The purpose of this study was to examine the effect of amnioinfusion on the incidence of meconium aspiration syndrome, perinatal asphyxia and obstetrical intervention rate amongst patients with moderate and thick meconium-stained liquor. A total of 112 patients were studied in a prospective fashion; 60 received amnioinfusion \ and 52 served as controls. We have shown that amnioinfusion is a simple, relatively inexpensive and safe procedure. It benefits the patient in terms of reduction in the likelihood of operative intervention (specifically Caesarean section) for fetal distress and improvement in neonatal outcome with reduced frequency of neonatal acidaemia
Meconium-stained liquor (MSL) occurs in 8% to 16% of all deliveries (1,2). It is found more commonly in cases of postmaturity (3). Meconium aspiration syndrome (MAS) occurs in approximately 1% to 3% of cases characterized by meconium-stained amniotic fluid (4). Although MAS is uncommon in modern obstetric practice, the presence of meconium in the amniotic fluid is associated with increased neonatal morbidity and mortality; the neonatal mortality asso- ciated with MAS may be as high as 28% (5,6). The presence of meconium at any time, but particularly in the intrapartum period, is an indication for careful observation, if not active intervention. A difference of clinical opinion exists regarding the significance of meconium passage in utero. Feton and Steer considered the passage of meconium a normal event with increasing fetal maturation especially beyond 38 weeks (7). Saling suggested hypoxia acts to stimulate peristalsis and sphincter relaxation (8).
A third possi- bility was proposed by Hon who considered that me- conium passage, particularly in the more mature fetus, may be a function of vagal stimulation secondary to umbilical cord compression (9). It is likely that one or more of these mechanisms is active in any individual case. Aspiration of meconium may occur before labour, during labour or at delivery. Tenaceous material, drawn into the airway by the fist respiratory effort, may result in lung collapse with other areas of overinflation due to a ball-valve effect of meconium plugs. The infant may have severe respiratory difficulty with complications such as pneumothorax and pneumomediastinum which may be fatal. no empirical approaches for prevention of meconium aspiration have been used in perinatal care. Gregory recommended immediate endotracheal suction of all newborns delivered through a thick meconium-stained field (2). Carson and colleagues, however, suggested that endotracheal suction may not be necessary if the obstetrician routinely suctions the fetal airway prior to delivery of the shoulders (1). A DeLee suction catheter should be passed through the nares to the level of nasopharynx and any meconium aspirated. If meconium is present in the oropharynx after delivery, the vocal cords must be visualized by direct laryngoscopy. Any meconium present at the level of the cords is an indication for direct suctioning of the trachea either with the DeLee catheter or under direct visualization through an endotracheal tube. Despite introduction of the aggressive tracheal suc- tioning method suggested by Carson which reduced the incidence of meconium aspiration syndrome, it still occurs in 1-2% of deliveries in our unit. In 1983, Miyazaki and Taylor first described the use of amnioinfusion with saline to relieve variable or prolonged decelerations (10,ll).
Later Nageotte and coworkers applied this prophylactic measure to prevent variable decelerations in patients with preterm rupture of membrane (12). Recently Sadorsky et a1 have evalu- ated the benefits of amnioinfusion in labours com- plicated by meconium in order to reduce the concen- tration of meconium. In. this setting, amnioinfusion significantly decreased the thickness of meconium, the likelihood of meconium below the vocal cords, and the incidence of neonatal acidaemia (13). From these studies, intrapartum dilution of mec- onium by transcervical amnioinfusion may reduce the risk of meconium aspiration occurring during labour or at delivery. It may also reduce neonatal morbidity in those suffering from meconium aspiration. Miller et a1 have shown that there is no significant difference in outcome between labour complicated by thin MSL and those with clear liquor (14). This study was designed to examine the effect of amnioinfusion on the incidence of meconium aspiration syndrome, perinatal asphyxia and obstetrical intervention rate amongst patients with moderate and thick MSL.
MATERIALS AND METHODS
Between April, 1991, and May, 1992, 112 patients at the Prince of Wales Hospital were studied in a pros- pective fashion. The patients included women in labour who were found to have more than a trace of meconium (moderate or thick meconium) on rupture of the membranes. Those with thin meconium were excluded. Additional inclusion criteria were singleton pregnancy, vertex presentation, at least 37 completed weeks of gestation, normal baseline and variability of fetal heart rate, and anticipated time to delivery of more than 2 hours (initial cervical dilation less than 8 cm). Exclusion criteria were fetal malpresentation, multiple gestation, fetal mal- formation, polyhydramnios, chorioamnionitis, cord prolapse, vaginal bleeding, moderate or severe variable or late decelerations, significant maternal disease states such as severe pregnancy-induced hypertension or insulin-dependent diabetes mellitus and any other situation that required urgent intervention.
Each patient was assigned to an amnioinfusion or the control group according to their own decision after full explanation of the risk of meconium aspiration and the objectives and technique of amnioinfusion. Inform- ed consent was obtained. Labour was managed in the left or right lateral recumbent position to avoid supine hypotension. Both the control and amnioinfusion groups received standard labour management by the midwives and obstetric on- call staff. In the amnioinfusion group, a normal saline solution was connected to an intrauterine pressure catheter. One litre of saline was infused at a rate of 10-15 ml per minute at room temperature (Miyazaki and Taylor noted that the blood warmer proved to be un- necessary as long as the infusion rate was limited to 10-15 ml per minute).
The amnioinfusion was stopped when completed or on reaching the second stage of labour or if labour was terminated earlier by Caesarean section. Amnioinfusion was repeated after a period of 4 hours if the patient remained undelivered. Continuous fetal heart monitoring was performed using external Doppler probes as a routine, using scalp electrodes only if external fetal heart monitoring was unsatisfactory. Fetal scalp blood sampling (FBS) was employed if an abnormal FHR pattern was detected. A fetal scalp blood pH greater than 7.25 was considered to be normal and labour allowed to continue. Where the pH was less than 7.20, immediate delivery of baby was indicated. If the pH was in the range of 7.20 to 7.25, this was considered a pre-acidotic stage and the sample was repeated within 30 minutes. Patients were allowed to labour, and a paediatrician was called to all deliveries. On delivery, cord blood gas values were obtained
The combined obstetric-paediatric suctioning approach to prevent meconium aspiration was used in all deliveries. The neonatal outcome was assessed by the paediatrician who was unaware of the group to which the patient had been allocated. It should be noted that the study was not blinded or randomized. In order to eliminate possible bias, the clinical investigators were not involved in the manage- ment of labour. Allocation of patients to amnioinfusion or control group was entirely at the patientkdecision after counselling by midwives assisted by a written explanation of the technique and its objective (in both English and Chinese versions). Data was analyzed by the independent 2 tailed Student t-test, chi X’ test or Fisher’s exact test where appropriate.