Measurement of flow in perfused organs, using microsphere techniques

Injection of radionuclide labelled microspheres into the circulation of animals can be used to measure the proportion of blood flow to any organ, or the regional distribution of blood flow within any organ. The use of this technique depends on the fact that these microspheres are distributed according to the blood flow and that they are completely trapped within the organ. It is important that they are adequately mixed within the flowing stream and also evenly distributed at branching sites. It has been shown that for the assessment of the regional distribution within an organ, microspheres of small diameter (10–15 μm) are preferable since they more accurately reflect true flow to all segments of the organ. The accurate measurement of the distribution of flow is also dependent on the absolute number of microspheres present in any organ or segment of organ under study.

Studies of regional distribution of flow during cardio-pulmonary bypass in rhesus monkeys have shown that the proportion of cardiac output distributed to the kidneys falls, while that to the gut increases. The haematocrit of the perfusing blood may modify the distribution of flow to organs; thus, cerebral blood flow was maintained while using blood of low haematocrit whereas with normal haematocrit it fell during perfusion. The regional distribution of the blood flow within the kidney has been shown to change markedly with artificial perfusion in situ with a striking increase in flow to the medulla and inner cortex during perfusion as compared to normal.

Circulatory changes during growth in the fetal lamb

The changes in circulation with advancing gestation were investigated in 44 fetal lambs in utero; gestational ages ranged from about 60 days to 150 days. Spinal analgesia was administered to the ewe, polyvinyl catheters were inserted into fetal vessels, and umbilical blood flow was measured by the steady-state diffusion Fick method during antipyrine infusion. Cardiac output, distribution of cardiac output, and actual organ blood flows were calculated from injections of nuclide-labeled microspheres (50µ diam) into a forelimb and into an umbilical or hindlimb vein.

Umbilical Po 2 , Pco 2 and pH did not change significantly during gestation. Umbilical blood flow and total cardiac output increased in proportion to fetal weight. The proportion of the combined ventricular output distributed to the placenta decreased from about 50% in the youngest fetuses to about 40% just before term. The proportion of the cardiac output distributed to the lungs, as well as the actual flow in relation to lung weight, increased throughout gestation, with a more rapid rise after about 120 days. There was also a late increase in intestinal flow. Cerebral blood flow increased gradually throughout gestation, both as a proportion of cardiac output and in relation to brain weight. There were no significant changes in percent of cardiac output, or flow related to weight in the kidney, heart, or skin and muscular tissues. The studies suggest that, since lung blood flow is a relatively small proportion of total cardiac output, it is not important in regulating distribution of blood flow, but that the peripheral circulation in skin and muscle, which receives a large percent of fetal cardiac output, is the site where vasomotor responses may effect major redistribution of the fetal circulation.

Bradykinin production associated with oxygenation of the fetal lamb

Previous studies showed that synthetic bradykinin produced in-vitro constriction of the ductus arteriosus of the fetal lamb and guinea pig, constriction of isolated human and lamb umbilical vessels, and pulmonary vasodilatation in the fetal lamb.

The present study showed that bradykinin precursor, kininogen, is present in arterial blood of the fetal lamb by 61 days of gestation and that its concentration increases toward term. We studied the bradykinin-generating system in 6 exteriorized fetal lambs. Kininogen concentrations in left atrial blood decreased within 1 to 2 minutes after beginning ventilation with O 2 , and free bradykinin was detected in left atrial blood. Pulmonary arterial blood kininogen concentration was not significantly altered. Kininogen concentration in left atrial blood did not fall in 4 other lambs ventilated with N 2 but did fall after subsequent ventilation with O 2 . To study the effects of increased oxygenation in utero without lung expansion, 6 pregnant ewes were exposed to hyperbaric O 2 at 3.63 atmospheres absolute. Associated with a mean increase in brachial arterial blood Po 2 to 44 mm Hg, a value equivalent to that in the ventilated exteriorized fetus, kininogen concentration fell, and free bradykinin was detected in the brachial arterial blood of 3 of the 6 fetuses. In 4 other exteriorized lambs we measured kininogen concentration in brachial arterial and right ventricular blood at frequent intervals up to 30 minutes after ventilation with O 2 . The concentration in brachial arterial blood fell before that in right ventricular blood.

The hypothesis is presented diat bradykinin is produced from kininogen in the lungs of the fetal lamb when oxygenated, and that the maximal rate of production occurs during the first few minutes after expansion of the lungs or exposure of the ewe and fetus to hyperbaric O 2 .

Effect of exteriorization of the sheep fetus on its cardiovascular function

Exteriorized sheep or goat fetuses, with placental circulation continuing, have been extensively used for measurements of fetal cardiovascular function. However, the effects of this procedure on fetal cardiovascular function have not been assessed. We inserted vinyl catheters into an umbilical vein, an umbilical or fetal hindlimb artery, and a hindlimb vein. We measured umbilical arterial and venous blood gas tensions, umbilical blood flow using the steadystate diffusion antipyrine method, and fetal arterial and umbilical venous pressures before and after delivery by cesarean section. Following exteriorization, placental vascular resistance consistently increased and umbilical blood flow decreased. Umbilical venous Po 2 increased, but umbilical arterial and venous pH and Pco 2 and arterial Po 2 did not change significantly. Thus exteriorization of the fetus produces major changes in the fetal circulation, but these changes are not indicated by any major alterations in fetal blood gases and pH.

The circulation of the fetus in utero. Methods for studying distribution of blood flow, cardiac output and organ blood flow

Techniques are described for insertion of vinyl catheters into the umbilical and limb vessels of the fetus of the sheep or the goat through small uterine incisions, with the ewes under spinal analgesia. The catheters are exteriorized and the fetus can be studied in its normal intrauterine environment. During constant infusion of antipyrine into a fetal limb vein, placental arteriovenous difference of antipyrine was measured, and fetal umbilical blood flow was calculated by the Fick method. "Carbonized" microspheres (50-µ diameter) labeled with various nuclides were injected into different venous sites in the fetus. The distribution pattern of the microspheres was used to determine the relative distribution of blood flow. Experimental evidence is provided that (1) there is no significant recirculation of microspheres, (2) the distribution of spheres is proportional to flow, and (3) circulatory physiology is not altered by injection of spheres. Quantitative data on the distribution of umbilical venous and superior and inferior vena caval return were obtained. It was possible to determine the actual blood flow to each of the fetal organs by relating the proportions of nuclide in each organ to that in the placenta. Total cardiac output was then calculable, taking into consideration the hemodynamic arrangement of the fetal circulation.

Validation of the antipyrine method for measuring fetal umbilical blood flow

Vinyl catheters were inserted into an umbilical vein, a hindlimb vein and either an umbilical or dorsalis pedis artery in 9 lamb fetuses with gestational ages of 110 to 147 days. The lambs were then delivered from the uterus, but placental flow was maintained. Cannulating electromagnetic flow transducers were inserted into both umbilical veins. Antipyrine solution was constantly infused into the hindlimb vein, and after equilibration, umbilical blood flow was calculated by the Fick method. The values for umbilical flow measured by the antipyrine technique were comparable with those recorded simultaneously with electromagnetic flowmeters. There was also no difference in antipyrine concentrations in the peripheral and main umbilical veins. The study thus validates the use of the Fick method with continuous antipyrine infusion for measuring fetal umbilical blood flow.