Discussion
We found that the mortality rate of preterm infants with MAS was 33.7%, which was similar to that in the study of Louis et al [8]. As mentioned above, this life-threatening condition remains a relatively high case fatality rate. Identifying the specific prediction risk factors of poor prognosis might be helpful to improved survival of preterm infants affected by MAS.
In this study, persistent pulmonary hypertension was considered as an independent risk factor associated with mortality in preterm infants with MAS. It was reported that PPHN increased the risk of death in infants diagnosed with MAS in the study of Louis and colleague[8]. PPHN is more commonly associated with diverse lung pathologies including MAS, perinatal asphyxia, congenital diaphragmatic hernia, pneumonia and respiratory distress syndrome[13]. MAS cause parenchymal lung disease with variable degrees of chemical pneumonitis, surfactant inactivation and release of inflammatory mediators, leading to hypoxemia, acidosis and hypercapnia, ultimately results in pulmonary vasoconstriction[14,15]. In clinical MAS, persistent pulmonary hypertension of the newborn is the leading cause of death in MAS. The overall mortality of infants with PPHN has been reported to be 12%-29% [16-18].
Pulmonary hemorrhage was another independent risk factor associated with mortality in preterm with MAS in our study. It occurs mainly in preterm infants with severe respiratory distress syndrome. Risk factors for pulmonary hemorrhage include severity of associated illness, intrauterine growth restriction, prematurity, PDA, coagulopathy and surfactant therapy [12,19]. Meconium aspiration may lead to uneven airway obstruction and dysfunction of ventilation, finally result in pulmonary hemorrhage. Pulmonary hemorrhage is associated with significant high mortality, which is as high as 50% to 82% [20-22].
Female was found to be another risk factor associated with mortality in preterm infants with MAS in our study, whereas relevant evidence was insufficient. The development and prognosis of some neonatal diseases (eg, neonatal respiratory distress syndrome, glucose-6-phosphate dehydrogenase deficiency, and congenital diaphragmatic hernia) were considered gender-related [23-25]. Although we can’t find any strong evidence to explain the effect of gender on increasing the rate of mortality in preterm infants with MAS. Our finding is meaningful and deserves further exploration in the future research.
Lin et al [10] analyzed 314 cases of MAS between 1995 and 2001, and found that infants who required resuscitation in the delivery room, with birth asphyxia, developed persistent pulmonary hypertension and pneumothorax were associated with increased mortality in MAS. Another retrospective study reported that myocardial dysfunction, birth weight, and higher initial oxygen requirement were independent predictors of mortality in neonates with MAS[8]. These above studies included infants irrespective of gestational age, additionally, our study included a relatively small quantity of patients, which may explain the different conclusions of our studies.
Our study has several limitations. The sample size of our study is relatively small. As a case–control study, the inherent shortcomings of a case-control design (eg, selection bias and recall bias) were difficult to avoid. Some of the patients were transferred to our center from other hospitals, and information with regard to the details of treatment protocols performed outside of our hospital were limited.