A 65 y/o patient presents with significant thirst, restlessness, hyperreflexia, and a serum Na = 158 mEq/L. The patient has no other significant medical history and is on no other medications. Other labs are WNL and the patient is cardiovascularly stable. After a thorough history and physical, the patient is diagnosed with isovolemic hypernatremia. The medical student wants to administer 'free water' and asks you to order ¼ normal sodium chloride (0.225% sodium chloride) IV @ 100 ml/hour. (FYI - Normal Saline = 0.9% Sodium Chloride)

Do you write the order or not? Why or why not?

In a brief review of the literature, I couldn't find a 'magic' number regarding the lowest acceptable osmolarity or tonicity of an IV solution. Most people have been taught 0.45% sodium chloride is the lowest concentrations that can be used. 0.225% sodium chloride (1/4 normal saline) is NOT available commercially. There are no clinical conditions where the use of ¼ normal saline is commonly accepted. For these reasons, there is very little information available about safe infusion rates for this product.

Hypotonic and hypertonic solutions may be infused in small volumes and into large vessels, where dilution and distribution are rapid. Normal osmolarity of blood/serum is about 300-310 mOsm/L. Solutions differing greatly from the normal range may cause tissue irritation, pain on injection, and electrolyte shifts. Just like the treatment of hypernatremia, correction of the serum sodium should be accomplished slowly. Correction at a rate of no greater than 1 mEq/L per hour has been recommended. Too rapid an infusion of hypotonic saline (and correction of serum sodium) can cause serious CNS side effects such as cerebral edema and seizures. Remember, the brain would also be hypertonic and since free water would go to the area(s) of high osmolarity, the brain would/may swell.

Some general guidance on osmolarity of solutions for peripheral use are:

    -    Not to exceed (NTE) 600 mOsm/L for peripheral IV solutions
    -    NTE 900 mOsm/L for peripheral nutrition solutions
    -    NTE 1200 mOsm/L for peripheral nutrition solution containing fat emulsions

Feedback/comment on last week's Pearl about rebound headaches from an AF neurologist:

"1) OVER-THE-COUNTER ANALGESICS ARE THE MOST COMMON CAUSE FOR ANALGESIC REBOUND. Many providers do not ask about the frequency of use of Tylenol, Excedrin, etc. I estimate 25% of all the headache referrals I get are associated in part with analgesic rebound, and I usually never find documentation on OTC analgesic use from the referring provider.

2) Prophylactic medications are not effective while a patient takes excessive amounts of analgesics.  Starting appropriate care for their headaches can be delayed at least 1 month if I am the one who has to tell them to stop their excessive analgesic use first (since it takes up to a
month to get in to see me).

3) Often, with stopping an OTC analgesic "cold turkey", a patient's headache severity will get worse for 1-2 weeks, then start to subside.  Many patient, though, cannot tolerate this.  A reasonable alternative is to schedule a slow taper off of OTC analgesics over 4-6 weeks.

4) Ask patients to keep track of all of their abortive medication use by using a diary or log.  This places some responsibility on the patient for their own health care, and is very valuable to the provider in deciding what to do regarding medications. (Assuming, of course, that the diary is
reliable.)

5) For difficult cases, sometimes admission for 3 days of IV dihydroergotamine (DHE) can be used to help break a daily headache cycle (the headaches do not necessarily need to be migranous)."