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great article: Meconium Aspiration, Prevention and ManagementFrom: GIN11153@aol.comThu Mar 19 23:47:53 2009
go to the site to see the charts, links and diagrams because I don't think the listserv servers let them through: _http://emedicine.medscape.com/article/1413467-overview?src=emed_whatnew_nl_0# MAS_ (http://emedicine.medscape.com/article/1413467-overview?src=emed_whatnew_nl_0#MAS) Meconium Aspiration, Prevention and Management Author: Taylor L Sawyer, DO, Instructor in Pediatrics, Uniformed Service University of the Health Sciences; Assistant Clinical Professor of Pediatrics, University of Hawaii, John A Burns School of Medicine; Associate Fellowship Director, Neonatal-Perinatal Fellowship Program, University of Hawaii, John A Burns School of Medicine, Tripler Army Medical Center, Kapiolani Medical Center for Women and Children Coauthor(s): Mark W Thompson, MD, Chief of Pediatrics, Pediatric Consultant to Surgeon General, Tripler Army Medical Center; George Graham, MD, Assistant Professor, Maternal Fetal Medicine, University of Hawaii Introduction Meconium is a viscous green-black substance that consists of denuded intestinal epithelial cells, ingested lanugo hair, mucus, digestive enzymes, bile acids, and water. The term meconium is derived from the Greek word mekoni, which means poppy juice or opium. This is in reference to either its tarry appearance or to Aristotle's belief that it induced sleep in the fetus. Meconium constitutes the first stool of a newborn infant. The passage of meconium typically occurs within 48 hours after birth; however, it can occur in utero. Intrauterine meconium passage has been linked to fetal hypoxia and is associated with fetal acidosis, abnormal fetal heart tracings, and low Apgar scores._1_ (javascript:showcontent('active','references');) In preterm pregnancies, intrauterine meconium passage has been associated with fetomaternal stress and/or infection._2_ (javascript:showcontent('active','references');) In term and post-term infants without fetal distress, intrauterine meconium passage may result from normal gastrointestinal maturation or from vagal stimulation from head or cord compression._3_ (javascript:showcontent('active','references');) Meconium staining of the amniotic fluid occurs in approximately 13% of live births; this percentage increases with increasing gestational age at delivery._3_ (javascript:showcontent('active','references');) _Meconium aspiration syndrome_ (http://emedicine.medscape.com/article/974110-overview) (MAS) occurs when meconium-stained amniotic fluid (MSAF) is aspirated into the lungs of an infant prior to, during, or immediately after birth. Intrauterine gasping resulting in aspiration of meconium has been shown in animal models exposed to hypoxia._4, 5, 6_ (javascript:showcontent('active','references');) MAS occurs in approximately 5% of infants born through MSAF._3_ (javascript:showcontent('active','references');) Even with modern neonatal intensive care, the mortality rate from MAS remains as high as 3-5%._1, 7_ (javascript:showcontent('active','references');) Many perinatal risk factors have been associated with meconium aspiration, including placental insufficiency, maternal hypertension, _maternal diabetes mellitus_ (http://emedicine.medscape.com/article/127547-overview) , _preeclampsia_ (http://emedicine.medscape.com/article/953579-overview) , _oligohydramnios_ (http://emedicine.medscape.com/article/405914-overview) , and maternal tobacco use._3_ (javascript:showcontent('active','references');) But perhaps the most significant risk factor for meconium aspiration is post-term delivery. In one prospective clinical study, a decrease in the incidence of MAS from 5.8% to 1.5% over an 8-yr period was attributed to a reduction in births at more than 41 weeks gestation._8_ (javascript:showcontent('active','references');) MAS occurs along a continuum from mild to severe._9_ (javascript:showcontent('active','references');) Mild MAS is seen in infants born through MSAF who have mild respiratory symptoms; it likely reflects mild parenchymal irritation from aspirated meconium. Moderate MAS presents with more pronounced pulmonary symptoms, including moderately high oxygen requirements and possible need for mechanical ventilation. This may represent a more significant meconium load or the aspiration of thicker meconium into the lungs._9_ (javascript:showcontent('active','references');) Infants with severe MAS require mechanical ventilation with high settings and may need alternative therapies, such as inhaled nitric oxide and, possibly, _extracorporeal membrane oxygenation_ (http://emedicine.medscape.com/article/904996-overview) (ECMO). These cases likely represent a combination of meconium aspiration and _persistent pulmonary hypertension of the newborn_ (http://emedicine.medscape.com/article/898437-overview) (PPHN). PPHN in these cases is thought to arise from chronic fetal compromise (hypoxia) with resultant pulmonary vascular remodeling. Recent evidence suggests that chronic in utero hypoxia with resultant PPHN, rather than the aspiration of meconium per se, may be the primary pathologic problem in newborn infants diagnosed with severe MAS._10, 11_ (javascript:showcontent('active','references');) (javascript:showcontent('active','hiddenlayerd26e564');) Pathophysiology of meconium aspiration (adapted from Wiswell T, Bent RC. Meconium staining and the meconium aspiration syndrome. Pediatric Clinics of North America.1993;40(5):955-981). Pathophysiology of meconium aspiration (adapted from Wiswell T, Bent RC. Meconium staining and the meconium aspiration syndrome. Pediatric Clinics of North America.1993;40(5):955-981). Several procedures have been used in the past to prevent MAS. None has strong evidence of proven benefit. This article focuses on the one preventative procedure that is still supported by the American College of Obstetricians and Gynecologists (ACOG) and the Neonatal Resuscitation Program (NRP): selective intubation and tracheal suctioning of the nonvigorous infant._12, 13_ (javascript:showcontent('active','references');) Selective intubation of the nonvigorous infant, rather than the prior practice of universal intubation of all infants born through MSAF, is based primarily on the work of Wiswell et al. A large, multicenter, prospective, randomized controlled trial involving 2094 apparently vigorous infants born through MSAF showed no decrease in the incidence of MAS with universal intubation compared with no intubation._14_ (javascript:showcontent('active','references');) A Cochrane Systematic Review of 4 other randomized and quasi-randomized trials also failed to show a difference in the incidence of MAS between intubated and nonintubated apparently vigorous newborns._15_ (javascript:showcontent('active','references');) Several other preventive measures commonly used in the past to prevent MAS, including amnioinfusion and intrapartum oronasopharyngeal suctioning, have been abandoned after results of recent randomized controlled trials. Amnioinfusion involves the infusion of isotonic fluid, either normal saline or lactated Ringer's solution, into the amniotic cavity via a transcervical intrauterine pressure catheter in an attempt to dilute the MSAF. Results of previous trials using amnioinfusion to prevent MAS have been mixed. A 2002 Cochrane Systematic Reviews concluded that amnioinfusion was effective in decreasing the incidence of MAS, especially in centers where perinatal surveillance was limited._16_ (javascript:showcontent('active','references');) However, given the heterogeneity of the studies included, and the small number of patients in each study, these results must be interpreted with caution._1_ (javascript:showcontent('active','references');) In a recent multinational, multicenter, randomized controlled trial involving 1998 women with thick MSAF, amnioinfusion failed to show an effect on the incidence of MAS or death._17_ (javascript:showcontent('active','references');) These findings, and the supposition that a large number of infants born through MSAF will have aspirated meconium before an amnioinfusion can be performed, prompted an ACOG opinion stating that "routine prophylactic amnioinfusion for the dilution of meconium-stained amniotic fluid should be done only in the setting of additional clinical trials."_18_ (javascript:showcontent('active','references');) In this same statement, however, ACOG noted that "amnioinfusion remains a reasonable approach in the treatment of repetitive variable decelerations, regardless of amniotic fluid meconium status."_18_ (javascript:showcontent('active','references');) Intrapartum oropharyngeal and nasopharyngeal suctioning with a DeLee suction catheter before delivery of the shoulders, combined with intubation after delivery, has been the standard practice to prevent MAS since the late 1970s. Although routine suctioning of the oropharynx is appropriate, recent evidence has argued against the efficacy of intrapartum suctioning of meconium using a DeLee suction catheter._19_ (javascript:showcontent('active','references');) In a multicenter, randomized controlled trial involving 2,514 full-term women with MSAF, Vain et al failed to show a benefit of intrapartum suctioning with a suction catheter on the need for endotracheal intubation, incidence of MAS, need for mechanical ventilation, and neonatal mortality._20_ (javascript:showcontent('active','references');) Based on this evidence, the ACOG Committee on Obstetric Practice recently recommended that "infants with MSAF should no longer receive intrapartum suctioning. If meconium is present, and the newborn is depressed, the clinician should intubate the trachea and suction meconium from beneath the glottis."_12_ (javascript:showcontent('active','references');) Since meconium aspiration can occur prior to the time of delivery because of chronic asphyxia and infection, perhaps the most important preventive strategy is good prenatal care, including the detection and prevention of fetal hypoxemia and the avoidance of post-date deliveries. Indications The indication for selective intubation and tracheal suctioning of the nonvigorous infant includes any infant born through meconium-stained amniotic fluid (MSAF) who is nonvigorous. The NRP defines a nonvigorous infant as an infant who meets one or more of the following conditions:_13_ (javascript:showcontent('active','references');) 1. Depressed respirations 2. Depressed muscle tone 3. Heart rate <100 bpm The consistency of the meconium in the amniotic fluid (thin versus thick) is no longer used to determine the need for tracheal suctioning._13_ (javascript:showcontent('active','references');) Contraindications A contraindication to selective intubation and tracheal suctioning of an infant born through meconium-stained amniotic fluid (MSAF) is apparent vigor. The NRP defines a vigorous infant as one with all of the following:_13_ (javascript:showcontent('active','references');) 1. Strong respiratory effort 2. Good muscle tone 3. Heart rate >100 bpm
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