Electrolyte and metabolic disturbances in critically ill patients

Electrolyte and metabolic disturbances are common in the critically ill patient. Hyponatraemia is most frequently caused by the syndrome of inappropriate antidiuretic hormone secretion. Over-rapid correction of hyponatraemia is associated with osmotic demyelinating syndromes, and sodium rise should usually be restricted to less than 10 mmol/day. Hypernatraemia is caused by a loss (e.g. diabetes insipidus after head injury) or inadequate administration of free water. It is treated by correction of the free water deficit with either intravenous 5% dextrose or water via a nasogastric tube. Prompt administration of desmopressin in diabetes insipidus will prevent severe hypernatraemia. Hypokalaemia is a frequent electrolyte disorder, associated with cardiac arrhythmias and is treated with intravenous infusions of potassium chloride of up to 40 mmol/hour. Hypomagnesaemia often co-exists and is associated with cardiac arrhythmias, muscle weakness and seizures. Correction of hypophosphataemia has been associated with improved cardiac and respiratory muscle function. The hyperglycaemic syndromes include diabetic ketoacidosis (DKA) and hyperosmolar non-ketotic hyperglycaemic syndrome (HNHS). DKA arises because of an absolute insulin deficiency causing hyperglycaemia and ketogenesis. This leads to dehydration, hypovolaemia and a metabolic acidosis. HNHS is due to a relative insulin deficiency from peripheral insulin resistance and is associated with profound hyperglycaemic dehydration without ketosis and acidosis. Both conditions are treated with intravenous fluid replacement, insulin infusion and correction of associated electrolyte disorders, including hypokalaemia and hypophosphataemia. There will be an underlying condition precipitating the hyperglycaemic syndrome (e.g. sepsis, acute coronary syndrome), which will also need to be treated.