Jeff,

Good on you for starting oxygen therapy... Keep it up and you’ll never be sorry.  Depending on the max Kip-pain level during oxygen therapy, I'm guessing your abort times are between 12 and 18 minutes.  A higher flow rate can cut that in half or thirds.  Check your PM.

As you've probably figured out, the oxygen therapy is not giving you rebounds...  The reasoning is simple.  A rebound is a physiological/pharmacological resistance build up to a medication that's being used too frequently for to long a time.  If you developed a resistance to oxygen, you would assume room temperature.

What you are having is similar to a re-attack.  A re-attack generally occurs between 15 to 45 minutes following a successful abort with oxygen therapy, but not at regularly scheduled attack times.  What is happening with a re-attack is the oxygen therapy works to abort the pain but not the triggering mechanism.  When the vasoconstrictive effects of hyperoxia from the oxygen therapy wear off, and the triggering mechanism is still present in your system, the attack resumes. 

A re-attack is also a good indication the oxygen therapy is working properly. 

In your case, with the attacks coming like clockwork every two hours, it's almost like the oxygen therapy has set the timer for your attacks at two hours.  This happens to a lot of us.  Like re-attacks, the frequency of regularly scheduled cluster headache attacks will start to decrease after two to three weeks of oxygen therapy if there are no other conditions interfering with it.  Again, a higher oxygen flow rate may be helpful.

I quit taking imitrex over two years ago and use only oxygen therapy now. When I was taking it, I would split a 50 mg tablet and take half at night along with the oxygen therapy and get 4 to 6 hours PF sleep.  If you're using a third of a 6 mg imitrex injection, you’re only 2 mg, and the half-life or clearance rate of basic imitrex is 2 hours to the 50% serum level.  The 25 mg tablet will not work as fast in aborting your attacks, but it will work longer.

Not all triptans are created equally. Check with your doctor.  A longer half-life triptan, like eletriptan (Repax) or naratriptan (Amerge) may be an alternative to the more expensive imitrex injections and give you more PF time, as long as oxygen therapy is effective.

You also might want to pick up some pH strips and start tracking your pH. Measuring the pH of your saliva is easy and it will provide a good, but not exact indication of arterial pH.  A low arterial pH (too much acid) tends to increase the frequency and intensity of cluster headache attacks.  You can pick up some reasonably priced pH test strips from pH-ion on line at:
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If your pH is low, you may want to check out some dietary tips on how to raise it at the following link:

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Mineral supplements (Calcium and Magnesium with Zinc) and simple diet changes from the link above can raise your system pH and this may help to reduce the frequency and intensity of your cluster headache attacks.

Take care,

V/R, Batch