Measurement of umbilical blood flow in fetal lambs in utero.

We continuously measured umbilical blood flow in fetal lambs in utero by placing an electromagnetic flow transducer around the common umbilical artery. Umbilical arteries originate from a short common segment as the terminal branches of the descending aorta. This segment was isolated by a retroperitoneal surgical approach and encircled with a specially constructed electromagnetic flow transducer. Catheters were also placed in fetal vessles to monitor pressure and derive flow values by the radionuclide-labeled microsphere technique. The fetus and ewe were allowed to recover for two days before studies were performed. Average umbilical blood flow obtained in 11 animals with the transducer was 199 ml/kg per min. In seven animals flow measurements obtained with the transducer were compared with those derived from microsphere injections. Paired measurements varied by an average of only 5.3%. This technique makes possible the accurate and instantaneous measurement of umbilical blood flow in fetal lambs in utero over a prolonged period.     

Effects of reducing uterine blood flow on fetal blood flow distribution and oxygen delivery.

We examined the effect of graded reduction in uterine blood flow on distribution of cardiac output and oxygen delivery to fetal organs and venous blood flow patterns in 9 fetal sheep using the radionuclide-labeled microsphere technique. We reduced uterine blood flow in two steps, decreasing fetal oxygen delivery to 70% and 50% of normal, and compared the results with those from a similar study from our laboratory on graded umbilical cord compression. With 50% reduction in fetal oxygen delivery, blood flow and the fraction of the cardiac output distributed to the brain, heart, and adrenal gland increased and that to the lungs, carcass, skin, and scalp decreased. Oxygen delivery to the brain and myocardium was maintained, while that to the adrenal doubled, and that to the brain stem increased transiently. The decrease in oxygen delivery to both carcass and lower body segment correlated linearly with oxygen consumption (P less than 0.001). The proportion of umbilical venous blood passing through the ductus venosus increased from 44.6% to 53% (P less than 0.05). The preferential distribution of ductus venosus blood flow through the foramen ovale to the heart and brain increased, but that to the upper carcass decreased so that ductus venosus-derived blood flow to the upper body did not change. Hence, the oxygen delivered to the brain from the ductus venosus was maintained, and that to the heart increased 54% even though ductus venosus-derived oxygen delivery to the upper body fell 34%. Abdominal inferior vena caval blood flow and its contribution to cardiac output decreased, but the proportion of the abdominal inferior vena caval blood distributed through the foramen ovale also increased from 23.0 to 30.9%. However, the actual amount of inferior vena caval blood passing through the foramen ovale did not change. There was a 70% fall in oxygen delivery to the upper body segment from the inferior vena cava. A greater portion of superior vena caval blood was also shunted through the foramen ovale to the upper body, but the actual amounts of blood and oxygen delivered to the upper body from this source were small. Thus, graded reduction of uterine blood flow causes a redistribution of fetal oxygen delivery and of venous flow patterns, which is clearly different from that observed previously during graded umbilical cord occlusion.     

Measurement of blood flow and oxygen consumption in the pelvic limb of fetal sheep

In order to determine blood flow and oxygen consumption in the pelvic limb of fetal sheep, we applied the Fick principle of measurement of oxygen consumption in seven paired experiments in seven fetal sheep under normal conditions and after treatment with pancuronium bromide. Catheterization procedures, which minimized interference with the study limb circulation, avoided changes of catheter tip position during fetal movements,n and prevented collateral circulation to and from tissues not located in the pelvic limb, were utilized. Blood flow through the external iliac artery was measured by means of a transit time ultrasonic method. Six sample sets for oxygen content were drawn from the external iliac artery and vein during 45-min control period and repeated after neuromuscular blockade. Normal oxygen consumption under these experimental conditions was determined to be 20.7 +/- 1.9 (mean +/- SEM) mumole.min-1.100 g-1. Neuromuscular blockade caused oxygen consumption to decrease significantly (P less than 0.01) by 12% to 18.1 +/- 2.1 mumole.min-1.100 g-1 and decreased the average coefficient of variation from 15 to 8%. The data demonstrate that spontaneous skeletal muscle activity accounts for a significant amount of oxygen consumption, the level of which can vary widely over brief periods of time. These results suggest that such tissues with significant spontaneous changes in metabolic activity require repeated blood flow measurements with simultaneous determination of substrate arteriovenous differences to best describe metabolism under normal conditions.     

Simulation of pressure drop and energy dissipation for blood flow in a human fetal bifurcation.

The pressure drop from the umbilical vein to the heart plays a vital part in human fetal circulation. The bulk of the pressure drop is believed to take place at the inlet of the ductus venosus, a short narrow branch of the umbilical vein. In this study a generalized Bernoulli formulation was deduced to estimate this pressure drop. The model contains an energy dissipation term and flow-scaled velocities and pressures. The flow-scaled variables are related to their corresponding spatial mean velocities and pressures by certain shape factors. Further, based on physiological measurements, we established a simplified, rigid-walled, three-dimensional computational model of the umbilical vein and ductus venosus bifurcation for stationary flow conditions. Simulations were carried out for Reynolds numbers and umbilical vein curvature ratios in their respective physiological ranges. The shape factors in the Bernoulli formulation were then estimated for our computational models. They showed no significant Reynolds number or curvature ratio dependency. Further, the energy dissipation in our models was estimated to constitute 24 to 31 percent of the pressure drop, depending on the Reynolds number and the curvature ratio. The energy dissipation should therefore be taken into account in pressure drop estimates.     

Measurement of renal blood flow in the rat.

The radioactive microsphere technique is a simple method for measurement of RBF and intrarenal blood flow distribution in the rat that does not require surgical manipulation of the kidney or general anesthesia. The results are reproducible and compatible with other established techniques.     

Measurement of gastric blood flow with radioactive microspheres.

The validity of using radioactive microspheres (15 +/- 5-mum diameter) to measure gastric blood flow and its partition between gastric wall layers was investigated in anesthetized dogs with a chambered segment of gastric corpus. Total flow measured by a venous effluent technique demonstrated close correlation with microsphere-measured flow (r = 0.98, slope = 0.95) in 12 dogs given histamine, gastrin, or isoproterenol. In 12 histamine-stimulated dogs, mucosal flow measured by aminopyrine clearance and by microspheres also showed good agreement (r = 0.96, slope = 0.83). No evidence was found to indicate that microspheres altered hemodynamic or gastric function. In all experiments less than 1% of the total gastric radioactivity passed through arteriovenous shunts. The mucosa always contained a statistically adequate number of spheres (greater than 400), but the submucosa and muscularis frequently did not. Microspheres of all sizes mixed adequately in large arteries, but a significant difference was found in the distribution of 16- and 26 mum spheres between mucosa and submucosa, presumably because of streaming of the larger spheres past mucosal arteries. It was concluded that, with the techniques developed in our laboratory, microspheres could be a highly useful tool for quantitating gastric regional blood flow under a variety of experimental conditions.     

Cerebral blood flow in the fetal guinea-pig

To measure brain blood flow in the fetal guinea-pig, radioactive microspheres were injected in the lateral saphenous vein whilst a reference sample of blood was withdrawn from the right axillary artery. Measurements were made near term of pregnancy, on the 60th-66th day, during anaesthesia with pentobarbitone and diazepam. Fetal blood pressure was 4.25 +/- 0.12 kPa and fetal heart rate was 250 +/- 7 beats per min. The arterial oxygen content varied between 1.9-5.1 mmol 1(-1). Blood flow to the whole brain (mean 1.13 +/- 0.14 ml min-1 g-1) was significantly correlated to the reciprocal of arterial oxygen content (r = 0.84). Four regions of the brain were examined: the cerebral hemispheres, the cerebellum, the thalamus and midbrain, and the pons and medulla. In each region blood flow was inversely related to arterial oxygen content (r = 0.80-0.83) but the rate of perfusion of the brain stem was greater than that of the cerebral hemispheres or cerebellum.     

A software tool for fetal blood flow analysis

Doppler ultrasonography is a widely used technique for determination of the fetal blood flow pattern. Determination of the waveform qualities was done manually, with considerable inter- and intraobserver variations. In order to limit the variations and the time-consuming data entry, a Fetal Blood Flow Analysis software program was developed to facilitate ease of determination of Doppler signals. This article describes the development and unique features of the software program, made specifically to meet the obstetric and gynecology department's needs.     

Behavioral state transition and local cerebral blood flow in fetal sheep.

Local cerebral blood flow in four near term fetal sheep was evaluated continuously before and after natural alternations in fetal behavioral state. Measurements were made in fetuses several days following an aseptic surgery to place electrodes for behavioral state recordings as well as heated and reference thermojunctions in cortical and subcortical tissue. These thermojunctions were used to qualitatively assess local cerebral blood flow. The time of transition between rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) was based on visual inspection of strip chart recordings of electrocortical, electroocular, and neck electromyographic activity and application of published criteria for their assessment. To confirm that transition had occurred, the amplitude of the spectrum of the electrocorticogram in one-third octave bands centered around 1 Hz and 20 Hz was measured before and after the transition point. Mean cerebral blood flow rose significantly by 24 s (P less than 0.05) after the transition from NREMS to REMS and fell by 12 s after the transition from REMS to NREMS (P less than 0.05).     

Ovine fetal electrocortical activity and regional cerebral blood flow

Fetuses of 12 near-term sheep were prepared for microsphere determination of cerebral blood flow. Experiments were performed 5 days postsurgery. The regional blood flows were measured in successive high (HV), low (LV) and high voltage electrocorticographic states. Comparisons were made between the observations made in the LV and averaged flanking HV cycles. Total cerebral blood flow was 95 +/- 8, 119 +/- 11 and 100 +/- 9 ml/min/100 g in HV, LV and HV, respectively. Low voltage electrocortical activity increased average cerebral blood flow by 22% (P less than 0.01). Significant changes were seen in all regions except the occipital cortex. The maximum change was observed in the thalamus in which the flows were 152 +/- 23, 243 +/- 35 and 138 +/- 20 ml/min/per 100 g tissue, respectively. The increase was 68% (P less than 0.001). The percent changes seen in the cerebrum are as follows: Frontal grey + 18%, frontal white + 22%, parietal white + 22%, temporal + 18%. A + 17% change was seen in the cord (P less than 0.03). It is concluded that in low voltage electrocortical activity all of the brain, except the occipital region, shows an increase in cerebral blood flow. This is probably secondary to a variance in cerebral activity. This preparation may be useful in localizing function in the fetal brain.     

Effect of ACE inhibition on the fetal kidney: decreased renal blood flow.

The kidneys of nine fetuses whose mothers were chronically hypertensive were examined microscopically. Three of these mothers used antihypertensive agents throughout pregnancy including one who used an angiotensin-converting enzyme (ACE) inhibitor. The tubular defects found in these kidneys were compared to the kidneys of 20 normal controls, 13 fetuses with various multiple malformation syndromes and six cases of the twin to twin transfusion syndrome. Evidence from these cases as well as the literature suggest that the primary mechanism by which ACE inhibitors affect development of the fetal kidney is through decreased renal blood flow.     

Adaptation of laser-Doppler flowmetry to measure cerebral blood flow in the fetal sheep.

The purpose of this study was to devise a means to use laser-Doppler flowmetry to measure cerebral perfusion before birth. The method has not been used previously, largely because of intrauterine movement artifacts. To minimize movement artifacts, a probe holder was molded from epoxy putty to the contour of the fetal skull. A curved 18-gauge needle was embedded in the holder. At surgery, the holder, probe, and skull were fixed together with tissue glue. Residual signals were recorded after fetal death and after maternal death 1 h later. These averaged <5% of baseline flow signals, indicating minimal movement artifact. To test the usefulness of the method, cerebral flow responses were measured during moderate fetal hypoxia induced by giving the ewes approximately 10% oxygen in nitrogen to breathe. As fetal arterial PO(2) decreased from 21.1 +/- 0.5 to 10.7 +/- 0.4 Torr during a 30-min period, cerebral perfusion increased progressively to 56 +/- 8% above baseline. Perfusion then returned to baseline levels during a 30-min recovery period. These responses are quantitatively similar to those spot observations that have been recorded earlier using labeled microspheres. We conclude that cerebral perfusion can be successfully measured by using laser-Doppler flowmetry with the unanesthetized, chronically prepared fetal sheep as an experimental model. With this method, relative changes of perfusion from a small volume of the ovine fetal brain can be measured on a continuous basis, and movement artifacts can be reduced to 5% of measured flow values.     

Heterogeneity of human adipose blood flow

Background

The long time pharmacokinetics of highly lipid soluble compounds is dominated by blood-adipose tissue exchange and depends on the magnitude and heterogeneity of adipose blood flow. Because the adipose tissue is an infinite sink at short times (hours), the kinetics must be followed for days in order to determine if the adipose perfusion is heterogeneous. The purpose of this paper is to quantitate human adipose blood flow heterogeneity and determine its importance for human pharmacokinetics.

Methods

The heterogeneity was determined using a physiologically based pharmacokinetic model (PBPK) to describe the 6 day volatile anesthetic data previously published by Yasuda et. al. The analysis uses the freely available software PKQuest and incorporates perfusion-ventilation mismatch and time dependent parameters that varied from the anesthetized to the ambulatory period. This heterogeneous adipose perfusion PBPK model was then tested by applying it to the previously published cannabidiol data of Ohlsson et. al. and the cannabinol data of Johansson et. al.

Results

The volatile anesthetic kinetics at early times have only a weak dependence on adipose blood flow while at long times the pharmacokinetics are dominated by the adipose flow and are independent of muscle blood flow. At least 2 adipose compartments with different perfusion rates (0.074 and 0.014 l/kg/min) were needed to describe the anesthetic data. This heterogeneous adipose PBPK model also provided a good fit to the cannabinol data.

Conclusion

Human adipose blood flow is markedly heterogeneous, varying by at least 5 fold. This heterogeneity significantly influences the long time pharmacokinetics of the volatile anesthetics and tetrahydrocannabinol. In contrast, using this same PBPK model it can be shown that the long time pharmacokinetics of the persistent lipophilic compounds (dioxins, PCBs) do not depend on adipose blood flow. The ability of the same PBPK model to describe both the anesthetic and cannabinol kinetics provides direct qualitative evidence that their kinetics are flow limited and that there is no significant adipose tissue diffusion limitation.