| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Departments of 1Physiology and Pharmacology, 5Obstetrics and Gynecology, and 2Medicine, 3Heart Research Center, Oregon Health and Sciences University, and the 4Portland Veterans Affairs Medical Center, Portland, Oregon
Submitted 30 April 2006 ; accepted in final form 3 August 2006
 |
ABSTRACT |
|---|
 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
fetal sheep; blood pressure; placental flow; systemic flow; placental resistance; systemic resistance
When fetal intravascular volume is experimentally expanded by means of continuous plasma infusions, the fetus responds with a steep rise in arterial blood pressure that takes several days to develop although the fetal levels of angiotensin-II decline (11, 14). This response was consistent with the Borst-Guyton model if it may be assumed that the increase in fetal placental capillary pressure that is caused by the fall in angiotensin level was insufficient to fully compensate for the increase in oncotic pressure of the fetal plasma that was caused by the increase in plasma protein concentration (1, 9, 14). The reasoning was that the observed increase in fetal venous pressure signified an increase in fetal circulatory volume. No such changes occurred in control twins infused with similar volumes of lactated Ringer solution (14).
When these experiments were followed up by similar experiments in which fetal placental blood flow was recorded also, it was found that the increase in fetal arterial pressure was in part mediated by a large (50%) increase in fetal placental resistance (11). The present experiments were undertaken to determine if the increase in fetal arterial pressure during plasma infusion was also due to an increase in systemic resistance.
|
METHODS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
A catheter was placed in the carotid artery of the fetus, and the tip was advanced to the level of the aorta. A catheter was placed in the jugular vein, and the tip was advanced to a level just cranial of the right atrium. Catheters were placed in the amniotic fluid space. All catheters were sewn to the fetal skin. The fetal chest was opened, and a Transonic flow transducer (Ithaca, NY) was placed around the common brachiocephalic artery. A second Transonic flow transducer was placed around the descending thoracic aorta, distal to the ductus arteriosus. Because, in the fetal sheep, no branches arise from the aorta between the brachiocephalic artery and the ductus, the combined flows of these two sensors constitute the entire fetal biventricular cardiac outputs except for pulmonary blood flow (<5%) and coronary blood flow. We will refer to this combined flow as the biventricular output. The flow probe cables exited the chest, and the chest was closed in anatomic layers.
The fetal abdomen was opened from the left flank. A third Transonic flow transducer was placed around the terminal aorta distal to the iliac arteries just before the terminal aorta bifurcates into the two umbilical arteries. The flow transducer cable exited the abdomen, and the abdomen was closed in anatomic layers. The catheters and flow probe cables exited the uterus, and the uterus was closed, forming a tight seal of the amniotic cavity. The catheters were tunneled underneath the skin of the ewe, and the abdomen was closed. The flow transducers and the catheters emerged at the ewe's flank and were stored in a pouch sewn to the skin. One million units of penicillin G (Bristol-Meyers Squibb, Princeton, NJ) were instilled in the amniotic space at the conclusion of the procedure, anesthesia was terminated, and the ewe was allowed to recover. The ewes received routine postoperative pain medication (0.6 mg bupremorphine, 2 times/day) for 2 days. The ewes and their fetuses were given a postoperative recovery period of 6 days (range 5–7 days).
Experimental protocol. During the experiments, the ewes were kept in stanchions in the laboratory. The baseline experiments were performed at an average gestational age of 128 days (range 125–131 days). After checking all zero values, we recorded fetal arterial blood pressures, central venous blood pressures and amniotic fluid pressures, and brachiocephalic, descending aortic, and umbilical flows continuously for the next 7 days. Fetal arterial blood samples were taken daily for determination of fetal blood gases and pH, for plasma protein concentration, and plasma renin activity.
On day 0, after baseline measurements had been made for 60 min, an intravenous infusion of adult sheep plasma solution was started using a Gilson Minipuls 3 roller pump. The plasma infusion rate was started at 229 ± 5 (SE) ml/day and increased by 3.5%/day to correspond to the daily increase in fetal mass due to growth. Experiments for the current study demonstrated that infusion of this volume of (protein-free) fluid in the fetus results in no changes in blood pressures or heart rate (14). Zeros were checked every morning, after which the data taken over the next 60-min period were averaged and recorded for later analysis. Heart rates were obtained from arterial pressure tracings.
Instrumentation. Pressures were measured with Abbott Transpac pressure transducers (Abbott Park, IL) and a computerized recording system. The system was calibrated against a mercury manometer to a scale value of better than 1% and rezeroed for drift before each measurement. Repeat calibrations established an accuracy of 0.5 mmHg. All fetal intravascular pressures were referred to amniotic fluid pressure as zero. Fetal blood flows were measured using a Transonic T 403 flowmeter with timing synchronized to measure three flows continuously.
Laboratory measurements. Arterial blood samples (2 ml) for determining plasma renin activities were collected in EDTA (Becton-Dickinson) and immediately separated in a refrigerated centrifuge. The plasma samples were stored in a freezer. Plasma renin activities were measured with a DiaSorin (Stillwater, MN) Gammacoat plasma renin activity kit. Arterial blood samples for other analyses were anticoagulated with heparin. A refractometer was used to obtain rapid estimates of plasma protein concentrations during the experiments. However, the plasma protein concentrations reported below were measured with a commercially produced version of the Lowry method (Sigma Diagnostics). Blood gas partial pressures and pH were determined in an Instrumentation Laboratories model IL 1610 blood gas analyzer at 39°C. Hb concentrations and Hb oxygen content were determined in an IL 842 cooximeter.
Plasma source. Sheep plasma was obtained from ewes that were to be killed as previously described (11, 14). In short, the ewes were medicated as described above for induction of anesthesia and given additional doses of ketamine to maintain a deep level of anesthesia. About 10,000 units of heparin were administered intravenously. A large-bore sterile catheter was inserted in a carotid artery, and the animal was exsanguinated in 1-liter sterile bottles. Each bottle contained 10,000 units of heparin. The collected blood was immediately centrifuged in sterile containers, and the plasma was separated. Our main concern with the methodology was to not raise the peripheral resistance of the plasma-infused fetuses by microembolization. For this reason, and to ensure final sterility, all plasma was subjected to ultrafiltration through a sterile Corning cellulose acetate 0.22-µm filter. Since capillary diameters are typically 20 times larger, this eliminated physical blockage by emboli as a cause of concern. We previously observed that two fetuses that were given intra-arterial infusions via the femoral artery (for lack of venous access) responded like the other fetuses (11). Since such arterial infusions could have embolized only a small fraction of the systemic vascular bed, they confirmed that embolization was not a significant concern. One million units of Penicillin-G per liter were then added, and the plasmas were stored at room temperature until use. Cultures confirmed the sterility of the prepared plasmas.
Statistical methods. All statistical analyses were performed with GraphPad Prism (GraphPad Software, San Diego, CA). Repeated-measures ANOVA with Dunnett's multiple-comparison test was used to determine if and when the daily measurement value differed from the preinfusion baseline, day 0 value. A level of P < 0.05 was required for any change to be considered statistically significant. Results are presented as means ± SD, except for Figs. 1–4, which show means ± SE.
|
|
|
RESULTS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
Figure 2 shows that fetal systemic flow and placental flow did not change. Fetal systemic flow was 851 ± 128 ml/min before and 834 ± 152 ml/min after 7 days [not significant (NS)], and fetal placental flow was 675 ± 154 ml/min before and 622 ± 214 ml/min after 7 days of plasma infusion (NS).
|
Figure 3 shows the effects of plasma infusion on total resistance (exclusive of pulmonary and coronary vascular beds), upper body (brachiocephalic) resistance, systemic resistance (exclusive of the coronary bed), and placental resistance. During the 7 days of infusion, fetal systemic resistance increased gradually from 0.047 ± 0.008 to 0.075 ± 0.016 mmHg·min–1·ml–1 (P < 0.01) and fetal placenta resistance from 0.065 ± 0.012 to 0.111 ± 0.049 mmHg·min–1·ml–1 (P < 0.01).
|
|
DISCUSSION |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
It has previously been shown that increases in fetal arterial pressure and venous pressure and increases in placental resistance to flows that are as large as those that were observed in the present study can also be obtained with chronic infusion of angiotensin (8, 10). That and the fact that the infused maternal plasma could have contained high concentrations of angiotensin and renin should be considered. However, angiotensin in the fetus is almost completely cleared in a single pass through the microcirculation, and its half-life, therefore, is much too short for it to persist (19). The fetal concentrations of renin were shown to be steeply decreased. We conclude therefore that the observed increases in pressures and resistances were not due to the renin-angiotensin system.
A further noteworthy finding is that these increases in resistance occur without an increase in blood flow, or at least a statistically demonstrable change in blood flow. Local control of resistance is variously ascribed to autoregulation in response to changes in flow (6, 7, 15) or myogenic regulation (4), the further vasoconstriction under influence of a rising arterial pressure. The slight, but statistically insignificant, increase in systemic flow in the first few days of the infusion (Fig. 2) would be consistent with an explanation in which a myogenic regulation takes over after a small initial rise in pressure due to autoregulation of flow. However, even if this were the case, it would be difficult to prove it in a manageable number of experiments because of the great sensitivity of local control (15). The fact, however, that excess oncotic pressure leads to a large increase in systemic and placental resistances is a major factor in the control of fetal arterial pressure and the development of its circulation and its heart.
Because only a small part of blood viscosity is due to plasma protein, whereas most of it is due to the presence of red cells, and because the normal (initial) fetal plasma protein concentration is only one-half of the adult value, we considered the increase in viscosity due to protein infusion to be a negligible contributor to the increase in resistance.
Both fetal heart filling pressure (preload) and arterial pressure (afterload) increased over the 7-day plasma infusion. Under normal fetal conditions, the heart is relatively insensitive to increased preload. Researchers have shown that rapid infusion of volume has little effect on fetal cardiac output or right ventricular stroke volume (12, 13, 17, 20). On the other hand, the fetal heart is very sensitive to increased afterload. Methoxamine-induced increased arterial pressure results in a marked decrease in fetal cardiac output (13). Phenylephrine-induced increase in arterial pressure is associated with a decrease in right ventricular stroke volume and right ventricular output (20). Similarly, when pulmonary arterial pressure is increased using a variable hydraulic occluder around the pulmonary artery, right ventricular stroke volume and output are decreased (3). These observations may explain the effects of plasma infusion on fetal cardiac output. Plasma protein-induced increased diastolic filling pressure along with an increase in cardiac chamber size due to normal cardiac growth could contribute to an increased stroke volume over the 7-day period. At the same time, arterial pressure (afterload) is increasing. Because of the afterload sensitivity of the fetal heart, the net effect of plasma infusion-induced increased cardiac preload and of increased cardiac afterload was dominated by the afterload effect with no change in cardiac output (biventricular output).
We have previously shown that plasma infusion increases both fetal venous and arterial pressures and that increased arterial pressure is at least in part mediated by a large increase in fetal placental resistance (11), in sharp contrast to the decrease in placental resistance that occurs in normal gestation (18). The present experiments were undertaken to determine if the increase in fetal arterial pressure during plasma infusion was also due to an increase in systemic resistance. We conclude that plasma infusions produce fetal arterial hypertension as a result of both an increase in fetal somatic resistance and placental resistance, whereas cardiac output does not change significantly.
|
GRANTS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
|
REFERENCES |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  S. S. Jonker, J. J. Faber, D. F. Anderson, K. L. Thornburg, S. Louey, and G. D. Giraud Sequential growth of fetal sheep cardiac myocytes in response to simultaneous arterial and venous hypertension Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2007; 292(2): R913 - R919. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
