Infant surgery

Infant electrolytes

Pyloric stenosis

Older children

Specific losses

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Infants with surgical problems frequently present with fluid losses and are variably dehydrated. This is where you need to think about the physiology of the congenital anomaly. Remember your goals in providing fluid therapy: replace losses, provide for maintenance requirements and estimate ongoing losses. For instance, there is not much reason to suspect that an infant promptly diagnosed with esophageal atresia /TEF is tremendously dehydrated ( remember he/she just came off placental bypass and hasn’t needed to depend on GI absorption yet.
 In contrast, the infant with gastroschisis has an immediate potential for significant fluid losses - in patients with relatively normal looking bowel surfaces that are immediately enclosed, those losses are probably less significant that in infants with a weeping/inflamed serosal surface. Gastroschisis 
This is where it is important to assess the individual patient and not make cookbook decisions and this is where experience is essential. Still,  you can start by making educated guesses and frequently reassessing the outcomes of your intervention. These measures will be those of adequate perfusion - heart rate, blood pressure, capillary refill, skin color and most importantly urine output. This requires attention to detail and a lot of number crunching. Inadequate perfusion is as treacherous as fluid overloading. Remember,  the ductus arteriosus is supposed to close postnatally, triggered by the reduced resistance in the pulmonary bed. Simplistically, think of sudden excess fluid as resulting in a full pulmonary bed, triggering shunting, which unless reversed leads to hypoxemia and worsens the pulmonary hypertension. Therefore, be careful about the rate of bolusing fluids and anticipate losses so that the baby doesn’t face repeated sudden volume expansion.
 Large left-sided CDH; note stomach (NGT) and liver (umbilical vein catheter) in chest (arrows)  Some infants, such as diaphragmatic hernia (CDH) babies, are set-ups for pulmonary hypertension. Although they need an adequate circulating volume for good perfusion and the avoidance of metabolic acidosis, this is often balanced with some inotropic support from dopamine or dobutamine to artificially keep systemic arterial pressures and -resistance greater than their pulmonary coutnerparts and avoid ductal shunting.
These examples give you a baseline appreciation for the variability in neonatal fluid management and hopefully will stimulate curiosity about the individual physiologic concerns. They also point out that babies can be categorized as euvolemic, mildly dehydrated or significantly dehydrated and that general principles can then be applied. Thinking of volumes per unit body weight is much more meaningful in this instance than speaking in terms of total fluid rates or volumes.

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