Background: Hyponatremia is associated with increased hospital morbidity and mortality and occurs in about 25% of hospitalized children. Vasopressin has become increasingly used to treat hypotensive shock following cardiac surgery due to its unique vasoconstrictive properties compared to catecholamines. However, vasopressin use has also been associated with increased incidence of hyponatremia, of which neonates are at particular high risk due to immature renal function. We hypothesized infants receiving vasopressin post-cardiac surgery are more likely to have hyponatremia than infants who do not receive vasopressin, and aimed to identify risk factors for hyponatremia in infants who receive vasopressin.  

Methods: 75 consecutive infants < 90 days of age who underwent cardiac surgery at a single center between 2018-2020 were included in this retrospective analysis. Maximum and minimum serum sodium levels were collected for the first 5 postoperative days as well as daily vasopressin dose ranges and cumulative vasopressin dose. Additional data points collected included patient age, weight, total sodium intake, diuretic dose, and surgical variables including the use of modified ultrafiltration.  Comparisons across postoperative days within- and between- patients who received vasopressin were examined using mixed effects linear regression.

Results: 36 of the 75 patients received vasopressin during the study. There was an overall decrease in serum sodium among all patients relative to baseline serum sodium levels, ranging from a 5.6-9.6mmol/L decrease (p<0.001). The vasopressin group had an increased fall in sodium from preoperative levels compared to the no vasopressin group that was significant beyond the first postoperative day and increased over time (p<0.001). There were 7 events of moderate-severe hyponatremia (sodium <130mEq) in the vasopressin group and 2 in the control group (p=0.10). The vasopressin group also showed increased variability between daily sodium minimum and maximum levels on day 1 and 2 (POD1 1.31mmol/L, p = 0.002 and POD2 1.227mmol/L, p = 0.003) relative to control.  Analysis of risk factors showed that total vasopressin was significantly correlated with sodium variability. The vasopressin group received more total intravenous fluids during the study. Additional analysis showed that overall sodium level and sodium variability did not differ based on age, modified ultrafiltration, or total diuretic dose. These findings indicate greater reduction in overall sodium levels as well as wider sodium fluctuations in the vasopressin group. 

Conclusion:
These results indicate that vasopressin use in infants after cardiac surgery is associated with overall increased risk of hyponatremia as well as increased variability in serum sodium levels in the early postoperative period. Sodium levels appeared to decrease in both groups in a progressive manner throughout hospitalization, though the vasopressin group had a greater decrease from baseline. The vasopressin group was also at an increased risk to have wide fluctuations in daily sodium early postoperatively. This effect on variability later decreased over time, suggesting progression to sodium equilibrium. Hyponatremia appears to also be associated with cumulative vasopressin administration during the postoperative period. Age, preoperative weight, and total diuretic dose were not associated with increased sodium variability or risk for hyponatremia. These results suggest that patients receiving vasopressin postoperatively may require closer monitoring of serum sodium in the early postoperative period.