Distribution of cardiac output in fetal and neonatal lambs with acute respiratory acidosis

Impact of chronic hypoxemia on blood flow to the brain, heart, and adrenal gland in the late-gestation IUGR sheep fetus

In the fetus, there is a redistribution of cardiac output in response to acute hypoxemia, to maintain perfusion of key organs, including the brain, heart, and adrenal glands. There may be a similar redistribution of cardiac output in the chronically hypoxemic, intrauterine growth-restricted fetus. Surgical removal of uterine caruncles in nonpregnant ewe results in the restriction of placental growth (PR) and intrauterine growth. Vascular catheters were implanted in seven control and six PR fetal sheep, and blood flow to organs was determined using microspheres. Placental and fetal weight was significantly reduced in the PR group. Despite an increase in the relative brain weight in the PR group, there was no difference in blood flow to the brain between the groups, although PR fetuses had higher blood flow to the temporal lobe. Adrenal blood flow was significantly higher in PR fetuses, and there was a direct relationship between mean gestational PaO2 and blood flow to the adrenal gland. There was no change in blood flow, but a decrease in oxygen and glucose delivery to the heart in the PR fetuses. In another group, there was a decrease in femoral artery blood flow in the PR compared with the Control group, and this may support blood flow changes to the adrenal and temporal lobe. In contrast to the response to acute hypoxemia, these data show that there is a redistribution of blood flow to the adrenals and temporal lobe, but not the heart or whole brain, in chronically hypoxemic PR fetuses in late gestation.

Cerebral blood flow and metabolism in the developing fetus

The inaccessibility of the human fetal brain to studies of perfusion and metabolism has impeded progress in the understanding of the normal and abnormal systems of oxygen substrate supply and demand. Consequently, current understanding is based on studies in fetal animals or in the premature infant (ex utero fetus), neither of which is ideal. Despite promising developments in fetal magnetic resonance imaging (MRI) and Doppler ultrasound, major advances in fetal neurodiagnostics will be required before rational and truly informed brainoriented care of the fetus becomes feasible.

The effects of repeated exposure to hypercapnia on arousal and cardiorespiratory responses during sleep in lambs

Arousal and cardio-respiratory responses to respiratory stimuli during sleep are important protective mechanisms that rapidly become depressed in the active sleep state when episodes of hypoxia or asphyxia are repeated: whether responses to repeated hypercapnia are similarly depressed is not known. This study aimed to determine if arousal and cardio-respiratory responses also become depressed with repeated episodes of hypercapnia during sleep and whether responses differ in active sleep and quiet sleep. Eight newborn lambs were instrumented to record sleep state and cardio-respiratory variables. Lambs were subjected to two successive 12 h sleep recordings, assigned as either sequential control and test days, or test and control days performed between 12.00 and 00.00 h. The control day was a baseline study in which the lambs breathed air to determine spontaneous arousal probability. During the test day, lambs were exposed to a 60 s episode of normoxic hypercapnia (Fractional inspired CO2 (F(ICO2)) = 0.08 and Fractional inspired O2(F(IO2)) = 0.21 in N2) during every quiet sleep and active sleep epoch. The probability of lambs arousing during the hypercapnic exposure exceeded the probability of spontaneous arousal during quiet sleep (58% versus 21%, chi2 = 54.0, P < 0.001) and active sleep (39% versus 20%, chi2 = 10.0, P < 0.01), though the response was less in active sleep. Exposure to hypercapnia also resulted in a significant increase in ventilation in quiet sleep (150 +/- 22%) and active sleep (97 +/- 23%, P < 0.05), though the increase was smaller in active sleep (P < 0.05). Small (< 5%) blood pressure increases and heart rate decreases were evident during hypercapnia in quiet sleep, but not in active sleep. Arousal and cardio-respiratory responses persisted with repetition of the hypercapnic exposure. In summary, although arousal and cardio-respiratory responses to hypercapnia are less in active sleep compared with quiet sleep, these protective responses are not diminished with repeated exposure to hypercapnia.

Effect of rhIGF-I infusion on whole fetal and fetal skeletal muscle protein metabolism in sheep

Insulin-like growth factor I (IGF-I) has been shown to have significant anabolic effects in the regulation of fetal protein metabolism. To investigate the tissue-specific effects of IGF-I on fetal skeletal muscle metabolism, we infused recombinant human (rh) IGF-I directly into the hindlimb of nine chronically catheterized, late-gestation fetal sheep. Substrate balance and amino acid kinetics were measured across the hindlimb and were compared with the effects at the whole body level before and during a 3-h infusion of rhIGF-I into the external iliac artery at 150 microgram/h. Infusion of rhIGF-I resulted in increases in IGF-I concentrations by 2- to 5. 75-fold in the ipsilateral iliac vein and by nearly 3-fold in the abdominal aorta. In the study limb, IGF-I had no effect on protein synthesis (phenylalanine rate of disposal 0.88 +/- 0.13 before vs. 0. 73 +/- 0.19 micromol/min during IGF-I) or breakdown (phenylalanine rate of appearance 0.67 +/- 0.13 before vs. 0.60 +/- 0.17 micromol/min during IGF-I) and did not alter net phenylalanine balance. IGF-I also did not affect hindlimb oxygen or glucose uptake. In contrast, at the whole body level, the rate of appearance of leucine, indicative of fetal protein breakdown, decreased during IGF-I infusion (rate of appearance of leucine 41.1 +/- 3.3 to 37.6 +/- 2.7 micromol/min) as did fetal leucine oxidation (8.4 +/- 0.8 to 6.8 +/- 0.6 micromol/min). There was no change in the umbilical uptake of leucine, and although not statistically significant, fetal leucine accretion increased 2.4-fold. These results provide further evidence that IGF-I promotes fetal protein accretion; however, its site of action is in tissues other than skeletal muscle.

Cerebral hemodynamics and distribution of left ventricular output during inhalation of nitric oxide

OBJECTIVES

Inhaled nitric oxide is being utilized as a selective pulmonary vasodilator in the treatment of persistent pulmonary hypertension of the newborn. However, the effects of inhaled nitric oxide on cerebral hemodynamics and distribution of left ventricular output in newborn subjects have not been studied. This study was designed to measure quantitatively the effect of inhaled nitric oxide on the distribution of left ventricular output and on cerebral hemodynamics in a perinatal animal model.

DESIGN

Prospective, controlled, experimental study.

SETTING

Research laboratory.

SUBJECTS

Eight fetal sheep.

INTERVENTIONS

Each animal was exposed to three separate study periods: a) mechanical ventilation with low FIO2 (maintaining fetal levels of PaO2); b) inhalation of nitric oxide (20 parts per million) during mechanical ventilation and low FIO2; and c) mechanical ventilation with an FIO2 of 1.0.

MEASUREMENTS AND MAIN RESULTS

Left ventricular output and cerebral blood flow were measured with radiolabeled microspheres. Cerebral oxygen delivery and consumption variables were calculated using measurements of arterial and cerebral venous (sagittal sinus) oxygen content. Total left ventricular output did not differ among the three treatment groups: 235 +/- 16 mL/min/kg with hypoxic ventilation; 283 +/- 13 mL/min/kg with nitric oxide inhalation; and 242 +/- 17 mL/min/kg with an FIO2 of 1.0. Lung blood flow increased 2.7-fold with inhaled nitric oxide and 1.6-fold during mechanical ventilation with an FIO2 of 1.0. With a left ventricle microsphere injection, increased lung blood flow is indicative of increased systemic-to-pulmonary shunt across the ductus arteriosus. Whole brain blood flow did not differ between the three groups: 49.6 +/- 6.7 mL/min/100 g with hypoxic ventilation; 46.4 +/- 7.4 mL/min/100 g with nitric oxide inhalation; and 36.4 +/- 3.8 mL/min/100 g with an FIO2 of 1.0. Cerebral oxygen delivery increased during inhalation of an FIO2 of 1.0 when compared with nitric oxide inhalation (p < .007); fractional extraction of oxygen decreased (p < .004 compared with hypoxic ventilation, p < .0005 compared with nitric oxide inhalation). Cerebral oxygen consumption did not differ between the three groups (1.11 +/- 0.12 microns/min/100 g with hypoxic ventilation, 0.95 +/- 0.12 microns/min/100 g with nitric oxide inhalation, and 0.96 +/- 0.08 microns/min/100 g with an FIO2 of 1.0).

CONCLUSION

Acute pulmonary vasodilation caused by inhalation of nitric oxide does not change left ventricular output, cerebral blood flow, or cerebral oxygen consumption, despite an increased systemic-to-pulmonary shunt across the ductus arteriosus.

Rapid accumulation of elastin and collagen in the aortas of sheep in the immediate perinatal period

While characterizing developmental changes in aortic wall composition in sheep, we observed very rapid accumulation of elastin and collagen in the immediate perinatal period. Thoracic aortic elastin content increased by 41%, and collagen content increased by 49% in approximately 1 week, between 140 days gestation and 3 days postpartum (term = 145 days). Even larger changes were observed in the abdominal aorta. Elastin content increased by 66%, and collagen increased by 57%. The pronounced increase in wall tissue accumulation near birth preceded a marked postnatal increase in arterial pressure. We propose that this elastin and collagen accumulation is a preadaptive response in preparation for the later increase in pressure. The prenatal and postnatal events that initiate this synthesis and accumulation are not known. We also found that, in the 3 weeks after this initial rapid increase, accumulation of elastin and collagen was markedly reduced in the abdominal, but not the thoracic, aorta. This latter finding may be linked to the dramatic decrease in flow through this vessel that results from the loss of the placental circulation. Finally, we observed that relatively high smooth muscle cell replication rates in the abdominal aorta postpartum resulted in no net DNA accumulation. This finding indicates that cell turnover plays an important role in postnatal arterial growth and development.

Effects of lactic acid infusions and pH on cerebral blood flow and metabolism

To determine the effects of lactic acidemia versus lactate on CBF, we infused lactic acid, either buffered with NaOH (L + NaOH) or with added NaCl (L + NaCl), to attain similar osmolalities in 18 piglets. CBF (microsphere technique), pH, blood gases, plasma osmolality, and cerebral arteriovenous differences of O2 content and lactic acid concentrations were measured prior to, at 30 min of a lactic acid infusion, and 15 and 90 min after completion of the infusion. Control arterial pH was comparable between groups (7.50 +/- 0.02 vs. 7.49 +/- 0.02, X +/- SE); during and following L + NaCl and L + NaOH, values were (p less than 0.05) 7.09 +/- 0.03, 7.35 +/- 0.02, and 7.46 +/- 0.02 vs. 7.58 +/- 0.03, 7.61 +/- 0.01, and 7.57 +/- 0.03, respectively. PaCO2 remained unchanged and osmolality rose by 15% in both groups during infusions and persisted throughout the study period. For L + NaCl piglets, CBF (ml/min.100 g) rose from 136 +/- 15 to 198 +/- 26 (p less than 0.05) at 30 min of infusion and remained elevated at 201 +/- 25 and 207 +/- 28 at 15 and 90 min following the infusion, respectively. Similarly, for L + NaOH piglets, CBF rose from 130 +/- 25 to 196 +/- 31 (p less than 0.05) with the infusion and was 174 +/- 17 and 166 +/- 21 at 15 and 90 min afterward, respectively. Although lactic acid infusion increases CBF, the associated metabolic acidemia is not responsible for changes in CBF.

Maturation of circulatory system in three mammalian models of human development

The review surveys the literature on maturation of vasoconstrictor and vasodilator functions in cerebral, renal and intestinal circulations of three non-primate models of human development. An ovine model has been refined for use at both fetal and neonatal stages of development. Important variables controlling regional circulations in the lamb fetus at term include arterial O2 content and pCO2 (brain), angiotensin-II (kidney) and norepinephrine (small intestine). Blood flow autoregulation to decreasing perfusion pressure has been inferred for the renal circulation of the neonate. A canine model has been employed in the postnatal period, usually later than the first week after birth. Important variables controlling regional circulations in the young puppy include arterial pO2 and pCO2 (brain) and epinephrine and angiotensin-II (kidney). Blood flow autoregulation to decreasing pressure has been demonstrated in the cerebral circulation at birth and in the renal circulation at one week thereafter. The intestinal circulation has not been studied with respect to blood flow control. A porcine model has been examined from birth through at least two months of postnatal life. Important variables controlling regional circulations in swine at birth include adrenergic nerve stimulation, arterial pCO2 (brain), angiotensin-II (kidney) and norepinephrine (kidney and small intestine). Blood flow autoregulation to decreasing perfusion pressure has been demonstrated in the brain by the fourth day, in the kidney by the end of the second week and in the small intestine by the end of the first month after birth. The advantage of each model for further investigation of functional maturation of regional circulatory control is summarized.