Size selectivity of lung protein accumulation in preterm ventilated lambs

The 1-hour net accumulation of four labeled proteins of different sizes (6.5, 29, 69 and 150 kD) from the vascular space into the lungs and airspaces was measured in preterm ventilated lambs at 132 days gestational age. Lambs treated with Survanta, a surfactant prepared from bovine lung, were studied at 1, 3, 5 and 8 h after birth, while lambs not treated with this surfactant were studied up to 5 h of age because of severe respiratory failure. The labeled proteins were lost from the vascular space more rapidly over the first 1 h of life than at later times (p less than 0.01). Labeled protein recoveries were similar at 1 and 3 h in surfactant and control lambs and decreased by 8 h in surfactant-treated lambs (p less than 0.05). In both the surfactant-treated and control animals, there was a sequential decrease in labeled protein recoveries based on protein size (p less than 0.01). There was no change with time in size selectivity for accumulation of the labeled proteins into the lungs for either the control or surfactant-treated lambs, although surfactant treatments decreased accumulation of the 6.5 and 29 kD proteins at 5 h when compared to the control group (p less than 0.05). Labeled protein recoveries in alveolar washes demonstrated less size selectivity. These studies documented that size selectivity of the vascular endothelium did not change over the first 8 h of life in preterm ventilated lambs, a pattern that was not indicative of progressive lung injury.     

Blood pressure responses to care procedures in ventilated preterm infants

Responses of mean aortic blood pressure to sequences of routine care procedures in 22 ventilated, preterm infants were studied daily for the first 3 days of life. In the first 11 infants standard care procedures were used, whereas the next 11 infants were preoxygenated by a preceding 10% increase in inspired oxygen concentration; in these infants, chest physiotherapy was entirely omitted while the frequency of endotracheal suctioning was reduced. A total of 259 blood pressure responses were recorded. In general, responses were biphasic, consisting of an initial blood pressure drop followed by a greater blood pressure rise of longer duration. Baseline blood pressure, as well as the minimum and maximum blood pressure during the care procedures, increased with gestational age and with postnatal age. The blood pressure drop was most pronounced in the infants requiring the most intensive ventilatory support and was reduced by modifying the care procedures. The blood pressure rise was the least in the infants receiving pancuronium and phenobarbitone. Eight infants, 4 in each group, had intraventricular haemorrhage; in these infants, the care procedures induced more pronounced blood pressure drops in the first day of life when compared to the infants without haemorrhage.     

Activation of the inflammatory reaction within minutes after birth in ventilated preterm lambs with neonatal respiratory distress syndrome

To study the activation of the inflammatory reaction within minutes after birth, we measured parameters of inflammation before and immediately after birth. To assess whether respiratory distress syndrome (RDS) or birth itself initiates activation, we compared preterm ventilated lambs with term nonventilated lambs. Preterm lambs were delivered by cesarean section at 132 days gestational age (term 145 days) and were ventilated by conventional ventilation (n = 9). Before clamping the cord, 5, 10 and 15 min after birth, blood was sampled from umbilical catheters. Term lambs (n = 9) were born spontaneously after 140-145 days gestational age. Immediately after birth, a venous umbilical catheter was inserted. Blood was sampled before the first breath and 5, 10, 15 and 20 min after birth while the lamb was breathing spontaneously. Blood was analyzed for AP50 (complement activation), number of polymorphonuclear leukocytes (PMNs) and beta-glucuronidase (released from activated PMNs). In preterm lambs, we found a decreased number of PMNs and increased levels of beta-glucuronidase already at 5 min after birth. In the term lambs, we found only a short-term mild decrease in PMNs and short-term increase in beta-glucuronidase. We conclude that systemic activation of the inflammatory reaction can be found in ventilated preterm lambs with RDS within 5 min after birth. This very early activation is mild, transient and less pronounced in term-born spontaneously breathing lambs compared with preterm, ventilated lambs with RDS.     

Antenatal betamethasone changes cord blood monocyte responses to endotoxin in preterm lambs

Corticosteroids are routinely administered to women at risk for preterm delivery to induce fetal lung maturation. Antenatal corticosteroids have immunomodulatory effects on fetal immune cells that are poorly understood. We hypothesized that maternal betamethasone would alter in fetal monocytes both the initiation of inflammation in response to pro-inflammatory stimuli and the resolution of inflammation by phagocytosis of apoptotic neutrophils. Preterm lambs at 124 d gestation were delivered 15 h, 1 d, 2 d, or 7 d after 0.5 mg/kg maternal intramuscular betamethasone. Monocytes from cord blood were isolated and cultured and results were compared with monocytes from preterm lambs exposed to maternal saline or monocytes from adult sheep. Phagocytosis of Escherichia coli was not changed, however, phagocytosis of apoptotic neutrophils was low in fetal monocytes but increased after 7 d exposure to maternal betamethasone to the level found in adult monocytes. Hydrogen peroxide production after endotoxin stimulus was significantly reduced to 7.1 +/- 2.2 micromol at 5 h, 8.7 +/- 2.9 micromol at 24 h, and 4.1 +/- 1.9 micromol at 48 h versus 16.4 +/- 3.6 micromol in control animals; at 7 d, the hydrogen peroxide production increased to 74.3 +/- 19.7 micromol (p < 0.05, per 10(6) monocytes). IL-6 production was reduced at 15 h after maternal betamethasone but at no other time point. Maternal betamethasone initially suppressed several fetal monocyte functions, however, at 7 d, measurements of initiation and resolution of inflammation were increased to levels similar to monocytes from adult sheep. The time-dependent changes in maternal betamethasone modulation of the responses of fetal monocytes may influence immune function of the preterm lamb after delivery.     

Surfactant treatment effects on lung structure and type II cells of preterm ventilated lambs

We evaluated surfactant treatment effects on lung morphology and alveolar type II cells of preterm ventilated lambs. Lambs were ventilated for 10 h following treatment of the right lung with natural surfactant. Lung parenchyma from the surfactant-treated right and the untreated left lung was compared morphometrically. Mechanical ventilation without surfactant resulted in distention of alveolar ducts accompanied by shallowing and loss of well-defined alveoli without disruption of collagen or elastin fibers. Surfactant treatment almost completely prevented these changes. The percent of normal parenchyma was 82 +/- 7% in surfactant-treated lobes and 26 +/- 5% in the nontreated lobes (p < 0.05). Type II cells became flatter in lungs ventilated without surfactant, and cell shape was preserved by surfactant treatment. The volume densities of lamellar bodies and multivesicular bodies in alveolar type II cells were not changed by surfactant treatment. With or without surfactant treatment, mechanical ventilation was associated with a shift in lamellar body distribution to a smaller size and a decrease in glycogen content of type II cells. Surfactant treatment of the preterm lung prevents alveolar distortion and atelectasis, but does not result in changes in subcellular organelles in immature type II cells.     

Haemodynamic responses and population pharmacokinetics of midazolam following administration to ventilated, preterm neonates

Objective: To evaluate the effects of intravenous midazolam on haemodynamic variables and cerebral blood flow velocity (CBFV) and to determine the pharmacokinetics using a population approach in very low birthweight (VLBW) ventilated infants.
Methodology: Physiological variables were measured at predetermined times in 10 infants with birthweight ≤1500 g following a bolus dose of intravenous midazolam (0.1 mg/kg). Heart rate, mean arterial blood pressure (MAP) and transcutaneous CO₂ (TcPCO₂) were recorded and CBFV was assessed by Doppler ultrasound. Midazolam concentrations were also measured and pharmacokinetic parameters determined using a population modelling package.
Results: No change in heart rate occurred during the study period, while the MAP decreased by 3 mmHg 5 min after midazolam administration compared to baseline values. A non-significant fall in TcPCO₂ was seen at 20 min. Mean CBFV decreased from the baseline by 12% at 5 min, then returning to predose values. Midazolam concentrations were in the range shown to be effective in sedation of paediatric intensive care infants with the elimination being delayed in comparison to older children.
Conclusions: As only minor cerebral and haemodynamic effects were found with the use of midazolam in stable ventilated preterm infants, it appears to be a safe, short-term sedative agent.     

Antenatal glucocorticoids attenuate activation of the inflammatory reaction and clotting in preterm lambs

Recently we have shown that activation of inflammatory reaction and clotting can be found immediately after delivery in preterm lambs ventilated for respiratory distress syndrome (RDS). To investigate whether antenatal glucocorticoids would attenuate postnatal activation of the inflammatory reaction and clotting, we studied ventilated preterm lambs delivered by cesarean section, 24 h after antenatal administration of betamethasone or placebo. Blood was sampled before clamping the cord, 5, 10, and 15 min after delivery, and 2-hourly afterwards. Blood was used to determine oxygenation index, alveolar - arterial partial O(2) difference (AaDO(2)), AP50 titer (see text), polymorphonuclear leukocytes (PMNs), beta-glucuronidase, thrombin inhibition, activated partial thromboplastin time, and clot lysis time. Bronchoalveolar lavage fluid was sampled before clamping the cord and 30 min and 1, 2, 4, 6 and 8 h after delivery and was analyzed for elastase, thrombin, and protein. After removal of the lungs, static compliance and water content of the lungs were determined. We found that betamethasone-treated lambs had lower oxygenation index and AaDO(2) than controls. At birth, PMN levels were higher, and the beta-glucuronidase level was lower after betamethasone treatment. PMNs and beta-glucuronidase did not change in betamethasone-treated lambs, in contrast to controls. Thrombin inhibition, activated partial thromboplastin time, and clot lysis time did not change in betamethasone-treated lambs, in contrast to controls. In both groups, elastase and protein levels in bronchoalveolar lavage fluid increased; the thrombin level increased in controls. The static compliance was better, and the water content of the lung was lower in the betamethasone-treated lambs. We conclude that early systemic activation of inflammatory reaction and clotting in preterm lambs with RDS are attenuated by antenatal betamethasone administration. Whether this is a direct effect of betamethasone on the inflammatory reaction or a result of a reduced ventilatory support because of less severe RDS after antenatal betamethasone treatment remains to be elucidated.     

Pulmonary lavage in preterm lambs

Pulmonary function was studied before and after bilateral lung lavage with oxygenated FC-80 fluorocarbon liquid in seven perterm lambs, 134 days of gestation. Measurements of transpulmoary pressure, airflow, tidal volume, and functional residual capacity (FRC) enabled calculation of lung resistance and compliance, specific compliance, and work of breathing. Immediately postlavage, arterial oxygen tension (PaO2) decreased significantly (P less than 0.05) by 41% from control values. In addition, at 1 hr postlavage, lung compliance significantly decreased (P less than 0.01) by 43% from control levels. Lung resistance, FRC work of breathing arterial carbon dioxide tension, and pH postlavage were not significantly different from prelavage values. The mean volume of FC-80 remaining in the lungs at 1 hr postlavage was 32% of the instilled volume. These data indicate that lung lavage with a low surface tension liquid has a relatively small effect on lung mechanics of the premature lung.     

Impaired parasympathetic response to feeding in ventilated preterm babies

BACKGROUND: Premature very low birthweight (VLBW) infants are born with an underdeveloped parasympathetic nervous system (PNS) which may limit their ability to respond adequately to feeding and may limit their capacities for extrauterine growth and development. OBJECTIVES: To describe the patterns of autonomic response to feeding and identify relationships between change in heart period variability measures over time with selected infant characteristics. METHODS: Individual growth curve analysis techniques were used to describe the patterns of change over time in sympathetic and parasympathetic tone as measured by low and high frequency heart period power. RESULTS: Sixteen mechanically ventilated VLBW infants with a mean corrected gestational age of 30.4 weeks participated in the study. The low frequency (LF) power slope was -17.67 (p = 0.0002) and the high frequency (HF) power slope was -0.92 (0.0003). There was a significant relationship between HF slope and birth gestational age (r = -0.49, p = 0.05). CONCLUSIONS: HF power, representing primarily parasympathetic activity, did not increase with enteral feeding as anticipated. LF power, an indicator of sympathetic tone, decreased during and after feeding suggesting the anticipated effect of inhibition of the sympathetic nervous system in response to the gut stimulus. Critically ill VLBW infants possess an overriding sympathetic response, but may not have adequate PNS tone development.     

Permissive hypercapnia to decrease lung injury in ventilated preterm neonates

Lung injury in ventilated premature infants occurs primarily through the mechanism of volutrauma, often due to the combination of high tidal volumes in association with a high end-inspiratory volume and occasionally end-expiratory alveolar collapse. Tolerating a higher level of arterial partial pressure of carbon dioxide (PaCO2) is considered as 'permissive hypercapnia' and when combined with the use of low tidal volumes may reduce volutrauma and lead to improved pulmonary outcomes. Permissive hypercapnia may also protect against hypocapnia-induced brain hypoperfusion and subsequent periventricular leukomalacia. However, extreme hypercapnia may be associated with an increased risk of intracranial hemorrhage. It may therefore be important to avoid large fluctuations in PaCO2 values. Recent randomized clinical trials in preterm infants have demonstrated that mild permissive hypercapnia is safe, but clinical benefits are modest. The optimal PaCO2 goal in clinical practice has not been determined, and the available evidence does not currently support a general recommendation for permissive hypercapnia in preterm infants.     

Lung and systemic inflammation in preterm lambs on continuous positive airway pressure or conventional ventilation

Intratracheal lipopolysaccharide (LPS) causes acute inflammation and injurious mechanical ventilation results in pulmonary and systemic inflammation. We aimed to determine in preterm lungs if continuous positive airway pressure (CPAP) protects against pulmonary and systemic inflammation, compared with conventional mechanical ventilation (CMV) after intratracheal LPS. Preterm fetuses were exposed to maternal betamethasone and Epostane 36 h before delivery at 133 d gestational age (term = 150 d). Lambs were intubated and randomized to receive gentle CMV (tidal volume 8 mL/kg) or CPAP with 8 cm H2O pressure. Surfactant (10 mg/kg) mixed with 1 mg LPS or saline was instilled into the trachea at 15 min. Blood gas status, ventilation variables, and arterial pressures were recorded for 3 h. Static pressure-volume curves and lung and systemic inflammation were assessed postmortem. CPAP lambs had elevated Paco2 and minute ventilation compared with the CMV lambs. Cytokine mRNA was increased in the lungs and liver of CPAP and CMV lambs relative to unventilated controls. Intratracheal LPS amplified the cytokine mRNA responses of IL-1beta, IL-6, and IL-8 in the lung and liver. Blood neutrophils decreased similarly after LPS in CPAP and CMV groups. Cytokine markers of lung injury or the systemic response to intratracheal LPS were not decreased by CPAP relative to CMV, in preterm lambs     

Comparison of four surfactants: in vitro surface properties and responses of preterm lambs to treatment at birth

Natural sheep surfactant, rabbit surfactant, human surfactant, and surfactant TA were compared for in vitro surface properties and for responses of preterm lambs to treatment. Equivalent amounts of sheep, rabbit, and human surfactants were needed to lower the surface tension to less than 10 dynes/cm, whereas four times less surfactant TA similarly lowered the surface tension. Surface-spreading rates were similar for the surfactants. The surface adsorption of the batch of human surfactant tested was much slower than was adsorption of the other surfactants. Ventilation was significantly improved in all surfactant-treated lambs relative to the control lambs, indicating the general efficacy of the surfactant treatments. Overall, surfactant TA had the best in vitro characteristics, yet the preterm lambs treated at birth with surfactant TA had lower PO2 values and higher ventilatory requirements than did the sheep surfactant-treated lambs. The in vivo responses to rabbit surfactant were intermediate between the responses to sheep surfactant and to surfactant TA. Human surfactant resulted in the least effective clinical response. More of the phosphatidylcholine associated with human surfactant and surfactant TA was lost from the alveoli and lung tissue after four hours of ventilation than was lost from sheep or rabbit surfactant-treated lambs. More intravascular radiolabeled albumin leaked into the alveoli of the surfactant TA-treated lambs than sheep or rabbit surfactant-treated lambs. The four surfactants also had different sensitivities to the effects on minimum surface tensions of the soluble proteins present in alveolar washes. The study demonstrates that the range of clinical responses was not predictable based on the in vitro surface properties that we measured.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exhaled nitric oxide in chronically ventilated preterm infants

Exhaled nitric oxide (eNO) levels were measured in eight ventilated infants, mean gestational age 25.8 (SD 1.7) weeks and postnatal age 55 (SD 39) days, before and after three days of dexamethasone treatment. The eNO levels fell from a mean of 6.5 (SD 3.4) to 4.2 (SD 2.6) parts per billion (p = 0.031) and the mean supplementary oxygen levels from 62% to 45% (p = 0.0078).     

Oesophageal pressure measurements in ventilated preterm babies.

Oesophageal pressure measured with an air-filled, thin latex balloon on a 6 French gauge catheter can accurately measure intraoesophageal pressures in ventilated preterm babies. Intraoesophageal pressures and intrapleural pressures are equivalent. Less than 10% of the applied positive ventilator pressure is transmitted to the oesophageal balloon and the intrapleural catheter (intercostal drain). The oesophageal traces show that ventilated babies breath independently of the ventilator. The largest deflections recorded from the oesophageal trace are from spontaneous inspiratory activity. During paralysis with pancuronium spontaneous respiration is inhibited, peristaltic waves are still recorded, and there is little transmitted pressure from the ventilator to the oesophagus or intercostal drain.     

Lung fluid balance in hypoxic lambs

In spontaneously breathing newborn lambs, alveolar hypoxia increases lung microvascular pressure, which causes lung lymph flow to increase and the concentration of protein in lymph to decrease. To see if this response derives from hypoxia itself rather than from the change in breathing pattern that occurs during hypoxia, we measured lung vascular pressures, pleural pressure, cardiac output, and lung lymph flow in 12 anesthetized lambs that were ventilated at a fixed rate and tidal volume, first with air, then with 10-14% O2 in nitrogen. Alveolar hypoxia did not affect pleural pressure, but pulmonary arterial pressure increased from 19 to 32 torr, lung lymph flow increased from 2.20 to 3.83 ml/h and lymph protein concentration decreased from 3.4 to 2.8 g/dl. To be certain that the increased lymph flow associated with hypoxia is not simply the result of an acute release of fluid from the lungs and to assess the effects of carbon dioxide on lymph flow during hypoxia, we next studied six unanesthetized lambs kept hypoxic for a total of 12 h. After a 2-4-h period in air the lambs breathed 9-11% O2 in nitrogen for 2-4 h, then 8-11% O2 and 3-5% CO2 in nitrogen for 8-10 h. In these lambs we injected intravenously radioactive albumin and measured its uptake in lymph to see if sustained hypoxia alters microvascular permeability to protein in the lungs. In these experiments pulmonary arterial pressure increased from 17 to 37 torr, lung lymph flow increased from 1.74 to 3.28 ml/h, and lymph protein concentration decreased from 3.8 to 3.1 g/dl during hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)