Pulmonary hemorrhage in neonates is mostly hemorrhagic, the pathophysiological changes this time is mostly about increase vascular filtration pressure, increased ventricular preload . Hemorrhaging thus leads to hypovolemia, and if there is a lack of relatively aggressive fluid resuscitation during this period and an untimely response to hypovolemia, dramatic circulatory volume fluctuations and inadequate perfusion of vital organs such as the heart and brain may lead to rapid progression of the conditions . Empirical and stereotyped correction of acidosis and transfusion may cause fluid overload. Increased secretion of Endothelin-1 (ET-1) of the vascular endothelial cells is triggered in response to PH. Contraction of smooth muscle and aggravated pulmonary edema can result in elevated pulmonary arterial pressure. Hypoxemia resulted from PH may cause hypoxic myocardial injury and cardiac dysfunction. All these factors lead to increased circulatory burden, causing tissue edema, organ dysfunctions and circulatory failure. Consequently, relatively limited fluid intake to alleviate the circulatory burden and thus reverse the process of pulmonary edema may avoid the development of cardiac failure, eventually reduces mortality .
Therefore, the introduction of volume assessment to guide fluid therapy in neonatal PH is crucial. In contrast, preterm infants are inherently characterized by unclosed ductus arteriosus, unclosed foramen ovale, immature lung development, relative pulmonary hypertension, right ventricular predominance, and unstable circulatory status . Moreover, indicators more frequently used in clinical practice such as heart rate, blood pressure, urine volume, and lactate concentration are easily affected by many factors such as stimulation, body temperature and respiration, making it difficult to assess volume status accurately. In order to obtain dynamic indicators such as central venous pressure, pulmonary capillary wedge pressure, per beat variability and pulse pressure variability, invasive operations are required with complications associated with infection, thrombosis, hematoma, pneumothorax, and operation failure, making it even more difficult to obtain in preterm infants.
Recent years’ clinical practice and scientific research revealed the effectiveness of applying IVC-CI in dynamically assessing the volume status, making it a comprehensive evaluation index of cardiac functional status. Because of the fact that IVC-CI can be acquired in a non-invasive manner, repeated assessment can be performed to evaluate the effectiveness of fluid therapy . Zhu Weihua et al. found that compared with patients breathing spontaneously, IVC-CI can more accurately evaluate fluid load and fluid responsiveness in patients on mechanical ventilation, producing timely report on the effectiveness of the therapeutic approaches . The fast respiratory rate and the short distance from the inferior vena cava to the atrium in preterm infants make accurate measurement of IVC-CI relatively difficult. In contrast, children with pulmonary hemorrhage are critically ill and poorly responsive, or are passively ventilator-dependent due to sedation, which is suitable for the application of IVC-CI to assess volume status. In this study, hypoxia, hypercapnia, elevated lactate concentration, and increased heart rate were found in all patients with PH, with no significant difference between the two groups. The inferior vena cava collapse index was found to be between 50% and 75% in the early stage of PH in 22 patients in the experimental group, and Figure 1 reflects a positive correlation between the IVC-CI and the degree of hemoglobin concentration decline in the early stage of PH. Similar to the findings of E Wilkman et al. who reported that IVC-CI can accurately assess volume status at positive end-expiratory pressure ventilation mode .
The study on the relation between IVC-CI and fluid status is relatively scarce. A foreign study with cross-sectional analysis of neonates found that IVC-CI correlated well with central venous pressure, but not with either gestational age or birth weight. Evidently, IVC-CI was not influenced by birth weight or gestational age . The results of this study were similar to those of adult studies in that the IVC-CI correlated well with CVP  .
In the experimental group of this study, patient’s urine volume, heart rate, blood pressure, capillary filling time, lactate, and left ventricular ejection fraction (LVEF) were taken into overall evaluation, while referring to the IVC-CI practices in adult patient in making therapeutic attempts (Limited fluid resuscitation when IVC-CI < 15%, active fluid resuscitation when IVC-CI > 50%)  . Our study revealed that IVC-CI usually drop by 10-15% 6 hours after the onset of PH. This result was thus used to limit the fluid intake of patients in experimental group. We retrospectively reviewed the cases in control group and discovered that the volume intake in the first 6 hours and between the 6th to 24th hour after the onset of PH was 60±4ml/Kg and 93±4ml/Kg respectively, suggesting the experimental group received more fluid during the first 6 hours than the control group, fewer fluid in the following 18 hours, both with statistical significance. The following day witnessed no particular difference in terms of fluid intake. When comparing the time taken for improvement of lactate concentration and oxygenation index in both groups, the experimental group was shorter than the control group, and the difference was significant.
This study showed that ultrasound-guided measurement of IVC-CI can promptly assess the volume status, provide targeted guidance for fluid therapy, shorten the improvement time of lactate concentration and oxygenation index, and facilitate the recovery of the disease. It is similar to the findings of Clive N May et al  who reported that ultrasound-guided fluid therapy shortened the duration of ICU stay and the duration of mechanical ventilation in patients with severe sepsis.
It is worth noticing that we selected a different ultrasound probing point for measuring the IVC-CI compared to adult patients. A routine measuring point is in between the confluence of the hepatic vein into the inferior vena cava and the confluence inferior vena cava into the right atrium . while we select 0.5-1 cm below where the hepatic vein meets the inferior vena cava was selected as the measurement point. This alteration was made because of the anatomical short distance between the confluence of hepatic vein into the inferior vena cava and the confluence of inferior vena cava into the right atrium in neonates, and the routine measuring point is subject to the influence of heart beat, consequently affecting the accuracy of the measurement. Moreover, the measurement should not be performed when patients experience respiratory distress, which may lead to an abnormally elevated IVC-CI, thus causing misdiagnosis and unnecessary treatment.
Due to the small number of cases included into the study and an untraditional measurement point of the IVC-CI, there may be certain limitations about the conclusion of the study. Hopefully such limitation can be alleviated through future studies which include more samples, leading to a more robust, timely and accurate assessment of the effectiveness of fluid therapy in neonates.