Bob,

Good question.  The short answer is we can breathe at oxygen flow rates of 25 liters/minute and higher because it’s easy.  If you jogged around the block, I would venture your 15-year younger lungs would have no problem breathing at these rates.  The full answer why flow rates like this are needed is a bit longer so please let me start by saying there are no absolutes when it comes to cluster headaches...  We're all wired differently and some therapies tend to be more or less effective on an individual basis.  

This same rule applies to oxygen therapy, but only up to a point. Respiratory physiology is a homeostatic process where the body automatically regulates respiration and pulse rates as the primary mechanism to maintain an equilibrium or balance of oxygen and carbon dioxide at normal levels.

Having said that, there are also physical and medical conditions or factors that tend to bias that equilibrium in one direction or another, and in the process, inhibit or enhance the abortive power of oxygen therapy accordingly.  For example, a low cardiac output, metabolic acidosis, or a COPD disorder like asthma are examples of medical conditions that tend to inhibit the beneficial effects of oxygen therapy and make it less effective.

There should be no question that an oxygen flow rate of 7 to 9 liters/minute is more than sufficient to oxygenate blood hemoglobin to 100% in the arterial bloodstream at a normal respiration rate.  Having said that, the physiology of respiration is based on the exchange of two fundamental gasses within the lung's alveoli, oxygen and carbon dioxide.  While breathing 100% oxygen at 7 to 9 liters/minute represents a 5-fold increase in the oxygen concentration inhaled, it’s the respiration rate and tidal volume of inhaled air that regulates CO2 levels.  

If you're using a properly functioning non-rebreather mask that prevents any room air from being inhaled, your respiration rate will be limited by the regulator's flow rate.  Depending on the level of CO2 being generated, an oxygen flow rate of 7 to 9 liters/minute may not provide sufficient lung ventilation to remove or cast off excess CO2.

In other words, a flow rate of 7 to 9 liters/minute represents a minute-volume of 7 to 9 liters where the minute-volume equals the amount of air or oxygen inhaled in one minute.  Depending on the level of physical activity, a minute-volume of 7 to 9 liters may not be sufficient to ventilate the lungs with enough volume to cast off excess CO2 in order to maintain its equilibrium at normal levels.  

As CO2 is the primary byproduct of metabolizing blood sugar in the presence of oxygen, the amount of CO2 generated depends on the level of physical activity.  Just like the motor in your car, increasing physical activity increases the rate of metabolism and that increases the amount of CO2 generated.

The same thing happens when you step on the accelerator in your car.  Both systems consume more fuel and oxygen, and that results in an increase in the output of exhaust.  This is easy for a car’s engine as it’s equipped with separate intake and exhaust manifolds…  

We on the other hand, need to breathe much faster as our lungs must accomplish both functions.  We breathe faster when chemoreceptors in the brain stem and heart detect a rise in the CO2 level and in turn signal an increase in respiration and pulse rates in a homeostatic process to regulate the CO2 level back to normal.  

The other point to consider is CO2 works both ways…  Too much CO2 is bad and too little is good.  

When the CO2 level rises above normal due to constrained lung ventilation caused by oxygen therapy at lower flow rates with a NRB mask, this condition acts as a very powerful vasodilator that not only negates the beneficial vasoconstrictive/abortive effects of elevated oxygen levels, it also increases the time to abort.  In some cases, this condition (hypercapnia) makes an abort with oxygen therapy impossible.  

When we cast off CO2 faster than our bodies generate it by hyperventilating (hypocapnia) on 100% oxygen, we go into respiratory alkalosis, the arterial pH rises, and as hemoglobin has a greater affinity for oxygen as the pH rises, it caries more oxygen to the brain increasing the abortive effect.

Getting back to aborting your attacks with an oxygen therapy at a flow rate of 7 to 9 liters/minute…  What is your success rate and what is the average time to abort your attacks?  Are you able to remain seated and motionless while using oxygen therapy, or do you get up and dance as the pain increases?

Most of us who use oxygen flow rates high enough to support hyperventilation are able to abort our cluster headache attacks at consistently higher success rates and at a third the time it took at lower oxygen flow rates that did not support hyperventilation.  Most of us are also able to remain quietly seated during oxygen therapy up to Kip-6, but find the level of physical activity starts to increase rapidly at Kip-7 and above.  By Kip-8 to Kip-9 we’re in leg shake and rocking rapidly or up and dancing the tarantella or cha cha at a high tempo around the O2 cylinder.

Again, good question.  I hope this helps.

Take care,

V/R, Batch