Objective To determine whether prematurely born infants exposed to chorioamnionitis compared to those not exposed have poorer lung function and are more likely to develop severe bronchopulmonary dysplasia (BPD).
Design Results were analysed from consecutive infants born at <33 weeks gestation with placental histology results and lung function measurement results on days 2 and/or 7 after birth and/or at 36 weeks postmenstrual age (PMA).
Setting Tertiary neonatal intensive care unit.
Patients 120 infants with a median gestational age of 29 (range 23–32) weeks were studied, 76 (63%) developed BPD and 41 (34%) had been exposed to chorioamnionitis and/or funisitis.
Interventions Chorioamnionitis was diagnosed histologically.
Main outcome measures Lung function was assessed by measurement of lung volume and compliance and resistance of the respiratory system. If the infants remained oxygen dependent beyond 28 days, they were diagnosed at 36 weeks PMA to have mild BPD (no longer oxygen dependent), moderate BPD (required less than 30% oxygen) or severe BPD (required more than 30% oxygen and/or positive pressure support).
Results No significant differences were found in the lung function results between the chorioamnionitis and non-chorioamnionitis groups at any postnatal age. There was no significant relationship between chorioamnionitis and the occurrence or severity of BPD. Regression analysis demonstrated BPD was significantly related only to birth weight, gestational age and use of surfactant.
Conclusion In prematurely born infants, routinely exposed to antenatal steroids and postnatal surfactant, chorioamnionitis was not associated with worse lung function or more severe BPD.
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Bronchopulmonary dysplasia (BPD; chronic oxygen dependency) is a common adverse outcome of very premature birth1 and affected infants have chronic respiratory morbidity.2,–,4 Nowadays, BPD can occur in very prematurely born infants who initially had minimal or even no signs of lung disease.5 At postmortem, however, affected infants have fewer and larger alveoli due to arrest in acinar development.6 It has been proposed that this may represent a maldevelopment sequence resulting from interference/interruption of normal development signalling for terminal maturation and alveolarisation of the lungs.7 A possible mechanism for the interference/interruption of normal lung development could be exposure to antenatal infection/inflammation.8 In an animal model, a single dose of intra-amniotic endotoxin treatment resulted in an increase in the alveolar volume of approximately 20%, but a decrease in the total alveolar number of almost 30%.8 In a case–control study, in which infants were matched for various factors including gestational age and birth weight,9 chorioamnionitis was associated with an increased risk of BPD if the infants developed postnatal infection or required mechanical ventilation for more than 7 days.9 The authors suggested that antenatal inflammation increases the susceptibility of the lung to postnatal injurious events and proposed a multi-hit model for BPD development.9 We, therefore, hypothesised that prematurely born infants exposed to chorioamnionitis compared to those not exposed would have poorer lung function during the perinatal period and prior to discharge from the neonatal unit and be more likely to develop severe BPD. Funisitis with chorioamnionitis compared to chorioamnionitis alone is associated with upregulation and shedding of umbilical cord adhesion molecules10 and hence infants exposed to funisitis may be at particular risk for adverse outcome. The aim of this study was to test those hypotheses, as if our hypotheses were proven this would emphasise the importance of identifying strategies to reduce chorioamnionitis in order to improve the respiratory outcome of prematurely born infants.
What is known on this topic
▶ Chorioamnionitis may increase the likelihood of bronchopulmonary dysplasia (BPD) but only as part of a multi-hit model.
▶ Infants with new BPD have larger but fewer alveoli; this can be replicated in an animal model by intra-amniotic injection of endotoxin.
What this study adds
▶ In prematurely born infants routinely exposed to antenatal steroids and postnatal surfactant, chorioamnionitis did not increase the likelihood of severe BPD.
▶ There were no significant differences in lung function in the perinatal period or at 36 weeks PMA between infants exposed or unexposed to chorioamnionitis.
Materials and methods
Results were analysed from consecutive prematurely born infants (32 weeks of gestational age or less at birth) who had placental histology results and lung function measurement results on days 2, 7 and/or 36 weeks postmenstrual age (PMA). Data were analysed from infants entered into a prospective study assessing the impact of chorioamnionitis on lung function (not previously reported) and from infants entered into two previous studies.11 12 Forty-seven of 82 infants were eligible from the prospective study and 33 of 12611 and 40 of 7812 from the previous two studies. During the time period in which the three studies were undertaken, there were no changes in routine policies regarding, for example, antenatal steroid or postnatal surfactant use (Survanta was used throughout), nor were there any changes in the use of ventilatory modes. Parents gave informed written consent for their infants to take part and all three studies were approved by the King's College Hospital Research Ethics Committee.
Examination of the placentae was undertaken by a perinatal pathologist (EP) who was unaware of the clinical outcome of the infants. The placentae were macroscopically examined and preserved in 10% buffered formalin. Sections for microscopic examination were taken from the placental membranes and from both the placental and the fetal ends of the umbilical cord. Chorioamnionitis was defined as the presence of polymorphonuclear leucocytes in the placental membranes or the fetal plate.13 Funisitis was diagnosed when neutrophils had infiltrated the umbilical cord vessel walls of Wharton's jelly.10
Lung function was assessed by measurement of lung volume and compliance and resistance of the respiratory system. Lung volume was assessed by measuring functional residual capacity (FRC) using a helium gas dilution technique (FRCHe) and a specially designed infant circuit (total volume 95 ml). The FRC system (Series 7700, Equilibrated Biosystems, Melville, New York) included a 500 ml re-breathing bag containing a mixture of helium (10%) and oxygen. The bag was connected to a three-way valve to which a facemask or an endotracheal tube could be attached. In ventilated infants, the ventilator manifold was connected to a port on the three-way valve, such that with the valve in the standby position, the infant was ventilated directly from the ventilator. The valve was actuated at end expiration, which switched the infant from being directly supported from the ventilator to the rebreathing bag. Positive pressure support was maintained by the simultaneous redirection of airflow from the ventilator to the airtight cylinder containing the rebreathing bag. If the infant was not ventilated, a facemask attached to the three-way valve was held snugly over their nose and mouth; silicone putty was used around the mask to achieve an airtight seal. The helium concentration was displayed in real time and equilibration was defined as when there was no change in the helium concentration over a 15 s period. The initial and equilibration helium concentrations were used in the calculation of FRC, which was corrected for oxygen consumption (assumed to be 7 ml/kg/min)14 and for body temperature, pressure and saturated conditions. Only recordings during quiet breathing were analysed and if the infant cried or sighed, the measurement was terminated and repeated. There was an interval of 10 min between measurements. FRC was estimated three times in each infant. The FRC was expressed as the mean of the three measurements and related to body weight. The coefficient of variation for the measurement was 5.2%.
Measurement of compliance (Crs) and resistance (Rrs) of the respiratory system was performed using a single breath occlusion technique. Airflow was measured using a pneumotachograph (Mercury F10L, GM Engineering, Kilwinning, UK) which was either placed between the infant's endotracheal tube and ventilator circuit or, if they were non-ventilated, attached to an appropriately sized facemask placed over the mouth and nose to achieve an airtight seal. The pneumotachograph was connected to a differential pressure transducer (range ±2 cm H2O; MP45, Validyne, Northridge, California, USA). Airway pressure was measured from a side port on the pneumotachograph using a differential pressure transducer (range ±100 cm H2O; MP45, Validyne). The signals were amplified (CD280; Validyne) and displayed in real time on a computer running Labview software (v 4.0; National Instruments, Austin, Texas, USA) with 100 Hz analog-to-digital sampling (DAQ 16XE-50, National Instruments). Tidal volume was obtained by integration of the flow signal using the Labview software. Occlusions were performed at end inspiration, which was identified from the flow signal. In ventilated infants, a pneumatic sliding valve (Hans Rudolph, Kansas, Missouri, USA) inserted into the ventilator circuit was used. The pneumatic valve was arranged such that during an end inspiratory occlusion, the infant expired to room air. In non-ventilated infants, the distal end of the pneumotachograph was manually occluded. Only breaths with a pressure plateau of at least 100 ms were used in the calculation of Crs and Rrs. The mean Crs and Rrs were calculated from 10 technically acceptable occlusions. The compliance was related to body weight and expressed as ml/cm H2O/kg. The mean intrasubject coefficients of variation for Crs and Rrs were 11.8% and 11.8%, respectively.
Infants were diagnosed as having BPD and the severity of BPD determined according to the National Institutes of Health consensus definition.15 Infants were diagnosed as having BPD if they remained oxygen dependent beyond 28 days after birth. At 36 weeks PMA, respiratory status was assessed. If the infant was no longer on oxygen they were described as having mild BPD, moderate BPD if they required less than 30% oxygen and severe BPD if they required more than 30% oxygen and/or positive pressure support (continuous positive airway pressure or mechanical ventilation).
Demographic data were collected from the maternal and infants' notes. Antenatal variables recorded were histologically proven chorioamnionitis and/or funisitis, premature rupture of the membranes (PROM) and maternal antibiotics and/or antenatal corticosteroid administration. Postnatal variables were gender, gestational age, birth weight, use of surfactant, postnatal infection (positive blood culture or suspected clinical infection with a raised C reactive protein, increased or decreased neutrophil count and/or decreased platelet count16), patent ductus arteriosus (PDA) requiring treatment (medical or surgical), the duration of mechanical ventilation, the duration of supplementary oxygen dependency and BPD status. Infants were treated medically for a PDA if they had respiratory compromise as a result of the PDA. If they remained symptomatic due to a PDA after two courses of medication, the PDA was surgically ligated. Postnatal corticosteroids were given to infants who, after 2 weeks of age, remained ventilator dependent and on high inspired oxygen concentrations.
Differences were assessed for statistical significant difference using the χ2 test, Student t test, Mann–Whitney U test or Kruskal–Wallis test (with Dunn's multiple comparison test) as appropriate.
Stepwise regression analysis was used to identify the relationship between BPD and possible predictors including antenatal (chorioamnionitis, antenatal steroid use) and postnatal (gestational age, birth weight, surfactant use, postnatal infection, PDA requiring treatment and the duration of mechanical ventilation) variables.
Comparison of the results of 18 patients in each group allowed detection of a difference equivalent to 1 SD of each measurement (FRC 1.6 ml/kg, Crs 0.09 ml/cm H2O/kg, Rrs 5.4 cm H2O/l/s) with 80% power at the 5% level.
A total of 120 infants were included in the study with a median gestational age of 29 (23–32) weeks and a birth weight of 1.12 (0.53–1.93) kg. A natural surfactant was given prophylactically to all babies who, on transfer from the labour ward, were ventilated and required at least 30% oxygen.
Overall, 41 infants had been exposed to chorioamnionitis and/or funisitis. They did not differ significantly regarding their demographics from those not exposed except that more of the chorioamnionitis/funisitis group had been exposed to PROM (p<0.001), maternal antibiotics (p<0.001) and antenatal steroids (p=0.049) (table 1). Histology revealed funisitis in 25 cases. There were no statistically significant differences in the demographics of infants exposed to chorioamnionitis with or without funisitis (data not shown). No significant differences were found between the lung function results at any postnatal age between infants exposed or unexposed to chorioamnionitis/funisitis (table 2). There were no statistically significant differences in any aspects of lung function between the infants exposed to chorioamnionitis with or without funisitis (data not shown).
Similar proportions of infants exposed and unexposed to chorioamnionitis/funisitis developed BPD and there were no significant differences in the severity of BPD between the two groups (table 3). There were no significant differences between the proportion of infants developing BPD who were exposed to chorioamnionitis with or without funisitis (data not shown). Infants who developed BPD were significantly more immature, of lower birth weight, required more respiratory support and were more likely to have had postnatal sepsis or a treated PDA (table 4). Regression analysis demonstrated that BPD development was significantly related to gestational age, birth weight and surfactant use, but not to chorioamnionitis.
We have demonstrated no significant differences in lung function results measured during the perinatal period or at 36 weeks PMA between infants exposed or unexposed to chorioamnionitis/funisitis, nor did the severity of BPD differ between the two groups. Chorioamnionitis has been reported to be associated with either a similar17 or a reduced risk of respiratory distress syndrome (RDS)18,–,21 in five studies, in which data were adjusted for confounding factors. It might then have been expected that chorioamnionitis would be associated with significantly better lung function in the perinatal period. The majority of the infants in this study, however, were exposed to antenatal steroids and postnatal prophylactic surfactant and there were no significant differences in the use of those treatments between the two groups. Both antenatal steroids and postnatal surfactant significantly reduce the occurrence and severity of RDS and this likely explains the lack of differences in lung function results between the two groups. At 36 weeks PMA, lung function is influenced by BPD status,22 thus the lack of difference in the 36 weeks PMA lung function results may be explained by the similar incidence of BPD in our two groups. Our results are supported by those of Latzin et al23 who examined FRC, ventilation inhomogeneity and tidal breathing parameters at 44 weeks PMA in preterm infants with and without BPD. Multivariate linear regression demonstrated no significant influence of chorioamnionitis on lung function. A possible explanation for the apparent lack of effect of chorioamnionitis on subsequent infant lung function may be the type of organisms known to cause chorioamnionitis in humans. These include Ureaplasma urealyticum, Mycoplasma hominus and Gardnerella vaginalis,24 and may be less injurious to the fetal lung than the endotoxin of Escherichia coli.8
Our results demonstrating no significant association of chorioamnionitis with BPD are at variance with those of Watterberg et al25 who demonstrated a higher incidence of BPD in intubated infants of birth weight <2000 g who had been exposed to chorioamnionitis. The chorioamnionitis group, however, was of significantly lower gestational age and the data were not adjusted for that difference.25 An increased incidence of BPD associated with chorioamnionitis has only been reported in six of 18 subsequent studies,26 and gestational age adjustment was performed in only one of those studies. In the remaining studies, multivariate adjustment generally showed no difference in BPD risk.26 In our cohort, there was no significant difference in the gestational age at delivery of those exposed or unexposed to chorioamnionitis. None of the patients reported in Watterberg's study25 received antenatal steroids and it has been suggested that the inability of many later studies to demonstrate an increased risk of BPD following chorioamnionitis may be attributed to the increased use of antenatal steroids.26 In our study, more than 90% of infants and 95% of the chorioamnionitis group were exposed to antenatal steroids.
Clinical or histopathological definitions of chorioamnionitis have been used in previous studies. A histological definition of intrauterine inflammation, that is identification of polymorphonuclear cellular infiltrates, more closely reflects antenatal inflammatory exposure than the clinical definition.27 As a consequence, we diagnosed chorioamnionitis using histological criteria and showed no statistically significant relationship of lung function abnormalities or severe BPD to chorioamnionitis. The effect of fetal inflammation, thought to be a marker of more severe chorioamnionitis, has also been investigated.28,–,32 None of the studies reported an increase in the incidence of BPD when the effect of fetal inflammation was compared to maternal inflammation only. The absence of any significant association between BPD and fetal inflammation remained after adjustment for potential confounders.31 In this study, we demonstrate that funisitis was not associated with worse lung function or increased the risk for BPD or the severity of BPD.
In conclusion our results suggest that, in infants routinely exposed to antenatal steroids and postnatal surfactant, chorioamnionitis does not adversely affect lung function in the perinatal period or at 36 weeks PMA. In addition, infants exposed to chorioamnionitis compared to those not exposed did not develop more severe BPD. Chorioamnionitis is associated with premature delivery, brain injury in preterm infants and adverse neurological outcome and we therefore should strive to reduce/minimise the effects of chorioamnionitis where possible. These results suggest reducing chorioamnionitis, however, may not improve the respiratory outcome of prematurely born infants.
Funding Dr May was supported by the Charles Wolfson Charitable Trust.
Competing interest None.
Ethics approval This study was conducted with the approval of the King's College Hospital Research Ethics Committee.
Provenance and peer review Not commissioned; externally peer reviewed.
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