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Comparison of the composition of arterial, venous, and capillary blood of the fetal monkey during labor
Comparison of the composition of arterial, venous, and capillary blood of the fetal monkey during labor KARLIS
ADAMSONS,
RICHARD
W.
RONALD San Juan,
E. Puerto
BEARD, MYERS,
M.D.,
PH.D.
M.D. M.D.,
PH.D.
Rico
The composition of blood obtained simultaneously from the carotid artery, jugular r,ein, and capillary bed of the scalp of the fetal rhesus monkey during labor revealed good correlation between the corresponding values of pH, Pro,, PO, and red cell concentration. The agreement was good not only with the fetus in a normal acid-base .state, but alsq under conditions of severe asphyxia. The mean of the differences between the pH of capillary and venous blood was 0.017 and that between capillary and arterial blood was -0.047.
blood obtained from the fetal scalp, carotid artery, and jugular vein in a species close to man and under conditions mimicking those present during labor.
s I N c E T H E introduction of sampling Of capillary blood from the presenting part of the human fetus by Saling,5 concern has prevailed regarding the representativeness of blood obtained from this source. It is known that the composition of blood obtained from central and peripheral sites is not the same in the immediate postnatal period, even in the vigorous newborn infant in whom collection of capillary blood is less likely to be subject to vasoconstriction, stasis, and edema of the subcutaneous tissues.4 The purpose of this study was to examine the relationship between the composition of
Maferial
and
methods
For the study, 11 pregnant rhesus monkeys from the caged breeding colony of the Laboratory of Perinatal Physiology were used. In all but one case, the gestation was more than 160 days (term is at 168 days). Following premeditation with pentobarbital (10 mg. per kilogram, intramuscularly), the mothers were anesthetized with a mixture of Fluothane and nitrous oxide. The peritoneal cavity was opened through a midline incision and the uterus was exposed. The head of the fetus was then delivered through an elliptical incision in the lower uterine segment, care being taken to avoid placental margins. Through a small incision in the neck, the left carotid artery and the right jugular vein were exposed, and polyvinyl catheters of appropriate dimension were inserted. The tip of the arterial catheter was advanced to the arch of the aorta while the tip of the venous
From the Laboratory of Perinatal Physiology, National Institute of Neurological Diseases and Stroke, National Institutes of Health, United States Public Health Service, Department of Health, Education and Welfare: and the Department of Obstetrtcs and Gynecology, Columbia University, New York. Supported in part by Contract No. PH43-63-25 of the United States Public Health Service administered by the University of Puerto Rico School of Medicine and in part by United States Public Health Service Grants GM09069 and ND 118. 435
436
Adamsons,
Beard,
and
Myers
catheter rested approximately 5 mm. above the right atrium. The scalp of the fetus was shaved in order to facilitate sampling of capillary blood. ‘The amniotic membrane over the internal OS was then ruptured and the cervical canal was gradually dilated to approximately 2 cm. with an obturator. After obtaining blood samples as simultaneously as possible from the carotid artery, jugular vein, and fetal scalp, the fetal head was returned to the uterine cavity and the incisions in the myometrium and in the abdominal wall of the mother were closed. Administration of Fluothane was then discontinued and the animal was maintained under sedation with either pentobarbital or nitrous oxide anesthesia. In most instances, uterine contractions ensued shortly after the discontinuation of Fluothane and led to a progressive dilatation of the cervix. In some cases, labor was stimulated with intravenous infusion of oxytocin in a deliberate attempt to produce fetal acidosis. Capillary blood from the fetal scalp was obtained using the same instruments and techniques as those used in the human subjects,’ except for the elimination of spraying the scalp with ethyl chloride. It was found in the initial experiments that blood flow during the pha.se of the “rebound hyperemia” did not compare favorably with the flow observed prior to cooling. Because of the thinness of the scalp in the fetal rhesus monkey, one to 2 minx:ltes were required to collect a sufficiently large sample from the capillary bed for complete determination of acid-base components. In these cases the specimens obtained from the carotid artery and the jugular vein were also withdrawn at lower rates to cover the corresponding time interval. Blood from all 3 sites was collected in heparinized ;glass capillary tubes. Hydrogen ion activity was determined with a glass electrode at 38’ C. using the radiometer expanded scale pH meter. Carbon dioxide tension was calculated from pH measurements of the sample ‘equilibrated at 2 known CO, tensions using the nomogram by Siggaard-Andersen, Engel, a.nd their colleagues.6 Oxygen tension was determined with a
Amer.
June J. Obstet.
1, 1970 Gynec.
radiometer microelectrode, and the red cell concentration was measured after centrifuging a triplicate sample of about 10 ~1 at 10,000 g for 5 minutes. Results
A total of 37 comparisons were made between capillary blood and that present in the central circulation. In 19 cases triplicate samples were obtained, while in 18 cases the comparison was made only between capillary and either arterial or venous blood. In 27 cases the capillary sample was obtained transvaginally and in 10 cases it was obtained while the head was still exteriorized. The mean value of the initial, pH obtained from the carotid artery at the time of the operation was 7.233, with a range of 7.032 to 7.325. During the course of labor, particularly when it was stimulated by oxytocin, there was a progressive fall in pH in all fetuses. The lowest value for pH observed in the carotid artery was 6.750. It was noted that sampling of capillary blood became increasingly more difficult with the progression of fetal acidosis, presumably due to intense cutaneous vasoconstriction. The relationship between the pH of arterial and capillary blood is given in Fig. 1. Closed circles identify capillary samples obtained transvaginally; open circles refer to samples obtained from the exteriorized fetal head. It is evident that a high degree of correlation exists between the two variables which extends over the entire pH range studied. The correlation coefficient for the combined population is 0.969, whereas that for the samples with fetal head in situ is 0.955. Statistically, there is no demonstrable difference between the pH values of arterial blood and those obtained from the exteriorized scalp. The pH of samples obtained from the fetal scalp transvaginally was, on the other hand, lower than that of the arterial blood, the mean of the differences being -0.047. There was a tendency of the values for capillary pH to depart more from the arterial pH at lower pH values. The correlation between pH of the capillary blood and that of the jugular vein is
Volume h’umber
107 3
Monkey
fetal
blood
during
labor
437
z400 7.30-
0* W.
?.20i g
7.10
0
-
02
.*
0
iA & zi 2
7.00
0
-
l
0 0. 0
6.90-
vf-i
0 00 6.80-
l*
r.=0.955
0
y.= /.029x
- 0.248
0 6.70
I 6.70
I 6.90
I 6.80
ARTERIAL
I 100
I 710
I 720
1 730
1 740
BLOOD
1. pH of blood obtained simultaneously from capillary bed of scalp and carotid artery Iof fetal monkeys during labor. Open circles (0) refer to samples obtained at time of hysterotomy.
IFig.
given in Fig. 2. Open circles refer to samples obtained while the feta1 head was still exteriorized.
Again
a satisfactory
agreement
between the two variables is observed. In the majority
of instances,
the pH value
of capil-
lary blood was higher than that in the jugular vein,
the mean
of the differences
being
0.017.. The correlation coefficient between the two variables is 0.965. The relationship between the corresponding Pcoz values in arterial and capillary blood is given in Fig. 3. The values are scattered rather evenly around the line with a 45 degree slopeoriginating from the intercept of the coordinates. A certain scatter is anticipated due to t.he error of measurement intrinsic to this method of determination Pooz. There is no tendency of the
of Pcoz
values to be relatively lower than arterial values at higher tensions of Pco,, which would have been suggestive of loss of CO, during the sampling of capillary blood. The relationship between Paz of the capillary, arterial, and venous blood is given in Fig. 4. Open circles refer to arterial blood; closeIdcircles denote venous blood. Consider-
able scatter of the values around the 45 degree slope line is evident. In 2 instances the values for capillary blood are higher than those for arterial blood. The mean of the red cell concentration of the arterial blood of the fetus at the beginning of the experiments was 40.1 per cent. There
was a good
agreement
between
the
values from the 3 sampling sites. In most cases the difference was 1 or 2 per cent only, which is not greater than the expected difference due to the limits of reproducibility of this method. No significant differences in red cell concentration were observed during the course of labor in spite of the development of progressiveacidosis.In most instances there was a slight reduction in red cell concentration at the end of the experiment ranging between 1 and 4 per cent. Since the total quantity of blood removed from the fetus during the study amounted to between 5 and 10 per cent of the total blood volume, the change in red ceII concentration represents a composite picture of hemodilution due to blood loss opposed by hemoconcentration in responseto asphyxia.
438
Adamsons,
Beard,
and
Myers
Amer.
June J. Obstet.
7 40-
2300+ z20g
210-
Li > 5.
zoo-
% * 00 0 *.
=! 6.90-
0
0
:
2i u
r*= 0.973
l 6.80
-
6.70
-
y,= 0992x
I 6.70
1 6.80
1 6.90
I 100
VENOUS Fig. 2. pH of blood obtained of fetal monkeys during labor.
simultaneously from (Symbols as in Fig.
ARTERIAL
I 710
I 720
+ 0 073
I 230
I 240
BLOOD capillary 1.)
BLOOD
bed
of scalp
and
(mm
jugular
vein
Hg)
Fig. 3. PCO, of blood obtained simultaneously from capillary bed of scalp and carotid of fetal monkeys during labor. (Symbols as in Fig. 1.) The lines enclose the region represents the error of measurement (25 per cent).
artery which
1, 1970 Gym.
Volume Number
107 3
Monkey
ARTERIAL
AND VENOUS
BLOOD
fetal
blood
(mm
during
labor
439
Hg)
Fig. 4. PC,, of blood obtained simultaneously from capillary bed of scalp, carotid artery (O), and jugular vein ( l ) of fetal monkeys during labor. The lines enclose the region which represents the error of measurement (‘2 per cent).
Comment This study has demonstrated that the composition of capillary blood of the fetus obtained from the scalp during the course of labor correlates well with that of blood entering and leaving the central nervous system. Before the onset of labor and with the fetal scalp not subject to compression by the maternal birth canal, the composition of scalp blood is virtually identical with that of the carotild artery. This pertained to essentially all constituents studied. During labor the composition of capillary blood resembled more closely that of the jugular venous blood. Except for a few occasions the difference was so small as to be of no significance regarding the assessment of the condition of the fetus from the clinician’s point of view. It appears that the differences are due to changes in scalp perfusion resulting from vasoconstriction or compression by the amnioscope rather than to changes in blood com-
position resulting from the exposure of capillary blood to air. It has been estimated? that exposure of fetal blood in the form of a drop to air for 5 seconds leads to a rise of pH by not more than 0.01, with a concomitant fall in Pooz tension even at high absolute Pco, values of not more than 2.5 mm. Hg. Another reason why the composition of capillary blood obtained from the fetus during labor may differ slightly from that present in the central circulation is that the collection of blood from the capillary bed is favored during the contraction period. It is known” that pH of blood obtained immediately at the end of contraction may be as much as 0.05 lower than that observed at the peak of contraction. Of particular clinical importance was the observation that capillary blood remained representative even under conditions of a degree of asphyxia seldom encountered in clinical practice. The fact that pH of capil-
440
Adamsons,
Beard,
and
Myers
lary blood more closely resembled that of the jugular venous blood rather than that obtained from the carotid artery is at least theoretically an advantage because the composition. of venous blood is determined not only by that of the arterial but also by the relationship between flow rate and metabolic needs of a given organ. Thus venous blood is much more informative than the arterial one regarding the operational conditions of the tissues. The pH of arterial blood was consistently higher than that of the venous. The difference ranged from 0.012 to 0.058, with a mean of 0.038. This difference is smaller than that re:ported for a.rterial and venous blood obtained from the umbilical cord at the time of delivery.’ In general, the difference between artery and vein was larger when the arterial pH was less than 7.10. Since a given difference of pH values denotes larger quantity of hydrogen ion at lower absolute values, this observation suggests that during acidosis of this degree flow rate is reduced; consequently, the volume of blood into which CO, or hydrogen ion, generated from lactic acid, had been distributed was less than that present under normal conditions or in states of moderate acidosis. Considering the technical difficulties, in obtaining capillary blood samples in the monkey fetus,, it was unexpected to iind that the correlation between pH of capillary and arterial blood was at least as satisfactory as that previously reported for the newborn human in whom warmed heel was used as a sampling site for capillary blood.4 The correlation between the corresponding Pcoz values was also satisfactory, particularly when one considers the limited ac-
.4mer.
June J. Obstet.
1, 1970 Gynec.
curacy of the indirect method for Pco? determination. The limited correlation between Po? values in capillary, arterial, and venous blood is most likely due to methodologic difficulties. Although it may seem plausible that blood at low Paz is likely to be altered in composition upon exposure to air, this has not been found to be the case under conditions similar to those prevailing during capillary blood sampling from the fetus. ACtual measurements on a drop model have demonstrated that over a period of 5 to 10 seconds there is no measurable difference in blood P oz.’ It must be emphasized that, although collection of the capillary blood sample occasionally took as long as 1 or 2 minutes, the exposure of fetal blood to air was probably less than a few seconds since it was collected in the capillary tube essentially at the rate of its emergence from the incision site. It is generally appreciated that Po, of capillary blood is of limited value in assessing oxygen supply in the newborn. This appears to pertain, perhaps to a greater extent, to the fetus. Scalp edema does not seem to interfere with obtaining representative blood samples. In our series there was no statistically significant difference between the red cell concentration of blood obtained from fetal scalp and that from the large blood vessels. Similar findings have also been reported for the human subject.2 The authors wish to convey their appreciation to Drs. Ermelando Cosmi, Sheila Haworth, and Ingemar Joelsson for their invaluable help in the conduct of the experiment and to Mrs. Stina Joelsson for her expert technical assistance.
REFFRENCES
1.
2.
3. 4.
Adamsons, K., Beard, R. W., Bowe, E. T., and James, L. S.: Bull. Sloane Hosp. Women 13: 11, 1967. Bowl:, E. T., Beard, R. W., Finster, M., Poppers, P. J., Adamsons, K., and James, L. S.: AMER. J. OBSTET. GYNEC. In press. Bretxher, J., and Saling, E.: AMER. J. OBSTET. GYNEC. 97: 906, 1967. Gandy, G., Grann, L., Cunningham, N.,
5. 6.
7.
Adamsons, K., 34: 192, 1964. Saling, E.: Z. 1963. Siggaard-Andersen, K., and Astrup, 12: 172, 1960.
Zernickow, 1966.
and
James,
Geburtsh.
P.:
von K.:
L.
S.:
Gynaek.
Pediatrics 161:
262,
O., Engel, K., Jorgensen, Stand. J. Clin. Lab. Invest.
Gynaecologia
161: 277,
ADAMSONS,
RICHARD
W.
RONALD San Juan,
E. Puerto
BEARD, MYERS,
M.D.,
PH.D.
M.D. M.D.,
PH.D.
Rico
The composition of blood obtained simultaneously from the carotid artery, jugular r,ein, and capillary bed of the scalp of the fetal rhesus monkey during labor revealed good correlation between the corresponding values of pH, Pro,, PO, and red cell concentration. The agreement was good not only with the fetus in a normal acid-base .state, but alsq under conditions of severe asphyxia. The mean of the differences between the pH of capillary and venous blood was 0.017 and that between capillary and arterial blood was -0.047.
blood obtained from the fetal scalp, carotid artery, and jugular vein in a species close to man and under conditions mimicking those present during labor.
s I N c E T H E introduction of sampling Of capillary blood from the presenting part of the human fetus by Saling,5 concern has prevailed regarding the representativeness of blood obtained from this source. It is known that the composition of blood obtained from central and peripheral sites is not the same in the immediate postnatal period, even in the vigorous newborn infant in whom collection of capillary blood is less likely to be subject to vasoconstriction, stasis, and edema of the subcutaneous tissues.4 The purpose of this study was to examine the relationship between the composition of
Maferial
and
methods
For the study, 11 pregnant rhesus monkeys from the caged breeding colony of the Laboratory of Perinatal Physiology were used. In all but one case, the gestation was more than 160 days (term is at 168 days). Following premeditation with pentobarbital (10 mg. per kilogram, intramuscularly), the mothers were anesthetized with a mixture of Fluothane and nitrous oxide. The peritoneal cavity was opened through a midline incision and the uterus was exposed. The head of the fetus was then delivered through an elliptical incision in the lower uterine segment, care being taken to avoid placental margins. Through a small incision in the neck, the left carotid artery and the right jugular vein were exposed, and polyvinyl catheters of appropriate dimension were inserted. The tip of the arterial catheter was advanced to the arch of the aorta while the tip of the venous
From the Laboratory of Perinatal Physiology, National Institute of Neurological Diseases and Stroke, National Institutes of Health, United States Public Health Service, Department of Health, Education and Welfare: and the Department of Obstetrtcs and Gynecology, Columbia University, New York. Supported in part by Contract No. PH43-63-25 of the United States Public Health Service administered by the University of Puerto Rico School of Medicine and in part by United States Public Health Service Grants GM09069 and ND 118. 435
436
Adamsons,
Beard,
and
Myers
catheter rested approximately 5 mm. above the right atrium. The scalp of the fetus was shaved in order to facilitate sampling of capillary blood. ‘The amniotic membrane over the internal OS was then ruptured and the cervical canal was gradually dilated to approximately 2 cm. with an obturator. After obtaining blood samples as simultaneously as possible from the carotid artery, jugular vein, and fetal scalp, the fetal head was returned to the uterine cavity and the incisions in the myometrium and in the abdominal wall of the mother were closed. Administration of Fluothane was then discontinued and the animal was maintained under sedation with either pentobarbital or nitrous oxide anesthesia. In most instances, uterine contractions ensued shortly after the discontinuation of Fluothane and led to a progressive dilatation of the cervix. In some cases, labor was stimulated with intravenous infusion of oxytocin in a deliberate attempt to produce fetal acidosis. Capillary blood from the fetal scalp was obtained using the same instruments and techniques as those used in the human subjects,’ except for the elimination of spraying the scalp with ethyl chloride. It was found in the initial experiments that blood flow during the pha.se of the “rebound hyperemia” did not compare favorably with the flow observed prior to cooling. Because of the thinness of the scalp in the fetal rhesus monkey, one to 2 minx:ltes were required to collect a sufficiently large sample from the capillary bed for complete determination of acid-base components. In these cases the specimens obtained from the carotid artery and the jugular vein were also withdrawn at lower rates to cover the corresponding time interval. Blood from all 3 sites was collected in heparinized ;glass capillary tubes. Hydrogen ion activity was determined with a glass electrode at 38’ C. using the radiometer expanded scale pH meter. Carbon dioxide tension was calculated from pH measurements of the sample ‘equilibrated at 2 known CO, tensions using the nomogram by Siggaard-Andersen, Engel, a.nd their colleagues.6 Oxygen tension was determined with a
Amer.
June J. Obstet.
1, 1970 Gynec.
radiometer microelectrode, and the red cell concentration was measured after centrifuging a triplicate sample of about 10 ~1 at 10,000 g for 5 minutes. Results
A total of 37 comparisons were made between capillary blood and that present in the central circulation. In 19 cases triplicate samples were obtained, while in 18 cases the comparison was made only between capillary and either arterial or venous blood. In 27 cases the capillary sample was obtained transvaginally and in 10 cases it was obtained while the head was still exteriorized. The mean value of the initial, pH obtained from the carotid artery at the time of the operation was 7.233, with a range of 7.032 to 7.325. During the course of labor, particularly when it was stimulated by oxytocin, there was a progressive fall in pH in all fetuses. The lowest value for pH observed in the carotid artery was 6.750. It was noted that sampling of capillary blood became increasingly more difficult with the progression of fetal acidosis, presumably due to intense cutaneous vasoconstriction. The relationship between the pH of arterial and capillary blood is given in Fig. 1. Closed circles identify capillary samples obtained transvaginally; open circles refer to samples obtained from the exteriorized fetal head. It is evident that a high degree of correlation exists between the two variables which extends over the entire pH range studied. The correlation coefficient for the combined population is 0.969, whereas that for the samples with fetal head in situ is 0.955. Statistically, there is no demonstrable difference between the pH values of arterial blood and those obtained from the exteriorized scalp. The pH of samples obtained from the fetal scalp transvaginally was, on the other hand, lower than that of the arterial blood, the mean of the differences being -0.047. There was a tendency of the values for capillary pH to depart more from the arterial pH at lower pH values. The correlation between pH of the capillary blood and that of the jugular vein is
Volume h’umber
107 3
Monkey
fetal
blood
during
labor
437
z400 7.30-
0* W.
?.20i g
7.10
0
-
02
.*
0
iA & zi 2
7.00
0
-
l
0 0. 0
6.90-
vf-i
0 00 6.80-
l*
r.=0.955
0
y.= /.029x
- 0.248
0 6.70
I 6.70
I 6.90
I 6.80
ARTERIAL
I 100
I 710
I 720
1 730
1 740
BLOOD
1. pH of blood obtained simultaneously from capillary bed of scalp and carotid artery Iof fetal monkeys during labor. Open circles (0) refer to samples obtained at time of hysterotomy.
IFig.
given in Fig. 2. Open circles refer to samples obtained while the feta1 head was still exteriorized.
Again
a satisfactory
agreement
between the two variables is observed. In the majority
of instances,
the pH value
of capil-
lary blood was higher than that in the jugular vein,
the mean
of the differences
being
0.017.. The correlation coefficient between the two variables is 0.965. The relationship between the corresponding Pcoz values in arterial and capillary blood is given in Fig. 3. The values are scattered rather evenly around the line with a 45 degree slopeoriginating from the intercept of the coordinates. A certain scatter is anticipated due to t.he error of measurement intrinsic to this method of determination Pooz. There is no tendency of the
of Pcoz
values to be relatively lower than arterial values at higher tensions of Pco,, which would have been suggestive of loss of CO, during the sampling of capillary blood. The relationship between Paz of the capillary, arterial, and venous blood is given in Fig. 4. Open circles refer to arterial blood; closeIdcircles denote venous blood. Consider-
able scatter of the values around the 45 degree slope line is evident. In 2 instances the values for capillary blood are higher than those for arterial blood. The mean of the red cell concentration of the arterial blood of the fetus at the beginning of the experiments was 40.1 per cent. There
was a good
agreement
between
the
values from the 3 sampling sites. In most cases the difference was 1 or 2 per cent only, which is not greater than the expected difference due to the limits of reproducibility of this method. No significant differences in red cell concentration were observed during the course of labor in spite of the development of progressiveacidosis.In most instances there was a slight reduction in red cell concentration at the end of the experiment ranging between 1 and 4 per cent. Since the total quantity of blood removed from the fetus during the study amounted to between 5 and 10 per cent of the total blood volume, the change in red ceII concentration represents a composite picture of hemodilution due to blood loss opposed by hemoconcentration in responseto asphyxia.
438
Adamsons,
Beard,
and
Myers
Amer.
June J. Obstet.
7 40-
2300+ z20g
210-
Li > 5.
zoo-
% * 00 0 *.
=! 6.90-
0
0
:
2i u
r*= 0.973
l 6.80
-
6.70
-
y,= 0992x
I 6.70
1 6.80
1 6.90
I 100
VENOUS Fig. 2. pH of blood obtained of fetal monkeys during labor.
simultaneously from (Symbols as in Fig.
ARTERIAL
I 710
I 720
+ 0 073
I 230
I 240
BLOOD capillary 1.)
BLOOD
bed
of scalp
and
(mm
jugular
vein
Hg)
Fig. 3. PCO, of blood obtained simultaneously from capillary bed of scalp and carotid of fetal monkeys during labor. (Symbols as in Fig. 1.) The lines enclose the region represents the error of measurement (25 per cent).
artery which
1, 1970 Gym.
Volume Number
107 3
Monkey
ARTERIAL
AND VENOUS
BLOOD
fetal
blood
(mm
during
labor
439
Hg)
Fig. 4. PC,, of blood obtained simultaneously from capillary bed of scalp, carotid artery (O), and jugular vein ( l ) of fetal monkeys during labor. The lines enclose the region which represents the error of measurement (‘2 per cent).
Comment This study has demonstrated that the composition of capillary blood of the fetus obtained from the scalp during the course of labor correlates well with that of blood entering and leaving the central nervous system. Before the onset of labor and with the fetal scalp not subject to compression by the maternal birth canal, the composition of scalp blood is virtually identical with that of the carotild artery. This pertained to essentially all constituents studied. During labor the composition of capillary blood resembled more closely that of the jugular venous blood. Except for a few occasions the difference was so small as to be of no significance regarding the assessment of the condition of the fetus from the clinician’s point of view. It appears that the differences are due to changes in scalp perfusion resulting from vasoconstriction or compression by the amnioscope rather than to changes in blood com-
position resulting from the exposure of capillary blood to air. It has been estimated? that exposure of fetal blood in the form of a drop to air for 5 seconds leads to a rise of pH by not more than 0.01, with a concomitant fall in Pooz tension even at high absolute Pco, values of not more than 2.5 mm. Hg. Another reason why the composition of capillary blood obtained from the fetus during labor may differ slightly from that present in the central circulation is that the collection of blood from the capillary bed is favored during the contraction period. It is known” that pH of blood obtained immediately at the end of contraction may be as much as 0.05 lower than that observed at the peak of contraction. Of particular clinical importance was the observation that capillary blood remained representative even under conditions of a degree of asphyxia seldom encountered in clinical practice. The fact that pH of capil-
440
Adamsons,
Beard,
and
Myers
lary blood more closely resembled that of the jugular venous blood rather than that obtained from the carotid artery is at least theoretically an advantage because the composition. of venous blood is determined not only by that of the arterial but also by the relationship between flow rate and metabolic needs of a given organ. Thus venous blood is much more informative than the arterial one regarding the operational conditions of the tissues. The pH of arterial blood was consistently higher than that of the venous. The difference ranged from 0.012 to 0.058, with a mean of 0.038. This difference is smaller than that re:ported for a.rterial and venous blood obtained from the umbilical cord at the time of delivery.’ In general, the difference between artery and vein was larger when the arterial pH was less than 7.10. Since a given difference of pH values denotes larger quantity of hydrogen ion at lower absolute values, this observation suggests that during acidosis of this degree flow rate is reduced; consequently, the volume of blood into which CO, or hydrogen ion, generated from lactic acid, had been distributed was less than that present under normal conditions or in states of moderate acidosis. Considering the technical difficulties, in obtaining capillary blood samples in the monkey fetus,, it was unexpected to iind that the correlation between pH of capillary and arterial blood was at least as satisfactory as that previously reported for the newborn human in whom warmed heel was used as a sampling site for capillary blood.4 The correlation between the corresponding Pcoz values was also satisfactory, particularly when one considers the limited ac-
.4mer.
June J. Obstet.
1, 1970 Gynec.
curacy of the indirect method for Pco? determination. The limited correlation between Po? values in capillary, arterial, and venous blood is most likely due to methodologic difficulties. Although it may seem plausible that blood at low Paz is likely to be altered in composition upon exposure to air, this has not been found to be the case under conditions similar to those prevailing during capillary blood sampling from the fetus. ACtual measurements on a drop model have demonstrated that over a period of 5 to 10 seconds there is no measurable difference in blood P oz.’ It must be emphasized that, although collection of the capillary blood sample occasionally took as long as 1 or 2 minutes, the exposure of fetal blood to air was probably less than a few seconds since it was collected in the capillary tube essentially at the rate of its emergence from the incision site. It is generally appreciated that Po, of capillary blood is of limited value in assessing oxygen supply in the newborn. This appears to pertain, perhaps to a greater extent, to the fetus. Scalp edema does not seem to interfere with obtaining representative blood samples. In our series there was no statistically significant difference between the red cell concentration of blood obtained from fetal scalp and that from the large blood vessels. Similar findings have also been reported for the human subject.2 The authors wish to convey their appreciation to Drs. Ermelando Cosmi, Sheila Haworth, and Ingemar Joelsson for their invaluable help in the conduct of the experiment and to Mrs. Stina Joelsson for her expert technical assistance.
REFFRENCES
1.
2.
3. 4.
Adamsons, K., Beard, R. W., Bowe, E. T., and James, L. S.: Bull. Sloane Hosp. Women 13: 11, 1967. Bowl:, E. T., Beard, R. W., Finster, M., Poppers, P. J., Adamsons, K., and James, L. S.: AMER. J. OBSTET. GYNEC. In press. Bretxher, J., and Saling, E.: AMER. J. OBSTET. GYNEC. 97: 906, 1967. Gandy, G., Grann, L., Cunningham, N.,
5. 6.
7.
Adamsons, K., 34: 192, 1964. Saling, E.: Z. 1963. Siggaard-Andersen, K., and Astrup, 12: 172, 1960.
Zernickow, 1966.
and
James,
Geburtsh.
P.:
von K.:
L.
S.:
Gynaek.
Pediatrics 161:
262,
O., Engel, K., Jorgensen, Stand. J. Clin. Lab. Invest.
Gynaecologia
161: 277,