BCC Lam, WK To, CY Yeung

Abstract

This is a prospective study of the perinatal characteristics of 427 consecutive infants who were born with meconium stained amniotic fluid (MSAF). They were managed by combined obstetric and paediatric suctioning of the airways at birth. The incidence of meconium aspiration syndrome (MAS) was 11 500 of infants born with MSAF. Meconium aspiration syndrome (n=49) especially severe MAS (n=6) were more likely to occur in several conditions, viz (1) liquor was thickly meconium-stained, (2) infant was apnoeic before intubation suctioning, (3) infant with lower apgar score at 5 minutes and (4) infants who needed intermittent positive pressure ventilation for resuscitation at birth. Among 49 infants with meconium aspiration syndrome most recovered well. Fourteen infants (28.5%) required oxygen supplement, six (12%) required mechanical ventilation and eight (18%) developed air leak complication. Four (8%) progressed to persistent pulmonary hypertension as confirmed by echocardiogram. Identification of perinatal risk factors associated with MAS may enable close monitoring and early intervention.

Introduction

Meconium staining of the amniotic fluid is a common condition affecting 13% (range 5.6% - 24.6%) of all deliveries.¹ The importance of intra-uterine passage of meconium was first investigated by Jesse as early as 1888.² Since then, the clinical significance of meconium stained amniotic fluid has generated considerable debate in obstetric and neonatal literature. Passage of meconium in-utero can be a sign of fetal hypoxia,³ however, it can also represent a normal physiologic maturation of the fetal gastro-intestinal tract under hormonal and neural control.⁴ In-utero passage of meconium can also result from vagal stimulation from transient umbilical cord entrapment and the resultant increased peristalsis.⁵ Even the etiologies appeared multiple, the fact that MSAF may associate with fetal compromise; the detection of meconium in the amniotic fluid often provokes obstetric and neonatal anxiety.

It has been variously reported that as many as 7-22% of infants born through meconium stained fluid develop meconium aspiration syndrome (MAS), representing 1-3% of all liveborn infants.^6-7 Many variables including fetal heart rate abnormalities, thick meconium stained amniotic fluid, low apgar score have been associated with higher risk of MAS.⁸ This study was undertaken to determine (1) the incidence and the perinatal factors associated with meconium stained amniotic fluid; (2) the frequency of MAS with the currently accepted practice of airway suctioning; and (3) to develop a model of prenatal and perinatal factors that may identify the fetus and newborn at risk of MAS for possible early intervention.

Patients and Methods

The study group consisted of infants delivered through meconium-stained amniotic fluid at Tsan Yuk Maternity Hospital from Jan 1995 - Dec 1995. Inclusion criteria were: delivery through meconium stained amniotic fluid, birth weight >2500gm, and gestational age >34 weeks. The perinatal characteristic of these deliveries were compared with labours with clear amniotic fluid.

The following treatment protocol was routinely practised for all infants. When meconium stained amniotic fluid was identified at rupture of the membranes or at delivery, the paediatrician was called to stand-by for the delivery. As soon as the head was delivered, the mouth and the pharynx were suctioned by the mid-wife or the obstetrician. After delivery, the cord was clamped and the baby was handed over to the pediatrician immediately. The trachea was suctioned via an endotracheal tube (size 3mm - 3.5mm) by connecting to a meconium aspirator which was connected to wall suction at a negative pressure of l0IKiPa. If thick meconium was present, repeated tracheal intubation and suctioning was performed. For babies who had cried vigorously at birth, direct laryngoscopy was performed, intubation was done only when there was visible meconium above the vocal cord. The nature of the meconium, its presence in relation to the trachea, the infant breathing effort before intubation and the Apgar score were recorded. The meconium was recorded as:- (I) thin - clear yellow and watery; (2) moderately thick- thick but still watery; (3) Very thick - green with particulate matter.

All babies were admitted to the neonatal unit for observation with oxygenation saturation monitoring. Meconium aspiration syndrome was defined as the development of respiratory distress in the first 6 hours after birth not explained otherwise and an abnormal chest roentgenogram.

All babies with symptoms of MAS were treated with intravenous antibiotics after cultures including surface swabs (ears, umbilical, eyes), gastric aspirates and the blood were taken. For babies with significant hypoxaemia requiring oxygen supplement, echocardiogram was performed to diagnose persistent pulmonary hypertension (PPHN). Echocardiographic diagnosis of PPITIN was based on the demonstration of tricuspid incompetence, right-to-left shunt at ductal level or atrial level by doppler study. Indication for mechanical ventilation included severe respiratory distress with Fi0₂ requirement of more than 0.5 to maintain Pa0₂ >= 8KPa and / or arterial PaCO₂ >= 7KPa.

Statistical Analysis

Both X² analysis and the student t test were used to define statistical significance between groups. Stepwise logistic regression analysis was used to develop a model of variables that would predict MAS.

Result

During the study period 437 infants (13%) of the 3360 live births were delivered through meconium stained amniotic fluid. When comparing labours associated MSAF (n=437) with clear AF (n=2923), there was no difference in maternal age, the proportion of primiparous or multiparous mothers, the incidence of major antepartum complications such as antepartum haemorrhage, gestational hypertension, gestational diabetes millitus and the duration of labour (Table I). Labours with MSAF were more likely to be associated with post-term gestation, intrapartum fetal blood sampling, caesarean section deliveries (CS) and instrumental deliveries, CS for fetal distress and epidural anaesthesia (Table II).

Of the 437 infants born through MSAF, ten infants were excluded because of incomplete data, or the turbid amniotic fluid was subsequently attributed to intra-uterine infection. The study group thus consisted of 427 infants. The clinical characteristics of these infants were shown in Table III. The occurrence of thick, moderate, thin MSAF was 37.4% 29.2%, & 33.4% respectively. 65% of the infants had tracheal suctioning before the first breath and 25.6% had moderate to copious amount of MSAF aspirated from the trachea. Forty-nine (11.5% of infants born through MSAF) infants developed meconium aspiration syndrome (MAS). When comparing the perinatal characteristics of these 49 infants with those infants without MAS, meconium aspiration was more likely to be associated with thick MSAF, lower median 5 minute Apgar score and apnoea before intubation (Table IV).

Of the infants with MAS, the majority (58%) recovered well with conservative management including antibiotic treatment. Eight infants (16%) had air leak complications on chest radiograph. Fourteen (28%) needed oxygen therapy in first 48 hours of life and 4 (8%) had echocardiographic evidence of PPHN. Six infants required mechanical ventilation for progressive respiratory failure (severe MAS). Babies who passed through thick MSL and required intubation for intermittent positive pressure ventilation were more likely to progress to severe MAS (Table V). There were no mortality in this study cohort.

Discussions

Traditionally, passage of meconium by the fetus in utero has been considered as a sign of fetal distress from hypoxia.⁹ Others suggested that this may represent normal physiologic maturation of the fetal gastrointestinal tract. Regardless of the explanation, MSAF is a common finding among term births and it occurred in 13% of the pregnancies in this study. The importance of meconium as an obstetric risk factor is difficult to interpret when so many pregnancies demonstrate this finding, yet so few have adverse perinatal outcomes. In this study, pregnancies with MSAF were not more likely to be associated with adverse prenatal risk factors such as hypertension or other maternal complications. However, it did provoke considerable obstetric anxiety regarding the status of fetal health. This explains our findings that labours with MSAF were exposed to more obstetric interventions including intrapartum fetal blood sampling, cesarean section and instrumental deliveries (Table II).

Recently, new obstetric intervention like amnioinfusion has been advocated to prevent MAS. Previous meta-analysis involving 5 publications concerning amnioinfusion¹⁰ showed a decreased incidence of meconium found below the vocal cords and a decreased incidence of MAS. However, more recent work^11-13 did not showed amnioinfusion to be associated with better apgar score, or decreased incidence of MAS. However, it is associated with more fetal heart rate abnormalities, and hence increased rate of cesarean section and instrumental deliveries. Even more worrying, the procedure may be associated with higher rates of postpartum endometritis and increased rate of neonatal sepsis. Therefore, there were no prenatal factors or intervention at present that can effectively identify or prevent MAS. Prenatal intervention does not appear to be a cost effective approach.

Combined obstetric and paediatric suctioning of the airway at birth has been associated with a decrease in the prevalence rate and the severity of MAS.^14,15 Despite these, 11.5% of the infants with MSAF who have been managed by these procedures developed MAS. This suggests that MAS may not be an entirely postnatal occurrence. Boddy & Dawes¹⁶ have demonstrated that hypoxic fetus developed deep intrauterine gasping following apnea attack, which places the fetus with MSAF at risk of aspiration. Evidence of in utero aspiration of amniotic fluid has been demonstrated at autopsies of human stillbirths.^6,17

All these findings suggested that MAS is not entirely preventable either prenatally or by intervention at birth. The purpose of this study was to determine the frequency of occurrence and the severity of MAS in a tertiary referral teaching hospital and to identify perinatal factors that can predict increased risk for MAS so that early interventional treatment such as close monitoring, early surfactant replacement therapy, or surfactant lavage can be implemented before the disease progresses to an advanced stage. The results of our study indicate that 49/427 (13%) of infants delivered through MSAF had meconium aspiration and 6/427 (1.4%) of infants had severe MAS requiring mechanical ventilation. These infants were more likely to be apneic at birth and have a lower median 5 minutes apgar score suggesting the occurrence of intrauterine hypoxia. The character of the observed meconium also has prognostic significance for perinatal outcome. The results of the current study and others have confirmed that thick meconium is an important risk factor for severe MAS.^8,18

In spite of recent development of new methods for treatment of severe MAS like high frequency ventilation, exogenous surfactant, inhaled nitric oxide, severe MAS continues to be the most common indication for extracorporeal membrane oxygenation and continues to cause considerable mortality. These new treatment modalities were employed only when the disease has progressed to an advanced stage. Recent studies in animals^19,20 and infants²¹ have shown that tracheo-bronchial lavage with diluted natural surfactant solution, when performed early, is a safe and effective method to prevent progression of severe MAS. Hence, identification of those infants who may progress to severe disease may enable early intervention.

In summary, in spite of the present practice of combined obstetric and paediatric suctioning of the airway, the incidence of MAS is 11.5% and severe MAS occurs in 1.4% of all babies born with MSAF. Babies with thick MSAF, apnoea at birth and low 5 minutes apgar score should be monitored closely for MAS and early intervention should be considered for severe MAS.

References

1. Wiswell TE, Bent RC: Meconium staining and the meconium aspiration syndrome. Pediatr Gun North Am 1993;40:955-871.

2. Schulze M. The significance of the passage of meconium during labor. Am J Obstet Gynecol 1925;10:38-8.

3. Walker J. Foetal anoxia. J Obstet Gynaecol Br Common W 1953;61:162-80.

4. Matthews TG, Warshaw JB. Relevance of the gestational age distribution of meconium passage in utero. Pediatrics 1979;64:30-1.

5. Hon EH, Bradfield AH, Hess OW. The electronic evaluation of the fetal heart rate. Am J Obstet Gynecol 1961;82:291-300.

6. Brown BL, Gleicher N. Intrauterine meconium aspiration. Obstet Gynecol 1981;57:26-9.

7. Gregory GA, Gooding CA, Phibbs RH, Tooley WH. Meconium aspiration in infants - a prospective study. J Pediatr 1974;85:848-52.

8. Urbaniak KJ, McCowan MW, Townend KM. Risk factors for meconium aspiration syndrome. Aust N Z J Obstet Gynaecol 1996;31:4:401-6.

9. Desmond MM, Moore J, Lindley JE, et al. Meconium staining of the amniotic fluid - a marker of fetal hypoxia. Obstet Gynecol 9:91,1957-60.

10. Dye T, Aubry R, Gross S, et al. Amnioinfusion and the intrauterine prevention of meconium aspiration. Am J Obstet Gynecol 1994;171:1601-5.

11. Lo K W K, Rogers M. A controlled trial of arnnioinfusion The prevention of meconium aspiration in labor. Aust N Z J Obstet Gynaecol 1993;33:51-5.

12. Spong CY. Amnioinfusion : Indications and controversies. Contemp OB/Gyn 1997;42:138-42.

13. Usta IM, Mercer BM, Aswad NK, et al. The impact of a policy of amnioinfusion for meconium stained amniotic fluid. Obstet Gynecol 1995;85:237-42.

14. Carson BS, Losey RW, Bowes WA, et al. Combined Obstetric and pediatric approach to prevent meconium aspiration syndrome. Am J Obstet Gynecol 1976;126:712-15.

15. Ting P, Brady JP. Tracheal suction in meconium aspiration. Am J Obstet Gynecol 1975;122:767-71.

16. Boddy K, Dawes GS. Fetal breathing. Br Med Bull 1975;31:3-7.

17. Manning FA, Schreiker J, Turkel SB. Fatal meconium aspiration "in utero": a case report. Am J Obstet Gynecol 1978;132:111-3.

18. Rossi ME, Philipsm EH, Williams TG, Kalhan SC. Meconium aspiration syndrome : Intrapartum and neonatal attribute. Am J Obstet Gynecol 1989;161:1106-10.

19. Ohama Y, Itakura Y, Koyama N, et al. Effect of surfactant lavage in a rabbit model of meconium aspiration syndrome. Acta Paediatr Japonica 1 994;36:236-8.

20. Paranka MS, Walsh F, Stancombe BB. Surfactant lavage in a piglet model of meconium aspiration syndrome. Pediatr Res 1992;31:625-8.

21. Lam BCC, Yeung CY. Surfactant lavage for meconium aspiration syndrome. J Pediatr Child Health 33 (supplement): S77, 1997 Proceedings on "Hot Topics" in Neonatology. Special Ross Conference Washington D.C. Dec 1997.