily. Furthermore, evidence of a satisfactory response of both electrode and tissue was provided by demonstrating an increase in fetal tissue PO, during oxygen administration in labor in ewe 1, and a close relationship between fluctuations of fetal arterial POz and fetal tissue (dura) PO2 in ewe 2. The possibility that oxygen consumption of the electrode itself led to falsely low values for tissue PO* must also be considered. Gilroy and Bessman (9) have measured 0, consumption of the galvanic electrode and have calculated that it is less than 2% of that of an equivalent area of tissue I mm thick. They therefore concluded that the electrode itself would not significantly affect the POz of any tissue that it is attempting to measure. Thus, in summary, we believe that the low values for fetal tissue PO? recorded during labor are reliable and this view is reinforced by an increase of tissue PO* after birth to values close to those anticipated in adults. We intend, however, to extend our observations to determine the normal range of fetal tissue PO, during labor and the range compatible with intact survival of the lamb. Administration of 100% oxygen appears to be an appropriate method of testing the response of the tissue and the electrode in vivo. Lack of response may represent failure of the electrode or, alternatively, failure of oxygen to reach the extracellular fluid surrounding the electrode. Lack of response to oxygen in viva despite acceptable recalibration values after removal of the electrode suggests the presence of a problem with oxygen supply to the tissue. Examples of such problems include tissue trauma with accumulation of blood, fibrin, tissue debris, or pus around the electrode, or impairment of tissue blood flow by epinephrine and similar vasoactive agents. We have previously shown in healthy fetuses that tissue PO* increases by 52% and arterial PO* by 40% in response to oxygen administered to the ewe (10). The results of oxygen administration to ewe 1 are consistent with these findings. The large increase in arterial and tissue PO> in the mother compared with the fetus can be explained by the limitation of oxygen transfer imposed by the placenta attributable to the high oxygen consumption of healthy trophoblastic tissue (11). It is also of interest that the increase in tissue PO? when oxygen was administered after birth was very much larger in the lamb which did not survive compared to the lamb which was vigorous and healthy. It is not known at the present time whether this difference in response is of any prognostic significance for survival or whether there is any physiological mechanism for limiting the supply of oxygen to newborn tissue in the first few days of life. It should be noted that tissue PO2 was lower than central venous PO1 in the fetus (Fig. 2) but was higher than venous PO, in the newborn lamb (Fig. 4). This can be explained by the fact that, before birth, central venous blood represents a mixture of oxygenated blood from the placenta and deoxygenated blood from the tissues. However, after birth, it represents the sum of venous blood draining the peripheral tissues and can, therefore, be expected to show a different relationship to tissue PO, than in the fetus. In summary, this report deals with implantation of galvanic oxygen electrodes in maternal and fetal tissues in the pregnant ewe. Fetal tissue PO1 during labor
188
TOWELL
ET AL
was less than 3 mm Hg but rose after birth to levels comparable to those found in adult tissues. Administration of 100% oxygen to the ewe or newborn lamb can be used as a test of the net response by tissue and electrode to a change in oxygenation. SUMMARY
Tissue PO, was measured with galvanic oxygen electrodes chronically implanted in maternal and fetal tissues of two pregnant ewes. Labor commenced after 3 days in one ewe and after 11 days in the other. Tissue PO: was measured in both newborn lambs. Fetal tissue PO2 during labor was less than 3 mm Hg but rose to values similar to those found in adult tissues in the newborn period. Function of the electrode and response of the tissue to change in oxygenation was challenged by administration of 100% oxygen to the pregnant ewe or newborn lamb, and by administration of intravenous epinephrine to the ewe. These tests suggested that all electrodes were functioning satisfactorily in W’VOand that tissue PO2 was responding as anticipated. Recalibration of electrodes after removal from the tissues confirmed that their response in ~irro was similar to that obtained before implantation. ACKNOWLEDGMENTS We gratefully acknowledge the assistance of Dr. J. Gregg. Umversity of British Columbia and 111 L. Belbeck, McMaster University with care of the animals and Mrs. Dianne Johnston for assistance with the manuscript.
REFERENCES 1. Bessman, S. P.. and Schultz, R. D.. Trcrrls. &?IP~. 5)~ .4,.r[/: 0r’grrn.r IY, 361 ( 1973). 2. Layne, E. C., Schultz, R. D.. Slama. G., Sayler, 1). F.. and Bessman. S. P.. IXtrberc.\ 25, 81 (1976). 3.
4. 5. 6. 7.
Towell, M. E., Lysak, I.. Layne. E. C.. and Bessman. S. P.. J. Appi. P’hysid. 41. 245 ( 1~76). Towell. M. E., and Salvador. H. S., Amer. J. Ohster. G,vrrecol. 118, I I?4 ( 1974). Towell, M. E., Regier. I.. Lysak, I.. Sayler. D. F.. and Bessman. S. I’.. “Oxygen 1‘ranspctr.t to Tissue--IV” (H. I. Bicher and D. F. Bruley. Ed>.). pp. 37-48. Plenum. New York. 19x3. Caldeyro-Barcia, R., Pose, S. V., Poseiro, J. J., Mender-Bauer. C.. Escarcena. I.,., and Behrman. R., in “Intrauterine Asphyxia and the Developing Fetal Brain” (I.. Gluck. I3.i. Year Book Med. Pub.. Chicago. 1977. Staisch. K. J.. Nuwayhid, B., Bauer. R. 0.. Welsh. I... and Brinkman. c‘. R.. Ohsrcr. c'/\'~.~.~I/. 47, 587 (1977).
8. Walker, Brit. 9. Gilroy, 10. Towell,
A., Maddern, L., Day. E.. Renou. P.. Talbot. T.. and Wood. (1.. J. Oh.ticr. (;v,rtar ,i/ Commons. 78. I (1971). K., and Bessman. S. P.. J. Med. Eng. 7~d7rrd. 2, 3 I I 1978). M. E., Johnson. J.. Smedstad. K.. Andrew. M.. and Vu. I.-L... J. I)c,r. Plr~,vr~,i. 6, 17~
(1984).
I I. Campbell, A. G. M.. Dawes. G. S.. Fishman. A. P.. Hyman, A. I.. and James, G. 13.../. /~/~~,c~,i 182, 439 (1966).
MEDICINE
33.
l80-
188
(1985)
Maternal, Fetal, and Newborn Tissue PO, in Sheep Measured with Galvanic Oxygen Electrodes’ M. E.
TOWELL,
J.
JOHNSON,
1. LYSAK,
Received
II.
August
F‘. SAYLER.
1.
AND
S. P. BESSMA~
19X3
The galvanic oxygen sensor, developed by Bessman and Schultz (1) which forms the basis of their glucose sensor for the artificial pancreas (2). has proved to be a useful tool to examine oxygenation in animal tissues (3). We have nou had an opportunity to study maternal and fetal tissue PO, in pregnant ewes under a variety of conditions including labor and delivery. This report deals with the performance of galvanic PO2 electrodes implanted in maternal and fetal tissue5 in two pregnant ewes in late gestation. The electrodes continued to function throughout the course of labor and delivery and in the newborn period afterdelivery. METHODS Galvanic oxygen electrodes. The galvanic oxygen electrode consists of a hilvet cathode and lead anode housed in an acrylic disc 1 cm wide and 3 mm deep covered with a polypropylene membrane. The electrodes were constructed as previously described (I) and were calibrated prior to implantation. Initially. we used a two-point calibration, i.e.. in nitrogen-equilibrated saline (PO2 = 0 mm Hg) and in saline equilibrated with air (PO-, = 150 mm Hg). Our present technique is to calibrate the electrode over the range of PO, values found in living tissues using a three-point calibration in saline equilibrated with 1. 2.5, and 5%~oxygen. All calibrations are performed in a water-jacketed tonometer maintained at a temperature of 39°C. i.e.. a temperature close to that of maternal and fetal tissues in the pregnant ewe. Electrode output was measured with specially constructed microammeters in our initial experiments. We now find it more convenient to measure the output by connecting the terminals of the electrode to the input of a continuous recording
’ Supported by grants David Berk Foundation.
from B.C. Heart Foundation and Ontario Heart l-oundatton. and Glycolysis Fund of the University of Southern California. I80
0006-2944185 Copyright .411 nghtr
$3.00 Q 1985 by Acadrrmc Pre\\. of reproduction m any firm
Inc. rcwrred
Minnie
,mtl
OXYGEN
PRESSURE IN NEWBORN
SHEEP
181
polygraph (Beckman R612) with a 220 kQ resistor in parallel in the circuit. With appropriate amplification, the output of the cell can be measured in millivolts and displayed on the polygraph. Postcalibration is performed after removal of the electrode from the tissues, using the same technique as for precalibration (Fig. 1). We have used a variety of techniques for sterilization including cold sterilization in benzalkonium chloride I:750 solution and gas sterilization with ethylene oxide. We now prefer to use gamma irradiation with 3 Mrad. This dose can be relied upon to sterilize the contents of the cell and does not alter the calibration slope. Intravascular oxygen electrode. A commercially available oxygen electrode (Searle venous oxygen probe) was used to measure arterial PO1 in the fetus. This electrode consists of a standard Clark-type oxygen electrode in the tip of a double-lumen catheter 1.5 m long. It is connected to an ammeter, the output of which is recorded on a continuous recording polygraph via an interfacing unit which electrically isolates the subject from the recorder. Calibration is performed in viva by obtaining a blood sample through the double-lumen catheter, measuring PO2 in vitro at 39°C with a Radiometer PO, electrode, and adjusting the gain of the ammeter if necessary. Calibration was performed at least once a day in this study. After removal from the fetus, zero output of the electrode was checked with the electrode immersed in nitrogen-equilibrated saline at 39°C. Preparation of animals and implantation of electrodes. The electrodes were implanted in two pregnant ewes, one of unknown gestational age and the other at 130 days gestation (term = 147 days). The surgical techniques used to implant the electrodes and catheters were carried out with full aseptic precautions and have been previously described (4S). Ewe 1. Silastic catheters were introduced into the fetal carotid artery and jugular vein, and the maternal femoral artery. Galvanic oxygen electrodes were implanted on the fetal sternomastoid muscle, and maternal omentum (peritoneal cavity), and muscle (groin). Samples of maternal and fetal blood were obtained EIectrode#1391
5’ t’o
,‘5- io
is
jo PO?
j, mmtig
FIG. 1. Pre- and postcalibration slopes for galvanic electrode implanted on fetal dura mater in ewe 2. Postcalibration 18 days after precalibration.
182
TOWELL
ET AL..
intermittently and PO2 was measured in vitro using a Radiometer PO, electrode at 39°C. Ewe 2. An intravascular oxygen electrode (Searle venous 0: probe) was in. traduced into the carotid artery of the fetus. The catheter portion of the doublelumen catheter was thereafter continuously infused with a solution of heparin 20 units/ml at a rate of 1.8 ml/hr until the time of delivery. A galvanic oxygen electrode was implanted on the surface of dura mater overlying the cerebral cortex. Fetal electrocorticogram (FECOGI leads of Teflon-coated braided stainlesssteel wire (Cooner AS 632) were implanted over parietal dura on either side ot the midline. Similar leads were sutured into the myometrium of the uterine wall to record electromyographic (EMG) activity. An open-ended catheter was fixed to the fetal chest wall to record amniotic fluid pressure. Delitlery c?f‘lamhs. Both ewes went into spontaneous labor. l:we I delrveicti spontaneously on the third postoperative day (gestation unknown) while ewe :I was delivered by Caesarean section after 8 hr of labor on the I I th postoperative day (141 days gestation). Catheters were tied off immediately before birth and electrode wires cut close to the point of emergence on the mother’s flank. They were carried with the fetus as it delivered and connections to recording equipment subsequently reestablished to monitor the newborn lamb. RESULTS Stubility of’ gul~wzic oxygen rlrctrotlrs. Calibration values fbr the galvanic oxygen electrodes implanted in ewe I showed little change over the h-day period. pre- and postcalibration values being very similar (Table I). Four-point calibration lines for the galvanic oxygen electrode implanted in NC 2 are shown in Fig. I. There was a linear response over the PO, range from 0 to 35 mm Hg. The slope of the pre- and postcalibration lines was almost identical. However, the intercept changed and postcalibration output of the cell was lower for the same PO? values resulting in downward displacement of the line. These results are typical for galvanic electrodes which appear to be functioning
Results .~
TABLE 1 of Pre- and Postcalibration of Galvanic 0, Electrodes Hg) and in Saline Equilibrated with Air (PO. : Oxygen
At PO, Electrode
site
Precalib. 0 (I 0
Maternal muscle Maternal peritoneum Fetal muscle ” Postcalibration
z= 0 mm Hg
is 6 days
after
precalibration.
Po5tcalib. 0 0 I .4
in O,-FI-er Solution (fJ07 IS0 mm Hg) at YC,
ammeter I’,; scakl
readings
ti ,nm
OXYGEN
PRESSURE IN NEWBORN
183
SHEEP
Tissue reaction to implantation of oxygen electrodes. The tissue from which
the electrodes were removed appeared healthy with little or no tissue reaction to the electrode. After removal of the dural electrode (ewe 2) the blood vessels of the pia mater and cerebral cortex could clearly be seen through the thin dura. This electrode had been implanted a total of 16 days. Ewe 1. Recordings of maternal and fetal tissue POz were made daily from this ewe until delivery which occurred spontaneously on the third day after implantation of electrodes. Recordings were also obtained from the newborn lamb after delivery. The lamb which weighed 2.08 kg appeared healthy at birth but did not survive more than 48 hr. Basal values for arterial PO, and tissue PO? (mean & SD) are shown in Table 2. Maternal tissue PO, was higher in peritoneum (omentum) than muscle. Fetal tissue POz declined from a mean of 11.5 mm Hg on the first day to 2.6 mm Hg on the third day after implantation, when the ewe went into labor. It rose to 25.5 mm Hg in the newborn lamb 3 hr after delivery, i.e., to values comparable to those found in adult tissues. Response of the tissue was challenged daily by administration of 100% oxygen to the ewe for periods of 15 or 30 min. Results of experiments on the second and third days after implantation of electrodes are shown in Figs. 2 and 3. There was an increase in tissue PO1 in both mother and fetus on each occasion. A bolus of epinephrine given to the ewe intravenously during hyperoxia (Fig. 3) led to a temporary decrease in maternal tissue PO1 with little or no effect on arterial blood PO, in the mother or blood and tissue PO? in the fetus. The response of the newborn lamb to 100% oxygen breathing is shown in Fig. 4. Note that central venous POz was higher than tissue PO? before birth but lower after birth, and the response to oxygen breathing was augmented after birth. TABLE 2 Basal Values for Blood and Tissue PO? following Implantation of Electrodes in Ewe I” First day
Second day
Third day
Mean 2 SD (mm Hg)
n
Mean + SD (mm Hg)
II
Mean 2 SD (mm Hg)
Mother Arterial blood Muscle Peritoneum
80.5 6.4 k 2.68 42.3 k 2.92
1 94 23
85.0 13.3 t 2.69 27.8 k 5.32
I 25 96
82.5 @O/85) 2 17.7 ir 5.08 52 -Not measured-
Fetus Arterial blood Venous Muscle
15.5 8.0 11.5 ” 1.26
I 1 92
18.0 14.5 5.1 t 1.65
I 1 96
19.5 t 19/20) 16.0 2.6 + 1.10
2 I 52
61.5 22.0 25.5 k 6.39
I 1 32
Newborn Arterial blood Venous Muscle
” n = No. of blood samples or No. of observations of tissue PO? at I-min intervals.
,I
184
TOWELL
0
!
ET AL
..W....“..,..~*i-. --
Perltoneol
llllllllllllllllHllllll
o
0
10
20
30
40
MlWkS FIG. 17. Effect of 100% O2 breathing by ewe 1 on maternal and fetal PO, in blood and tissue on the second postoperative day. Tissue PO2 obtained at I-min intervals from continuous records.
Ewe 2. The intravascular PO2 electrode continued to function until delivery (11 days). The galvanic tissue PO? electrode functioned throughout the period of implantation (16 days) and was removed from the newborn lamb on the fifth day of life. The lamb weighed 3.60 kg and appeared vigorous and healthy until it was euthanized on the fifth day of life.
FIG. 3. Effect of 100% 0, breathing by ewe I on maternal and fetal PO2 in blood and ussue on the third postoperative day when the ewe was in labor. Tissue PO: obtained at I-min intervals from continuous records. Note the effect of epinephrine 200 pg (5 pg/kg) bolus iv to ewe at *
OXYGEN
PRESSURE IN NEWBORN
SHEEP
185
Fro. 4. Effect of 100% 0: breathing by newborn lamb (ewe I) on blood and tissue PO, 3 hr after birth. Tissue PO, obtained at I-mm intervals from continuous records.
Recordings of fetal arterial and tissue PO2 as well as a variety of other variables were recorded during labor in this ewe (Fig. 5). Fetal dural PO* ranged from 1.3 to 2.7 mm Hg during labor. Fetal arterial PO* ranged from 16 to 23 mm Hg and there was a close temporal relationship between fluctuations in arterial and tissue POz despite wide disparity in absolute values. The relationship of these changes in fetal arterial and tissue PO* to a wide variety of other variables such as fetal electrocortical activity and uterine activity is currently under investigation.
30
mina
5. Part of continuous recording from ewe 2 during labor showing fetal arterial POz, fetal dural PO,, fetal electrocorticogram (FECOG), uterine electromyogram (EMG), and amniotic fluid pressure. Note close correspondence between fetal arterial and dural PO,: also relationship between bursts of uterine EMG activity and decline of PO,. Arrows indicate change in position of ewe from standing to sitting J and sitting to standing T FIG.
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ET AL
Response of dural PO, in the newborn lamb to 100% oxygen administratnm. on the second and fifth days of life is illustrated in Fig. 6. Dural PO, ranged from 3.4 to 9.1 mm Hg on the second day, and increased by 10 mm Hg with oxygen administration. By the fifth day, dural PO: had risen to a basal level ot about 16 mm Hg and oxygen administration led to a further increase of I6 mm Hg. Dural PO1 fell to near zero values when the lamb was euthanized. COMMENTS The design of the galvanic oxygen electrode used in this study has remarned essentially similar to that of the original description (1) with only minor modifications. We have previously reported that over 50% of electrodes implanted in maternal and fetal tissues of the pregnant ewe for periods of up to 73 days will have acceptable recalibration values when removed from the tissues (5). We consider that our present technique of calibrating electrodes with gas mixtures of low oxygen concentration is the most appropriate for electrodes which are to be used for implantation in living tissue. Downward displacement of the calibration slope after implantation (Fig. I) is typical for these electrodes and may represent an aging process. This report contains the first description of tissue PO, recordings obtained from fetal lambs with chronically implanted galvanic oxygen electrodes which have been followed through labor and delivery and into the newborn period. Basal tissue PO2 values found in our fetal lambs were lower than average values reported by other investigators in fetal monkeys (6), fetal lambs (7), and human fetuses (8). Fetal tissue POz during labor was less than 3 mm Hg despite arterial blood values within the normal range of 16 to 23 mm Hg. Furthermore, PO, was similar in both fetuses despite differences in location of the electrode and in the interval since implantation. The rise of PO? after birth to values which have been reported in adult tissues (3) suggests that the low fetal values measured during labor cannot be attributed to technical problems such as electrode failure, tissue trauma. or oxygen conNewborn
2
Lamb
41
42
IS
days
old
min.
40 5
days
-~
~._
Old
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\-.,
o”f PE
_ ‘\ ‘\ --
_-
---
0k 43 15min.
FIG. 6.
Dural PO, in newborn lamb delivered from ewe 2. Note effect of 100% Oz breathing for 15 min. Arrows indicate the following events: (1) lamb taken out of cage (note slight decrease in tissue POz): (2) lamb fell asleep (note increase in tissue POz); (3) iamb euthanized (note tissue PO1 fell to near zero level).
OXYGEN
PRESSURE
IN
NEWBORN
SHEEP
187
sumption of the electrode itself. Postimplantation calibration results suggest that the electrodes were responding satisfactorily. Furthermore, evidence of a satisfactory response of both electrode and tissue was provided by demonstrating an increase in fetal tissue PO, during oxygen administration in labor in ewe 1, and a close relationship between fluctuations of fetal arterial POz and fetal tissue (dura) PO2 in ewe 2. The possibility that oxygen consumption of the electrode itself led to falsely low values for tissue PO* must also be considered. Gilroy and Bessman (9) have measured 0, consumption of the galvanic electrode and have calculated that it is less than 2% of that of an equivalent area of tissue I mm thick. They therefore concluded that the electrode itself would not significantly affect the POz of any tissue that it is attempting to measure. Thus, in summary, we believe that the low values for fetal tissue PO? recorded during labor are reliable and this view is reinforced by an increase of tissue PO* after birth to values close to those anticipated in adults. We intend, however, to extend our observations to determine the normal range of fetal tissue PO, during labor and the range compatible with intact survival of the lamb. Administration of 100% oxygen appears to be an appropriate method of testing the response of the tissue and the electrode in vivo. Lack of response may represent failure of the electrode or, alternatively, failure of oxygen to reach the extracellular fluid surrounding the electrode. Lack of response to oxygen in viva despite acceptable recalibration values after removal of the electrode suggests the presence of a problem with oxygen supply to the tissue. Examples of such problems include tissue trauma with accumulation of blood, fibrin, tissue debris, or pus around the electrode, or impairment of tissue blood flow by epinephrine and similar vasoactive agents. We have previously shown in healthy fetuses that tissue PO* increases by 52% and arterial PO* by 40% in response to oxygen administered to the ewe (10). The results of oxygen administration to ewe 1 are consistent with these findings. The large increase in arterial and tissue PO> in the mother compared with the fetus can be explained by the limitation of oxygen transfer imposed by the placenta attributable to the high oxygen consumption of healthy trophoblastic tissue (11). It is also of interest that the increase in tissue PO? when oxygen was administered after birth was very much larger in the lamb which did not survive compared to the lamb which was vigorous and healthy. It is not known at the present time whether this difference in response is of any prognostic significance for survival or whether there is any physiological mechanism for limiting the supply of oxygen to newborn tissue in the first few days of life. It should be noted that tissue PO2 was lower than central venous PO1 in the fetus (Fig. 2) but was higher than venous PO, in the newborn lamb (Fig. 4). This can be explained by the fact that, before birth, central venous blood represents a mixture of oxygenated blood from the placenta and deoxygenated blood from the tissues. However, after birth, it represents the sum of venous blood draining the peripheral tissues and can, therefore, be expected to show a different relationship to tissue PO, than in the fetus. In summary, this report deals with implantation of galvanic oxygen electrodes in maternal and fetal tissues in the pregnant ewe. Fetal tissue PO1 during labor
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TOWELL
ET AL
was less than 3 mm Hg but rose after birth to levels comparable to those found in adult tissues. Administration of 100% oxygen to the ewe or newborn lamb can be used as a test of the net response by tissue and electrode to a change in oxygenation. SUMMARY
Tissue PO, was measured with galvanic oxygen electrodes chronically implanted in maternal and fetal tissues of two pregnant ewes. Labor commenced after 3 days in one ewe and after 11 days in the other. Tissue PO: was measured in both newborn lambs. Fetal tissue PO2 during labor was less than 3 mm Hg but rose to values similar to those found in adult tissues in the newborn period. Function of the electrode and response of the tissue to change in oxygenation was challenged by administration of 100% oxygen to the pregnant ewe or newborn lamb, and by administration of intravenous epinephrine to the ewe. These tests suggested that all electrodes were functioning satisfactorily in W’VOand that tissue PO2 was responding as anticipated. Recalibration of electrodes after removal from the tissues confirmed that their response in ~irro was similar to that obtained before implantation. ACKNOWLEDGMENTS We gratefully acknowledge the assistance of Dr. J. Gregg. Umversity of British Columbia and 111 L. Belbeck, McMaster University with care of the animals and Mrs. Dianne Johnston for assistance with the manuscript.
REFERENCES 1. Bessman, S. P.. and Schultz, R. D.. Trcrrls. &?IP~. 5)~ .4,.r[/: 0r’grrn.r IY, 361 ( 1973). 2. Layne, E. C., Schultz, R. D.. Slama. G., Sayler, 1). F.. and Bessman. S. P.. IXtrberc.\ 25, 81 (1976). 3.
4. 5. 6. 7.
Towell, M. E., Lysak, I.. Layne. E. C.. and Bessman. S. P.. J. Appi. P’hysid. 41. 245 ( 1~76). Towell. M. E., and Salvador. H. S., Amer. J. Ohster. G,vrrecol. 118, I I?4 ( 1974). Towell, M. E., Regier. I.. Lysak, I.. Sayler. D. F.. and Bessman. S. I’.. “Oxygen 1‘ranspctr.t to Tissue--IV” (H. I. Bicher and D. F. Bruley. Ed>.). pp. 37-48. Plenum. New York. 19x3. Caldeyro-Barcia, R., Pose, S. V., Poseiro, J. J., Mender-Bauer. C.. Escarcena. I.,., and Behrman. R., in “Intrauterine Asphyxia and the Developing Fetal Brain” (I.. Gluck. I3.i. Year Book Med. Pub.. Chicago. 1977. Staisch. K. J.. Nuwayhid, B., Bauer. R. 0.. Welsh. I... and Brinkman. c‘. R.. Ohsrcr. c'/\'~.~.~I/. 47, 587 (1977).
8. Walker, Brit. 9. Gilroy, 10. Towell,
A., Maddern, L., Day. E.. Renou. P.. Talbot. T.. and Wood. (1.. J. Oh.ticr. (;v,rtar ,i/ Commons. 78. I (1971). K., and Bessman. S. P.. J. Med. Eng. 7~d7rrd. 2, 3 I I 1978). M. E., Johnson. J.. Smedstad. K.. Andrew. M.. and Vu. I.-L... J. I)c,r. Plr~,vr~,i. 6, 17~
(1984).
I I. Campbell, A. G. M.. Dawes. G. S.. Fishman. A. P.. Hyman, A. I.. and James, G. 13.../. /~/~~,c~,i 182, 439 (1966).