Association Between the Method of Delivery and Respiratory Outcomes in Extremely Preterm and Extremely Low Birth Weight Infants: a Retrospective Cohort Study in Newborns From 2000-2003

Abstract

Background: Extremely preterm (gestational age (GA) <28 weeks) and extremely low birth weight (ELBW) infants (<1000g) are at risk to develop respiratory morbidities like respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD) and pneumothorax. The delivery method also influences respiratory outcomes, with cesarean sections (CS) being associated with a higher incidence of RDS, pneumothorax and ventilatory treatment. To develop new preventive or early respiratory treatment strategies for high-risk neonates, knowledge about risk factors for respiratory outcomes in this group is necessary. This study aims to investigate the association between the delivery method and respiratory outcomes in extremely preterm and ELBW infants.

Methods: We performed a retrospective patient record cohort study of all extremely preterm and/or ELBW neonates born between 2000-2003 in a tertiary center in Maastricht, the Netherlands. Differences in RDS, BPD, pneumothorax, invasive mechanical ventilation (MV), continuous positive airway pressure (CPAP), nasal oxygen, duration of invasive MV, CPAP and total respiratory support, nitric oxide, surfactant and postnatal steroids with pulmonary indication between vaginal delivery (VD) and CS were assessed.

Results: A total of 172 infants were included. The CS group (n=119) had a higher mean GA, percentage males and prevalence of multiple gestations compared to the VD group (n=53) (p<0.05). Furthermore, birth weight, birth weight standard deviation score (SDS) and weight at discharge were lower in the CS group (p<0.05). Fetal distress and moderate 1-minute Apgar scores (4-6) were more common in the CS group, as was premature rupture of membranes. Low 1-minute Apgar scores (0-3), persistent ductus arteriosus and intraventricular hemorrhage occurred more frequently in the VD group, as did pre-eclampsia or ‘hemolysis elevated liver enzymes low platelet count’-syndrome. After correcting for GA and birth weight SDS, a significant association between CS and receiving surfactant was found. Other primary respiratory outcomes and treatments did not show any associations.

Conclusions: After correcting for GA and birth weight SDS, the method of delivery is not associated with adverse respiratory outcomes in extremely preterm and ELBW infants. Due to changes in standard operating procedures, additional prospective research should focus on infants born more recently.

Trial registration: Not applicable.

Background

Yearly, approximately 1 million babies in developed countries are born preterm (before 37 weeks of gestational age (GA)), which corresponds to 8.6% of all births in these countries. Trends show that rates of prematurity are increasing, which makes it a substantial burden [1, 2]. Prematurity and low birth weight (LBW) are known to be important risk factors for neonatal morbidity and mortality [3, 4]. Low birth weight can be defined as very low (VLBW) and extremely low (ELBW) when birth weight is lower than 1500 or 1000 grams, respectively [5]. Complications of preterm birth lead to an estimated 35% of deaths among newborns, making them the leading cause of neonatal deaths [6]. Respiratory morbidities in particular are the number one cause of neonatal intensive care admissions [7]. Neonatal respiratory morbidity is inversely related with GA and birth weight, since lung development is not fully completed at the time of delivery, making infants born extremely preterm and those with an ELBW belong to the most vulnerable group [4, 7, 8]. Preterm birth is a complex entity in which the adaptation from a fluid filled uterus to air breathing is essential. A mature surfactant system and a perinatal catecholamine surge are necessary for effective lung fluid clearance at birth [7–11].

The disturbances caused by an underdeveloped respiratory system may cause respiratory distress syndrome (RDS), a disease of progressive alveolar collapse which occurs in 60–80% of extremely premature (< 28 weeks GA) and ELBW infants [3, 7]. These infants often have a need of ventilation with positive pressure to increase surfactant production and secretion [9]. Optimal treatment includes oxygen administration and mechanical ventilation (MV), which is nevertheless accompanied by several risks like atelectotrauma, volutrauma and the forming of free radicals [7]. These affect vascular and alveolar development and thus injure the immature lung. The lung damage in preterm and VLBW infants caused by requiring MV and oxygen therapy for prolonged periods, can lead to the development of bronchopulmonary dysplasia (BPD) [3, 7, 11]. BPD is characterized by alveolar hypoplasia, saccular wall fibrosis and variable smooth muscle hyperplasia causing a reduction in the surface area available for gas exchange. The exact pathogenesis remains unknown, but immaturity of the pulmonary vasculature, inflammation and high airway pressure are thought to play a key role in the progression of lung damage [7, 12, 13]. BPD occurs mostly in extremely preterm (< 28 weeks GA) and/or ELBW infants [7, 13, 14] and with more of these infants surviving, the incidence of BPD among this group is increasing [15]. Another complication of MV is overinflation, which may lead to alveolar rupture and subsequently pneumothorax, a potential life-threatening condition with a high mortality rate [7].

Next to prematurity and ventilatory treatment, the method of delivery may also contribute to adverse respiratory outcomes in neonates. Cesarean section (CS) is known to reduce perinatal passive lung fluid clearance from the airspaces, impeding gas exchange [8, 11]. Furthermore, these infants are known to reach lower catecholamine levels than infants born vaginally, leading to a lower surfactant secretion [11]. The method of delivery may be associated with mechanical ventilation, a known risk factor for respiratory morbidities [16–18]. An association between CS and the risk of RDS and pneumothorax was found [19, 20], but large studies on the method of delivery associated with BPD are not available. Known risk factors for respiratory morbidities, such as CS, are thought to act differently in different GA groups [21]. Earlier studies confirm the association between CS and respiratory morbidities in moderate and late preterms (GA 30–37 weeks) and suggest that lower GA further increases this risk [22–25], as does low birth weight independently [21, 26, 27]. A study in Norway investigated CS in very preterm newborns (≤ 29 weeks GA) with outcomes of pneumothorax and the need for continuous positive airway pressure (CPAP) and MV [28]. BPD has only been taken into account as a separate outcome in a group of breech-presenting neonates of 23–24 weeks GA [29] and in twins between 26–31 weeks GA [30].

There is only limited knowledge about the influence of the method of delivery on respiratory outcomes in extremely preterm and ELBW infants, a high-risk group, with a lack of information on the diagnosis BPD. The aim of the current study was to investigate whether there is an association between the method of delivery and respiratory morbidity and ventilatory treatment in extremely preterm and ELBW infants. Knowledge about whether there is an association between the method of delivery and respiratory treatment, MV in particular, and developing RDS, pneumothorax and/or BPD in extremely preterm and ELBW infants can contribute to an earlier identification of newborns at risk in whom (new) preventive and/or early supportive treatment strategies could be most effective.

Methods

Results

Discussion

In order to develop new, preventive and/or early supportive respiratory treatment strategies for extremely preterm newborns, registration and correlation of the clinical data on risk factors and clinical respiratory outcomes is necessary. We therefore studied the association between the method of delivery and respiratory outcomes in extremely preterm and ELBW infants, who were born between 2000–2003 in the MUMC+. A non-equal distribution of GA and birth weight SDS was found between VD infants and CS infants, with different risk profiles for preterm birth indication and respiratory outcomes. This finding was unexpected. However, the standardized operating procedures (SOPs) which were applied in the Netherlands during those years were different from the SOPs which are currently applied. Preterm CS with an indication of IUGR were almost never conducted below a GA of 28 weeks because of a ‘no, unless’-policy [37], resulting in most of the births < 28 weeks being VD.

Since preterm birth is directly associated with poorer neonatal outcomes, we were not surprised to find different incidences for PDA and IVH [38]. Furthermore, LBW correlates with a lower functional residual capacity (FRC) and causes a more compliant chest wall, leading to more difficulties in drawing the first breath [7]. This makes growth restriction an important confounder for respiratory outcomes. Therefore, as expected, these differences disappeared after correcting for the most important confounders: GA, birth weight SDS, antenatal steroids, fetal distress, 1-minute Apgar score and 5-minute Apgar score. An independent association was found between the method of delivery and surfactant, after adjusting for GA and birth weight SDS. This confirms the knowledge that CS may lead to a lower endogenous surfactant secretion due to lower catecholamine levels, causing these infants to be in need of exogenous surfactant therapy [11]. The higher catecholamine levels during fetal distress on the other hand, explain why this association disappears after correcting for fetal distress [39].

Surprisingly, no other independent associations were found between the delivery method and respiratory morbidity, invasive MV and CPAP. This is in contrast to the study by Werner et al. who did find an association between CS and RDS [40]. However, the latter study included a different patient group, namely preterms (25–34 weeks GA) who were small for GA (birth weight < 10th percentile of standard birth weight for their GA). In another study in preterms (≤ 30–36 weeks GA), a higher incidence of pneumothorax was reported in the CS group and preterms in this group were more in need of CPAP and MV [28]. The discrepancies in results between these studies and ours could be due to the different study populations. Still, our findings were consistent with Sangkomkamhang et al. [41], who reported no difference in the incidence of RDS, although they also included a different study population (infants born at GA ≤ 36 weeks). Since a difference in surfactant treatment was found between our two groups, we speculated that preventive treatment might have been applied more effectively in the CS group. This was tested by additionally adjusting for surfactant in all the models [Additional file 3], which should eliminate differences in RDS and pneumothorax diagnosis but not in BPD incidence [42]. However, an influence of surfactant on respiratory morbidity and respiratory support was not found. Another possible explanation was the lack of maternal medical information, which might contain confounders. No information about the onset of labor was taken into account, because this information was unavailable. The CS infants of the mothers who were already in labor are thought to have better respiratory outcomes due to a higher catecholamine release and a higher uterine tone because of present contractions. The surfactant system is activated during labor and an increase of cortisol and thyroid hormones as a reaction to the birth process promote the neonatal transition from fetal to extrauterine lung function [9, 43]. A large proportion of emergency CS in our cohort could have influenced the magnitude of our results. We therefore speculated that the birth process of the CS group in this study was more stimulating regarding the neonatal respiratory transition, than the birth of infants born by CS at a higher GA. This could then explain why there were less respiratory complications in our group than expected. To further investigate this, future studies should focus on respiratory outcomes after CS, adjusted for different GA groups.

One of the strengths of this study was that it was conducted in the only regional tertiary center in the Dutch province of Limburg, where care has been based on SOPs for attending policy. Multiple outcomes were documented in a database, combining several documents from the medical files of the patients, which improves the quality of the data. In order to reduce individual variation in care, the Netherlands have embraced SOPs very early [44]. Therefore, we rated the possibility of high variation in the clinical decision-making among the attending physicians neglectable. This was in particular highlighted in the SOP of surfactant treatment, in which precise cut offs for oxygen therapy mandating surfactant treatment are given. Furthermore, performing statistical corrections for potential confounders made our results more truthful. A limitation, however, was that data were collected retrospectively, leading to an inability to control exposure or outcome assessment and the need to rely on the accurate recordkeeping of others. Also, the 3-year interval of data collection and sample size were limitations. The diagnosis BPD has difficulties in itself, because the current definitions require a postmenstrual age of 36 weeks before classification can be done accurately [45]. The limitation of having an assessment at one time point is that infants who have died prior to this age, but who would have developed BPD later on, could not be included in the analyses. This may have led to seemingly better outcomes in BPD infants, because of missing recording of the outcomes of infants in the worst condition and clinical course. We have corrected for the problem of missing the recording of infants who were transferred or discharged before 36 weeks’ postmenstrual age, and who had thus not been diagnosed with BPD yet, by collecting data from medical records until the age of five. We, however, refrained from a combined outcome of death or BPD, which has been used to deal with this shortcoming [46].

Conclusions

Extremely preterm and ELBW infants belong to the group of neonates which is most at risk of adverse respiratory outcomes. We included two groups of preterm infants, who were delivered by different methods on account of the SOPs at that time and who proved to have different risk profiles for respiratory outcomes. The clinical implication of the group differences must be further studied in particular with long term outcomes at adult age. Our results suggest that after correcting for GA and birth weight SDS, the method of delivery is not associated with adverse respiratory outcomes in this high-risk group. Since SOPs have changed since 2003, additional prospective research should focus on infants born more recently.

Abbreviations

BPD = bronchopulmonary dysplasia

CPAP = continuous positive airway pressure

δ = difference

ELBW = extremely low birth weight

FRC = functional residual capacity

GA = gestational age

HELLP = hemolysis elevated liver enzymes low platelet count

HFOV = high frequency oscillatory ventilation

HFV = high frequency ventilation

IPPV = intermittent positive-pressure ventilation

IQR = interquartile range

IUGR = intrauterine growth restriction

IVH = intraventricular hemorrhage

LBW = low birth weight

LNR = Landelijke Neonaten Registratie (= Dutch national neonatal registration)

MUMC+ = Maastricht University Medical Centre +

MV = mechanical ventilation

NEC = necrotizing enterocolitis

nIMV = nasal intermittent mandatory ventilation

PDA = persistent ductus arteriosus

PROM = premature rupture of membranes

PVL = periventricular leukomalacia

RDS = respiratory distress syndrome

SD = standard deviation

SDS = standard deviation score

sIMV = synchronized intermittent mandatory ventilation

SOP = standardized operating procedure

TTN = transient tachypnea of the newborn

VD = vaginal delivery

VLBW = very low birth weight

Declarations

• Ethics approval and consent to participate: The study received a non-WMO declaration by the ethical review committee of the MUMC+ (research number: 2019-1311).

• Consent for publication: Not applicable.

• Availability of data and materials: The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

• Competing interests: The authors declare that they have no competing interests.

• Funding: Not applicable.

• Authors' contributions: Concept of study: BK, EV, RB, WG; Collected data from medical records: EV; Interpreted data and performed statistical analyses: EV, RB, BK, JN; Wrote manuscript: EV; Edited and approved manuscript: EV, RB, BK, WG, JN.

• Acknowledgements: The authors want to thank M.A.H.B.M. van der Hoeven, MD PhD, for assisting in obtaining the medical records.

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