A large number of clinical studies have confirmed that early use of PS combined with ventilator assisted ventilation is an effective method for the treatment of NRDS [1, 2, 3], but there is no clear indication of the appropriate time limit point for the clinical application of PS. Prospective studies  have shown that FiO2 is significantly reduced in children who received PS treatment earlier than 3 hours after PS administration, and the improvement in oxygenation was related to the increase of ventilation/perfusion (V/Q) caused by the rise of the functional residual gas (FRC). SR treatment changes lung function first by stabilizing the alveoli with ventilation function, and then collecting a new gas exchange unit to improve FRC, increase the gas exchange area, and increase the V/Q, so as to improve gas exchange. This study showed that there was no significant difference in blood gas analysis parameters between the two groups before treatment. After 12 hours and 72 hours of PS treatment, FiO2 and PaCO2 in the two groups were significantly decreased, while PaO2 and OI were significantly increased, both of which were significant improved compared with that before PS treatment (P<0.05). The decrease in blood gas PaCO2 in group A before and after treatment was more significant than that in group B (P<0.05), but the improvement of PaO2 and OI in group B was better than that in group A (P<0.05). General statistics of the two groups demonstrated that except that the birth weight of group A was significantly lower than that of group B (P<0.05), there was no statistically significant difference between the two groups. However, in group A with low body weight, the decrease of blood gas PaCO2 was more significant than that in the group B after PS treatment.
An improvement in PaCO2 also led to local and/or systemic hemodynamic changes which will affect the function and operation of other organs of premature infants. A previous study of bedside echocardiographic monitoring for children with NRDS found that systemic blood flow changes occurred in children 10 minutes after PS administration (including superior vena cava flow, right ventricular output, PDA diameter, and vhanges in blood flow in the foramen ovale) long before FiO2 to reduce . Although this hints in the clinical application of SR can improve the patient's lung ventilation and oxygenation, but after PS treatment, pulmonary vascular resistance decreased, pulmonary blood flow increased, PDA and body lung circulation pressure gradient increased, systemic blood flow and terminal organ perfusion decreased , which made PS potentially problematic, such as transient airway obstruction, bradycardia, decreased oxygen saturation, intracranial hemorrhage (IVH), pulmonary hemorrhage (PH), etc.
Pulmonary hemorrhage (PH), which is one of the complications of early death in children with NRDS during clinical treatment, is concerned with the risk of PH in preterm infants with RDS after SR. Premature infants with RDS after SR have an increased risk of PH. Possible causes include: vaginal delivery, low birth weight, low gestational age, male and PDA. After SR, the horizontal left to right shunt of the catheter will increase, and the pulmonary blood flow will increase rapidly, leading to high flow and high-pressure vascular bed injury, which is related to the occurrence of PH . A recent summary also suggests that early indomethacin closure of PDA (3-12h postnatal) can reduce the incidence of PH in preterm infants at 30 weeks of gestational age . In this study, it was shown that the incidence of PDA in group A treated with PS within 3 hours was significantly lower than that in group B, which made the incidence of PH in group A lower than that in group B despite the low birth weight. Thus, to some extent, the high mortality rate and hospitalization time of group A high-risk group with low body weight were avoided.
SR is also associated with changes in brain hemodynamics that may lead to intracranial hemorrhage (IVH) and periventricular white matter softening, but the mechanism of action is unclear. Studies have shown that after surfactant replacement（SR）, child blood pressure, CBF and CBV change, PaCO2 increases and EEG are inhibited . PaCO2 fluctuation can directly mediate brain injury by changing cerebral blood flow, and interaction between high oxygen exposure and PaCO2 fluctuation will increase the risk of BPD/death . In addition, excessive lung inflation after PS treatment can also impede venous return and promote IVH in premature infants who lack autonomic regulation of cerebral blood flow . In this study, two groups of children were treated with PS in a timely manner and strictly according to the clinical manifestations and blood gas analysis of the children, and the results of chest radiograph adjustment of ventilator assisted treatment ventilation parameters. There were no 3-4 degree intracranial hemorrhage complications, 1-2 There was no significant difference in the incidence of intracranial hemorrhage between the two groups (P>0.05). It should be said that there is a certain relationship with the reasonable control of the fluctuation of PaCO2 and the avoidance of high concentration of oxygen inhalation.
Statistical results showed that there was no significant difference in the incidence of complications NEC and ROP between group A and group B (P>0.05). The increase in the incidence of NEC may have been due to systemic hemodynamic changes after SR, decreased circulating blood flow and terminal organ perfusion, decreased gastrointestinal blood flow, tissue hypoxia, etc. ROP occurs mainly with retinal dysplasia in premature infants and long-term elevation of oxygen levels. In this study, FiO2 was significantly decreased in both groups A and B after treatment with PS, PaO2 and OI were significantly increased, and significantly improved compared with PS before use (P<0.05). All these are beneficial to reduce the incidence of NEC and ROP. Although the duration of oxygen therapy in group A is relatively long, ROP can be effectively avoided as long as strict control is exercised to avoid high concentration of oxygen.
It is important to note that the incidence of BPD in group A was significantly higher than in group B (P<0.05). However, this does not prove that using PS earlier does not have any benefit in preventing the occurrence of BPD. Due to the fact that basic data analysis of the two groups of children showed that the birth weight of group A was significantly lower than that of group B (P<0.05), and the premature infants with low birth weight, especially those smaller than gestational age, were more likely to be congenital. The inevitable congenital disadvantages of poor bronchopulmonary development, weak respiratory muscle compensatory capacity, primary apnea, feeding difficulties and growth retardation are also a reality in preterm babies.