Veronica - Do you allow complete recovery before each send? One of the reasons I do the hypoxic swims is because I kayak and it is very handy to be able hang out underwater for extended periods without freaking out and exiting the boat. I have spent a fair amount of time underwater which gives rise to feelings of panic, alarm and face/helmet strikes to submerged rocks. Knowing that I'm not going to pass out from the discomfort of holding my breath for extended periods gives me time to rearrange the river bed with my face a bit before I remember to keep my head down and get a decent roll. The mental aspect of gutting out that last few meters before you get to the wall is a big part of what the hypoxic swims develop.
Rather than resting for a given period of time, select a number of lengths and time how long it takes you to complete that number. I do 20 lengths and felt pretty good about myself for doing it in something under 12 minutes until Eric and Matt torched off 9 somethings. Freaks. Anyway, keep score and whittle down your total time by resting less between sends.
You'll go a bit faster if you swim with your hands extended out in front of you. Breathe out very sharply and try to get rid of as much C02 as possible between swims. If you get the sparklies and start seeing fireflies as your vision narrows SURFACE IMMEDIATELY as you are about to have a shallow water blackout. Swim with someone or make someone aware of what you are doing...just in case. Be careful to no pressurize your lungs too hard as it can hasten unconsciousness.
Hmmm...why are we doing this again?