NASA's are slightly cooler than the average bear, since theirs uses three-dimensional imaging.  I've done these tests about 6 times now, and despite being covered with electrodes and gooped up with transducer gunk, it never gets old... I have an endless fascination with watching the ultrasound screen where I can see the live image of my heart beating.  They press buttons and it rotates so they can take freeze-frames of various angles.  Awesome.

For this test, we have a thin, flexible catheter tube inserted into an arm vein. They turn off the lights (and their cell phones) for a 20-minute "quiet period" where we are told to keep our hands by our sides, not cross our legs, etc., after which an initial blood sample is taken from the arm.  Through a special apparatus with a snorkel-like mouthpiece, we breathe 100% oxygen from a tank for 1 minute.  A small amount of carbon monoxide (about the equivalent of 2 cigarettes) is piped into the breathing equipment and after 10 further minutes, a second blood sample is drawn for testing.

I am not actually sure how many times they continue to take readings, because after a few moments, voices and other sounds seem to tune out.  I always get warm, and have a mild feeling of dizziness, like I just want to close my eyes and sleep.  After a few more minutes, however, it feels like someone is crushing your head between a couple of bricks.

The first time I had nitro, I had a monster of a headache for most of the day.  Ditto the second time, only slightly less thunderous.  The third and fourth times?  Nothing.  Symptoms were entirely gone within an hour.  I thought I had grown accustomed to it, or built up a tolerance -- but I guess it can come and go.  This morning was my 5th nitro, and I've spent most of the day feeling like my skull was hit with a hammer.  I have an icepack on my head as I'm typing this... !