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Biomedical subjects

G G Power

Publications and source records attributed to G G Power.

At least 91 records · Page 5Linked to original sources
PubMed1977

Carbon monoxide exchanges between the human fetus and mother: a mathematical model.

A mathematical model was developed to calculate maternal and fetal carboxyhemoglobin concentrations, [HbCO], as functions of time during and after exposure of the mother to various inspired CO concentrations. Effects of variation in alveolar ventilation rates, pulmonary and placental fiffusing capacities, cardiac output, endogenous carbon monoxide production and other factors were studied. Following a change in the inspired CO concentration, fetal HbCO lags behind maternal HbCO by several hours. During CO uptake, fetal HbCO eventually overtakes maternal, and approaches an equilibrium value as much as 10% higher than the mother's. During CO washout the fetal levels again lag behind the mothers. Results indicate that treatment of pregnant women who have elevated HbCO levels with 100% oxygen reduces the time necessary to reduce the maternal HbCO level as expected, but that the rate of fetal CO elimination is not increased as much as that of the mother. Changes in maternal and fetal HbCO were also calculated for a representative exposure to changing inspired CO levels produced by fluctuating levels of air pollution. Finally, the effects of carboxyhemoglobin on fetal oxygenation were studied, including the effects of high altitude and exercise.

Carbon Monoxide↗
PubMed1977

Umbilical vascular compliance in sheep.

To explore the mechanical properties of umbilical vessels and to test for possible interactions between maternal and fetal circulations, we recorded pressure-volume curves from isolated in situ placentas of 17 sheep. We estimated static umbilical compliance by extrapolating to infinitely slow rates of volume change. We found that compliance averaged .231 +/- .014 SE ml/mmHg per kg fetal wt under control conditions, a value that increased 26.5% when maternal arterial pressure was lowered 85 mmHg (clamping the aorta), but did not change when venous pressure was raised 37 mmHg (clamping IVC). After replacing blood with kerosene which does not penetrate small vessels because of interfacial tension, we found arteries accounted for 22% and veins for 41% of total compliance, leaving 37% attributable to small placental vessels and surrounding tissue. We conclude that umbilical vessels are about one-half as compliant as adult vessels on a body-weight basis, but only slightly less compliant than vessels elsewhere in the fetal body. When maternal vessels expand they interact with surrounding placental tissue, displacing fetal blood and altering the apparent compliance of umbilical vessels.

Animals↗
PubMed1977

Fetal O2 changes in response to hypoxic stress: a mathematical model.

We developed a mathematical model to compute the time course of PO2 changes in fetal blood vessels during hypoxia. The model represents the circulation and major organs as a system of paths and nodes. We calculated outflow O2 for a path from its inflow O2 content and its distribution of transit times. The O2 content at a given node equals the flow-weighted sum of O2 delivered by different paths. Placental O2 transfer and organ O2 consumption are related to their arterial PO2 levels. We simulated the effects of uterine contractions with Gaussian-shaped decreases in placental O2 transfer. Increasing the intensity and increasing the duration of hypoxic episodes have comparable effects. Liver O2 consumption decreases more than that of other organs during hypoxic episodes. At the peak of a contraction, fetal systemic PO2 values decrease only about one-fourth as much as those in end-capillary placental blood. This indicates that despite rapid circulation times, fetal O2 reserves protect it against severe, short term hypoxia.

Brain↗
PubMed1977

Rate of pH changes in blood plasma in vitro and in vivo.

The rate of pH decrease in dog blood after addition of CO2 (dissolved in saline) was measured in vitro with a combination pH electrode and fast-response amplifier. The pH change was found to have an apparent half time of 6.0 +/- 0.5 s at 38 degrees C, similar to that predicted by previous mathematical simulations on the basis of uncatalyzed CO2 hydration kinetics and the buffering characteristics of plasma. Measurements were also made in vivo in dogs by withdrawing blood rapidly from a carotid artery through a temperature-controlled chamber containing a pH electrode. When the flow was stopped, we measured the pH change which occurs in blood (after it leaves the lungs) as the slow dehydration of bicarbonate continues. The measured half time was 7.3 +/- 0.6 s, again agreeing well with predictions. Essentially the same half time was found when the direction of approach to chemical equilibrium was reversed by adding CO2 to the inspired air. The rate of the reactions could be increased markedly by hemolyzing the blood to release carbonic anhydrase into the plasma.

Animals↗
PubMed1975

Placental diffusing capacity in unanesthetized rabbits.

The purpose of this study was to determine the placental diffusing capacity of CO in unanesthetized rabbits. A standard dose of CO was administered to 18 near-term rabbits, and at time intervals varying from 5 min to 4 h maternal and fetal blood samples were analyzed for carboxyhemoglobin saturation (HbCO). Following administration of CO, the maternal HbCO rose rapidly within the 1st min and remained nearly constant for the remainder of the experiment. The fetal HbCO after CO introduction to the mother rose as CO crossed the placenta from maternal to fetal blood. After 15 min the fetal HbCO rose to equal the maternal HbCO averaging 18.5%. The fetal HbCO after 1 h averaged 33% saturation, 1.7-fold greater than the maternal HbCO at 1 h. Thereafter there was little further fetal rise and apparently equilibrium was reached. The average placental diffusing capacity for CO was 2.3 ml/(min times mmHg times kg), a value fourfold higher than has previously been reported in anesthetized sheep and dogs. Possible reasons are species difference, effects of posture, influence of anesthesia and surgery, and effects of an open versus intact uterus.

Animals↗
PubMed1975

Factors affecting O2 transfer in sheep and rabbit placenta perfused in situ.

In these experiments we have studied three factors that affect placental O2 transfer. The fetal artery of an isolated cotyledon of the sheep placenta (or one of the umbical arteries in flow studies in rabbits) was perfused in situ with blood of varying PO2 and at different flow rates while the ewe was administered varying inspired O2 concentrations. Measurements were made of the PO2 of inflowing and outflowing umbilical blood, and the O2 transfer rate was calculated by the Fick principle. Changes in individual factors could be studied, since most compensations tending to maintain normal O2 delivery were not operative in the isolated preparation. Results indicate that a 20% change in umbilical arterial PO2, a 14% change in umbilical blood flow, and a 20% change in maternal arterial PO2 would be equivalent in causing a 10% change in placental O2 transfer. Small changes in umbilical arterial PO2 are sufficient to maintain the rate of placental O2 transfer equal to the rate of fetal consumption. Maternal arterial PO2 becomes progressively more critical to fetal oxygenation as its level falls. The experimental results are compared to those predicted by a mathematical model of placental exchange.

Animals↗
PubMed1975

Hematocrit of the fetal rabbit placenta.

51Cr-labeled erythrocytes and [125I]RISA were used simultaneously to measure the fetal rabbit whole-body and placental hematocrits (Hct) and to find the ratio of placental transit times of erythrocytes and plasma. The levels were injected into the heart of 21 fetal rabbits of 26--28 days gestation, and about 60 s mixing time, the placenta and a fetal blood sample were assayed with a gamma well-type scintillation counter. Erythrocyte and plasma activity per milliliter were determined from a standard dilution of the isotopes. Large-vessel Hct was measured as the corrected packed erythrocyte percentage in capillary tubes. Large-vessel Hct was 37.3 (+/- 3.7 SD) %, whole-body Hct was 31.3 (+/- 3.5) %, and the placental Hct was 25.3 (+/- 4.0) %. The placental/large-vessel Hct ratio was 0.676. The ratio may be estimated as 0.765 when corrected for the loss of albumin in time. The transit time of erythrocytes in the placenta was calculated to be 0.682 of that for plasma. The shorter erythrocyte transit time implies that there is less time for O2 and CO2 exchange than previously thought.

Animals↗
PubMed1975

Fetal circulation times and their implications for tissue oxygenation.

In an effort to understand O2 delivery to fetal tissues, we measured circulatory transit times using dye dilution methods in near-term fetuses of 19 ewes. Times from dye injection to peak response (+/-1 sec) were: femoral vein to carotid artery 2.2, to femoral arter 2.6, to umbilical artery 3.7; jugular vein to carotid artery 1.7, to femoral artery 2.6; umbilical vein to carotid artery 1.9, to femoral artery 3.4, to umbilical artery 5.1, and umbilical artery to umbilical vein 5.4 sec. These rapid transits suggest that changes in placenta and peripheral tissues will quickly affect one another. The time for a complete circuit of blood through the fetus was 12.6 (+/-1.0 SEM) sec. Following dye dilunjections into an umbilical vein double peaks, separated by 4.9 sec, were recorded in peripheral arteries (44 observations), and the divergence localized to the ductus venosus. Based on relative areas under the two peaks, ductus flow averaged 0.425 +/- 0.052 SEM of total flow returning from the placenta with the remainder taking a slow course through liver parenchyma.

Animals↗