Hyponatremia is among the most common electrolyte abnormalities encountered in the perioperative period [10]. Hyponatremic encephalopathy is a serious complication that can result permanent neurological injury or even death [11]. The threshold for seizure activity and abnormal neurological examination secondary to hyponatremia is variable; however, symptoms are more likely to occur at sodium levels less than 125 mEq/L (< 125 mmol/L) [12]. Sodium levels between 130 and 135 described as mild hyponatremia in our study may not have any clinical significance especially in neonatal population. A varied incidence of hyponatremia has been described in pediatric patients undergoing surgical procedure depending on the definition of hyponatremia, patient selection, and the setting of postoperative care [13, 14]. The cause of postoperative hyponatremia is predominantly by a combination of nonosmotic stumuli for ADH release including volume depletion, pain, nausea, stress, narcotics, and the administration of hypotonic fluids [15]. Incidence of hyponatremia is even higher in neonatal population. Pooled data from literature in neonatal intensive cares shows a very high mean incidence of hyponatremia (40%) [13]. The population of neonates undergoing TEF repair may be especially vulnerable to this entity because most of them are sick and ventilated in postoperative period. The basis for this patient selection is based on our assumption that in thoracic surgeries restrictive fluid regimes can lead to less interstitial edema which can facilitate early extubation. Also, this group of neonates is at a higher risk of developing SIADH.
Hyponatremia was more common in our study in immediate 24-h postoperative period as compared to later because of possible contribution of SIADH. Another reason could be dilutional hyponatremia secondary to extra fluid received by these neonates during surgery. Urine output was significantly less in neonates who developed hyponatremia despite receiving comparable average fluid during the postoperative period. SIADH can be the most plausible explanation for this finding.
There has been a paradigm shift in the fluid management of sick children over the last decade. Isotonic fluids are now used routinely for intraoperative maintenance in children [16]. Karen Choong et al evaluated the risk of hyponatremia following administration of isotonic compared to a hypotonic fluid for 48 h to postoperative pediatric patients. Hypotonic fluids significantly increased the risk of hyponatremia, compared with isotonic fluids (40.8% vs 22.7%; relative risk 1.82) [17]. There are several other trials and guidelines recommending isotonic fluids in sick pediatric population. However, there are a very few trials comparing isotonic versus hypotonic fluids in neonates. A consensus has not yet been reached due to concerns regarding the ability of the neonate to handle salt solutions. The clinical practice guidelines of the American Academy of Pediatrics have also excluded the neonates from the consensus to use of isotonic maintenance fluids in neonates due to lack of data. We also continue to use hypotonic fluids in our unit but with volume restriction.
It may not be sufficient to use isotonic fluids alone to decrease the incidence of hyponatremia in critically ill children. Even with isotonic maintenance, more than 20% of children were diagnosed with hyponatremia [18]. Fluid restriction along with the use of isotonic fluids can decrease the risk of hyponatremia in sick children. As per the NICE guidelines, if there is a risk of water retention associated with non-osmotic ADH secretion, consider restricting fluids to 50–80% of the routine maintenance needs [19]. As free water excretion is altered for all children in the postoperative period, reduction in the volume of maintenance fluid therapy to half the previously recommended volume has been suggested by a few authors [12]. Probably, the incidence of hyponatremia in our unit where we are using hypotonic fluids was comparable to other units using isotonic fluids because of our policy of restricting the fluids. As per most neonatal protocols, fluids are started at 60 ml/kg in term neonates and 80 ml/kg in preterm neonates. This maintenance is hiked by 15–20 ml/kg to reach a fluid infused at 150 ml/kg/day [6]. But we have a protocol to keep the intravenous fluids restricted even with the advancement of age unless clinically indicated. Frusemide has also been used in a large number of neonates in the postoperative period to maintain a restrictive intravascular volume.
Various outcomes were compared among neonates with and without hyponatremia. Hyponatremia in the postoperative period is an important risk factor for mortality which was evident in our study too [20]. Sepsis was also significantly more in neonates with hyponatremia. Association between sepsis and hyponatremia has been vastly described in Literature [21].
The incidence of seizures was more in children with hyponatremia, even though the neurological status of these babies may not be adequately assessable due to concomitant sedation. There may be a concern about impaired organ perfusion with a restrictive fluid regime. However, we did not observe worsening renal functions and urine output also remained adequate. Lactate level was also not affected because of this practice of fluid restriction.
The major limitation of this study is the absence of a control group receiving non-restricted fluid, however, considering our unit practice it would have been unethical to give extra fluids to the neonates whom we consider to be at an increased risk for developing SIADH. We do not routinely do urine osmolality which could have strengthened our hypothesis.
We assume that the incidence of severe postoperative hyponatremia in our unit is less than documented worldwide due to our policy of fluid restriction and use of Lasix however, the incidence can further be reduced if we use isotonic fluids as per the current guidelines in sick pediatric patients. Further studies are required to be conducted in the neonatal population to evaluate the benefits of using isotonic fluids in the perioperative period.