In adults and children low or high blood chloride levels are linked to the risk of death. The aim of this observational study is to determine whether there is a relationship between low or high blood chloride levels and the risk of death or long-term lung problems. We will also learn the risk factors and associated conditions of high or low blood chloride levels. We will include infants born before 32 weeks of pregnancy or have a birth weight of less than 1500 grams in the study. The main question it aims to answer is:

Is there a relationship between low or high blood chloride levels in the first 4-6 weeks of life and risk of death or long-term lung problems in premature babies? We will examine the medical reports of babies who were followed up in neonatal intensive care unit over the past 5 years.

Chloride balance usually parallels that of sodium, and it is strictly correlated to the extracellular volume balance. In addition plasma chloride and bicarbonate concentrations are inversely regulated through the chloride-bicarbonate exchange pump in renal collecting ducts, independent of sodium. Consequently, plasma chloride levels are closely correlated with pH (hypochloremia/metabolic alkalosis, hyperchloremia/metabolic acidosis).

In adults, dyschloremia is associated with mortality, acute kidney injury, and prolonged hospital stay. Hyperchloremia is often associated with severe sepsis. It has been shown that resuscitation with high-chloride fluids (eg: saline solution) instead of balanced fluids (e.g., Ringer's lactate) increases the need for inotropes in critically ill adults. Similarly, hyperchloremia in septic pediatric patients is linked to acute renal injury requiring dialysis, increased inotropic support, and mortality. In 1979, infants fed with chloride-deficient formula were reported to develop impaired head growth and neurologic sequelae.

Although serum chloride is routinely measured in neonatal intensive care units, its clinical significance is often overlooked. For this reason unlike sodium, literature on chloride metabolism in preterm infants is exceedingly limited.

Advancements in perinatal care over the past 50 years have significantly improved survival rates in preterm infants. However, a large proportion of very preterm infants who survive the neonatal period develop bronchopulmonary dysplasia (BPD). BPD, a chronic lung disease, manifests clinically through persistent respiratory support and/or oxygen dependency. Despite efforts to optimize neonatal care -such as controlled oxygen usage, non-invasive ventilation, volume-guarantee techniques, high-frequency ventilation, and caffeine therapy- BPD remains the most common complication among preterm infants and is associated with increased morbidity and mortality.

It has been suggested that extracellular volume expansion (edema) plays a role in the pathophysiology of BPD. Perlman et al.'s 1986 study demonstrated that infants who died from BPD exhibited hypochloremia, metabolic alkalosis, and complications like inadequate head growth, more frequently than those who survived. These findings highlight the critical importance of chloride imbalance in neonates, similar to findings in adult and pediatric populations.

This study aims to investigate the relationship between chloride balance and two major outcomes-mortality and the development of BPD-in preterm infants. Infants <32 weeks postmentruel age (PMA) or weighing less than 1500 grams will be enrolled in the study. We also plan to explore the relationship of dyschloremia with other outcomes of prematurity (eg: patent ductus arteriosus, ventilator dependency, retinopathy of prematurity (ROP), necrotizing enterocolitis, intraventricular hemorrhage, periventricular leukomalacia, hospital stay. Lastly we plan to explore the risk factors and clinical/laboratory conditions that are associated with chloride level abnormalities.

If this study yields findings that underscore the clinical significance of chloride levels in preterm infants, similar to adults and children, then close monitoring of chloride balance and appropriate interventions could potentially reduce mortality and BPD incidence in preterm infants. Conversely, if results indicate that chloride balance lacks clinical significance in preterm infants, this will also contribute valuable information to the current literature.

Inclusion Criteria:

• Infants born <32 weeks PMA or <1500 grams
• Infants admitted to NICU within the first 24 hours

Exclusion Criteria:

• Infants with major congenital anomalies
• Infants with chromosomal anomalies
• Infants who have undergone enterostomy operation
• Infants admitted to NICU after the first 24 hours

Study population includes infants admitted to the Neonatal Intensive Care Unit at Kanuni Sultan Suleyman Training and Research Hospital over the past 5 years.

We are not sure that local ethics committee that approved our study will allow IPD sharing