Preterm Baboons Research Articles

Direct fetal glucocorticoid treatment alters postnatal adaptation in premature newborn baboons.

Abnormalities of premature newborn adaptation after preterm birth result in significant perinatal mortality and morbidity. We assessed the effects of short-term (24 h) fetal betamethasone exposure on preterm newborn baboon pulmonary and cardiovascular regulation and renal sodium handling during the first 24 h after birth. Male fetal baboons (Papio) (124-day gestation, term 185 days) received ultrasound-guided intramuscular injections of saline (n = 5) or betamethasone (0.5 mg/kg; n = 5). Fetuses were cesarean delivered 24 h later, treated with 100 mg/kg surfactant, and ventilated by adjusting peak inspiratory pressures to maintain PCO2 values of 35-50 mmHg for 24 h. Betamethasone- vs. saline-treated mean +/- SE newborn body weights (0.45 +/- 0.02 vs. 0.41 +/- 0.01 kg) were similar. Although prenatal betamethasone did not affect postnatal lung function (PCO2, arterial/alveolar O2 gradient, or dynamic compliance), plasma hormone (cortisol or thyroxine), or catecholamine levels, mean arterial pressure (25 +/- 1 vs. 32 +/- 1 mmHg), plasma sodium concentration (132 +/- 2 vs. 138 +/- 1 meq/l), glomerular filtration rate (0.07 +/- 0.02 vs. 0.16 +/- 0.02 ml.min-1.kg-1), and renal total sodium reabsorption (1.5 +/- 0.5 vs. 16.0 +/- 3.0 mu eq.min-1.kg-1) values were significantly lower in saline-treated than in betamethasone-treated newborns at 24 h. We conclude that despite the fact that there are no pulmonary and endocrine effects, antenatal glucocorticoid exposure alters premature newborn baboon vascular and renal glomerular function and improves sodium reabsorption after preterm delivery.

Surfactant Metabolism Studied with Stable Isotopes in Preterm Baboons Exposed to Prenatal Betamethasone † 1619

Surfactant Metabolism Studied with Stable Isotopes in Preterm Baboons Exposed to Prenatal Betamethasone † 1619

Hypoxia initiates Ductus Arteriosus Remodeling. • 101

After birth, the ductus arteriosus actively constricts in response to the rise in PaO2 and undergoes extensive histologic changes that prevent it from subsequent reopening. We used 7 full-term (1-5 d.o.) and 21 preterm (0.67 gestation) newborn baboons to test the hypothesis that following the initial constriction of the ductus, a region of hypoxia develops within the vessel wall that initiates the subsequent histologic changes. Twenty-four hours prior to sacrifice, we injected the baboons with the drug EF5, a nontoxic 2-nitroimadazole, which binds irreversibly to intact cells at a rate that increases 100-fold as the PO2 decreases from air to N2. EF5's plasma half-life is similar in both preterm and full-term baboons. Using a fluorochrome-conjugated mAb to bound EF5, we can detect changes in EF5 accumulation within the tissue by immunohistochemistry. In the full-term baboons, pulsed-Doppler exam on day 1 showed that the ductus lumen was closed. In the 1-day-old ductus, an intense zone of hypoxia (EF5 staining) was detected in the middle of the muscle media. In addition, all the smooth muscle cells in this intense hypoxic zone were undergoing apoptosis (TUNEL technique). By day 2 and 3, vascular endothelial growth factor (VEGF) was expressed in the muscle media on either side of the hypoxic zone; endothelial cells of the ductus lumen and vasa vasorum began to proliferate (began to express proliferating cell nuclear antigen [PCNA]) and invade the muscle media toward the hypoxic zone (which was becoming acellular). Among the 21 preterm baboons, pulsed-Doppler exams found that only 8 had closed their ductus by 6 d after birth. At necropsy on day 6, there was either no or negligible hypoxia(EF5 binding) even in those with a closed ductus on Doppler exam. There was no apoptosis, or staining for VEGF or PCNA. In contrast with the full-term ductus, the preterm ductus, despite Doppler evidence of closure, fails to develop a zone of hypoxia in the muscle media and fails to remodel, making it susceptible to later reopening.

Effect of ductal patency on organ blood flow and pulmonary function in the preterm baboon with hyaline membrane disease.

To examine the effect of early ductal ligation vs. maintenance of ductal patency on vital organ perfusion and pulmonary function in premature baboons with hyaline membrane disease. Randomized, controlled interventional study to compare early ligation with formalin infiltration of the ductus arteriosus. Animal care facility at a dedicated research foundation. Eighteen premature baboons delivered by hysterotomy at 140 +/- 2 day gestation. Nine premature baboons underwent formalin infiltration of the ductus arteriosus (group 1), and nine underwent ductal ligation (group 2). Surgical ligation or formalin infiltration was performed at 2 to 4 hrs of age. Animals were maintained on mechanical ventilation and ventilator parameters were adjusted to maintain PaO2 and PaCO2 within the physiologic range. Left ventricular output indexed to body weight and vital organ perfusion were measured at 24 hrs of age by the radiolabeled microsphere method. Lung mechanics, including lung wet/dry weight ratio, total lung water, static compliance and functional residual capacity were measured immediately following euthanasia. Total pulmonary blood flow was significantly lower (p = .0001) in group 2 (mean = 94 mL/min/kg), compared with group 1 (mean = 287 mL/min/kg). Systemic blood flow and effective pulmonary blood flow were higher in group 1 (p = .07). No significant difference between groups in absolute organ blood flow was noted, although flow as a percent of left ventricular index was significantly higher in all organs except the kidney in group 2. There was no difference in arterial blood gas values, parameters of mechanical ventilation, percent lung water, or postmortem measurement of lung mechanics between groups. Early ductal ligation did not result in improved cardiac output, increased organ blood flow, or improved pulmonary function. We postulate that gradual constriction of the ductus arteriosus may play an important role in successful cardiovascular adaptation in the premature infant. While it is clear that premature infants with symptomatic patent ductus arteriosus often benefit from ductal closure, we question the practice of prophylactic early ductal closure.

Brainstem auditory evoked response development in preterm and term baboons ( Papio hamadryas)

Brainstem auditory evoked responses were recorded longitudinally from 11 neonatal baboons ( Papio hamadryas), 6 of which were preterm. Recordings were made in unsedated animals from day 161 to day 362 after conception (term = 182 days). The pattern of development of both waveform morphology and of wave latency was consistent with that seen in the human neonate, with a rapid maturation of the response during the perinatal period, and then a slower development to adult values. Brainstem conduction time was measured from the wave I to wave IV interval, and this demonstrated a similar pattern, with a rapid decrease in latency up to term, and then decreasing more slowly to reach adult values by 4 months of age in the baboon.

Surfactant replacement therapy in utero for prevention of hyaline membrane disease in the preterm baboon

A previous study of intraamniotic administration of surfactant in preterm rabbit fetuses demonstrated that exogenous surfactant can be taken up into the lungs from amniotic fluid in quantities sufficient to alter pulmonary mechanical properties. On the basis of these findings we chose to test the hypothesis that intraamniotic administration of surfactant to the preterm baboon 24 hours before delivery will prevent the development of clinical and pathological aspects of hyaline membrane disease. Characteristics of hyaline membrane disease in the preterm baboon model include atelectasis, the formation of hyaline membranes in the airways and distal air saccules, overexpansion of distal airways, and disruption of airways by barotrauma associated with neonatal intensive care practices. Nine preterm baboons were treated with either saline solution (n = 4) or surfactant (n = 5) by intraamniotic injection on 136 to 137 days' gestation. One day later fetuses were delivered by cesarean section and maintained for 24 hours with standard neonatal intensive care techniques. All neonates completed the protocol, and surfactant-treated animals had better clinical courses as documented by Pao2/PAO2 (p < 0.05), FIO2 (p < 0.005), and Paco2 (p < 0.05). Significant radiographic differences (p < 0.02) were noted to be the result of surfactant treatment. No differences in ventilator pressures or neonatal cardiovascular parameters were found. Significant differences in pulmonary pathologic conditions (p < 0.01) were also found, but all animals had some degree of pathologic pulmonary changes associated with hyaline membrane disease. Therefore a single treatment with surfactant in utero significantly improved the clinical course but did not completely prevent hyaline membrane disease in an established animal model.

Left ventricular dysfunction following ligation of the ductus arteriosus in the preterm baboon : A.F. Taylor, W.R. Morrow, K.P. Lally, et al. J Surg Res 48:590–596, (June), 1990

Left ventricular dysfunction following ligation of the ductus arteriosus in the preterm baboon : A.F. Taylor, W.R. Morrow, K.P. Lally, et al. J Surg Res 48:590–596, (June), 1990

Left ventricular dysfunction following ligation of the ductus arteriosus in the preterm baboon

We evaluated the effect of early closure of the patent ductus arteriosus (PDA) on cardiac performance in a preterm baboon model of hyaline membrane disease. Eleven baboon fetuses were delivered at 75% gestation, resuscitated, and treated with conventional ventilation. The animals were divided into two groups, and underwent left thoracotomy at 2 hr of age. Group 1 ( N = 5) had ductal infiltration with formalin to maintain ductal patency. Group 2 ( N = 6) had ductal ligation. Hemodynamic parameters were compared between groups before and after surgical intervention. Examination of cardiovascular function was performed at intervals over a 24hr period using two-dimensional, M-mode, and pulsed Doppler echocardiography. Ventricular performance was significantly worse in the ligation animals as demonstrated by reduced left and right ventricular outputs and reduced left ventricular shortening. However, pulmonary disease and arterial blood gases were not different. Early ligation was also associated with significantly increased systemic vascular resistance. We conclude that early ductal ligation leads to impaired ventricular performance in the premature primate. We hypothesize that the PDA may protect the immature ventricle by preventing pressure overload during adaptation to postnatal life.

A Comparative Evaluation of the Transport of H2-Receptor Antagonists by the Human and Baboon Placenta

Using a single cotyledon perfusion model, the placental transport of four H2-receptor antagonists, cimetidine, famotidine, nizatidine, and ranitidine, was determined and compared using normal term human and normal preterm baboon placentas. In both the human and baboon placentas, the transport of each agent was similar whether administered singly or in combination with the other drugs. Drug transport was the same in both directions, maternal-to-fetal and vice versa, indicating a lack of preferential transfer. The H2-receptor antagonists were transported at about 40% the rate of the freely diffusable reference compound, antipyrine. There were no significant differences between the human and baboon in any of the parameters of placental function evaluated. Placental glucose and oxygen consumptions, and lactate production were comparable in the human and baboon preparations. The transport and clearance of each of the H2-antagonists were similar in each species.

A nonpulmonary complication of high-frequency oscillation.

High-frequency ventilation (HFV) has been suggested as an alternative to standard positive-pressure ventilation with positive end-expiratory pressure (PPV/PEEP) in the treatment of infants with hyaline membrane disease. To assess the relative safety and efficacy of HFV, we compared standard PPV to HFV delivered by oscillation (HFO) and HFV delivered by a flow interrupter (HFFI) in a previously validated preterm baboon model of hyaline membrane disease. At necropsy, the livers of several animals were unexpectedly found to have severe fatty change and/or cytoplasmic vacuolization and edema. There was a strong correlation between use of HFO and the development of liver lesions. The hepatic lesion may reflect decreased cardiac output with a concomitant fall in splenic perfusion and/or alterations in normal intrathoracic pressure that increase resistance in the hepatic vasculature. Although the etiology cannot be determined precisely, these data suggest the potential for nonpulmonary complications of HFV in the immature newborn and mandate further investigation before its widespread clinical application.

1552 COMPARISON OF TWO ANIMAL MODELS IN ARTIFICIAL SURFACTANT THERAPY FOR HYALINE MEMBRANE DISEASE (HMD)

Preterm baboon (B) and lambs (L) have been used as models for different surfactant studies. Varying responses have been recorded. We studied and compared the responses of these models to S-TA therapy. Animals delivered prematurely (B:76% and L:83% term) by C-section were used. All had HMD. S-TA 100 mg/kg was instilled into trachea at 2 hrs. of age. Controls received no S-TA. Sequential a/APO2 data are shown. Treated B had sustained improvement in a/APO2 up to 16 hrs. and mean airway pressure dropped from 14.5±1.1 to 10.2±0.5 cmH2O p<.001. In lambs improvement in a/APO2 was significant but transient lasting only 2-3 hrs, with deterioration by 4-5 hrs. Pressure vol. curves at autopsy showed significantly larger hysteresis in treated baboons than lambs. Two other treated baboons could be maintained alive for 36 hrs. We conclude: 1) Differences in response to S-TA therapy may be species related, 2) Baboon HMD model may be better for studies of long term effects of surfactant therapy than the lambs.

1443 IS ULTRA EARLY (UE) SURFACTANT (S-TA) TREATMENT IN HYALINE MEMBRANE DISEASE (HMD) BENEFICIAL?

Although S-TA has been used in the treatment of HMD, the optimal time of therapy is not established. We compared the physiologic responses to ultra early (UE) and late (LT) treatment of S-TA therapy in preterm baboons (76% of term). After C-section they were intubated and ventilated. S-TA 100 mg/kg was instilled at 10' in UE group and at 2 hrs. in LT group. All were treated to maintain normoxia, normocarbia, and normal pH range. Sequential data are shown below. Improvement in compliance (CL) was significantly better in UE than LT group. Mean airway pressure (MAP) dropped significantly in UE group reaching 5.2±2.2 cmH20 at 14 hrs. P-V curves of lungs at autopsy showed significantly higher lung vol. (at P5 deflation, 19.3±5.2 vs 11.6±2.8 ml/kg. P<.02) in UE group. These data suggest UE treatment establishes rapid alveolar stability and improves lung compliance facilitating rapid drop in MAP. better than the late treated group.

1737 FUNCTIONAL RESIDUAL CAPACITY (FRC) CHANGES IN PRETERM BABOONS WITH HYALINE MEMBRANE DISEASE (HMD)

FRC, related to alveolar stability at end expiration, would be expected to improve with recovery from HMD. Intubated, paralyzed preterm (140±2 days) baboons with HMD were evaluated with serial FRC measurements by the He dilution method on Day 0, 3, 7 and 9. Values on Day 0 were obtained within 4 hrs. of birth when all animals were ventilated by conventional ventilation (CV). Animals were then randomized to CV or high frequency ventilation (HFV). Mean values (±SD) obtained were: On Day 0, FRC was not significantly different (ANOVA for repeated measures) between ventilator groups indicating groups of equivalent initial disease severity. Analysis of differences due to ventilator type and interaction between ventilator type and time proved nonsignificant. FRCs on Day 3, 7 and 9 were significantly improved (P< 0.01) over Day 0 values. Changes in FRC appear to reflect recovery in surviving preterm baboons with HMD treated with CV or HFV.

340 EFFECT OF PHENOBARBITAL ON REGIONAL CEREBRAL BLOOD FLOW IN THE NEWBORN BABOON

The effect of phenobarbital (P) on regional cerebral blood flow (rCBF) was studied in 6 preterm newborn baboons (gest. age 149/184 - 168/184 d: x = 160.2 d; birth weight: 0.69 - 1.06 kg, x = 0.89 kg) to evaluate possible mechanisms underlying protective effect on intracranial hemorrhage. Polyvinyl catheters were placed in the left ventricle, abdominal aorta with umbilical or femoral artery, sagittal or internal jugular vein within 1 hr. following cesarian section birth. Arterial and venous blood gases, P, glucose, lactate, hemoglobin and rCBF, measured by radio labelled microspheres (141Ce, 51Cr, 85Sr) before and at 30 and 60 min. following P, 20 mg/kg I.V. Using the baboons as their own control, results show that P produced a transient but variable decrease in total CBF at 30 min. (- 27.2 ± 28.2% of control values). Decreased rCBF 30 min. post P was noted in all 16 brain regions examined including superior and inferior colliculi (x ± SF: - 33.8 ± 25.6%), thalamus, (- 32.2 ± 25.7%), medulla (- 29.6 ± 25.1%), frontal lobe (- 28.9 ± 25.6%), occipital lobe (- 28.5 ± 28.8%), and parietotemporal lobe (- 26.3 ± 28.1%). rCBF values in all regions examined returned or exceeded baseline 60 min. post P; regions with the least fall in rCBF showed best recovery in rCBF. Data suggest that protective effect of P may minimally be mediated by direct effect on rCBF but largely via other mechanism (eg. sedative effect and stabilization of systemic blood pressure).

1802 PULMONARY FUNCTION TESTS (PFTs) IN PRETERM BABOONS WITH HYALINE MEMBRANE DISEASE (HMD): PROGRESSION AND RECOVERY

Serial PFTs were performed in a baboon model to characterize changes during the course and recovery of HMD. Preterm baboons delivered abdominally at 140±2 days gestation were intubated at birth and stabilized. An esophageal balloon was inserted. The anesthetized, paralyzed animals were placed in a constant volume pressure plethysmograph for PFTs. Dynamic compliance (Cdyn) at 0.5 Hz, total lung resistance (RL), inspiratory capacity (IC) at 30 cmH2O pressure, static compliance (Cst) and functional residual capacity (FRC) by helium dilution were measured. Mean values (±SD) obtained were: (Paired t–test).Significant (P<0.05) improvement occurs in FRC by day 1; IC, Cst and Cdyn by day 3; and RL by day 9. Serial PFTs may serve as a basis for evaluating complications and therapy for HMD.

PRETERM BABOONS WITH HYALINE MEMBRANE DISEASE (HMD) DEMONSTRATE SIGNIFICANT IMPROVEMENT IN OPENING AND PEAK PRESSURES BY THE THIRD DAY

Ventilated preterm baboons (140±2da) with HMD show a significant reduction in opening and peak pressures on pressure volume curves on the third day (74±2 hr) compared to initial studies (3±1hr). Thirteen preterm baboons (140±2da) were delivered by Cesarean section, resuscitated, and ventilated with a conventional neonatal ventilator (Bear Cub) or one of 2 high-frequency ventilators. After ketamine and succinylcholine intravenously the intubated animals were tested in a constant volume plethysmograph with an esophageal balloon in place. The inspiratory limb of the pressure volume curves at 0.5 Hz were analyzed for the opening pressure, defined as the pressure at the point of departure from an isovolumetric transpulmonary pressure change. The peak transpulmonary pressure was defined as the highest pressure required to ventilate the animal. Opening and peak pressures were significantly improved by the third day. Differences due to ventilator type were not apparent. Changes in opening and peak pressures may be a sensitive measure of improvement in HMD in preterm baboons.

A baboon model of bronchopulmonary dysplasia: I. Clinical features

Investigation in bronchopulmonary dysplasia (BPD) has been seriously hampered by the lack of a suitable animal model. The consistent development of BPD in preterm baboons ( Papio cynocephalus) with hyaline membrane disease (HMD) who were treated with 95–100% inspired oxygen and supported with mechanical ventilation for more than 1 week is reported. One term (173 days) and nine preterm (134–147 days) pregnancies were delivered by cesarean section with birth weights 495–988 g. Amniotic fluid lecithin: sphingomyelin (L/S) ratios ranged from 0.47 to 1.00. The six animals with L/S ratios ⩽ 0.62 developed HMD. The clinical and radiologic course was indistinguishable from HMD in preterm humans. HMD was confirmed pathologically in two animals dying acutely. One of the remaining four was supported with supplemental oxygen as needed and remains a long-term survivor of HMD. The other three were maintained in 95–100% oxygen. A clinical and radiographic picture similar to that of human BPD developed in each and was pathologically confirmed. The preterm baboon appears to be a suitable animal model for investigation of the etiology, pathophysiology, prevention, therapy, and long-term sequelae of HMD and BPD.

1325 NEONATAL BABOON MODEL FOR STUDIES IN HYALINE MEMBRANE DISEASE (HMD)

The preterm neonatal baboon (Papio cynocephalus) at 74-82%(134 to 147 days) of term gestation (180 days) with an amniotic fluid (AF) L/S ratio of 24 hrs followed by clinical improvement, usually rapid. Symptomatic patent ductus arteriosus was not encountered clinically. HMD was confirmed pathologically in 2 animals who died of complications <5 days. Four of eight animals were successfully weaned from the ventilator. This study shows that the preterm (<147 day) baboon naturally develops HMD when the L/S ratio is<0.7 and that when supported by methods presently used for human neonates with HMD they have a reasonable chance of recovery and survival. The preterm baboon may serve as a valuable model for the study of not only acute HMD but also its course and consequences. (Supported by the Ladies Forum of the SWFRE)

1651 THE PRETERM BABOON WITH HYALINE MEMBRANE DISEASE (HMD) AS A MODEL FOR BRONCHOPULMONARY DYSPLASIA (BPD)

BPD could be produced clinically and confirmed pathologically in preterm neonatal baboons (Papio cynocephalus) with HMD who were supported with mechanical ventilation (IPPV) and treated with 100% O2 for >1 week. Ten dated pregnancies were delivered at 74-100% of term, weighing 495-988 gms. They were managed with techniques and equipment used for human neonates. Of 9 preterm animals 6 had HMD by clinical and X-ray criteria and 3 survived to develop pathologically confirmed BPD. HMD was pathologically confirmed in 2 animals who died <1 week. It appears that the major factors thought to contribute to the development of BPD in human neonates may also produce BPD in neonatal baboons: prematurity, HMD, high FIO2, IPPV and time. With the development of an animal model for BPD, the etiologic variables should be amenable to future investigation. (Supported by the Ladies Forum of the SWFRE)

1258 BODY WATER SPACES IN BABOON NEONATES

Body water spaces were estimated within 2 hours of birth in 15 baboon neonates delivered by cesarean section. Birthweights ranged from 495 to 1001g, gestational ages ranged from 134 to 174 days and all animals were normally-grown for gestational age. Total body water (TBW) ranged from 503 to 803 ml, extracellular water (ECW) ranged from 201 to 496 ml and intracellular water (ICW) ranged from 225 to 330 ml. All values were within 1 SD of the mean expected for human neonates of similar weights. In proportion to body weight, mean (±SD) TBW for term (764±59.4 ml/kg) and preterm (853±79.3 ml/kg) animals were quite similar to mean TBW reported for human neonates (753 and 816 ml/kg respectively). Mean ECW (460±38.6 and 531±142.7 ml/kg) were somewhat higher than in humans (388 and 431 ml/kg) but most values were within the human range. Mean ICW (318±16.3 and 423±129.6 ml/kg) were again similar to human means (360-386 ml/kg) and individual values were within the range of human values. Mean plasma volumes for term (42±7.9 ml/kg) and preterm (43±9.7 ml/kg) baboon, were almost identical to human means (43 and 46 ml/kg), and so were mean blood volumes (89±13.2 and 85±13.7 ml/kg vs. 98 and 88 ml/kg). These data suggest that the body composition of baboon neonates at a given gestational maturity is quite similar to that of equally mature human babies. Baboon neonates may therefore constitute an adequate animal model for studies of intrauterine growth and development.