Hi,
This is a complex issue, which isn't solved by just suggesting a certain medication that can be given. Sodium levels in the body can be altered by a number of physiologic states, and diseases. Normal levels are usually in the 135-145 mEq/L range, depending on the lab. The reason the soduim level is low is related the the often total body water (eg if the total amout of body water is high, the sodium level can be diluted and measure low).
The real issue is to find out what the cause is, and correct the cause... if that is not done, simply replacing the sodium will not solve the problem. Some causes of hyponatremia are congestive heart failure, liver cirrhosis, kidney disease, Addison's disease. Obviously if this patient is in the ICU, there may be a number of underlying issues causing this symptom.
You should speak to the patient's physician about what the cause is and I am sure he/she will share with you what they are trying to do to correct it.
Good luck,
JMK MD