Effect of carbon dioxide elevation on renal blood flow in the fetal lamb in utero F.
The effect of elez’ationt of arterial carbon dioxide tension on rennl blood pow has been ermaluated with the use of a semichronic fetal lamb preparation and the radioactiile microsphere method. Hypercapnia caused a decrease in renal blood flow and nn increase in renal z,ascular resistance. The renal perfusion pressure remained \-table throu~ghout the experiment.
flow in the fetal, neonatal, and adult sheep. The secretory capacity of the fetal tubules for PAH is not known and, accordingly, the interpretation of these results was difficult. Other authors’!, ‘, ‘I have used electromagnetic flowmeters in acute fetal lamb preparations. Hovvcver, it has been demonstratedI that the esterioraization of the lamb could alter the fetal physiologic responses. The radioactive microsphere method described in 1967 by Rudolph and Heymann”’ permitted studies of the fetal circulation in the intrauterine environment and obviated the immediate effects of the anesthetic and surgical stress. During this study, the previously described semichronic lamb fetus preparation’l and the radioactive microsphere technique were used in an attempt to evaluate the response to elevation of carbon dioxide in fetal renal blood flow.
T 1%E 15F F E c T of hypercapnia on renal blood flow in adult animals is well documented”, i. 11. 1;1,li. L’I-L’li; there is a reflexly mediated increase in renal vascular resistance with concomitant decrease in renal blood flow as carbon dioxide tensions are elevated. On the other hand, because of the technical difficulties inherent to the study of thr fetal renal function in utero: our knowledge about the effects of asphyxia, hypoxia, and hypercapnia on the fetal kidney is very limited, Clearance of p-amino-hippuric acid (C,,,,) has been studied’, L’ to estimate renal blood From the Department of Obstetrics Gynecology, Los Angeles CountyUnir’ersity of Southern California Medical Center.
This study was supported by Grant R-231-72, IJnited Cerebral Palsy Research and Education Foundation, Inc., New York, New York.
Twelve Dorset and Ramouillet sheep with singleton fetuses between 123 and 141 days’ gestation were used in this study. The animals were maintained in air-conditioned stalls and givren prophylactic antibiotics (penicillin 1.0 million units and streptom):tin 1.0 Gr11. daily) for a period of four to seven days prior to sur,qery.
Reprint requests: E. J. Quilligan, M.D., Los Angeles County-Uniuersity of Southern California Medical Center, Women’s Hospital, 1240 N. Mission Rd., Los Angeles, California 90033. *Resenrch Fellow supported by Stiftung fiir biologisch-meditinische Stipendien, Switzerland. Present address: DBpartement de Gynicologie et Obste’trique, HGpital Universitaire. Geneua, Switzerland. 530
Table I. Measurements and renal
(A) (B) (C) (D) (E) (F) (G) Unpaired
(J) (K) (L) (M)
on renal blood
Fetus gestational (days) Paired
PO2 mm. Hg
B. E. mEq./L.
Perfusion pressure (mm. Hg)
Renal blood flow (ml./min./lOO Gm. of tissue)
Hg/ml./ Gm. of tissue)
123 Base High 132 Base High 129 Base High 136 Base High 141 Base High 129 Base High 129 Base High
0.48 10 0.9874
125 Base 137 Base 137 Base 128 Base 140 Base
Surgery was performed under local lidoCaine anesthesia. The fetal head was delivered and placed in a saline-filled rubber glove. The following devices were placed on ( 1) fetal shoulder silver-silver the fetus: chloride ECG electrode, (2) fetal carotid artery Micron flow transducer, and (3) P.E.50 polyethylene catheters inserted into the submandibular branch of the carotid artery, the external maxillary branch of the jugular vein, and the umbilical vein. An infant-feeding tube was placed in the amniotic cavity, and a Tygon plastic tube was inserted into a maternal femoral artery. The fetus was returned to the uterine cavity and the uterus and abdominal walls were closed. The lost amniotic fluid was replaced with saline
containing 1.0 million units of penicillin via the infant-feeding tube. The experiments were performed 24 hours after surgery according to the procedure previously described.l’s I3 The electrode, flow transducer, and catheter were attached to the appropriate transducers and connectors on a seven-channel recorder (Brush Mark 200 Recorder, Clevite Corporation, Brush Instruments Division, Cleveland, Ohio). Maternal arterial, fetal arterial, and venous blood samples were obtained at several intervals during the procedure for determination of pH, PO*, Pco*, base deficit, and hematocrit. The tensions and pH were corrected to 38’ C. to approximate intrauterine conditions. A maternal tracheotomy was per-
July 1, 1974 Am. J. Obstet. Gynecol.
Fig. 1. Renal blood flow as a function of the arterial carbon dioxide tensions.
formed under local anesthesia. Blood flow studies were accomplished using 35 p radioactive microspheres labeled with either chrcmium-5 1 or cerium-41 (Minnesota Mining and Manufacturing Company, St. Paul, Minnesota). In seven animals, the first nuelide vvas administered to the fetus at baseline carbon dioxide conditions and the second after the ewe had breathed a gas mixture containing 20 per cent CO,, 20 per cent O,, and 60 per cent N, via an endotracheal tube for two to three minutes (paired data j . In the five remaining animals experimental problems enabled us to have only a simple carbon dioxide tension measurement (unpaired data). At the conclusion of monitoring, the ewe was given a concentrated solution of potassium chloride. The fetus was removed, weighed, and measured. The radioactivity of the fetal organs was determined by a Nuclear-Chicago Gamma Systems two-channel counter. Calculation of the renal blood flow was performed according to the organ blood-flow determination described in earlier publications.12, ?* It was assumed in the calculations that aortic pressure was equal to renal pressure and that
superior vena cava pressure was equal to renal vein pressure. Renal vascular resistance was calculated as perfusion pressure across the kidney divided by renal blood flow. Results
The results are listed in Table I. At baseline carbon dioxide conditions the mean arterial Pcoz was 37.5 and the mean arterial pH was 7.31. In animals with paired data the mean arterial Pco2 increased from 37.3 to 75.6, the mean arterial pH decreased from 7.31 to 6.98, and the mean arterial PO, remained about the same level. The mean base deficit was -8.07 + 3.66 mEq. per liter at the lower Pco? levels and -12.7 ? 5.35 mEq. per liter at the higher Pco2 levels. There was no significant change in base deficit. The mean value of the fetal renal blood flo,w at base-line dioxide conditions was 162.3 + 74.9 ml. per minute per 100 Gm. when related to the wet weight of the kidneys and 11.81 i 6.22 ml. per minute per kilogram when related to the fetal body weight. The renal blood flow decreased as the
on renal blood
Fig. 2. Changes in renal blood flow as a function animals with paired data. carbon dioxide tensions were elevated (Fig. 1). A linear regression of this relationship showed that this decrease of flow was significant (p < 0.05). There was a decrease in the renal blood flow of 1.87 ml. per minute per 100 Gm. of kidney for each mm. Hg increase in the carbon dioxide tension. The t test applied to the slopes of paired data showed that the correlation was highly significant (p < 0.005) (Fig. 2). There was an increase in vascular renal resistance with increased carbon dioxide tension but this relationship was of questionable significance (p < 0.10) (Fig. 3). However, the t test applied to the slopes of paired data showed that the correlation was significant (p < 0.05) (Fig. 4). There appeared to be no significant changes in the perfusion pressure over a broad range of carbon dioxide tensions. In the animals with paired data the perfusion pressure changes ranged between -11 and +4 mm. Hg. Comment
Several PAH or
authors,l-“’ 8, I1 using clearance of electromagnetic flowmeters, have
of the arterial
carbon dioxide tensions in
studied fetal renal blood flow in the lamb and have found considerably lower values than are present in the adult animal. These differences were attributed to the fact that the largest portion of the cardiac output bypasses the fetal body and is channeled directly toward the placenta. Measurements of renal vascular resistance in term fetal lambs were reported as being extremely high.3, x For instance, Alexander and Nixon’* z have found in fetuses at term a very low Cr.hrr ( 1.3 ml. per minute per kilogram of body weight) in comparison with that in the adult sheep (12.7 ml. per minute per kilogram). Other authors,” 8s I1 using electromagnetic flowmeters, later confirmed that renal blood flow and renal fraction of cardiac output in the fetus were significantly below adult levels. All these data were obtained in acute experiments. The mean renal blood flow found by Rudolph and Heymannz3 using the radioactive microsphere technique and less disturbed fetus preparations, was 173 ml. per minute per 100 Gm. of tissue. The placenta was found to receive the largest portion of the cardiac output (40 per cent) whereas the kidneys received a rela-
Am. J. Obstct.
1, 1974 Gynecol.
Fig. 3. Renal vascular resistance as a function of the arterial carbon dioxide tensions. tively small portion ( 1.9 per cent). These results are comparable to ours at base-line carbon dioxide conditions. Converted into milliliters per minute per kilogram of body \veigl;:. our mean rtnal blood flow is 1 I .81, a value compatible with low normal le,,els in the adult animal. Frtal kidney weights are not specified in the publication of Kudolph and Heymann but, since these authors studied renal blood flow in term lamb fetuses, one can assumr that their results are similar to ours---closer to adult level ranges than prcviously reported \2lues. Keccntly, Gresham and associates,“’ using chronic lamb preparations, have found that the fetal filtration rate (GFK ) measured by inulin-C-14 clearancr \vas approximately 30 to 50 per cent of that reported in the nctvborn lamb or in the adult sheep. ‘l’trc rtznal blood flow \vas not measured in this study but. according to the hypothesis of the authors, a small GFR would reduce the quantities of sodium reabsorbed by the tubules, in turn rcduring renal 0, consumption. A low renal 0, consumption would have the additional ad\.antage of requiring a relatively low renal blood flow. It is difficult to determine if the differcncps in the results of several
authors are due to animal preparations (acute, semichronic, chronic), to measurement techniques (clearances, flowmeter. microspheres) , or to both factors. However, it has been demonstratedI that anesthesia, and exteriorization or marsupializasurgery, tion of the lamb could considerably alter physiologic fetal conditions. It has to be emphasized that renal blood flow was calculated in this study with the use of the measured cephalic blood flow and the ratios of microspheres in the head and kidneys. It was thus assumed that the microspheres kvere in equal concentrations in blood arriving at the head and at the kidneys. An error in calculation of renal blood flow has to be taken into account because the role of the ductus arteriosus shunt was ignored in this determination. However, our results concerning the renal blood flow at baseline carbon dioxide conditions are in complete accordance with the results of Kudolph and Heymann.?” Resides using the figures given by Dawes” for fetal blood flow through the inferior vena cava and its distribution into the atria plus pulmonary and superior vena caval blood flow, and making the assumption of homogenous distribution of the
Fia. 4. Changes in renal vascular resistance as a function of the arterial carbon dioxide tensions in-animals w%h paired data. microspheres in the fetal inferior venal caval blood, one may calculate microsphere dilutions above and below the ductus arteriosus. The calculated dilution by the ductus arteriosus would be between 20 and 25 per cent. With this in mind, our calculated renal blood flow would be increased by 20 to 25 per cent or approximately 200 ml. per minute per 100 Cm. An effect of CO, on ductus arteriosus hemodynamic had never been clearly demonstrated. The effect of hypercapnia on renal blood flow in adult animals is well documented. Brooker and associates’ and Stone and associates’” have demonstrated that carbon dioxide inhalation in dogs causes a marked decrease in renal blood flow and an increase in renal resistance despite a slight fall in blood pressure. This is accompanied by olor anuria. Hypercapnia is probably iguria also an important factor contributing to reduction in renal blood flow during diffusion respiration (respiratory arrest induced by curare or anesthetic overdose) .‘, ” It has been suggested 21 that fetal animals may react to asphyxia in a manner like that of adult diving animals. In seals during diving and in do,qs, ducks, and sheep during im-
mersion’4s 19, lR, zi the renal blood flow is profoundly reduced with impaired GFR, the hypercapnia obviously playing a major role in the production of that response. A complete cessation of renal function has even been observed in trained seals during diving.?O The mechanism by which hypercapnia causes a reduction in renal blood flow has been studied by several authors in adult animals. In dogs the pharmacologic denervation of the kidneys has been shown to prevent or to reduce the marked decrease in renal blood flow and the oliguria observed in the intact kidney in response to high CO, tensions.“, ZG It has also been demonstrated in adult rabbits that renal vasoconstriction concomitant to the fall in renal blood flow was induced by anoxic stimulation of the IQ Thus, the reduction aortic chemoreceptors. in renal blood flow observed during hypercapnia and acidosis appeared to be essentially reflexly mediated. The preponderant effect of hypercapnia in these experiments was a reflex increase in renal vascular resistance without elevation in arterial blood pressure. During fetal asphyxia there is a redis-
tribution of cardiac output by vasodilatation in cerebral and coronary beds and vasoconstriction in other vascular beds. Chronic or acute hypoxia causes, according to several authors, a decrease in fetal renal blood flow. Blanc” achieved experimental fetal growth retardation in pregnant rats by Iigation of the uterine vessels at the lower end of one horn on the seventeenth day of pregnancy. The kidneys were, with the liver and lungs, the most stunted organs, whereas the brain and heart were least affected. In arute mature fetal lamb preparations. Campbell and associates’ found a renal flow of 2.0 to 2.3 ml. per minute per kilogram of body weight at basic conditions. The renal flow decreased to 0.6 to 0.9 ml. per minute per kilograni during partial asphyxia caused either by otelusion of the umbilical cord in the absence of pulmonary ventilation or by stopping the respiration pump after tying the cord. The fall in renal blood flow was preceded by an increase in arterial pressure, probably due to the occlusion of umbilical arteries. Behrman and associates’ induced fetal distress in term primate fetuses by decreasing maternal 0, tensions and found a statistically nonsignificant decrease from 190 to 120 ml. per minute per 100 Gm. of tissue in the renal blood flow during hypoxia. Our results clearly shove that a rise in Pco, has the same effect on renal blood flow and renal resistance as a fall in PO,. As in the adult, hypoxia hypercapnia, and asphyxia cause a fall in renal blood flow and a rise in renal vascular resistance.
I. 2. 3.
Alexander, D. P., and Nixon, D. A.: Br. Med. Bull. 17: 112, 1961. Alexander, D. P., and Nixon, D. A.: Nature 194: 483, 1962. Assali, N. S., Bekey, G. A.. and Morrisson, L. W.: In Assali, N. S., editor: Biology of Gestation, New York and London, 1968, Academic Press, Inc., Vol. II, p. 51. Behrman, R. E., Lees, M. H., Peterson, E. N., DeLannoy, C. W.. and Seeds, A. E.: AM. J. OBSTET. GYNECOL. 108: 956, 1970. Blanc, W. A.: Pediatr. Res. 1: 218, 1967 (abst.).
It is difficult to know if the mechanism by which asphyxia causes a reduction in renal blood flow is the same in fetuses as in adult animals, However, it is well established that the autonomic nervous control of fetal cardiovascular function is well developed and operative at the end of gestation. It has also been demonstrated that the activity of the systemic arterial chemoreceptors plays a major role in this control.!‘, “’ Kecently, Dunne and associates” have made some interesting observations during the stimulation of aortic rhemoreceptors with minimal doses of sodium cyanide in acute fetal lamb preparations. They found an increase in renal arterial pressure, a slight decrease in renal blood flow, and an increase in renal vascular resistance. In two animals the same experiment was perfornied follo\ving section of the vagus nerves: the renal vasoconstriction was much less pronounced than in the intact animals. It is therefore likely that the decrease in renal blood flow and renal vasoconstriction are reflexly induced by anoxic stimulation of the chemoreceptors. Our rcsuits seem to confirm the hypothesis of 1)unne and associates that the activity of the systemic barorereptors is not an important factor in the development of renal vascular response to hypoxia or hypercapnia. As in the adult, the preponderant effect of hypercapnia and the accompanying acidemia on the fetal kidney appears to be a refles increase in renal resistance in the face of a stable arterial blood pressure.
Bohr, V. C.. Rails, R. J.. and Westerrneyer, R. E.: AM. J. PHYSIOL. 194: 143, 1958. Brooker, W. J., Ansell, J. S., and Brown. E. B., Tr. : Surer. Forum 10: 869. 1960. Campbell, A. E. M., Dawes, d. S., Fishman, A. P., and Hyman, A. I.: Circ. Res. 21: 229, 1967. Dawes, G. S.: Foetal and Neonatal Physiology, Chicago, 1968. Year Book Medical Publishers, Inc. Dawes, G. S.: In Perinatal Factors Affecting Human Development, Washington, D. C., 1969, Pan American Health Organization, p, 199.
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17. 18. 19. 20.
Dunne, J. T., Milligan, J. E., and Thomas, B. W.: AM. J. OBSTET. GYNECOL. 112: 323, 1972. Dunnihoo, D. R.: Doctoral dissertation, University of Southern California, Los Angeles, Calif., 1972. Dunnihoo, D. R., and Quilligan, E. J.: AM. J. OBSTET. GYNECOL. 116: 648, 1973. Elsner, R., Franklin, D. L., Van Citters, R. L., and Kenney, D. W.: Science 153: 941, 1966. Elsner, R., Hammond, D. D., and Parker, H. R.: Yale J. Biol. Med. 42: 202, 1969. Gresham, E. L., Rankin, J. H. Cl., Makowski, E. L., Meschia, G., and Battaglia, F. C.: J. Clin. Invest. 51: 149, 1972. Heymann, M. A., and Rudolph, A. M.: Circ. Res. 21: 741, 1967. Johansen, K.: Acta Physiol. &and. 62: 1, 1964. Korner, P. I.: Circ. Res. 12: 361, 1963. Murdaugh, H. V., Jr., Schmidt-Nielsen, B.,
22. 23. 24.
on renal blood
Wood, J. W., and Mitchell, W. L.: J. Cellular Comp. Physiol. 58: 261, 1961. Quilligan, E. J., Dunnihoo, D. R., and Anderson, G. G.: AM. J. OBSTET. GYNECOL. 109: 706, 1971. Rudolph, A. M., and Heymann, M. A.: Circ. Res. 21: 163, 1967. Rudolph, A. M., and Heymann, M. A.: Circ. Res. 26: 289, 1970. Scholander, P. F.: In Walker, J., and Turnbull, A. C., editors: Oxygen Supply to the Fetus, Oxford, 1960, Blackwell Scientific Publications, p. 267. Stone, J. E., Irwin, R. L., Wood, C. D., Draper, W. B., and Whitehead, R. W.: J. Appl. Physiol. 14: 405, 1959. Stone, J. E., Wells, J., Draper, W. B., and Whitehead, R. W.: Am. J. Physiol. 194: 115, 1958. Vaughan, D., Kirschbaum, T, H., Bersentes, T., Dilts, P. V., Jr., and Assali, N. S.: J. Appl. Physiol. 24: 135, 1968.