Jimmy,

Brew's comments about needing a minimum oxygen flow rate of 25 liters/minute to support hyperventilation are spot on.  An oxygen flow rate of 12 liters/minute with a non-rebreathing mask can actually cause a build up of CO2 if you're experiencing any additional physical activity like doing the CH dance.  A slight elevation in arterial CO2 can completely negate the benefits of oxygen as a vasoconstrictor causing even more vasodilation.

I'll add to that by saying the three most important factors in effective oxygen therapy as an abortive for cluster headache are:

(1) Hyperoxia - breathing 100% oxygen as it acts as a vasoconstrictor;

(2) Hypocapina -  hyperventilating with 100% oxygen to cast off CO2 faster than your body is generating it through normal metabolism until you reach a condition called respiratory alkalosis that also elevates arterial pH above normal with symptoms of paresthesia (a slight tingling or prickling of the fingertips, lips, or back of the neck as well as a sensation of dizziness).  Hypocapnia and an elevated pH are also vasoconstrictors; and,

(3), An elevated arterial pH increases hemoglobin''s affinity for oxygen.  This is a respiratory physiology phenomenon called the Bohr effect, that can super oxygenate the blood flow to the brain increasing it's vasoconstrictive effect.

All three of these factors increase the effectiveness of oxygen therapy's capacity to abort a cluster headache as fast as possible... Seven minutes is a good average abort time for pain levels 3 through 9 with attacks at the lower pain levels 5 and below aborting faster than 7 minutes and attacks at pain level 7 and above taking longer than 7 minutes.

Having said that, there will be times when even higher flow rates up to 40 liters/minute will take much longer than normal to abort an attack. 

It's just a theory at this point, but it appears the increased difficulty in achieving an abort even when hyperventilating with 100% oxygen may be due to a low arterial pH - a higher than normal acid content. 

This low arterial pH condition can be caused by diet or other conditions like allergic reactions that bias our metabolism towards acidosis and cause increase inflammation.

A few of us have had success countering this condition with a regimen of calcium citrate tablets that also contain vitamin D3, magnesium and zinc washed down with sugar free lemonade, limeade, or Baja Bob's sugar free margarita mix.

Hope this helps.

Take care,

V/R, Batch